Spitfire Combat Radius (range) evolution, limitations?

[tomo pauk]

1,4701 hp, 380 mph @ 22,500 ft for the Mk Vb.

1,720 hp, 408 mph @ 22,000 ft where, with the cube root of the power increase rule, we'd expect 400 mph. So, the new nose, 4-bladed prop, and whatever "cleanup" was done added 8 mph to the expected top speed. Not bad, but not great, either. Still, it WAS a useful jump in performance.

At 22500 ft, the Merlin 45 or 50 on the Mk.V will be making about 1000 HP with ram, not 1470. Not even the Merlin 46 or 47 (the ones with big S/C) will do more than 1100 at 22500 ft. See your chart.
The pilot of Spitfire VB will be very happy if it can fly at 360 mph at 22500 ft straight and level.
The Merlin 66 will do perhaps 1550 HP at 22500 ft with ram. FWIW: data sheet

 

[pbehn]

There is a chart here of a Spitfire V using 16 lbs boost doing 378 MPH at 15-17,000 ft and circa 370 at 22,500ft http://www.wwiiaircraftperformance.org/Spitfire_V_Level_Speed_RAE.jpg

 

[GregP]

Those must be sea level speeds.

At 22500 ft, the Merlin 45 or 50 on the Mk.V will be making about 1000 HP with ram, not 1470. Not even the Merlin 46 or 47 (the ones with big S/C) will do more than 1100 at 22500 ft. See your chart.
The pilot of Spitfire VB will be very happy if it can fly at 360 mph at 22500 ft straight and level.
The Merlin 66 will do perhaps 1550 HP at 22500 ft with ram. FWIW: data sheet

Hi Tomo.

The formula is: New Speed = (cube root [new power / old power]) * old speed for estimate of speed increase due to power alone.

It may surprise you that I have spreadsheets with the Merlins, Griffons, and Allisons, etc. in them, but not all of them report power at the same altitudes. I don't mind doing a power chart at all, but I sort of flinch when someone wants me to go to the trouble of doing power charts for all models of two separate engine designs. It isn't that important since we basically KNOW what these airplanes will do.

For the purposes of estimating a change in performance due to power increase alone, knowing the power of one of them at some altitude does nothing unless we know the power of the other one at the same altitude. Since I see and work on warbirds almost every week, I'm not that interested in spending time digging around for no good reason, and there isn't in this case. 80%+ of the increase in performance of the MK. IX over the Mk. V was from the increased engine power. I wouldn't be surprised at all if it turned out to be 90%+. Take a MK. IX and put in a Merlin 45 and you'd have almost an exact clone of a Mk. V.

And there's nothing new to either you or me in the above at all.

Edit:

Sorry, Tomo.

When I re-read the post above, it doesn't come off very nice. Unintended originally. So, apologies and I attach one of my Merlin charts with Horsepower versus altitude for P.V. 12 up through Merlin 66. From there, we could make some decent estimates. The chart I made from the original Rolls-Royce chart is on the second tab.

 

[pbehn]

Those must be sea level speeds.


Hi Tomo.

The formula is: New Speed = (cube root [new power / old power]) * old speed for estimate of speed increase due to power alone.

It may surprise you that I have spreadsheets with the Merlins, Griffons, and Allisons, etc. in them, but not all of them report power at the same altitudes. I don't mind doing a power chart at all, but I sort of flinch when someone wants me to go to the trouble of doing power charts for all models of two separate engine designs. It isn't that important since we basically KNOW what these airplanes will do.

For the purposes of estimating a change in performance due to power increase alone, knowing the power of one of them at some altitude does nothing unless we know the power of the other one at the same altitude. Since I see and work on warbirds almost every week, I'm not that interested in spending time digging around for no good reason, and there isn't in this case. 80%+ of the increase in performance of the MK. IX over the Mk. V was from the increased engine power. I wouldn't be surprised at all if it turned out to be 90%+. Take a MK. IX and put in a Merlin 45 and you'd have almost an exact clone of a Mk. V.

