Ultimate-Jets

Barbed fittings vs push-to-connect fittings

I have seen an increasing number of people using push-to-connect fittings on the suction side of the fuel pump.
Most people do this because it is very practical and straightforward.
However I would like to bring some important cons to people's awareness and emphasize on technique.

First of all what is the pros and the cons of using barbed fittings vs push-to-connect fittings?

Barbed fittings:

Pros ( if safety wired ):
Secure
Leak proof
Durable
Several types of tube ( material ) can be used.

Cons:
Requires safety wiring
The tube will be somewhat difficult to pull when comes the time to remove it.

Push-to-connect fittings:

Pros:
Fast to insert
Fast to extract ( if done properly )

Cons:
Can only be used with PU tubing of shore A60 and above ( no tygons ! )
Can promote leaks after a while ( see below )
Can exhibit reduced life ( see below )

So what about the risk of leak and shorter life of push-to-connect fittings?
We this is mostly due to one fact:

The plastic push-to-connect fittings most people and manufacturers use ( QS type Festos fittings) are not designed to be used with fuels.

They rated by Festo to mostly accept air and water.
What's the problem here?

1. The body of the plastic connector is made of polybutylene therephthalate ( PBT ).
https://en.wikipedia.org/wiki/Polybutylene_terephthalate
Although this polymer exhibits good resistance to diesel/ kerosene/ gas, it does get affected by it on the long run. The material swells slightly and gets more brittle.
2. The o'ring inside the fitting that is sealing the tube is made of nitrile ( NBR ). Although nitrile also exhibits good resistance to fuels, it does harden slowly when exposed to them and get brittle as well.

Here is a cross section of a push-to-connect fitting:

The o'ring is fitted inside the polymere body, close to the tube end.

The retention ring is a spring type nut that prevents the tube from sliding out of the fitting. it pushes against the pube and is only effective if the tube is rigid enough.

After a long exposure to fuels, the PBT fitting body will swell a bit and the nitril o'ring will harden. the sealing of the tube will not be as good as before. In positive pressure, the slight deflection of the tube should be sufficient enough to prevent leaks in most cases. However in case of suction, the thinner air is more likely to leak inside the system.

 

Push-to-connect fittings associated with soft tubes on the suction side of the pump might leak air.

Her is what can happen: A soft tube could be deflected inwards under the action of vacuum.

This could lead to a deficit in tube pressure against the seal and get some tiny bubbles of air get sucked inside the fuel system. Here  is a schematics showing what could happen:

 

However, if the fitting is on the pressure side of the pump, the tube will be pushed against the o'ring and sealing will be improved:

 

 

Altogether,  don't recommend using push-to-connect fittings on the suction side of the pump for these reasons. Barbed fittings are much more reliable and relatively easy to setup. They require no servicing.

If anyway you are willing to go for push-to-connect fittings, please consider the following:

1. Use our Legris fittings which are made of fiber reinforced nylon ( very strong resistance to fuel ) and use NBR seals ( very good resistance to fuel ).

http://www.ultimate-jets.net/collections/fuel-hardware/products/legris-hiflow-push-on-fitting

2. Inspect the fitting/ tube on the pressure side of the pump for wet areas. Change the fitting when the tube interface becomes wet ( fuel leak )

3. Change the fitting on the suction side of the pump every two years ( unless using Legris fittings ). Leaks cannot be detected other than appearing small bubbles/ unexplained flameouts!

 

Written by Olivier Nicolas — August 26, 2016

New generation air traps and avoiding cavitation.

 

1.     Introduction

The air trap market has long been dominated by the BVM UAT and its derivatives. However with the coming of bigger and more powerful engines, this little device that was designed about 15 years ago is reaching its limits.

Quite a few companies have come to the market with new generation air traps, using a completely different approach from the original UAT. These units are much more suited for high flow engines and some of them will protect your fuel system very efficiently from the dangers of cavitation as I will explain below.

2.    The concept

 

The venerable BVM UAT was designed many years ago to protect the best ducted fans and the first JPX jet engines from sucking air bubble that would almost certainly lead to a flameout. The idea behind this device was to be able trap a small air bubble that would otherwise make its way to the engine and create a flameout condition.

2.1. Imperfect fuel pickup

The main reason why air would get into your fuel line is not because of a leak but because the fuel tank clunk is not a perfect system. Although it is heavy and mounted on a flexible line to stay in the fuel as much as possible, there will be situations when it will get out of the fuel and suck air! This will happen towards the end of the flight, as the tanks get mostly empty and/ or while performing aerobatics.

The BVM UAT was designed for fairly reasonable fuel flows by today’s standards. Just imagine that the biggest engines available on the market nowadays can burn 1 liter of kerosene per minute, whereas the first JPX turbines were burning a quarter of this amount.

So as good as the older system are, they only offer a few seconds of maximum thrust fuel flow buffer. However, for hard core aerobatics flyers and people who like to fly at full thrust for a large portion of the flight and keep high thrust level in the dive, the amount of air that is sucked by the clunk can be up to 15 seconds of full thrust fuel flow!

For this reason I strongly recommend choosing and air trap that offers 30 seconds of full thrust fuel flow buffer capacity.

 

Here is a video example illustrating the fact. This video is taken on board an aerobatic plane. During this 30 second sequence, one can see that the fuel clunk is out of the liquid for exactly 50% of the time, pickup up air instead of fuel during this period!

 

2.2. Cavitation

What happens with these high fuel flow is that the pump is required to deliver a lot of pressure to the engine from the 4 mm PU tubing. This means that the suction upstream the pump, ie vacuum level,  can be very high. This may lead to a phenomenon that is not very well known to the modelers: fuel cavitation.

Kerosene is very aerophile. It can absorb over 10 % of its volume of air. This air is dissolved into the fuel until the condition for releasing is met: namely depression. This depression does not have to be very high to create a fuel cavitation condition. You can see it by using a plastic syringe filled with fuel and pulling the piston while closing the tip. The kerosene will foam quite quickly. The air dissolved releases.

What happens in our models is that air bubbles are created by both a certain level of vacuum AND turbulence (this is very close to what happens in a can of beer. As soon as you open the container, its pressure drops and thousand of micro bubbles of CO2 start coming to the surface). The reason behind this is that the micro swirl in turbulence creates local point of lower vacuum where the vapor pressure of air dissolved in kerosene equals the local level of vacuum. This is where the liquid "explodes" in a micro bubbles. Thousands of micro swirls create thousands of micro bubbles. Then the high energy stream keeps on swirling passed the creation point and prevents the bubbles from re-dissolving.

Real size passenger aircraft are constantly prone to fuel cavitation. In fact, every time the aircraft is climbing, the fuel is literally “boiling” in the tank above 30,000 feet for a certain amount of time until all the dissolved air is released. This is why commercial aircraft have got their low pressure fuel pumps  mounted in the tanks. These pumps are designed to take a mix of air and fuel and re-pressurize it considerably in the downstream lines so that all the air bubbles are forced to dissolved again before the fuel reaches the engines and their high pressure pumps.

