Bearcat vs Corsair

[drgondog]

Confusion about V1650-9 power settings

The data indicates that the V1650-9 produced 2200+ hp with 90" Hg using water injection and about 1600 hp with 67" Hg dry using 130 octane fuel. Something is missing here: what is the power output using 145 octane fuel without water injection? Wouldn't it be about 2000 hp? And shouldn't the comparison being made be between the P-51D using 145 octane fuel (widely available since mid-1944) at 75 Hg and the P51H using 145 octane fuel at 80 Hg, and also using water at 90Hg?


Ray - to get caught up you need the NA-8284A-A Report- which has the Power Calcs for all boost settings.

Then when examining the Revised Power Charts for the 1650-9, note that the charts are expressed as HP vs a.) Static, b.) 500mph ratings and c.) 400 mph ratings. Thus there are no 'simplistic' HP ratings as a function of Boost.

When water ran out for the P-51H, it would still be able to produce power at 80 Hg, somewhat better than the P-51D at 75 Hg.

Another careful distinction - in most cases when comparing Boost vs Boost between the 1650-7 and the 1650-9, the 1650-7 has more HP up to FTH for High Blower - then the -9 (and -3) start to have an advantage.

From other sources, the max speed of the P-51D using 75 Hg was about 450MPH. The P-51H (not a P-51 at all but a complete redesign based on the P-51) at the same power setting was about 10MPH faster than the P-51D. We can now answer the central question: what part of the improved performance of the P-51H was due to improved aerodynamics and what part was due to using water injection. The P-51H gained about 10MPH from better aerodynamics and 10+MPH from using water injection. Was the overall improvement, 20+MPH, worthwhile? Greg says no. But what if the war in Europe had continued and the allies has had to face numbers of 335's and 152's. The extra speed would have been useful and the higher rate of climb even more so. Wouldn't have helped against the 262 though.

Simple question but complicated answer. The P-51H was superior aerodynamically across the full spectrum of Reynold's Number but for Boost settings below WI and Dry when using the Simmons Manifold Pressure System/Speed Density Pump, the 1650 was more powerful below 25000 feet at 67, 61 and 46' HG boost

What about the future? The Hawker P1030 and the Supekrmarine 391 both had projected speeds of about 510MPH using the 3500 hp RR Eagle engine. That seems to represent the absolute limit for a combat aircraft that the combination of reciprocal engine and propellor was capable of, at least using a WWII wing.

Thin line between advanced development tested with combat loads and an operation aircraft purchased and placed into service.

The XP-51G was clocked 1t 498MPH with 2000 HP Rolls Engine. If production ready the 14 S.M. would have been used instead of the 1659-9.

 

[RayB]

Drgondog - The point you're making seems decisive. At the same power setting, the P-51H was about 20MPH faster than the P-51D. Why then did the P-51H in SAC service have a max speed of only about 470MPH when using water? I believe I've read that the P-51D had a max speed of about 450MPH when using 75" Hg. Seems like the P-51H in SAC sevice should have had a max speed closer to 490 than 470.

 

[wuzak]

Drgondog - The point you're making seems decisive. At the same power setting, the P-51H was about 20MPH faster than the P-51D. Why then did the P-51H in SAC service have a max speed of only about 470MPH when using water? I believe I've read that the P-51D had a max speed of about 450MPH when using 75" Hg. Seems like the P-51H in SAC sevice should have had a max speed closer to 490 than 470.

You have to look at the altitudes as well as teh top speed.

For instance, the top speed of the H at 90inHg was 471mph @ 22,700ft, at 80in Hg was 466mph @ 25,700ft and at 67inHg was 447mph @ 29,800ft.

P-51H Performance

For the V-1650-7 in the D the maximum speed at 67inHg was at a lower altitude - 440mph~@ 25,000ft.

This is because the different altitude ratings of the engine (different gear ratios in the supercharger drive).

 

[drgondog]

Drgondog - The point you're making seems decisive. At the same power setting, the P-51H was about 20MPH faster than the P-51D. Why then did the P-51H in SAC service have a max speed of only about 470MPH when using water?

The top speed with 90" for full combat load out of all ammo and fuel but with external racks attached at 22.7K is 410kts/472mph and 406kts/467mph at 25K. The Light version at 8283 pounds GW at take off is 412/474 and 408/470 respectively for those altitudes. It is in the 'Light" config that ROC is 5850 fpm at SL

I believe I've read that the P-51D had a max speed of about 450MPH when using 75" Hg. Seems like the P-51H in SAC sevice should have had a max speed closer to 490 than 470.

