Page 1 of 1

Myth: I can fully deflect the controls below maneuvering spd

PostPosted: 04 Jun 2015 17:19
Myth: I can fully deflect the controls below maneuvering speed!
The wing structure in light planes is usually certified to take +3.8 G’s, -1.52 G’s (plus a certain safety factor). Put more load on the wing than that and you should consider yourself dead.
But here is the nice part: Below a certain speed, the wing simply cannot put out a full 3.8 G’s of lift! It will stall first! This speed is called Maneuvering Speed or Va.
Maneuvering Speed is defined as the maximum speed the plane can be flying at and still stall before the wing breaks no matter how much you pull back on the stick. If you are going slower than the Va and you pull the stick all the way back, the wing will stall without braking physically.
If you are going faster than the Va and you pull the stick all the way back, the wing can put out so much lift that it can be expected to break. Therefore people think they can deflect the stick as much as they desire below Maneuvering Speed and stay alive.
Wrong! The Maneuvering Speed is based on pulling back on the stick, not pushing it forward!
Note what was said above: The Va is defined as how fast you can fly and not be able to put out more than 3.8 G’s of lift. But while the plane is certified for positive 3.8 G’s, it is only certified for a nega- tive G-load of 1.52 G’s! In other words, you can fail the wing in the negative direction by pushing forward on the stick well below the Va! Few pilots know this.
Also, for airliners, certification basis require that the rudder can be fully deflected below Maneuvering Speed, but only if the plane is not in a sideslip of any kind! (e.g. crab method of ap- proach) Does this make sense at all? Why would you need to fully deflect the rudder if not to re-establish wings-level flight?
In a wonderfully-timed accident shortly after Sept. 11th, 2001 of which everybody thought might be an act of terrorism, an Airbus pilot stomped the rudder in wake turbulence while the plane was in a considerable sideslip. The combined loads of the sideslip and the deflected rudder took the vertical stabilizer to it’s critical load. A very simple numerical analysis based on the black box con- firmed this. The airplane lost it’s vertical stabilizer in flight and you know the rest.
Also, if you are at your maximum allowable g-limit (e.g. 3.8) and you deflect the ailerons even slightly, you are actually asking for more lift from one wing than the allowable limit!
Therefore combined elevator and aileron deflections can break the plane, even if the elevator is positive only!
Please reconsider this myth and also look at the Vg diagram and the aircraft’s limitations to prove it to yourself.

Courtesy Pipstrel aircraft POH

Re: Myth: I can fully deflect the controls below maneuvering

PostPosted: 05 Jun 2015 01:24
by GlowWorm
Cool - I learnt something today. Thx!!!