Ex. 12 - Stalls

A. This is the basic definition of a stall. That's why we can talk about wings being "barely" stalled and "deeply" stalled - it all depends on how much that critical angle of attack has been exceeded. The greater the angle, the more dramatic the stall. Do note, however, that even a stalled wing is producing lift - just not enough to sustain level flight.



B. It is important to understand that a plane can be stalled at any airspeed. In straight unaccelerated flight, each angle of attack generally corresponds to a certain airspeed, so it's easy to refer to "stall speeds", since we have an airspeed indicator, but no angle of attack indicator. However, a plane can be stalled at any airspeed. For example, in steep turns, a higher angle of attack is required to produce a certain airspeed compared to level flight, so a steeply banked plane will stall at a much higher airspeed (but the same angle of attack).

C. Stalling planes has everything to do with wings, and little to do with the engines.

D. First of all, if the wing is only "lightly" stalled, you might not get a noticeable nose drop. Also, once stalled, you will probably be flying at a low airspeed, since a high airspeed will typically imply a low angle of attack. Finally, it is important to not confuse attitude with angle of attack. Angle of attack is not just about where you are pointing; it's about where you are pointing vs. where you are going. Consider the picture below: the plane is pictured in drastically different attitudes, yet at the same angle of attack:

Correct! Lowering the nose by pushing forward on the yoke (or relaxing the back pressure) is in essence lowering your angle of attack, which equals destalling the wings.

This is what happens with the airflow as the angle of attack increases (note that the actual angles will vary for different airplanes, though 17-18 degrees is a typical angle at which a light trainer like a Cessna 172 will stall):





It's no wonder you might experience some buffeting!

(Some planes are also equipped with a stick-shaker, which is a stall warning device that shakes the pilot's controls at the onset of stall - do not confuse this with buffeting, which is airframe shaking due to to turbulent airflow over the wings.)

Rudder is the most effective control surface in a stall, so that's what's used to correct a wing drop in a stall.
Your ailerons must stay NEUTRAL at the beginning of the stall recovery procedure: when the wing is stalled, they won't be very effective and in fact may make the wing drop worse, since they would more deeply stall an already stalled wing. Practice this during power-on stalls, because it will be critical at your next lesson - spins!
Pulling back on the yoke does not make sense - that's what got you in the stall in the first place - it just increases the angle of attack stalling the wings more deeply.

Pulling the nose back up will only serve to stall the plane again! Lowering the flaps is actually dangerous because now that your plane is unstalled, it's gaining airspeed and you could easily overspeed the flaps, damaging them. Maintaining best glide speed will ensure you won't lose altitude too fast, but adding power will actually allow you to stop altitude loss and begin the climb. So, since your objective is to prevent altitude loss, don't wait too long before applying the power on recovery.

A common mistake student mistake on stall recoveries is to point the nose down far too much, producing a rapid dive which results in unnecessary airspeed build-up and considerable altitude loss. Your objective is to recover with minimum loss of altitude (since if you stall inadvertently, it will likely be close to the ground on take-off or landing), so you want to push the nose down enough to break the stall, but not so much that you end up in an unchecked high-speed dive. For a typical power-off stall in most trainer aircraft entered from straight and level flight, that would be just a bit below the horizon. Save your instructor's nerves, don't point the thing straight down! :-)

B looks like a plausible answer, but it is reversed: in actuality, washout is the decrease of the angle of incidence towards the wingtips, so that they stall after the wing roots. This is done so that even if most of the wing is stalled, the wing tips are not. In other words, the wing tips are NOT yet at the critical angle of attack, so the increase in an angle of attack will produce an increase in lift, so ailerons will still work as expected. When the wing tips are actually stalled, the increase in an angle of attack would cause a decrease in lift, so turning the yoke to the left might actually make the plane roll to the right!

A power-on stall will occur at a lower airspeed than a power-off stall, for a couple of reasons. First, the turning propeller will create additional airflow over the wing roots (which are normally supposed to stall first due to washout, remember?) This means the wing can avoid being stalled for a little bit longer. Another reason is that the thrust generated by the engine has a bit of an upward component, which acts as extra lift, which helps delay the nose drop.

However, because the wing root stall is delayed, once the stall occurs, the wing roots and the wing tips will stall at about the same time, which results in a more deeply stalled wing and thus a more dramatic stall. Moreover, if one wing ends up being a bit more stalled than the other, it will drop, so you are more likely to encounter a wing drop along with a nose drop in a power-on stall.

CAUSES CONTRIBUTING TO POWER-ON STALL DELAY:

See the CONSIDERATIONS slides in the Ex. 12 presentation for the explanations. (Or contact us if you are confused about this (or any other exercise!)

A stall warning horn will sound when a certain angle of attack is exceeded (typically a few degrees below the critical angle of attack), no matter what airspeed you are flying at. To test it, ask your instructor to demonstrate a steep turn at a relatively low airspeed and note when the stall warning horn sounds - it will go off at a much higher airspeed than during normal stall practice.
In severe icing conditions, however, the whole aerodynamics of the wings changes, so that they will stall at a much lower angle of attack than normally. So a stall may occur well before the angle of attack increases sufficiently to actuate a stall horn. (This is just one of many reasons why flying with ice or frost anywhere on the control surfaces is extremely dangerous.)