Elevator Position During Tailwind Taxi

[Realms09]

It seems the textbook recommendation during this regime is to move the elevator down (stick forward) to expose a surface such that the wind acts to keep the tail down. Why is this case? Is not the elevator more under the influence of the propeller slipstream than a tailwind for at least a certain range of wind speeds?

 

[MusketeerMan]

I don't see propeller slipstream being much of a factor during a taxi...

 

[tgrayson]

It seems the textbook recommendation during this regime is to move the elevator down (stick forward) to expose a surface such that the wind acts to keep the tail down. Why is this case? Is not the elevator more under the influence of the propeller slipstream than a tailwind for at least a certain range of wind speeds?

Good question. Let's play with it. Using the following formula

Induced velocity = sqrt(T/(2*A*Rho))

​

from Airplane Aerodynamics and Performance, by Jan Roskam, with the values of

T = 100 lbs (Thrust, this is high, I suspect)
A = 28.27 (Area of prop arc)
Rho = .002377 (air density)


I come up with an induced velocity of 27.3 fps, or 16.2 knots.

If the math is accurate (suggest you check it), then any tailwind would have to be over 16 knots to be felt by the airplane, and much greater to be a significant risk.

If I use a more reasonable number for taxi thrust, say, 50 lbs, the induced velocity goes down to 11.4 knots.

 

[DE727UPS]

Sometimes I'm glad I'm not smart.

 

[darrenf]

Good question. Let's play with it. Using the following formula

Induced velocity = sqrt(T/(2*A*Rho))​

from Airplane Aerodynamics and Performance, by Jan Roskam, with the values of

T = 100 lbs (Thrust, this is high, I suspect)
A = 28.27 (Area of prop arc)
Rho = .002377 (air density)


I come up with an induced velocity of 27.3 fps, or 16.2 knots.

If the math is accurate (suggest you check it), then any tailwind would have to be over 16 knots to be felt by the airplane, and much greater to be a significant risk.

If I use a more reasonable number for taxi thrust, say, 50 lbs, the induced velocity goes down to 11.4 knots.

What about T-Tails Mr. Smartypants?? :nana2:

 

[Foxcow]

Sometimes I'm glad I'm not smart.

Same here, I never explained it that way to my tailwheel students. Speaking of that, is the Luscombe still accumulating dust?

 

[tgrayson]

What about T-Tails Mr. Smartypants??

Excellent point!

 

[tgrayson]

Same here, I never explained it that way to my tailwheel students. Speaking of that, is the Luscombe still accumulating dust?

The issue seems relevant. Most of the experienced tailwheel pilots I've talked with emphasize the stick should be in the gut while taxiing, regardless of head- or tailwind. This does not correspond to the recommended control deflection for tailwheels. Their reasoning is that the propwash is dominant over the tailwind; also, if the tail starts to rise, a quick burst of power can fix that.