The top Calculated speed from NAA was 487mph at 90"/3000RPM without bomb/fuel tank or rocket racks, and based on the Performance spec for the -9 engine dated September, 1944. The data for sea level for the engine and all subsequent altitudes was adjust by combination of calibration, more flight tests and is published in the November 1946 report I referred to above, and is attached to the NAA Report following the May, 1945 Flight Test report on Mike Williams' site. It was lower than expected. AFAIK the use of ADI was never fully sorted out and the when the Simmons
speed density boost pump was sorted out, it remained and that version became known as the V-1650-9A.

I suspect, but do not know, that the 1949 and 1950 Tables were derived from the use of the 9A

 

[GregP]

Hi,

I decided to visit this thread one more time and throw in some aerodynamics for those who like it.

My premise was that if they took the P-51D and installed the V-1650-9 engine in it instead of the V-1650-7, they would have had substantially the same aircraft instead of the total redesign that the P-51H incorporated. To that end, I have calculated the top speed at altitude had the V-1650-9 been installed in the P-51D. It assumes no increase in drag, which is reasonable since the engines are the exact same size when you put them side by side. We HAVE at the Planes of Fame.

I took the power versus altitude for the V-1650-9 from: http://www.wwiiaircraftperformance.org/mustang/p-51h-powercurve.jpg

I took the power versus altitude for the V-1650-7 from: http://www.wwiiaircraftperformance.org/mustang/V-1650-3_7_power_curve.jpg

I got the speed versus altitude from the same sources in different tables. The speed versus altitude comes from: http://www.wwiiaircraftperformance.org/mustang/na-46-130-chart.jpg for the P-51D and the speed versus altitude for the P-51H comes from the link just above the power table for the H.

Almost everyone in here knows the speed gain with horsepower increase comes from the old speed times the (new power divided by the old power) raised to the 1/3 power. Everyone who knows aerodynamics knows it at any rate. Physics rules the world in real life.

The data looks like this:

My calculations. The second chart is speed versus altitude for the D and H and the third is speed versus altitude for the D, H, and new model. Speed is mph, altitude is feet, HP is HP. Simple.

So, the New P-51D with the -9 engine gets you 60 to 80% of the performance gain of the P-51H from sea level to 18,000 feet or so and again from 24,000 feet up to 32,000 feet or so. Between those altitudes it gets from 20 to 50% of the performance gain … just like I said it did.

And it does this IMMEDIATELY for almost no cost since the -9 engine fits the same engine mounts and avoids the entire cost of the P-51H's new airframe development. It ain't exactly a P-51H but it gets into combat with substantially better performance before the war ends and costs almost nothing to accomplish except installing a new Merlin variant. I've worked on changing Merlin variants, and it isn't a big deal if it fits.

And it DOES fit.

What it doesn't get you is a static change in C.G. I could live with that, even if I was flying it. Almost everone else did ... but, to be fair, not all.

The point is and was not to say the P-51H was a bad plane, it wasn't and I never said it was. I said it was useless to WWII since it didn't get into combat.

The intent was to get a better-performing P-51 into the war before it ended. Changing the engine DOES, without doubt, at least in my mind. If I were running the war, and I wasn't, I'd rather have had a P-51D with a -9 engine before the war ended in combat than a P-51H that flew around at 250 mph impressing someone's girlfriend and doing airshows after the war was over.

 

[FLYBOYJ]

Greg, good information, but again we're looking at straight up linear speed and altitudes. Acceleration and maneuverability needed to be factored in as well and there were other refinements to the P-51H that "would have" more than justified it's production had the war continued.

"Almost all the parts from the D line were not usable in the P-51H. This was the first production P-51 with a complete overhaul. The wing did not have that famous leading edge kink in it. The landing gear was visibly smaller. The profile shows new lines with a taller tail (later versions). The fuselage was a bit more slender and the length was increased to 33.33 feet. The wingspan stayed the same. The belly scoop inlet profile was not angled any longer but was now square again like the first P-51s. The chin scoop for the engine was decreased in size.

