olnico

Dear all,

Following the release of my article about heat management

https://www.ultimate-jets.net/blogs/...avs-and-models

I wanted to share some basic principles and tips with you.


Heat energy transfers to its surrounding essentially via three ways:
.conduction,
.convection
.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 occur in a jet powered UAV/ model.




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

My blog details all the products available on the market and their service temperature in a large table.

I have a very extensive base of hobby and professional UAV manufacturers who are regularly coming to me for advice and products related to heat management.

The most questions I get are related to the electrical and fuel systems.

So, here are a few considerations.
Fuel system: bear in mind that this is the main cause of fire after a crash.
Electrical system: it is usually the cause of the crash in heat related problems.

So, both areas need to be addressed to increase the safety related to heat transfer hazard. As always, prevention is of the essence and remember that in aviation, the chain principle prevails. A system is only as good as its weakest link.

1. Fuel system:
1.1. Tubing: Remember that Tygon is a PVC tubing. It is a very low temperature material in terms of fire resistance. Yet, it carries the most flammable liquid! Tygon is rated for 74c service temperature. This can be reached easily with poorly managed heat spreading around a jet engine. Also, Tygon tends to age faster and cracks when subject to these temperatures on a regular basis. Finally, Tygon formulation changed a couple of years ago ( you probably noticed the horrible smell of the new product ) and it tends to "sweat" silicon compound. This silicon by-product actually deposits in the fuel injection capilliaries and increases fuel burn deposits/ cocking in the nozzle and bearings.

For heat protection, our USA made Frethane fuel lines have a slightly higher service temperature ( 80c ) and do not expell any by-product in the fuel. We have used them extensively for years with great success.
https://www.ultimate-jets.net/produc...ct-fuel-tubing

However, I recently commended a large tubing company to produce a specific PTFE tubing for aircraft fuel systems. Since we now have a very large tubing output, I managed to get a custom specification on the PTFE formulation that eliminates the biggest problem with PTFE tubes: wall hardness and kinking. Those of you who have use PTFE tubing in vacuum systems know what I am talking about. PTFE is generally quite stiff and kinks very fast. Well, was until now. Our new PTFE fuel tubing does not kink within the usual turn radii seen in RC jets and professional UAVs. Additionally, the tubing is extremely resistant to tear and cuts as well as much more resistant to heat ( service temperature of 260c ).
https://www.ultimate-jets.net/collec...re-fuel-tubing

In order to maximize the safety of the fuel system around the engine and/ or inside the bypass duct, I have also specifically required the tubing company to make a 4 mm OD/ 2.5 mm ID one. This will replace the original manufacturer Festo PU tube with a much more heat and flame resistant product.
The other size I have on the shop at the moment is 6 mm OD/ 4 mm ID. The 8 mm OD/ 6 mm ID tube is 100% sold to some industrial customers for the moment, but will become available on the shop within end of February.
BTW, because I order these tubes by batch of 6000 ft, I am able to get a very decent pricing on them. These come about 30% cheaper than Tygon...

Although the tube is a little bit stiffer than Tygon, once clipped with our specific fuel tubing clips, it super easy to set and stays in any position. As a reminder, our PYCABS clips are designed to be glued in seconds with CA gel and are very durable ( service temperature of 100c ).
https://www.ultimate-jets.net/collec...n-pycabs-clips

Also, for people looking at MIL spec products or at higher temperature rating ( remember, the chain principle ), I also make clips rated to a matching 250c service temperature. These are made of MIL spec/ FAR 25.853 rated PEEK. By the way, we produce some of these clips for aircraft interiors as well as to the automotive industry ( have a look at the Antelope project https://www.facebook.com/52346948099...type=3&theater).
https://www.ultimate-jets.net/produc...uel-line-clips

1.2 Design
Another thing to keep in mind for fuel systems is the position of the flammable liquid mass versus the heat source in case of crash.
It is a good design practice to place the fuel system below the thrust line and generally speaking to ensure that if the engine is propelled forward in a crash, it will not pass through or fracture the fuel tank. This might seem trivial, but I see so many model designs that are a very real fire threat to the environment in case of crash.