We do not have wet pumps in our models. The created air bubbles will not dissolve immediately when the vacuum/ turbulence condition stops because they are made of air, not kerosene. Condition for the bubbles to re-dissolve would be to agglomerate into a bigger bubble with a lower tension surface into a locally calm area of the stream. In any case, re-dissolving would take between one to two minutes. Way too slow to avoid bubble propagation into the engine.

additionally, old generation air traps are small in volume and made of a soft material by design. This combined with what I call a high suction drag ( too much vacuum upstream the pump creating fluid drag) could lead the pump to vacuuming the air trap. You’d see the walls of the air trap slowly bending until they touch the filter bag. At this point, the bag would get its usable surface decreasing rapidly and start to cavitate itself: air bubbles would form downstream the bag as if it was letting the bubbles through. In fact, what happens in this case is that the clogging of the bag creates a restriction that leads to increasing the suction downstream it. When this suction reaches the cavitation point, it looks like the bag is bleeding air bubbles.

 

This phenomenon can also happen in any point of the fuel system upstream the pump when a restriction occurs.

So having that in mind, some manufacturers have decided to come up with new designs to improve the resistance of the unit to cavitation as a secondary function of the air trap, while improving the primary function ( air buffer ) by increasing the available  volume.

 

2.3. Summary

To resume,  the functions of a modern air tap are:

1. Act as a buffer for the non perfect clunk ( air trap )

2. Reduce the likeliness of cavitation

3. Act as a first level of filtration ( raw filtration that will not substitute to the post pump fine filter )

 

I will analyze here for you the Hanson ( BVM ) , PST and GBR Jets and Tom Cook ( JMP ) solutions.

 

3.    Devices description

 

3.1.                    BVM Universal Air Trap

The BVM air trap is the “ancestor” but is still widely used around the globe and sold from our shop as kit bundle on some variants. I remember using this unit as fast as 1995 back with my ducted fans. At this time the concept was revolutionary. It is made of a semi clear semi soft plastic tank and an automotive synthetic fuel filter bag commonly called “Kuss”. The fittings are fastened through the plastic, and the cap is sealed with a few turns of Teflon plumber tape. The optimum orientation is 45 degrees nose up.

 

 

 

3.2.                    The Hanson SuperTrap

The Hanson super trap is a declination of the venerable BVM UAT that came on the market in 2006.

As such it uses the same soft Nalgene bottle and the Kuss automotive filter bag.

The main difference is that the bag is safety wired onto an aluminium assembly that provides two advantages: to keep the bag open is all circumstances, and to enable the use of a ¼” SMC push fitting, or large barbed fitting. This assembly also houses a micronic filter so that this unit can be used as a filtering device as well. 

 

 

 

A Hanson Super trap dismantled. It includes from left to right: a ¼” push on fitting, an aluminium housing, a micronic filter, a filter retainer and the Kuss bag.

I found two problems with this unit: firstly the micronic filter is quite restrictive and not really required considering the fact that the Kuss bag also provides a fairly good level of filtration ( in fact as much as a standard aluminium turbine filter ). Secondly, although it is a very good idea to use a large push on fitting, it appears that this SMC unit is quite restrictive. The inner diameter is only 2 mm, whereas the aluminium housing minimum  inner diameter is 2,8 mm.

So this makes this unit optimally suited for 60 to 80N engines as such. The solution to improve the flow of this unit is to:

1)      Remove the micronic filter while retaining the Kuss bag by just unfastening the inner filter retainer

2)      Redrill the push on fitting to 2,8 mm and clean it  thoroughly, or go for the barbed fitting option.

 

 

 

The SMC push on fittings and filter retainer inner diameters are shown here. The top fitting has been redrilled to 2,8 mm and compares to the filter retainer on the right. The bottom fitting is stock and has an inner bore of 2 mm.

The optimum orientation of this unit is 45 degrees nose up. This is to allow an optimum purging of the air during refill.

A last note about the Nalgene bottle cap. It is a screw-on cap that has quite a large and loose drive. To avoid introducing air via the screw, it is wrapped with a few turns of Teflon tape. It is important to check this area every year for tightness and replace the tape if in doubt. This also applies to the UAT.

 

 

The teflon tape is being renewed on this air trap. 4 to 5 turns of plumber tape tightly wrap are required.

A leak in this area would introduce air directly downstream of the air trap…

Note that after having tested both the original BVM UAT and modified Hansen air trap, we realized that they actually perform the same. So the ancestor actually holds quite well compared to some younger contendants!

 

3.3.                    The PST Air Trap

The PST air trap is a bit more recent and was released on the market in 2008.

The PST air trap is a very clever device that uses materials issued from other fields of activity  ( like the UAT ). So don’t be surprised to discover that the body is in fact a baby bottle!

The advantage of this design is that it is inexpensive to produce and the bottle is quite strong and completely transparent. Additionally, the tolerance is such that the sealing of the bottle neck is achieved with 100% success by an o’ring system with no risk of subsequent air leak.

The filtering unit is comprised of a sintered brass pickup wrapped up with a geotextile felt. This assembly is very effective and has the advantage to increasing the capitation surface, thus reducing  the suction drag and the possibility of cavitation.

 

 

 

The core of the PST air trap: the filtering felt assembly that is housed within an anodized cylinder. Note the size of the fittings and the large sealing o’ring.

 

The bottle top is made of an aluminium core that includes the fuel line fittings and a hard connection tube to the bronze pickup. It acts as a cap and the sealing is made by a large  o’ring forced along the side of the bottle. It is very effective. The top plastic cap is only used to keep this aluminium assembly in the bottle. It does not provide any sealing.

 

 

 

The filtering felt removed shows the sintered brass pickup and the large brass tube. The restrictive point on this designed is the brass pickup.

The PST air trap comes without any mounting device. However, one can purchase the plywood mount kit. His brings the price up substantially and is quite heavy. The optimum orientation of this unit is nose up.

3.4.                    The GBR Jets Composite Air Trap

 

The GBR Jets Composite Air Traps were designed a few months ago by Marc Scully, after the release of my fuel article series on RC Universe and RCJI. They have been designed with purposely chosen materials and feature a very high quality of fabrication, 100% from the UK.

These air traps have been designed to combine all the advantages of the previous systems while being as light as possible. As such, the units are non dismountable glass fiber tanks that enclose a fixed brass tube and brass/paper felt filtering assembly. They come in 4 different sizes:

1)      Small for engines from 20N to 60N

2)      Medium for engines from 60N to 120N

3)      Large for engines from 120N to 300N

4)      Extra large for engines at and above 300N

 

 

 

The CAT range, from small to extra large UAV type

 

The brass fittings as well as the inner pickup tube are optimized for high flow with no restriction along the path of the fuel. The units are made of FR4 glass fiber and glued with Hysol 9462. They are coming with a CNC cut plywood mount and a complete package including a user manual.

 

 

 

The comprehensive CAT package. Note the super strong cardboard shipping box.

The tank itself is a cylinder and is very rigid. The dimension is computed to provide a buffer of 30s of fuel usage at full thrust. This is why GBR jets is providing different unit sizes. The size, shape and position of the filter assemblies is matching the different sizes of the tanks. These points are extremely important to allow an optimum volume of buffer liquid around the filter that will give a good flow capability in case of cavitation.

 

 

 

The extra large filter assembly. A very neat device made of brass and paper.

 

The FR4 tank wall is thin enough to allow seeing the liquid and detecting a cavitation condition.

Ultimate Jets adds a mark on the CAT wall, along the diameter. This mark is aimed at giving you an indication of how strong the suction drag is upstream the unit. If the fuel level reaches the mark when you are doing a full power test for one minute, then there is a danger of cavitation at this point. The restriction upstream is too strong and you need to check your fuel system again.

 

 

 


The test mark designed to show a restriction condition upstream the tank.

