Spitfire Combat Radius (range) evolution, limitations?

[Wild_Bill_Kelso]

The RAF census as of February 1943 only has PR.III, IV. XI and XIII, with 30/30 III ordered/delivered, 229/232 IV, 152/31 XI (yes, more delivered than ordered) Nett conversions are 18 to III, 68 to IV and 4 to XIII. For June 1944 it is PR III, IV, X, XI, XIII, XIX and Spitfire PR (Griffon 66) with 30/30 III, 229/229 IV, 471/335 XI, 22/22 XIX and 229/0 Spitfire PR (Griffon 66) Nett conversions are 13 to III, 71 to IV, 16 to X, 25 to XIII.

I imagine the PR Spits went away pretty quickly as they certainly seemed to have a high attrition rate, at least in the Med. Not as bad as the Tac-R Hurricanes, but pretty bad. P-38 / F-4/5 recon planes too, very, very dangerous job. The Luftwaffe seemed to press far more resources into killing recon birds than to stopping bombers. It speaks to the importance of reconnaissance! The original military mission of aircraft.

 

[ThomasP]

Hey Shortround6,

Sorry, I should have posted a link to the following:

"http://www.wwiiaircraftperformance.org/mosquito/hx809.pdf"
"http://www.wwiiaircraftperformance.org/150grade/157sqdn-ORB-25lbs-3july44.jpg"

and

Somewhere on the internet . . . there is a very good write-up on the Mosquito using +25 lb boost operationally. IIRC it was mainly used in the anti-V1 (ie 'diver') and cross channel intruder roles. Unfortunately I do not remember where it is.

 

[Shortround6]

It wouldn't be the first time a set of pilot's notes listed something that is nearly impossible to do.
Pilot's manual for an A-36A says that you can pull 52in with RAM at 2500rpm.
The engine chart (estimated performance) without RAM says max power at sea level was about 1450hp at just over 50in.

To put this into perspective the manual says you can hit 1325hp military power at 5400ft at 47in using RAM.
The engine chart says that without ram you hit about 1350/60hp at 2500ft about 46.75in.

You need a crap load of RAM to hit the book numbers.


I don't doubt that some of those Mosquitos exceeded 18lbs of boost using 150 fuel. But getting close to 25lbs of boost means running the engines at full throttle while diving (even a shallow dive) or over revving the engines or operating very close to sea level (or a combination of all three). It might also require ability to override the supercharger shift device to engage Hi gear below normal altitude?

 

[SaparotRob]

Is that an Imperial crap load or a Metric crap load?

 

[GregP]

I'd imagine the altitude at which you can GET +25 psi boost was pretty low. So, it was very likely used as a V-1 chaser. The Merlin 25 was a Merlin 24 that turned the opposite direction. It was rated at 9,250 feet.

You certainly weren't going to get +25 psi at typical ETO combat heights. Power usually reduces by about 3% per 1,000 feet in the absence of boost but, when a supercharged engine reached critical altitude, it reduces that way anyway.

 

[Zipper730]

The MkXIV could fight with the 90G combat tank fitted and outperform the Me109 and FW190 with it attached.

Fascinating, but could the Mk.VII/VIII? While it had less fuel up front, it also had a heavier engine further forward, which might give it a center of gravity that would allow you to get away with stuff you couldn't do otherwise.

Trouble is that the cube root rule needs a correction factor once you get into the high 300mph range.

While I knew eventually mach effects would start to throw a monkey-wrench into that formula (almost always you'll never see something simple apply over a large array of conditions), I'm curious what correction factor is that?

 

[eagledad]

Gentlemen

From the Australian Archive check pages 140 to 147 in the PDF frame. There is data for the Mosquito FB VI at 25 pound boost.

FYI

Eagledad

147

 

[wuzak]

The Merlin 25 was a Merlin 24 that turned the opposite direction. It was rated at 9,250 feet.

No, the Merlin 25 had a "reversed flow cooling system". Though the actual internal flow was the same, the external cooling system was rearranged to suit the Mosquito's radiators.

The 25 turned the same way as most Merlins - the exceptions being engines like the 131 for the Hornet, which was the same as the 130 on the other wing but with reversed rotation.

 

[EwenS]

No, the Merlin 25 had a "reversed flow cooling system". Though the actual internal flow was the same, the external cooling system was rearranged to suit the Mosquito's radiators.

The 25 turned the same way as most Merlins - the exceptions being engines like the 131 for the Hornet, which was the same as the 130 on the other wing but with reversed rotation.

The reversal of the rotation in the Merlin 131, 133 and 135 was achieved by use of an extra gear in the reduction gear casing. The crankshaft turned in the same direction as the Merlin 130, 132 & 134 and all other Merlins.

 

[GregP]

No, the Merlin 25 had a "reversed flow cooling system". Though the actual internal flow was the same, the external cooling system was rearranged to suit the Mosquito's radiators.

The 25 turned the same way as most Merlins - the exceptions being engines like the 131 for the Hornet, which was the same as the 130 on the other wing but with reversed rotation.

My bad. You are correct ... similar to Merlin24 / 224 except for reverse cooling, not reverse rotation. I was one line off.

I'm really surprised to see a Merlin 25 with a line for +25 psi boost at 25,000 feet, and I'd not guess you could get that. That's 80.82 inches of Mercury, and that's a lot for a WWII engine at 3,000 rpm. I wonder if that was at 3,200 or 3,400 rpm? You can get 90 inches from an Allison, but it would be a G-series with the big impeller, turning at 3,200 - 3,400 rpm on the best fuel you can make ... at LEAST 140 or so. I see in the text that it is a short-duration power setting, but it is still surprising.

