Discussion
Which of the following would not improve directional stability?
Fitting a tractor propeller further forward
Enlarging the fin
Lengthening the fin's moment arm
Adding a ventral fin
Answer: D
Correct answer: Fitting a tractor propeller further forward
Moving a tractor propeller further forward makes directional stability worse, not better. Its side force in sideslip acts at the disc, ahead of the centre of gravity, so lengthening that moment arm increases an already destabilising moment.
The other three all help. Enlarging the fin increases the side force available; lengthening the fin's moment arm increases the moment that force produces; and a ventral fin adds side area aft and below, where it stays clear of the wake at high angle of attack. Area and arm aft of the centre of gravity help; area and arm forward of it hurt.
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