Angle of Attack and Stall

Maintaining Control at All TImes

It has been said that the essence of flying an airplane is "Control Yaw and Don't Stall". Another way off saying this is "Control Yaw, Angle of Attack AND Speed" at all times.

Pilots are often taught that air speed determines a stall. This is sometimes true, under certain conditions, however it is NOT true under all conditions. Many a pilot has died trying to pull out of dive by pulling back on the stick too far and essentially stalling the wing, even at very high speed. It is hard to remember to only pull the stick back only enough to arrest the descent while avoiding excess angle of attack as well as avoiding excess g forces.

Loss of control often occurs because a lack of awareness that the use of ailerons will often cause the wing with the down aileron to stall and lose all lift because of the increased angle of attack caused by the change in the airfoil's shape with the down aileron. This is the cause of the infamous stall-spin-invert loss of control during "turn to final". 

Angle of Attack AND Airspeed together creates lift, enabling the plane to fly. Too little angle of attack at a given speed and the plane will descend. Too much and the wing/s will stall, lose lift and descend. The pilot's job is to control lift by adjusting speed and the angle of attack---indirectly with the elevator in the case of a traditional three axis airplane, or more directly by tilting the wing on a trike. 

Other than pointing the plane is the direction you want it to go, controlling yaw has to do with keeping the air flowing over the wing perpendicular to the leading edge and minimizing drag. Uncontrolled yaw will cause increased drag and may result in an unanticipated loss in speed. In addition, the rudder that is used to control yaw, also causes one wing to rise and the other to fall.

But Angle of Attack works together with Speed, so uncontrolled Yaw and increased drag, reduces speed and therefore reduces lift.  And adverse yaw (in the opposite direction of travel) creates and need to increase aileron input, increasing the angle of attack on the high wing. 

I fine point often misunderstood, if the concept of "relative wind" over the wing. To illustrate, let's use an example. Assume the plane is flying at 50 mph and the pilot reduces speed by reducing power, and maintains the same nose up/down attitude using elevator--the plane will descend without changing nose up/down attitude--the angle of attack seen by the wing by the relative wind will increase. A high drag ultralight will slow rapidly and in order to avoid too rapid a descent, the pilot may be tempted to pull the stick back---resulting in an further increase in angle of attack to the point of stall. To avoid this, unstable scenario, the descent in an ultralight should always be with sufficient nose down elevator to maintain a safe "margin" of "reserve angle of attack". 

Ultralights are often capable of very steep climb attitudes. When climbing the relative wind tends to decrease the angle of attack. This can create a dangerous situation: 1) With a sudden loss of power; or 2) With a sudden change in wind conditions---as angle of attack could increase suddenly causing a stall. 

Bottom Line---Determine the "safe" nose up attitude for climbs and nose down for descents, devise a way of determining these attitudes and use this to maintain a "reserve angle of attack" for descent, level flight and climb. 

Keep in mind that most planes have a "built in" angle of attack to allow "level" attitude at cruising speed. An increase in power and a change to a nose up attitude will cause the plane to climb. A decrease in power and a change to a nose down attitude will cause the plane to descend. Remember BOTH air speed and nose attitude will determine angle of attack.

 

 

Angle of Incidence + Pitch  =

Angle of Climb + Angle of Attack

For example if Angle of Climb is held constant, a change in pitch equals (created by stick back and reduction in throttle) an exactly equal change in Angle of Attack. 

Another example (with negative pitch and negative climb) If Angle of Climb reduced to descend, then Angle of Pitch must be reduced by same amount with stick forward in order to keep Angle of Attack unchanged. Without moving stick forward to reduce Angle of Pitch, Angle of Attack would increase.

To make planes "safer", most general aviation planes incorporate a feature called "Wing Washout" where the wings have a higher angle of attack (2-3 degrees) near the root than at the tips. This causes the wings to stall "smoothly" with the wing first stalling at the root, allowing the ailerons to still be effective.  

Most ultralights (including the Aerolite 103) do not have any washout. Hence, the stall will occur more suddenly across the entire wing simultaneously, with less "warning" and with the high probability that one wing will drop, with the plane entering a down spiral or spin--especially if the plane's rudder is not perfectly centered.  Hence managing angle of attack and maintaining a "reserve angle of attack" on an ultralight is very important.

Here are two ways to measure AOA without fancy electronics:

The Wright Brothers used a similar method on their first flights.

The Concept of TOP RUDDER 

Whenever an ultralight encounters a situation where a stall does occur and the plane enters an down spiral or spin--remember that abrupt input of top or opposite rudder is the only input that will allow the pilot to regain control. Generally, the best position for the ailerons during this "recovery" is neutral, and of course whenever there is a stall, the stick should be moved forward to increase speed and airflow over the wing.  The use of ailerons when stalled or in slow nose up high angle of attack flight can be ineffective and/or dangerous as they change the angle of attack on each wing differently.