Teaching Power on Stalls

[ryan1234]

This phenomena also occurs in a single engine in a high power high AoA... The ball will not stay centered.

This is not an accelerated stall right?

Only accelerations cause the ball to be out-of-center, not steady motion.

Most light aerobatic aircraft won't have a ball movement intially if you just pull hard into the vertical from the level flight(acceleration)... if there is a movement it would be slightly to the left going up and slightly to the right when pushing over and going opposite as the airspeed slows (gyroscopic precession in western aircraft).

Usually, like most, I don't look at the ball too much doing acro, just heading... but for an experiment I kept my feet flat on the floor in the CJ-6 and did a series of accelerated stalls (4-5g) vertical and turning... neither moved the ball at all really.... the only time the ball moved was slow at a high alpha... but accelerated stalls didn't really have any effect on the ball (granted the prop spins the other way... but I don't think that mattered too much overall).

 

[tgrayson]

Most light aerobatic aircraft won't have a ball movement intially if you just pull hard into the vertical from the level flight(acceleration)...

Accelerations in the vertical plane have no component along the lateral axis and would have no effect on the ball.

 

[🦈💜]

Accelerations in the vertical plane have no component along the lateral axis and would have no effect on the ball.

I think he's referring to the gyroscopics.

If you're in a Super D and you pull to a vertical line with no correction, you will probably score badly as you'll be off line due to the rotation to the right around the vertical axis induced by the translation of the spinning propeller disc, where the effective forward force on the bottom of the propeller disc is rotated 90˚ around the crankshaft in the direction of rotation, and then applied. The opposite is true when raising the tail on takeoff or pitching down abruptly. The rate of change dictates the amount of force, which among other factors is one of the reasons it's much easier to snap roll to the left in 'conventionally-engined' airplanes.

I believe that is that to which he was referring, with regard to a hard pull to vertical from level flight.

-Fox

 

[Blackhawk]

The snap roll really requires two very specific inputs at nearly perfect timing with a reserve amount of energy to have the needed control effectiveness. Acrofox has a point where the correct input in any case is to ease up on the accelerated stall and unload the airplane...never really understood how people manage to do something like that unintentionally... usually you can really feel the airplane (a light recip) talking to you in an accelerated stall... telling you to ease up the pull.

But - as far as teaching power on stalls go... 'falling leafs' work pretty well in showing what happens when you mash on a rudder to try to 'raise' the wing...even in a Pitts, I don't think I've been able to snap roll it without having a decent amout of airspeed and pulling up (or down) hard.. Also another big thing is the effect of gyroscopic precession when doing accelerated stalls... or anytime pulling up or pushing down.

Some talk to you, some don't. Depends on the airplane type and the situation. I learned very quickly in the SU-29 to pull a little easier from a down line and to keep the airplane coordinated or it would quickly snap roll during the pull. No warning.
Again, the snap roll I have seen in Cessnas is hardly a competition snap roll but it is also not really a "vertical" spin. Usually results from a wing dropping and a pilot responding with a panicked stomp on opposite rudder.

 

[tgrayson]

I think he's referring to the gyroscopics.

He quoted my comment about accelerations and then pointed out that pitch-ups don't, by themselves, produces a ball movement. My reply was to point out that I wouldn't have expected them to. I do agree that gyroscopic procession will produce a yaw, hence, potentially, a slight lateral acceleration.

 

[ryan1234]

I think he's referring to the gyroscopics.

If you're in a Super D and you pull to a vertical line with no correction, you will probably score badly as you'll be off line due to the rotation to the right around the vertical axis induced by the translation of the spinning propeller disc, where the effective forward force on the bottom of the propeller disc is rotated 90˚ around the crankshaft in the direction of rotation, and then applied. The opposite is true when raising the tail on takeoff or pitching down abruptly. The rate of change dictates the amount of force, which among other factors is one of the reasons it's much easier to snap roll to the left in 'conventionally-engined' airplanes.

I believe that is that to which he was referring, with regard to a hard pull to vertical from level flight.

-Fox

The Super D really feels pronounced with the need for left rudder on a pull to a 45 upline going to right rudder... have to do it again in the Pitts and watch, but I don't think I remember it being as necessary... if so maybe only slightly (maybe the rudder is just more effective)...but with the CJ the gyroscopics pretty much keep the ball centered in hard turns and vertical pulls. For a 6g overhead, I haven't really had the need to use rudder other than to expedite the initial roll into the bank... the slower the more rudder coming around base/final. I guess...hmm... I'll go out and mess around with it some more.. haven't really thought too much about it, just went on feeling...

interesting conversation though...

 

[ryan1234]

Some talk to you, some don't. Depends on the airplane type and the situation. I learned very quickly in the SU-29 to pull a little easier from a down line and to keep the airplane coordinated or it would quickly snap roll during the pull. No warning.
Again, the snap roll I have seen in Cessnas is hardly a competition snap roll but it is also not really a "vertical" spin. Usually results from a wing dropping and a pilot responding with a panicked stomp on opposite rudder.

