British Young Pilots · Principles of Flight · Module POF-6.1
A moment is a turning effect. Balance a ruler on a pencil and put a coin near the middle: it tips a little. Slide the same coin out to the end: it tips a lot. Same force, further from the pivot, bigger turning effect. That is why a door handle sits far from the hinge and a long spanner shifts a stiff nut. An aeroplane in the air turns about its centre of gravity, so that is its pivot.
Source: Pooleys Vol 4 (10th ed.) p.67 and p.69.
In straight and level flight the opposing forces are equal: lift equals weight and thrust equals drag. With no resultant force the aeroplane carries on in steady, unaccelerated flight. Lift and weight are much the larger pair.
The four forces do not act through one point, though:
The turning moment of any force on the aeroplane depends on the size of the force and its distance from the CG.
Source: Pooleys Vol 4 (10th ed.) p.65–66 and p.69.
Turn a steering wheel with two hands: one hand pushes up, the other pulls down. Open a jam jar: fingers push one way, thumb the other. Two equal forces, pointing in opposite directions, that do not line up. They cannot cancel out. They twist. Pilots call that pair a couple. Make the forces bigger, or move them further apart, and the twist gets stronger. Try it: grip a pencil with two fingers close together, then far apart, and twist.
Source: Pooleys Vol 4 (10th ed.) p.66–67.
A couple is a pair of equal, parallel forces acting in opposite directions along different lines of action. The pair produces no resultant force, but it does produce a turning moment, whose size depends on the magnitude of the two forces and the distance between their lines of action.
For the two moments to match, the big forces (lift and weight) need their lines of action fairly close together, while the much smaller thrust and drag need rather more distance between theirs.
STRETCHLift and weight are far larger than thrust and drag. How can two couples built from such different forces produce the same turning moment?
Source: Pooleys Vol 4 (10th ed.) p.66–68.
FOUNDATIONSThe four forces make two twisting pairs. On many aeroplanes lift sits just behind weight: that pair twists the nose down. Drag pulls back along a line above thrust: that pair twists the nose up. Swap a pair round and its twist swaps too. The designer sets one twist against the other. Clever bonus: if the engine stops, the nose-up twist fades and the nose lowers itself into a glide.
In many aeroplanes the lines of action are arranged so that the thrust-drag couple pitches nose-up and the lift-weight couple pitches nose-down, with the distances chosen so the two moments cancel. The practical pay-off: if thrust is lost, the nose-up couple weakens and the lift-weight couple lowers the nose into a gliding attitude without any action from the pilot. Add power and the nose tends to pitch up.
Source: Pooleys Vol 4 (10th ed.) p.66–69.
The two twists hardly ever match perfectly. Usually a little nose-down is left over. The tailplane (the small wing at the back) fixes it with a gentle push down on the tail, which lifts the nose. Remember the see-saw: the tail is a long way behind the centre of gravity, so a small force does a big job. That is why the tailplane can be so much smaller than the wings.
The centre of gravity moves when people, bags and fuel move, and that changes the twists. It is why pilots load an aeroplane carefully: see the module Weight and the Centre of Gravity (POF-1.2).
Source: Pooleys Vol 4 (10th ed.) p.65–66 and p.68–69.
A perfect balance between the four forces and their two couples is rarely possible, so there is usually a residual pitching moment. The tailplane (horizontal stabiliser) counteracts it. It is simply another aerofoil, usually of symmetrical section, that can produce an upward or downward force depending on its angle of attack to the local airflow.
The residual moment is usually nose-down, so the tailplane normally produces a downward force, giving a nose-up moment. Because its moment arm from the CG is long, a small force is enough, which is why the tailplane is small compared with the mainplanes. The pilot varies the force with the elevators (held by hand or by trim) or, on some types, by moving the whole tailplane.
Aeroplanes are designed so the forces and couples are in approximate equilibrium in the cruise, leaving the tailplane only a small balancing job. How loading moves the CG is covered in POF-1.2.
Source: Pooleys Vol 4 (10th ed.) p.68–69.
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