1.3 Tanks
One discussion that I have recently had with FAA officials is about fuel bladders. The reasoning behind the AMA ban on fuel bladders is the fact that most fuel systems are placed in the fuselage. Old style bladders, in this case were bursting easier than rigid walled tanks. However, one of the main purpose of using fuel bladders is to be able to place them in the wings! I have recently demonstrated the decrease of the fire hazard on such wing mounted bladder systems as part of a MIL project. Also, our new multi-layered safety fuel bladders are much more resistant and effective than the old plasma bags we were using 30 years ago.
https://www.ultimate-jets.net/collec...-fuel-bladders
This could be a consideration for scale competitors who intend to meet a weight target and fly in competitions outside of the USA.

Most Kevlar tanks are set in front of the engine in a cold area. However, some planes like our F-8E Crusader have fuel tanks under the engine to avoid CG excursions.
In this case, some principles need to be observed.
1. Use a full bypass system, made of high temperature epoxy.
2. Protect the exposed tank surfaces with a layer of ceramic blanket.
https://www.ultimate-jets.net/collec...eramic-blanket

We sell this ceramic blanket by the foot up to 100 ft length at once!
It is extremely light and resists 100 c ! So, no excuse not to use this amazing material.

1.4. Fittings
Another consideration on fuel systems is the safety of fuel connections. Please, please, please, safety wire all you barbed fittings. If you feel lazy or have difficulties accessing some areas, use high quality push-to-connect fittings.
https://www.ultimate-jets.net/collec...raight-fitting
Yes, most barbed connections will stay put in normal use when new. But what about the silicon sweating effect on the new Tygons that actually lubricates the connection? What if the tubing is pulled during aerobatics? What if the tubing heats up and expands on the fitting?


A lot of people have argued that a push-to-connect fitting is a risk of introducing air bubbles in the system upstream the pump due to the vacuum levels that can be reached. Well , this is true on soft walled tubing like Tygons of Sullivans. However, this will not happen on our Frethane fuel lines of PTFE tubing. These can actually be used on vacuum system for molding parts without leak! We effectively use both of these tubes for large volume infusion ( Frethane ) / high temp autoclave process ( PTFE ) !
All our industrial customers use our push-to-connect fittings in MIL spec projects with 100% reliability levels. We have Middle-East customers running large UAV engines at 260 l/ h flow without any problem on Pisco connectors/ PTFE tubing.

1.5. Organizing
Finally, even if you use heat resistant fuel tubing, remember to clip all you lines in the plane. This has several advantages:
. It looks super clean and tidy
. It reduces the risks of having a line touching a pipe or hot engine component ( even if PTFE tubes do not melt at 200c, the fuel will boil inside the line and generate and engine flameout )!
. It allows you to see what is happening with your system and troubleshoot a problem/ detect a problem before it is too late ( a mess of wires and tubes will not help maintenance an troubleshooting )

In a second part, I will talk about the electrical systems, procedures and accessories that one can use to protect it from heat transfer.

FenderBean

I was looking at your bladders, other than build a box how would you attach the bladders?

olnico

This depends on the shape of the structure/ skin around it.
On circular shapes with a radius close to the bladder balooned state ( full ), then velcro strips around the bottom work well.
If the surface around is flat, then we use carbon fiber rods that slide in the side tabs. Because the bladder inflates considerably like a baloon when full, one cannot use velcros that would unfasten on flat surfaces.

There is a considerable amount of considerations going into bladders design. I have been making and using very large ones consistently for the past 30 years on RC jets and UAVs on our European operations.
For example, wing bladders are very specific in that they should not blow the skins out when full! Also, fill and pickup point are very specific. FInally, a fueling system with pressure sensor and bubble purging cycle has to be used.

There is an aerospace company called ATV that makes bladders in California for race cars and space vehicles. However these are completely out of the hobby range in terms of weight and price.

Twin-B0

My Feedback: (1)

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dbsonic

My Feedback: (3)

Interesting stuff Oli. A good read as always. What are your thoughts on using ceramic paints for heat protection?

olnico

Ceramic is usually white ( good reflectivity ) and has a high service temperature. It is also a poor heat conductor.
So in theory, it is a great material for insulation. However one word of notice. We did some extensive tests with a ceramic spray designed as a heat insulation.
The ceramic coating was sprayed onto carbon/ epoxy monolithic plates and subject to torch test (1000c ), and heat gun test (400c ).
It did not work so good.
The reason why is that the spray was too thin. Heat was transferring too fast to the substrate, although the ceramic material was doing its job. Actually, the ceramic coating was holding great, while the laminate behind was burning...