The tank is mounted on a very clever plywood assembly and fastened with two o’rings.

 

 

The CAT on its mounting plate. The two o’rings are super easy and fast to remove.

The optimal orientation of the tank is horizontal with the vent fitting placed on the upmost position. However, any angle from horizontal to vertical is acceptable as long as the vent fitting is on the upmost position.

 

3.5.                    The JMP Air Trap

 

This is an evolution of the above described CAT.

It uses the same concept but pushes to newer levels of quality.

The cylinder is made of significantly stronger glass fiber.

The trap caps are made of anodized aluminum instead of G10.

The membrane system if very much the same compared to the CAT product.

This product is offered with different types of fittings ( regular or large flow ) and different types of caps allowing for horizontal or vertical mounting.

 

 

4.    Devices comparing

 

 

 

3 generations of air traps side by side. From left to right: the Hansen air trap, the PST air trap, the GBR jet medium size air trap.

 

4.1.                    High flow and cavitation workbench test

 

Each device was plugged to a Hausl pump controlled by a Jetcat ECU in an open loop. The fuel was directly taken from my Jersey Modeller jerrican and returned there. I placed a Festo ball valve on the feed line to simulate an increase of the suction drag. I also placed a vacuum gage on the other feed line of the air trap device to be able to measure the feed line vacuum charge generated. I finally plugged a restrictor of my design on the return line downstream the pump. This restrictor is designed to simulate the injectors charge of a 160N thrust engine.

 

 

 

The basic cavitation test setup. The air trap is looped back into the open jerrycan. The fuel pump is controlled by the Jetcat GSU in device test mode.


I then ran the benchmark for each device, doing a series of tests.

1)       full power test with no restriction.

This test enables me to check the depression generated in the air trap at the maximum pump output of 6,2V and to verify that no cavitation condition exists at this stage.

2)      Full power air retention test.

This test is designed to see how the air trap manages with air in the outer compartment in all flight positions. I do a full power test with the air trap gradually emptied from full and moved it in all the positions. I look at the pump line to see if any cavitation bubble shows up.

It is important to characterize the minimum level available for cavitation protection because when slowly reaching the cavitation point, the tank will gradually empty till reaching the minimum level, at which point the air will be released in the system. So knowing the minimum level will give you an idea of the time frame available with a proper protection level. This is also why a big air trap will give you a better protection.

3)      Cavitation test.

I run the system with the ball valve ¾ closed on the feed line and see at what voltage the system starts to cavitate. This is a fast pump increase, contrary to the previous test.  I note the depression value and level drop at this point to characterize the absolute cavitation point of the air trap.

 


 

A vacuum gage is used to monitor the depression in the air trap. This gives a precise indication of the restriction created by the partly closed ball valve on the feed line.

 

4.1.1.   Hanson Super Trap/ BVM UAT

1)      Full power test.

The Hanson air trap/ BVM UAT did not cavitate at 6,2 V with no restriction. The depression generated in the air trap was 5 inches of mercury ( in Hg )

2)      Full power air retention test

The Hanson air trap/ BVM UAT did not send cavitation bubble at ¾ and ½ full. However it started to cavitate when put vertically and 1/4 full (which corresponds to a 50 degrees climb in most circumstances ). This is a good result.

 

 

The minimum acceptable level in the Hanson air trap before cavitation. This picture shows the air trap after the pump had stopped, with no more vacuum applied.

 

3)      Cavitation test

The Hanson air trap/ BVM UAT started to cavitate with the pump set at 3 V. This corresponded to a restriction equivalent to 10 inHg. At this point the walls of the air trap had started to deplete under the action of the suction.

So when you test your system at full thrust, wait one minute at this power level and check you air trap. If you see the walls depleting on this unit, this is an indication that something is wrong upstream the air trap and that you need to work out your fuel system again.

4.1.2.   PST air Trap

1)      Full power test

The PST air trap did not cavitate at 6,2V with no restriction. The depression generated in the air trap was 3 inHg. So this unit generates less suction drag than the Hanson air trap.

 

 

 

The PST air trap in the test. Note the fully open ball valve for the full power test.

2)      Full power air retention test

The PST air trap started to send cavitation bubbles into the pump at ¾ full in the vertical nose down position. This corresponds to an inverted flight in most cases. This could be a problem since it means that as soon as you deplete the air trap due to high fuel flows and suction drag, you cannot do any aerobatic maneuvers anymore. In other words, this system does not tolerate a high suction drag ( restriction in the system ) on an aerobatic aircraft and 200N class engine or more.

3)      Cavitation test

The PST air trap started to cavitate at 3,5 V with a depression of 13 inHg. This is slightly better than the results found with the Hanson air trap/ BVM UAT. The level drop in the air trap at the cavitation point was 20 ml or 2cm from the top / 1 cm at the cylindrical section.

So if you want to setup this system correctly, just do a full power test for 1 minute and see if the level in the air trap drops by less than  1 cm. If not, you need to rework your fuel system upstream the air trap.

 

4.1.3.   CAT medium size

1)      Full power test

The medium size CAT did not cavitate at 6,2V with no restriction. The depression generated in the air trap was 2 inHg. So this unit generates less suction drag than the previous ones.

 

 

 

The medium size CAT ready for testing.

2)      Full power air retention test

The medium size CAT did not send any air into the system at ¾, ½ and ¼ full. It is an excellent result that gives an optimal protection. The air retention capability is only compromised if the fuel quantity reaches the bottom of the filter. There is no detrimental position for this unit since the volume taken by the filter is perfectly optimized.

3)      Cavitation test

The medium CAT just started cavitating with the ball valve closed at ¾, when reaching  6,2V. This is an amazing result and means that this unit can accept twice the flow and restriction compared to the other units before cavitating. I was honestly not expecting such an amazing result. I believe that it is due to the pleated paper filter characteristics.

The equivalent depression was 15 inHg and the fuel level dropped slightly below the test mark, which means that it is suitable for the purpose.

4.1.4.   CAT large size

1)      Full power test

The large size CAT did not cavitate at 6,2V with no restriction. The depression generated in the air trap was 2 inHg.

 


2)      Full power air retention test

The medium size CAT did not send any air into the system at ¾, ½ full. It did cavitate a ¼ full in the vertical position nose up and nose down. It is an excellent result that gives an very good  protection . This CAT being designed for 120 to 300N, you will probably never end up  ¼ full unless you have consumed the last drop of fuel in your tanks, or your running a nearly completely blocked fuel system for a long time.

This result is slightly different from the medium CAT because the filter is slightly shorter for the size and the volume of fuel left at the flats ends of it is slightly larger than on the sides.

3)      Cavitation test

The large CAT did not cavitate with the ball valve closed at ¾, when running at full  6,2V! I had to close the valve further to 80% to get it cavitate. This is an equally amazing result and means that this unit can accept probably three times  the flow and restriction compared to the other units before cavitating.

The equivalent depression was 17 inHg and the fuel level dropped slightly below the test mark, which means that it is suitable for the purpose.

 

4.1.5.   CAT extra large size 

1)      Full power test

As expected, the large size CAT did not cavitate at 6,2V with no restriction. The depression generated in the air trap was 1 inHg.

 

 

The extra large CAT in the test.

