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There are a couple airplanes out there that use the winglet as the rudder, but why doesn't the winglet have a rudder on it to increase the turn of the aircraft?

enter image description here

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    $\begingroup$ why can't, again. Will you learn to ask why don't and stop making false assumptions? $\endgroup$ – Federico Aug 27 '15 at 5:34
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    $\begingroup$ @Federico perhaps you could phrase it more kindly ? Please bear in mind that users might be young or non native English speakers. This kind of difference can be quite subtle and not easy to get right for everyone. $\endgroup$ – Antzi Aug 27 '15 at 9:28
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    $\begingroup$ @Antzi this is most definitely not the first (nor second or third) time this gets mentioned to him. One can allow for age and all other kinds of issues, but if advice keeps falling on deaf ears kindness and empathy start to wear off. $\endgroup$ – Federico Aug 27 '15 at 10:38
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The purpose of the rudder is to control the heading of the aircraft. It turns the aircraft by creating an aerodynamic moment about the vertical axis of the aircraft. The moment is product of the aerodynamic force and the arm.

enter image description here

image: NASA

The aerodynamic force is the the side force on the rudder, the arm is the distance between the rudder and the centre of gravity of the aircraft.

If the rudder would be on the winglets, the side force of the rudder would act at a short distance behind the centre of gravity, reducing the effectiveness very much.

It would be possible the have airbrakes on the winglets / wingtips and use differential braking to create the rotating moment. This is done for example on the B2 which doesn't have a vertical stabilizer. The added complexity and weight increase makes it an unattractive option for conventional aircraft.

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    $\begingroup$ Note that the Horten flying wings had small airbrakes close to the wingtip for yaw control. They were OK at high speed, but had so little effect at low speed that the second prototype of the Horten IX was lost when one engine quit. $\endgroup$ – Peter Kämpf Aug 27 '15 at 18:04
  • $\begingroup$ Of course, if the wing is either very far back compared to on a typical aircraft (such as with the Long-EZ in the OP's photo) or is very long and highly-swept, then the winglets can easily be far enough behind the COM to be useful vertical stabilisers/rudder mounts. $\endgroup$ – Sean Jan 24 at 1:10
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Short answer:

  • Vertical tails create more yawing moment per unit of drag.
  • Vertical tails create less adverse rolling moment for a given yawing moment.

The rudder works by creating a side force, just like a wing creates lift. This incurs a small amount of drag (about a tenth of the side force).

Even on a swept wing their lever arm in lengthwise direction is small compared to the wingspan. If the winglets would be used as rudders, they would be most efficient as drag devices, because then their force will act on a longer lever. This has been done on flying wings (note that the SB-13 tailless glider has a maximum rudder deflection of 70° in order to create drag).

SB-13 in flight with Hans-Jürgen Berns at the controls

SB-13 in flight (picture source)

This makes the use of a rudder vastly more efficient for creating a yawing moment than using winglets.

The deflection of winglet rudders will also change the lift distribution on the wing, so they do not only create a yawing moment, but also a rolling moment. Vertical tails wich are high above the roll axis also add a rolling moment, but this is comparatively small to the rolling moment of a winglet with rudder on a high aspect ratio wing. In both cases, the rolling moment acts against the desired roll direction, so it is preferable again to create the yawing moment with the vertical tail.

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    $\begingroup$ Couldn't you use both winglet-rudder at the same time to prevent the rolling moment? $\endgroup$ – ROIMaison Aug 27 '15 at 19:58
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    $\begingroup$ @ROIMaison: If both are behind the center of gravity and try to contribute to the desired yawing moment, their side force has to point away from the center of the desired circle. Thus, the outer winglet will induce a downforce on the wingtip, and the inner one additional lift on the opposite wingtip (if the winglets point up). Both will produce their own adverse rolling moment. $\endgroup$ – Peter Kämpf Aug 27 '15 at 20:29
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    $\begingroup$ Ah yes, I overlooked this fact. Thanks for the explanation $\endgroup$ – ROIMaison Aug 27 '15 at 20:33
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It is possible, but just not that useful.

