Heat management in jet powered UAVs and models

Heat management in jet powered UAVs and models

0. Introduction

Heat management is essential to jet powered UAVs or models.

The safety of the vehicles and property or persons on the ground can be affected in many ways by heat related issues. Bush fires ( fire bans in the summer ), direct burns due to jet efflux/ hot parts exposure, airframe destruction in flight, loss of control due to airframe deformation or electronics burning, airframe explosion due to jet pipe implosion are some examples among many other.

The main source of heat in the model is obviously the jets engine. This type of powerplant is basically burning the fuel as hot as possible and converting the heat energy into kinetic energy for propulsion. This create many heat hazards. To understand them better, let's talk a bit about theory.

1. Theoretical considerations

1.1 Heat transfer theory

Heat energy transfers to its surrounding essentially via three ways: conduction, convection and radiation.

Conduction occurs through a solid ( engine or pipe mount for example ), convection via the air ( flow of hot air spreading to its surrounding ), radiation via electro-magnetic waves ( heat felt from a red hot object at distance ). All 3 phenomena occurs in a jet powered UAV/ model. 

 

This implies that several areas have to be considered for heat related dangers and several methods/ products can be utilized to mitigate the risks of using a jet engine in an enclosed airframe.

1.2. Heat transfer modes applied to jet powered airframes.

1.2.1. Risks associated with heat conduction.

Heat conduction occurs through a solid when atoms of material excite their neighbours by proximity. Heat spreads a bit like a wave in the material.

The areas where heat conduction can occur are:

Mounting of engine ( transmission via the tabs )

Mounting of the pipe ( transmission via the tabs )

Touching elements ( wires, tubes components touching the engine and/ or pipe walls )

All these points are subject to heat damage/ melting and must be addressed.

 

 

1.2.2. Risks associated with convection

Heat convection occurs when hot gas or liquid mix with cooler one.

Heat convection can occur around the jet efflux from the engine.

But the same effect is used extensively to cool down the internals ( bypass air and venturi effect ).

 

1.2.3. Risks associated with radiation

Heat radiation occurs when the electro-magnetic energy from the heated element spreads to the nearby object. Generally speaking the phenomenon is present when one "feels" the heat of red hot object at a distance.

This would occurs in vacuum as well ( no convection ) and is how our sun heats up Earth.

The radiating elements in a jet are usually the engine and the pipe.

 

2. Practical considerations 

2.1. Internal elements protection

Internal elements must be protected against heat conduction, convection and radiation. Most of these have a limited maximum continuous temperature and peak temperature.

Maximum continuous temperature shall be considered for normal use ( most of the time, the peak temperature is reached on the ground while taxiing at idle or close to idle )

Peak temperature can be used to plan for resistance to a wet start with flames exiting the pipe from either the front bellmouth or the rear end.

 

Here is a table showing the heat resistance of some products:

 

Designation

Material

Continuous temp c

Peak temp c

Standard PVC servo wire PVC/ copper 105 150

Tygon fuel tubing

PVC

74

100

Frethane fuel/ air tubing

PU Frethane

80

120

PYCABS clips

ABS-Polycarbonate

100

180

High temp clips

High temp SLA resin

130

280

ABS electronics enclosure

ABS

80

150

Silicon servo wire

Silicon/ copper

200

300

PTFE servo wire

PTFE/ copper

300

600

Carbon fiber plate

Post-cured epoxy

80

110

Aluminum plate

Al 6061

400

660

PEEK aerospace grade sleeve

PEEK

300

400

Wrap around sleeve

PET-UL94

125

250

Kevlar/ nomex sleeve

Kevlar

350

600

Aerospace grade Ceramic blanket

Alumina oxyde fibers

1090

1500

 

As you can see, the fuel lines are one of the most restrictive elements regarding heat management. They carry fuel and melt at relatively low temperature! This has created many fuel fires in the past, resulting in a complete destruction of the aircraft.

