gboulton
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Joined: 5/28/2005
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From: La Vergne,
TN, USA
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DISCLAIMER : Everything that follows is based upon the presumption that we're dealing with a constant crosswind, in both speed and direction. Since this is rarely the case, please treat the following more as "insight" than 'step by step directions" or "predictions of actual behaviour"
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Blueapplepaste has given a good answer above for the wing lifting question you asked about originally. We'll leave that one alone, and address your 2nd question:
quote:
And one more question, during the final approach, how do you crab the airplane?
The response is : Why do you want to?
This isn't to say that there's not good reasons for doing so...just be aware of what they are...they likely AREN'T what you think.
The reality is that the airplane has absolutely no idea there's any wind. It doesn't "feel a push", or "feel the wind" or anything of the sort. Aside from the obvious fact that it's an inanimate object, that statement would be true even if it were a living, breathing creature.
There simply is NO wind from the AIRPLANE'S point of view.
All it cares about, above and beyond anything else, is the speed of the air over its wings...its airspeed. It has no way of indicating, and no reason to CARE about, its relationship to fixed objects (like YOU) on the ground.
What all this means is this:
Flying an approach at 20 mph, heading due west, with 0 wind, and an approach at 20mph, heading due west, with a 10mph wind out of the north is, TO THE AIRPLANE, precisely the same approach. All it knows is that it is moving forward, in a body of air, at 20mph.
What YOU, the pilot, recognize is that the body of air in which it resides is ALSO moving...relative to you, the trees, the runway, etc...10mph from north to south. It is, obviously carrying the airplane right the heck along with it.
Understanding that difference...between ground track and path through the air, is CRITICAL to understanding the whole idea of 'crabbing' into a cross wind.
The short answer is, if your intent is, as it usually is when pilots ask this question, to line up on the runway...you don't need to "crab" at all.
"Crabbing" the airplane...that is, applying rudder to yaw the nose "into" the wind, has exactly the effect we DON'T want. What we've got, very simply, is an airplane that...WITH RESPECT TO THE AIR (remember...the airplane couldn't care less about the ground until it's on it)...is not following its nose. It's quite literally flying slightly sideways! This uncordinates the airplane, which has 2 pretty scary consequences:
1) We increase drag, thus slowing the aircraft.
2) We're uncoordinated when/if we stall, almost ENSURING a spin.
Now...all that theory is nice...but will it accomplish our purpose...lining up on the runway?
Sure..if done 'correctly'. But it's something like using lateral force and a parking break to slide a car around a corner. Sure..it'll work...(oddly for many of the same reasons *heh*) but there are certainly safer, more reliable, and more controllable ways of navigating a turn.
The answer is to TURN the airplane...not yaw it with rudder only, but TURN it...with both ailerons and rudder in a coordinated fashion as always...to "face" the wind. In other words, you want an airplane that...again, with respect to the AIR...is moving forward, directly behind its nose...and we're letting the movement of its pocket of air bring the GROUND track in line with the runway. We're left with an airplane that has kept its airspeed, maintained the full lift capabilities of its wings, AND will behave more predictably if we stall it.
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Having said all of that, IS there a time to "crab" the airplane, in the classical sense of what pilots mean when they use the term?
Absolutely. It's called a slip. And it's a FANTASTIC way to lose altitude in a hurry WITHOUT gaining airspeed.
That's a GREAT trick, btw, for deadsticks. You're up high, but not high enough to go around the field. Problem : You point the nose down to get to the runway, but the airplane picks up speed, and zooms right on past the runway, out into the grass, over the fence, through the trees, and off to grandma's house.
Next time it happens, try a slip...or a "crab" if you prefer.
Kick the rudder over (doesn't matter which way, whatever you're comfortable with), and apply enough OPPOSITE aileron (left rudder, right aileron) to keep the wings level. On some planes, a bit of down elevator may be required to keep the nose down enough to stay flying.
The airplane...now flying QUITE sideways, will drop like the proverbial rock. The beauty of it is, however, that the speed of the air OVER THE WINGS (the only airspeed that matters) won't climb rapidly (if done correctly, it won't climb at ALL). The airplane will come right down to a manageable altitude, take the slip out of it, and land as normal.
Again...can this trick be used to land crosswind? Absolutely. Lots of us use slips in full scale crosswind landings all the time. Personally, I find it an easier approach in full scale...i can keep my eye on the runway the entire time, instead of trying to "predict" the drift of my ground track. Heck, I've been known to intentionally fly a high approach with NO x-wind, JUST so I can throw a slip in on final...i think they're fun! 
Remember, however, there's a few distinct differences between full scale and RC:
1) I'm IN the 100%er...looking down at the runway...not on the runway, looking up at the airplane.
2) I've got all kinds of instruments to TELL me my airspeed, my sink rate, how uncoordinated I am, etc etc...in RC, we have none of that...no way to know any of those things except
by guessing.
3) In a full-scale, I'll FEEL the impending stall WAY before it shows up...with RC, the stall's already HAPPENED before we SEE it.
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Note in all of this...i never said "don't crab", or "don't slip" or whatever. I just shared what i teach all my students...that the normal concept of 'crabbing' is NOT what we often think it is, or at least doesn't DO what we think it does.
It's certainly not "wrong" to manage x-wind landings in EITHER fashion...both have their merits and their challenges.
It's simply best to understand exactly what the airplane, its body of air, and the runway are actually DOING with respect to each other, so you're prepared for the potential challenges of each.
< Message edited by gboulton -- 7/21/2009 5:18 AM >
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Crosswind landings - 
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