Spitfire Combat Radius (range) evolution, limitations?

Wild_Bill_Kelso · Apr 8, 2022

I don't know enough about the production arrangements, but I know that they could have cut a few less promising projects like the Defiant, Fairey Albacore, Bristol Blenheim, Hawker Henley etc., and also make a few hundred less Spit Mk Vs, and maybe you could get something like the Mk VIII into service a bit sooner, and perhaps enhance it a bit further.

I love the Mk IX and those were extremely useful, but I think the VIII is a bit more so because of the enhanced fuel capacity. Once you are at Mk.XIV of course you can rush strait into that.

Of course, hindsight and all that. This seems to be one way to get to a longer ranged spit though.

Wild_Bill_Kelso · Apr 8, 2022

GregP said:
Not really. They got the general Spitfire up to 408 mph, but it wasn't the same as a Mk V airframe with modifications. Later Griffon Spitfires were also changed airframes and were longer and sleeker. So, yes, they could make a Spitfire go faster, but no, it wasn't a modified Mk. V airframe anymore.

A MK. V was 259 ft 11 in long and came in at 6,525 pounds loaded. Top speed was 371 mph at 20,000 ft and 350 mph at 5,900 ft. This from either 1,470 or 1,585 hp, depending on whether it has a Merlin 45 or a Merlin 50.

A Spitfire XIV was 32 ft 8 in long and came in at 8,574 pounds loaded. Top speed was 449 mph at 24,500 ft. This from a Griffon 65 of 2,050 hp.

Let's take the 371 mph. Using the standard cube root of the power increase, we'd expect the XIV to go 414 mph just due to the power change. Since it went 449 mph, the difference was a change in drag. They weren't much like the same airframe, with the exception of the wing and, rather naturally, the XIV would not turn or handle like a MK. V since it was a ton and half heavier with essentially the same wing. It DID go faster, largely with a huge does of horsepower, but also with reduced drag. That didn't come from a series of small mods but rather from a general cleanup of the fuselage when the Griffon was fitted with a 5-bladed prop and the new tail and overall streamlining were changed.

Wow! 259 feet? Much longer than I thought

From what I understand, actual factory production Spit Vs ranged from top speed of ~350 mph to 380 or so, and this depended on the engine type and maintenance, but also a lot on the finish and various drag related modifications, for example some of them had external BP window, some internal, some had flat rearview mirror, some fared, etc.

I think small mods clearly do work, and did, but a general cleanup of the fuselage works too, no doubt.

Jugman · Apr 8, 2022

drgondog said:
Interesting. Couple of comments:
The S3 wing was 11.7 T/C Max at semi span 63%, 14.7% T/C Max at 25%. Extrapolating to Root chord puts it in the 16% T/C Max range - about the same as P-51.

The Cd profile drag (pure shape, no friction) a low CL (High Speed) in the RN=19 x 10^6 range is 0.0062 for that puttied, primed and sanded smooth wing.

The P-51D wing CD profile drag is 0.0070 at RN=2x10^6, at RN=19x10^6, the P-51D wing CD profile drag =~0.0040.

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

tomo pauk · Apr 8, 2022

GregP said:
A MK. V was 259 ft 11 in long and came in at 6,525 pounds loaded. Top speed was 371 mph at 20,000 ft and 350 mph at 5,900 ft. This from either 1,470 or 1,585 hp, depending on whether it has a Merlin 45 or a Merlin 50.

A Spitfire XIV was 32 ft 8 in long and came in at 8,574 pounds loaded. Top speed was 449 mph at 24,500 ft. This from a Griffon 65 of 2,050 hp.

Since we're discussing aircraft, stating the altitude where the speed was achieved and horsepower was available might've been handy. Eg. Spitfire XIV using 2050 HP will do 417 mph: data sheet.

GregP said:
Let's take the 371 mph. Using the standard cube root of the power increase, we'd expect the XIV to go 414 mph just due to the power change. Since it went 449 mph, the difference was a change in drag. They weren't much like the same airframe, with the exception of the wing and, rather naturally, the XIV would not turn or handle like a MK. V since it was a ton and half heavier with essentially the same wing. It DID go faster, largely with a huge does of horsepower, but also with reduced drag. That didn't come from a series of small mods but rather from a general cleanup of the fuselage when the Griffon was fitted with a 5-bladed prop and the new tail and overall streamlining were changed.

