LatitudeDancer
Well-Known Member
You always hear “it is important that no attempt be made to force the airplane to become airborne with a deficiency of speed; the recommended takeoff speed is necessary to provide adequate initial climb performance” regarding ground effect.
In ground effect, the lift coefficient and drag change so the required angle of attack and thrust required decrease but the velocity at which the plane will fly remains the same since the wing isn’t changing? If so, since the change in upwash, downwash, and wingtip vortices usually create a higher pressure near the static source on smaller aircraft (leading to an artificially low indicated airspeed reading vs. true airspeed), would it be more accurate to say “no attempt should be made to force the airplane to become airborne until the necessary TRUE airspeed is reached (vs. your indicated reading, since it might show the lower, ‘correct’ airspeed, given the installation/position error)?
I have an old “Flight Training Handbook” (AC 61-21A) that reads like the reason you can get off the ground at an indicated airspeed less than that normally required for flight is because of the installation/position error due to the pressure change while in ground effect while my PHAK reads like the reason you might be able to get the plane airborne below the recommended airspeed is a reduction of drag due to ground effect (which I think only effects thrust not velocity).
If you look at both book’s chart on thrust required (y-axis) vs. velocity (x-axis) in and out of ground effect, the curve lowers on the chart when in ground effect (less thrust required) but does not appear to shift left (so not required velocity increase at the low end). So that to me kind of indicates it’s the instillation/position error + the pressure change surrounding the static source while in ground effect that gives the illusion of being able to fly at a slower KIAS but not an actual ability to fly at a true lower speed?
In ground effect, the lift coefficient and drag change so the required angle of attack and thrust required decrease but the velocity at which the plane will fly remains the same since the wing isn’t changing? If so, since the change in upwash, downwash, and wingtip vortices usually create a higher pressure near the static source on smaller aircraft (leading to an artificially low indicated airspeed reading vs. true airspeed), would it be more accurate to say “no attempt should be made to force the airplane to become airborne until the necessary TRUE airspeed is reached (vs. your indicated reading, since it might show the lower, ‘correct’ airspeed, given the installation/position error)?
I have an old “Flight Training Handbook” (AC 61-21A) that reads like the reason you can get off the ground at an indicated airspeed less than that normally required for flight is because of the installation/position error due to the pressure change while in ground effect while my PHAK reads like the reason you might be able to get the plane airborne below the recommended airspeed is a reduction of drag due to ground effect (which I think only effects thrust not velocity).
If you look at both book’s chart on thrust required (y-axis) vs. velocity (x-axis) in and out of ground effect, the curve lowers on the chart when in ground effect (less thrust required) but does not appear to shift left (so not required velocity increase at the low end). So that to me kind of indicates it’s the instillation/position error + the pressure change surrounding the static source while in ground effect that gives the illusion of being able to fly at a slower KIAS but not an actual ability to fly at a true lower speed?
