What is a strong crosswind?

JoeFromKS said:
Right at liftoff there is a transitional period where the wind is working against the inertia of the plane to accelerate the plane to the x wind component. During that transition you would be weathervaning about the center of gravity.
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I'm open to that, but I still doubt it. Could be convinced, though.

Good one for Mythbusters!

If only we had a treadmill!!!
 
OK...

If you could levitate a plane, and blew on it from the side, would it rotate around its center of gravity into the wind or just move sideways?

I can see it either way, but still vote for sideways.

Any aeronautical engineers of physicists to help out here?
 
Absolutely it will rotate. From the side, there is more surface area in the rear, and the center of gravity is in the front. But you don't have to take my word for it, imagine holding a model airplane in the air at a 90° bank and dropping it. Rotates and lands nose first.

I'm a mechanical engineer, close enough? ;)
 
JoeFromKS said:
But you don't have to take my word for it, imagine holding a model airplane in the air at a 90° bank and dropping it. Rotates and lands nose first.

I'm a mechanical engineer, close enough? ;)
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You're winning me over, but...

If you had a model airplane sitting on a frictionless surface and turned a fan on from the side, would it rotate to point into the wind?
 
Yes. And I should have said earlier, it will rotate AND move away. And different designs will have more tendency to rotate than others. (Cessna vs F-117)
 
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JoeFromKS said:
Yes. And I should have said earlier, it will rotate AND move away. And different designs will have more tendency to rotate than others.
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I have often taken exception with the concept that banking the airplane causes it to turn. That would caused only translational movement with the nose still pointed straight ahead if not for something else at work. Linear forces in different directions acting on the same point (the center of lift) cause a resultant translation, not rotation. Rotation requires a force couple, two opposite forces separated by a moment arm. What turns an airplane, IMO, is weathervaning, i.e. the force couple caused by the horizontal component of lift in a bank opposed by the center of sidewise resistance to the air being well behind the center of lift, this caused by the abundance of side surface area to the rear. Rich Stowell likes to say that the elevator turns the airplane but it would be more correct to say the empennage and tailfeathers turn the airplane, divided between vertical and horizontal parts of those depending on angle of bank.
 
JoeFromKS said:
Yes. And I should have said earlier, it will rotate AND move away. And different designs will have more tendency to rotate than others. (Cessna vs F-117)
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Na, not buying it. If this were the case, you would be "rotated" anytime you changed direction. Once you are in the air, you are moving relative to that airmass. At least that is my limited < mach 1 experience.
 
brian];1557125 said:
Na, not buying it. If this were the case, you would be "rotated" anytime you changed direction. Once you are in the air, you are moving relative to that airmass. At least that is my limited < mach 1 experience.
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You are rotating every time you change direction, that is the whole point of the exercise.
 
brian];1557125 said:
Na, not buying it. If this were the case, you would be "rotated" anytime you changed direction. Once you are in the air, you are moving relative to that airmass. At least that is my limited < mach 1 experience.
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Joe is talking about that very short period as the plane accelerates relative to the ground to assume the velocity of the surrounding air mass.

I just don't think I've ever noticed a tendency of the plane to weathervane on its own into the wind once in the air. Maybe it's because I'm virtually always turning into the wind after a crosswind takeoff to establish a crab angle anyway, and that masks it.

Still want to see the frictionless surface demo!
 
Don't forget about the currents and othe turbulence associated with a strong wind. This does have the feeling of being weather veined...

I'm thinking that you would have to be going incredibly fast for the mass of an airplane to dominate your forward position for the weather vane effect to be delayed much after the last wheel leaves the ground. Well beyond my financial abilities....

Edit: and I'm still not sure... Where can we submit this to mythbusters??
 
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brian];1557370 said:
Don't forget about the currents and othe turbulence associated with a strong wind. This does have the feeling of being weather veined...

I'm thinking that you would have to be going incredibly fast for the mass of an airplane to dominate your forward position for the weather vane effect to be delayed much after the last wheel leaves the ground. Well beyond my financial abilities....

