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.