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What are the disadvantages of a tailless ultralight design with say 20 deg swept back wings with wing tip rudders, 20-60mph, max 25 hp?

I'm not an aerospace engineer, so I stand to be corrected:

The only 2 disadvantages I can think of are:

  1. Increased wing area/more weight/stronger spar

I understand the wing area would need to be larger as the "aerodynamic wing span" would be smaller than the physical wing span. I understand the wing only produces lift with the velocity component perpendicular to the leading edge ( which I understand would be V0*cos(radians(20)))

  1. Possible reduced or no side slip angle.

I understand you're not supposed to side slip a Velocity aircraft as the leeward wingtip vertical tail may stall ( rudders only move outward). I understand only the right rudder deflects outward when turning right ( the left rudder acting only as a vertical stabilizer, in this case) I'm attributing this to too thin of a vertical tail airfoil ( low stall angle ) with too high of an aspect ratio. ( I stand to be corrected on all the above). I understand Velocity reduced this problem by reducing the max angle of rudder deflection.

Are there any other disadvantages in terms of stability, cross wind landing, etc?

See pic below of the Mitchell Flying Wing:

Mitchell Fying Wing

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  • $\begingroup$ The last paragraph might better be a separate question. Also tell us, when you give a right rudder input in a Velocity, which tip-rudders move in which direction? Some aircraft with tip rudders move the rudders only inwards or only outwards. If you don't know the answer, maybe grounds for yet another question. $\endgroup$ – quiet flyer Apr 1 '20 at 16:04
  • $\begingroup$ It seems Not clear....helicopter,plane? $\endgroup$ – L'aviateur Apr 1 '20 at 16:47
  • $\begingroup$ Yes, I meant vertical. Corrected. $\endgroup$ – Fred Apr 1 '20 at 17:10
  • $\begingroup$ The answer has to be yes there is a tradeoff but-- I don't have more details ready at present. The Aerianne Swift ultralight sailplane is one example of a successful aircraft along these lines. $\endgroup$ – quiet flyer Apr 1 '20 at 17:21
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When we designed the SB-13, we also thought that it's not so much different from a regular layout. Boy, were we wrong.

It starts with ground handling: Regular aircraft have some distance between front and main wheels, and even more between main wheels and tail skid in taildraggers. This makes them fairly stable in pitch on uneven surfaces. When both wheels are close together because there is little of a fuselage to space them apart, every bump on a grass field translates into heavy pitching. This is annoying.

It gets worse once you lift off: The little pitch damping (now I am talking of the aerodynamic sort) means that every gust makes the tailless aircraft enter into a noticeable short-period mode. This is normally so much damped that the pilot doesn't even notice. In a tailless plane the damping is just enough to stop the motion once a considerable change in pitch has happened. Of course, the pilot will try to steer against it, but with the unavoidable delay of a human in the control loop. That means the counter action will now make the aircraft pitch against the initial motion just when natural stability does the same, so an overshoot follows. Either the pilot suppresses the learned reflex of actively stopping the pitch motion or a pilot induced oscillation follows. This is even more annoying.

So much for the annoying stuff. The outright dangerous would be:

  • coupling between fast period mode and wing bending. Flutter at low speed is the consequence.
  • Nasty stall characteristics once the combination of sweepback and aspect ratio surpasses a certain threshold. For 20° sweep this would be an aspect ratio bigger than 7.

You are right about the larger wing area, but this actually helps to make the spar lighter since there is more height to place it in. Unless you need to stiffen the spar in order to shift its bending eigenfrequency above that of the fast period mode in your allowed speed range (plus 20% safety margin).

