Exceeding the critical angle of attack in a nose-down pitch

[HPNPilot1200]

I completely understand how (aerodynamically) an airplane wing stalls; when it exceeds its critical angle of attack. Many factors such as weight and load factor though may change the pitch and/or speed that a stall may occur at.

With that being said, I also know that an average light airplane wing (such as a C172 or Cherokee) will reach its critical angle of attack at approx. 16 - 18º

...so now comes my question Can an airplane wing stall at a negative pitch that exceeds the airplane wing's critical angle of attack? IOW: Can you stall an airplane if you pitch below 16 - 18º nose down?

The only instance I know of a stalled wing in a nose-down attitude/pitch is during a spin, where the plane is pitched down with one wing more thoroughly stalled than the other.

Any answers/explanations are greatly appreciated!

Thanks,
Jason

 

[GaryO]

You can exceed an airfoils critical angle of attack in any pitch attitude. As an example, if you pitch down into a very steep dive and try to pull up sharply. You can easily stall while still pitched down.
The airfoil does not know what your pitch attitude is. What matters is the direction of the airflow in relation to the wings.
If you stabalize in a 60 degree nose down dive and abruptly pull up to a 30 degree nose down dive, your angle of attack will be about 30 degrees. That will very likely cause the airfoil to stall.

 

[wbarnhill]

I completely understand how (aerodynamically) an airplane wing stalls; when it exceeds its critical angle of attack. Many factors such as weight and load factor though may change the pitch and/or speed that a stall may occur at.

With that being said, I also know that an average light airplane wing (such as a C172 or Cherokee) will reach its critical angle of attack at approx. 16 - 18º

...so now comes my question Can an airplane wing stall at a negative pitch that exceeds the airplane wing's critical angle of attack? IOW: Can you stall an airplane if you pitch below 16 - 18º nose down?

The only instance I know of a stalled wing in a nose-down attitude/pitch is during a spin, where the plane is pitched down with one wing more thoroughly stalled than the other.

Any answers/explanations are greatly appreciated!

Thanks,
Jason

http://www.centennialofflight.gov/essay/Theories_of_Flight/Two_dimensional_coef/TH14G3.htm

According to this image, yes, "Negative Lift" does exist, and past the AOA, you would encounter a "Negative Stall". Guess what happens though in a "Negative Stall". The airflow separation would do the opposite of a regular stall, and the airfoil's "Negative Lift" will decrease.

 

[wbarnhill]

You can exceed an airfoils critical angle of attack in any pitch attitude. As an example, if you pitch down into a very steep dive and try to pull up sharply. You can easily stall while still pitched down.
The airfoil does not know what your pitch attitude is. What matters is the direction of the airflow in relation to the wings.
If you stabalize in a 60 degree nose down dive and abruptly pull up to a 30 degree nose down dive, your angle of attack will be about 30 degrees. That will very likely cause the airfoil to stall.

Of course with the airspeed you're likely to have, who is going to notice?

 

[fgcason]

...so now comes my question Can an airplane wing stall at a negative pitch that exceeds the airplane wing's critical angle of attack? IOW: Can you stall an airplane if you pitch below 16 - 18º nose down?

Absolutely.

The plane can stall at any attitude in relation to the horizon. The plane only reacts to the air going over the wings (AOA) which is completely independent from what the horizon is doing. It'll stall anywhere in 360° of pitch or 360° of roll. The only difference between off level and level is which way the plane is traveling in relation to gravity when the wing quits flying.

All the wing needs is enough + or - AOA to exceed the stalling AOA and it'll quit going where the controls are telling it to go.

Ex: The plane is coming through vertical in a loop or such. You want to tighten the loop and start pulling harder. Once the wing reaches the critical AOA it dumps lift and quits pulling the nose up even though the nose can be 60° or more below the horizon. The same concept applies for being at extremely high nose attitudes above the horizon.

Of course there's the psychological side of pushing a CE172 or such to some horrific nose down attitude to create such a situation in the first place... Your average non aerobatic type pilot isn't going to be comfortable doing that (cause it feels like straight down even though it's not) so we're not normally exposed to that situation very often if ever.

 

[wbarnhill]

I would also say that in a negative situation, you wouldn't necessarily lose control surface function. So we get another question on stall speed. What is the stall speed while you are in a negative attitude, and would you be able to exceed the reach the stall speed?