Edit: and I'm still not sure... Where can we submit this to mythbusters??
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It is easy to demonstrate. Next time you are flying along, set up the maximum forward slip that you can. Then continue holding the bank but release the cross-control rudder you are holding. See how fast the airplane weathervanes around to something closer to a coordinated turn. That is what happens when you lift off while holding ailerons into the wind.
 
brian];1557370 said:
Edit: and I'm still not sure... Where can we submit this to mythbusters??
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An aviation themed episode would have been awesome, but since Kari is leaving the show, what's the point? :(
 
I'm conjuring up a demo involving our ATV and a model airplane and a GoPro or iPhone.

Mount the model perpendicular to the direction of travel. Launch it into the air, somehow.

See what happens.

Idle hands are the devil's workshop!
 
alfadog said:
It is easy to demonstrate. Next time you are flying along, set up the maximum forward slip that you can. Then continue holding the bank but release the cross-control rudder you are holding. See how fast the airplane weathervanes around to something closer to a coordinated turn. That is what happens when you lift off while holding ailerons into the wind.
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But isn't that the relative wind produced by the prop the "external" force creating the moment. Sorry - not trying to be obtuse on this one.

Thinking about this one last night, there is one external condition that might turn the aircraft into the "wind" while in the air: a Thunderstorm. In the end, you need some kind of pressure gradient along the aircraft to cause a moment to rotate the aircraft. The slip is causing this condition, but the effect is created by the aircraft and not the external wind.

Maybe I'm just too focused on the "wind" being external and I'm drawing a bunch of force vectors in my (admittedly less than capable) engineering mind..



(Kari is leaving!! Need more cute girls in science to attract the next generation of guys away from the internet ... and into the real world and good science based careers!)
 
A thunderstorm would definitely do it, but all you need is to be in an air mass moving differently than the aircraft's inertia. When you transition from the ground to the air, or from an air mass moving one direction to an air mass moving a different direction. Once the transition is complete, the aircraft's inertia will match the air, and the effect will be gone.
 
As the air mass changes direction/orientation, so will the aircraft.
 
brian];1557581 said:
But isn't that the relative wind produced by the prop the "external" force creating the moment. Sorry - not trying to be obtuse on this one.

Thinking about this one last night, there is one external condition that might turn the aircraft into the "wind" while in the air: a Thunderstorm. In the end, you need some kind of pressure gradient along the aircraft to cause a moment to rotate the aircraft. The slip is causing this condition, but the effect is created by the aircraft and not the external wind.

Maybe I'm just too focused on the "wind" being external and I'm drawing a bunch of force vectors in my (admittedly less than capable) engineering mind..

(Kari is leaving!! Need more cute girls in science to attract the next generation of guys away from the internet ... and into the real world and good science based careers!)
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No, this would happen in a glider also. When you are in a forward slip, you are presenting the side of the aircraft to the relative wind. That is the purpose of a forward slip on landing. Since the centroid (geometric center of a two-dimensional region) of the side of the aircraft is behind the point where the horizontal component of lift acts on the wing, you create a force couple that rotates or weathervanes the aircraft into the relative wind. The same would happen when you lift the wheels off the pavement while holding aileron into a crosswind. The tires were preventing the weathervaning and then that is released as if you released opposite rudder in a forward slip.
 
I don't have a "Mythbusters budget" - or talent for that matter.

But I came up with this:

15259501311_0424b47d86_z.jpg


Enough weight from the nuts on the nose to put the CG where you see it.

My plan is to get up to 20 mph or so with the model perpendicular to the wind - it has stops to keep it there.

Then pull the fishing leader which should release the spring and launch it into the airstream.

If Joe is right it should point into the relative wind - i.e. the direction the car/ATV is going.

If I'm right, it should just blow back with no particular tendency to align itself with the wind.

Am I on the right track?
 
FastEddieB said:
Am I on the right track?
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Ummm, does the spring just release the stops? if so what the model does depends on the location of the center of pressure relative to the pivot.
 
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You think weighting the bottom might keep it upright?

I can just move the "nut ballast" lower to start.

For this exercise, wings seem unnecessary.

