British Young Pilots · Principles of Flight · Module POF-7.1
Sit up straight and try this. Nod your head: that is pitch. Tip your head towards one shoulder: that is roll. Shake your head "no": that is yaw.
Those are the only three ways an aeroplane can turn. Each one happens about an imaginary line through the aeroplane's balance point, its CG, so there are three axes. Now hold your hand out flat like an aeroplane and do all three: nose up and down, one wing down, nose left and right.
Source: Pooleys Vol 4 (10th ed.) p.87.
Every aeroplane has control systems so the pilot can manoeuvre it, and trim it, about each of its three axes. The turning forces that do this are called moments. They are produced by changing the airflow around the aerofoils: altering an aerofoil's shape, or moving its position. The three motions are pitch (nose up and down), roll (one wing down, the other up) and yaw (nose left and right).
The circle-and-dot at the CG is the third axis, pointing straight up out of the page. Source: Pooleys Vol 4 (10th ed.) p.87.
Each motion has its own control in the cockpit, and each control moves its own hinged surface on the aeroplane:
Hold an invisible stick and put your feet on invisible pedals. Nose up: stick back. Wing down: stick over. Nose left: left foot forward.
Source: Pooleys Vol 4 (10th ed.) p.87.
There are usually three sets of primary controls and three sets of control surfaces, all linked to the cockpit:
Note the wording: "longitudinal control" here means control in the pitching sense, and "lateral control" means roll. Say pitch, roll and yaw and you cannot go wrong.
STRETCHIdeally each control would make a moment about one axis only. In practice it is rarely that tidy: deflecting the ailerons to start a roll also produces adverse yaw. Which pedal do you think fixes it?
Source: Pooleys Vol 4 (10th ed.) p.87.
Why are the elevator and rudder right at the back, and the ailerons out near the wingtips? Try a door. Push it right next to the hinges and it hardly moves. Push at the handle, far from the hinges, and it swings with one finger.
The aeroplane turns about its balance point, the CG, so the control surfaces are put as far from it as possible. A long lever means a small surface can give a big turning push. The controls are the aeroplane's door handles.
Source: Pooleys Vol 4 (10th ed.) p.87.
The surfaces the pilot moves are usually hinged and sit near the extremities of the aerofoils, far from the centre of gravity. That gives them a long moment arm and the greatest leverage. Deflecting one changes the airflow and pressure distribution over the whole aerofoil, not just the moving part: the lift of the combined aerofoil and control surface changes, and that produces the moment. How effective the controls are is the aeroplane's controllability.
Stability opposes controllability. Too much built-in stability makes an aeroplane resist change and hard to manoeuvre, so the designer picks a balance for the job: an airliner wants more stability, a fighter wants more controllability. In that sense the controls are destabilisers. Stops limit control travel, and control locks (a pin through the column, or blocks around the surface) protect parked aeroplanes from the wind. They carry flags because they must come out on the walk-round, before flight.
Source: Pooleys Vol 4 (10th ed.) p.87–88.
The elevator as the worked example. Column forward moves the elevator down. That changes the shape of the whole tailplane-and-elevator section, so the tail now produces a reduced downward force, or even an upward one if the deflection is large enough. The tail force acts a long way behind the CG, so the change creates a pitching moment about the CG that lowers the nose. Column back does the reverse: elevator up, a larger downward force at the tail, nose up.
Every primary control works the same way in principle: a hinged surface reshapes its parent aerofoil, the force on that aerofoil changes, and the long arm to the CG turns that force change into a moment.
Source: Pooleys Vol 4 (10th ed.) p.87–88. The elevator, stabilator and anti-balance tab get their own treatment in POF-7.2.
Tap a card to flip it.
Three dashed lines cross at the CG: nose to tail, wingtip to wingtip (the short one, pointing at you) and top to bottom. Tap a word, then tap the box for the motion about each line, and the box pointing at each control surface.