I see what you're saying - interesting what some people will do... That Sukhoi must be fun!

He quoted my comment about accelerations and then pointed out that pitch-ups don't, by themselves, produces a ball movement. My reply was to point out that I wouldn't have expected them to. I do agree that gyroscopic procession will produce a yaw, hence, potentially, a slight lateral acceleration.

Right!

 

[Blackhawk]

The ball doesn't directly measure slip, only a yaw string can do that, which is why I always have one attached during ME training.

Same here. It can be very instructive for showing that zero side slip is not necessarily a 5 degree bank.

 

[nosehair]

The ball is only an indicator of side slip when that side slip causes a lateral acceleration.

As you raise the nose, and increase AoA, you are inputting right rudder to off-set increasing P-Factor. This causes a continuous acceleration to the left. When approaching a stall, you are not in a steady state condition; the AoA is constantly increasing towards the stall angle, and the P-Factor is constantly increasing, and therefore, all flight control inputs are constantly increasing: more aft elevator, more right rudder, and more aileron input to keep the wings level.

Also, this is a theoretical aerodynamic discussion, and the other forces, gyroscopic precession, engine torque, & corkscrew effect have not been discussed, nor do I have the knowledge to accurately discuss them, ...but,.. to the OP, the point I am making is that the ball is not the measure of the maneuver.

That is my point. He thinks the ball should be centered, when we know it will change because of a variety of aerodynamic forces happening during a stall, so quit teaching to look at the ball in a straight ahead stall.

 

[tgrayson]

That is my point. He thinks the ball should be centered, when we know it will change because of a variety of aerodynamic forces happening during a stall, so quit teaching to look at the ball in a straight ahead stall.

The lateral acceleration at the stall will be negligible; the only innate tendency to stall asymmetrically will be due to the side slip to the left, which will not be measured by the ball. While the ball isn't perfect, it's the best approximation that you have, certainly better than looking at a cloud. If the AoA increase is smooth and slow, the airplane will stall straight ahead. While stalling straight ahead isn't a criterion for the maneuver, it's certainly pretty.

Note that the purpose of this maneuver is the teach stall detection and stall recovery, not how to stall the airplane. Placing too great of emphasis on the "proper" way to set up the stall is a bit silly, except within the limits required to actually successfully complete the maneuver. It's much better pedagogy to teach people to use the flight controls in a consistent manner, not to change their function among different maneuvers. In normal flight, we maintain heading with coordinated aileron and rudder, and this works just fine when setting up a stall. Teaching it differently serves no purpose, and suggests a rote memorization of a technique the flight instructor was taught.

 

[nosehair]

Note that the purpose of this maneuver is the teach stall detection and stall recovery, not how to stall the airplane.

OK, here's my take on that.
"Stall detection" encompasses a range of conditions. I'm gonna simplify it at first by separating the "purpose of the maneuver into two purposes; one to detect the stall in normal inflight conditions which are supposed to be simulated in the PTS standards, the other is to detect the stall during the landing phase, or in other more specific terms, how the airplane feels, and how the control inputs are in a constant state of change during the approach, flare, stall/touchdown, and after-touchdown roll. Partial power, and power off stalls which simulate the landing approach and stalled touch-down with the power recovery being done as in a go-around with minimum loss of altitude is primary training and practice for take-offs, landings and go-arounds.

The secondary purpose, is stall detection and recovery during power on climbs, climbing turns, and the infamous base-to-final turn, and these maneuvers are what the PTS standard stall demo is supposed to show.

During the take-off and landing phase, the heading is held constant with rudder, and the bank is controlled with aileron. No attempt is made in "coordinating rudder with aileron" on landing with a cross wind. The take-off and landing stalls should be done in the same way that you actually make takeoffs and landings.

 

[Fly_Unity]

I just went out and did some stalls to take a look at the ball (since I rarely look at the ball with students).

Doing a power off stall with the ball centered, it stalled straight ahead every time.

Did quite a few Power on stalls clean config. EVERY SINGLE TIME I stalled with the wings level and ball centered, one wing would drop dramatically, and the nose would be sliding across the horizon.

Doing them without looking at the ball, but looking outside with wings level, and using rudder input to prevent the nose from sliding across the horizon, stalled straight every time. The ball was NOT centered, rather about a half a ball out.

This was done in a C172S

There has got to be some torque factor, p factor, or even spiraling slipstream factor causing this as it only happens with full power.

Tried it in a SR22, and the ball always stayed centered with no skid or slip, and stalled straight ahead

 

[tgrayson]

Did quite a few Power on stalls clean config. EVERY SINGLE TIME I stalled with the wings level and ball centered, one wing would drop dramatically, and the nose would be sliding across the horizon.

Well, my private students don't have that problem, normally. And I don't know what you mean by "nose sliding cross the horizon". If you've neutralized the yaw, you'd be in a side slip to the left, but the horizon is so far away that you really wouldn't see any relative motion, since the side slip velocity is so low. If the nose is sliding one way or the other, it suggests to me that rudder is being applied either too quickly or two slowly. That's why I stressed that the increase in AoA needs to be slow and smooth and you need to hold the attitude that will induce the stall, rather than continuing to increase the attitude.

