Two Unrelated Technical Questions

[mhcasey]

1. How do most manufacturers determine "max demonstrated crosswind?"

2. I'm having trouble visualizing the force relationships in a twin engine, single engine out scenario. Anyone got any good explanations/visual aids that might help me understand what becomes balanced when I "raise the dead" and have the ball deflected towards the good?

[staplegun]

Originally Posted by mhcasey

1. How do most manufacturers determine "max demonstrated crosswind?"

Watch this!



Kevin

[tgrayson]

Originally Posted by mhcasey

1. How do most manufacturers determine "max demonstrated crosswind?"

Supposedly, it the maximum encountered crosswind during flight testing. I'm skeptical that this is the entire story, though. Seems a little coincidental that the maximum wind encountered during testing is so close among aircraft of the same market segment. My suspicion is that the flight testers seek out particular wind velocities that are sufficient for marketing purposes and have little motivation to put the airplane to its limits.

BTW, Part 23 only requires a capability of .2Vs0.

what becomes balanced when I "raise the dead" and have the ball deflected towards the good?

[staplegun]

Originally Posted by tgrayson

Supposedly, it the maximum encountered crosswind during flight testing. I'm skeptical that this is the entire story, though. Seems a little coincidental that the maximum wind encountered during testing is so close among aircraft of the same market segment. My suspicion is that the flight testers seek out particular wind velocities that are sufficient for marketing purposes and have little motivation to put the airplane to its limits.

Max demonstrated crosswind in the B-777 is 38 knots...


We did it in the simulator last night; 38 knots 90 degrees off of runway heading and it landed just fine!





Kevin

[MidlifeFlyer]

Originally Posted by tgrayson

My suspicion is that the flight testers seek out particular wind velocities that are sufficient for marketing purposes and have little motivation to put the airplane to its limits.

Your suspicion and mine are identical.

[moxiepilot]

Originally Posted by mhcasey

1. How do most manufacturers determine "max demonstrated crosswind?"

I'm not an engineer, but this sounds good to me.

The max x-wind is determined by the surface area of the rudder because that surface yaws the aircraft. However, the surface area of the rudder is not the only condition. It would also have to be based on max gross weight because the rudder must be able to yaw the mass of the aircraft.

This is why a 747 rudder the size of a cessna still only allows a max x-wind of 38 kts.

My .02 - but again, I'm just using logic, not any factual support

[MidlifeFlyer]

Originally Posted by moxiepilot

I'm not an engineer, but this sounds good to me.

The max x-wind is determined by the surface area of the rudder because that surface yaws the aircraft. However, the surface area of the rudder is not the only condition. It would also have to be based on max gross weight because the rudder must be able to yaw the mass of the aircraft.

This is why a 747 rudder the size of a cessna still only allows a max x-wind of 38 kts.

My .02 - but again, I'm just using logic, not any factual support

The math may work if your are trying to determine the max crosswind in which the rudder is physically capable of maintaining longitudinal alignment in a slip. But that's not what it is. It's really much looser:

The reg that deals with it is 23.233(a): "A 90 degree cross-component of wind velocity, demonstrated to be safe for taxiing, takeoff, and landing must be established and must be not less than 0.2 VS0."

Other than establishing a bottom line minimum, that's pretty broad language. AC 23-9B, Chap 2, para 107(a)(1) explains it a bit further:

==============================
Crosswind. This regulation establishes the minimum value of crosswind that must be demonstrated. Since the minimum required value may be far less than the actual capability of the airplane, higher values may be tested at the option of the applicant. The highest 90-degree crosswind component tested satisfactorily should be put in the AFM as performance information. If a demonstrated crosswind is found limiting, it has to be introduced in Section 2 of the AFM.
==============================

So, other than the minimum, how high to go is up to the manufacturer, at least up to the point where the airplane is truly uncontrollable, at which point you would have a limitation (which these numbers generally are not).