The wheels now had disk brakes. The oil cooler was moved from inside the belly scoop to in front of the oil tank ahead of the firewall. This eliminated the oil lines that ran from the engine to the old location in the scoop. The oil was now cooled by a heat exhanger mounted next to the engine intercooler.

The engine mounts were incorporated into the structural engine cradle, thus saving weight. The engine would not be the newer RS.14.SM Merlin as in some of the lightweight prototypes. The Rolls Royce Merlin V-1650-9 was chosen. Take-off horsepower was actually down from the -7 series to 1,380. But, the new -9 Merlin used water/alcohol injection and was able to up the war emergency power to 2,200 at 10,200 feet. This was the fastest production P-51 clocking 487 mph at 25,000 feet.

The propeller of the P-51H was the Aeroproducts 11'1" 4-blade Unimatic otherwise know as the "H prop". This prop is even lighter than the K model Aeroproducts but it looks much different. The blades are wider and keep approx. the same width almost the whole blade. The tips are rounded.

Armament was the same as in the P-51D. Removable ammo boxes and a redesign of the ammo doors were added. This saved time reloading and must have eased up on the laminar flow killing scratches and scuffs on the wings. The earlier models had to be loaded by hand out of portable ammo boxes. The top surfaces of the wings were taking a huge beating and disrupting the true laminar flow of the wing surface. I honestly doubt the crews in the field either knew or cared much about that.

Fuel in the fuselage tank was decreased to 55 gallons max. The fuselage skins were lighter and thinner, made from a new alloy. The cockpit panel was improved and simplified. The canopy was redesigned and the "hump" moved further forward. The pilot sat higher for a better angle using the gunsight."

P-51 Mustang Variants - P-51H - MustangsMustangs.com

 

[GregP]

I believe the H was a good decision had the war continued, and don't believe I ever said otherwise. What I said was it didn't make it into combat in the war.

The "new" D model I suggested would have been a stop-gap unit, improving the breed until the H got into the war, which it never did, but at least the new D would have made the war.

It's all in whether or not the end of the war was predictable. It really wasn't because even the nuclear scientists weren't really sure if the bomb was going to work when it was dropped for real. If it hadn't worked, the war would have drug on for another 8 months to a year or more and the H would have been in the fray. Had that happened you would have seen hordes of the US Navy version of the V-1, the JB-2 Loon, raining down on Japan. As it is, they (and WE) got away with many fewer casulaties the way the war really ended than would have otherwise been possible.

 

[pbehn]

P-51 Mustang Variants - P-51H - MustangsMustangs.com

Flyboy from your quoted article it says this, which surprised me (apart from the spelling of "Brittish")
quote
Brittish fighters were lighter than U.S. fighters. Schmued ask for detailed weight statements from Supermarine on the Spitfire. Schmued wanted to know why the Spitfires were so much lighter than the P-51. Supermairne did not have such data on the Spitfire, so they started weighing all the parts they could get a hold of and made a report for Schmued. The Brittish had design standards that were not at strict in some areas of design as the U.S. Landing gear, angle of attack and side engine design loads were higher in the U.S.
unquote

I am no expert but I cannot believe that it took until the design of the "H" to notice that British fighters were lighter, the P51 was always heavier than a Spitfire with the same engine. I think the above quote was really the US going to a lower design standard that had worked in practice to allow a weight reduction not allowed under USA design regs. I personally find the statement "all the parts they could get hold of" amusing, since the US operated Spitfires and the lease lend agreement meant sharing all British technology there is no part of a Spitfire that wasnt available to any US scientist/engineer who the US wanted to have it, I think the weighing and evaluating of components was to see just how much (or little) they could get away with. This is a soft ball soft Flyboy, upon your reply depends the status of the P51H, did North American really have to weigh parts of a Spitfire to get the design right? I think they did an engineering/design revue to see where weight could safely be saved using other almost comparable designs as a precedent/base for study.


edit...from the original post I cannot really tell who did the weighing, Supermarine or US engineers working for Schmeud?

 

[GregP]

The US standards are and were not lower or higher than British standards. They worked to different set of requirements. The Spitfire was many things, but one of them was fragile. So was and is the Hurricane, as fabric-covered planes must necessarily be. It also makes them very repairable in the case of minor damage that has a lot a fabric tears. US fighters are not and were not fragile.