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  • $\begingroup$ Significant washout, with tip downforce, can cure the stability and stalling issues. Indeed, the first aeroplanes to be certified fully stable by the British authorities, even when flown hands-off, were of this design. The drag penalties are less than expected, at least in part because the root bending moment and accompanying structural weight are reduced. $\endgroup$ – Guy Inchbald Apr 1 '20 at 18:44
  • $\begingroup$ Yes, Dunne and Prandtl wing. I know. Reduces maximum lift even more $\endgroup$ – Peter Kämpf Apr 2 '20 at 10:06
  • $\begingroup$ I would assume that one of the problems of GA aircraft is there is no damping/use of shock absorbers ( probably due to weight) in either a bungee chord or any other suspension system. A motorcycle has a small wheel base too, but does not suffer from this pitching. Is this correct thinking? $\endgroup$ – Fred Apr 3 '20 at 19:13
  • $\begingroup$ @Fred: I agree that the motorcycle suffers much less. The SB-13 originally had an undamped front wheel which turned out to be inadequate and got replaced by one with a damper. Still, off-road motorcycles need more damper stroke but still shake when driven over uneven ground. $\endgroup$ – Peter Kämpf Apr 4 '20 at 1:15
  • $\begingroup$ @ Peter Kampf: With the SB-13, I assume the rudders were drag rudders? Is this correct? If so, why is that? I assume a smaller moment arm ( less wing sweep) was chosen for better wing efficiency and a drag rudder creates a bigger moment than a classical rudder. Is this correct? $\endgroup$ – Fred Apr 28 '20 at 16:20
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This has been a contentious question since 1910 and shows no sign of being settled yet. Several of your intuitions about the problems of the tailless swept wing are common mistakes.

Perhaps the best way to approach the design is "putting the tail on the end of the wing".

Some observations on its benefits:

  • Yes there is a greater wing area but there is no tail structure so the balance of benefits vs disadvantages is far from obvious. Although extrapolation to supersonic speeds is tenuous, we may note that most record-breaking jets have been tailless.
  • Prandtl showed around 1930 that sufficient washout for the tips to exert a modest downforce will actually reduce structural loads compared to a conventional wing, and NASA have recently been revisiting his analysis.
  • Wing sweep confers some stability in yaw. tip anhedral combined with washout and downthrust adds more. For the Dunne D.7 of 1911 this was sufficient and it was officially certified as stable.

Some observations on its disadvantages:

  • Rudders are pretty ineffective. The best solution is to use drag rudders - split control surfaces on the trailing edge of the outer wing, sometimes combined with the ailerons. In the Mitchell design shown, only the inner rudder is moved for a turn and it operates more as a drag rudder than as a conventional one; they can even be operated together, in opposite senses, as an airbrake.
  • There is high sensitivity to pitching forces, due to the low inertia in pitch. However pitch control is difficult to make effective for the same reason as rudders. Geoffrey de Havilland remarked of the DH.108 that "The sensitivity is there, but not to any great extent". But without very careful design, the end result can be a bit lively and prone to Dutch roll; famed test pilot "Winkle" Brown described the GAL 56 as the worst-handling plane he had ever flown.
  • Longitudinal trim is sensitive to CG and so CG range is small. The changes in angle of attack from CG changes do nothing for the pitch instabilities.
  • Sharper sweep helps, but also increases sideways flow and the associated inefficiencies.
  • It is difficult to keep the tips off the ground when taking off. had Peter Kampf and his friends (see his answer) read the autobiography of Westland's test pilot Harald Penrose, they would have been forewarned of the unpleasant ride via the Westland-Hill Pterodctyls.
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  • $\begingroup$ And now we know why probably 997 out of about 1,000, or 99.7% of 1,000 current GA aircraft designs have a tail. Oh well. $\endgroup$ – Fred Apr 2 '20 at 12:59
  • $\begingroup$ I assume the Mitchell wing tip rudders are too low as they are in bathed in the vortex of the end of the wing, which is probably the reason the twin Velocity VTs are so high: the bottom half of the the VT acting as an end plate/winglet, the top of the rudder being more effective. Is this correct thinking? $\endgroup$ – Fred Apr 3 '20 at 19:09
  • $\begingroup$ I think the max cross wind landing of a Velocity is 28knts, about double a C172, so we know they work. :) $\endgroup$ – Fred Apr 3 '20 at 19:15
  • $\begingroup$ @Fred Not really. The Velocity has a large side area in front of the CG so it needs plenty of side area behind it to maintain stability. Handling characteristics of a given design may be a deliberate design decision or a surprise. $\endgroup$ – Guy Inchbald Apr 3 '20 at 19:16
  • $\begingroup$ So why do you say the rudders on a Mitchell are pretty ineffective? This must be more than just too small a VT volume/moment, no? $\endgroup$ – Fred Apr 5 '20 at 12:48

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