Landing patterns and reverse thrust!

I don't know how long of a distance it takes for most of you to land, but when diving in below the treeline from 200 feet I pick up tremendousspeed on my EasyStar to the point of hitting 50 mph and overshootingthe runway, ending up in the trees.

Of course I can land in 500 feet easy, but making my autopilot do it wasnot. I was curious if anyone wanted to chime in on how they solvedtheir landing issues and minimizing the length of space required.

I was able to get mine down to 500 feet diving in from 200 feet and leveling off. The attached photo is my landing pattern.

1) Circle the landing zone, sample the winds
2) Go downwind
3) Turn for final approach
4) DIVE! with a feedback loop on airspeed able to do reverse thrust
5) flare and land.

My reverse thrust is done with a car speed controller. I can get +1 lbthrust as well as -1 lb of thrust. (Wasn't expecting that either.) Thisis just by running a typical 5x5 prop backwards!

The end result is that I slow down from 50 mph to 20 mph in a few seconds after the dive.

This is my personal autopilot/IMU/Modem combo, but the math will work anywhere :) There isn't really instability. The tail's downforce is reduced with reverse throttle ON, basically having the same effect as reducing your elevator deflection. I only notice issue if I'm trying to hold my nose up over -8 degrees. Reverse throttle works well when you don't need much tail, and that's during dives. Yeah, it's the Easystar from hell with the huge antenna and gorilla glue everywhere. I even have bumper stickers on it now. Going strong after 3 years now.

Attitude is maintained because you reduce the maximum reverse thrust as your pitch command goes higher. For me, it turns off with commands higher than about -5, scaling to 100% at -30 pitch. This lets it work during the dive, and during the approach, but not during flare and the final stage of landing.

You need to be a member of diydrones to add comments!

Join diydrones

Comments

Ritchie September 20, 2010 at 3:36pm

It is not the Easystar from hell its the Epicstar.
I guess the critical part of the reverse thrust isn't getting that to work its where it works.
The UAVDev team have successfully created an IMU wind vector estimation file so perhaps that could be used to create a landing pattern setup with the Ardu in mind. I'm gonna enjoy this.
Tj Bordelon September 20, 2010 at 11:24am

This is my personal autopilot/IMU/Modem combo, but the math will work anywhere :) There isn't really instability. The tail's downforce is reduced with reverse throttle ON, basically having the same effect as reducing your elevator deflection. I only notice issue if I'm trying to hold my nose up over -8 degrees. Reverse throttle works well when you don't need much tail, and that's during dives. Yeah, it's the Easystar from hell with the huge antenna and gorilla glue everywhere. I even have bumper stickers on it now. Going strong after 3 years now.

Attitude is maintained because you reduce the maximum reverse thrust as your pitch command goes higher. For me, it turns off with commands higher than about -5, scaling to 100% at -30 pitch. This lets it work during the dive, and during the approach, but not during flare and the final stage of landing.
Ritchie 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?
Tj Bordelon 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.
Moderator

Gary Mortimer 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?
Ed Stewart September 19, 2010 at 8:51am

Excellent work, looking forward to the video's...
Tj Bordelon September 19, 2010 at 8:10am

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

http://well.In/
Ed Stewart 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.
Roman Krashanitsa 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.

of 4

This reply was deleted.