2)      Full power air retention test

The extra large size CAT performed as the large one . The length of the filter relative to the length of the CAT is the same. So it did not send any air into the system at ¾, ½ full. However it did cavitate a ¼ full in the vertical nose up and nose down position. It is an excellent result that gives an very good  protection. Since the CAT has a volume of 400 ml, you’d have to use 300 ml of the CAT fuel before reaching the cavitation point on low level.

 

 

 

This is the minimum level available for cavitation protection on the extra large CAT.

 

3)      Cavitation test

 

The extra large CAT did not cavitate with the ball valve closed at ¾, when running at full  6,2V. I had to close the valve further from the large CAT to get it cavitate. This is a great result and means that this unit can accept well over  three times  the flow and restriction compared to the other units before cavitating.

The equivalent depression was 19 inHg, which is a huge restriction and the fuel level dropped slightly below the test mark, which means that it is suitable for the purpose.

In essence, this air trap is a big buffer tank that can allow very big engines to run on a very restrained fuel system for a long time.

4.1.5.   JMP medium and large size

1)      Full power test

As for the large size CAT, it did not cavitate at 6,2V with no restriction. The depression generated in the air trap was 1 inHg.

2)      Full power air retention test

The medium and  large size JMP units performed as the CAT units . The filter units being the same, this was expected. The minimum level available for air retention is 1/4 capacity in both sizes.

 

3)      Cavitation test

 

The JMP units did not cavitate with the ball valve closed at ¾, when running at full  6,2V. I had to close the valve as for the extra large CAT to get it cavitate. This is a great result and means that these units can accept well over  three times  the flow and restriction compared to the other units before cavitating.

The equivalent depression was 19 inHg, which is a huge restriction and the fuel level dropped slightly below the test mark, which means that it is suitable for the purpose.

In essence, this air trap is a big buffer tank that can allow very big engines to run on a significantly restrained fuel system for a longer time.

 

 

4.2.                    Devices weights

 

Hanson Super Trap ( modified )/ BVM UAT:  61 grs

PST air trap:  93 grs

CAT small: 23 grs

CAT medium: 33 grs

CAT large: 43grs

CAT extra large: 115grs

JMP small: 33 grs

JMP medium: 43 grs

JMP large: 53grs

JMP extra large: 125grs

 

4.3.                    Pros and cons

 

Hanson Super Trap/ BVM UAT:

Pros: very good filtering capability, translucent, low price

Cons: soft wall, small capacity, average resistance to cavitation, no mounting device included.

 

PST air trap

Pros: very good filtering capability, hard wall, medium capacity, average resistance to cavitation, completely transparent.

Cons: heavy, low air separation characteristics, no mounting device included, performance to price ratio.

 

GBR Jets CAT

Pros: good filtering capability, hard wall, optimum capacity, very high resistance to cavitation, translucent, mounting device included, weight, performance to price ratio.

Cons: Filter not serviceable. Visual inspection of the filter for slime detection not possible ( see my maintenance advice below ). Walls are brittle ( can crack with shocks and improper installation ). O'ring system not perfect ( the o'ring will crack after a while leading to a possible inverted pickup leading to fuel transfer ).

 JMP air traps

Pros: good filtering capability, hard wall, optimum capacity, very high resistance to cavitation, translucent, mounting device included, weight, performance to price ratio. Stronger wall. Very strong mounting system. Very strong overall design. Swappable fittings.

Cons: Filter not serviceable. Visual inspection of the filter for slime detection not possible ( see my maintenance advice below ). 

5.    Avoiding restriction

 Having a perfect air trap for your system is great, however there are a number of modelers using the venerable BVM air trap in their model who've had perfect results for years. So, how do they achieve this?

The answer is avoiding restriction and minimizing suction drag in their system. The fuel system follows the chain rule. It is only as good as the weakest link. So the same attention to details must be used for the whole fuel system and some important guidance must be followed, from the vent line to the suction side of the pump:

. Use large tubing ( 3/16" or 1/4" ID Tygon )

. Use large fittings ( 3/16" or 1/4' ID bras tubes and fittings )

. Avoid any reduction in your ID ( Inner Diameter ) on the fuel path from the clunk, to the fuel stopper down to the air trap.

. Avoid tight turns in your fuel tubing ( avoid to pinch it ) and keep the tube path very clear and neat

We are offering a line of UHF ( Ultra High Flow ) accessories specifically designed to help you on this matter.

. UHF fuel clunk

. UHF fuel stopper

. UHF viton tubing to plumb your clunk and plunger

. UHF Tygon tubing

. UHF fittings and brass tubes

. Specific PYCABS Tybon tube clips that allow for a very clean and easy setup of your system

 Although, air has a 30 times lower dynamic viscosity ( 15 cSt vs 0.5 cSt for hydrocarbons ), it is also good practice to keep the vent line and fitting of the same large diameter as he remaining of the line.

All of this is of the utmost importance to keep your system healthy and avoid putting too much load on the suction side of the pump.

6.    Fuel pump considerations

 Our fuel pumps are geared type, electrical motor driven.

Brushed motors are typically driven by voltage. The voltage value applied to the motor will translate to a certain flow ( non linear relationship ). Well, in theory.

That is because geared pumps have a specific characteristics: they are weak on the suction side and strong on the pressure side ( meshed gear will struggle at grabbing your fuel, but once it is entrapped, will excel in compressing it ).

In practice, if the suction drag on the pump side is too high, the pump will see a cavitation condition occur at the entry of the gear mesh. If this happens, there is a great probability that the cavitation will propagate beyond the gear, to the engine. Also the pump will see and RPM increase as well as a drop in the downstream flow.

All these would most certainly lead to a flameout. However, bear in mind that pump cavitation usually occur at higher vacuum value than air trap cavitation.

 

7.    Servincing considerations

 Even if you have created the best system in the World, you will need to keep it that way.

Fuel systems tend to degrade over time. This can be due to several factors:

. Fittings oxidization

. Fuel lines hardening

. Dust accumulation ( filters, vent lines )

. Slime ( kerosene/ diesel algae )

For this reason, I recommend the following:

. Fittings oxidization: use the best quality brass fittings ( our fitting, tubes and clunks use quality traced low oxidation brass alloy ) and inspect them before the beginning of the season. Change when oxidation is visible.

. Fuel lines hardening: Festo lines will slowly darken and harden with time when immersed in diesel and kerosene. when you see that the line becomes brownish and hard, change it. Do not use Festo lines inside the tanks ( clunk plumbing ) but Viton lines that are very flexible and virtually unaffected by hydrocarbons.

. Dust accumulation: Flush the air trap reverse several times at the en of the season to clean it.Inspect your vent lines at the same time and on a regular basis if you operate in dusty environment. Evaporated fuel will leave a greasy deposit in the vent line that will eventually get dust to stick to the line and possibly clog it. Rinse the line with non mixed diesel or kerosene if this occurs, then flush with alcohol to facilitate a dry tube.

Slime: inspect your filter on a regular basis. If you see a gel substance accumulating, clean and mix some kerosene with fuel bugs killer. Leave for a couple of days and flush the system. I have used Marine 16 fuel treatment for 10 years now with great success. I also always fill my air trap with this mix at the end of the season.

 

 

8.    Conclusion

The choice of a proper air trap and fuel hardware is not always as obvious as it might seem when you include suction drag reduction in the equation.

The first criteria, however, should be size/ fuel buffer capacity.

The second criteria should be resistance to cavitation

Although choosing a modern air trap will help you in reducing suction viscosity, the fuel system follows the chain rule. It is only as good as the weakest link. So keep the same attention to details when setting up the whole system and use appropriate hardware that are designed to work with each other.

