I'm starting a high altitude return-to-sender balloon launched glider project. I want to keep it simple, and I think my first decision is airframe. I'm wondering if it would be feasible to use a small glider like this mini swift for rudder-only control. The idea would be to have a fixed elevator, set for a fairly steep glide ratio (like around 5:1). It seems like this is how thegps boomerang works.
Would an airframe like the swift be stable enough to recover and glide without stabilization?
Why rudder only? A glide ratio of 5/1 means a sudden arrival which will probably demolish the plane, and anything in it!.If you are at the landing site you might need more than rudder to avoid obstacles also. Have you actually flown any R/C models?
Thanks for the input. The gps boomerang uses rudder only and seems to work very well, so that's what I'll try. My goal is to only add complexity where absolutely necessary. 5/1 will not destroy the plane and anything in it. This isn't a Boeing.
It is my understanding that the boomerang is a mature and well-tested system.
For example, I am sure that you looked at the data from one of its test flights:
Yes, those results are what have me optimistic. I looked very closely at the photos, and it's pretty clear that the only control surface is the rudder. Also the weight is listed as 9.5 oz, so I'm guessing there isn't a lot of hardware on board. I could be wrong, but I'm guessing that it depends on a stable airframe rather than some sort of IMU. I wish I could find further documentation on the design, but I'm hoping that it's simple.
I agree, the system probably depends on an inherently stable airframe for pitch stability. It really does not need elevator control. Also, the airframe design itself solves the issue of transitioning from very thin air to more dense air.
So, yes indeed, the only control is the rudder. What could be simpler?
As far as yaw stability is concerned, the boomerang may or may not have a single gyro, its hard to tell from the website, but I think there would be a problem with navigation stability without one.
Several years ago, when I first started out in this hobby, I tried doing navigation with a GPS only. That just did not work at all, because of GPS latency.
The next step was to add a yaw gyro. That helped a great deal, and I was able to implement navigation with GPS and a yaw gyro. So, you really don't need a full IMU if you are willing to tolerate some lateral error.
Since my first post, I have also reconsidered against the swift for a number of reasons. But mainly as you have mentioned, the boomerang already works, so that's where I'll start. It will provide better insulation and more room for electronics. I have some extruded polystyrene and a foam cutter, and I'm already doing tests on a foam box in the freezer to see how much insulation I'll need. I'll cut the airframe to roughly the same dimensions as the boomerang. I happen to have a tall bridge near my house that I can toss a glider from, so I'll be able to do some stability testing. As for creating the control hardware and software, that's where the fun is for me, so I'll definitely be rolling my own. I've had some experience with autonomous ground vehicles, so at least I won't be starting at zero. Thanks for your input.
I started a project like this about 2 years ago, its going very slowly, But unfortunately we didn't have a tall bridge nearby, I built a large twin engined electric plane to launch the glider from. And am only now getting into Ardupilot. Video is quite interesting though!
Just to be clearer on a point that I glossed over...I do not recommend the mini swift and building your own controller. There are a lot of engineering challenges, and it would be hard to develop the controls without doing the actual flights. The main issue for the airframe is the low air density at high altitudes. The big risk would be the swift would reach a very high velocity in the thin air, and then the wings would snap off when it reached more dense air.
Rather, what I am recommending is that you start with the gpsboomerang.
I've just started on this same project.
A quick background since this is my first post.
I'm an electronics engineer and a member of my local ham radio club. We're all very active with a variety of projects and as of late we've been doing a few balloon launches. One of the projects on the bench is to bring the balloon payload back to launch. Or to a reasonable landing area.
Most members of the club have experience in electronics, programming, avionics and some are pilots themselves amoung other skills. So we have very little reason for not being able to do something we put our minds too. We also have some useful data from our previous launches.
We're just at the beginning of this little project and we're starting to select an airframe.
A lifting body is high on our list of air frames to test. Our first test is going to be a simple styrofoam free flying brick. ok... a aerodynamic brick that should provide some interesting data itself.
It will be taken up with some of the local skydivers and left to it's own devices while we track it and hopefully recover the wreckage.