And there's nothing new to either you or me in the above at all.

As I understand it a single stage Merlin could produce the same power as a two stage Merlin if it had the fuel and uprated parts and maybe bigger radiator(s), but why would you do that, a lot of time and effort to make a sows ear out of a sows ear?

 

[GregP]

A single-stage Merlin could produce about the same power at sea level and, likely be decently close up to it's critical altitude. The single-stage engine might even make a few more hp down low since it is not driving the same supercharger and therefore uses less power than the 2-stage engine supercharger.

After that critical altitude for the single-stage, the single and two -stage engine will stay decently close until the altitude where the pilot changes supercharger gears in the 2-stage aircraft. Then, the 2-stage guy will pull away dramatically and continue to perform quite well until his critical altitude, after which he will gradually lose performance like a normal engine does until he reaches the altitude where the max power he can make is just the power required for level flight. That will be absolute ceiling.

The above assumes the pilot has oxygen and/or a pressure cabin or he will likely pass out before reaching critical altitude.

Edit:

The above is theoretical and makes some assumptions. The assumptions made are as follows:
1. Both engines are running the same fuel.
2. Both engines are running the same boost.

So, a single-stage Merlin 45 made 1,480 hp at 12,500 feet.
A 2-stage Merlin 63 made 1,710 hp at 8,500 feet, and did so be virtue of running more boost due to have an extra compressor stage.

These engines were about a year apart, and development usually results in better numbers up until the limit has been reached. This assumes a real development effort and not just an experiment.

But, in real life, they didn't follow assumption 1 and 2. In WWII, Merlins were cleared for more boost on a VERY regular basis. If you look at the single-stage engines and 2-stage engines, almost every new Merlin model gave an increase in boost and HP. Not EVERY one, but most of them. By way of example, the Merlin 45M could make 1,580 hp and the Merlin 61 only made 1,565. But the Merlin 45 was well-tweaked for HP and the Merlin 61's main intent was to make better HP up a lot higher. It DID that, but the Merlin 45M was the superior engine at 3,000 feet ... assuming you were flying a Spitfire at 3,000 feet, that is. If you were in Mk. IX, you were much more likely at 18 - 25,000 feet.

 

[PAT303]

Having small improvements add up to a total speed increase is not my experience with warbirds at all, and it also doesn't work very well with civil aircraft, either, say a Mooney 201. I have a good friend who has one. There are lots of mods out there and each has a speed increase associated with it. Adding them all will NOT give you an additive speed increase.

You have lost me here, you only have to look at Reno racers to see the effect all the little improvements make.

 

[PAT303]

There was a tactical aspect too. The commander, an Australian Ace named Clive Caldwell,

He was not a very liked man, too many times he had Spitfires endlessly circling until he was happy everyone formed up under him before attacking while the Betty's flew off into the distance. No aircraft could catch a Betty with a 10 mile head start in that situation.

 

[PAT303]

I'm surprised they never added aft fuel to that design then...

So is everyone else considering how blindingly obvious it was

 

[PAT303]

When carrying the 90 or 175 gal tank the aircraft was restricted, once airborne and at cruising altitude, to straight and level flight.

So was every other aircraft when loaded with drop tanks or bombs, the much vaunted P51 would stall and pancake if not flown straight and level when fueled up, I remember reading once about a red tail pilot that would take off with the canopy unlocked so he had a chance to get out at low level if his plane stalled. The MkXIV could fight with the 90G combat tank fitted and outperform the Me109 and FW190 with it attached.

 

[GregP]

You have lost me here, you only have to look at Reno racers to see the effect all the little improvements make.

You must be kidding.

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.

 

[tomo pauk]

Hi Tomo.

A simple "yes, I'm wrong, Merlin 45 was not near 1470 HP at 22500 ft" would've been just fine.

