Did the USAAF Design Standards kill the P51's Climb rate?

[timmy]

Interesting reading

The fastest Mustangs to be built owed their existence to Mustang designer's Edgar Schmeud's exploration of weight issues with American aircraft; e.g. that they were too heavy. Upon being notified by the Air Force of this concern, Schmeud had North American's Field Service Department in England check with the British aircraft manufacturers, especially Supermarine to obtain detailed weight statements of their aircraft.

To Schmeud's surprise, no British company at the time had any detailed weight statement for their aircraft; they literally did not know how much specific parts of their aircraft weighed! To solve this problem, Schmeud had NAA's Field Service Department go out to where Spitfires were repaired and had them start to weigh all of the parts they could obtain, and soon Schmeud finally got a weight statement for the Spitfire.

Studying the statement, Schmeud finally learned why British aircraft turned out to be lighter than American aircraft built for the same roles and requirements.

1.) The Angle of Attack Load Factor in the USAAF was 12.0, while in the RAF it was only 11.0
2.) The Side-Load Factor on the engine in the USAAF was 2.0 g's, while the British did not have this requirement; due to side loading on the engine mount not being a real problem in normal flight.
3.) The Landing Gear Load Factor in the USAAF was 6.0 g's, but only 4.0 in the RAF.

Armed with this information, Schmeud began to design a lightweight Mustang built around RAF requirements, not USAAF requirements. This project resulted in the XP-51F and the XP-51G Mustang.


While the XP-51F resembled the earlier Mustangs externally; internally it was a virtually new aircraft structurally, with an empty weight 2,000 pounds less than a P-51D and carrying the same powerplant as the earlier P-51D, rated at 1,695 hp. The refinements in the -51F gave it a top speed 26 MPH higher than the P-51D, some 4,000 feet higher; and it first flew in February 1944.

North American's Lightweight Mustangs (P-51H/P-51M/P-51L)

Sounds like Edgar Schmeud wasn't to happy with his P51's climb rate compared to the spitfire
I know the P51B/C wasn't bad in that area, but the Heavy D model suffered. A shame because if
this plane came out with the British specifications, this plane would have been unbeatable

 

[Njaco]

It was almost unbeatable as it was and did the job. I think thats all you can ask. But I wouldn't know if enhanced performance would have changed things .

 

[timmy]

It was almost unbeatable as it was and did the job. I think thats all you can ask. But I wouldn't know if enhanced performance would have changed things .

Well it might not have changed the result to much. Considering the P-51D being a High altitude escort was more likely flying down hill against the Germans rather than Up hill. In which case no one ever doubts the P51's Dive or Zoom climb performance. Still would have helped the A-36 in the early days I think if it had the lighter weights

 

[drgondog]

Climb rate is a function of Wing Loading and excess Power Available over Power Required.

The design limit and ultimate Limit Load did contribute to Weight - which is built into ROC equations so there is something to what you say - but NAA didn't design to USAAF spec, per se. The standard criteria of 8 and 12 G was an industry standard for high performance aircraft in the civil and military industry pre-war though Vietnam.

The XP-51F which morphed into G and J were basically the same airframe with different engines. All were developed under the XP_51F contract

Back to your point, however, the A-36 and P-51A were fast because they needed far less HP than say a Spit or a 109 because of the aerodynamics. The next evolution was the B/C/D/K which grew in weight over the P-51A but more than made up for it by increasing the power by as much as 500Hp depending on fuel octane and altitude - hence a 10% gain in weight was offset by nearly 20% increase in available Power.

Then the H was result of weight reduction program as you mentioned but it also had another jump of 300-400 Hp at 90" Boost and WI... which is why it had a climb rate rivaling an F8F and more than a Spit XIV for comparable mission loading.

 

[drgondog]

Another interesting point about the P-51H. It had 11G Ultimate and 7.25 G Limit Load for stress ceilings on critical structure, but it was designed to 'planned design Gross Weight' of 8700 pounds IIRC. The XP-51 Gross design was 8,000 pounds for 12 and 8G but by the time the P-51B and D were flying combat, the Gross weight was about 9200 and 9600 pounds respectively until internal fuselage fuel and external tanks were added.

So the actual limit load for the P-51D had shrunk to 64,000 (i.e. 8g x 8,000 pounds) divided by actual Gross Weight of 9600 = 6.66 G instead of 8G back in 1941.

The comparable limit loads for the P-51H was 8700 x 7.25 and x 11. for 63,163 and 95,700... so mission creep from 8700 to 9000 for full fuselage with no external tanks = 63,163/9000 = 7.0 G Limit load in combat situation with full internal load including Fuselage fuel tank. Add 85 gallons to the P-51D and you are up to 10,200 pounds - reducing the comparable P-51D stress limit attainment at 64000/10200= 6.27 G.

Pretty significant difference = advantage P-51H even though designed to lower G load than the XP-51

 

[davparlr]

Contrary to popular belief, the P-51 was not a slow climbing aircraft when loaded at equivalent weights (the mustang carried 180 gal of internal fuel normally, the Spitfire carried 122(US)gallons internally, and the Bf-109 carried 105 gallons, the Fw-190, 138 gallons). Even at this heavier weight, the P-51B, post May,'44, was capable of over 4400 ft/min climb at SL, the D capable of over 4000 ft/min climb. Very comparable to contemporary aircraft. Even the pre May,'44 Mustang was not a slow climber when equally loaded.