When you use rudder to maintain directional control during the maneuver, it's not at all surprising that you stall straight ahead...it's true by definition. Any yaw left or right, you fix by using rudder. My point is that this isn't a virtue. This whole maneuver is artificial and ensuring a straight ahead stall transfers no useful skill to the pilot, nor does it meet any PTS requirement. Should a pilot encounter an actual stall in real life, there is no way to predict what his slide slip state will be and it's very unlikely that he will be trying to keep his nose fixed on some reference point. The single most important skill that he he can apply to lower his AoA and fixed his heading once the aircraft is unstalled.

 

[Fly_Unity]

Well, my private students don't have that problem, normally. And I don't know what you mean by "nose sliding cross the horizon". If you've neutralized the yaw, you'd be in a side slip to the left, but the horizon is so far away that you really wouldn't see any relative motion, since the side slip velocity is so low. If the nose is sliding one way or the other, it suggests to me that rudder is being applied either too quickly or two slowly. That's why I stressed that the increase in AoA needs to be slow and smooth and you need to hold the attitude that will induce the stall, rather than continuing to increase the attitude.

When you use rudder to maintain directional control during the maneuver, it's not at all surprising that you stall straight ahead...it's true by definition. Any yaw left or right, you fix by using rudder. My point is that this isn't a virtue. This whole maneuver is artificial and ensuring a straight ahead stall transfers no useful skill to the pilot, nor does it meet any PTS requirement. Should a pilot encounter an actual stall in real life, there is no way to predict what his slide slip state will be and it's very unlikely that he will be trying to keep his nose fixed on some reference point. The single most important skill that he he can apply to lower his AoA and fixed his heading once the aircraft is unstalled.

Im just saying that the ball isnt always that accurate. What would you tell me if I showed you a youtube video of a power on stall with the ball completely centered and a wing drops, and another one of a power on stall with the ball half out, and it stalls straight ahead? I know it only happens on this one particular airplane we have and maybe its out of rig or something, I dont know. But I do know what the OP is talking about, and ignoring the ball and using the horizon WILL fix the problem.

 

[ryan1234]

Im just saying that the ball isnt always that accurate.

I don't know if the ball isn't accurate or it's the design of the aircraft, but a lot of the warbirds have the same problem.... do a stall with the ball perfectly centered and many will drop a wing.. the T-28 does it for sure.

 

[tgrayson]

But I do know what the OP is talking about, and ignoring the ball and using the horizon WILL fix the problem.

My point is that there isn't a problem to be fixed. All things being equal, a ball-centered stall ought to roll off to the right, because the airplane is in a side slip to the left. The roll off is mild and probably more reflective of a real-world, accidental stall, and the recovery is simply push the yoke forward. If you don't want the roll-off, then a slow, smooth pitch up normally suppresses this behavior. I object to the concept of maintaining directional control with the rudder because this conflicts with the normal use of flight controls, which disrupts the student's mental model of how the airplane works. I could live with that, if it truly served a purpose, as it does in a spin, but since it doesn't, I can't.

 

[///AMG]

Every airplane has its quirks at or around the stall. Have flown plenty that tended to drop a wing, while others were smoothly wings level unless provoked. I wouldn't really overthink it too much....if you are trying to maintain a coordinated stall, when the wing drops, just smoothly add a little rudder to level it back out. I would agree that the slip indicator can be inaccurate; best to use the seat of your pants and/or some sort of landmark/object in front of you for reference

 

[nosehair]

I object to the concept of maintaining directional control with the rudder because this conflicts with the normal use of flight controls, which disrupts the student's mental model of how the airplane works.

Maintaining directional control with the rudder does not conflict with the normal use of the flight controls during landing - during flare, float, drift, touchdown, after touchdown roll, takeoff roll initial liftoff before turning to crab, all these maneuvers, the all-important, close-to-the-ground maneuvering is done with rudder to heading, aileron to drift; ie. maintain heading with rudder, and wings level with aileron.

I know you teach that for T.O. & Landings, don't you? Do you try to get your student to be at or near stall at touchdown? Aligned with, and over the centerline?

 

[tgrayson]

don't you? Do you try to get your student to be at or near stall at touchdown? Aligned with, and over the centerline?

Note that I said "if it truly served a purpose". No such purpose exists when performing a power-on stall, except an artificial one set up by the flight instructor. It's a simple maneuver made needlessly complicated, which is the opposite of what instruction is supposed to achieve. The more phenomena that you can roll into one overarching explanation, the more effective the instruction and learning. I throw this into the same category as teaching that the method of controlling of airspeed varies depending on the particular flight regime, i.e, cruise vs approach, or power on vs power off. This sort of rote memorization cripples both understanding and correlation.

 

[nosehair]

I throw this into the same category as teaching that the method of controlling of airspeed varies depending on the particular flight regime, i.e, cruise vs approach, or power on vs power off.

Ah! OK, never mind. 'nuff said.