P-51 yaw instability: just how much of a problem was it?

[Nick - A2A]

Hello all,

Like many here, I received a nasty surprise the first time I tried to take to the skies in the A2A P-51D. I pretty-much took for granted that I'd be able to keep any aircraft that FSX could throw at me on the runway during take off. The A2A Spit had never presented too much of a problem provided I used proper technique with smooth and gradual introduction of power.

However, the P-51 was all over the place! With a bit of practice I could get into the air alive in it most of the time; however, with a full fuel load and external stores, take off can still be an interesting affair...

What puzzles me a bit is this: if the P-51D and previous marques really demonstrate yaw instability which is this poor—and I'm not suggesting it doesn't—why did they build ten-and-a-half thousand of 'em before they introduced the taller vertical stabilizer on the P-51H?

I appreciate that the fillet forward of the fin that was introduced with the P-51D was an attempt to address the yaw instability (particularly I gather with the addition of the fuselage fuel tank and cut down rear fuselage). However, given the redesigned tail on the 'H' model it was presumably never really considered to be solution to the issue. I'd guess it was felt that increasing the size of the vertical stabilizer in the P-51D would compromise performance too much at a time when every knot was important. In general terms, modifications to the tail surfaces of WWII types seems to have been one of the most common alterations, though perhaps more so in multi-engine types.

I'm rather curious as to how daunting a prospect take off in a fully-fuelled, fully-armed P-51B/C/D was for even an experienced pilot. How prone to take off accidents was the P-51? I still find take off to be more of a hazard than landing, and it's perhaps the only aircraft in FSX in which I've genuinely 'bought it' on take off when not doing anything ostensibly silly*.

Nick

*Such as fiddling with the undercarriage selector on the P-40 and not paying attention to airspeed...

[DHenriques_]

Hello all,

Like many here, I received a nasty surprise the first time I tried to take to the skies in the A2A P-51D. I pretty-much took for granted that I'd be able to keep any aircraft that FSX could throw at me on the runway during take off. The A2A Spit had never presented too much of a problem provided I used proper technique with smooth and gradual introduction of power.

However, the P-51 was all over the place! With a bit of practice I could get into the air alive in it most of the time; however, with a full fuel load and external stores, take off can still be an interesting affair...

What puzzles me a bit is this: if the P-51D and previous marques really demonstrate yaw instability which is this poor—and I'm not suggesting it doesn't—why did they build ten-and-a-half thousand of 'em before they introduced the taller vertical stabilizer on the P-51H?

I appreciate that the fillet forward of the fin that was introduced with the P-51D was an attempt to address the yaw instability (particularly I gather with the addition of the fuselage fuel tank and cut down rear fuselage). However, given the redesigned tail on the 'H' model it was presumably never really considered to be solution to the issue. I'd guess it was felt that increasing the size of the vertical stabilizer in the P-51D would compromise performance too much at a time when every knot was important. In general terms, modifications to the tail surfaces of WWII types seems to have been one of the most common alterations, though perhaps more so in multi-engine types.

I'm rather curious as to how daunting a prospect take off in a fully-fuelled, fully-armed P-51B/C/D was for even an experienced pilot. How prone to take off accidents was the P-51? I still find take off to be more of a hazard than landing, and it's perhaps the only aircraft in FSX in which I've genuinely 'bought it' on take off when not doing anything ostensibly silly*.

Nick

*Such as fiddling with the undercarriage selector on the P-40 and not paying attention to airspeed...

I never flew the H but I've flown the 51D in just about every attitude it's possible to fly it in and I've never once had a lateral stability issue.
The dorsal was added along with the change to a reverse rudder tab at the same time and helped to solve issues that were being noticed during specific violent maneuvering, not with takeoff.
The 51 is really no different than any other powerful propeller driven fighter as that would apply to handling that power.
Increasing the vertical stabilizer and rudder area was done on several fighters in efforts to improve lateral stability. Notable among these changes would be the modifications done to the D by Cavalier in Florida, the change done by North American on the H, and the change done between the -1 and -2 Bearcat by Grumman.
Dudley Henriques

[Nick - A2A]

I never flew the H but I've flown the 51D in just about every attitude it's possible to fly it in and I've never once had a lateral stability issue.

