Posted a new article for student pilots, titled "Remembering Correct X-Wind Taxi Technique"
Remembering Correct X-Wind Taxi Technique
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Greg Bockelman
FWIW, I think maybe a short Bio would be good on your site. As it stands now, I wonder about credibility. Although the material is good, I like to know about who is writing it.
dmccormack
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Not quite -- if the wind is 90 degrees right, for example, you put in full crosswind correction on taxi, and expect to do the same early on the takeoff roll and towards the end of the landing roll.
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Which direction? Right or left? What if the winds are variable?
Reality is, VERY SELDOM will the wind be straight off a wingtip. When it is, I suppose best guess as to where to place the ailerons.
Well, yeah.
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Dan,
I certainly do that on early takeoff and late landing roll, and I used to do that on taxi, until I read somewhere (now I gotta go research where) that for exact sideways wind on taxi, neutral aileron is recommended.
Do post if you find some reference -thanks!
Greg,
I have clarified on this forum earlier in answer to a direct question that I am a student pilot yet to solo. I am no instructor, and I have a disclaimer on my site saying that I do not intend to provide flight instruction. My site is meant for student pilots like me, who can learn from the same things that I have learned.
If I had a bio worth putting up on my site, I would have.
That being said, why do you worry about credibility when you say the material is good?
- No offense, I asked that in jest!
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EdFred
Taxi to Parking
I've found that generally, if you need left crosswind correction on takeoff (aileron deflected up), you'll taxi to/from the runway with the ailerons in that position the entire time.
dmccormack
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I guess I don't understand your question, but my point is that a 90 degree crosswind isn't a no deflection mode (as mentioned in the article) -- it's full deflection to keep the unwind side down.
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I guess I missed that thread.
Ok. Haven't found it yet.
Nothing wrong with that.
Well, you do, actually. Just because you don't have a lot of letters behind your certificate does not mean you have nothing to "brag" about.
why do you worry about credibility when you say the material is good?
Because when you DO make a mis step and post something on your site that is totally out in left field, I will know that you are a SP and take it in that context.
Understood. :smile:
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Greg Bockelman
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Ok, but if the wind is variable, slightly fore to slightly aft of direct crosswind, how do you deal with that? Because if the wind is slightly in FRONT of the right wingtip, you would want a different correction than if it was slightly AFT of the right wingtip. What do you do? Flop the ailerons back and forth?
In strictly a theoretical 90 degree crosswind, how will the aileron deflection make a difference at taxi speeds? The wind is near parallel to the control surface.
If I look at the diagrams in the POH, they seem to leave the 90 degree crosswind situation not marked, like it is one that requires no elevator/aileron deflection from neutral.
I agree that it seems to be a logical extension of the starting takeoff roll/ending ground roll situation, but would it make any difference at the speed of "brisk walk"?
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EdFred
Taxi to Parking
Dan Thomas
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No?
The airplane responds to air over its surfaces, even when sitting still. A strong wind can lift a wing (or the tail) even if the airplane isn't moving across the ground. So you must fly the airplane ALL the time or it will eventually bite you unless you only fly in light or no winds. Assuming that control deflection is necessary only when taking off or landing could be dangerous.
And taxiing too fast in windy conditions is asking for trouble, too, especially in taildraggers. For instance, one of the more common bangups involves backtracking the runway too fast downwind, then when turning around the centrifugal force combines with the wind against the side of the airplane. The upwind wing (which is inside the turn) lifts, the wind gets under both it and the stab/elevator, and the downwind wing hits the runway and starts to drag. That drag plus the wind under the tail makes the tail come up and the airplane goes up onto its nose and maybe right over onto its back. It has happened here and it isn't cheap to fix.
Taxiing too fast on slippery surfaces is dumb in any airplane, windy or not.
Dan
dmccormack
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The only time you will have an exact 90 degree crosswind is if you're not moving. Assuming a brisk walk taxi -- what happens to the wind vector now?
This is very theoretical for heavy nosewheel airplanes -- not so much with light taildraggers.
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He was being facetious.
Doggtyred
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Greg,
That being said, why do you worry about credibility when you say the material is good?
Because people have a nasty habit of believing everything they read on the internet.
Its helpful to know if the source is speaking with authority, or simply parroting what may be good (or not good) advice. Authority and Experience give you the background from which to critique the material that you glean your information from.