Finally, think in terms of durability and servicing. Don't forget the slime problem neither!

All of this will help you in achieving a perfect system that will get you to enjoy trouble free flights for years.

Written by Olivier Nicolas — November 29, 2012

Fuel system for RC jets considerations

 

0.    Introduction

 

The fuel system is one of the most critical component of our model jets for several reasons.

Firstly a badly setup fuel system will directly affect the reliability of the engines. I have witnessed a countless number of engine shut down due to a too small air trap buffer capacity or cavitation bubble generation. A system could work at the limit in summer conditions with good quality fuel, then generate cavitation in cold OAT, low pressure and/or with lower quality fuel.

Secondly the fuel system will evolve with time and need some specific on condition servicing. Not monitoring the fuel system performance with time will once again lead to a significant degradation of the powerplant reliability.

Thirdly, a bad fuel system could lead to a destructive on board fire in case of a leak close to the engine tailpipe or of a fuel pump runaway.

I have posted several articles on the RCU jets forum and RCJI in the past about this matter and collected a significant number of feedback from users of different systems as well as from my own experience after 25 years flying jet models. Here is a link to the RCU thread:

http://www.rcuniverse.com/forum/m_9232173/mpage_1/key_/tm.htm

You’ll find in this article a summary of all the accumulated data and experience.


1.    Setting up the system

 

The first step in the process of building a good jet fuel system is to correctly size the components and set it up according to the plane requirements.

The biggest two issues that we can have on our fuel systems are air pickup and cavitation bubbles. Most of the engines available on the market are very intolerant to air bubbles and would flame out without notice if some were to appear in the lines downstream the air trap. The air bubble syndrome comes from two main causes: air pickup or cavitation. Air leak is very unlikely if the system is setup properly.

The main reason why air would get into your fuel line is because the fuel tank clunk is not a perfect system. Although it is heavy and mounted on a semi flexible line to stay in the fuel as much as possible, there will be situations when it will get out of the fuel and suck air! This will happen towards the end of the flight, as the tanks get mostly empty and/ or while performing aerobatics.

Here is a video example illustrating the fact. This video is taken on board an aerobatic plane. During this 30 second sequence, one can see that the fuel clunk is out of the liquid for exactly 50% of the time, pickup up air instead of fuel during this period!

 

In-flight fuel tank video from Oli Ni on Vimeo.

 

Following the guidance given below will allow you to properly design, plan and setup your fuel system and will certainly help you in avoiding a rapid or regular decrease in the powerplant reliability.

1.1.         Fuel system requirements vs engine size

 

The most important point that you should understand  when planning for the fuel system is the maximum fuel flow your engine will need at full thrust

This is clearly a function of the  engine size and type and typically ranges from 80 ml/min for the latest micro turbines to 3000 ml/min for the biggest 1300 N thrust powerplants.

This is a factor 37 and you can imagine that the system requirements and sizing will be quite different on each sides of this scale.

1.1.1.   Air leak

The fuel system upstream of the fuel pump is subject to to the pump suction. This segment of the fuel system is being put under vacuum. Therefore,  if the fuel system components are not leak proof, air bubbles could find their way into the fuel lines. The use of an air trap should help in avoiding these bubbles to reach the engine to a certain extend, but the best cure is prevention.

There are a few very simple rules to follow to avoid air leaks:

  • Use good quality push-to connect fittings on the pressure side of the fuel: Festo, and Legris are suitable brands ( although Festo fitings are less resistant to fuel than Legris ). Avoid using unnamed Chinese fittings. It is important to ensure that the push-on fittings are being used with the proper tubes  with proper outer diameter.  Poor quality/ soft tubes with poor diameter tolerance can lead to air leaking from  the fitting. If you are not sure of the tube/fitting combination, then use standard barbed fitting instead.

In any case it is extremely important to understand that push-to connect fittings are designed to be used with Shore A60 or above tubes ( Shore D40 or above ). Tygon tubes are way too soft for this purpose. It is extremely risky to use Tygon tubes on these fittings at the walls risk to collapse under vacuum at the fitting opening and create an air leak.

 

  • Use proper the barbed fittings on the suction side of the pump.Chose ones that are suitable for the tube size selected, to ensure that all the tubes are tight on the fittings. Single barbed fitting offer a much better leak proofing but will require safety wiring as the retention effect of a single barb will be lower ( in other words, high number of bars = more retention effects = less leak proofing ). We offer the best quality single barbed fittings  on the market.
  • Safety wire all your tubes on the barbed fittings. Use proper stainless steel or Chromel wire. 0,015” is optimal in most cases. Secure it with proper auto return twisting pliers with catcher or auto return twisting pliers without catcher. This way you will avoid over tightening the wire which could result in cutting the soft tube and create an air leak!.

 

 

1.1.2.   Safety wiring.

Here is a technique to safety wire a fitting properly.

Use a good length of wire of at least 3 inches . Coil the wire around tube already pushed on the fitting  twice. 

 Use the pliers to twist the wire for about ½ inch. Do not apply too much tension in the twist or the wire could break.

You should stop twisting the wire when it cannot rotate any more around the secured tubing.

When the twisting process is complete, cut the  excess wire and fold the twist inward on itself so that it cannot hurt your fingers are wandering around. This would also help in avoiding damaging the soft line. We recommend the use of a plier with catcher incorporated in the jaw  to avoid the risk of having a tiny bit of stainless steel wire dropping in the airframe and getting eventually sucked through the FOD screen.

 

Some professional tools used: aero grade twisting pliers and 0,016” stainless steel wire. I reckon that 0,016” or 4/10 mm is the best gauge for our hobby needs.

 

 

 

1)      The wire is coiled twice around the line before twisting. This ensures a leak free and tight assembly.

 

 

 

2)      The wire is twisted using the plier. As the wire tension increases around the line, bring the two coils together for a perfect leak free twist.

 

 

 

 

3)      Continue twisting the plier until the tension at the coil is high enough so that the twist cannot rotate around the line any more. Note that the twist gets much tighter than on the previous picture.

 

 

 

4)      Then cut the twist at about 1/2 an inch and bend it inward to the line. That way there will be no risk of harming yourself or damaging the soft fuel line passing around with the sharp twist cut. Note the large 3/16”  Tygon line used here on our HF fuel clunk.

  •      Make sure that the cap of the air trap is tightly fastened, and that the Teflon sealing tape is wrapped properly and in good condition. If you are using a BVM UAT or equivalent, change the Teflon tape after each season.
  • If the cap is not properly set or the Teflon tape is not in good condition, you might get a very nasty air leak, since it would introduce air downstream of the air trap bag..


 

 

 

 

The old sealing PTFE tape is removed from this Hansen air trap after having changed the filter bag.

 

 

 

Wrap a new PTFE Teflon tape around the neck clockwise. Use 4 to 5 turns.

 

 

 

 

The cap is ready to be tightly secured on the air trap bottle. 

  • Finally, make sure that all the fastened  fittings are well tightened and sealed. A good flange sealant like Loctite 518 will provide excellent sealing capability and fuel proofing.

    1.1.3.   Cavitation

     

    Cavitation is an interesting phenomenon. In our case it could lead you to believe that you have an air leak, where you don’t. It is basically the formation of gas bubbles in a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure.

