Stalls

[Switchfoot]

Scenario:

Flying a light, high-wing trainer. Straight ahead, co-ordinated stall. With 1300 RPM or less, aircraft stalls and drops the left wing. Same situation but with more power (1400 RPM +) and the right wing drops first. Everytime.

(*note* This is not a trick question; weight and balance correct, fuel load equal.... )

Why?

Maybe it was just the student or the instructors piloting skills, but I don't think so!

[shimmydampner]

Overcompensating for the increased power with too much right rudder maybe?? I dunno, just a guess.

[mcrit]

Gyroscopic precession. The prop is a gyroscope (spinning mass). When you try and tilt the axis of a gyroscope it will behave as though the force was applied 90 degrees (in the direction of rotation) from the site of application. The prop spins clockwise as seen from the rear. During a stall the nose pitches down, which tilts the axis of the prop. The prop acts like the force was applied 90 degrees in the direction of rotation. The net result is that the prop tries to twist the a/c to the right. If you want a not so fun example of this in action try and force the tail of a taildragger up to fast on t/o. It will do a wicked right yaw. I PROMISE that that is a mistake you will make only ONCE (I sure haven't done it twice)

[Wasn't Me]

There are many causes of YAW Torque, asymetric thrust, slipstream etc. some cause forces to the right some to the left. I'm sure if you could measure each force you would find that at the power setting the overall force was to the left.

Or maybe the student pushed a peddle or control colum differently.

[looproll]

If you want a not so fun example of this in action try and force the tail of a taildragger up to fast on t/o. It will do a wicked right yaw. I PROMISE that that is a mistake you will make only ONCE (I sure haven't done it twice)

funny, for me it goes LEFT due to precession. Lifting the tail up is like pushing on the top of the prop, and in most North American engines spinning clockwise, gyroscopic principles dictate a force felt 90º in the direction of rotation hence like pushing on the right side of the prop disc resulting in a yaw to the left! Correct me if I'm wrong.

[Airtids]

looproll, you're right. When the nose drops, the prop acts as if force was applied to the back of the right side of the prop, pushing the nose left. This would explain why the left wing drops under a normal stall for switchfoot. Why the right wing drops with power is a mystery to me. I'd suggest keeping one eye on the T/C and ensure the a/c is actually coordinated. Lots of these old planes don't fly so straight anymore, so anything can happen!

[mcrit]

looproll: You are right, that situation I described (forcing the tail up on a taildragger) will put you off to the left very hard and very fast. My bad. In fact, it should also take you off to the left when the nose drops in a stall. Again, my bad.

[Benwa]

I'll go with shimmydamper... too much right foot as the aircraft stalls.

[hz2p]

mcrit is right - it's likely gyroscopic precession creating the yaw to the right as the nose is raised.

Slipstream, asymmetric thrust and torque will all cause a yaw/roll to the left at slow speed with power on. They're obviously being overpowered by the gyroscopic precession in this configuration.

An easy way to confirm this is to fly a variant of the aircraft that has a composite-blade (usually 3-blade) instead of the metal 2-blade.

[Tango01]

1. Uncoordinated entry.

2. Improperly rigged a/c.

3. Lateral imbalance.

4. Specific loading scenerio.

T01

[shimmydampner]

mcrit is right - it's likely gyroscopic precession creating the yaw to the right as the nose is raised.

Slipstream, asymmetric thrust and torque will all cause a yaw/roll to the left at slow speed with power on. They're obviously being overpowered by the gyroscopic precession in this configuration.

An easy way to confirm this is to fly a variant of the aircraft that has a composite-blade (usually 3-blade) instead of the metal 2-blade.

Like airtids and looproll said, gyroscopic precession causes yaw to the left. Nose down=force applied at top of prop arc, translated 90 degrees in the clockwise direction=force applied at right of prop arc=yaw left.
At any rate, as long as yaw is being properly controlled with the rudder (ie. no yawing action taking place) the aircraft should stall straight ahead. Power setting is essentially irrelevant, it just means you have to compensate more. No power or full power, as long as yaw is controlled, you shouldn't get a wing drop, which is what would suggest to me that it's a feet problem.
I'm curious as to why the prop material and number of blades would make a difference. I've never heard that.