However, another sensitive element is the standard servo wire that is PVC insulated. This melts at a relatively low temperature as well and once melted, is is almost certain that the servo wire will short itself. On regular non line protected receivers, this will short the entire RC system and lock the aircraft flight controls, resulting in a loss of the model control. This has been seen many times in the past and can be extremely dangerous to persons and property on the ground.

To protect internal elements of the aircraft from heat transfer, several strategies need to be implemented:

2.1.1 Avoid:

Try to avoid routing heat sensitive elements around the heat emitters. For example, avoid placing an ECU on your engine bracket, unless the engine is enclosed in a bypass.

Similarly, avoid routing servo wires around your engine and pipe if possible. However in many cases, this is just unavoidable. In this case, you will need to mitigate the heat transfer risks by using other strategies as described in the paragraph below.

Finally, to avoid having components touching heat sources and melting by conduction, you will need to fix the components.

2.1.2 Use heat proof components:

We offer a large range of heat resistant components for jet powered planes

Use them!

Specifically, we offer two types of HD servo wires for the hot areas with silicon insulation and PTFE insulation. Our PTFE wire is by far the most heat resistant and sustains torch exposure!

2.1.3. Mitigate:

If neither of the two first strategies are possible or if one wants to put an additional layer of safety in heat proofing, the following rules can be applied:

2.1.3.1. Protect it:

Different materials and devices can be used for this:

Heat protecting sleeves can be used to protect wires harnesses or tubes. We offer tubular kevlar sleeves and MIL SPEC flame retardant wrap around sleeves. Kevlar sleeves sustain higher temperatures but are a bit looser woven, are harder to cut and require specific provisions to pass the bundle in them.

Kevlar fiber cable hose flame test. from Oli Ni on Vimeo.

 

MIL spec flame retardant wrap around sleeves are less heat resistant but super convenient to pass the wire or tubes bundle and super easy to set up. 

 

 

Aero grade loom tutorial: wires protection. from Oli Ni on Vimeo.

 

Aerospace grade ceramic blanket can be used to shield the elements from convective heat ( insulation from airflow ) or conductive heat ( by spacing two elements that are in contact with a insulation layer ).

Aluminum high temperature tape can also be used to reflect heat radiating to the component. The tape must use a high temperature glue so that it does not un-stick in hot environment.

2.1.3.2. Fix it.

Use heat resistant clips to avoid have loose components possibly touch a heat emitter and melt by conduction. We have designed a range of heat resistant clips and heat resistant ECU/ receiver holders for this purpose.

 

 

 

2.2. Airframe protection

2.2.1. Cooling the radiating elements by convection:

The best strategy for airframe protection is to ensure cooling of the heat radiating elements is adhequate. This must be done through proper venturi effect. The venturi effect is drawing cold air against the hot parts ( engine, thrust tube ). Cooling occurs by convection.

 

 

2.2.2. Skin protection:

The skin of modern UAVs or jet models is not just designed to make the shape of the plane. It is also an integral part of the structure and must be kept to an acceptable level of stiffness. Additionally, a lot of UAVs are using Airex core laminates, which can de-gas at higher temperatures and create internal delaminations. 

Therefore skin must be kept at a temperature significantly below its tg limit ( transition to glass where the resin starts melting ) at all times. This must be done by ensuring that the radiating elements are not too hot ( properly designed engine bypass, and pipe cooling ) and the internal airframe volume is ventilated ( convective cooling ). A properly designed airframe should be staying below 50 degrees c skin temperature as taken from the outside at all times.

 

 

If the plane is not properly designed, mitigation will consist in using ceramic blanket in the zones that are too hot. The blanket must be glued with high temperature CA gel or zirconia glue.

 

2.2.3. Bulkheads protection:

Bulkhead that are made from airex/ carbon fiber will sustain maximum 80 degrees c. Note that cheap sourced airex copy very often start melting at 60 c instead of 80.

In the case of a hot rear section, the bulkhead must be protected as well with ceramic blanket. Once again, the best strategy here is to ensure a proper cooling of the radiating elements ( venturi effect and double wall pipe ) rather than loading the rear of the plane.

