So I have tested quite a few different platforms to use for UAV work, including a few flying wings, but, I can not seem to understand why in the SUAS world so many platforms are flying wings?
From my testing here are the pro's and con's I have found:
- Good MTOW for wing span
- decent crash resistance
- Easy to fit wide cameras
- Difficult launching, can be dangerous as prop is usually in the rear
- Very CG dependent
- Less efficient in practice
- No flaps
- Tip stalls/death spirals
- difficult to adjust CG when changing payloads
- Less stable in wind (No yaw control)
Am I missing something as to why so many fixed wing UAV's coming from the DIY community are flying wings?
Our testing has shown cargo plane style twins with fairly high aspect ratio and high aspect ratio gliders to be the best all around platforms for UAV work (carrying payloads)
I am curious to hear from the community on your thoughts and what designs other people find effective or what I am missing about the flying wing craze.
Easy to chuck on the back seat of a car. What on earth makes you think a wing is less stable. Also they are very easy to launch if set up correctly.
In windy conditions all of the flying wings we have tested when setup for UAV work are much less stable in terms of holding a camera stable looking down at the ground. picture quality is less almost every time. What makes me think this is lack of yaw control so any correction for wind changes has to be done solely with elevon inputs vs. the pixhawks excellent yaw corrections when a rudder is present. Maybe all of the flying wings (3 different models) we have tested so far are bad?
How are you launching? With full mapping equipment installed when we launch it is very scary and we have resorted to bungie launches for the flying wings as it is to prone to failure with hand launch. Our traditional style planes launch much easier, especially since we can safely launch under power. Maybe we're doing it all wrong though?
We started out with higher hopes for flying wings since there seems to be so many in use, but, unless we are doing something completely wrong, we have failed to see the benefits in the area's that are most important to us of a flying wing over a high aspect ratio traditional style plane/glider. The key things being easy and reliable take off/landing, stability in flight for the sensors, and efficiency as we're mapping large area's.
But I still question myself and our experiences since it seems there are so many flying wings for UAV work and more people keep developing them. I feel like I am missing some benefit or not understanding something.
This is not meant to bash flying wings by any means, I am extremely open to any airframe design which offers the best performance/mission requirements. Just trying to understand what I am missing.
You have it all correct - Boeing still flies with a tail on the newest machines. The pros are all mission dependent.
If you want to fly straight and far with little fuel burn a wing is a good option. They suffer in maneuvering and in all the ways you say. The intermediary step from wing to conventional platform is the canard. more maneuverable but picky with CofG too. They are a good compromise with range because all the lift is up (no conventional tail pushing down etc.)
But of course you all know this already!
more interesting in my mind is the low vs high aspect ratio compromise.
I find the low vs. high aspect ratio compromise interesting as well, especially at the speeds we are typically running missions at. at the 15-20 m/s speed, how much advantage is the higher aspect ratio plane getting over the lower aspect ratio with thicker chord plane?
The advantages of high aspect ratio are due to drag created at the wing tips being considerably less, however, since drag is a squared function, and we are flying at such low speeds compared to traditional full scale aircraft, I wonder how much of a benefit it has for our applications?
One just has to make two wings for the same fuselage (did this many times as a teenager flying slope soaring gliders). The outcome is very surprising. the size of my aircraft were small. about 1M in span with about 6" chord. the low aspect ratio wings tapered in the middle to mount and had 75% area of the larger wings. all of my small wings were elliptical in shape or tapered in a cranked fashion to approximate an ellipse.
If the wing can be made structurally sound - high aspect is the way to go every time (admittedly not good in a crash - or while flying through trees!). I fly in wide open spaces with nothing to hit (Lake Ontario).
Math is less telling than models in this regime. lets get back to the point - flying wings.
The aerodynamic moment of the elevator and rudder is very short on a flying wing. This is why they are less stable, and also why flying wings usually have a very high sweep angle with vertical stabilizers at the tips of the wings. There's no real advantage to high sweep angle on subsonic aircraft, but a flying wing needs to have the control surfaces as far aft as possible, and the steep sweep angle allows the tips of the wings to be as far aft as possible.
If you imagined a perfectly rectangular flying wing, during a stall it would likely just begin to rotate and tumble end-over-end. The longer aspect ratio wings would be very prone to this. A flying wing has to sacrifice a high aspect ratio wing for stability, while adding drag and losing lift as a result.
A very stable configuration would be a sufficiently-sized wing at the CG with a very long, narrow tail and relatively small control surfaces far to the rear. The closer the tail is to CG, the larger the control surfaces would need to be to overcome the moment forces. If the tail was too long, it would actually be too stable, and difficult to maneuver.
I agree with most points you make about flying wings. The main big advantage is simplicity and ruggedness. In practise this is a huge factor. No good having perfect aerodynamics if the aircraft breaks easily. However a flying wing can have flap, as shown in the video below. In this , my wing, you counteract the flaps tendency to pitch the aircraft down as it is activated by mixing in some proportional up elevon, which works very well. The flap is most useful for takeoff when it slows the required takeoff speed a great deal. and for flying slowly
You can have flaps and as many as 6 control surfaces spread over the T.E.s.
Centre of gravity: the percentage of the root chord that the C of G is ahead of the Neutral point decides the stability. e.g 3% would be almost uncontrollable; 5% is better, 7.5% to 10 % would be more stable. Look up http://www.b2streamlines.com/; the definitive website about flying wings.