a “soft field takeoff,” the effect on distance is again small, except to the extent that holding the nose up tends to get you airborne at the earliest possible moment.
for example, shows the aeroplane rotating 5 knots slower at 2,400 pounds than at 2,950 and getting off the ground, at sea level, in 440 feet rather than 705.
The thing to bear in mind about getting airborne at a very low speed is that you may be unable to climb until you have gained more speed in ground effect. The takeoff roll may be shorter, but the distance to clear an obstacle is no different.
The Skylane POH says that every 9 knots of headwind reduced takeoff distance by 10 percent. That’s a useful rule of thumb, but couldn’t they both be 10? Either way, the rule goes astray in really strong winds, since in a 55-knot wind a Skylane’s takeoff distance would be zero, not two or three hundred feet.
The big factors in takeoff distance are the ones you’d expect: density altitude, weight, wind, surface condition and gradient. For unturbocharged aeroplanes, density altitude reduces the power and thrust available and therefore the rate at which the aeroplane accelerates. According to the program – your results may vary – takeoff distance increases by about 8 or 9 percent for every 1,000 feet of density – not pressure! – altitude. You can find presumably exact information about density altitude and weight in the POH, so it’s not necessary to memorise any rules of thumb. Weight is a critical factor because it affects both acceleration and liftoff speed. The Skylane POH,
Tailwinds are a more sensitive matter, since they increase the liftoff groundspeed and so make the aeroplane eat up more runway at the departure end of its roll. The Skylane book says to add 10 percent for every two knots of tailwind, which seems extreme; a Mooney 231 chart shows a 26% increase for a 10-knot tailwind. My program predicts a 34 percent increase for a 10-knot tailwind for a hypothetical Skylane-like aeroplane. So you see we’re all over the map. Various kinds of runway surfaces present different resistances to the rolling aeroplane, but it’s impossible to categorise surfaces in any precise way. My program, whose surface
Downhill and downwind - or uphill and into wind?
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December / January 2021/22