80%+ of the increase in performance of the MK. IX over the Mk. V was from the increased engine power. I wouldn't be surprised at all if it turned out to be 90%+.

Bingo.

Take a MK. IX and put in a Merlin 45 and you'd have almost an exact clone of a Mk. V.

A Mk.IX with Merlin 45 would've been faster than the Mk.V, featuring less draggy exhausts, less draggy BP glass layout, no ice guard, and a bit better fit & finish.

As I understand it a single stage Merlin could produce the same power as a two stage Merlin if it had the fuel and uprated parts and maybe bigger radiator(s), but why would you do that, a lot of time and effort to make a sows ear out of a sows ear?

A single-stage Merlin could produce about the same power at sea level and, likely be decently close up to it's critical altitude. The single-stage engine might even make a few more hp down low since it is not driving the same supercharger and therefore uses less power than the 2-stage engine supercharger.

1600+ HP for the latest 1-stage Merlins, 2000+ HP for the latest 2-stage Merlins (both for the cases with latest fuel and no water-alcohol injection, and at low altitude).

After that critical altitude for the single-stage, the single and two -stage engine will stay decently close until the altitude where the pilot changes supercharger gears in the 2-stage aircraft. Then, the 2-stage guy will pull away dramatically and continue to perform quite well until his critical altitude, after which he will gradually lose performance like a normal engine does until he reaches the altitude where the max power he can make is just the power required for level flight.

The pilot of an aircraft with a 2-stage engine already has the advantage down low.
There is such a thing as an 1-stage 2-speed supercharged engine, too.

 

[Shortround6]

Trouble is that the cube root rule needs a correction factor once you get into the high 300mph range.
Some planes will start showing a difference due to airfoil or local air flows/ compressability even if they tracked the cube root rule pretty well over a range of several hundred mph.

 

[Shortround6]

The single stage Merlins topped out at 18lb of boost.
You might be able to get more low down without a boost limiter.

The two stage engines started out at 12lbs of boost and worked their way up to 25lbs of boost.

There were a few things that limited power at times. The single stage engines didn't get to use 18lb of boost until the supercharger drive system was beefed up. Somewhere between 14lbs and over 16lbs it was possible to start breaking the drive shaft and or clutches. The up graded drive system didn't arrive until after the 2 stage supercharger showed up.
A two stage system actually makes things a bit easier on the engine. A two stage supercharger will actually take less power to drive for the same manifold pressure and heat the intake charge less. And that is without the intercooler. The 2 stage engine is flowing more air ( it is denser) that a single stage engine using the same manifold pressure.

The corrections for different bobs and bits on the Spitfire (outside bullet proof glass, IFF aerials, snow guards,etc)
Need a bit of interpretation to. As the speed goes up from 360-370mph to over 400mph the actual effect will go up slightly, it was just easier to use a fast mph correction than figure out the percentage.
Two of fittings can actually make a difference in power. I have never really seen any charts or much talk about the difference in thrust between exhaust manifolds. We are told that one style had less drag than another but believing that both had the same amount of thrust is a bit harder to swallow. But since exhaust thrust varies with both altitude and speed of the aircraft even with the engine burning the same amount of air it gets simplified in most explanations.
The "snow guard" is another piece that has some different effects. We are told it was worth about 6mph.
But what it did was disrupt the airflow going into the intake and lower the ram effect. This lowered the critical altitude (FTH) of the engine. So the engine is making less power at a given height than a plane without the snow guard, this is somewhat altitude dependent.

 

[tomo pauk]

The single stage Merlins topped out at 18lb of boost.
You might be able to get more low down without a boost limiter.

The two stage engines started out at 12lbs of boost and worked their way up to 25lbs of boost.

There were a few things that limited power at times. The single stage engines didn't get to use 18lb of boost until the supercharger drive system was beefed up. Somewhere between 14lbs and over 16lbs it was possible to start breaking the drive shaft and or clutches. The up graded drive system didn't arrive until after the 2 stage supercharger showed up.