I don't think anything happens to the wind vector. With respect to the center of gravity of the airplane, it stays where it was. The reason is that the wind vector is applied over a much wider front than the length of the airplane, for any relative change in position of the airplane while taxiing to be a factor. Essentially, the length of the airplane is infinitesimal while the slope and width of the wind front are both infinite (straight line parallel to airplane's longitudinal axis - tan 90). So the moving airplane (Vector-A) can and will remain at 90 degrees to the wind (Vector W, which represents wind direction and hence is at 90 degrees to the slope of the wind front) over taxi distances. One can visualize this as the longitudinal axis of the plane being parallel to the wind front, or, in other words, perpendicular to the wind direction.
Vector A and Vector W will add to produce resultant vector which would tend to make the airplane go in a diagonal path, but this resultant will be balanced by the friction vector from the wheels on concrete in normal circumstances; hence the airplane will move straight ahead and not crab.
However, whether the plane crabs or not, as long as its nose is not allowed to yaw, the airplane's side will continue to present a zero degree front to the relatively wide wind front, or 90 degrees to the wind direction.
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I think you lost us with all that vector stuff.
The point is this: A plane standing still and a wind blowing straight in from the wingtip, the wind is 90 degrees to the longitudinal axis of the airplane. If the airplane starts taxiing, the wind is from the same direction, but the "RELATIVE" wind is actually some degrees in front of the wing, or something LESS than 90 degrees. In other words, a quartering headwind of some degree.
BUT, if the wind is X degrees ahead of the wing to begin with, as you start to taxi, the relative wind moves back to 90 degrees to the longitudinal axis.
The reality is this is more a theoretical mental exercise rather than practical, because how often is the wind EXACTLY 90 degrees to the longitudinal axis?
mantakos
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If your nose is pointed 360, and you're standing still, and you have a 20kt left crosswind, from 270, then the wind is obviously at a 90 degree angle to your nose, and it isn't clear what purpose deflecting the ailerons in either direction will hold, as the wind will flow across them side-to-side.
But if you begin your takeoff roll on runway 36, once you reach a 20kt forward speed, the air flowing over your wings will appear to be a 28kt, 45 degree left-quartering headwind, it will appear to come from 315. Clearly, in this scenario, the ailerons should be deflected to the left.
-harry
I am sorry about the vectors, but please break down the scenario into infinitesimal time slots t0, t1, t2... tn separated by arbitrary distances d0, d1, d2... d(n-1), the sum of d1 through d(n-1) being much much less than the width of the wind front, which for argument's sake say is 2 miles.
Now draw the vectors for each position marked by t. Since the wind front is 2 miles long and the airplane is only 26+ feet, you will find that your diagrams are identical over any distance less than or equal to the width of the front. The airplane will be at 90 degrees to the wind in all your diagrams.
This is probably worse than the vectors... I promise to argue no more.
Now draw the vectors for each position marked by t. Since the wind front is 2 miles long and the airplane is only 26+ feet, you will find that your diagrams are identical over any distance less than or equal to the width of the front. The airplane will be at 90 degrees to the wind in all your diagrams.
This is probably worse than the vectors... I promise to argue no more.
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mantakos
Final Approach
Okay, you're sitting in a car, the car is pointed North, your arm is extended out the passenger's side window, holding a little flag. The car is parked. There's a 5mph breeze coming in from the East, so your flag is flying to the West.
Now the car pulls away, and drives onto the autobahn, still headed North. You still have that 5 mph breeze coming at the car from the side, but you're now driving forward at 180 mph. What's your flag doing?
-harry
You generated wind (relative to the car)while you drove the car. You do not generate significant wind at taxi speeds so there is no additional wind vector to introduce.
The situation is quite different during takeoff roll. Not only are you generating an additional wind vector, but the airplane is producing lift, so that the resultant vector of the crosswind and the airplane's movement is less and less balanced by friction between the wheels and concrete, and the airplane will crab and slide off the runway unless you apply aileron correction.
The situation is quite different during takeoff roll. Not only are you generating an additional wind vector, but the airplane is producing lift, so that the resultant vector of the crosswind and the airplane's movement is less and less balanced by friction between the wheels and concrete, and the airplane will crab and slide off the runway unless you apply aileron correction.
mantakos
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If you have a 10kt wind from the West, and you're taxiing at 5kts along a taxiway to the North, how should you hold your ailerons?
-harry
jesse
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If the airplane crabs far enough into the wind you will not slide off the runway. On an icy runway, you might find yourself in a situation where the only way to remain on the runway is by crabbing into the wind.
Neutral, per POH.
Clark1961
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Nope. The effective crosswind is forward quarter so the controls are held into the wind.
My question is why would a student pilot fight so hard against those who actually fly taildraggers????? The world wonders????????????
Just to clarify (hopefully), the term "relative wind" is what is normally used for what you are referring to as effective wind. In other words, it's what the air mass is doing relative to the airplane that counts.
