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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" ![]() 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. |
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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" ![]() 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 |
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