    You can do a very simple experience at your workshop to characterize it. Take a transparent syringe and fill it with jet fuel. Then block the outlet with your thumb and pull the syringe piston. You will see thousands of small bubble appearing in the fuel passed a certain level of depression. The liquid is “foaming” ! This is "cavitation". Also note how long it takes for the air bubbles to dissolve again in the fuel ( minutes if not hours )

    Jet fuel is able to absorb significant quantities of air in dissolved form. If the pressure in the fuel drops below the vapor pressure, it will release its dissolved air in form of bubbles. In our case this cavitation phenomenon is mostly made of air bubbles, not gaseous kerosene bubbles ( so strictly speaking the term "cavitation" should be replaced by "air bubbles release under vacuum conditions" but this obviously not very convenient )

    On our RC jets systems, the fuel pump is perfectly capable of creating the depression required to make the fuel cavitate. As a general rule, any pump voltage in excess of 3,0V could create this type of vacuum if there is enough restriction in the system upstream the pump. A restriction in the fuel lines will generate fluid drag in the stream that will lead to a slight overpressure upstream the obstacle and a slight underpressure downstream of it. If the restriction is high enough, the pressure drop will make the fuel cavitate or foam. That restriction might be caused by  a valve, a junction, or a pinched line. You will see bubbles appearing at this location as if there was an air leak…

    Another interesting phenomenon is the air trap cavitation. Soft Nalgene air traps have a tendency to collapse in case of a restriction and big pump demand. The walls of the air trap could touch the sides of the air trap bag, thus reducing the filtering area. Additionally if some air bubbles are trapped upstream of this bag, they will cover it. The filtering area will be further reduced because the micro air bubbles will stick to the bag, coating it with “restrictive obstacles”. This will increase the flow drag and the vacuum downstream the filter. At some point the filter will cavitate and it will look exactly as if the bubbles were making their way through the air trap bag!

     As a general rule, cavitation can be generated in a fuel system for engines of 120N thrust or more. It can be avoided by carefully choosing the fuel system components and planning for proper fuel lines routing to avoid tight bends. I will come back to this later.

    1.1.4.   Static electricity discharge

    With some engine brands, another problem can arise in our jets: accumulation of static electricity in the fuel, that will discharge through the ECU, reset it and create an engine shutdown ( phenomenon called “watchdog” on Jetcat engines ).

    This static electricity discharge appears under certain atmospheric conditions: on dry and dusty days. The electricity is captured by the engine parts while the air charged particles are travelling through it. When a certain potential difference is reached, the electricity will travel backward through the fuel lines into the pump and ECU and shut it down.

    This phenomenon is exponential with the quantity of air displaced by the engine. Some 200N+ thrust engine types of certain brands are prone to this type of problem creating a "watchdog" ECU reset.

    Some engines like BF and Behotec are very resilient to static discharge.  However I still recommend bonding for safety issues. Strong static discharge in empty tanks have generated explosion in the center tank of the old generation 737.

    Bonding is the fact of using that static electricity dispenser additives and metalizing the different components of the engine to fuel interface.

    • Antistatic Fuel additives are available from specialized jet model shops.
    • Metalizing the fuel to engine interface is easy. Connect the engine, tailpipe, fuel pump body and main fuel tank together with a thin electrical wire ( a single strand of AWG 22 servo wire works well). The main tank connection is made via the brass fittings. Use black static dispersive festo tube from the engine to the fuel pump.

     The engine metalizing wiring: a thin yellow wire is used at the bottom of this picture to connect the stainless steel pipe to the P-200SX engine bracket, to the fuel pump and to the fuel tank.

     

    Note that the "watchdog" function can also be activated if static electricity discharges from the aircraft body into the ECU when the plane’s wheels lift from the ground. Large quantities of fuel pumped into the plane with a significant electrical potential difference can also create a problem. Metalizing the plane as described above shall give you a good defense against all these phenomenon.

    1.2.         Tank considerations

    A good fuel system needs to use good tanks. They must be leak free and the tank arrangement shall be such that the flow drag is reasonable. As a general rule, try to avoid having two tanks in series. Parallel them if you can. If this cannot be done, then use large bore tank internals and interconnecting lines.

    As a good practice, I recommend reinforcing the tank seams with a Kevlar tape.

    The tank leaks can be detected by moderately inflating them with air in and putting them in a water bucket ( do not use a powerful compressor to inflate them ! do it with the mouth: max 10 PSI here). Mark the leaks ( usually pin holes ) with a water proof marker. Then apply E-20HP epoxy resin to the marked areas with a piece of plastic bag  and push on, to fill the pin holes.

     

     

     

    The Kevlar tank is SLIGHTLY pressurized ( not more than 10 PSI )

    Another area to watch is the tank cap or fuel stopper. A lot of ARTFs use rubber caps expanding into the bare fiber. This will introduce leaks on the long run. I strongly recommend using aluminium tank stopper rings glued with Hysol 9462 on the tank end like the one used on our UHF fuel stopper.

     

    1.2.1 Traditional semi rigid high flow tank plumbing

    This method is used to avoid reverse bending the line that could get stuck at the front of the tank.

    Our UHF fuel stopper is used to accept 3/16' ID or 1/4" ID fuel lines.

    Our UHF fuel clunk is used in 3/16" or 1/4" ID size.

    1. Measure the length of your tank from the stopper to the end wall. Substract 1/2" or 10 mm.

     

    2. Cut two sections of 3 " o four ultra flexible Viton tubing. They will be used to connect the stopper tube to the long brass tube, to the clunk. Cut a section of brass tube in the measured length minus 2".

    3. Insert the Viton lines on both ends of the tube by 1". Spray the tube with Zip Kicker before inserting. This will ensure that the line is lubed along the tube and slides properly. Zip Kicker has no silicon and does not contaminate kerosene ( higher risks of fuel stick clogging ).

     

     

    4. Insert the fuel clunk on the line. The clunk has an aggressive barb and will require more pushing than the tube.

     

    5. Safety wire all connections as per chapter 1.1.2 above.

     

     

     

     

     

     

     

    Note that with this setup, the clunk will never be at the front of the tank. This means that on long dives, the pickup line will suck air. This is acceptable if you put your engine to idle in your dives. However hardcore aerobatics pilots should not use this setup as it will put most air traps to their limits.

     

    Here is the seating of the clunk in the tank in the horizontal position:

     

    In the vertical nose up position:

     

    It is imperative to verify that the clunk stays 1/2" away from the wall. Otherwise there would be a risk of sucking the clunk flat against the wall and immediately starve the engine.

     

     

    The vertical nose down position:

     

     

    In this position, the clunk will feed air to the air trap. At this point the air trap will play its role and provide buffer fuel to the engine. If you like to keep your engine running full power in the dive, then we'd recommend that you use the next setup.

     

    1.2.2 New full flexible high flow tank plumbing

    This method was not recommended previously as there were no proper heavy clunks and flexible enough lines to avoid reverse bending the pickup. It would eventually get stuck at the front of the tank, resulting in a fixed pickup point ( flameout at the end of the flight guaranteed ). Additionally most plastic lines ( including Tygons ) would harden with time and render this scenario more likely.

    However our new specific UHF clunk is heavy weight while compatible with regular fuel stopper diameters and allows for a super flexible Viton line to never get stuck in a reverse bend scenario. It is also optimally designed to avoid pinching the thin wall Viton line. Additionally the super high quality Viton tubes that we use are completely unaffected by immersion in kerosene or diesel and will never harden.