The engineering may give some guidelines of what to look for. but it's really a flight test created number that probably represents a compromise between what the test pilot really can do with the airplane and what the manufacturer guesses the average pilot who will fly the airplane can handle comfortably.

[Dugie8]

Other things to consider with max crosswind are bank angle and engine pod clearance. I don't know if the regs deal with that specifically or if it becomes and individual airplane limitation.

[tgrayson]

Originally Posted by moxiepilot

The max x-wind is determined by the surface area of the rudder because that surface yaws the aircraft. However, the surface area of the rudder is not the only condition. It would also have to be based on max gross weight because the rudder must be able to yaw the mass of the aircraft.

Your logic is really addressing the maximum possible crosswind, not max demonstrated, as Midlifeflyer said. Even for the max possible crosswind, gross weight isn't really a factor for rudder authority for two reasons:

1) If the increased mass were located at the CG, there would be no difference in the yawing capacity of the aircraft, since it yaws *around* the CG. What you would really be interested in is the distribution of mass, not its absolute quantity. This is addressed by the concept of moment of inertia. A large moment of inertia means that the mass is distributed toward the extremities of the object, rather than at the center. (Think of a dumbbell.)

2) Even if there were a high moment of inertia (resistance to yawing), it would merely affect the acceleration of yaw, not the maximum quantity of yaw achievable. The aircraft would respond sluggishly to the rudder, but it would respond.

This is why a 747 rudder the size of a cessna still only allows a max x-wind of 38 kts.

The use of the rudder during a crosswind landing is to neutralize the tendency of the vertical stabilizer to weathervane the aircraft. Since the 747 has a very large vertical stabilizer, it needs a very large rudder to neutralize it.


For those of you flying transport category aircraft, how many of these actually have an operating limitation on crosswind vs a max demonstrated? (Assuming uncontaminated runways.)

[Dugie8]

Originally Posted by tgrayson

Your logic is really addressing the maximum possible crosswind, not max demonstrated, as Midlifeflyer said. Even for the max possible crosswind, gross weight isn't really a factor for rudder authority for two reasons:

1) If the increased mass were located at the CG, there would be no difference in the yawing capacity of the aircraft, since it yaws *around* the CG. What you would really be interested in is the distribution of mass, not its absolute quantity. This is addressed by the concept of moment of inertia. A large moment of inertia means that the mass is distributed toward the extremities of the object, rather than at the center. (Think of a dumbbell.)

2) Even if there were a high moment of inertia (resistance to yawing), it would merely affect the acceleration of yaw, not the maximum quantity of yaw achievable. The aircraft would respond sluggishly to the rudder, but it would respond.


The use of the rudder during a crosswind landing is to neutralize the tendency of the vertical stabilizer to weathervane the aircraft. Since the 747 has a very large vertical stabilizer, it needs a very large rudder to neutralize it.


For those of you flying transport category aircraft, how many of these actually have an operating limitation on crosswind vs a max demonstrated? (Assuming uncontaminated runways.)

DC8 has a chart for max crosswind vs weight (ie Vref). Less weight equals slower Vref equals less rudder, BUT more pitch which allows for more bank. There is a trade off from a heavy weight to a lighter weight, ie dimminshing returns. The "demonstrated" crosswind is 30kts.

[tgrayson]

Originally Posted by Dugie8

DC8 has a chart for max crosswind vs weight (ie Vref). Less weight equals slower Vref equals less rudder,

Good point; I guess I should have said that weight doesn't directly affect rudder authority. For most light airplanes, I suspect that few are touching down at slower speeds when light.

BUT more pitch which allows for more bank. T

How is that? Your wingtips are closer to the ground the greater the pitch attitude, so I would have thought the opposite.

[Dugie8]

DC8 has some long legs (landing gear) the limitations I posted were for -7x models, those have the CFM engines, the inboard pod is only 15-18 inches off the ground nose level, the more you pitch up the more clearance you have on that inboard pod. The wingtips are also fairly close to the CG of the aircraft so they dont "dip" quiet as much as something like a Citation X or even an MD80 series. The citation on the other hand has a max bank of IIRC 14 degrees or you will drag the wing tip, short landing gear, relatively long wing.