Spitfire V going 370+ mph was probably the one with less drag, if just a tad less...
At 26000 ft, where, with ram, the XIV have had 1780 HP, it indeed did just under 450 mph. Spitfire V with Merlin 45 or 50 will have under 900 HP there, even with ram, and do 330-350 mph, depending on carb and state of aircraft?

GregP · Apr 8, 2022

Wild_Bill_Kelso said:
Wow! 259 feet? Much longer than I thought

From what I understand, actual factory production Spit Vs ranged from top speed of ~350 mph to 380 or so, and this depended on the engine type and maintenance, but also a lot on the finish and various drag related modifications, for example some of them had external BP window, some internal, some had flat rearview mirror, some fared, etc.

I think small mods clearly do work, and did, but a general cleanup of the fuselage works too, no doubt.

Seems like maybe a bit longer than the actual 29 ft, doesn't it?

I am not exactly the best typist in the world, as I'm sure you recognize ....

But, I WOULD like to fly ANY Spitfire, particularly the Mk.IX. It was the Mk. V with a dose of 2-stage power and was likely the best of the Spitfires for eacy handling combined with high climb rate and decent speed.

GregP · Apr 8, 2022

Hi Tomo.

I can find Mk. V at 371 mph at 20,000 feet and 350 mph at 9,500 feet with a stronger engine ... but the specs don't show the Merlin 50 variant at 20,000 feet. I suspect that it was a tad faster due to an extra 100 hp, but was not going to do the complete lookup of the performance envelope for a short post stating that speed increases are not strictly additive in general. That's pretty obvious to anyone who had tried it in their airplane. It gets faster, but not nearly as much as we want it to and hope it will.

My point was that the speed increase was due to a a combination of a few mods and general better finish (I think of it as a "cleanup"), but a Spitfire going faster than 408 mph or so was a combination of the Griffon horsepower and a new fuselage that was cleaner than the Mk. V / IX was, combined with the big, 5 or 6-blade Rotol prop / contra-prop for the Griffon variants.

tomo pauk · Apr 8, 2022

GregP said:
I can find Mk. V at 371 mph at 20,000 feet and 350 mph at 9,500 feet with a stronger engine ... but the specs don't show the Merlin 50 variant at 20,000 feet. I suspect that it was a tad faster due to an extra 100 hp, but was not going to do the complete lookup of the performance envelope for a short post stating that speed increases are not strictly additive in general. That's pretty obvious to anyone who had tried it in their airplane. It gets faster, but not nearly as much as we want it to and hope it will.

Merlin 50 was making about the same power as the Mk.45. The 'merlin in perspective' boo even gives a bit less power for the Mk.50.

GregP said:
My point was that the speed increase was due to a a combination of a few mods and general better finish (I think of it as a "cleanup"), but a Spitfire going faster than 408 mph or so was a combination of the Griffon horsepower and a new fuselage that was cleaner than the Mk. V / IX was, combined with the big, 5 or 6-blade Rotol prop / contra-prop for the Griffon variants.

Mk 14 was dirtier than the Mk.IX, due to the much bigger radiators.
Mk.IX was cleaner than the Mk.V in a number of details, bar the cooling system (where the Mk.V was less draggy), making the Cd0 of the Mk.V a bit better than of the Mk.IX. See here.
Brute force coming from the 2-stage Griffon engine made all the difference; granted, a much better better prop was needed for the 2-stage Griffon than for the 2-stage Merlin, let alone for the 1-stage Merlin.

Wild_Bill_Kelso · Apr 8, 2022

GregP said:
But, I WOULD like to fly ANY Spitfire, particularly the Mk.IX. It was the Mk. V with a dose of 2-stage power and was likely the best of the Spitfires for eacy handling combined with high climb rate and decent speed.

Me too brother!!!

GregP · Apr 8, 2022

Mk XIV cannot be dirtier than the Mk. IX because the power increase alone will not give 449 mph. Cannot be possible. If you look at an XIV side by side with a Mk. IX, the cleanup will be rather obvious. The XIV just fits better and has fewer edges and things to produce drag. Well. at least the ones I have seen up close.

The museum used to display a Mk. IX, a freshly-restored Mk. III, and an XIV, sometime side by side. Alas, the Spitfires all departed for their owner's homes along with the Hurricane and the Wildcat painted as a Martlett. Now, they're all in Texas getting quite warm in the hangar.

Zipper730 · Apr 8, 2022

EwenS said:
Supermarine were the sole producer of the Mk.VIII. Production of that model ended in Dec 1944.