 

[PAT303]

The lion's part of the speed increase is the power increase. If a stock P-51 makes a 360 mph lap with 1,490 hp (pretty darn close to a stock P-51D lap) at Reno and makes NO OTHER changes other than bumping the power to 3,850 (can you say "Strega" or "Voodoo"?) the expected speed is 494 mph using the standard cube root rule. Voodoo's fastest lap is 512 with MOST in the 495 mph range, so all the little "extra" mods made a whole 18 mph bump after the 134 mpg bump that came from the power increase.

So the obvious streamlining, cut down canopy, blended joints and overall perfect fit and finish only added 18mph?.

 

[PAT303]

Fascinating, but could the Mk.VII/VIII? While it had less fuel up front, it also had a heavier engine further forward, which might give it a center of gravity that would allow you to get away with stuff you couldn't do otherwise.

It didn't need too, there is more than enough space in the leading edge to increase the tank capacity from 26G to 50G, add 75G in the rear, 96G main and a 90G dropper and you have the range to fight deep over the continent.

 

[GregP]

View attachment 664337
So the obvious streamlining, cut down canopy, blended joints and overall perfect fit and finish only added 18mph?.

Over and above the expected increase from the addition of power? Yes.

But, Reno is not a straight line. In a straight line, either Voodoo or Strega can probably make 540 mph at full tilt boogie. But, and here's the thing to note: we don't know the atmospheric conditions where it will make 540 mph (I figure warm and higher than sea level), so we don't know what a stock P-51D will do in a straight line at whatever altitude that is.

I DO know a stock P-51D will make about 360 mph around Reno with a completely stock engine. And we know Voodoo's and Strega's record speeds there. I made the calculation, but it's possible the speed would be higher for the P-51 than for, say, Rare Bear, because rare Bear has lower aspect ratio wings after race mods, and I don't know how much of the speed increase is due to a higher aspect ratio wing than Rare Bear, especially considering Rare Bear will ALSO make 540 mph in a straight line at full power.

So, it's an educated guess, but it's decently close.

 

[PAT303]

All the power in the world is pointless without the aerodynamic improvements to go with it, the Spitfire's speed improved with every little mod, not much, 5mph here and there but add up a couple and the gains, as well as more power are worthwhile. To give an example the Seafire Mk111 was 12mph faster than the MkII just by deleting the cannon stubs, reducing the cannon bulges by half and changing from triple ejectors to individual fishtail exhausts.

 

[drgondog]

Fascinating, but could the Mk.VII/VIII? While it had less fuel up front, it also had a heavier engine further forward, which might give it a center of gravity that would allow you to get away with stuff you couldn't do otherwise.


While I knew eventually mach effects would start to throw a monkey-wrench into that formula (almost always you'll never see something simple apply over a large array of conditions), I'm curious what correction factor is that?

Mach correction factor. The rough border between Incompressible Fluid and Compressible Fluid for aero is about M=0.3. Afterwards the Mach correction factor is applied to all Parasite and Form drag. 1/(1-M^2) is the multiplier. Refined in wind tunnel.

 

[GregP]

All the power in the world is pointless without the aerodynamic improvements to go with it, the Spitfire's speed improved with every little mod, not much, 5mph here and there but add up a couple and the gains, as well as more power are worthwhile. To give an example the Seafire Mk111 was 12mph faster than the MkII just by deleting the cannon stubs, reducing the cannon bulges by half and changing from triple ejectors to individual fishtail exhausts.

Of all the changes you can make to go faster, power is far and away the biggest. If you believe streamlining is more important, stay away from Reno, which is the very temple of power changes.

 

[wuzak]

The reversal of the rotation in the Merlin 131, 133 and 135 was achieved by use of an extra gear in the reduction gear casing. The crankshaft turned in the same direction as the Merlin 130, 132 & 134 and all other Merlins.

Yes, I should have said reverse rotation propeller.

 

[pbehn]

Of all the changes you can make to go faster, power is far and away the biggest. If you believe streamlining is more important, stay away from Reno, which is the very temple of power changes.

Well I think it is about fruit picking. Initially the quickest and cheapest way to make a P-51D go faster is to take all the guns and ammo out along with everything else you dont need, cover holes smooth and fill in gaps etc, but you can only do that once. The aerodynamic improvements have now been made, making a substantially better version than Voodoo would cost a fortune and is probably against the competition regs. All the low hanging fruit has been picked, only more power/thrust remains.

 

[Zipper730]

Mach correction factor. The rough border between Incompressible Fluid and Compressible Fluid for aero is about M=0.3. Afterwards the Mach correction factor is applied to all Parasite and Form drag. 1/(1-M^2) is the multiplier. Refined in wind tunnel.

So you'd cube root, then multiply by (1/(1-(M^2))? Or do you convert the airspeed to mach then do that?

 

[MikeMeech]

Hi
The Harleyford book, 'Spitfire - The Story of a Famous Fighter', despite being rather 'old' now does have some things of interest, including a list of Spitfire/Seafire 'variants':

The book 'Spitfire, The History' has much more detail and drawings on many of these variants. The SAM Publications Modellers Datafiles No. 3 'The Supermarine Spitfire Part 1: Merlin Powered' and No. 5 'Part 2: Griffon-Powered' also have many of the 'official' drawings in that helps to illustrate the variations and improvements, including added fuel tanks and drag improvements.

Mike