In article ,
Jim Carriere writes:
Dudley Henriques wrote:
"Don McIntyre" wrote in message
oups.com...

Dudley,
I don't quite understand how the fuel tank installation relates to
the dorsal fairing. Wasn't the fuel tank related to the "ass-heavy" CG
on the B-model (or did that also apply to the D)? The airflow off the
canopy makes a lot more sense to me.
I'm not trying to pick nits here, just curiousity has reared it's
ugly head. 8-)


I wouldn't disagree with this . Just mentioned it because it was a NA
engineer who threw it out there to us at one time.
In my opinion it was the canopy change that necessitated the need for
the dorsal extension. I remember questioning him at the time as well.

It sort of makes sense. The "ass-heavy" rear fuel tank take away
stability, the dorsal fairing contributes to stability. The bubble
canopy upset the airflow behind it and in front of the fin, the
dorsal fairing may have improved this.

So the question is whether the dorsal fairing was added for the first
reason, the second, or both. The airplane may be too old a design to
get a definitive answer, but I wouldn't be surprised if they added
the fairing for both reasons.

The directional stability of an airplane depends, basically, on where
you put it's side area - area ahead of the CG is destabilizing, and
area behind it adds to the stability. When they cut down the aft
fuselage of the P-51 to put the bubble canopy on hte "D" models, they
lost some ditectional stability. (Yaw) The added the dorsal extention
to the rudder to try to remedy this, and in the later H-models and the
Temco and Cavalier builds put a taller fin on the airplane.

The data for this still exists. Buried in the uncatalogued files on
the NACA Technical Reports Server are the results of the wind tunnel
tests used to determine the H-models fin shape. It's also got the
stock D-model data in the report.

Note that the P-51 wasn't the only airplane theat needed its
directional stability punched up a bit after getting the bubble
canopy. A dorsal fin was added to late model P-47Ds, Ms, and Ns, and
the Spitfire got a brand new fin & rudder.

The fuselage auxilliary tank moved the CG aft, right to, or perhaps a
bit beyond, the practical limit for an aft CG. This had a small
effect of directional stability, but a huge effect on pitch
(longitudinal) stability. The airplane tended to be unstable in
pitch, very, very light on the stick at low Gs (Something like 1.5
lbs/G have been reported) and with a felt force reversal somewhere
around 4 Gs. This led to NAA and the USAAF devising a bobweight
system in the elevator circuit that increased the feel of the airplane
in pitch.

--
Pete Stickney

Without data, all you have are opinions