Though I do have some model airplanes I could press into service.
 
I like it. I hope there is a video of this coming.
 
FastEddieB said:
Leaning towards a leaf blower now.

Pop the "plane" perpendicular up into the "wind" and see if it weathervanes.

Make it easier to get on video!
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Just make sure that the model is scaled correctly as regards the location of the pivot point. It should be about 1/3 of the way back from the front. You look to have closer to 1/2.
 
FastEddieB said:
I'll go ahead and move it forward a bit, though with more vertical surface aft of the CG
it still should have worked, right?
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Yes. Remember that when you blow at it from the side, you will have two things at work. First the wind will try to blow it laterally. You should not let that happen. In an airplane we oppose that force by banking the wings into the wind. Then the fact that the side centroid is not coincident with the center of lift of the wings sets up the force couple that rotates or weathervanes the airplane.
 
Each aircraft responds differently, especially taildraggers. A Stearman has a lot of side area and low wings which make a crosswind landing very dicey above 15 -20. Especially if it's gusting.....( it usually is.) a champ is much lighter as is a t craft and can be a handful in these same winds if the pilot is afraid of a one wheel roll on. They are lousy aircraft to fly in windy conditions as you get bounced around, head hits overhead, etc. no enjoyment for me in this type flying. ( I'm speaking of a direct crosswind, not ten or twenty degrees. )
 
Let me restate that I am perfectly willing to accept that there may be a brief period right after takeoff when a crosswind can weathervane a plane.

But let me make one of two observations...

On zero-zero practice instrument takeoffs and departures, I never recall anyone telling me to look out for weathervaning right after takeoff that would need to be corrected. And I don't think I ever felt a need to warn my students about that.

For instance, taking off on RWY5 with an easterly wind, we would set the heading indicator to the runway heading and apply gradually decreasing right aileron into the wind, maintaining 050° with rudder. By takeoff, if no WCA was called for ("Maintain runway heading") a firm rotation would have the plane climbing wings level on a heading of 050°. I don't ever recall the plane "weathervaning" to 060° or 070° and needing to be steered back to the left to maintain runway heading.

But maybe it's subtle and I missed it. Do any CFII's recall having to teach that weathervaning after takeoff was something that one needed to routinely correct for?

Second observation to follow...
 
Observation(s) the second...

I have logged about 4,500 hours of dual given over the years, including lots of crosswind takeoffs.

And so I've seen my fair share of takeoffs with inadequate crosswind technique applied.

It sure seems to me that when that happens, the plane just gets "pushed" downwind on whatever heading it's on. Absent any turn to a wind correction angle to compensate for drift, it has not seemed to me that the crosswind "assists" the student in any way by making his WCA for him, via weathervaning or whatever.

Curious as to what other instructors have found.
 
FastEddieB said:
Observation(s) the second...

I have logged about 4,500 hours of dual given over the years, including lots of crosswind takeoffs.

And so I've seen my fair share of takeoffs with inadequate crosswind technique applied.

It sure seems to me that when that happens, the plane just gets "pushed" downwind on whatever heading it's on. Absent any turn to a wind correction angle to compensate for drift, it has not seemed to me that the crosswind "assists" the student in any way by making his WCA for him, via weathervaning or whatever.

Curious as to what other instructors have found.
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Not an instructor, but I think I know the feeling being described. When I lift off in a crosswind, using xwind technique and all that, it *feels* so natural to level the wings and establish the crab once airborne that one could almost imagine the plane weathervaning. But, as you say, it isn't. But I know the feeling.

Weathervaning is a ground concept, not a flight concept. Once the airplane is in flight, it doesn't know what wind is. It flies through the air and cares not whether said air happens to be moving relative to the ground.

Wind is only relevant with reference to the ground (WCA to follow a course, ground speed, crab angle to follow a course, final approach course, etc). The airplane doesn't GAF about wind while airborne unless there is a shear but shear matters in flight because it is a sudden change in the speed of the air passing over the aircraft. It just responds to the molecules passing over the flight surfaces.