 

2.2.4. Glue joints considerations:

 

This is something that gets sometime skipped. Glue joints between the bulkheads and the skin are essential structural elements and must be protected like the rest of the airframe is internal cooling is deficient. Once again, ceramic blanket is a good strategy here. It is just a matter of extending the skin and bulkhead protection to the glue joints.

 

2.2.5. Efflux heat:

The hot gas exhaust can be as high as 600 degrees c during operations and 900 degrees c during a flameout or a wet start.

It is essential that the designer of the airframe makes provisions so that no engine efflux is allowed to escape into thhe airframe.

This is ensured by properly designing the engine bypass and the pipe. We recommend to systematically fit an full engine bypass in a UAV or jet model.  The bypass should be molded from carbon fiber ( which melts at about 1000 c ) and high temperature resin.This will protect the airframe in case of flameout or wet start/ fuel dripping during start. The carbon fiber will keep a certain level of integrity in case of large flames exiting the engine and spreading due to a wet start, even after the epoxy resin has started burning.

 

2.2.6. Heat radiation and venturi effect:

There are four venturi effects happening along a jet engine duct. One at the duct intake, one at the engine intake, one at the engine nozzle and one at the exhaust.

At the intake, the air is being forced from an infinite volume ( outside ) to a constrained volume ( intake section ).

The venturi effect at the engine nozzle is the most significant one regarding heat transfer. It is driven by the jet efflux and is forcing the bypass air to accelerate. This makes a strong Bernouilli effect where vacuum is generated and propagates along the thrust tube.

The low pressure at the pipe bellmouth forces some cool air from the engine intake to be diverted to the engine exhaust. If a proper bypass volume distribution has been implemented, a flow of cold air will pass around the engine casing ( which radiates at an average of 150 c in operations ) as well as the nozzle ( which can radiate at up to 700 c at full thrust ) and reduces the bypass outer skin temperature ( which gets increased by radiating heat transfer ) via convection effect.

This is extremely convenient for the designer, as it helps in keeping one of the most important source of radiating heat from affecting internal heat too much. So, the proper design of bypass volume distribution is essential to heat management.

The other noticeable and important venturi effect occurs at the thrust tube exhaust.

A properly designed thrust tube must be dual walled. A flow of cold air must travel within the dual walled tube to cool down the inner tube by convection. This is achieved by creating a venturi effect at the rear of the pipe. To do so, the inner pipe must be stopped inside of the outer pipe by a certain distance. This distance depends on a number of factors like the inter-wall distance and exhaust velocity, but a value of 10 to 15 mm is usually optimum.

With a properly designed venturi, the outer pipe temperature should stay with 60 to 80 c.

 

2.2.7. Dangers of pipe implosion:

These venturi effects however have a down side: they create a vacuum in the pipe. This vacuum can be quite strong in some cases and create a danger of pipe implosion.

This is a very serious threat to the aircraft as an implosed pipe can result in two problems: internal components melting and airframe blowing out if the pipe obstructs the back of the airframe.

Because of the danger of pipe implosion, it is a good practice to protect the servo wires running at the back of the plane from extreme heat.

In case of pipe collapse, one should have the time to stop the engine and steer the plane away from persons and properties on the ground before the servo wires melt.

This is achieved by using either silicon servo wires, or even better PTFE wires protected in a kevlar sleeve. Similarly, I recommend using high temperature sleeve clips around the pipe so that in case of implosion and hot exhaust gas blowing inside the airfame, the cable protecting sleeve does not get pulled off its location by melted clips failure.

2.2.8. Heat soaking

This is a convective phenomenon where hot air gets entrapped in a closed are and transfers heat to its surrounding. This can typically occurs at the back of the plane in the fin/ elevator/ top of the fuselage area. The risk is to slowly get to epoxy Tg and beyond by lack of cooling and end up damaging the structure or getting a rupture in flight. To avoid heat soaking, I recommend closing the areas where active cooling cannot be achieved. Generally, aluminum foil will work perfectly well for this purpose.