Doesn't mean one way or the other is wrong, it just means we spcified different things be strressed to different design criteria. The British specs worked for the ETO and resulted in a great aircraft. We got the same in other aircraft on our own. Together they carried the airwar in the ETO.

US aircraft ALWAYS have a detailed weight breakdown and I'm sure the North American guys were quite surprised that British didn't have one done for the Spitfire already. If you donlt HAVE one, how do you know where to start looking for weight savings? It's a case of different thinking, not superior or inferior design.

 

[wuzak]

The Spitfire was fragile?

 

[GregP]

Quite fragile relative to a US fighter. You might expect that given how much lighter it is. If you didn't expect that mauybe you didn't think about it in terms of more weight means more metal.

I say that having had one for more than 6 years at the museum. The difference in care that must be taken during servicing it versus a P-51 is an eye opener. Land a Spitfire on a carrier a few times and it starts coming apart if you aren't careful at touchdown. It jsut wasn't designed for hard landings. British squadron commanders were famous for requiring their pilots to 3-point the Spitfire, even after battle. It may technically be the correct way to do it but it almost always results in a touchdown at the lowest possible airspeed, too. Best way to get a soft landing.

There's nothing wrong with it being a bit fragile, but you DO have to think about it if you are used to servicing a P-40 or P-51 and find yourself servicing a Spitfire. Mind you it's unkilely since US planes don't have Whitworth fasteners and you'd need a new set of tools ... but it is noticeable.

 

[tomo pauk]

R. W. Gruenhagen states on the pg. 117, about E. Shmued's visit to the UK in 1943:
After visiting the Spitfire construction plants, it was obvious to Schmued that an airframe weight reduction was necessary in the Mustang. A staff was assembled to assist him in acquiring data on the construction of the Spitfire. The British system of engineering did not provide the answers needed by Schmued and the team had to weight and measure the airframe components to satisfy their needs. This was a tedious task but the information was finally assembled for transfer to the Inglewood.

However, same author says that already by Jan 2nd 1943 NAA planed to construct two experimental light weight fighters using British load factors as a guide to construction. After Schmued's visit to the UK and consolidation of the data, the number was increased to 5 aircraft.
NAA techicians also examined Bf 109, per same author.

Hopefully an owner of the book 'Mustang designer' could provide additional info, especiailly since Gruenhagen's book, for all it's qualities, is woefully short of footnotes and sourcing.

 

[rochie]

Jeez Greg where do you get this crap from, never heard a Spitfire or Hurricane described as fragile before ?

Hurricanes were fabric covered because they were progression of Hawker's earlier designs but we're turned into carrier aircraft and fired off CAM ships with rockets.
Spitfires were comparable with the best prop driven fighters from its birth to its retirement which was a lot longer than most.
And all the time it was hardly recorded they were any weaker or more fragile than anything else around !


Beginning in late 1943, high-speed diving trials were undertaken at Farnborough to investigate the handling characteristics of aircraft travelling at speeds near the sound barrier (i.e., the onset of compressibility effects). Because it had the highest limiting Mach number of any aircraft at that time, a Spitfire XI was chosen to take part in these trials. Due to the high altitudes necessary for these dives, a fully feathering Rotol propeller was fitted to prevent overspeeding. It was during these trials that EN409, flown by Squadron Leader J. R. Tobin, reached 606 mph (975 km/h, Mach 0.891) in a 45° dive. In April 1944, the same aircraft suffered engine failure in another dive while being flown by Squadron Leader Anthony F. Martindale, RAFVR, when the propeller and reduction gear broke off. The dive put the aircraft to Mach 0.92, the fastest ever recorded in a piston-engined aircraft, but when the propeller came off the Spitfire, now tail-heavy, zoom-climbed back to altitude. Martindale blacked out under the 11g loading, but when he resumed consciousness he found the aircraft at about 40,000 feet with its (originally straight) wings now slightly swept back.[113] Martindale successfully glided the Spitfire 20 mi (32 km) back to the airfield and landed safely.[114] Martindale was awarded the Air Force Cross for his exploits.

Not bad for a fragile one trick pony !

 

[GregP]

I don't get this crap from anywhere, I observe it. Go do the same and come back. A Spitfire in a hangar is not going to appear fragile. But go maintain it for awhile and the difference is quite clear.

If you don't know, don't call crap on it, guy, 12 thousand posts or not.

Maybe ask Joe? He might know, being in the industry.

