What's the pitch ossilation during the stall?

[Pi1otguy]

What's the pitch oscilation during the stall?

During some departure stall practice in the Archer (PA-128-181?) I noticed that it’s pretty darn hard to get a full "drop out of the sky" stall with a forward CG. With full aft elevator I did get a stall followed quickly by what I can only call a pitch oscillation similar to the C172's 40* flaps & slip pitching but above the horizon. During this time there was no dramatic loss of altitude and the cfi called it as a "secondary stall". It seems like the aircraft could continue forever in this mode if I really wanted to.

Did the wings block airflow of the elevator similar to the flaps on the 172? Did the critical AOA on the elevator get exceeded and therefore stalled?

 

[Missa]

During some departure stall practice in the Archer (PA-128-181?) I noticed that its pretty darn hard to get a full "drop out of the sky" stall with a forward CG. With full aft elevator I did get a stall followed quickly by a what I can only call a pitch ossilation similar to the C172's 40* flaps & slip pitching but above the horizon. During this time there was no dramatic loss of altitude and the cfi called it as a "secondary stall". It seems like the aircraft could continue forever in this mode if I really wanted to.

Did the wings block airflow of the elevator similar to the flaps on the 172? Did the critical AOA on the elevator get exceeded and therefore stalled?

Welcome to power-on stalls in an archer. I've actually gained altitued durring a power on stall in an archer... I know that it will continue for at least 2-3 minutes or until you get tired of holding all that back pressure and just let the plane start flying normally agian.

 

[poadeleted20]

Light planes like the C-152/172, Warrior/Archer, AA-5-series, etc., are just too stall-averse to stay solidly in a stalled condition. They typically exhibit a low rate oscillation (more of a cycling than an oscillation to my sense -- a "roller coaster" type ride) in pitch when the elevator is held full trailing edge up (stick full aft) after the stall occurs. What's happening is the wing stalls and lift is lost, the nose drops (due to the loss of lift with no change in elevator downforce), speed picks up a little, elevator effectiveness increases, the nose pitches up, speed reduces, elevator effectiveness decreases, the noses falls, speed picks up, etc. This is especially true when the 4-seaters are loaded for training with two up front and nobody in the back -- near the forward cg limit so there's a lot of lever arm between the cg and the center of lift. Note that with the elevator held full aft (nose up), the wing remains stalled the entire time, but pitch varies due to changing elevator effectiveness.

Despite 40 years of slipping 172's with full flaps (including the older 40-flap models), I've never experinced the "dreaded" C-172 slip/flap pitch oscillation, so I can't tell you what it's like or how it compares with the Archer's normal stall response. However, I understand it has a reasonably quick oscillation rate (way less than flutter, but not a "roller coaster" effect, either) much faster than the stall oscillation described above.

BTW, the term "secondary stall" is used by the Airplane Flying Handbook (FAA-H-8083-3, page 5-8) to describe a different situation. When the stall occurs, the pilot moves the yoke forward to reduce the angle of attack. If this is done too aggressively, a significant nose-down attitude may develop. If the pilot responds to that by pulling back too hard before adequate speed has built up again, the critical angle of attack may be exceeded before the aircraft reaches a level pitch attiude, and the aircraft again stalls. This is the "secondary stall" -- occurring when the stall has been fully broken by reducing AOA after which the AOA is then increased again.

The big danger of a secondary stall is that unlike the primary stall in which stall occurs with the nose well up, in the secondary stall, the nose is below the horizon when the stall occurs. There is a tendency for the pilot, who knows that the primary stall was broken by the initial pitchdown, to try to correct this distinctly nose-low attitude by pulling up instead of breaking the secondary stall by reducing back pressure or even making a postive nose-down input. The secondary stall is particularly insidious during spin recoveries when the aircraft's attitude at the point where the stall is broken appears nearly vertical nose-down, and there is an especially strong compulsion to pull too hard in the recovery back to level flight.

 

[Pi1otguy]

That's all, a simply a loss of elevator effectiveness? Sounds alot simplier then what I expected but makes perfect sense. I was bracing for some disrupted airflow explaination.

Despite 40 years of slipping 172's with full flaps (including the older 40-flap models), I've never experinced the "dreaded" C-172 slip/flap pitch oscillation, so I can't tell you what it's like or how it compares with the Archer's normal stall response.

I've got fewer years by far, but the dreaded C-172 slip/flap pitch oscillation seems to require a very particular CG, slip angle, & attitude as I've only gotten it once or twice. It is however the only time before this that I encountered any pitch cycling, flutter, or oscillation making it my frame of reference.

During future checkouts I'll ask the cfi if the a/c has any attitude issues to expect during a particular manuever.