Pitch moment is created by lift created by the horizontal stabilizers via the elevators.
But then main wings also create lift, why does not this lift enough to create pitch moment?
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1$\begingroup$ The wings of most aircraft are located at or near the centre of gravity. Aircraft like the Concorde do have the pitch moment created my the main wing because it also acts as the horizontal stabilizer. $\endgroup$– SMS von der TannFeb 14, 2016 at 19:23
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$\begingroup$ Lift moment is a force (the lift) and a lever (the distance between the aerodynamic center and the CG). As long as they are not null, they induce a pitch change. Fortunately the lift moment of the horizontal stabilizer and the elevators cancels it. See Pitching moment. $\endgroup$– minsFeb 14, 2016 at 19:32
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2 Answers
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Of course it creates a pitching moment! Now we need to define around which reference point this moment should be measured.
If the reference point is the center of gravity, it is even equally strong as the pitching moment of the elevator, it only has the opposite direction.
If you use the aerodynamic center as the reference point, the moment will be less strong. If the wing airfoil is symmetric, the moment will actually be zero (ideally), but with a cambered airfoil there is still a measurable moment left.
A moment is always a combination of a force and a lever arm, measured perpendicular to the direction of the force. Only when the lever arm is zero will the moment disappear. If we use the center of gravity as the reference point, the weight will have no lever arm and will contribute no moment, but the distance between the wing's lift and the center of gravity is large, so this moment is large, too.
answered Feb 14, 2016 at 20:07
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1$\begingroup$ Would it be correct to interchange orthogonally and perpendicularity? $\endgroup$– TerryFeb 14, 2016 at 20:25
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$\begingroup$ @Terry: Yes, I guess. Which would be more natural for you native speakers? $\endgroup$ Feb 14, 2016 at 20:38
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1$\begingroup$ My guess is that American English speakers not technically educated would often hesitate upon encountering orthogonally. I probably would have said "measured perpendicularly" or "measured perpendicular", but I'm not sure if both of those are grammatically correct. $\endgroup$– TerryFeb 14, 2016 at 21:32
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1$\begingroup$ Native English speakers conversant in higher mathematics would understand "orthogonal", but might think it an odd choice of words; "perpendicular" is the far more common term. "Orthogonal" is often used by mathematicians to denote a relationship more general than mere perpendicularity, like orthogonality of basis vectors in a vector space. Computer scientists on the other hand think of orthogonality as the ability to alter parts of a system without worrying that the change will impact other parts, and that is certainly not the sense meant. "Perpendicular" is the right word choice here. $\endgroup$ Feb 14, 2016 at 23:07
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$\begingroup$ @PeterKämpf Why do they cancel it? CG is ahead of wings toward cockpit. Now lifts produced by both main wings and elevator are behind CG which should mean they both are creating the moment in same direction. Then why do you say they are in opposite direction? May be I am wrong somewhere, please correct me. $\endgroup$ Feb 15, 2016 at 16:37
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Pitching moment is usually defined relative to the centre of gravity. The main wing does provide a pitching moment, since it has both a force and a moment arm relative to CG.
The reason to define pitch relative to CoG is that in a free response in vacuum, the CoG is the centre of pitch rotation. But moments can be defined relative to any point, also to the centre of lift of the main wing, which holds the aeroplane up when flying. And if we do that, we can see why the tailplane lift often points down, not up:
- Total pitching moment must be zero in steady flight.
- In the depicted situation, if the tailplane stalls, its lift and pitching moment contribution is cancelled - the n.p.$_{fixed}$ moves forward to the a.c.$_w$. There is now a nose-up pitching moment, tending to want to stall the aeroplane.
- Therefore, relative to the a.c.$_w$, the fail-safe situation is a gravitational moment nose-down and an aerodynamic moment nose-up: c.g. in front of a.c.$_w$, tailplane lift pointing down.
As mentioned in this answer this is mainly a safe configuration at low airspeeds. At higher speeds the upwards lift of the tailplane is fine and will create an aerodynamically stable configuration.