[Murph]

uncoordination of someform id say, either right rudder or left aileron(maybe anticipating the wing drop?) could have be just some freak occurence though, maybe the left wing passed through a thermal at the moment of stall entry causing it to rise, or inequal fuel burn, or something like that

[hz2p]

It's amazing how something so simple can be misunderstood .... when the nose is raised for the stall, that's the same as pushing at the BOTTOM of the disc, which rotates counter-clockwise when viewed from behind ... with the 90 degree lag, that ends up acting like a force pushing the LEFT hand side of the disc forward, causing a yaw to the RIGHT, which is what you get during the stall entry, when someone's probably already got a bootful of right rudder in to compensate for the effects of slipstream, asymmetric thrust during the deceleration.

This is the OPPOSITE of what happens when the tail is raised on takeoff (eg with taildragger) which causes a yaw to the LEFT ... ie push on TOP of disc, 90 degree lag, causes effective push on RIGHT of disc, causing a yaw to the LEFT on takeoff, requiring right rudder to compensate.

I'm also amazed that people don't think the material of the prop blade makes a difference ... after all, it's the rotation of the blades that matters, right? If the blades did not have any mass whatsoever, there would be no gyroscopic precession. Hence the lighter the blades, the less gyroscopic precession.

Look any an unlimited-class aerobatic airplane - they ALL have 3-blade composite props. Oh I forgot, there are only a handful of unlimited-class aerobatic airplanes in all of Canada.

IIRC the formula for polar moment of inertia is the integral of radius squared dm (dee-em), which I'm sure that any Transport Canada Civil Aviation Inspector could tell you.

Does anyone here ride a motorcycle? I can only imagine what would happen if this crowd tried to turn above 50 mph - they'd be wrestling pretty hard with the handlebars!

[Airtids]

hz2p:
Your comments make sense for the entry to the stall, similar to what happens when the airplane is rounded out, and flared. However, at the point of the stall when the nose drops as CofP moves aft, you are pushing on the TOP of the prop, with the effect of the nose being pushed to the left (like lifting the tail wheel off, as has been pointed out). Left wing slows further (or right wing speeds up) which would cause that side to drop.

[hz2p]

As the nose is raised - and the angle of attack is increased - for the stall with power on, the gyroscopic precession causes the aircraft to yaw right, thus slowing down the right wing, causing it to reduce the amount of lift produced, and it stalls and drops.

Now, you're saying after the stall/incipient spin to the right is started, the nose going down during the recovery should yaw the aircraft left, which I suppose might help in the recovery. But that's too late - you've already stalled the right wing when the nose went up.

This is really not all that complicated, despite apparent efforts to make it seem so.

[Benwa]

Which all comes back to lazy footwork...

As you approach the stall with a nose high / high power setting, you are applying a lot of right rudder to compensate mainly for the assymetric thrust. as the aircraft stalls, you end up overcompensating...

[Airtids]

Nobody is trying to make this discussion difficult. I agree with all your points hz2p, I'm just trying to play CFI and help out switchfoot here. His conundrum is not as simple as merely discussing the physics of what happens in relation to p-factor during a stall exercise. That would indeed be pretty straightforward. We have to try and think about the human factor as well.

Let's assume that the student has at least marginal rudder control ability. As the nose is raised slowly to the point of stall, the right yawing tendency is fairly easily controlled because it is slow. At the point of stall, however, the nose drop is more abrupt, and (as pointed out by benwa) it's less likely that the novice pilot is going to quickly correct for the now left yawing tendancy with his/her feet just the right amount. What you're saying, hz2p, is the problem rests with poor coordination on the entry to the stall, what I'm saying is that the airplane can be completely co-ordinated right up until entry, and then poor rudder work at the point of the stall will cause the left wing to drop. It depends on where the poor rudder control is taking place. And therein may lie the solution to switchfoots problem!

