Fuel system for UAV and 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. On that matter, I have covered the subject of heat and fire protection in a separate article.
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.
Professional users, please refer to our UAV fuel systems page:
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 a rigid 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! Obviously, this does not happen with a bladder system, which we highly recommend to improve fuel system delivery, and we will talk about this later.
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 MIL spec push-to-connect fittings on the pressure side of the fuel: Pisco is a superior brands. Their fittings resist corrosion for a lot longer than other brands. Avoid using unnamed Chinese fittings. It is important to ensure that the push-on fittings are being used with the proper tubes with proper wall stiffness and 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 fittings 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 as 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 barbed fitting properly ( not required on push-to-connect fittings, obviously...
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.
Air trap safety consideration: old style cap system.
- 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.
- Use branded high quality lines. We recommend Tygon for barbed fittings, ester-based 1/8” and 1/4” polyurethane tubing for the imperial push-on fittings and 3, 4 or 6 mm ester-based PU tubing for metric push-on fittings.
- 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.
Our new UFBK air trap system is immune to these leak problems. It features a large threaded aluminum cap that fastens on a glued flange. Sealing is provided by compressing a large Viton o'ring.
1.1.3. Cavitation or foaming
Cavitation/ foaming 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 refered to as "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.
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.
When ever legal or possible, we highly recommend the use of bladder systems for your jet or piston engines.
The reason for this is that these systems are air free. This allows to feed the engine with no risk of air bubble ingestion at any point of the flight, in any position and supply a continuous positive flow of fuel ( atmospheric pressure pushes on the bladder at all times ).
This is turn allows the removal of air traps,with saves a lot of complexity, risk of failure and about 1 to 3 lbs of dead weight.
The latest professional units we sell are super fuel resistant and very strong. They consist of a kerosene resistant inner bladder, reinforced by a puncture proof/ crack proof/ UV resistant outer layer.
For rigid tanks, as a good practice, I recommend reinforcing the walls 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. For a bladder system, a tank leak can be sealed by welding.
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/bladder high flow tank plumbing
For all tank plumbings, it is essential to use Viton material. The reason is that Viton tubes are designed to stay immersed permanently in fuel and stay flexible. This is essential to keep the clunk mobile and moving towards the fuel. Also, Tygon chemical composition got changed a couple years back and the tubing sweats chemical after a few years. Wee found out that this sweating could obstruct the capilliary injection needles of the jet engine.
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.
Example of wrong technique: in the picture below, the Tygon line will harden relatively fast. The small/ lightweight clunk will not be able to follow the fuel during aerobatics. This setup will send a lot more air into the air trap.
Note that for semi-rigid 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.
Here is an example of plumbing on a box type bladder:
1.3. Air trap considerations
1.3.1 Description
Air traps are not necessary for bladder systems, provided you refuel with an air purging pump unit.
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.
Our new generation UFBK air traps can be set horizontally or vertically.
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 or 4 mm ID lines and fittings on the suction side of the pump.
For engines of 200 N and more, use 1/4" ID or 6 mm ID lines and fittings on the suction side of the pump.
For engines of 500 N and more use 5/16" ID or 8 mm ID lines and fittings on the suction side of the pump.
For UAV engines of 1000 N and more use 10 mm 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:
2 mm ID/ 4 mm ID/ 6 mm ID/ 8 mm ID push-to-connect fuel tubing.
Or our new line of high temperature PTFE fuel tubing.
The latest Frethane tubing is very light, quite flexible and very resistant to kinks. It does not harden with time as fast as previous generation tubing and is a lot more resistant in crashes than Tygon ( huge reduction in risk of fires ). However, it is will not resist for long to high temperature conditions. On the contrary, the PTFE tubing is incredibly resistant to high temperature, high pressure and puncture. However it is very hard and does not sustain bends when cold. It must be either hot bent, or set with L connectors. It is incredibly easy to setup both type of tubing with the Pisco push-to-connect fittings and our line of PYCABS fuel line clips. or PEEK ultra high temperature line clips.
Fuselage vent fitting: UJ vent in HF ( 3/16" ), UHF ( 1/4" ) size or XHF size ( 5/16" )
Straight fittings, elbow fitting, thread-to-barb fitting and tubes: Pisco push-to-connect fittings.
XHF, UHF or UF fuel stopper system
Plated paper filter clunks can be used in some specific cases in lieu of air trap.
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.
UFBK 500
air trap
|
Item designation |
ID |
OD |
20N to 160N suitable |
120N to 160N suitable |
160-250N suitable |
250 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 |
|
|
air trap |
4mm |
6 mm |
yes |
yes |
No |
No |
No |
|
|
air trap |
6mm |
8mm |
yes |
yes |
yes |
Yes |
No |
|
|
air trap |
8mm |
10mm |
yes |
yes |
yes |
yes |
No |
|
|
air trap |
10 mm |
12 mm |
yes |
yes |
yes |
yes |
yes |
yes |
|
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 |
|
|
8mm |
10mm |
yes |
yes |
Yes |
Yes |
Yes |
||
|
6mm |
8mm |
yes |
yes |
Yes |
Yes |
Yes |
||
|
4mm |
6mm |
yes |
yes |
Yes downstream the pump |
Yes downstream the pump |
Yes downstream the pump |
||
|
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 |
|
|
8mm |
10.5mm |
yes |
yes |
yes |
yes |
yes |
||
|
6.6mm |
9.5mm |
yes |
yes |
yes |
yes |
No |
||
|
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 system 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 designed to be glued with CA gel!
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!
It is also important to keep the fuel system segregated from the electrical system. On that matter we have lots of electrical harness clips custom designed for wrap around sleeved bundled from PYCABS or PEEK, for professional use.
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
PU line 12 mm OD: max 900 mb of vacuum
PU line 10 mm OD: max 900 mb of vacuum
PU line 8 mm OD: max 1000 mb of vacuum
PU line 6 mm OD: max 1100 mb of vacuum
These can effectively be used for vcuum bagging.
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:
Note that the softer tubing is more enclined to collapse/ kink in the turn under vacuum. This is the reason why we recommend the use of more rigid wall PU tubing in hobby jets where the installation often requires many turns.
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 anti-slime MIL fuel treatment syringes 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.