 

 

2.3. Persons and properties protection

2.3.1. Jet efflux risks:

The jet efflux coming out of the engine nozzle is dangerous. It is a high velocity/ high energy stream that can be at up to 700 c. But even at idle, the efflux will be at 400c on average. Such a high energy stream can burn one’s skin instantly. So it is important to ensure that the back of the plane is never pointing at persons of equipment when the engine is running. Bare legs of people can get burned in a matter of seconds.!

2.3.2. Direct burns risks:

The next danger is direct burn by touching the heat radiating elements inside the plane. Do not attempt to work on a running engine when a bypass is removed. Similarly, do not work around the thrust tube with a running engine.

 

2.3.3. Fire hazard:

A running jet engine represent a fire hazard at all times. Keep an extinguisher and fire blankets close to the model at all times. I recommend using a CO2 extinguisher rather than a powder one as the latter can be chemically destructive for your composite layup/ paint and servo wires.

 

 

3. Testing

 3.1. Testing before maiden: 

I highly recommend testing the airframe for heat transfer performance before the maiden. This will give a good idea on the risks to the plane and will usually cover the flight scenario. The airframe tends to run cooler in flight than on the ground.

There are essentially two ways of testing heat transfer modes live: probing and imagery.

3.1.1 Probe testing.

There are many different types of thermometers available on the market for heat testing. Some are analog, some digital, some direct reading and some telemetry enabled.

I developed an ASSI/ CAN BUS system with Carsten that is able to broadcast a number of temperature readings to a Jeti/ Futaba transmitter in real time.

 

This system allows the use of up to 8 K type thermo-couples ( up to 1500 c ) and broadcast via industrial CAN bus as well as Futaba SBUS 2 and Jeti EX BUS.The sensors can be extended and placed wherever necessary in the airplane to map the heat signature of the design. This includes positioning the thermo-couple on the engine nozzle/ engine casing or inside the thrust tube!

I recommend placing internal probes around the engine bypass, on the pipe outer wall, and at proximity of the electrical harness in the hotter section. If servos are present at the back of the plane, a probing near them will be crucial to ensure reliable operation of your controls. Similarly, probing structural bulkheads like elevator or fin tube formers can reveal some problems that could happen on the long run.

This system has been designed for professional UAV designers and has already delivered an impressive amount of invaluable data to our teams.

3.1.2. IR imagery

Infra-read imagery is also extremely valuable and fast to obtain. Many options are available like the smarphone-plugging FLIR One Pro.

Thermal imagery is a very good choice for checking skin temperature. However, live multiple temperature readings inside the plane must be done with a telemetry system.

3.2. Test protocols on the ground:

Idle test: This is the base test for temperature reading. Start the engine and record temperatures at idle thrust. Note that the position of the plane into the wind should influence your readings. They should read higher in tailwind. The idle test can be quite difficult for the plane as the venturi effects are lower at idle.

For the first idle test, I recommend to bring an electric blower with you and be ready to ventilate the airframe in case the outcome gets really hot!

Taxi test: This is a second test that can be done once the idle test is finished and airplane cooled down. Taxiing will require various levels of thrust and will generate different cooling flows within the plane. A telemetry reading is recommended for this one in order to be able to stop the egine if the situation becomes critical while being far away.

Full thrust static test: This will allow to see how the plane will perform cooling wise at the beginning of the takeoff run. As the plane accelerate on the runway, thermals should improve with increasing speed.

3.3. Test protocols in the air:

In flight: The only way to test the thermals in the air is to have a telemetry system on board.

After flight: Once the flight is finished and as the plane returns to the stand, it is a good practice to keep looking at the telemetry values and monitor how the thermals increase.

 

4. Conclusion

Heat threats management can be a lot more involving that one thinks and requires good knowledge of the theory and some specific hardware as well as soft skill.

As always with aero designs, remember that the system follow the chain rule. It is only as good as the weakest link. If one decides to invest in a better technology in some area, then all the other areas must follow. No need to use super high tech ceramic blanket insulation if the servo wire are un-protected PVC types.

We hope that this article will give you the tools to increase the safety of your UAV and models.

Oli and the UJ team.