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

Shortround6 · Apr 8, 2022

Zipper730 said:
I'm surprised they never added aft fuel to that design then...

If you can fit it in you can fly with it???

From wiki.

Provision was made to allow the Mk VIII to carry a single "slipper" drop tank of 30, 90 or 170 gal capacity. With a 170 gal tank, the aeroplane could fly over 1,500 mi (2,400 km). When carrying the 90 or 175 gal tank the aircraft was restricted, once airborne and at cruising altitude, to straight and level flight.

Now perhaps better pilots could be allowed a bit more latitude but there was difference between flying a certain distance and fighting with a certain fuel load over and above the "normal fuel load. The MK VIII had the bigger fuselage tanks and had the 11-12 gallon tanks out in the wing. Tanks in back of the pilot tended to screw up the center of gravity. With a tank behind the pilot how much do you want to screw up the CoG even once you get rid of the drop tank.

Lets Remember that the they had been trying to put more fuel into/under a Spitfire since at least 1940.

At least 3 squadrons tried out this configuration of MK II.

Wild_Bill_Kelso · Apr 8, 2022

Spitfire. Still manages to be pretty even with that ugly bump on the wing.

Zipper730 · Apr 8, 2022

Shortround6 said:
From wiki.

Provision was made to allow the Mk VIII to carry a single "slipper" drop tank of 30, 90 or 170 gal capacity. With a 170 gal tank, the aeroplane could fly over 1,500 mi (2,400 km). When carrying the 90 or 175 gal tank the aircraft was restricted, once airborne and at cruising altitude, to straight and level flight.

Then why did they fit all that tankage to the Mk.IX? They added like 66-75 gallons of fuel aft on that design!

wuzak · Apr 9, 2022

GregP said:
Not really. They got the general Spitfire up to 408 mph, but it wasn't the same as a Mk V airframe with modifications.

Mk IX got 408mph with Merlin 63.

404mph with Merlin 66 (LF.IX).

416mph with Merlin 70 (HF.IX).

And it really was a modified Mk V. Most of the modifications were forward of the firewall.

GregP said:
Later Griffon Spitfires were also changed airframes and were longer and sleeker. So, yes, they could make a Spitfire go faster, but no, it wasn't a modified Mk. V airframe anymore.

Early Mk XIIs were modified Mk.Vs.

XIVs were modified Mk.VIIIs. The "prototypes" were Mk.VIIIG.

GregP said:
A MK. V was 29 ft 11 in long and came in at 6,525 pounds loaded. Top speed was 371 mph at 20,000 ft and 350 mph at 5,900 ft. This from either 1,470 or 1,585 hp, depending on whether it has a Merlin 45 or a Merlin 50.

A Spitfire XIV was 32 ft 8 in long and came in at 8,574 pounds loaded. Top speed was 449 mph at 24,500 ft. This from a Griffon 65 of 2,050 hp.

The Griffon was longer, and mounted more forward. In the XIV the engine was tilted down at the front for better pilot visibility. That is the reason for eh 5 blade prop.

And, of course, the wider rudder added to length.

GregP said:
Let's take the 371 mph. Using the standard cube root of the power increase, we'd expect the XIV to go 414 mph just due to the power change. Since it went 449 mph, the difference was a change in drag. They weren't much like the same airframe, with the exception of the wing and, rather naturally, the XIV would not turn or handle like a MK. V since it was a ton and half heavier with essentially the same wing. It DID go faster, largely with a huge does of horsepower, but also with reduced drag. That didn't come from a series of small mods but rather from a general cleanup of the fuselage when the Griffon was fitted with a 5-bladed prop and the new tail and overall streamlining were changed.

The XIV did have bigger, draggier radiators, but I agree that there was an improvement in drag.

wuzak · Apr 9, 2022

GregP said:
The museum used to display a Mk. IX, a freshly-restored Mk. III, and an XIV, sometime side by side. Alas, the Spitfires all departed for their owner's homes along with the Hurricane and the Wildcat painted as a Martlett. Now, they're all in Texas getting quite warm in the hangar.

There were 40 PR.Mk.IIIs (redesignated PR.Mk.I Type C) built - all converted from Spitfire Mk.Is.

There were two Mk.IIIs built. One was a new build and one was later converted from a Mk.V. The original became the first with a Merlin 61, and was the prototype for the IX.

Which Mk.III do you have?