Of course at this point I'm speculating, but enjoying the question

 

[GaryO]

I would also say that in a negative situation, you wouldn't necessarily lose control surface function. So we get another question on stall speed. What is the stall speed while you are in a negative attitude, and would you be able to exceed the reach the stall speed?

Of course at this point I'm speculating, but enjoying the question

The controls may still change the airplanes attitude but it's very likely that you would no longer be able to pull out of the dive. Raising the nose more would just make the AOA even worse.

Also, stall speed increases as weight (or relative mass) increases. In a sharp pull up there would be high G load and therefore the stall speed would also be quite high.

 

[MauleSkinner]

I used to stall Stearmans when pointed straight down at the ground all the time...I'd do loops with passengers on board, and in order to keep the g-load down, I'd be very slow over the top, and hold full up elevator until the airplane stalled--pointed straight down. At that point, I'd ease off the back pressure to reduce angle of attack just below the stall and keep it there as I accelerated downhill. By the time I got back to level flight, the stick was back at neutral.

No, the loops weren't round, but they never exceeded 2 g's for the comfort of first-time aerobats.

Also, if you go to airshows, you'll occasionally see a snap-roll on a vertical down line. Snap rolls are initiated with a stall.

Remember, though, in order to do this without overstressing the airplane, you have to start out at a pretty low speed.

Fly safe!

David

 

[RotaryWingBob]

Jason, pitch angle doesn't count. The only thing that counts as AOA. You can take an airplane (not mine please), point is straight at the ground, pull up on the stick and stall the wing. Never mind that the pitch angle (relative to the horizon( is negative...

 

[HPNPilot1200]

Thanks for all your answers, guys. I thought I made it clear in my first post that I also knew pitch in reference to the horizon is not a factor, as a wing can be stalled as long as it exceeds its critical angle of attack.

Thanks for your help!

Jason

 

[mgkdrgn]

Relative Wind

Your direction of travel doesn't matter. The direction of the wind over the wing, and that relative angle, does.

I completely understand how (aerodynamically) an airplane wing stalls; when it exceeds its critical angle of attack. Many factors such as weight and load factor though may change the pitch and/or speed that a stall may occur at.

With that being said, I also know that an average light airplane wing (such as a C172 or Cherokee) will reach its critical angle of attack at approx. 16 - 18º

...so now comes my question Can an airplane wing stall at a negative pitch that exceeds the airplane wing's critical angle of attack? IOW: Can you stall an airplane if you pitch below 16 - 18º nose down?

The only instance I know of a stalled wing in a nose-down attitude/pitch is during a spin, where the plane is pitched down with one wing more thoroughly stalled than the other.

Any answers/explanations are greatly appreciated!

Thanks,
Jason

 

[HPNPilot1200]

If you stabalize in a 60 degree nose down dive and abruptly pull up to a 30 degree nose down dive, your angle of attack will be about 30 degrees

In this case, would you recover from the stall similar to how you recover from a dive (or a spin after you stop the rotation of the spin)? (IOW: Do you close the throttle, reduce the elevator-back-pressure, and pull out of the dive smoothly?)

 

[gismo]

I completely understand how (aerodynamically) an airplane wing stalls; when it exceeds its critical angle of attack. Many factors such as weight and load factor though may change the pitch and/or speed that a stall may occur at.

With that being said, I also know that an average light airplane wing (such as a C172 or Cherokee) will reach its critical angle of attack at approx. 16 - 18º

...so now comes my question Can an airplane wing stall at a negative pitch that exceeds the airplane wing's critical angle of attack? IOW: Can you stall an airplane if you pitch below 16 - 18º nose down?

The only instance I know of a stalled wing in a nose-down attitude/pitch is during a spin, where the plane is pitched down with one wing more thoroughly stalled than the other.

Any answers/explanations are greatly appreciated!

Thanks,
Jason

First, I think you are confusing AOA with pitch attitude. Consider the extreme case where an airplane in a flat spin. The longitudinal axis is nearly parallel to the horizon (IOW the nose is about where it would be in cruise). The pitch attitude is nearly zero, but the AOA is nearly 90 degrees. Now for the reverse consider an airplane climbing straight up. The pitch attitude is +90 degrees but the AOA is zero (symmetrical airfoil) or slightly negative (cambered airfoil).

