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Low speed airfoil and relative wind.

Low speed airfoil and relative wind.

I'm looking for clarification about low speed flight (<60kts)and relative wind in real world conditions. My background is sailing so it's difficult to transfer all that information into an object with no contact to water or ground. We were taught that the relative wind or air flow over an airfoil is always in the opposite direction of flight, it was implied that this meant the exact direction air would take over the wing would always be the same when in level flight even if the aircraft was perpendicular to a severe cross wind. My thought is that since an aircraft is not a gas molecule, it would not behave like one and travel precisely with the air column (drag, mass, bow shock etc). I've also noticed a tendency flying RC planes that they seem more prone to dipping a wing tip unexpectedly when landing in a cross wind (I always interpreted as a small gust, instead of causing lift if heading straight into the wind, more greatly disturbing the direction of airflow over the wing). Is the direction of travel over the wing not skewed a little in the direction of the true wind if the aircraft is flying perpendicular to it, even if only a few degrees? Thanks in advance.

RE: Low speed airfoil and relative wind.

At low speed, you have to 'crab' the plane to take account of strong cross wind i.e. the direction of flight does not match the direction of the fuselage.

Then as your wheels hit the runway you have to pull it back around quickly.

This is one of the things that makes landing a plan kind of tricky.

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RE: Low speed airfoil and relative wind.

Thank KENAT, I've seen some interesting videos of large (747-ish) aircraft make those maneuvers in a high crosswind, certainly looks like white knuckle flying.

I guess my question is more about the fluid dynamics of the wing interacting with the air column, in straight and level flight. Would the direction of airflow be bent in the direction of the true wind (x, z axis, not related to angle of attack), or is there some other force acting on an aircraft in a crosswind that causes a change in behaviour? (e.g. gust direction, unintentional crabbing, etc)

RE: Low speed airfoil and relative wind.

A wing is only affected by how it moves relative to the air around it. A steady wind is no different than still air as far as the wing is concerned. The wind will affect the ground speed/direction of a plane but not the air speed. So, a 10 mph wind will simply add 10 mph to the ground speed of the plane, in the direction of the wind.
Gusts are a different matter due to the inertia of the plane, and there will be momentary changes in air speed/direction.

RE: Low speed airfoil and relative wind.

for a cross-wind, consider the component of wind along the flight-path as being useful (in creating lift) and the component across the flight path as being a nuisance/hazard (creating difficulties for the pilot). Will the airflow at the wing really have a spanwise component? yes, from sweep; possibly from remote wind direction.

Remember truisms like the one you've quoted are mostly true most of the time, possibly completely true some of the time, but probably never completely true all of the time.

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RE: Low speed airfoil and relative wind.

Crosswinds are problematic because the fuselage partly shields one wing from the crosswind, so the lift on the wings would be different.

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RE: Low speed airfoil and relative wind.

IRstuff, that can only happen when the plane's wheels are on the ground. In the air, a plane always points into the "wind" (airflow). In flight, if the wind changes the pilot will not even feel it except that the ground path will change. The same istrue for a boat moving through a cross current. Rudder can be used to cause only a little side slip, where the plane is not pointed straight into the airflow.

RE: Low speed airfoil and relative wind.

This problem has vexed every student pilot from the days of the Wright brothers. The early pioneers did not even bother with it.
They just used a square field and landed directly into the wind. As aircraft landing speeds got higher and the takeoff and landing distances increased, the cost of real estate started playing a part in the equation. The result was techniques had to be evolved to land in less than ideal landing sites. The two most basic methods are the lowered wing side slip where the aircraft is deliberately cross controlled/ yawed to fly in a diagonal flight path with the amount of the sideslip corresponding to the amount of the crosswind. This also reduces the available lift on the downwind wing because of the shading effect of the fuselage, and increases the drag, this results in a steeper glide path, which can be desirable.
Or the crabbing method where the aircraft is pointed at an angle to the runway with the wings level, and requires the pilot at the moment of touchdown to push " Downwind" rudder to kick the aircraft straight to the desired direction of the runway. Because of the “shading effect of the fuselage, strong aileron deflections are required to prevent the “Upwind” wing from being lifted by the crosswind.
To cope with this problem some large aircraft are fitted with “crosswind landing gear” this is an undercarriage which castors to allow the aircraft to land diagonally. Other aircraft like the Ercoupe which could not be cross controlled just had an over designed landing gear that allowed the aircraft to touch down diagonally without ill effect. Gusting conditions often make the transition from air to ground tricky by lifting the aircraft or dropping it before the pilot is ready. I am a licensed pilot so I know about a lot of this first hand.

You are judged not by what you know, but by what you can do.

RE: Low speed airfoil and relative wind.

This doesn't answer the question from OP directly, but it is a fantastic resource for private pilot related subject matter.

More specifically about maneuvers (such as crosswind landings and slow flight), the FAA manual is available, free, and goes into sufficient detail to answer most questions:

I have a bunch of free training links I've accumulated over the years (was a CFI/CFII for a long time ... tried to save my students money instead of buying books). If you want to chat more, please feel free to PM me.


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