So, I recommend to use liquid ceramic compound and build up some thickness. It is better if the ceramic liquid has some fibers/ microspheres in it to increase the insulation factor.
Typically, BVM heat shield is best suited for this application.
However, because is the manual nature of the job, one cannot guarantee an even application of the product. And evenness is of the essence here ( remember, the chain principle... ).

So, for covering tricky non flat area ( like bulkheads ) where heat transfer is likely to be low, it is OK.
However, in critical applications, I recommend using our calibrated ceramic blanket.

olnico

Also, a word of caution with aluminized foils.
Aluminum is a high reflectivity material. However it is a good heat conductor.
So it should be used where high radiation transfers are a concern, but it SHOULD NOT be used alone.
As a good practice, a layer of ceramic blanket or several thick coats of BVM heat shield should be placed under the aluminum foil as a conduction and convection barrier.
Also, beware of the type of glue used on the foil. It has to sustain high temperatures. In some cases, zirconia glue is required.
Finally is important to place the reflective element first in the transfer path, then the insulating coats.

David Gladwin

joeflyer

My Feedback: (48)

I use Permatex Copper Spray-A-Gasket Hi-temp Sealant to install ceramic blanket https://www.permatex.com/products/gasketing/gasket-sealants/permatex-copper-spray-a-gasket-hi-temp-sealant/

Just spray it on lightly and use it like contact cement. It works well and is very durable. Available at most automotive stores.

olnico

Nice find!
Thanks, Joe. I will give it a try. 500F should be sufficient for most applications.

olnico

Actually, this one might be slightly better for our applications:
https://www.permatex.com/products/ga...asket-sealant/

It is the same product, without copper.
Slightly cheaper as well.
Copper being conductive, this might act as a shielding coat for 2.4 Ghz receiver.

olnico

Thanks for the kind words!

joeflyer

My Feedback: (48)

I've been using the copper stuff for years without issue. It's still holding up in 10 - 12 year old jets.

You raise a good point though. I'll try the other stuff on my next build.

Joe

olnico

It looks like it is the exact same product without copper.
I will try it, but have no means of comparing with the copper one.
Let me know from your side,
Thanks.

sidgates

My Feedback: (2)

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Does it matter which side you glue to the fuse, rough side or smoother side? I will use glue the smooth side if I don't hear otherwise.

olnico

Depends if you have any air flow streaming on the ceramic blanket.
If not, glue the smooth side on the fiber. You will use less glue.
Note that I tried the regular Permatex spray and it works well on our blanket.

sidgates

My Feedback: (2)

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I will glue the smooth side, I also bought the reg. Permatex.

sidgates

My Feedback: (2)

I first sprayed the blanket with 3-4 passes with the Permatex. It seemed like it aborsbed all the glue so hit it with 3-4 more passes and have placed the blanket in the fuse. I will test in 24 hours and see if it is secure.

joeflyer

My Feedback: (48)

Try spraying the blanket from 1 to 2 feet away. That way it isn't so wet when it hits the blanket. Also put a light mist on the fuse. Let it dry for a few minutes then install the blanket while the glue is still tacky.

olnico

I have done 2 passes on the ceramic blanket and 1 pass on the glass skin.
Waited a bit after the first pass on ceramic, during that time sprayed the fuselage, then did the second pass on the ceramic, then applied still tacky.
Basically treated the product as contact glue spray.

erbroens

Interesting thread.

One important point about heat management (if not the most) is how the air flows around the turbine/pipe system . With a well designed installation we can take advantage of the turbine exhaust gas flow to get rid of the hot air inside the fuselage even with the jet standing still. Hot fuselage spots can be eliminated improving streamlining inside it. The old AMT turbine and Jetcat manuals shows information about using this technique making heavy termal blankets/ceramic coating unnecessary. in most if not all jets.

About tanks and fire... well. In 20 years witnessing jet crashes with bladders, pet plastic coke bottles, fiberglass and kevlar tanks,,the ugliest and less professional looking ones are by far the strongest and most resilient in crashes. The pet bottles are almost indestructible and it is needed a big crash to make a fireball with them.

Heat checking on the fuselage can be simply done by hand...you dont need a thermal fancy camera.. if you can keep your hand on it, then is OK. However if the paint is changing color, or it is bubbling (LOL) then you have a heat problem.