One other thing: As hz2p has alluded to in relation to the weight of the prop, one must also consider the speed of prop rotation as that will also affect how much yaw is created by moving the nose up or down. This in turn affects how much rudder input is required to keep co-ordinated. This is one of the reasons why folks who fly tail-draggers from long, paved runways can get into trouble when they get to grass/gravel/mud. The added power carried in a soft landing changes the amount of rudder required when changing landing attitudes, and the pilot may be caught unawares.

By the way: I've found that poor rudderwork in a student is one of the biggest signs of a lazy instructor who sits there counting hours.

[Tango01]

Part of the problem is that we tend to think that GP has a left turning tendency, but if you raise the nose (lower the tail) the aircraft will yaw to the right (as long as the propeller spins clockwise as seen from behind the airplane). This probably explains why an increase in RPM caused the right wing to drop (greater GP). At lower RPMs, slipstream, torque and P-factor where greater components than GP and therefore you were more likely to get a left wing drop. As you increased the RPMs, GP was able to overcome the all the left turning tendencies. That's what I think.


Tango01

[Airtids]

Tango, that's a good point as well. We need to consider how different aspects of aircraft design, and physics of flight affect EACH OTHER as well as the aircraft in total.

[Joey Jo Jo]

By the way: I've found that poor rudderwork in a student is one of the biggest signs of a lazy instructor who sits there counting hours.

Airtids, is there an effective method you have found to get this through to your students?

[Switchfoot]

Keep up the good discussion on this subject!

AIRTIDS:

Yes, lazy rudder work is a common trait by many students but I can assure you that I am one instructor who ensures that they keep the airplane co-ordinated as much as possible in each phase of flight. Even more importantly, I'm in the cockpit to teach and not just build time. I demand attention to detail and safety, and I'm quite certain my students would agree. (However, it is a problem when a student does not understand the importance of using their feet....teaching in a tail-wheel aircraft helps to ensure the importance of using rudder).

I also try as often as possible when practicing and reviewing upper air work to show them what happens when the aircraft stalls or they attempt a manouvere with yaw present. An eye opener for them sometimes, but it sure teaches the importance of the subject! I'd rather have them stall/spin at altitude than turning base to final. And in a controlled environment.

Back to the original question though, with as near as I can tell accurate co-ordination, the airplane would stall and drop left wing with low power, and right wing with high power. Still thinking about that one....



Switchfoot.

[Airtids]

Switch;
By no means was I trying to infer that you are hobbs-watching. Appologies if that's how it came off. Rudder instruction is so obvious because it requires the instructor to CONSTANTLY be harping about it, which gets tiring as an instructor. Eventually, however, the student gets it (feels it in their hips) and then you can start to let up.

Joey;
As I have just noted to switch, the biggest thing is that you've got to keep on them about it. Keep referring them to the t/c, and make sure they understand that the pilot poilnt the nose in the direction they want to go, not mother nature. Might be a little easier here in the rocks, because the horizon is so much closer that it's easy for the student to see when the nose wanders off even just a smidge. Insufficient instruction in the first few lessons (straight and level, and turns) will come back and bite you in the ass. Spend the time on the early phases, and the proficiency will make your life a lot easier when dealing with more advanced stuff, like landings, especially in a cross-wind.

[hz2p]

You can teach effective rudder use in a nosewheel aircraft. No one bothers doing this any more, but ...

Enter slow flight - just barely. 5 miles/knots slower than Vx (bottom of the power curve) will do ya.

Feet off the pedals with the wings level. With the nose-high attitude, the ball will be out and the aircraft will be steadily yawing left. Emphasize the need for right rudder to maintain co-ordinated flight when the power goes on, and the nose goes up (eg asymmetric thrust, contracted slipstream, etc). This is applicable after takeoff to maintain tracking over the runway centerline. "Right rudder", I have said entirely too many goddamned times in my life.

