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Look at this one:

strange plane

The thrust force from the engine should generate some clockwise moment, right? Why won't it turn and just crash then? I guess the wings generate some sort of anti-clockwise moment? Can someone explain this?

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    $\begingroup$ Just a note, this is not an airplane (or seaplane as its tagged) its a Ground Effect Vehicle en.wikipedia.org/wiki/Ground_effect_vehicle in particular this is a A-90 Orlyonok. GEV's have similar but different aerodynamics than an airplane as they are made to work only in ground effect. Some however can climb to significant altitude and they can all work over land as well as water. $\endgroup$ – Dave Apr 14 '15 at 14:41
  • $\begingroup$ @Dave yes, being a WIG will change the magnitude of the effect, but the principle is the same $\endgroup$ – Federico Apr 14 '15 at 14:46
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    $\begingroup$ @Federico I know I just thought I would mention that its not a plane mainly because i love how ekranoplan sounds :) $\endgroup$ – Dave Apr 14 '15 at 14:48
  • $\begingroup$ Doesn't the question apply to any single-engined airplane? $\endgroup$ – jamesqf Apr 14 '15 at 18:44
  • $\begingroup$ @jamesqf it applies to any aircraft whose thrust axis does not go through the center of gravity, including multi-engine ones. $\endgroup$ – Federico Apr 14 '15 at 19:24
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The thrust force from the engine should generate some clockwise moment, right?

If we take your photo as a reference (and thus we intend "clockwise in the picture plane"), yes, it is. It is called pitch moment, because it would change the pitch of the aircraft (in your case, a A-90_Orlyonok), and in the case shown in your picture is negative, because it would make the aircraft pitch down.

Why wont it turn and just crash then? I guess the wings generate some sort of anti clock wise moment?

Because the wings and the horizontal stabilizer produce a total net pitch moment that counteracts the one generated by the engine.

They do so by having the center of lift in a different location w.r.t. the center of gravity.


The "normal" configuration for airliners would have an opposite pitch moment due to the engines, since in that case the engines are usually below the wings (and, being usually low wings, below the center of gravity). It is well illustrated in this image taken from the FAA:

enter image description here

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  • $\begingroup$ With the A90_Orlyonok, when changing the thrust is there a mechanism or electric unit to change the amount of "counter pitch" produced by the horizontal stabilizer? And is there something like that for airliners as well? $\endgroup$ – Maverick283 Apr 14 '15 at 22:48
  • $\begingroup$ @Maverick283: The horizontal stabilizer is mounted to counteract the nominal pitch moment from the engines. The elevators are adjustable and provide pitch control for all needs, including weather, desired attitude, and changes in engine torque. $\endgroup$ – Ross Millikan Apr 15 '15 at 3:56
  • $\begingroup$ Okay but does the elevator adjust automatically or does the pilot have to do it manually when the engine torque changes? $\endgroup$ – Maverick283 Apr 15 '15 at 8:36
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    $\begingroup$ @Maverick283 depends if there is an autopilot and is a different question that I encourage you to eventually ask. $\endgroup$ – Federico Apr 15 '15 at 8:37
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If you look closely there are two propellers on the tail, and they are contra-rotating, so one spins clockwise and the other counterclockwise so there is no roll moment produced.

See: http://en.wikipedia.org/wiki/A-90_Orlyonok

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    $\begingroup$ the question is about pitch moment, your answer is about roll moment. $\endgroup$ – Federico Apr 14 '15 at 17:48
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    $\begingroup$ @Federico: While you are correct about the probable intention of the questioner, the answer is valid for another way of interpretation. $\endgroup$ – Peter Kämpf Apr 14 '15 at 21:03
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There are also engines in the nose with their exhaust pointing below the leading edge of the wing. Although, according to wikipedia, these engines can be turned off during cruise, when on, these engines would provide a pitching moment to counteract the nose-down pitching moment from the turboprop.

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No matter the exact design and careful balancing that goes into an aircraft, there is no general guarantee that all the moments cancel out. The direction and magnitude of pitch/roll/yaw moments vary with speed and altitude. Aircraft typically require active control surface inputs to stay level; these inputs can be re-centered or "trimmed" using trim tabs (which are usually located on the trailing edges of the control surfaces) so the pilot doesn't have to actively run the inputs.

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    $\begingroup$ Not to mention putting a bottle of duty free Scotch in the overhead bin will shift the C of G upward, and necessitate a change in control input... $\endgroup$ – DJohnM Apr 14 '15 at 21:23

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