But the pilot sees the runway on takeoff and wants to fly the upwind course and naturally establishes that crab almost without thinking about it and imagines that the airplane weathervaned. But the pilot did it, not the wind.
 
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petrolero said:
But the pilot sees the runway on takeoff and wants to fly the upwind course and naturally establishes that crab almost without thinking about it and imagines that the airplane weathervaned. But the pilot did it, not the wind.
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The CFI said "very nice" and I was like "WHAT????? the plane did it" and the aircraft does have a pivot point with one wheel still on the ground in a proper crosswind take-off. Of course it is up to the pilot to catch it at the correct moment.
 
petrolero said:
...

Weathervaning is a ground concept, not a flight concept. Once the airplane is in flight, it doesn't know what wind is. It flies through the air and cares not whether that wind is moving relative to the ground.

Wind is only relevant with reference to the ground (WCA to follow a course, ground speed , crab angle to follow a course, final approach course, etc). The airplane doesn't GAF about wind while airborne. It just responds to the molecules passing over the flight surfaces.

But the pilot sees the runway on takeoff and wants to fly the upwind course and naturally establishes that crab almost without thinking about it and imagines that the airplane weathervaned. But the pilot did it, not the wind.
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The airplane always knows where the wind is - the relative wind. When you bank the airplane, you introduce a relative wind that strikes the top and side of the airplane and it weathervanes to turn. You fine-tune that weathervaning with rudder and elevator.
 
FastEddieB said:
Observation(s) the second...

I have logged about 4,500 hours of dual given over the years, including lots of crosswind takeoffs.

And so I've seen my fair share of takeoffs with inadequate crosswind technique applied.

It sure seems to me that when that happens, the plane just gets "pushed" downwind on whatever heading it's on. Absent any turn to a wind correction angle to compensate for drift, it has not seemed to me that the crosswind "assists" the student in any way by making his WCA for him, via weathervaning or whatever.

Curious as to what other instructors have found.
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Especially in the context of the CFI's Mantra:

"More right rudder."

How much more right rudder? Enough to make the airplane go straight.

You train student pilots to straighten out the airplane. The skipping occurs because you also have to train them to use the ailerons. I'll suggest this just happens at different times.
 
Clark1961 said:
The CFI said "very nice" and I was like "WHAT????? the plane did it" and the aircraft does have a pivot point with one wheel still on the ground in a proper crosswind take-off. Of course it is up to the pilot to catch it at the correct moment.
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Well there is weathervaning at low airspeeds with wheels connected to the runway. With proper xwind technique that pivot point should be applying very little force (tire and bearing friction) at takeoff speed.

Once actually airborne that pivot point is gone and the airplane no longer cares that there is a wind. It can't automatically establish the crab once airborne because "crab" is a ground reference concept and airborne aircraft doesn't know there is a ground. If you level the wings it will happily fly runway heading and not maintain the upwind course at all.

I think it seems like the wind is doing it because the wings are already banked into the wind but the nose is being held straight initially by friction with the runway. As soon as the nose lifts, you have a bank but no opposite rudder pressure to keep the nose pointed down the runway so naturally the airplane turns into the wind as a result of the bank (OK horizontal component of lift, etc) not as a result of weathervaning.
 
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petrolero said:
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I think it seems like the wind is doing it because the wings are already banked into the wind but the nose is being held straight initially by friction with the runway. As soon as the nose lifts, you have a bank but no opposite rudder pressure to keep the nose pointed down the runway so naturally the airplane turns into the wind as a result of the bank (OK horizontal component of lift, etc) not as a result of weathervaning.
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Don't know what you mean by "etc." but the horizontal component of lift alone can never turn an airplane - the nose would remain pointing straight ahead while the airplane flew forward at an angle to original flight path, basically a forward slip. I know this goes against how the subject is taught but, from a physics standpoint, there is no arguing against the fact that weathervaning is what turns an airplane. If that were not the case, we would not need all that surface area in the back and we would have more useful load.
 
alfadog said:
Don't know what you mean by "etc." but the horizontal component of lift alone can never turn an airplane - the nose would remain pointing straight ahead while the airplane flew forward at an angle to original flight path, basically a forward slip. I know this goes against how the subject is taught but, from a physics standpoint, there is no arguing against the fact that weathervaning is what turns an airplane. If that were not the case, we would not need all that surface area in the back and we would have more useful load.
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I've already presented the argument against weathervaning. Please show me a text that says weathervaning is what turns aircraft. :lol:

What you describe above, where the airplane continues straight, requires opposite rudder. Fact is, if the airplane is in a bank and the pilot is climbing, as in a takeoff, there is a horizontal component of lift that turns the airplane. I'd draw you a freebody diagram, but there is already one in your basic flight manual.

