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Flying in the relative wind


Ed Cesnalis

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In the autopilot thread it was said that there was a 30kt crosswind that was responsible for torquing the AP servos.

 

I don't think we have had the downwind turn argument either, since the concept is the same maybe we can get both issues resolved.

 

How do you best articulate the concept of flying in the relative wind.  How do you explain that you could be cruising (crabbing) along with a 30kt crosswind and yet the ball is centered indicating that the wind is on the plane's nose?

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As mentioned in my prior "Stick and Rudder" thread, in a crosswind the autopilot behaves exactly as it does in a no wind situation to maintain straight and level flight.

 

I think what you're bringing up is like when my Cirrus demo pilot suggested the plane was flying right wing low due to the right crosswind.

 

Best way to articulate it is, barring gusts, there is no wind acting on an aircraft in flight - it is simply moving through a parcel of air which is itself moving.

 

Regardless of the crosswind, an autopilot will simply fly straight and level on whatever heading is necessary so the resultant ground path, allowing for the movement of the air parcel, is the desired course.

 

Is that where you're going with this?

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A much much better way to simplify imagining how wind acts on an aircraft, is to think in terms of the air sitting still, and instead the ground is moving beneath it.

 

It's not a foolproof thought, but for basic understanding of how consistent wind velocity acts on an aircraft, it works well.

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As mentioned in my prior "Stick and Rudder" thread, in a crosswind the autopilot behaves exactly as it does in a no wind situation to maintain straight and level flight.

 

I think what you're bringing up is like when my Cirrus demo pilot suggested the plane was flying right wing low due to the right crosswind.

 

Best way to articulate it is, barring gusts, there is no wind acting on an aircraft in flight - it is simply moving through a parcel of air which is itself moving.

 

Regardless of the crosswind, an autopilot will simply fly straight and level on whatever heading is necessary so the resultant ground path, allowing for the movement of the air parcel, is the desired course.

 

Is that where you're going with this?

 

Thanks Eddie, flying in the relative wind is a point where we lose certain people every time.  I wasn't getting at your instructor explaining the low wing but that is one more to add to the list.  I think the wing was low because the rudder trim wasn't set for strait and level.

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A much much better way to simplify imagining how wind acts on an aircraft, is to think in terms of the air sitting still, and instead the ground is moving beneath it.

 

It's not a foolproof thought, but for basic understanding of how consistent wind velocity acts on an aircraft, it works well.

 

That's good, thanks

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With the many left turning tendencies of our aircraft in flight I would think the autopilot will fly slightly right wing down to maintain a specific heading in order to maintain ground track on a course. It has nothing to do with crosswind only the aerodynamic effects.  One of the UAV ' S I fly with a sophisticated autopilot will do a nice left circle with a couple mile radius when commanded wings level and coordinated.  In requires a 2-3 degree right roll to maintain a heading.  Its a twin centerline thrust counter rotating,  but that doesn't stop P-factor.  Those tendencies can be built out with some tweaking of incidence etc.

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With the many left turning tendencies of our aircraft in flight I would think the autopilot will fly slightly right wing down to maintain a specific heading in order to maintain ground track on a course. It has nothing to do with crosswind only the aerodynamic effects.  One of the UAV ' S I fly with a sophisticated autopilot will do a nice left circle with a couple mile radius when commanded wings level and coordinated.  In requires a 2-3 degree right roll to maintain a heading.  Its a twin centerline thrust counter rotating,  but that doesn't stop P-factor.  Those tendencies can be built out with some tweaking of incidence etc.

A properly designed plane will have everything in balance at normal cruise - any right wing down flight at cruise would indicate something wrong.

 

P-factor should also not be a factor in normal cruise, where the prop disc should be meeting the air perpendicularly.

 

Spiralling slipstream is always there, but the vertical stabilizer normally had a tiny bit of offset to counter that, again at cruise.

 

Or at least that's been both my understanding and experience.

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