Let's try one more. Flying along below Va in level flight we haul back on the stick/yoke abruptly. The pitch attitude is nearly level since the plane can't pitch up very rapidly due to inertia, but the AOA goes right to the stalling AOA and beyond.

If you're still with me, here's a little insight into stall behavior that you might not "completely understand". As you slowly approach a stall by decelerating in level flight, you keep increasing the AOA in order to maintain enough lift to support the plane. Once you reach the stalling AOA, the wing can no longer generate enough lift to support the plane and the plane starts to descend. In most planes if you hold the elevator in the full up position, the wings remain stalled even though the nose drops. At first blush you'd think that the nose dropping would decrease the AOA and terminate the stall, and if you only hold enough up elevator to barely make the wing stall this is exactly what happens: the plane bobs up and down going in and out of a stall (as long as you prevent any yawing). But if you hold the stick all the way back, the wing remains stalled because when the wing drops (loss of sufficient lift to support the plane in level flight) that downward motion increases the AOA more than the nose dropping decreases it and the wings remain stalled.

If OTOH your question was about stalling when the AOA (not pitch attitude) is negative the answer is simpler: Any wing has both a positive and a negative stalling AOA and you can reach the negative stall by trying to generate more negative lift than the wing is capable of at any given airspeed just like th positive case.

 

[gismo]

In this case, would you recover from the stall similar to how you recover from a dive (or a spin after you stop the rotation of the spin)? (IOW: Do you close the throttle, reduce the elevator-back-pressure, and pull out of the dive smoothly?)

To recover from any stall, you must "unload" EG move the AOA towards zero. I this case (stalling while in a steep dive) you would move the stick forward to unstall the wing. This might require forward pressure or just releasing backpressure depending on your trim and CG as well as the behavior of the elevator (or stabilator) in a stall. What you'd do after that would depend on your airspeed although in most cases you'd try to maximize lift (AOA just below stalling AOA) subject to the aircraft's (and the pilot's) G load limits in order to recover with the least amount of altitude loss.

 

[gibbons]

...so now comes my question Can an airplane wing stall at a negative pitch that exceeds the airplane wing's critical angle of attack? IOW: Can you stall an airplane if you pitch below 16 - 18º nose down?

Yes. The easiest way to see this is to watch someone do OUTSIDE snap rolls. The wings are stalled by a rapid push on the stick, snapping the airplane negative.

Chip

 

[Bill]

Any answers/explanations are greatly appreciated!

Other have explained this quite well. One thing I didn't do during my intro acro course was stall the plane out at the top of a loop. I'll have to go up with John sometime soon and try that...

 

[poadeleted20]

There are both negative and positive critical AOA's. The wing can produce negative lift (i.e. lift that pulls the plane down with respect to its normal vertical axis) at negative AOA's. In a plane with a symmetrical wing like many aerobatic types, the critical values are the same except for sign. Usually negative AOA comes into play only during sustained inverted flight, when the airplane is pitched nose-up with reference to the horizon in order to create lift that is up with reference to the earth but down with reference to the airplane's normal axes to keep the airplane from falling to the ground. In that case, you can have an inverted stall due to execessively negative AOA, and the proper response is to pull on the stick/yoke to bring the AOA back below the critical negative AOA value.

 

[RotaryWingBob]

Ron -

Is a negative AOA possible in normal flight, by shoving the stick forward, for example? If so what happens aerodynamically, and how is recovery from a stall done?

 

[gismo]

Ron -

Is a negative AOA possible in normal flight, by shoving the stick forward, for example? If so what happens aerodynamically, and how is recovery from a stall done?

There are two ways (with variations in between and beyond) to reach negative AOA in upright flight. One is to zoom upwards creating enough vertical momentum to counteract the pull of gravity and then push to negative g's. The other is to push hard enough in level flight without the zoom to achieve negative g loading. In the former case you can do this without significant altitude loss, for the latter, you will be accelerating towards the Earth and losing lots of altitude rapidly unless the negative g is very brief.

The aerodynamics are the same whether you are upright or inverted, the difference is negative g isn't sustainable for long when upright. As to the stall recovery, nothing changes (just unload the wing) there until you become unstalled at which point you may be dealing with an unusual attitude recovery.