1-stage Merlin* with 'normal'-sized impellers were making +16 psi max. The ones with cropped impeller were the ones allowed for +18 psi.

A two stage system actually makes things a bit easier on the engine. A two stage supercharger will actually take less power to drive for the same manifold pressure and heat the intake charge less. And that is without the intercooler. The 2 stage engine is flowing more air ( it is denser) that a single stage engine using the same manifold pressure.

Very true.

Two of fittings can actually make a difference in power. I have never really seen any charts or much talk about the difference in thrust between exhaust manifolds. We are told that one style had less drag than another but believing that both had the same amount of thrust is a bit harder to swallow. But since exhaust thrust varies with both altitude and speed of the aircraft even with the engine burning the same amount of air it gets simplified in most explanations.

The bigger bulk of the 3-per-side exhausts vs. the 6-per-side is apparent looking at pictures and drawings. Eg. here vs. here.
The 'individual' exhausts will certainly also give a bit better exhaust thrust, the 2-stage Merlin already making the better exhaust thrust on it's own than the 1-stage type.
Cutting the drag by some fixed percentage usually gives more extra speed than increasing the propulsive power by the same percentage. The cube root law is ... evil; or, the "drag is cruel" saying I've 1st heard by Bill Marshall.
Granted, many times in ww2 was easier to increase the power by a good deal than it was to cut the drag by a good deal.

The "snow guard" is another piece that has some different effects. We are told it was worth about 6mph.
But what it did was disrupt the airflow going into the intake and lower the ram effect. This lowered the critical altitude (FTH) of the engine. So the engine is making less power at a given height than a plane without the snow guard, this is somewhat altitude dependent.

Agreed again.

* goes for the Merlins usually found on Spitfires like the 40s and 50s series

 

[Geoffrey Sinclair]

The early Spitfire PR types are worthy of several here be dragon signs. Conversions, conversions of the conversions, conversions of the converted conversions, new builds along with designation changes. Starting with the letter codes, PR A, B, C, D, E, F, G, which are reported to later been called PR mark I, II, III, IV, V, VI, VII. The designations covering different fuel capacity, camera and engine arrangements.

In 1940 the mark III and IV were allocated to fighter versions but the III was cancelled and the IV redesignated mark 20 during 1941 to early 1942, clearing the way for the mark numbers to be used by PR types. When doing so the letter C and mark III was used to designate short range, while D and mark IV long range (wing leading edge tanks usually), regardless of engine and camera fit, with the official production total of thirty PR mark III and two hundred and twenty nine PR mark IV reported by both the RAF and Ministry of Aircraft Production, totals which exclude the two prototype mark III, which were type D, long range, P9551 (September 1940), P9552 (March 1941) to PRIII type D (leading edge wing fuel tanks), prototypes on contract S.B. 2415/C.23A.

The other PR.III were R7029 to 7034, X4332 to 35, X4383 to 4386, X4491 to 4505 and X4538, with the R serials delivered last.

So 32 III and 229 IV produced as such. Mark III production, 9 August/September 1940, then the remainder November 1940 to May 1941, with mark IV production starting in June, probably.

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.

 

[ThomasP]

Mosquito Merlin 25 (normal Ø10.25" impeller and 8.15/9.49:1 S/C ratios) 1-stage 2-speed low & high gear was rated at +25 lb boost with 150 grade
Spitfire Merlin 66 2-stage 2-speed low & high gear was rated at +25 lb boost with 150 grade

 

[tomo pauk]

Mosquito Merlin 25 (normal Ø10.25" impeller and 8.15/9.49:1 S/C ratios) 1-stage 2-speed low & high gear was rated at +25 lb boost with 150 grade
Spitfire Merlin 66 2-stage 2-speed low & high gear was rated at +25 lb boost with 150 grade

Of course you're right. I'll edit my post above.