 

[wuzak]

Contrary to popular belief, the P-51 was not a slow climbing aircraft when loaded at equivalent weights (the mustang carried 180 gal of internal fuel normally, the Spitfire carried 122(US)gallons internally, and the Bf-109 carried 105 gallons, the Fw-190, 138 gallons). Even at this heavier weight, the P-51B, post May,'44, was capable of over 4400 ft/min climb at SL, the D capable of over 4000 ft/min climb. Very comparable to contemporary aircraft. Even the pre May,'44 Mustang was not a slow climber when equally loaded.

The problem is that the Spitfire with the equivalent engine had an initial rate of climb over 5000ft/min.

 

[rinkol]

It is interesting that, as far as I know, the P-51D remained in service well after the end of WW2 whereas the P-51H, despite its outstanding performance did not. Perhaps familiarity and availability of spare parts contributed to the service life of the P-51D. That said, I understand that ht P-82 followed the structural design concepts of the P-51H.

 

[GrauGeist]

Contrary to popular belief, the P-51 was not a slow climbing aircraft when loaded at equivalent weights (the mustang carried 180 gal of internal fuel normally, the Spitfire carried 122(US)gallons internally, and the Bf-109 carried 105 gallons, the Fw-190, 138 gallons). Even at this heavier weight, the P-51B, post May,'44, was capable of over 4400 ft/min climb at SL, the D capable of over 4000 ft/min climb. Very comparable to contemporary aircraft. Even the pre May,'44 Mustang was not a slow climber when equally loaded.

The problem is that the Spitfire with the equivalent engine had an initial rate of climb over 5000ft/min.

Where do those figures come from?

The P-51B with a Packard Merlin (roughly 3,520 ft/min RoC) was comparable to a Spitfire Mk.VB equipped with a Merlin 45 (roughly 3,250 ft/min RoC)

Any P-51 prior to the P-51B-1NA was equipped with the Allison V-1710...those models being the Mustang Mk.I, P-51A and the A-36.

 

[tomo pauk]

Packard Merlin in Mustangs was of a two stage variant, the Merlin 45 was a single stage engine. Spitfires outfitted with 2 stage Merlins, like Mk.IX, were doing easy 4500 fpm (here). That is on +18 psi boost, ie. in 1943 with 100/130 fuel.

 

[fastmongrel]

The Spitfire MkVB (converted from a MkII) was in service early 1941 hardly comparable to a P51B.

 

[GrauGeist]

So then what would be a closer comparison, the LF Mk.IX with the Merlin 66 or parhaps the Mk.XII with the Griffon III?

 

[wuzak]

So then what would be a closer comparison, the LF Mk.IX with the Merlin 66 or parhaps the Mk.XII with the Griffon III?

The LF.IX.

 

[BiffF15]

It is interesting that, as far as I know, the P-51D remained in service well after the end of WW2 whereas the P-51H, despite its outstanding performance did not. Perhaps familiarity and availability of spare parts contributed to the service life of the P-51D. That said, I understand that ht P-82 followed the structural design concepts of the P-51H.

The H version stayed in the Guard until the 1950s. They were then put out to pasture when the Guard started receiving F-86 Sabers.

Cheers,
Biff

 

[drgondog]

Dave - the use of 150 octane fuel at 72" and 25+ boost moved the climb rate of a P-51B/RAF III with 1650-7 engine, in excess of 4000fpm in V-1 Interceptor role and nearly 400 mph at 4700 feet of altitude. No fuselage fuel and no external drop tanks were required. They probably cut the ammo supply in half also.

At Biff, the last Nasty Guard active duty P-51D's were retired in 1957, the last P-51H in 1955 and the last actice duty ADC F-82 was in the 1954 timeframe when the F-89 and F-86D units replaced them in Alaska for the All Weather role there.

 

[grampi]

Climb rate is a function of Wing Loading and excess Power Available over Power Required.

The design limit and ultimate Limit Load did contribute to Weight - which is built into ROC equations so there is something to what you say - but NAA didn't design to USAAF spec, per se. The standard criteria of 8 and 12 G was an industry standard for high performance aircraft in the civil and military industry pre-war though Vietnam.

The XP-51F which morphed into G and J were basically the same airframe with different engines. All were developed under the XP_51F contract

Back to your point, however, the A-36 and P-51A were fast because they needed far less HP than say a Spit or a 109 because of the aerodynamics. The next evolution was the B/C/D/K which grew in weight over the P-51A but more than made up for it by increasing the power by as much as 500Hp depending on fuel octane and altitude - hence a 10% gain in weight was offset by nearly 20% increase in available Power.

Then the H was result of weight reduction program as you mentioned but it also had another jump of 300-400 Hp at 90" Boost and WI... which is why it had a climb rate rivaling an F8F and more than a Spit XIV for comparable mission loading.

I think the H model actually had a higher climb rate than the F8F (maybe not at sea level, but it certainly did at altitude), unfortunately most people compare the climb rate of the D model to the F8F which is hardly a fair comparison...

 

[gjs238]

At Biff, the last Nasty Guard active duty P-51D's were retired in 1957, the last P-51H in 1955 and the last actice duty ADC F-82 was in the 1954 timeframe when the F-89 and F-86D units replaced them in Alaska for the All Weather role there.

What does Nasty Guard mean?
Just asking.
Does it mean the aircraft were in real nasty shape by that point?

George

 

[BiffF15]

Not sure what it means either. The ANG typically keeps their A/C in better shape than the active duty due to lack of churn with crew chiefs and jets.

 

[drgondog]

Nasty Guard was a term of affection for Air National Guard in the 50's.

 

[davparlr]

Nasty Guard was a term of affection for Air National Guard in the 50's.

If I remember correctly, the ANG often won inter-service fighter competitions.