Flying Wings - why not?

[richard]

Published on behalf of Tim Costello : -

I have often wondered why we don't see more Flying Wings amongst us soarers, even outside competition. They seemingly offer a very high degree of efficiency due to their lacking the drag of a fuselage and tailplane and I would like to open a discussion about them if I may.

I have been designing my own gliders for some years, always with an eye to the aerodynamics, and lately have been studying available information on the Flying Wing.  Incidentally, it is almost impossible to buy a reasonably sized, and priced, flying wing glider these days and I have searched the mainland European suppliers as best I can. They just seem to be out of fashion for the moment.

Many years ago somebody discovered that you actually can make a straight, 'plank' wing fly without a tailplane if you curl up the trailing edge and pop a big lump of lead on the nose. The problem was that the lump of lead was always fighting the curl-up, one trying to nose dive the aircraft and the other trying to keep its nose up. This flying plank also needed a huge vertical fin to control the model and keep the Law of the Lever happy. The drag was therefore very high with this set-up until another bloke said 'why not sweep the wings back and put smaller fins on the wingtips?'. Bingo, it worked and the model really looked cool too.

Then the aerodynamicists stepped in and actually confirmed that the swept flying wing was in fact very efficient, both in and out of the wind tunnel. The amount of trailing edge curl-up, or reflex, could be drastically reduced and, like the delta wing, needed little or no dihedral. They explained that a normal aircraft flies with the centre of gravity in front of the centre of pressure or lift.  This will cause the aircraft to nose dive unless corrected by sticking a tailplane way back from the wing on the end of the fuselage.

But here is the interesting bit: The wing-tips of a flying wing are nearly as far back from its C of G as the tailplane is on a conventional aircraft. So by curling the tips up slightly you get the same stabilising effect but without the drag of a fuselage and a seperate tailplane!

Of course it is difficult to pack a load of passengers into a full-size flying wing but for radio control models it is an ideal configuration.  Using modern Eppler airfoils, like the E-186, the lift is excellent and the drag very low indeed. The only disadventage might be orientation when the model is very high up, but by painting a foot or so of the port wingtip with white enamel, followed by a coat of Humbrol Flourescent Orange, you won't lose track of it.

So there is my tuppence-worth and I would love to hear other viewpoints about the Flying Wing.  In particular what is present day thinking about the need, or otherwise, for progressively changing the amout of reflex or twist of the airfoil?  Is the airfoil shape speed dependent?

Finally, can somebody tell me how on earth do those Multiplex and Phase 3 foam models fly so well?  Their wing sections seem to ignore all the rules of aerodynamics when flying either fast or slowly!

Tim Costello, Dublin.

[olivers dad]

Tim a man after my own heart. I think i am the only person in the uk to fly a F3K with a flying wing, I have not given up the idea but the wing i made was to heavy to get a great launch height. But when i flew it on the slope it was great in thermal turns. I have plans to make another. Here is my other flying wing  with 20/1 glide the more expensive version has winglets. The v tail is to help tumbling and is fixed angle, Spoilers for roll control and weight shift for pitch, And some nice flaps to help with landing and cruising between lift

                  Regards Nick

[mitch]

Fascinating beast's but very difficult to get to perform as a thermal soarer.
I played around with many types some years ago, but eventually gave them up in frustration.
There are so many compromises that a pure wing has to make just to be stable that makes it very very hard to get a thermaling performance anywhere near a standard config. glider.
Good fun on a slope though .
I still have a 2mtr 'Klinberg' wing (an American design)that was exciting to launch and tricky to fly off a bungy. I keep meaning to put a motor in it but never get round to it.

There are a quite a few kits available in Germany, this one is a nice build
http://www.aff-cnc.de/de/Model/Model/Nurfl%FCgler/
another model I fancy having a go at, but never get round to it.

Best of luck and keep us posted.

[olivers dad]

I have been tempted with that one myself pete for electric comp but its wood. I wouldn't know what to do
It's got me thinking about a project in the garage for f3k i had shelved

[mitch]

but its wood. I wouldn't know what to do
It's got me thinking about a project in the garage for f3k i had shelved

Is this is another example of the 'Revenge of Blue Peter'
Ah well, that programme may have stopped kids using sharp knives and proper wood, but it did give us Konnie Huq

Oh! and whats this secret project in your shed?

