Airplane Spins: why does the low wing have a greater angle of attack?

Question

Hi, I'm a CFI candidate, and can't figure this out, let alone explain it: why does the low wing have a greater angle of attack? That is, why isn't the relative wind the same on both wings? I need a simple way to picture this. Thanks!

Answer 1

Aaron has explained it well and is spot on .

Bravo Zulu....

Answer 2

The inner wing has greater angle of attack - because:

As the airplane autorotates in the spin, the "outside" wing travels a greater distance than the inner wing. Greater distance over time = higher TAS.

Put a higher TAS into the lift equation and you will get a smaller "alpha" or angle of attack.

I think you're hanging up on the relative wind, my advice is to try to think about it my way, if you want to answer your original question.

For your checkride, if the DE or whoever asks you about spins, use the terms "autorotation", "uncoordinated" "partial stall" and you'll be fine.

Spin aerodynamics are relatively complicated and you should concentrate on the practical aspects for your CFI ride.

If the DE wants to stump you, he will... don't freak out about memorizing every detail in the aviation world to try to prevent stumpage.

You can PM me if you want.

Answer 3

The outer wing travels in a larger circle over the same period of time, meaning that it is moving faster.

More airflow over the wing means it develops more lift.

Because the aircraft in a spin is uncoordinated, the aircraft is actually yawed one direction or the other.  This yaw, combined with the nearly vertical flight path of the aircraft (as the spin develops more fully) results in the lower wing's angle of attack being a little higher than the upper wing's.  This is both the cause of and the result of entering into a spin.

It is difficult to describe this in text form, but there is actually a great picture of it on page 5-101 in the Jeppesen Flight Instructor's Manual if you have that book.

Edit:  Trying to add clarification

Try to visualize where the relative wind is originating from.  The wing obviously does not "twist" itself into a higher AoA, so the increase in AoA stems from a difference in the relative wind rather than the physical position of the wing.

Answer 4

The "inner wing"--not the "low wing."  

The inner wing and outer wing have about the same sink rate, but different airspeeds.  And different angles of attack.

Draw the angle of attack diagrams for two wingtip airfoils.  Same sink rate.  But one is going forward through the air at 40 knots, and one is going forward through the air at 30 knots.  Compare the angles of attack.