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Touring Motor Gliders Association (TMGA)
Jim Lee

Rotax in-air restarts using a windmilling prop

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Jim Lee

I have been asked to address in-air restarts using a windmilling prop without electrical power starting capability.  Rotax says that the 912 cannot be started with a windmilling prop.  I have been told that this is a liability related answer.  So I will use the profound wisdom of Rotax, and also state that you can not restart the Rotax 912 by windmilling the prop.  Got it?  Ok good, now on to reality.

I have tested windmill starting my Phoenix 4 or 5 times back in 2010 or 2011.  I started it successfully each time.  If the engine is warm, i.e., just shut down, then a dive of "only" 80kts was sufficient to rotate the prop fast enough to start the engine, losing about 800'.  On other occasions with varying degrees (sorry) of cold engines, I have had to dive the Phoenix at over 100kts to start the engine.  Do you know what a 100kt dive looks like?  Well it is practically straight down, and the vertical speed is horrendous.  It is easy to eat up 2000 feet of altitude during this maneuver.  There is absolutely no way I would want to attempt this below 3000' instead of gliding to a suitable field or airport to make a forced landing.

So when do I anticipate needing to do a windmill start?  I am reluctant to use the term "never" but I cannot think of a time when I would need to do this rather than gliding to a safe landing.

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mikeschumann

A number of years ago I was at the Auxiliary Powered Sailplane flyin at Parowan and got a ride in a Ximango, which is also ROTAX powered.  At the time, I was looking at various motorglider options, including the Stemme.  During our flight, we were discussing one of the advantages of airstarting the ROTAX with the Ximango, which is impossible in the Stemme, given its folding prop system.

The obvious situation where this might be useful is when you have a dead battery.  About 30 minutes after we had this discussion in the Ximango, we were getting kind of low and decided to restart the engine.  It turned out we had a complete electrical system failure.  We quickly restarted the ROTAX by windmilling the prop and headed back to the airport without incident.

Upon landing we discovered that the main supply wire from the battery to the Ximango's fuse panel had a bad solder joint, which melted.  There was no mechanical attachment of this wire to the fuse panel, so it was dangling in mid-air behind the panel, a pretty scary prospect since there was no fuse between the bare wire and the battery.

Starting the engine by windmilling the prop saved our bacon.  You never know what unexpected stuff will hit you when you are dealing with aircraft, so this capability is definitely nice to have.

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Thermalseeker

I had to demonstrate an air start in my Ximango to earn my self launch endorsement. I don't know whether it was an official requirement or not, but the instructor wanted to see it since the aircraft was capable. I've done it many times since. It's a very nice feature to have and much lighter than a second battery. Here in the Sequatchie Valley I'm slightly north of Chattanooga Class C airspace. So, I always have the transponder squawking 1202 when I'm flying in hopes the airline, corporate and military aircraft that frequent KCHA will see me. I also have the vario, data logger, comm radio, GPS and XM radio boombox on, too. When the master switch is on you're also holding the avionics buss relay energized to the tune of about 1Ah. I've run my battery down several times on long soaring flights (usually 4 hours or more) and have always been able to air start, even with a cold engine. I push the nose over to 105 kts, flip on the ignition and unfeather the prop. The engine fires right up. I don't find the dive to be particularly steep and it only takes maybe 15 seconds to build up the necessary speed. The dive is about as steep as what I do when I do wingovers, although the entry speed on a wingover is more like 80-85 kts. I have had to use choke once or twice when the engine was really cold after being high in wave for a while. It takes about 700', but you recover most of that back. As such, I set a lower limit "hard deck" of 1500' agl to give myself ample altitude to air start should it be necessary, plus a safety margin. There's not shortage of landing fields here. Your mileage may vary. When I went to Rotax school there was some discussion as to whether air starting would damage the dogs in the slipper clutch, since they were being loaded from the opposite side during an air start (compared to using the starter). Nothing was ever shown that air starting was actually detrimental to the slipper clutch, though. I'll video one next time I'm out flying with the camera on the wing and post it.

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Jim Lee

John, since your findings are so much different than mine, I wonder if you have the 912UL engine (80hp and less compression)?  Also, we have the very thin Woodcomp prop for reduced drag, and is probably less chord than the prop on your Ximango which would also make it harder to spin the prop with air resistance. 

Thanks for the comments,

Jim

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Thermalseeker

Hey Jim,

No, I have a 912s, 100hp in my Ximango. Certified version. I have the Hoffman 3 position prop. You're correct. The Hoffman blade cord is fat compared to the Woodcomp. That's probably the difference.

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