Sorry, back to flyingwings...

[Simon V]

I'm quite into wings and have an electric Yoda as mentioned in the above links which I occasionally fly. It is the 2M variant.

It flies really well and has a wide speed range. Building it was not really a balsa bashing exercise but more like a slot together affair.

I also have a TGworks Chinook and red herring for hand launch, and am just finishing a sort of copy of Dave Jones's 'Raven' 100" flying wing for 100s contests.

With flying wings, I have found that you have to adjust the way you trim and fly to suit the model when compared to a conventional glider.

http://www.facebook.com/photo.php?pid=1716698&l=4082bc9093&id=664557775 for a picture of the yoda

[boffin]

My only experience with a flying wing was in the 80's with a Jeff Andriesson 60" span delta flying wing, Pteroway, run as power assist with a DC Wasp. The attraction for me was a fiendishly complex mechanical mixer for elevon control which worked well. This way before electronic mixing. The model was easy to fly had a good speed range and was capable of high AOA before stalling. Subjectively, the glide was not that good and it was a handful in a tight thermal turn. Eventually sold as I had satisfied my curiousity.

My feeling is that IF a flying wing layout gave lower drag than a tailed model then the F1A brigade would have fallen on the layout as they would sell their left arm for a 5 sec still air improvement. By the same argument, I do not see too many full size flying wings in competition.

Martin Simons (Model Aircraft Aerodynamics) P111 is rather dismissive of the layout.

A 2m Yoda appeared in the hangar on Sunday at the 2008 Nats at £60. I pondered the sale but the very small chord and tissue covering argued against an involvement. Strange decision as common sense does not usually prevail.

There are several Jetex/Rapier flying wings that fly at Old Warden. ......no 2 flights are the same and trim stability is questionable.

IMHO, a flying wing has good curiousity value, but the add-ons necessary for pitch stability put the drag beyond that of a conventional tailed model.

Flying Wings in competition?...........regrettably not for me.

Regards Ian

[JefO]

As some of you may know I am bodging together a simple Delta wing rocket glider, my investigations into the previous design work of others, have unearthed a document which makes me think about the Vulcan Bomber's development, but is in line with the subject of this thread.

The following is disgracefully plagiarised from a publication called "Flight" produced 10th August 1951. It raises hurdles (a to e below) which are pertinent to the design of a tailless thermal soarer, despite answering them (positively) from the perspective of a very different end, which is more relevant to my project...

174 FLIGHT, 10 August 1951
AERODYNAMICS OF THE DELTA . . .
A preliminary question is whether a tailplane is necessary.

From the earliest days of flying, the question has been raised as to whether aircraft can be flown satisfactorily without a tailplane. Confining attention only to the case of the high-speed jet aircraft, each of the functions of a tailplane will be examined in turn in relation to the delta wing aircraft. The functions are :

(a) To trim out changes of centre of gravity position according to the load carried and the consumption of fuel.
Investigation shows that a control surface at the trailing edge of the wing, provided that the latter has a large root chord (as has the delta), can cater for all but extreme CG movements.

(b) To deal with trim changes due to landing flaps, etc.
With the low wing-loading associated with the delta wing, take-off and landing speeds are moderate without the use of flaps, and this question does not, therefore, arise.

(c) To provide damping of pitching oscillations.
The reduction of damping of the pitching oscillation has led to difficulty on some tailless aircraft, but it does not arise on the delta, since the large chord near the root gives adequate damping.

(d) To deal with loss of stability or control power consequent on distortion of the wing structure at high speed. "Aeroelastic distortion"
At very high speeds, all aircraft structures distort to a greater or less extent under the high loads imposed, and this distortion alters the aerodynamic form. In extreme cases this leads to loss of stability or control power, making the aircraft dangerous or impossible to fly at high speeds. An aircraft with a high aspect-ratio, swept-back wing would need a tailplane to deal with this, but the shape of the delta wing makes it extremely stiff, both in bending and in torsion, and a tailplane does not appear to be necessary.

(e) To provide for spin-recovery.
Although this point has not been proved, it is expected that the controls on a tailless delta wing would not be powerful enough to ensure recovery from a fully developed spin. A tailplane appears to be the only way of dealing with the problem. This restriction is of small significance for transport or bomber-type aircraft.

