Ending the Argument

[wuzak]

The P 51D was not a peak performance aircraft in 1944, it was just a very good aircraft that could carry a huge internal an external load of fuel and so project a good performance over most of Europe. If ultimate performance was the aim then the P51 would have been fitted with a Griffon, as later racers were. However a Griffon engine P51 wouldn't have reached Berlin and returned to base. The US air force could have coped with a lower top speed rate of climb etc on the P51 they could not have coped with a reduced range.

Not sure about the range.

Certainly the endurance would be reduced, but the cruise with the Griffon should be faster, which would offset the time factor to some degree.

Rolls-Royce were keen to put the 2-stage Griffon in the Mustang, but NAA said there would be too much rework involved (at that stage of the war - 1942).

 

[BiffF15]

Does anyone have forecast data for what a Griffon powered Mustang might have done range / performance wise? The powers that be would have known we would be on the continent eventually, but then again the war in the Pacific required serious range for land based forces.

Cheers,
Biff

 

[pbehn]

Not sure about the range.

Certainly the endurance would be reduced, but the cruise with the Griffon should be faster, which would offset the time factor to some degree.

Rolls-Royce were keen to put the 2-stage Griffon in the Mustang, but NAA said there would be too much rework involved (at that stage of the war - 1942).

M opinion was based on the Griffon being bigger and heavier and having a larger swept volume. All of these tend to increase fuel consumption. It may be true that a Griffon engine P 51 would consume approximately the same while cruising to rendezvous. Personally I doubt it. However much of a P51s mission was on escort where consumption was in gallons per hour and the speed dictated by the bomber formation. On this part of a mission and in combat it would definitely consume more.

 

[Shortround6]

It shouldn't consume a lot more while cruising. Power required it pretty much dependent on drag. Yes the extra weight will cause a bit of extra drag ( higher angle of attack for wing at same speed) but that is going to be rather small. Larger radiator scoop? but airflow through the radiator will be in proportion to the power being made.

That leaves the actual fuel economy of the engine. The Griffon is 36% bigger in swept volume to be sure but it runs approximately 10% slower. It also has about 22% more scrubbed area of cylinder wall and pistons scrubbing cylinder walls are roughly 75-80% of the friction in an engine. Fiction goes up with the square of the speed soooo.......
Running an engine even 200rpm slower can make a difference in friction losses (one big reason for dropping the revs and using high boost (relatively) for cruising.
A clean Mustang (tanks gone) can cruise at over 300mph true at 1850, boost not given, at 25,000ft using 59 gallons an hour. I don't know what the power is but is seems to be well under 700hp. (2400rpm and 36in/3lb is 775hp at 22,500ft)

I have no idea what settings are needed to get 5-600hp out of a Griffon at 22-25,000ft.

 

[wuzak]

It shouldn't consume a lot more while cruising. Power required it pretty much dependent on drag. Yes the extra weight will cause a bit of extra drag ( higher angle of attack for wing at same speed) but that is going to be rather small. Larger radiator scoop? but airflow through the radiator will be in proportion to the power being made.

That leaves the actual fuel economy of the engine. The Griffon is 36% bigger in swept volume to be sure but it runs approximately 10% slower. It also has about 22% more scrubbed area of cylinder wall and pistons scrubbing cylinder walls are roughly 75-80% of the friction in an engine. Fiction goes up with the square of the speed soooo.......
Running an engine even 200rpm slower can make a difference in friction losses (one big reason for dropping the revs and using high boost (relatively) for cruising.
A clean Mustang (tanks gone) can cruise at over 300mph true at 1850, boost not given, at 25,000ft using 59 gallons an hour. I don't know what the power is but is seems to be well under 700hp. (2400rpm and 36in/3lb is 775hp at 22,500ft)

I have no idea what settings are needed to get 5-600hp out of a Griffon at 22-25,000ft.

Cruise settings all seem to refer to maximum cruise. Which is at 2,400rpm and over 1,200hp @ 25,000ft for a Griffon 65.

Regarding drag, I don't think the engine itself would lead to an increase in drag, for while it is bigger in area than the Merlin it is not bigger than the P-51's fuselage.

It would require larger oil coolers, intercooler radiator and engine coolant radiators, as you have noted. This would probably require a larger scoop, which would impact the drag slightly.

Also, with the escort duty the P-51s may not have escorted the B-17s for the entire journey. Rather, escorts were done in relays, with other aircraft, such as P-47s, providing the initial cover. This would allow the P-51s to get to the rendezvous at their optimum speed.

 

[Koopernic]

Not sure about the range.

Certainly the endurance would be reduced, but the cruise with the Griffon should be faster, which would offset the time factor to some degree.

Rolls-Royce were keen to put the 2-stage Griffon in the Mustang, but NAA said there would be too much rework involved (at that stage of the war - 1942).

If North American said the P-51 required too much modification to take the Griffon they meant it would be better and faster to start an all new aircraft. The P-51B had vast amounts of panel changes over the P-51A while the P-51H was new again.