It's the velocity of the air mass relative to the airplane that counts. That's the only thing that determines how the air interacts with the position of the flight controls. The velocity of the air relative to the ground is irrelevant.
Suppose there were no wind at all. When the airplane moves forward at five knots across the ground, the air mass can then be thought of as moving relative to the airplane in the opposite direction at five knots. From the point of view the airplane, the air mass is moving from the nose toward the tail at five knots. That's called relative wind.
It's valid to think of it that way because of Einstein's principle of special relativity, which says that the laws of physics are the same for all uniformly moving reference frames. His Theory of Special Relativity is based on it, and has been well confirmed over the past 100 years.
With a 10 knot 90 degree crosswind, when the airplane taxis at five knots, the velocity of the air relative to the airplane is now the vector sum of a five knot vector from the nose to the tail and the 10 knot vector from the side.
By the way, which POH says to hold ailerons neutral with a 90 degree crosswind?
dmccormack
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Student pilots taxi at a crawl (5-7 MPH) -- with good reason.
Once you get sufficient experience, you will taxi a bit faster (15 mph) on the long trip out to the runway.
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dmccormack
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Sorta....
The up aileron on the windward side achieves two things -- keeps the upwind wing down and causes yaw.
In lightweight taildraggers, the airplane wants to weathervane into the wind as soon as the tail lifts (which is quite early, unless you hold the tail down).
In a stiff crosswind you want to upwind wing to stay down as the downwind wing lifts -- the wheel on one side will be airborne before the other, and that's fine.
IF you're depending on wheel friction to keep you tracking straight, you're flying a Cherokee.
dmccormack
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Too much book learnin'...
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Uhh, tend to disagree with that. If you are moving, you are at least changing the relative direction of the wind regardless of how insignificant it is. In fact, the slower the wind velocity, the more the relative direction changes.
Which is reducing the relative crosswind component the faster the plane is going.
In an extreme case, that is true. Well, theoretically in all crosswind cases that is true, however in most crosswind situations, the airplane will be airborne before that becomes an issue. Might be ugly, but...
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Wrong answer. Sorry.
dmccormack
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I think he's deriving his answer from the ubiquitous Cessna crosswind taxi diagram...
rottydaddy
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Forgive me if I'm wrong, but I gather from this kind of description that you have never taxiied an airplane in conditions where control surface position was actually critical to safety.
There was a time when I used to fret and sweat and over-analyze this problem, until I actually found myself taxiing in wind that threatened to tip the airplane over.
That teaches you what you need to know: that is, to simply do what it takes to keep all three wheels on the ground. Start moving the controls in such wind and you will very quickly discover what it takes.
Vectors, infinitesimal time slots, etc. are the very last things you should be thinking about when taxiing an airplane, IMHO.
FWIW, there is a blanket rule that I was told about taxiing taildraggers (NOT during landing/takeoff roll, but taxiing; I repeat: NOT during takeoff/landing roll, but when taxiing!!!!)... it also applies pretty well with nosedraggers, although elevator position is not as critical with those.
It is this:
"Let the wind blow the stick". In other words, once you know "whence the wind" you pretend the wind is blowing against your hand, and thus moving the stick where it wants to be, for elevator and aileron position.
You can apply this to a yoke, as well... and it works, as a general rule.
Naturally, one does not want to show the underside of the upwind aileron to the wind, so in the case of a quartering headwind, you would not let the wind "blow" your hand all the way over to the downwind side; might even want to "turn into" the wind, as they say.
But even these simple rules-of-thumb seem like empty thought exercises after you find yourself really "flying" a plane on the ground... taxiing in a good stiff breeze, especially in a taildragger, shows you that the questions of vectors and time increments are best answered by intutition and "feel", not by formulae. "Let the wind blow the stick" or "dive away/turn into" are merely starting points... only by practice can we determine "how much" and for "how long". Just like making a good x-wind landing... there's no standard formula, and one has to remain flexible.
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Matthew
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I don't think about it much anymore, it's pretty automatic. But, as a student, the "dive-away/climb-into" reminder seemed to work well enough, even though the books say neutral elevator with a headwind in a nose-gear.
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I see the "Let the wind blow the stick" was posted just prior to my post. I hadn't heard it put that way before.
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I see the "Let the wind blow the stick" was posted just prior to my post. I hadn't heard it put that way before.
dmccormack
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Elevator neutral hardly ever works in a tailwheel unless you want it light (for sharp turns, for example)
95% of the time, stick/yoke is al the way back while taxiing.
Rottydadddy: exactly -- vectors and such are interesting, but when the wind's blowing, you need to keep that wing down.
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