Thanks for the reply Dudley. Must admit, I was under the impression that rather poor lateral stability was a known characteristic of the P-51. I guess whether one considers it an "issue" is dependent upon their skill! For me, it certainly was a problem during those first few take offs. In fact, I recall it was particularly notable during climb out at low speed with a heavy aircraft when I sometimes felt like the fin must have fallen off altogether sometime after brake release! My guess (often wrong ) was that the "yaw_stability=-0.750" entry in the aircraft.cfg file represented a somewhat ineffective vertical stabilizer on the real P-51D. (The same value is +1 in the Spit I think...)

Could be I'm confusing my terminology a bit here. I understand yaw/lateral stability to be the resistance of the aircraft to rudder input and the speed at which the resulting yaw is damped-out when rudder input is released?

I had a quick look for documents on the subject of yaw stability and this 1946 research memo: Flight Measurements of the Directional Stability and Control of a P-51D Airplane... may be of interest; I need to have a proper read of it myself at some point. This article by Joe Baugher also describes how "the distinctive taller tail was installed on the P-51H-5-NA and later production block aircraft and later retrofitted to earlier P-51H-1-NAs to eliminate the yaw instability that plagued earlier Merlin-powered Mustangs."

The feeling I got was that the A2A flight model realistically captured what was a difficult trait of the real P-51 when compared to early model Spits and so on. I read somewhere that it was poor lateral stability which led to the P-51 being rejected for carrier-based use by the USN, but can't seem find that now...

Cheers,
Nick

[DHenriques_]

I never flew the H but I've flown the 51D in just about every attitude it's possible to fly it in and I've never once had a lateral stability issue.

Thanks for the reply Dudley. Must admit, I was under the impression that rather poor lateral stability was a known characteristic of the P-51. I guess whether one considers it an "issue" is dependent upon their skill! For me, it certainly was a problem during those first few take offs. In fact, I recall it was particularly notable during climb out at low speed with a heavy aircraft when I sometimes felt like the fin must have fallen off altogether sometime after brake release! My guess (often wrong ) was that the "yaw_stability=-0.750" entry in the aircraft.cfg file represented a somewhat ineffective vertical stabilizer on the real P-51D. (The same value is +1 in the Spit I think...)

Could be I'm confusing my terminology a bit here. I understand yaw/lateral stability to be the resistance of the aircraft to rudder input and the speed at which the resulting yaw is damped-out when rudder input is released?

I had a quick look for documents on the subject of yaw stability and this 1947 research memo: Flight Measurements of the Directional Stability and Control of a P-51D Airplane... may be of interest; I need to have a proper read of it myself at some point. This article by Joe Baugher also describes how "the distinctive taller tail was installed on the P-51H-5-NA and later production block aircraft and later retrofitted to earlier P-51H-1-NAs to eliminate the yaw instability that plagued earlier Merlin-powered Mustangs."

The feeling I got was that the A2A flight model realistically captured what was a difficult trait of the real P-51 when compared to early model Spits and so on. I read somewhere that it was poor lateral stability which led to the P-51 being rejected for carrier-based use by the USN, but can't seem find that now...

Cheers,
Nick

The problem with lateral stability in the 51 was connected to the fact that it was possible for pilots to exert excessive rudder pressure by nothing more than their leg power. This was remedied by switching the trim tab to reverse boost. By doing that, the more pressure the pilot exerted on the pedal, the tab fought that pressure making it harder to use excessive rudder.

The carrier trials on the Mustang for the Navy were actually flown by a friend of mine, Bob Elder. The problem with the Mustang on the boat was the wing. Being laminar it required a fairly high approach speed for the 51 as opposed to the higher lift wings common to carrier fighters. There were other considerations of course but this was a serious issue that couldn't be solved without massive redesign consideration.
DH

[Nick - A2A]

Thanks for the clarification.

This was remedied by switching the trim tab to reverse boost. By doing that, the more pressure the pilot exerted on the pedal, the tab fought that pressure making it harder to use excessive rudder.

...Guess this is rather hard to represent properly in a desktop sim without some sort of force feedback rudder pedals. Presumably the lesson here is just to be particularly gentle with the rudder inputs in the 51?

