Expert: Paul Soderman - 6/15/2011

Question
QUESTION: Sir,
When a plane is at stall.It no longer generate lift.At that state is there any force on that plane?Can explain it in vector.Thank you.

ANSWER: When an airplane in horizontal flight stalls, the wing lift is reduced to a value less than the weight of the airplane so the airplane starts to fall.  There are still forces on the aircraft, namely lift, drag, thrust (usually), and gravitational force. It is just that the aircraft is no longer in stable flight, and the flight path is no longer horizontal.  The vector forces relative to the flight path are lift upward, thrust forward (more or less, depending on aircraft attitude) and drag aft; relative to the earth: weight downward.

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QUESTION: Sir,
Thank you.You said the flight path is no longer horizontal.What exactly the flight path going to?What about the velocity vector(without thrust)?Is there any possibilities that the plane fall vertically without thrust?Does all of this work on glider too?Thank you.

ANSWER: In stall, the aircraft will still have forward momentum but will enter a steeper and steeper descent path if stall condition is maintained. I assume it will be a parabolic trajectory.  Without thrust, the gravity vector will tend to dominate and the path will become steeper faster until in the limit the path is nearly vertical.  Often the aircraft will hit the ground before a truly vertical descent is reached.  All this is true for a glider too.  However, a well designed aircraft or glider will recover from stall if there is enough altitude because the tail will provide a restoring moment that brings the nose down and allows the aircraft to recover lift during the glide.  A poorly designed aircraft will have one wing stall before the other and the aircraft will roll into a descent. There is also a rare condition called deep stall where both the wing and tail are stalled and the aircraft becomes stable while falling.  Some T-tail aircraft have this characteristic because the wing wake can wash out the tail. Of course, many things have to go wrong at the same time including stall and loss of power.

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QUESTION: Thank you Sir for the information.For your information I use them for my oral test.What is the velocity vector JUST BEFORE the stall.I think it is the direction of the plane.I have to repeat the oral test until I pass.Till now I have gone for 8 times.Really need your help.Thank you.

Answer
The aircraft velocity vector is in the direction of the flight path.  In horizontal flight, that would be a horizontal vector.  Note that the thrust vector is not necessarily along the flight path but is close to it.  Likewise, the aircraft may be at an angle of attack that is not necessarily aligned with the flight path.