Now onto the Good Stuff (tm). Accelerate to around Vy to get rid of the need for constant right rudder, then demonstrate the following: bank right to 10 degrees of bank, neutralize for a moment, then bank left to 10 degrees of bank, neutralize for a moment, then bank right again to 10 degrees of bank. Repeat.

First demonstrate this maneuver with your feet on the floor. The adverse yaw will dish the nose terribly, opposite to the roll. Then repeat the maneuver, showing the student that when the aileron is deflected, so must be the rudder to oppose the effect of adverse yaw. The point of the maneuver is to keep the ball centered at all times.

Increase the angle of bank to make it more challenging. It is considerably more difficult to do perfectly than it appears.

This is sometimes erroneously called "Dutch Roll". It ain't dutch roll, which is a coupling/oscillation effect of swept wing aircraft.

Once this is mastered, move onto the "falling leaf" which is a sustained stall - stick or yoke all the way back - and the rudders (NOT AILERONS) being used to pick up a dropping wing.

It's a real pity more instructors couldn't be bothered to teach these mini-exercises. A student can learn a lot about rudder work in a lesson, but I guess people don't need to use the rudder much any more.

[shimmydampner]

I'm also amazed that people don't think the material of the prop blade makes a difference ... after all, it's the rotation of the blades that matters, right? If the blades did not have any mass whatsoever, there would be no gyroscopic precession. Hence the lighter the blades, the less gyroscopic precession.

Look any an unlimited-class aerobatic airplane - they ALL have 3-blade composite props. Oh I forgot, there are only a handful of unlimited-class aerobatic airplanes in all of Canada.

I was by no means disagreeing with you about the prop material. I just figured we were discussing the types of props found on your average light training aircraft, which I assumed would produce only very marginal differences. But you're absolutely right, the mass of the blade does make a difference, as does the prop diameter as well as the shape of the blade, or more correctly, how the mass of the blade is dispersed throughout the length.

Part of the problem is that we tend to think that GP has a left turning tendency, but if you raise the nose (lower the tail) the aircraft will yaw to the right (as long as the propeller spins clockwise as seen from behind the airplane). This probably explains why an increase in RPM caused the right wing to drop (greater GP). At lower RPMs, slipstream, torque and P-factor where greater components than GP and therefore you were more likely to get a left wing drop. As you increased the RPMs, GP was able to overcome the all the left turning tendencies. That's what I think.


Tango01

Yes, if you raise the nose (and there were no effects from slipstream etc.) GP would cause yaw to the right. However an increase in RPM would not cause greater GP. In fact, as you increase the rate of spin (RPM) the rate of gyroscopic precession decreases.

When I teach stalls I demonstrate the ideal stall (controlling yaw, A/C stalls straight ahead) and also the effects of what happens when you don't control yaw (keep the feet off the rudders). 9.5 times out of 10 in that situation, the aircraft drops the left wing. That's slipstream, assymetric thrust etc. doing its thing. It's been my observation (and maybe I'm way off here) that when you do a stall (wings level) without controlling yaw (especially power-on) if you pay close attention and watch the yawing action closely, it moves slowly left at first and then more rapidly as the stall is imminent. (Could it be assymetric thrust increasing as the nose is raised?) I have never seen an aircraft yaw to the right under these circumstances, it's always to the left. I always look outside to watch for yaw and compensate for it, never the TC which we all know reacts to more forces than just yaw. That's how I was taught, that's how I teach, and so far, it works great. Students drop stalls straight ahead just about every time.

Switchfoot is right, this is a good discussion. And no one has even started name-calling yet, is this avcanada?

[Tango01]

an increase in RPM would not cause greater GP. In fact, as you increase the rate of spin (RPM) the rate of gyroscopic precession decreases.

You're right, a faster rotating mass will have greater inertia thus resisting precession.

My bad

T