Thanks,
Nick

[Bomber_12th]

Some things to add to this conversation...

Pilots who have flown restored P-51B/C's today, without the dorsal fin fillet, always comment that the aircraft is more unstable in yaw than the P-51D with its dorsal fin fillet (all D-models flying today have to have the dorsal fin fillet fitted, in order to be certified under the "limited" category, where as the same doesn't apply to the P-51B/C). Jack Roush actually had a dorsal fin fillet added to his P-51B (though not of authentic design, like seen on the restored "Impatient Virgin"), only after spending several months flying it without the dorsal fin fillet, as he felt it didn't fly right/handle correctly compared to the P-51D.

The taller tail fin was actually first introduced by the NACA (precursor to NASA), working with North American. NACA, working with NAA, designed and fitted a taller fin cap on their P-51B's and D's they were using for dive-testing/airfoil testing, so as to provide better lateral stability while in the dive, approaching Mach 8/8.5. The NACA was also instrumental in paving the way for the taller P-51H fin. When Cavalier came about, one of the modifications they did to their P-51D conversions was to copy the same exact NACA-introduced taller fin cap.

These service bulletins of the period may be of interest, regarding the dorsal fin fillet.

[jcomm]

Interesting thread and comments!

My other p51d is the DCS module. I find it rather wobbling inflight, at the full speed / power range but specially at high power / low speed / high AoA. It turns aiming at the Me109 in front of you rather tricky, which I don't think is realistic.

A2A's p51d feels like a lot more stable platform, which is indeed as I would expect the real thing to be, specially those models with the dorsal fin.

[Nick - A2A]

Some things to add to this conversation...

John - thanks for the post and the service bulletins in particular. That link I posted above to the 'research memo' is actually a NACA document describing some of the testing you describe if I'm not mistaken. Here's one of the photos contained within; it shows two experimental P-51 mods: an extended tail and a horn-balanced rudder.


The photo was taken in late '45 I believe.

I haven't got round to reading the whole thing yet, but it does mention that "previous tests showed that the P-51D series airplanes with the original production tail had poor directional stability at small angles of sideslip and had a large change in rudder trim force with speed."

Cheers,
Nick

[renaissanceman]

Hi Nick,

As many on this forum know, my father flew P-51s for the 8th Air Force in WWII. I remember one of our discussions about the P-51, an aircraft he fell in love with after seeing a picture in an Air Corps magazine early in his training. One of his comments was that the P-51 was an excellent aircraft with one major and one minor weakness. The major weakness is common to all liquid cooled engines, a single small hit to the engine could knock it out. The minor weakness is the left yaw tendency, especially on take-off, that required much right rudder to counteract.

At another time he related to me a conversation he had with another former P-51 combat pilot. Both were in their 70's at the time. Dad told him he felt he could still fly a P-51 today even though it had been some 50 years since he had. The other gentleman told him he doubted either of them would have the strength in their right leg to manage a take-off.

Jim

[Nick - A2A]

Hi Jim,

Thanks for the comments. Yes, a strong right leg but also a sensitive one was/is requisite I guess. I wonder if P-51 squadrons produced good footballers? (soccer players for those of you in the US )

For what it's worth, I do find the yaw tendency/instability in the A2A P-51 to be very convincing in that it really requires focus on the take off. The A2A Spit—though I love it dearly—feels just a touch too 'safe' in this regard for a V-12 powered tailwheel aircraft. However, I appreciate it 'only' has around 1000hp so a good bit less than the Mustang.

My other p51d is the DCS module. I find it rather wobbling inflight, at the full speed / power range ...

I also have the DCS Mustang jcomm and had exactly the same impression on loading it up. Just breathing on the flight controls seemed to send it jittering around in all three axes. I was able to largely address this with changing the sensitivity curves, but to be honest I haven't spent much time with it so can't really comment on the FDE. (I've mostly been spending my sim time in the A2A GA stuff this last year or so.)

One thing's for sure though: during a couple of ground strafing runs I tried, the enemy ZSU-23-4s or whatever the hell they were (play fair guys! ) had little to fear from me...

Cheers,
Nick