GregP · Apr 9, 2022

wuzak said:
Mk IX got 408mph with Merlin 63.

404mph with Merlin 66 (LF.IX).

416mph with Merlin 70 (HF.IX).

And it really was a modified Mk V. Most of the modifications were forward of the firewall.

Early Mk XIIs were modified Mk.Vs.

XIVs were modified Mk.VIIIs. The "prototypes" were Mk.VIIIG.

The Griffon was longer, and mounted more forward. In the XIV the engine was tilted down at the front for better pilot visibility. That is the reason for eh 5 blade prop.

And, of course, the wider rudder added to length.

The XIV did have bigger, draggier radiators, but I agree that there was an improvement in drag.

Yes, including a new, smoother nose that was not exactly the same as the Mk. V nose. And, since it came with a new, baby 2-stage Merlin, it also came with a 4-bladed propeller and a new spinner. All of which combined to go from 380 mph or so at 22,500 feet in the Mk. V to 408 mph in the Mk, IX.

Let's use the old cube root of the power increase rule.

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.

Wild_Bill_Kelso · Apr 9, 2022

But what are they both making at altitude? Is the MK V really making that much at 22,500 feet? I'd think it would be a lot faster!

As I'm sure you are aware, a major factor in top speed is how much power the engine generates at altitude where the air is thin. Two fighters with the same HP output, but one has a low alt blower (like a cropped impeller) and the other a high alt or multi-speed or multi-stage blower, the latter is going to have a faster top speed, though the former is going to be faster down low.

GregP · Apr 9, 2022

wuzak said:
There were 40 PR.Mk.IIIs (redesignated PR.Mk.I Type C) built - all converted from Spitfire Mk.Is.

There were two Mk.IIIs built. One was a new build and one was later converted from a Mk.V. The original became the first with a Merlin 61, and was the prototype for the IX.

Which Mk.III do you have?

Don't have one anymore.

It was a private Spitfire for an owner in Texas, and is was absolutely beautiful. That owner had four airplanes at the Planes of Fame and some 6 years ago, he had them flown from Chino to Texas, where his home base is.

GregP · Apr 9, 2022

Wild_Bill_Kelso said:
But what are they both making at altitude? Is the MK V really making that much at 22,500 feet? I'd think it would be a lot faster!

As I'm sure you are aware, a major factor in top speed is how much power the engine generates at altitude where the air is thin. Two fighters with the same HP output, but one has a low alt blower (like a cropped impeller) and the other a high alt or multi-speed or multi-stage blower, the latter is going to have a faster top speed, though the former is going to be faster down low.

The Mk. V is a rather well-know animal and it wasn't faster. Spitfires got faster as they got engines that retained horsepower up higher, like all WWII fighters did as they were developed. Most of the speed came from maintaining horsepower to a higher altitude, new propellers, and new cowlings to hold the new engines. A small bit came from better streamlining and small mods.

You can see a basic chart of Merlin power versus altitude below. It was in another thread in a reply from member jerryw back in 2008, and I borrowed it from there, but you can find it in Google images, too.

DarrenW · Apr 9, 2022

Wild_Bill_Kelso said:
But what are they both making at altitude? Is the MK V really making that much at 22,500 feet? I'd think it would be a lot faster!

As I'm sure you are aware, a major factor in top speed is how much power the engine generates at altitude where the air is thin. Two fighters with the same HP output, but one has a low alt blower (like a cropped impeller) and the other a high alt or multi-speed or multi-stage blower, the latter is going to have a faster top speed, though the former is going to be faster down low.

I did some preliminary checks on www.wwiiaircraftperformance.org and found the maximum speeds of the Mk V and Mk IX at sea level to be as follows:

Spitfire Mk V with Merlin 45 @ 1175 hp (+12 boost) = 302 mph
Spitfire Mk IX with Merlin 66 @ 1575 hp (+18 boost) = 336 mph

This gives the Mk IX 400 more horsepower with a speed difference of 34 mph. To me it looks like the drag conditions of both marks were not very different in a useful sense, as the average WW2 piston engine fighter would need approximately 15 hp to increase top speed at sea level by 1 mph. Not a perfect calculation but it will probably get you in the ballpark. I'm also not sure if the four bladed propeller had anything to do with the speed difference as a three blade can be just as efficient depending on altitude, blade type, and the engine's particular ratings. So it seems like the performance increase of the Mk IX was primarily due to increased engine power output.

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