Protecting the loom from the engine exhaust and heat

 There is mostly one source of heat in remote control jet: the turbine.

Two types of heat will be found: readiating heat and direct exposure to the jet efflux.

The first one will generate moderate levels of heat, rarely above 100c/ 220F.

The second one could expose the equipment to temperatures up to 700c/ 1300F.

Although it can be relatively easy to hide the loom from hot areas due to radiating heat, it is much more difficult to protect it from jet efflux.

Radiating heat protection is just a matter of routing the loom appropriately and choosing the right cables. We recommend using high temperature silicon insulated heavy duty servo cables or PTFE insulated electrical cables.

However the second scenario would occur mostly in case of a catastrophic failure of the exhaust pipe. In this case the failure would be certainly recognized in flight by the pilot and should lead to a turbine shutdown. So it is reasonable to ensure protection to high temperatures for 3 to 5 seconds.

i have conducted a series of test on some MIL spec and aero grade cables and cable sleeves to understand a bit more how the equipment would hold in this case.

Here is a flame test conducted with the HD high temperature silicon servo cable. The cable is directly exposed to a 1000c propane torch flame:

Silicon servo wire flame test from Oli Ni on Vimeo.

The cable hold the flame for 3 seconds and the insulation integrity is still ensured, meaning that the conductive cores have not short cut.

Here is a test of the MIL spec cable in the same condition:

MIL spec cable flame test from Oli Ni on Vimeo.

The same result is found after a 3 seconds exposure at 1000c. The insulation has not melted.

Now let's see what happens to a regular high quality servo wire ( 20 AWG, PVC insulation ).

PVC insulated servo wire flame test from Oli Ni on Vimeo.

During the exposure, the bright glow shows that the insulation is burning. After 3 seconds, the insulation is totally melted and the copper cores have fused. The loom has completely short cut.

In general the high temperature cables would hold a jet efflux of 700c ( full thrust ) for about 30 seconds before melting the insulation and short cutting the cores. A standard PVC cable would not hold more than a few seconds.

So the idea would be to protect the system by using these cables anywhere downstream the engine. In case of catastrophic failure of the thrust pipe, there would be enough time to recognize the problem and cut the engine while maintaining the integrity of the electrical system.

However, standard PVC cables could still be used in this area by protecting them with flame resistant cable sleeves. Here is a test made with our high temperature kevlar sleeve:

The same PVC cable as before is now inserted in our kevlar sleeve and exposed to the propane torch for 5 seconds. The cable barely shows signs of overheat and the sleeve, although black, is still in one piece.

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

This type of protection is completely possible and would ensure the same level of protection as using high temperature cables on standard looms.

Now the next level is to use our high temperature cables in heat protection sleeves. In this case, the loom would be able to sustain jet efflux for minutes, probably longer than what an airframe could take, even protected with ceramic blanket...