Predator build and fly thread (tip stall warning!)

For those of you interested in the Nitroplanes/Hobbyking Predator (they're essentially the same plane), here's a great build thread on how to make it, along with a very sobering set of warnings and lessons learned the hard way about how it flies. Basically, tip stalling is a huge problem and you have to land it with long, straight and fast glide paths or it will crash (I've already lost one that way). Basically, it's a poor airframe for UAV/FPV/AP use; we just use ours to show off at conferences.

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Comment by tycinis on September 24, 2009 at 7:26am
Very very fast , not a plane to do Fpv like you said.
Comment by Peter Meister on September 24, 2009 at 9:53am
Sorry to hear of your problems. I have the exact same one, and I love it. I have over 250 flights on it, with Paparazzi Tiny 2.11 and a video FPV setup. There is a thread on it on RCGROUPs.

I used carbon fiber to stiffen the elevons, and I put a steerable nose gear in the front with a servo as well I build a removal wing solution for it so I could unplug them and plug them in rapidly.

I have not had a tip stall situation, not to say its not possible because the wing design lends itself to this potential due to the thin width and thing airfoil with tapering at the ends.

Mine lands slow as molasses, but I added flaps to it, so maybe that's why your comes in HOT.

Hope you fix it, also where was your CG. If its too tail heavy it will TIP STALL ALL DAY Long.

-Peter

3D Robotics
Comment by Chris Anderson on September 24, 2009 at 10:09am
Yes, flaps could make a big difference. Basically, people should know that the plane is not really usable as a UAV platform out of the box. Between the need to add a steerable nose gear (which I did), the difficulty in squeezing equipment into the nose, the CG issues, and the tendency to tip stall (also true for the full size Predator, BTW), this is a big project to make useful. It looks cool, but to my mind it's not worth the hassle for most people. (I've built two, and one is flying okay, but I have to be super careful with it--I much prefer the EasyStar)
Comment by Jack Crossfire on September 24, 2009 at 10:22am
With gyros now at $5 per axis, you'd think solving these problems with software rate damping would be a no brainer.
Comment by Peter Meister on September 24, 2009 at 10:34am
Jack,

I have FMA on mine for pure FPV only flight, and I never turn it off. It works like a charm on my other one, for paparazzi it has stability modes so I get the same effect as FMA with the paparazzi. Like ardupilot would provide if it had say AileronAssist like the UAV devboard has. Stability mode rocks for planes that are inherently not so stable. I have a delta wing UAV that I use FMA to maintain stability mode for aerial photography and video recording. This plane is a challenge to maintain stable flight, especially durring the hand launch. But with the fma copilot I turn it on on hand launch and its smooth sailing all day long. IR Sensor stabilization in any un-stable plane, or gyros will improve drastically the stability of that airframe.

3D Robotics
Comment by Chris Anderson on September 24, 2009 at 11:01am
Jack, once that tip stall starts it's essentially impossible to stop. The plane is going to fall 100ft before you can get control, and if you're less high than that you're going to crash. What determines a tip stall is a combination of airspeed and roll angle, so I suppose you could program an autopilot to keep that combination above a certain threshold. But it would be hard to tune, and hardly worth it when you can just switch to a better airframe that doesn't have this problem. And once you've gone into a tip stall, I don't see how an autopilot can save you since you're basically non aerodynamic at that point.
Comment by Peter Meister on September 24, 2009 at 11:55am
Chris,

A tip stall is recoverable at altitude though, it's not impossible to recover from a tip stall. An AP could be programmed to avoid a tip stall happening by settings and sensors noticing critically low speed - high angle of attack and wing dip. At that point the AP could inject correction, level out, dive a little and wing level to avoid the stall occuring. I have had so many different planes do tip stalls, especially my turbine yellowaircraft F-18 which is prone to it at high angles of attack on final approach. Its recoverable, just need to be able to see it, feel it, and respond quick enough with the right corrections. Can an AP do it, haven't seen a DIY one that could, but im sure someone could make it with wing sensors to sniff the rough air occuring at the wing tips.
Comment by Riccardo Kuebler on September 24, 2009 at 4:24pm
Quote: Hope you fix it, also where was your CG. If its too tail heavy it will TIP STALL ALL DAY Long.
Agree with that, and why not try with the old, always working rules:
- accurate wing incidence measuring.
- some light negative tip warpage (-0.5/-1 degree)
- ailerons up on landing (not down!)

Just a thought to make life easier.

Best

Ric

T3
Comment by Krzysztof Bosak on September 25, 2009 at 2:27pm

-turbulators
Comment by RaWe on October 17, 2009 at 5:46am

Am in process of installing FMA CoPilot on my TMPro Predator ARF from HobbyKing. Anyone here care to share their actual flight reports with this combination? My experiences are posted here; build thread

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