Ahh heck here it is from page 3-8 of the AFH:

Page 3-8 of the Airplane Flying Handbook said:
When an airplane is flying straight and level, the total lift is acting perpendicular to the wings and to the Earth. As the airplane is banked into a turn, the lift then becomes the resultant of two components. One, the vertical lift component, continues to act perpendicular to the Earth and opposes gravity. Second, the horizontal lift compo-nent (centripetal) acts parallel to the Earth's surface and opposes inertia (apparent centrifugal force). These two lift components act at right angles to each other, causing the resultant total lifting force to act perpendicular to the banked wing of the airplane. It is the horizontal lift com-ponent that actually turns the airplane—not the rudder.
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But maybe you are just trolling. :dunno:

K. After reading your post above, which i had missed, I am now pretty sure you are pulling a CTLSi. Whatevs. Physics overrules superstition.
 
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petrolero said:
I've already presented the argument against weathervaning. Please show me a text that says weathervaning is what turns aircraft. :lol:

What you describe above, where the airplane continues straight, requires opposite rudder. Fact is, if the airplane is in a bank and the pilot is climbing, as in a takeoff, there is a horizontal component of lift that turns the airplane. I'd draw you a freebody diagram, but there is already one in your basic flight manual.

Ahh heck here it is from page 3-8 of the AFH:
r
But maybe you are just trolling. :dunno:

K. After reading your post above, which i had missed, I am now pretty sure you are pulling a CTLSi. Whatevs. Physics overrules superstition.
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I am afraid that it is you that needs to brush up on physics, no insult intended. Specifically, the difference between rotation and translation.

Here is something to chew on:

http://www.av8n.com/how/htm/yaw.html#sec-roll-rudder

8.9 Rudder Usage During Rolls

Turning the airplane properly requires coordinated use of ailerons and rudder. Getting it exactly right is a bit tricky.

Remember that in an airplane, the direction you are moving is not necessarily the same as the direction you are pointing. There are several crucial things that happen during a turn:

1) You use the wings to change the direction you are going, i.e. to re-orient your momentum vector. I call this the MV-turn.

2a) You use the rudder to change your heading (i.e. to overcome yaw-wise inertia, i.e. to provide yaw-wise acceleration).

...

Item 2a is important because if the airplane didn’t have any vertical tail, banking would cause it to just slip off in the new direction without changing its heading. It is much nicer to yaw the plane to align its axis with the new direction of motion, so you apply the rudder, thereby creating a yaw rate that matches the MV-turn rate.6
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To review, yaw is rotation about the vertical axis. When that rotation is caused by the relative wind hitting the side of the copper rooster or the side of an airplane, it could be called weathervaning. Since we do not need to add rudder to to create yaw rate during a turn, it is clear the weathervaning adds the rotation to the translation.
 
Another idea while I work on my Mythbusters demo...

I know a lot of pilots here have GoPros or equivalent.

How about taking one along on the next crosswindy day?

If one can do so safely, rotate firmly on runway heading, making an effort to hold the wings level and just enough rudder to compensate for normal left turning tendencies. Then make a note of whether the crosswind "weathervanes" the plane into the wind right after takeoff, and if so, how many degrees.

My Sky Arrow was down for quite some time awaiting ignition modules, and the day I got it flying again they closed my airport for over a week for resurfacing. Opening this morning and if I can find an appropriate crosswind and the time I'll try it myself.

If it can be demonstrated repeatedly and clearly, I'll be happy to stipulate to a real phenomenon I just somehow missed.
 
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