And fragile doesn't mean it can't be repaired or isn't serviceable at all, it means it's easy to damage and doesn't tolerate rough handling. It didn't and doesn't.

 

[pbehn]

The US standards are and were not lower or higher than British standards. They worked to different set of requirements. The Spitfire was many things, but one of them was fragile. So was and is the Hurricane, as fabric-covered planes must necessarily be. It also makes them very repairable in the case of minor damage that has a lot a fabric tears. US fighters are not and were not fragile.

Doesn't mean one way or the other is wrong, it just means we spcified different things be strressed to different design criteria. The British specs worked for the ETO and resulted in a great aircraft. We got the same in other aircraft on our own. Together they carried the airwar in the ETO.

US aircraft ALWAYS have a detailed weight breakdown and I'm sure the North American guys were quite surprised that British didn't have one done for the Spitfire already. If you donlt HAVE one, how do you know where to start looking for weight savings? It's a case of different thinking, not superior or inferior design.

From the same article
Quote
The Brittish had design standards that were not at strict in some areas of design as the U.S. Landing gear, angle of attack and side engine design loads were higher in the U.S.
Unquote

The Hurricane and Spitfire started life designed around a 850HP engine a twin blade prop and carrying 4 x .303 mgs. During WWII the Spitfire approximately doubled its weight. Supermarine/Vickers must have known the weight of the aircraft and before any change must have considered the weight. In a detailed study you may weigh each nut, bolt and component or the weight of each assembly. If the British weighed each assembly then that data would not be of use to North American if they wanted to compare differences down to each nut and bolt.

 

[rochie]

I don't get this crap from anywhere, I observe it. Go do the same and come back. A Spitfire in a hangar is not going to appear fragile. But go maintain it for awhile and the difference is quite clear.

If you don't know, don't call crap on it, guy, 12 thousand posts or not.

Maybe ask Joe? He might know, being in the industry.

And fragile doesn't mean it can't be repaired or isn't serviceable at all, it means it's easy to damage and doesn't tolerate rough handling. It didn't and doesn't.

well that told me !

on the contrary i think spitfires look very fragile in a hanger.

i am glad we only used them on sunny days in august from long perfectly smooth concrete runways and didnt try to make a carrier fighter out of it or operate in the desert or rough european and paciific airfields, telling the pilots not to try and turn too fast in case the wings came off.

luckily we didnt try and re arm / re fuel in a hurry in case we scratched the paint as it seems that was all that was holding it together.

just as well we never actually fought in them as any thing more than a BB pellet would of had them falling from the sky in droves !

my point being i dont recall history labelling the Spitfire fragile, it might not have had the legendary toughness of a P-47 but hardly fragile either !

as i said before.

not bad for a fragile one trick pony !

 

[drgondog]

Hi,

I decided to visit this thread one more time and throw in some aerodynamics for those who like it.

So, in your 'aerodynamic thesis' of plug and play it doesn't look like you found a table set of Test data for the P-51D with full internal fuel load, nor does it look like you calculated the delta Induced drag due the missing 480+ pounds of weight differential for the P-51D-15 tested in June. Next Greg, could you reference the Parasite Drag vs Mach no. curves for the P-51D and show that you extended the rather steep drag rise curve for those extra MPH? You know, the one for CDm/CDp vs M for each CL value from <.1 to .3?

My premise was that if they took the P-51D and installed the V-1650-9 engine in it instead of the V-1650-7, they would have had substantially the same aircraft instead of the total redesign that the P-51H incorporated. To that end, I have calculated the top speed at altitude had the V-1650-9 been installed in the P-51D.

It assumes no increase in drag, which is reasonable since the engines are the exact same size when you put them side by side. We HAVE at the Planes of Fame.

The delta drag is due to the airframe differences Greg, as well as the difference between load outs in the NAA Performance calculations for both the P-51D and the P-51H. You failed to note that the 'Assumptions list' for the Calculated curves clearly states 180 gallons (of 269) for internal fuel load. So, if you have done anything of substance other than plug and play table data (extremely doubtful), then you will tell us how much CL and thus CL^^2/(Pi*AR*e)difference you applied to your numbers?

Then, Assuming there is Zero Parasite Drag difference between the two when there is in fact about 8% sends you back to the dumpster to recover your Purdue (?) undergrad notes.