Controllability

In order to maximize the moment incuded by the rudder (or keep the rudder as small as possible to provide this moment), you want to place the rudder as far (in longitudinal sense) from the center of gravity. This increases the moment induced, and thus the effectiveness of the rudder. In a normal airliner, you'll have a tube with wings in the middle, and thus the most aft part to place the rudder is at the end of the tube. On the aircraft in your picture, the most aft part is the wing tip, so it makes sense to put the rudder there.

Structural reasons

If you place the rudder at the end, this will mean there will be substantial forces induced at the tip of the wing if you use the rudder. This means that you will have a substantial bending moment added to the wing, making it much heavier. This is further discussed in Winglet vs span extension

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  • $\begingroup$ Nice answer. Perhaps you could add why the rudder is generally not at the front of the aircraft ? $\endgroup$ – Antzi Aug 27 '15 at 9:30
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    $\begingroup$ I could, but it's not related to the question, is it? $\endgroup$ – ROIMaison Aug 27 '15 at 9:36
  • $\begingroup$ Strictly, it isn't, unless you consider the broader question of positioning the rudder $\endgroup$ – Antzi Aug 27 '15 at 9:44
  • $\begingroup$ @Antzi: Look here for the answer. $\endgroup$ – Peter Kämpf Aug 27 '15 at 17:42
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The existing answers miss what seems to be a fairly obvious answer: the winglets are an afterthought, added to existing, proven designs (e.g. 737) to improve fuel economy. Putting rudders on them would require running control cables, perhaps strengthening the wing structure, &c, rather than just bolting on new tips.

You could perhaps design a new airliner from scratch to do this, but again, why abandon a proven basic design unless there are large gains to be had?

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    $\begingroup$ For example, the 787 was a brand new design, which had winglets right from the start. So all you have to say is, "Well, if they were any good, the 787 would have them." OK, so we conclude that they're probably not any good, but the question is why. $\endgroup$ – David Richerby Dec 3 '16 at 19:43
  • $\begingroup$ @David Richerby: For a brand new design, it looks quite suspiciously similar to the 757, 767, & 777, not to mention products from Airbus and others. Basically another iteration of the standard tube with wings and a tail, and a couple of engines hanging below the wings. When I said "new design", I meant perhaps something like the X48 and similar blended wing-body designs. $\endgroup$ – jamesqf Dec 4 '16 at 6:10
  • $\begingroup$ Now you're just contradicting yourself. If the 787 doesn't have rudders on winglets because it's a tube with wings, I put it to you that the 737 not having rudders on its winglets has nothing to do with the difficulty of retrofitting them to an existing design, which is the only reason you give in your answer. The 737 is also a tube with wings, you see. $\endgroup$ – David Richerby Dec 4 '16 at 10:35
  • $\begingroup$ @David Richerby: I don't see the contradiction. We (well, aeronautical engineers :-)) know how to build "tubes with wings", so each new iteration improves on that. E.g. the 787 reworks the basic design using composites (yes, I'm simplifying). But to put rudders on winglets, you have run control lines out to the tips, strengthen the outboard parts of the wings to handle the different stresses, decide what to do with the elevator now that you don't have a vertical tail... Which also applies to retrofits: why add rudders out there when you already have a perfectly functional tail? $\endgroup$ – jamesqf Dec 4 '16 at 18:22
  • $\begingroup$ You're claiming that the 737 and 787 are just tubes with wings, and therefore the same considerations apply to them. But the 737 has no rudders on its winglets for one reason (hard to retrofit to an existing design), while the 787 has no rudders on its winglets for some completely different reason (though you haven't clearly stated what). That is a contradiction: the 737 and 787 cannot simultaneously be the same and different. $\endgroup$ – David Richerby Dec 4 '16 at 22:49

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