It can, therefore, be concluded that for a delta-wing aircraft of the transport type a tailplane is unnecessary. Its deletion leads immediately to a considerable saving of weight and drag, and to a major gain in performance.

We have now shown that, compared with a conventional aircraft, the delta-wiog aircraft will be simpler by the omission of the following items : the tailplane; the rear fuselage necessary to carry the tailplane, and wing flaps and other high lift devices such as the drooped wing leading edge. There is a saving of weight, of design and manufacturing effort, and of maintenance when the aircraft is in service. These economies will have a considerable bearing on the initial cost and the man-power necessary to produce and maintain a number of aircraft.

Because of its shape, and the large root chord, the delta wing provides a large internal volume in relation to its surface area, even when using the thin sections which, as noted above, are essential for high-speed aircraft. Simple calculations show that for the same wing area, the delta wing has 33 per cent more internal volume than an untapered wing, while, if the inboard half of the wing only is considered (as this represents a more practical case from the point of view of the aircraft designer) the internal volume of the delta wing is more than twice that of the corresponding untapered wing.

It is found that without exceeding a wing thickness of as little as 8 to 10 per cent, it is possible on a moderate-sized delta-wing aircraft to bury completely the engines, the undercarriage, and sufficient fuel tanks for long range. The fuselage also has a tendency to disappear into the wing at the root. The result is the attainment of an aircraft consisting only of a wing, a fin and a rudimentary fuselage, representing a degree of aerodynamic cleanliness which has never before been reached. In fairness, it must be pointed out that this is achieved at the expense of a rather larger wing area than usual, but investigation shows that the drag of this area is less than that due to a conglomeration of items such as engine nacelles, tailplane, etc.

From the design point of view, the shape of the delta wing leads to an extremely stiff structure without the use of thick wing skins, and strength becomes the determining feature rather than structural stiffness. This avoids the inefficiency of conventional swept-back wings where the wing has to be made stronger than necessary in order that it shall be stiff enough.

Summing-up, it can be said that in order to meet the requirements of large loads for a long range, at high speed, the high performance transport or military aircraft of the future will cruise at a considerable altitude, at a speed not much below that of sound. The delta wing provides the only satisfactory solution to these requirements, for the following reasons:

(1) It meets the four features necessary for avoiding the drag rise near the speed of sound, i.e., it is highly
swept back, it can be  made very thin, the wing loading is low, and the aspect ratio is low.

(2) Extensive wind-tunnel and flight tests have shown that the low-aspect-ratio delta wing gives minimum
changes in stability and control characteristics at speeds near the speed of sound.

(3) In spite of the wing being thin, its internal volume is large, so that the engines, undercarriage, fuel, and
all the necessary equipment can be contained within the wing and a rudimentary fuselage.

(4) Adequate control can be obtained by control surfaces on the wing, thus eliminating the need for a
conventional tailplane. Together with item 3, this leads to a considerable reduction in the drag of the
aircraft, and, therefore, to high performance.

(5) Auxiliary devices such as flaps, nose flaps, or slots, and the all-moving tailplane, are unnecessary,
thereby saving weight and design effort, and simplifying manufacture and maintenance.

(6) The delta wing is very stiff and free from distortion troubles.

[bja]

There is no reason why flying wings could not be seen more in sport flying, other than I suspect most flyers needs are for a more all round model that will give a good account in contest flying as well. But this debate has been going on for ages and the facts speak for themselves.
In full size, most of the stabilty problems seem to have been ironed out, due to fly by wire, witness the stealth series of aircraft, where the flying wing type of aircraft has the advantage due to low radar signature.

Back in the 70's a model was published in the RC Modeller Magcalled a "Wind Freak". I remember Martin Garnett flying one in thermal Comps & at the time it was impressive. Although the ability to detect what it was doing  at height in a thermal, was a bit worrying & like most wings it was very touchy on the controls. I am told that putting it into a dive was veryexciting to say the least as the thin wing led to the speed increasing very FAST!!!
It was not the easiest to build either, certainly taxing the skills of a lot of modellers.. I have a plan & instructions for any one who wants a challenge, that I could get copied.