Hope this would help.. best regards everyone, and happy flights.

olnico

Not necessarily. Hence the distinction between conduction, convection and radiation.
You can have a relatively mild skin temperature ( convection cooling ) and internal components melting due to conduction or radiation.

olnico

Paint is more heat resistant than most epoxy/ polyester resins.
If you see your paint bubbling/ changing color from internal heat transfer, then you have reached destructive temperature on your skin. You can basically consider that the laminate in this area is destroyed. A bit of a late call I would say.

olnico

Here is the second part of the technical discussion about heat management.
As a reminder, you can read the full article on my technical blog here:

https://www.ultimate-jets.net/blogs/...avs-and-models

2. Electrical systems


Electrical system failures are usually fatal to RC models and UAVs, mostly due to relativly simple redundancy schemes.

When things get really hot around the thrust pipe or the engine, the plane electrical system can get really vulnerable.


2.1. Here are is a lists of factors to consider in heat managment :

. Conduction hazards : engine brackets, thrust tube are usually hot. If a cable is left to touch these parts, it could potentially melt.

. Convection hazards : hot air flowing inside the pipe can create convection problems. If the pipe implodes, lots of internal components will be directly exposed to the hot efflux. Even in normal conditions, hot air treams will be generated alng the thrust tube. These could pootentially heat soak the airframe in « dead ends » like the bottom of the fin. Any electrical component running in these areas will be subject to higher heat. Heat soakig might not blow away when the aircraft is flying if the area is really far and remote into the airframe.

. Radiation hazard :

The pipe and bare engine will radiate heat. A good way to avoid radiatin hazard is to enclose the engine in a full bypass system and to use a dual wall thrust tube. Heat soaking can also be the result oof radiating heat.

2.2. Avoidance strategies :

However, the following principles must always be considered first :

. Route all the wires away from area subject too heat transfer ( consider all 3 modes of transmission )

. Fix the servos outside of these areas as well.

However, these cannot always be followed due to a specific airframe geometry, weight contrains or other. In that case, mitigation strategies will be used.

2.3. Mitigation strategies :

These mostly consist in using high temperature rated accessories, and ensuring that a constant stream of cool air is available in flight to cool down the equipment.

This can be done by adding scoops to duct outside air inside the airframe, or opening up scale scoops. In this case, it is important to ensure that the air is allowed to escape at the back of the airframe.

However, even if the cooling is effective in flight, there will be cases where they do not work effectively. That is mostly on the ground or if the pipe implodes. In this case using high temperature accessorie could save the airframe. I consider this as a mitigation strategies that can be listed by mode of transmission :




.Conduction hazards:

If you want to avoid toouuching hot parts, make sure all your cable bundles are clipped. If you use high temperature cables/ cable sleeves, it makes sense to use high temperature PEEK clips ( remember, the chain principle ). Similarly, it can be a good idea to use high temperature servo plugs that clip sercurely, with screw-on female recptacles. That way, you will be sure that the servo plug does not float around the plane.




.Convection hazards :

. Use heat resistant wires. PTFE coated wires will sustain 250c continuous. To avoid radiation heat hazards, it is a good proctice to enclose the wire bundle in a wrap arond sleeve. In case of danger of direct efflux exposure ( critical servo cables passing along the thrust tube ) I recommend the use of kevlar sleeving that sustains 1000 c continuous.

. Sabilizers being close to thrust tubes usually expose the servos and pushrods.

.Protect the servos with ceramic blancket. A lightweight carbon channel covered with ceramic blancket can be very helpful.

. Use aluminum servo arms

. Use stainless steel heat calibrated ball links

. Use stainless steel threaded rods. Avoid any composite rods like carbon rod with glued threaded ends !

. Consider the use of high temperature compliant thread lock fluid.

. Make sure that stabilizer shafts are not made of composite. Carbon rods are not welcome close to a thrust tube !




. Radiation hazards :

Use aluminized tape, like Flite Metal, to isolate components that could suffer from radiatng hheat.

Cooling down :

The best way to fight against heat soaking is to use active cooling in flight. Once an area of the airframe has been identified to be subject to heat soaking, creating a fresh air intake and exhaust uppstream and downstream the area will help dramatically. However, for certain narrow area, another effective strategy can be to close the heat entry with a piece of ceramic blancket or a patch of flight metal.

F1 Rocket