LEADING EDGE - PETER GARRISON
INVERT TO UNTANGLE WHEN CLIMBING, YOU NEED SOME RIGHT RUDDER TO KEEP THE BALL CENTRED. IF YOU PERFORM A HALF ROLL AND CONTINUE TO CLIMB UPSIDE-DOWN, WHICH RUDDER WILL YOU HAVE TO USE TO STAY COORDINATED? WHY do we need right rudder in a climb,
is in a level attitude; but it is absent when the
anyway? Is it because of slipstream rotation?
aeroplane holds the same level attitude in flight.
The propeller drags some air around with it, and
So at least some of the pull to the left has to be
the aeroplane continually advances through this
due to something other than the aeroplane’s
slipstream of deflected air. The fin, being behind
pitch attitude.
the portion of the propeller disk where the blades are going left to right, feels a push to the right. But what about P factor? When an aeroplane
invoked: torque. Torque is the twisting force supplied by the
is nose-high, its propeller is tilted upward with
engine to make the propeller spin. The natural
respect to the direction of its travel through the
effect of torque, if we did not do something to
air, and a downgoing blade has a greater angle
prevent it, would be to spin the aeroplane in the
of attack than an upgoing one. The downgoing
opposite direction to the propeller, in the same
blade is on the right side, and so it tends to pull
way that a helicopter, deprived of its tail rotor,
the nose of the aeroplane to the left. I once
begins to rotate in the direction opposite to the
did some calculations that suggested that the
main rotor. Torque and slipstream rotation are
difference in blade speed that results from tilting
two sides of the same coin; part of the torque is
the propeller is at least as influential as the
imparted to the slipstream, making it rotate.
difference in angle of attack, but that doesn’t matter. The general principle remains the same. A big problem with the P factor explanation is
22
Another mysterious being is sometimes
In flight, torque is trimmed out by rigging and by imperceptibly small aileron deflections, but on the ground the job is done by a sideways
that the pull to the left is felt early in the takeoff
force on the tyres instead. That force tends, like
roll, when the aeroplane, if it is not a taildragger,
everything else, to make the aeroplane veer off
February 2021