In all the other cases of aircraft we're considering, one of our main issues is how to detect the ground to pull up or flare. It's more complex than this, but detecting ground proximity is just a simplified description of a variety of problems we face. And while we do have some ideas already, it would be interesting to hear what everybody from this forum has to say or suggest.
There are no discontinuities in the atmosphere, so the airframe won't suddenly arrive at the dense air and break up. I think people think this because they are imagining what happens in orbital and inter-planetary space travel where the velocities are absolutely enormous compared to airborne flight.
The airframe might break up for other reasons though...
One of the possibilities is flutter excited due to turbulence, control surface imbalance, aeroelasticity or excitation through vortex shedding. Having a rudder only is an interesting way of dealing with control surface balance - or not, depending on how you are looking at it.
One thing to remember is, you can get an airframe to fly at ultra-high altitudes... ...that is, probably - I've only worked with the theory, so far! In my opinion, you need to have an aerodynamically stiff airframe capable of low indicated speed flight.
Another thing to understand at high altitudes is the Reynolds number becomes very low, especially with small models. A chord Re of 10e4 is likely. If you have a look at the drag polar of nearly every aerofoil, you'll see that the laminar separation bubble (look it up) causes you great headaches at this level of Re. Ideally I think you need to stay north of Re = 10e5 to keep some predictability in the lift curve.
High Re equals long chords, so now we have an airframe that has to be stiff with a long chord. It starts to look a lot like a flying wing with a low AR. A quasi-delta wing or similar might be one ideal shape, especially if you can get vortex lift to be on your side.
The other thing that low Re and laminar separation bubbles seem to do is to make the pitching moment of the aerofoil and/or airframe a little unpredictable, so this may work against the usefulness of the delta planform. I don't know enough yet to say this with any certainty.
Just to make life complicated, I doubt that X-plane, XFLR5 or anything remotely simplicated in terms of aero analysis will give you meaningful data with lifting bodies and/or deltawings or anything remotely unconventional...
If you mean point the nose down to achieve 5:1 then you will probably arrive at a heck of a speed. Wings would have to be very strong.
I think I would just let it tumble for 20 or 30k until the air thickened.
Is that airframe big enough to carry any sensors and an AP?
Fir the first couple of tests, we're just randomly testing some shapes that "might" fly.
Very trial and error. Which is part of the fun. If we find something suitable that can be controlled later, then it will go up with the skydivers and left to it's own means. That test method is already causing a few people to laugh and while it is still in denser atmosphere, I do think it will provide some useful information on controlling the final design.
At higher altitudes it really doesn't matter much to us what the model does. The point of the first drop from a balloon would not to be attempt to glide at altitude, it's more about getting the payload back to the ground in one piece without a parachute.
We have considered a modern type wing shaped parachute and flying the payload back to the ground like a paraglider. This is surprisingly simple in terms of UAV. Keep turning the wing in the direction you would 'like' to fly. Though the ~20kph airspeed is not going to be much of a match for 120kph winds in the jetstream.
This is one of our latest balloons http://aprs.fi/#!mt=roadmap&z=11&call=a%2FVK2BTW-11&tim...
The winged parachute will probably fly on our next balloon, but we have to think about how to bring it back to the ground to a designated area. It's unlikely it is going to be able to fly back to launch, so somewhere downwind will probably be chosen.
The challenge is more to do it without any form of parachute and preferably to have it fly itself back to the launch site and land itself without destroying the payload. In future launches we might look at best glides, but for this first attempt it's more about getting back to the ground quickly and not having to go looking for the aircraft.
The actual way this version flies at altitude isn't so important. After all, we could just treat it like a guided bomb, and that is one of the models we have considered. So we don't want to smack the bomb into the ground, we want it to pull up and (relatively) soft land.
A delta wing or lifting body of sorts is something in between the conventional glider and a guided bomb.
Dethermalising the aircraft for the first part of the flight is another option. I don't think there is much we can do about laminar separation, but that is some of the other club members expertise.
I'm of the opinion of just getting the aircraft as a whole back to a lower altitude in one piece and under control is the focus of our first attempt at 'flying' something to or into the ground :)
But the more opinions we hear, the better.
I've already seen some good ideas from reading other threads on this forum.