     

    This plumbing technique is in fact very simple:

    Our UHF fuel stopper is used to accept 3/16' ID or 1/4" ID fuel lines.

    Our UHF cross drilled fuel clunk is used in 3/16" or 1/4" ID size.

    A long section of ultra flexible Viton tubing is used to connect the stopper tube to the clunk. The length should be so that the clunk is just 1/2" short of the tank end wall.

     

    When you are done with the plumbing, dry test the tank. Just turn it in all positions. you should hear the heavy clunk fall on the tank wall every time. The heavy weight of the clunk and fluidity of the line/ setup should be obvious. Safety wiring is essential here as the clunk is constantly falling and pulling on the Viton line...

    Note that with large tanks of 8" diameter or more, the fuel clunk line can reverse itself into the tanks, which is acceptable. Just make sure that the tank does not have any blobs of glue that could block the clunk in position.

     

    Also note that in this case, a cross slotted clunk is necessary to avoid having it sucked flat against a wall and completely blocking the system. This would not happen on the previous semi rigid scenario if setup as described.

    1.3.         Air trap considerations

    1.3.1 Description

     

    Many modern air traps will be suitable in your model.

    What matters is the size of the trap ( buffer fuel reserve ) and of the fittings ( 3/16" or 1/4" ).

    Please refer to the next article in this blog: " New generation air traps and avoiding cavitation "

    1.3.2. Placement

    The air trap shall be positioned as close as possible to the tanks. This is to minimize the suction drag upstream of the pump. Each air trap has a specific positioning recommended. The BVM UAT and equivalent shall be placed 45 degrees nose up.

    The JMP accumulator comes in two types: nose up or nose horizontal.

    The PST air trap is best used nose up. If not, ensure that the pickup nipples as placed as high as possible above the tank symmetry line.

    Modern cylindrical air traps can be placed horizontally or vertically. Just make sure that the vent line is at the top position.

     

     

     

    The GBRJets UAT dock designed by Marc Scully is a very elegant way of holding the UAT or similar at the correct angle.

     

    1.3.3. Model storage and transport.

    On the latest air trap designs, capillary drag is very law, allowing the fluid to travel between the air trap and the tank. This is especially true with the larger air traps I designed a few years ago that feature large plated paper filters.
    Plated paper is great in that it offers very little resistance to fluid travel, thus reducing risks of cavitation. However it also allows the fluid to escape from the air trap and the filter, thus allowing air to enter.
    This would occur if the level of the pickup line is placed somehow at the level of lower that the top of the fuel filter unit when the tank is empty.
    This can happen in case of rotation of cylinder type air traps, or if the model is place on its side for storage or transport.


    In the case shown above, air has entered the filter section and will enter the pump at some point. Even if the air is purged, micro bubbles can stick to the membrane and enter the pump. It the case above, after purging the air, a thorough high flow test while tapping on the air trap will be required to remove these bubbles.

     

     

    1.4.         Feeder tubes and vent tubes considerations

    1.4.1. Sizing.

    When setting up your system, you must watch very carefully the inner diameter of your lines and fittings ( ID ). Always remember that the fuel system follows the chain rule. It is only as good as the weakest link. So always match the lines and accessories IDs.

    For engines of 120 N and more, use 3/16" ID lines and fittings on the suction side of the pump.

    For engines of 200 N and more, use 1/4" ID lines and fittings on the suction side of the pump.

    For engines of 50 N and more use 5/16" ID lines and fittings on the suction side of the pump.

    Outside diameter ( OD ) does not really matter here. It is just an indication of the line wall thickness.

    Vent lines are important as an improperly setup can completely block the fuel system. Make sure that you use appropriate fittings of equivalent IDs as the feed lines. If you can, use 1/4" lines on the vent side of 3/16" systems and 5/16" for 1/4" systems.

    Over sizing the vent side will avoid blockage due to possible dirt or grass debris or accumulation of dust.

     

    I recommend the use of the following hardware for plumbing the suction side of the pump:

    Fuselage vent fitting: UJ vent in HF ( 3/16" ), UHF ( 1/4" ) size or XHF size ( 5/16" )

    Tygon 3/16”, 1/4" or 5/16" ID

    Straight fittings, elbow fitting, thread-to-barb fitting and tubes: HF ( 3/16" ), UHF ( 1/4" ) or XHF ( 5/16" ) size.

    XHF, UHF or UF fuel stopper system

    XHF, UHF or HF UJ fuel clunk. I do not recommend the use of filter type fuel clunks due to the increased suction drag.

    One component is also very important: it is the fuel filling closing system. On that matter, I use the simplest and most reliable system: the fuel dot.

     

     

    The fuel dot is the simplest, yet most reliable fueling closing system…

    All the sizing and thrust rating requirements are summarized in the table below.

     

    Item designation

    ID

    OD

    20N to 160N suitable

    120N to 160N suitable

    160-200N

    suitable

    200 N - 500N suitable

     

    500+ N suitable

     

    Sullivan vent fitting

    3,2mm

    5,2mm

    yes

    yes

    No

    No

    No

    JMP pitot vent

    3,2mm

    n.a.

