# Can someone explain to me why these planes just don't turn about their center of gravity?

Look at this one:

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?

• 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.
– Dave
Commented Apr 14, 2015 at 14:41
• @Dave yes, being a WIG will change the magnitude of the effect, but the principle is the same Commented Apr 14, 2015 at 14:46
• @Federico I know I just thought I would mention that its not a plane mainly because i love how ekranoplan sounds :)
– Dave
Commented Apr 14, 2015 at 14:48
• Doesn't the question apply to any single-engined airplane? Commented Apr 14, 2015 at 18:44
• @jamesqf it applies to any aircraft whose thrust axis does not go through the center of gravity, including multi-engine ones. Commented Apr 14, 2015 at 19:24

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:

• 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? Commented Apr 14, 2015 at 22:48
• @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. Commented Apr 15, 2015 at 3:56
• Okay but does the elevator adjust automatically or does the pilot have to do it manually when the engine torque changes? Commented Apr 15, 2015 at 8:36
• @Maverick283 depends if there is an autopilot and is a different question that I encourage you to eventually ask. Commented Apr 15, 2015 at 8:37
• @Maverick283 In a normal airplane is is usually not automatic unless autopilot is engaged. In the Ekranoplan it is automatic but is normally added on to the pitch moment of the top mounted engine - that is, ekaronaplans need to apply down-elevator most of the time thus the engine position actually aids the elevator. The reason for this is to prevent the ekranoplan from taking off like an airplane. The reason for that is that once out of ground effect the amount of lift generated by the wings would be drastically reduced out of GE and cause the plane to stall Commented Apr 9, 2020 at 21:10

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.

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.

• 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... Commented Apr 14, 2015 at 21:23

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.