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Old 2nd May 2012, 13:04
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Genghis the Engineer
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PFTS: Are you referring to stick force per knot from trim? If so, I find it equally surprising that Mil 8785C appears to be silent on the matter.
No. Apparent longstab, which is what you're describing, defines force (or displacement) per airspeed change largely provides ability to control and stabilise flightpath.

If you integrate apparent longstab between trim and the stall point you will get a total force - which I've termed PFTS (not a commonly used term, but it works). Very simply, it's how much pull a pilot has to make to stall the aeroplane.

This value arguably is an important factor in safety - stall avoidance being critical for obvious reasons. Research that exists suggests that a PFTF not less than 10lbf is appropriate to a light aeroplane, probably more for a big jet. An interesting bit of research that exists shows that the apparently very similar C150 and C152 have a factor 3 difference in PFTS, with the lower force C150 having a roughly 16 times greater rate of fatal accident with a contributory stall. In landing configuration mind you, both are well below the proposed 10lbf figure (around 0.5lbf for the C150 and around 1.5lbf for the C152).

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