Of course North American could have built a Griffon powered P-51H given the effort involved in the H but would that have yielded a better aircraft? Larger certainly but faster than 487 mph?

The reason Supermarine shoe horned the Griffon into a hybrid of the Spitfire IV and Spitfire VIII is because brute power was the only way to overcome the limitations of the early 1930s airframe and aerodynamic technology. A Merlin Mustang was as fast if not faster than Griffon Spitfire. A Griffon mustang may not be much faster.

 

[wuzak]

If North American said the P-51 required too much modification to take the Griffon they meant it would be better and faster to start an all new aircraft. The P-51B had vast amounts of panel changes over the P-51A while the P-51H was new again.

Of course North American could have built a Griffon powered P-51H given the effort involved in the H but would that have yielded a better aircraft? Larger certainly but faster than 487 mph?

The reason Supermarine shoe horned the Griffon into a hybrid of the Spitfire IV and Spitfire VIII is because brute power was the only way to overcome the limitations of the early 1930s airframe and aerodynamic technology. A Merlin Mustang was as fast if not faster than Griffon Spitfire. A Griffon mustang may not be much faster.

The Griffon had similar power at 20-25,000ft with 100/130 grade fuel and +18psi boost (67inHg MAP) as the V-1650-9 did with 90inHg MAP and ADI.

The Griffon-Mustang discussion was occurring when the Mustang X and P-51B programs were in their initial stages. The Griffon Mustang would have predated the P-51H by a year or so. Certainly the 100 series Merlins (of which the V-1650-9 was one) had yet to be developed at that time.

The Griffon Spitfire (XIV) was 30-40mph faster than the equivalent Merlin Spitfire (VIII), depending on the sub type (F, LF, HF) and the Merlin variant used (63, 66, 70). Certainly the Mustang was as fast as the Griffon Spitfire, but it was quite a bit faster then the Spitfire with the same engine.

 

[pbehn]

From what I have seen of the P51 production line it was true mass production, you cannot make small changes to such a system, any modification means in effect a new aircraft.

 

[Shortround6]

Another problem was the Griffon itself, where were hundreds or thousands of 2 stage Griffons going to come from in 1942?
I mean this form a planning perspective. First squadron with Spitfire MK XIVs didn't become operational until Dec of 1943.
To build Griffon powered Mustangs instead of Merlin Mustangs in 1943 (and Packard couldn't build Merlins fast enough in the Spring/summer of 1943) you need a source of 2 stage Griffons at a rate of hundreds per month by the middle of 1943.

 

[drgondog]

AFAIK the Griffon was never considered for the Mustang at NAA or AAFMC. As noted above there were major changes required to both the airframe as well as Packard (or new source) tooling and start up. The Merlin 100 was a candidate but deemed too late for B/D and too early in the development/refinement stage in late 1943. It was installed in NA-105B XP-51G with excellent results in 1944, but by that time the 1650-9 was selected.

The political forces at the AAFMC/GM Board of Directors were also a hidden hindrance to any 'new' engine coming from England, as Allison frantically striving to replace the Packard Merlin with a new two stage/two speed V-1710 installed in the XP-51J.

 

[BiffF15]

Bill,
As a CROT (crude rule of thumb) could one compare the equivalent Spitfires with Merlin / Griffon engines for fuel flow differences to get a "rough" hack at what a Griffon Mustang would have burned? And from that extrapolate a "rough" hack at it's range with or without external fuel tanks?
Cheers,
Biff

 

[YF12A]

From what I have seen of the P51 production line it was true mass production, you cannot make small changes to such a system, any modification means in effect a new aircraft.

Agree. This reminds me of the B-29 "Battle of Kansas" situation.

 

[tomo pauk]

...
The reason Supermarine shoe horned the Griffon into a hybrid of the Spitfire IV and Spitfire VIII is because brute power was the only way to overcome the limitations of the early 1930s airframe and aerodynamic technology. A Merlin Mustang was as fast if not faster than Griffon Spitfire. A Griffon mustang may not be much faster.

Since the Spitfire 'swallowed' the 2-stage Griffon without breaking a sweat, looks like the supposed early 1930s airframe have had more stretch than it's designer team ever reckoned. Modifications nothwitsanding.
Spitfire's aerodynamic technology was unmatched by many design tems in years to come. By 1944, the only thing that was aerodynamically probelmatic was layout of it's big radiators. Other people also went with brute power when available (including NAA, Republic, Hawker, Focke Wulf), so that point is moot.

 

[Juha2]

Hello
if somebody has enough info on CAC CA-15, that might give a clue on "Griffon P-51" consumption.

 

[pbehn]

Another problem was the Griffon itself, where were hundreds or thousands of 2 stage Griffons going to come from in 1942?
I mean this form a planning perspective. First squadron with Spitfire MK XIVs didn't become operational until Dec of 1943.
To build Griffon powered Mustangs instead of Merlin Mustangs in 1943 (and Packard couldn't build Merlins fast enough in the Spring/summer of 1943) you need a source of 2 stage Griffons at a rate of hundreds per month by the middle of 1943.