I took the power versus altitude for the V-1650-9 from: http://www.wwiiaircraftperformance.org/mustang/p-51h-powercurve.jpg

I took the power versus altitude for the V-1650-7 from: http://www.wwiiaircraftperformance.org/mustang/V-1650-3_7_power_curve.jpg

I got the speed versus altitude from the same sources in different tables. The speed versus altitude comes from: http://www.wwiiaircraftperformance.org/mustang/na-46-130-chart.jpg for the P-51D and the speed versus altitude for the P-51H comes from the link just above the power table for the H.

See above for the two major flaws in your comparisons... and your logic... and your understanding of the 'physics'.

Almost everyone in here knows the speed gain with horsepower increase comes from the old speed times the (new power divided by the old power) raised to the 1/3 power. Everyone who knows aerodynamics knows it at any rate. Physics rules the world in real life.

Almost everybody understands that in a low mach number fluid field, that the relationship is a decent 'rule of thumb'. The lesser 'lights' - or dimmer bulbs will pause for a deep breath when the vehicle/prop system ventures into the medium and high drag fluid flow region. To help illuminate the variables of interest, the tip speed to forward velocity reaches the non linear range sooner than the airframe for comparable Drag rise factor. The CDm/CDp ratios for the Mustangs (slightly lower for H than D)attain a value of ~ 1.5 at .5M, and ~ 1.25 at .6M for a Lift Coeeficient <= .1, and steeper for CL between .1 and .2 (which should be interesting to the intellectually curious when discussion 'aerodynamics' and 'physics' and throwing out terms like "everyone who knows aerodynamics knows this. Physics rules the world in real life." Did YOU know anything about the drag rise ratios Greg. Did you apply them. Did you break out the CL vs Induced drag differences from the NAA Report vs your tutorial on the comparisons?

Thought so.

The data looks like this:

View attachment 283217

My calculations. The second chart is speed versus altitude for the D and H and the third is speed versus altitude for the D, H, and new model. Speed is mph, altitude is feet, HP is HP. Simple.

Yes, Pretty simple - so how did you recalculate the NA Report for 269 gallons of fuel?


So, the New P-51D with the -9 engine gets you 60 to 80% of the performance gain of the P-51H from sea level to 18,000 feet or so and again from 24,000 feet up to 32,000 feet or so. Between those altitudes it gets from 20 to 50% of the performance gain … just like I said it did.

Yes. Impressive analysis.

And it does this IMMEDIATELY for almost no cost since the -9 engine fits the same engine mounts and avoids the entire cost of the P-51H's new airframe development. It ain't exactly a P-51H but it gets into combat with substantially better performance before the war ends and costs almost nothing to accomplish except installing a new Merlin variant. I've worked on changing Merlin variants, and it isn't a big deal if it fits.

I suspect that you realize that our DoD has a tendency to buy increased combat capability. The P-51H decision was made before the first P-51D rolled off the assembly line. The aircraft the P-51H was compared to was the P-51B in the context of 'Information at hand".

And it DOES fit.

What it doesn't get you is a static change in C.G. I could live with that, even if I was flying it. Almost everone else did ... but, to be fair, not all.

Ah, it (P-51D w/1650-9 vs P-51H) not only failed to solve the CG problem, it did not match performance across the board in ANY category, AND also carried with the migration a lower combat G capability, more yaw problems at take off and high speed flight, and slightly less Critical Mach speed than the lower drag P-51H wing.

The point is and was not to say the P-51H was a bad plane, it wasn't and I never said it was. I said it was useless to WWII since it didn't get into combat.

Greg - you are back pedaling from many more statements if you care to review the comments.

The intent was to get a better-performing P-51 into the war before it ended. Changing the engine DOES, without doubt, at least in my mind. If I were running the war, and I wasn't, I'd rather have had a P-51D with a -9 engine before the war ended in combat than a P-51H that flew around at 250 mph impressing someone's girlfriend and doing airshows after the war was over.

It did all that while the ETO battle was still being fought, Greg. Your hindsight is often 20/20 but even you have to feel a little silly pontificating on a decision You would have made in January/February 1944 when the NAA team was putting a wrap on discussions with the Brits and conceptualizing the H for presentations to AAF? Just a little bit?

