In-Spin Aileron
 

Simple quesiton...

In aircraft I am familiar with into spin aileron increases the rate of rotation. Why?

Is this the same for all aircraft? Looking for an aerodynamic explanation but please feel free to post aircraft/situations where something different is true.

It comes down to the ratio of pitching to rolling inertias B/A. The yawing inertia is the largest – approximately being defined by C~B+A, and so its actual value tends not be significant in determining the spinning characteristics. The two smaller rotational inertia values, B and A, and specifically their ratio, tends to define the spin mode and the response to roll (aileron) input during the spin.

When B/A<1, the aircraft is often referred to as “wing dominated” which tends to imply a spin-prone aircraft. When B/A>1, the aircraft can be referred to as “fuselage dominated” or “pitch dominated”, which tends to imply spin-resistance. So, biplanes or aircraft with wing-mounted engines will tend to be more spin-prone than monoplanes or aircraft with fuselage mounted engines; although this is a trend and not an absolute rule.

Ailerons act in the normal sense during the spin. So, if the aircraft is wing-dominant, in-spin aileron will tend to cause the roll to couple with yaw, increasing the spin rate (the Scottish Aviation Bulldog is well known for this, but far from alone), also tending to flatten the spin. The converse will also be true, so:

Fuselage dominant aircraft
in-spin aileron tends to reduce spin rate
out-spin aileron increases spin rate and flattens the spin.

Wing dominant aircraft
in-spin aileron tends to increase spin rate and flatten spin.
out-spin aileron tends to reduce spin rate.


Of-course, this is all gross generalisation, and I'm sure with a bit of effort you'll not have too much trouble finding aircraft tht contradict me, but it's a reasonable general rule.

G

In the mighty A4 Skyhawk, the aileron direction required for spin recovery depended on whether you were spinning inverted or upright.

Upright Spin Recovery
Aileron - Full into turn needle
Rudder - Full opposite turn needle
Elevator - Central to slightly aft

Inverted Spin Recovery
Aileron - Full opposite turn needle
Rudder - Full opposite turn needle
Elevator - Central to slightly aft

The ailerons are applied first and are the dominant spin recovery control due to their effectiveness. (A4 roll rate of 720 deg/sec)

In the case of the A-4 it was the strong adverse yaw reaction to aileron input that made the aileron the dominant spin recovery (as well as spin INDUCING) control. It was unlikely that you would ever get an A-4 to spin using rudder input only. OTOH, a bit of aileron at the top of a near-vertical 0-airspeed maneuver was almost sure to put you into a spin instead of a falling leaf...

FWIW, the full roll rate of the A-4 could not be achieved with aileron alone; significant rudder into the roll was needed as well.

Yep, I only spun an A4 once...........:ooh:

...which is about once more than most people who have flown it!

You need to be careful when talking about "in spin aileron" in an inverted spin. In an erect spin, roll and yaw rate are in the same direction and thus "in spin aileron" is in the same direction as both roll and yaw rate. However, in an inverted spin, roll and yaw rates are in opposite directions and, by convention, the direction of an inverted spin is defined by the direction of yaw. Therefore, in a left inverted spin right aileron is in the same direction as the roll rate and will, therefore, have the same aerodynamic effect as left aileron in a left erect spin (and vice versa). Hence the A4 advice regarding the direction of aileron input in an inverted spin. For inverted spinning, I refer to left and right aileron to avoid any ambiguity, and I always avoid the phrases pro- and anti-spin with respect to direction of aileron input.