[mhcasey]

Originally Posted by tgrayson

Supposedly, it the maximum encountered crosswind during flight testing. I'm skeptical that this is the entire story, though. Seems a little coincidental that the maximum wind encountered during testing is so close among aircraft of the same market segment. My suspicion is that the flight testers seek out particular wind velocities that are sufficient for marketing purposes and have little motivation to put the airplane to its limits.

BTW, Part 23 only requires a capability of .2Vs0.

Where's the picture from? Maybe a before and after picture would help, as in ball centered and no bank first, ball towards the good engine and banked second?

Is it safe to say that you're essentially slipping back into the good engine when you "raise the dead?" I'm still having trouble seeing why with the ball centered there is a net force (yet no yaw?) moving the entire aircraft in the direction of the bad engine. Is the force solely the result of rudder input to counteract the yaw from the good engine?

[tgrayson]

Originally Posted by mhcasey

Where's the picture from?

I made it.

Maybe a before and after picture would help, as in ball centered and no bank first, ball towards the good engine and banked second?

Hmmm....I have a "before" from the top:

Is it safe to say that you're essentially slipping back into the good engine when you "raise the dead?"

If you raise it high enough, yes. If you only raise it a little, you're just stopping the slip into the dead engine.

I'm still having trouble seeing why with the ball centered there is a net force (yet no yaw?) moving the entire aircraft in the direction of the bad engine.

You only have a net force for a brief time. The aircraft accelerates toward the dead engine until the drag produced by the sideslip is equal and opposite to the rudder lift. Once you reach the equilibrium position, the ball will be centered, because there is no longer any acceleration.

Is the force solely the result of rudder input to counteract the yaw from the good engine?

Yes.

[mhcasey]

Got it thanks a lot guys.

[mhcasey]

Alright...does normal P-Factor in a single engine airplane cause a slip during climb out with the ball centered? Seems like the same forces (minus the inop engine) are at work, just to a lesser extent because the thrust is less asymmetrical.

[tgrayson]

Originally Posted by mhcasey

Alright...does normal P-Factor in a single engine airplane cause a slip during climb out with the ball centered? Seems like the same forces (minus the inop engine) are at work, just to a lesser extent because the thrust is less asymmetrical.

The rudder to correct for the P-factor (and spiraling slipstream) does, just like the multi scenario. That's also why the right wing *usually* stalls first in a power on stall....you're sideslipping to the left.

[nosehair]

Originally Posted by tgrayson

The rudder to correct for the P-factor (and spiraling slipstream) does, just like the multi scenario. That's also why the right wing *usually* stalls first in a power on stall....you're sideslipping to the left.

That is, if you try to 'keep the ball centered" while inducing more P-Factor and side slipping while approaching the stall. However, if you bank slightly to the right - balancing the forces the same as a left engine out in a twin - the forward motion will be straight ahead and both wings will stall at the same time.

I don't teach keeping the ball centered during power-on stalls - it's a set-up to a spin entry to the left. I teach positive heading control with the rudder, and wings level or a couple degrees to the right, and the ball should be out to the right about a half-ball width.

[tgrayson]

Originally Posted by nosehair

However, if you bank slightly to the right...ball should be out to the right about a half-ball width.

Either one of those should work.

I teach positive heading control with the rudder, and wings level

Common approach, but I strongly dislike it because it's so artificial. Normal flying is ball-centered and changing your heading with the ailerons. Pedagocially unsound, in my opinion, to set up the stall any differently.

[nosehair]

Originally Posted by tgrayson

Normal flying is ball-centered and changing your heading with the ailerons. Pedagocially unsound, in my opinion, to set up the stall any differently.

