Comment by Mike Lombardi on September 13, 2010 at 5:36am

Sounds like a lot of fun doing these combat approaches! Maybe you could program the autopilot to reduce the reverse in proportion to the elevator command or maybe use less reverse for a longer time to keep control?

Comment by Tj Bordelon on September 13, 2010 at 6:45am

Great idea! That sounds like a good solution. I had thought of:

1) Limiting the reverse thrust to 50% of what it is now
2) Just cutting out the reverse thrust when I get within 50 feet of the ground

but yours sounds the best...

3) Scale maximum reverse thrust with elevator command.

I hope I can encourage others to try reverse thrust as a valid way to slow down in a hurry. It works reasonably well.

Comment by Roman Krashanitsa on September 13, 2010 at 6:04pm

I would like to point out that at the very last stage - actual landing and touch-down you need your airplane be very stable.
My suggestion is this:
Make your reverse-thrust stage even more agressive but start at mid-descent. I should end at the beginning of your runway.
Follow it with a normal landing throttle setting. This way you will decelerate at safe altitude and have a safe margin to a stall speed. Ideally your approach speed at the end of the reverse-thrust stage will be same as you would have at the end of a longer "normal" landing procedure.

Comment by Ed Stewart on September 15, 2010 at 8:32pm

Has anyone used their airspeed sensor tied to the throttle to maintain airspeed? Not having tried any of this myself it makes me wonder if one could incorporate this to drive your motor forward or reverse to govern your airpseed like cruise control regardless of your crafts inclination. By not building so much airspeed to start with it might not be so agressive as to cause your lose of control across the elevator.

Comment by Tj Bordelon on September 16, 2010 at 6:01am

@Ed - I'm using a feedback loop to command airspeed as you describe. PID loop on the pitot controlling throttle. Throttle can go negative for reverse thrust. It works great. It's opposite of what the purists seem to like, but it seems to control the aircraft much more precicely. For example, if I command 25mph and go from level flight to a climb, the aircraft will start to slow down and throttle will kick up to compensate. It works very well.

In the dive, thrust is full reverse, but it takes time to build up airspeed and "error" in the PID loop, so the full reverse doesn't really kick in until halfway down the dive. There is probably room for tightening up things.

I'll have to report back after this weekend and let you guys know if scaling back reverse thrust based on pitch command helped anything. I'm so close!

Comment by Tj Bordelon on September 19, 2010 at 8:10am

I landed with the backoff technique-- it worked perfectly. I'll post videos next week hopefully...

Comment by Ed Stewart on September 19, 2010 at 8:51am

Excellent work, looking forward to the video's...

Moderator Comment by Gary Mortimer on September 19, 2010 at 9:15am

What happens if its a really windy day and a gust drives the airframe backwards just before touch down?

Comment by Tj Bordelon on September 19, 2010 at 5:08pm

@gary - I haven't had a case of this happening... YET. I do want to make a change to the airspeed FB loop. Instead of locking to the pitot exclusively, I should probably do " speed = min(pitot_mph, gps_mph)"... and try to drive that speed to my 20 mph landing speed. That way if ground track is too slow, the throttle will just kick up to fight the wind, making my ground speed the same as if there were no wind. Fortunately I guarantee I never land going downwind, so that isn't a concern anymore.

Comment by Ritchie on September 20, 2010 at 6:42am

I am loving this. The feedback loop could be added to the current ardu code as calc_landthrot or something easily enough thanks to the excellent programming standards. The real question is how the attitude will be maintained during landing if the landing is set given the instability caused by the reverse thrust although having a TJ sense and land setup would greatly help. Is this setup on your behemoth easystar or an ardu based one?

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