Good read on the P-51

[Magister]

A common misconception is that the cutting down of the rear fuselage to mount the bubble canopy reduced stability requiring the addition of a dorsal fin to the forward base of the vertical tail. In fact, as described, stability problems affected the earlier Bs and Cs, as well as the subsequent D/K models; this was partly attributable to the 85 gallon fuselage fuel tank which had been installed during production of the P-51B-5-NA. [Note:While some existing aircraft do not have the dorsal extension fitted, many were equipped at some point in their service or refurbishment with a taller tail, which provided a similar increase in yaw stability. Also, civilian-owned examples often have newer, lighter radios, an absence of external munitions and drop tanks, removed guns and armor plate and an empty or removed fuselage tank — reducing the need for the dorsal fin.]

P-51 Mustang

 

[beaupower32]

Nice find. Thanks for sharing.

 

[marshall]

P-51 Mustang

The source is exactly the same text as on wikipedia

P-51 Mustang - Wikipedia, the free encyclopedia

 

[beaupower32]

lol, so it is.

 

[drgondog]

A common misconception is that the cutting down of the rear fuselage to mount the bubble canopy reduced stability requiring the addition of a dorsal fin to the forward base of the vertical tail. In fact, as described, stability problems affected the earlier Bs and Cs, as well as the subsequent D/K models; this was partly attributable to the 85 gallon fuselage fuel tank which had been installed during production of the P-51B-5-NA. [Note:While some existing aircraft do not have the dorsal extension fitted, many were equipped at some point in their service or refurbishment with a taller tail, which provided a similar increase in yaw stability. Also, civilian-owned examples often have newer, lighter radios, an absence of external munitions and drop tanks, removed guns and armor plate and an empty or removed fuselage tank — reducing the need for the dorsal fin.]

P-51 Mustang

The yaw issues and the aft cg issues were separate.

The key yaw issue was the tendency for the Mustang to yaw to the right in high speed compressibility dives and the addition of the tail dorsal helped mitigate that in the B/C/D/K.

The final stability solution was the P-51H which combined the taller vertical stabilizer (with 2 sq ft add'l area) AND a 14 inch extension of the fuselage plus adding 2 1/2 feet to horizontal stabilizer span as well as stronger vertical stabilizer spar.

With the extensions the P-51H no longer had an aft cg issue with full fuselage tank (50 gallons).

Those changes dramatically reduced torque roll on takeoff, yaw issues in dive, weakness of tail in slow roll and porpoising with external tanks.

 

[Ivan1GFP]

Hello drgondog,
You beat me to it. Please pardon me for restating in slightly different words. There are two DIFFERENT stability issues being discussed here.

The P-51 had a DIRECTIONAL (yaw) stability issue in all models with the worst being the early D models with reduced "keel" area because of the cut down fuselage. It finally was addressed with the H model with the larger fin. The added fillet on most D's didn't completely address the issue.

The LONGITUDINAL (pitch) stability issue was because there was a need for additional fuel capacity and a 85 gallon tank was added behind the cockpit. This moved the Center of Gravity so far back as to make the aircraft dangerously unstable. Many folks don't realise the tank is capable of holding 85 gallons because it was seldom filled to capacity because of the nasty handling that resulted. This tank had to be mostly empty in order for the aircraft to be safe to fight. Thus, the fuselage tank was to be used BEFORE using drop tanks when flying long range missions.

Late model Spitfires had similar longitudinal stability issues apparently. This may be obvious, but the best location for disposable loads is near the aircraft center of gravity so that trim doesn't change when the loads are expended. This applies to ammunition as well as fuel which is why guns crammed into the nose such as on a P-39 / P-63 aren't the best idea.

- Ivan.

 

[Magister]

"This tank had to be mostly empty in order for the aircraft to be safe to fight."

I wonder if this issue actually presented a problem and if so, how often.

 

[drgondog]

Hello drgondog,
You beat me to it. Please pardon me for restating in slightly different words. There are two DIFFERENT stability issues being discussed here.

The P-51 had a DIRECTIONAL (yaw) stability issue in all models with the worst being the early D models with reduced "keel" area because of the cut down fuselage. It finally was addressed with the H model with the larger fin. The added fillet on most D's didn't completely address the issue.

Ivan - while the 'tall tail' helped and seen today on many Cavalier modified Mustangs, it took the lengthening of the fuselage to totally solve both the yaw and the aft cg (different, as you noted) issues

The LONGITUDINAL (pitch) stability issue was because there was a need for additional fuel capacity and a 85 gallon tank was added behind the cockpit. This moved the Center of Gravity so far back as to make the aircraft dangerously unstable. Many folks don't realise the tank is capable of holding 85 gallons because it was seldom filled to capacity because of the nasty handling that resulted. This tank had to be mostly empty in order for the aircraft to be safe to fight. Thus, the fuselage tank was to be used BEFORE using drop tanks when flying long range missions.

The most common 'nasty' side effect was to suddenly (and violently) snap roll. 8th AF doctrine was split on full fuselage tank. My father's Group, the 355th, tended to put only 65 gallons in, then use to take off and form up - using the aft tank until down to at least 25 gallons before switching to external tanks.

Late model Spitfires had similar longitudinal stability issues apparently. This may be obvious, but the best location for disposable loads is near the aircraft center of gravity so that trim doesn't change when the loads are expended. This applies to ammunition as well as fuel which is why guns crammed into the nose such as on a P-39 / P-63 aren't the best idea.

- Ivan.

All true... but even the ammo on the 51 was near the mean aerodynamic center so it was still a little forward of cg.