    yes

    yes

    yes

    No

    No

    UJ XHF 5/16" vent

    8mm

    9mm

    yes

    yes

    yes

    Yes

    Yes

    UJ UHF 1/4" vent

    6.4mm

    7.4mm

    yes

    yes

    yes

    Yes

    Yes for vent/ double

    UJ HF 3/16" vent

    4.8mm

    5,8mm

    yes

    yes

    yes

    Yes for vent/ double

    No

    UJ XHF 5/16" brass tube

    8mm

    9mm

    yes

    yes

    yes

    Yes

    Yes

    UJ UHF 1/4" brass tube

    6.4mm

    7.4mm

    yes

    yes

    yes

    Yes

    Yes for vent/ double

    UJ HF 3/16" Brass tube

    4.8mm

    5,8mm

    yes

    yes

    yes

    Yes for vent/ double

    No

    4 mm brass tube

    3,3mm

    4mm

    yes

    No

    No

    No

    No

    PST air trap

    3,5mm

    6,1mm

    yes

    Drill to 4mm ID

    No

    No

    No

    JMP air trap HF M/L

    4.8mm

    5.8 mm

    yes

    yes

    yes

    Yes/ cruiser

    No

    JMP air trap UHF M/L

    6.4mm

    6,4mm

    yes

    yes

    yes

    Yes/ cruiser

    No

    JMP air trap XHF XL

    8mm

    9mm

    yes

    yes

    yes

    yes

    Yes/ cruiser

    UJ UHF 5/16" ID clunk

    8mm

    9mm

    yes

    yes

    yes

    yes

    yes

    UJ UHF 1/4" ID clunk

    6.6mm

    18mm

    yes

    yes

    yes

    yes

    Yes/ cruiser

    UJ HF 3/16" ID clunk

    5mm

    15mm

    yes

    yes

    yes

    Yes/ cruiser

    No

    Sullivan Super clunk

    3,2mm

    5,5mm

    yes

    Drill to 3,8 mm ID

    No

    No

    No

    XHF 5/16" Tygon

    8mm

    10.5mm

    yes

    yes

    yes

    yes

    yes

    UHF 1/4" Tygon

    6.4mm

    9.5mm

    yes

    yes

    yes

    yes

    Yes/ cruiser

    HF 3/16" Tygon

    4.8mm

    8mm

    yes

    yes

    yes

    No except parallel systems and vent

    No

    NF 1/8" Tygon

    3,2mm

    6,4mm

    yes

    yes

    No

    No

    No

    LF 3/32" Tygon

    2,4mm

    5,4mm

    No. Fueling line only

    No. Fueling line only

    No. Fueling line only

    No. Fueling line only

    No. Fueling line only

    PU  10 mm tube

    8mm

    10mm

    yes

    yes

    Yes

    Yes

    Yes

    PU  8 mm tube

    6mm

    8mm

    yes

    yes

    Yes

    Yes

    Yes

    PU  6 mm tube

    4mm

    6mm

    yes

    yes

    Yes downstream the pump

    Yes downstream the pump

    Yes downstream the pump

    PU 4 mm tube

    2,3mm

    4mm

    yes

    No

    No

    No

    No

    PU 3 mm tube

    2mm

    3mm

    No. Gas start only

    No. Gas start only

    No

    No

    No

    XHF 5/16" Viton clunk line

    8mm

    10.5mm

    yes

    yes

    yes

    yes

    yes

    UJ UHF 1/4" Viton clunk line

    6.6mm

    9.5mm

    yes

    yes

    yes

    yes

    No

    UJ HF 3/16" Viton clunk line

    4.8mm

    8mm

     

    yes

    yes

    yes

    No

    No

    XHF 5/16" brass fitting

    8mm

    10.5mm

    yes

    yes

    yes

    yes

    yes

    UJ UHF 1/4" brass fittings

    6.6mm

    9.5mm

    yes

    yes

    yes

    yes

    No

    UJ HF 3/16" brass fittings

    6.6mm

    9.5mm

    yes

    yes

    yes

    No

    No

     

     

    1.4.2. Line routing.

    Fuel line routing is also essential.

    Avoid tight turns and situations where the line could get pinched. Avoid throwing the lines in the plane but make then well separated and easy to see/ follow. A clean routing will make the ssytem easy to service and allow you to see is a line tends to bend/ pinch.

    One of the essential point about clean routing is the use of proper clips. I designed designed some PYCABS Tygon clips for this to let you place the fuel line and remove it easily but most importantly, these are using our unique PYCABS material that is 100% CA gel compatible!
    This makes installing the clips on the fuselage walls a matter of seconds and offer super strong bonding capability ( the glue always stays put on the PYCABS side when removing a clip ). They are sized to offer easy clipping of the line, yet allows it to slide and does not pinch it!

     

    1.4.3. Resistance to vacuum collapsing

    Most of the items are very strong and will resist the vacuum that a geared pump can create ( max 600 mb of vacuum for most pumps ). In any case, the bubbling point of kerosene is usually around 600 mb of vacuum, unless it has been degassed.

    The following items do partially resist vacuum:

    BVM UAT and flexible tanks: max 600 mb of vacuum

    1/4" Viton flexible line: max 700 mb of vacuum

    3/16" Viton line" max 800 mb of vacuum

    1/4" Tygon line" max 700 mb of vacuum

    3/16" Tygon line: 800 mb of vacuum.

    Here are some picture of the line testing at max vacuum. One can see that the line starts to collapse in the bending zones.

    1/4" Viton line:

     

    1/4" Tygon line:

     

     

    Any vacuum level below 600 mb is in the realm of professional composite bagging pumps...

     

    2.    Servicing considerations

    Even if you have created the best system in the World, you will need to keep it that way.

    Fuel systems tend to degrade over time. This can be due to several factors:

    . Fittings oxidization

    . Fuel lines hardening

    . Dust accumulation ( filters, vent lines )

    . Slime ( kerosene/ diesel algae )

    For this reason, I recommend the following:

    . Fittings oxidization: use the best quality brass fittings ( our fitting, tubes and clunks use quality traced low oxidation brass alloy ) and inspect them before the beginning of the season. Change when oxidation is visible.

    . Fuel lines hardening: Festo lines will slowly darken and harden with time when immersed in diesel and kerosene. when you see that the line becomes brownish and hard, change it. Do not use Festo lines inside the tanks ( clunk plumbing ) but Viton lines that are very flexible and virtually unaffected by hydrocarbons.

    . Dust accumulation: Flush the air trap reverse several times at the en of the season to clean it.Inspect your vent lines at the same time and on a regular basis if you operate in dusty environment. Evaporated fuel will leave a greasy deposit in the vent line that will eventually get dust to stick to the line and possibly clog it. Rinse the line with non mixed diesel or kerosene if this occurs, then flush with alcohol to facilitate a dry tube.

    Slime:

    We see a lot of people with slime problems. This is coming from a huge mis-conception in the jet crowd. People believe that jet fuel is immune to slime. This is incorrect. Jet fuel DOES NOT have antimicrobial additives as standard. It has Prist, which is a fuel icing inhibitor.
    The old Prist formulation prior to 1994 was certified to be a retardant to microbial growth thanks to its formulation mostly composed of EGMME.
    As regulation changed in 1994, EGMME was replaced by DEGMME which is not certified as microbial growth retardant.
    Jet fuel suppliers forbid long term storage for this exact reason.

     

    So, slime can develop in our systems, especially with clear model tanks where sun light is allowed to penetrate into the fuel. Organic growth would start pretty fast in this case. So the semi clear Kevlar and air traps in the model are great organic growth generators.
    Flushing the fuel back into the jerrycan will recycle this organic life back into the storing system. It will then slowly grow in the dark. Eventually the whole fueling chain will get contaminated.

    However it is really when the fuel is left in the clear tanks that slime develops the fastest. 

    So, inspect your filter on a regular basis. If you see a gel substance accumulating, clean and mix some kerosene with fuel bugs killer. Leave for at least 24 hours and flush the system.

    We have used Biobor JF MIL fuel treatment for 10 years now with great success. I also always fill my air trap with this mix at the end of the season. This product not only kills slime but also reduces it to microscopic levels that do not affect the filters or engine. leaving your system with this product during the winter season will completely immune it from this problem.

     

    3.    Conclusion

    The choice of a proper air trap and fuel hardware is not always as obvious as it might seem when you include suction drag reduction in the equation.

    The first criteria, however, should be size/ fuel buffer capacity.

    The second criteria should be resistance to cavitation

    Although choosing a modern air trap will help you in reducing suction viscosity, the fuel system follows the chain rule. It is only as good as the weakest link. So keep the same attention to details when setting up the whole system and use appropriate hardware that are designed to work with each other.

    Finally, think in terms of durability and servicing. Don't forget the slime problem neither!

    All of this will help you in achieving a perfect system that will get you to enjoy trouble free flights for years.

     

     

     

    Written by Olivier Nicolas — November 25, 2012

    Welcome to Ultimate Jets.

    We are the one and only USA based turbine powered UAV and hobby models manufacturer. Ultimate jets offers some of the best engines, kits, and accessories to the Industry. ................................................... We offer in-house design and manufacture services for military and hobby companies. Our expertise extends from aero dynamics design to carbon fiber aerostructures manufacture and 5 axis CNC milling and turning. ................................................... We are the USA distributors for Behotec, BF-Turbines, Mibo and Aviation Design and provide in-house support and service for these products. ................................................... We also are dealers for the following brands: Jets-Munt, Permagrit, Ultratool, Cotronics, Techflex and many other aerospace industry suppliers. ................................................... We finally house a unique technical blog that you will find below, providing industry reference articles. Please feel free to browse through these pages. Enjoy... Barry, Woody, Shaun and Oli.