I wrote it up for RCJI about a year ago.

To be clear, I am sure the Tamjets is an excellent unit and works very well for many people. However, for my big SkyGate Hawk with huge doors I don't want a possible uncommanded gear extension, possibly at high speed, ripping off the very large doors with perhaps a large pitch change at an inopportune time.

One of the first jets I flew, the Jet Provost and several other military jets such as the F4, extend the gear using compressed air in the event of hyd. failure to blow the gear down.

On my model system I do the same. The alternate air supply is the brake tank (or another dedicated tank in my Hawk which has the space) This air is supplied to the gear valve ONLY when the gear is selected down in emergency mode, it otherwise remains isolated from the system, protecting it from leaks. To achieve this a mixer is arranged on the Tx mixing gear to gear so that when the gear is selected down, activating the mixer causes the gear valve servo to overtravel slightly and open the air micro switch directing reserve air to the gear system. This air cannot dump into the main air tank because a Clippard one way valve is installed in the main air tank delivery line. I will also have a backup air supply to this backup using compressor bleed air from the Olympus. This pressure will be only about 40 psi but this should be enough to extend the gear and get the downlocks into position.

If you are using an electronic gear valve one can simple Y a small servo into the gear cable to operate the air micro switch if and when needed.

As the leak will have occurred when the gear is UP, this should result in gear extension.

The drill is simple. Extend gear, if nothing happens due air leak, wait until all sequencing timing is complete. Then select gear DOWN and immediately activate the mixer, gear should then extend on alternate air.

It may sound complex but in fact is very simple to install and operate. I have extensively tested it and it works perfectly

I am currently refining the hyd. system on my FC Mig 29 (about to be powered by the new JetCat 140 RXI, making engine installation so simple) Although the system has two hydraulic pumps ( a Hausl and a UMS) operating in parallel at 10 bar but isolated from each other by NRVs , I am working on an emergency system which will blow the gear down in the unlikely event of a total hydraulic failure. If I can get it to work I will pass on the details.

David G.