 

[GregP]

A simple "yes, I'm wrong, Merlin 45 was not near 1470 HP at 22500 ft" would've been just fine.



Bingo.



A Mk.IX with Merlin 45 would've been faster than the Mk.V, featuring less draggy exhausts, less draggy BP glass layout, no ice guard, and a bit better fit & finish.





1600+ HP for the latest 1-stage Merlins, 2000+ HP for the latest 2-stage Merlins (both for the cases with latest fuel and no water-alcohol injection, and at low altitude).




The pilot of an aircraft with a 2-stage engine already has the advantage down low.
There is such a thing as an 1-stage 2-speed supercharged engine, too.

I should have realized in doesn't matter. Noted.

 

[drgondog]

Per High-Speed Wind-Tunnel Tests of Dive-Recovery Flaps on a 0.3-Scale Model of the P-47D Airplane it 15%.

Just looked at the report and the model dimensions - Several model table dimensions are 'off' from 0.3 scale Model dimensions in Fig 1, and off from actual dimensions.
Actual vs Model values and ratios:
i.e Wing Area 300 ft. vs 27 ft (11.1:1); Wing Span 40' 10" vs 12.23 ft. (3.34:1); Wing Mean Aero Dyn Chord 87.5 in. vs 2.187 ft (26,25 in.) (3.33:1)
OTOH Tail Area (Stab and Elev) are 59.6sqft(actual) to 4.95ft (Table), Ratio of model to actual for Table is 12:1.
The Table Data for H.Tail Area should say "17.88sqft".
The H.Tail Span are 16ft (Actual) to 57.63in. (Fig 1) but the Table value is 6.786 ft for a ratio of 2.35:1. I believe the H.Tail Span in the Table should be = 57.63x 0.3 = 4.8ft.

The reference P-47C/D dimensions are from Dean - 1st value, the second value is from table in NACA Report. The Table data in the report do not all agree the physical dimensions of the model shown in Fig 1 so the errors are curious. I also suspect that 15% is measured from model MAC - obviously not root.

This report states that the Root Chord is 15% but that implies extraction from another document. The other NACA report that I commented on explicitly stated the T/C at the 25in WS is 14.7%, considerably down slope from the root to the tip. OTOH if the P-47 Root chord is at the edge of the fuselage - not C/L - then 15% probably is correct.

Just looked root chord is at Fuselage location - not C/L like Mustang.

 

[Shortround6]

Mosquito Merlin 25 (normal Ø10.25" impeller and 8.15/9.49:1 S/C ratios) 1-stage 2-speed low & high gear was rated at +25 lb boost with 150 grade
Spitfire Merlin 66 2-stage 2-speed low & high gear was rated at +25 lb boost with 150 grade

I would have my doubts about the single stage two speed engines making 25lbs of boost.

They were rated at 14lbs at 6000-6250ft and were rated at 18lbs at 2250ft in low gear. You either need a crap load of RAM or you have to be flying well below sea level to pick up another 7lbs (14 in?) of pressure.

Since high gear was supposed to be good for 18lbs at 9250ft that may be possible. However since the supercharger was taking 125hp more to run than when in low gear (and around 30% of the power was going into to heating the intake charge and only around 70% was actually compressing the air) you would be running on a very thin margin

Merlin type................................HP.......................Boost.......................Altitude.
XX-Low....................................1485........................14lbs.......................6000ft
XX-Hi........................................1490........................16lbs.....................12500ft
22/23 Low..............................1460.........................14lbs......................6250ft
22/23 Hi.................................1435..........................16lbs....................11000ft
24/25 Low..............................1635..........................18lbs....................2250ft
24/25 Hi..................................1510..........................18lbs....................9250ft

You might be able to it in high gear at under 3000ft? But even with 150 fuel you have to wonder if the detonation level is going to kick in before the you get to 25lbs of boost.

Remember that the two stage engine is heating the air just a bit less to begin with and the intercooler is taking several hundred degrees back out of the intake mixture.