...well, in my dreams and in X-plane only so far.
It's been quite interesting trying to get a flying wing to fly at such high altitudes. There are all manner of complicated instabilities to overcome. So far, the biggest improvement was adding a motor and propellor, not as any form of thrust device, but as a gyroscopic stabiliser!
Making the elevons operate only in the outer third or quarter of the span was also another leap forward in stability. The image above is just a flying .STL file and hidden behind that is a rather crude series of aerofoils to simulate the aerodynamics of the thing.
The APM can control it sometimes very well at altitude with it being nice and stable, but other times it seems to always go into some form of unrecoverable spin. I haven't figured it all out yet, but I will be doing so a fair bit more before committing to the foam. Operation at <400' (yeah, right!) is nigh on perfect in all but my most ham-fisted efforts!
Comments
Thanks for the tip Mauro - I'll check it out.
For Paul, here's a screenshot from X-plane with the flight model vectors made visible. It's easier to see what's going on, where the stablising downforce, normally provided by the empenage in a conventional aircraft, is delivered by the wing tips. The washout is what causes this effect and I think the outboard elevons add to it.
As far as weight and moment of inertia/radius of gyration, I have tested the model with increasing radius of gyration and it makes the problems worse. If anything, I think I should increase the sweep of the wing to make the stablising force of the tips more effective.
For those interested, here's the X-plane model with all the secret aerofoils revealed. It shows what I've done with the elevons too.
Paul, the oscillation issue you describe is exactly the main one I've been trying to overcome. Eventually when it gets bad enough, it goes into a full 360° tumble and then on to subsequent tumble/spin modes, depending on control deflections.
Things that inlfuence this are washout (a lot more than you'd normally use (I'm up to -4.7°) and as I mentioned above, the part-span elevons helped a lot. I'm sure that the two effects are related. The gyroscopic rigidity in pitch of the propellor was the last significant step I have found. Now I'm just tuning it to find the optimum of these features.
Paul, Yaw stability is a very, very minor problem now for me due to the rather clever wing tip design I borrowed from the guys at www.crashtesthobby.com. I have modelled it in X-plane as winglets with span of slightly less than the thickness of the main plane. I have modified it in several ways to improve the stability effect as well.
As far as stability concerns go, the main problem I had to try to overcome was pitch stability. I wouldn't say I've solved the problem entirely, but it is significantly improved on where I have started out. There are a few spin modes that look unrecoverable, but I reckon with the right amount of thought-power and coding, APM could recover from them (if it could measure the precession rates) - it's all in the timing of the control inputs.
Brandin, I'm using X-plane 9 to do this - see www.X-plane.com The package is cheap (29 bucks or so) and comes with Plane-maker which you can use to concoct your own aircraft models. It's a pretty comprehensive package and I believe even Burt Rutan's mob at Scaled Composites have used it in the past for evaluating new designs and ideas.