Last year I put together a"Mamba", see the Ace Models web site for details of the model & my review. This model was really nice to fly, with very benign tendencies. Proved to be very fast due to the weight of the electric drive/battery combination, but it did thermal very well. I reckon if you completed it as a slope soarer, with lower wing loading it would be even better. If it had an elevator function, rather than just elevons, the control would be even better, as the elevons restrict the control in certain aspects of flight namely rolling manourvers where you cannot apply a separate elevator to keep the roll axial about the centre line.
But the finish is superb & when purchased last year the price at £150 was very reasonable Sad to say it has now risen to £210 due to our rate of exchange being what it is

[milquetoast]

you can't have crow brakes or camber changing which makes them a poor proposition when you are talking about "why not" vs a 3m+ "normal" moulded glider. 

I would not agree that lack of landing flaps does not matter.  The drag of a fuselage and tailplane is hardly anything in comparision to a poor wing. all wings must have some reflex or other means of stabilisation.   

[JefO]

Whilst I agree that it is unusual, there is no reason why a flying wing cannot have crow brakes.

The Atos VX wing that I was aerotowed to 2000ft under last month (and Nick Chitty has a variant of), has inboard flaps and letterbox style tip spoilers - so it is set up very similarly to crow braking.

http://www.learn-to-fly.co.uk/hang_gliding.html

The 100"S Design flying wing 'Hatchet', had a 'split rudder' braking system. I have a plan of that model if anyone fancies building it.

Best regards to all,

Jef

[mitch]

Whilst I agree that it is unusual, there is no reason why a flying wing cannot have crow brakes.

Here's a trip down memory lane, I remember when I found crow brakes on flying wings is not a good idea.

Some years ago, I had a Multiplex ‘Cortina’ flying wing. This was a serious, true flying wing,with a small glass fibre pod to house the battery and rx. It was near 4mtrs span, obechi on foam, root chord was something like 16inch and it was equipped with ailerons, split elevator and top mounted airbrakes.

It weighed in the region of 8lb, was very exciting to fly, and had quite a few memorable Bungy launches and flights which greatly amused my fellow Fairlop club mates who witnessed them.

It was pretty quick and really suited to slope soaring rather than flat field thermal soaring and I eventually sold it on to a guy on the south coast (I wonder what happened to it?).

My transmitter at the time was a Futaba 1024 (remember them) and it’s mixing enabled me to try proper crow breaking without using the top surface blade airbrakes.
The result was trouser bothering .
It was almost impossible to avoid a violent pitch down when the break was deployed. The elevator always overpowered the aileron, no matter how careful I was. When a stable breaking attitude was established, it was then very difficult to guide. As both ailerons were up in brake mode, direction was controlled by dropping one aileron. Unfortunately the dropped aileron stalled the wing which usually turned the model the opposite way to that intended .

The same model using the top blade airbrake with elevator compensation was a doddle to land, which spoiled every ones fun .

Modern transmitters would probably be able to mix these problems out, but I think a proper flying wing (a wing without a separate stabiliser) would be a handful under crow breaking.

[olivers dad]

right guys flaps down to 70 degrees for landing.  you can see how big the flaps are on the trailing edge. the Vtail is fixed and is there to stop the glider tumbling/ spinning. I used to fly this glider without the tail. And it only spins if miss handled.

                   Nick

[milquetoast]

right guys flaps down to 70 degrees for landing.  you can see how big the flaps are on the trailing edge. the Vtail is fixed and is there to stop the glider tumbling/ spinning. I used to fly this glider without the tail. And it only spins if miss handled.

                   Nick

this plane has a tail

[milquetoast]

Whilst I agree that it is unusual, there is no reason why a flying wing cannot have crow brakes.

The Atos VX wing that I was aerotowed to 2000ft under last month (and Nick Chitty has a variant of), has inboard flaps and letterbox style tip spoilers - so it is set up very similarly to crow braking.

http://www.learn-to-fly.co.uk/hang_gliding.html

The 100"S Design flying wing 'Hatchet', had a 'split rudder' braking system. I have a plan of that model if anyone fancies building it.

Best regards to all,

Jef

you could aerotow a house brick to 2000ft though.  If it worked there would be lots of flying wings with brakes - there aren't. 

A tailed plane is inherently more stable. Notice how spears, arrows, missiles, rockets and other projectiles all have tail surfaces. Not to mention birds.

The problem with tailless aircraft is that they aren't as good as ones with a tail.  Which is why most planes have  tail.