Simply re write history. Give the plans for the Griffon to Napier and tell them to make it instead of the Sabre.. Give the plans to the P51 to Hawkers and say make it with a Griffon. No more Typhoons or Tempests but a lot of different P1s and Mustang XIVs for a variety of roles.

It means re writing commercial relations between UK and USA plus 20/20 hindsight though.

 

[davparlr]

he P 51D was not a peak performance aircraft in 1944, it was just a very good aircraft that could carry a huge internal an external load of fuel and so project a good performance over most of Europe.

? From mid 44, to the end of the war the P-51D with high octane fuel, was a class A fighter easily contesting the skies over Germany and Japan. Not until late 1944 did Germany generate any aircraft that could challenge the Mustang but never had the quantity to do it.

The report is somewhat confusing to me. I had never heard that the control forces for the P-51 could be very high. Indeed the Fighter Conference in late '44 never mentioned high control forces and selected the P-51 as second best all around fighter above 25k feet slightly behind the P-47, and also second best all around fighter below 25k feet almost equal to the F8F. Only one person rated the P-51 elevator forces poor while none rated the aileron forces as poor.

 

[GregP]

I'm not sure I read all of this thread yet, but fuel consumption shouldn't be too hard to get. It'll be the same as for a Griffon in a Spitfire at the same power level. Might be a different speed. but the gal/hour should be about the same. Speed at power level will be different, but fuel consumption should be for a Griffon engine more or less in a fighter airframe, and the Spitfire was very close.

From the Pilot's Notes for the Spitfire XIV:

Spitfire XIV has 4 tanks. They total 111 gallons. It consumed 22.5 gallons climbing to 20,000 feet, leaving 88.5 gallons.


IAS at 20,000 feet cruise (MAP not stated) at various rpm were as follows:

1800 rpm, 240 mph, 88.5 gallons: 4.65 air miles per gallon --> 411.5 miles --> 1.71 hours --> 51.8 gal/hour

2000 rpm, 240 mph, 88.5 gallons: 4.6 air miles per gallon --> 407 miles --> 1.70 hours --> 52 gal/hour

2200 rpm, 240 mph, 88.5 gallons: 4.45 air miles per gallon --> 394 miles --> 1.64 hours --> 54 gal/hour

2400 rpm, 240 mph, 88.5 gallons: 4.3 air miles per gallon --> 380 miles --> 1.58 hours --> 56 gal/hour

 

[Shortround6]

Most of these engines burned from 0.45lbs to 0.50lbs of fuel per horsepower hour in cruise settings (lean mixture) . Some settings (or combinations) were better than others. But as Greg's illustration shows, the Spitfire MK XIV varied about 8% from 1800rpm to 2400rpm (boost may have gone inverse. like -2lbs at 2400rpm and +4lbs at 2400rpm)
Without knowing the power required it gets increasingly into guess work as to the settings required for a particular aircraft to cruise at a given speed.

If you are depending on a 10% or less difference in fuel consumption to make to a particular target or group of targets on a regular basis (not a one off raid) then you are probably cutting things too fine. AN unexpected head wind could leave your planes just a few miles short of returning.

 

[MycroftHolmes]

It's a bit of a myth that the Mustang was significantly faster than a Spitfire with the same engine. The oft-quoted statement that the Mustang was '20-30 mph faster' applied to an early Spitfire IX with the Merlin 61 (nominally capable of 403 mph). By contrast the Spitfire HFIX with normal wing-tips and the Merlin 70 could do 419 mph, which suggests that a P51B would be ~10 mph faster if both aircraft were in pristine condition. Of course, the Mustang's wing was much more sensitive to dirt and dents and after a few missions the speeds would probably be more-or-less identical. The top speed of about 430 mph for the P51 is also quoted in Bowyer and Sharp's Mosquito.

 

[tomo pauk]

Hope that you don't mind butchering the post a bit

It's a bit of a myth that the Mustang was significantly faster than a Spitfire with the same engine. The oft-quoted statement that the Mustang was '20-30 mph faster' applied to an early Spitfire IX with the Merlin 61 (nominally capable of 403 mph). By contrast the Spitfire HFIX with normal wing-tips and the Merlin 70 could do 419 mph, which suggests that a P51B would be ~10 mph faster if both aircraft were in pristine condition.
The top speed of about 430 mph for the P51 is also quoted in Bowyer and Sharp's Mosquito.

The V-1650-3 engine was equivalent of the high-alt Merlins produced in the UK. The Mustang III (ie. P-51B) with that engine was good for 450 mph. data sheet ; chart (67 in Hg max boost on 130 grade fuel)

We also have a comparison between Spitfire V and Mustang II - on a worse engine the Mustang II was 30++ mph faster.

Of course, the Mustang's wing was much more sensitive to dirt and dents and after a few missions the speeds would probably be more-or-less identical.

I'll politely ask for surces about this sentence.