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A:
The aircraft stall and an engine stall are not the same thing!! In fact, the engine has nothing to do with the "aircraft stall"!
The airplane's wing stalls at a particular angle of attack. That’s the whole story. The stall is not a function of airspeed,
although we commonly practice stall entry at low airspeed, nor is it a function of attitude, although we generally associate the stall with a
nose up attitude as a result of our training sessions. A stall happens because the angle of attack of the wing—the angle between the chord
line of the wing and the relative wind—has exceeded the critical angle.
Stall recovery - How is it accomplished? Reduce the angle of attack. If we’re right side up, we move the (elevator) stick forward to lower
the nose. If we’re upside down, we pull the stick back and lower the nose. If we’re in a normal, upright turn (such as turning from
downwind to base or base to final), we ease the stick forward to reduce the angle of attack in relation to the relative wind. In any situation,
what we do to recover from the onset of a stall is reduce the angle of attack below the critical angle that is producing the stall. At the stall
itself, of course, the nose will drop as we lose lift and control response, but unless we are performing a stall on purpose we’d like
to recognize its approach and solve the problem before it occurs.
The arrival stall, sometimes described as a descending, turning stall, results when a RC airplane pilot, normally at low level, possibly on final
approach or turning base to final, allows the airspeed to decay and some yaw to develop. Some typical scenarios for the arrival stall are
turning harder to correct for an overshoot of the extended runway centerline; turning to avoid obstacles, birds or other aircraft; attempting to
stretch a glide to the runway by raising the nose and not applying power, gusty wind conditions; and/or distraction resulting in airspeed decay.
An accelerated stall is frequently entered from a level turn and results from, as with all stalls, too high an angle of attack. The added factor
we must consider with an accelerated stall is the increased wing loading or load factor resulting from acceleration. In a turn, we are
accelerating and the loading on the wings is increased by a function of the angle of bank.
Without beating the math to death, a 45 degree bank, level altitude turn produces a 1.41g loading; a 60 degree bank, level altitude turn produces a
2g loading and a 75 degree bank, level altitude turn results in a 4g loading. We can look these values up and save ourselves some effort,
or we can work them out ourselves. We can derive them either mathematically or geometrically. Mathematically, we find that G = 1/cos ?
(where ? = the angle of bank). One divided by the cosine of our angle of bank is the “G” force. The stall speed, also increases by the
square root of the “G” force, or load factor on the aircraft so that, in a 60 degree bank level turn our stall speed increases by the square root of
2 which is 1.41. If your normal, non-accelerated stall speed in level flight is 20 miles per hour, your stall speed in a 60 degree bank level
turn will be 28.2 miles per hour. For a 75 degree bank level turn, your stall speed will be 40 miles per hour!!
Have you ever seen (or experienced) an RC airplane appear to have "loss of controls" as the pilot turned from base to final? A lot of time, that
"loss of controls" was due to an accelerated stall due to increasing the angle of bank on the plane and pulling harder on the elevator to tighten
up the turn and being to slow to do it successfully!! Keep the accelerated stall in mind the next time you are flying at your "normal"
approach speed and start to overshoot the runway. Also, remember to increase your "approach speed" when flying in gusty winds to avoid a stall due to "loss of airspeed" if the winds decrease as you come in on approach and landing!
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