 

[drgondog]

The US standards are and were not lower or higher than British standards. They worked to different set of requirements. The Spitfire was many things, but one of them was fragile. So was and is the Hurricane, as fabric-covered planes must necessarily be. It also makes them very repairable in the case of minor damage that has a lot a fabric tears. US fighters are not and were not fragile.

Please do some research. The US Standard for Ultimate load was 12 for the symmetrical aero (read dive pull out) loading, the Brit standard was 11. The NAA team also designed for a lateral 2G load due to the engine. The XP-51F and subsequently the P-51H was designed to 11G ultimate/7.26G Limit. The P-51H was stressed for 11/7.26G at 9400 pounds GW. The XP-51 was designed for 12G/8G for 8000 pounds. By the time the B/D was in service at 9700 and 10,300 combat GW without external load, the rating was 6.59 and 6.21 Limit load respectively for those GW's.

Doesn't mean one way or the other is wrong, it just means we spcified different things be strressed to different design criteria. The British specs worked for the ETO and resulted in a great aircraft. We got the same in other aircraft on our own. Together they carried the airwar in the ETO.

The P-51H was the only US fighter designed to 11G Ultimate (aero) load, 4G (instead of six or more) for landing load. The other mainstream US fighters all grew in combat weight and their Ultimate/Limit N factors reduced accordingly.

Both somehow limped along and served well in all theatres, not just the ETO.

US aircraft ALWAYS have a detailed weight breakdown and I'm sure the North American guys were quite surprised that British didn't have one done for the Spitfire already. If you donlt HAVE one, how do you know where to start looking for weight savings? It's a case of different thinking, not superior or inferior design.

Do you perhaps have any data comparing the robustness of the Mustang to the 'Fragility' of the Spitfire?

 

[Balljoint]

Fragile is a tricky term, particularly with regard to Gs. A lightweight plane imposes lower forces on its structure at a given G loading. However, it may not do as well with someone walking on the wing or a stone strike. My opinion is that the Spit was plenty strong but perhaps not as robust as the P-51.

 

[drgondog]

Fragile is a tricky term, particularly with regard to Gs. A lightweight plane imposes lower forces on its structure at a given G loading. However, it may not do as well with someone walking on the wing or a stone strike. My opinion is that the Spit was plenty strong but perhaps not as robust as the P-51.

Balljoint - the way Limit Load analysis works for Aero loads (i.e - pulling out of a dive in which the lift load far exceeds W), for say N=8G and the airplanes is stressed for all areas pertinent to surviving the loads (.i.e the Spar and fuselage carry through structure first, then secondary items like the shear panels surrounding the spar which distribute the loads - or the engine mounts supporting the engine and beam the G load into the longerons) at the Design Gross Weight.

The P-51 started with a design Gross weight of 8000 pounds. The N=+8G says to the airframe stress engineer that the lift load to be absorbed is 8x8000 pounds distributed over the 'beam' (i.e Spar) with the distribution curve supplied by the Aerodynamics team.

When that airplane adds fuel, guns, more ammo it rarely goes back for re-design and re-engineering to accommodate the 'weight creep"- so that wing (in the case of XP-51-P51A/A-36 to P-51B to P-51D 'grew' in Gross Weight from 8,000 pounds to 10,300 pounds for the D for full fuel and ammo.

As a result, the aerodynamic loads at 8G translates to 82,400 pounds distributed along the span at the spar instead of the original design target of 64,000 pounds. If the D actually pulls 8G Now, then many stresses designed at Yield point move from the elastic range to the Plastic region and parts 'stretch' causing a dangerous movement toward failure of the subjected part of interest.

If the P-51D at 10,300 pounds GW it hits the design stress load of 64,000 pounds at 6.21G. (64000/10700), it hits the Limit Load. 50% more than that causes the structure to approach Ultimate where it normally 'Breaks'.

Back to the Spitfire. The original Mark I had a design Gross weight of about 5280 pounds. The Mark IX was about 6622 pounds. We do not know (at least I do not know whether any re-design and manufacture the wings and carry through structure to accommodate the increase in GW. If so then perhaps the Spit maintained the 11G ultimate Load limit for stress analysis as the weight increased from 5280 to 6622 pounds. If it remained unchanged then the original Ultimate load of N=11G translates to 58080 pounds of beam load on the wing but when it grew in weight, only 8.7G would load up the Mark IX wing to Ultimate failure range or about 5.8 Limit versus the P-51D Load of 6.21 for Limit G load.