P-51 Rudder Aerodynamics

[Hawkeye Hughes]

The rudder fitted to P-51 series aircraft is noticably thicker at its
hinge line than the vertical fin section just in front of it. Several
writers have characterized the benefits of this "bulged" configuration
as 1. reduced form drag at the juncture of fin and rudder, and 2.
improved airflow characteristics over the rudder, with resultant
increased control effectiveness and less tendency for the surface to
exhibit a "dead band" around the neutral point.

If this design element performs as described, why is it not more
widely employed in elevators and ailerons as well as rudder
applications, particularly in the faster homebuilts? I believe that
it has been used on the ailerons of some of the after-market Pitts
wings as well as some of the midwing acro designs. Have any downside
effects been encountered?

Hawkeye (Skyote NX8XX)

[Richard Lamb]

Hawkeye Hughes wrote:
>
> The rudder fitted to P-51 series aircraft is noticably thicker at its
> hinge line than the vertical fin section just in front of it. Several
> writers have characterized the benefits of this "bulged" configuration
> as 1. reduced form drag at the juncture of fin and rudder, and 2.
> improved airflow characteristics over the rudder, with resultant
> increased control effectiveness and less tendency for the surface to
> exhibit a "dead band" around the neutral point.
>
> If this design element performs as described, why is it not more
> widely employed in elevators and ailerons as well as rudder
> applications, particularly in the faster homebuilts? I believe that
> it has been used on the ailerons of some of the after-market Pitts
> wings as well as some of the midwing acro designs. Have any downside
> effects been encountered?
>
> Hawkeye (Skyote NX8XX)

Hey Hawkeye,

Jim Bede (8-) published a paper on this way back when.
He had a cool graph about it IIRC, was taken from the P-51 studies.
The BD-4 ailerons and flaps were done this way, about 10% thicker(?).
But the elevator was (sorry, IS) a stabilator.

Anyway, it gets back to one of the oddities of aerodynamics, the
boundary layer.

Thick boundary layers are usually not very desirable.
Especially in the vicinity of control surfaces.

The sudden (?) increase in thickness trips the boundary layer onto
the control surface. Lower drag, more effective controls, and yes,
a noticibly reduced dead band effect.

But something to keep in mind when thinking about that ancient
Mustang. It was and probably still is the ultimate flying machine.

EVERY detail was optimized toward that simple goal of making a fast
and furious fighting machine.

So, ok, that is one way...

But next time you get a chance, take a close look at the A-4 Skyhawk
rudder. Ed Heinneman found a completely different solution to the
same problem.


Richard