Ah, but 'normal flying', as you describe it, is a level cruising turn. Stalls occur in the 'landing phase', or when you are focused on 'heading-to-centerline' control with rudder; looking out over the nose keeping the nose straight with rudder and wings level with aileron. Which is how I teach, all the time, for continuity. I 'turn the airplane' with rudder, and I 'co-ordinate' enough bank, with aileron, to keep the ball centered. No matter what phase of flight I'm in, yaw is controlled by rudder, and bank (or slip) is controlled by aileron.

[tgrayson]

Originally Posted by nosehair

I 'turn the airplane' with rudder, and I 'co-ordinate' enough bank, with aileron, to keep the ball centered. No matter what phase of flight I'm in, yaw is controlled by rudder, and bank (or slip) is controlled by aileron.

Yes, I've seen you say that before. I'm amazed you could pass a CFI checkride with that explanation.

[MidlifeFlyer]

Originally Posted by tgrayson

Yes, I've seen you say that before. I'm amazed you could pass a CFI checkride with that explanation.

Who says he did? Parroting back the FAA's official explanation during a checkride and coming up with an explanation that fits the brain of a student are often two completely different things.

(Doesn't "centrifugal force" still appear in some FAA texts?)

BTW, I'm not agreeing with nosehair's explanation. But "it's not on the checkride" is hardly a valid criticism.

[nosehair]

Originally Posted by tgrayson

Yes, I've seen you say that before. I'm amazed you could pass a CFI checkride with that explanation.

I also do explain the theory of horizontal component of lift and how the rudder is to overcome the adverse aileron yaw. That's the theory of it. But in the practical side, most everyone who comes to learn to fly already has a 'drive-the-wheel' habit and will continue to drive the yoke and turn with aileron with insufficient rudder. But that only applies to turns before final approach. Then on final approach you're switchin' to "align-the-nose-with-rudder!" Huh? Wha?...So, in Primary Training, I focus on the Primary Yaw Control is the Rudder. And Bank Control is Aileron. Sometimes, you want to coordinate them, as in cruising flight, and especially in glides and gliding turns, or low power descending turns such as base-to-final,... but,.. when you are on final, throughout final, through the flare, touch-down or go-around and into the climb, you are pointing the nose with rudder and controlling bank with aileron and not having any connection in your mind about keeping them 'co-ordinated'. They will never be 'co-ordinated', even in a dead calm, due to the gyroscopic and P-factor forces occuring during pitching moments. But on a normal 3 to 4 knot varying to 6 or 7 knot wind, you are constantly crossing the controls. Constantly.

So it is this primary knee-jerk response to heading-with-rudder, bank with aileron, that I try to instill in the primary beginning student.

Or the student who cannot seem to keep it lined up on landing. Or cannot flare - because he cannot keep it lined up. He hasn't connected his steering mechanisim to his feet - he's still driving with his hands.

[tgrayson]

Originally Posted by MidlifeFlyer

coming up with an explanation that fits the brain of a student

The brain must be molded to fit the explanation.

It's certainly easier to teach that the sun revolves around the earth (fits the brain of the student), but the truth is the reverse and the brain must be molded to understand it.

(Doesn't "centrifugal force" still appear in some FAA texts?)

Yes, but I believe that they acknowledge that it's a fictitious force. And I think the force vector was removed from the diagram in the latest version of the Airplane Flying Hanbook.

But "it's not on the checkride" is hardly a valid criticism.

That wasn't meant to be a criticism, per se, but just an implication that the explanation was fundamentally screwed up (to use a technical term) and even an FAA examiner should realize it.

Radical concept, but I think a flight instructor must have some inkling of how airplanes fly.

[tgrayson]

Originally Posted by nosehair

Or the student who cannot seem to keep it lined up on landing. Or cannot flare - because he cannot keep it lined up. He hasn't connected his steering mechanisim to his feet - he's still driving with his hands.

The rest of us seem to manage it.

And the rudder isn't a steering mechanism even in this scenario. Where the nose is pointed has little to do with which way the airplane is going.

I don't accept that a butchered explanation of how airplanes fly is of a net benefit to a student, and it surely cripples his ability to achieve a more sophisticated understanding later in his career.