Escort Fighter Performance Comparison

[Geoffrey Sinclair]

That's been my argument from day dot, there's no reason after the MkIX came into service and helped turn the tide over France, Africa and the Med that they couldn't have started on immediately addressing the range issue

The Spitfire range issue had been addressed, the mark VIII upped fuel capacity from 87 to 124 gallons, the mark I range was 575 miles, the IX 434 miles and the VIII 660 miles on internal fuel using the RAF measuring system. In late 1942 and through mid 1943 the requests were for more Spitfires, not longer range, the fighting over France, Tunisia and Sicily had the Luftwaffe coming at the allied air forces around the front line. After that the bulk of the Luftwaffe day fighter force kept pulling back to where the western allied fighters could not reach or to at least cut the number of allied fighters likely to be encountered.

There were two technical issues that had to be addressed, the Spitfire was more sensitive to CoG issues than the Mustang, the Westland elevator design improved that, then comes the general strengthening required, at 7.11 pounds per imperial gallon for 100 octane, plus a rule of thumb 1 pound of self sealing tank weight per imperial gallon capacity a 75 gallon tank means 533.33 pounds of fuel plus 75 for that tank, 610 pounds. Given the heavy bomber escort mission extra oxygen will be needed as well. The 90 gallon external tank at 0.5 pounds per gallon of capacity plus fuel is 685 pounds. As far as I know the VIII was cleared to carry 1,000 pounds of bombs, even so this full fuel load adds another 300 pounds to the take off weight versus a full bomb load.

From an incomplete list of Spitfire Modification orders, the date is of course the order date, not the actual implementation date and the modifications were not always for all marks. Numbers 1293 and 1383 are present to help date 1335 and 1414, the rear fuselage tanks installations.

437 29-Aug-41 Introduce 90 gallon drop tank
729 22-Sep-42 Introduce rear fuselage 29 gal tank for ferrying
743 20-Oct-42 Fit Westland convex elevator
814 15-Dec-42 Trial installation mock up torpedo 50 gal o/ld tank
1099 19-Oct-43 Introduce 45gal o/ld tank
1231 02-May-44 Modify 90gal o/ld tank for use on any mark
1293 21-Mar-44 T.I. Lightweight air system
1363 20-May-44 Strengthen cannon fairings. Cancelled
1377 27-Jun-44 Introduce Mk VII bottom fuel tank
1436 22-Aug-44 Gyro gunsight Mk IID

So, when WWII started, only the P-38 and to a degree the Bf 110, was a big airframe with easy capability to extend range.

The P-38 started with 400 or 410 US gallons unprotected, dropped to 300 gallons protected, then went to 410 gallons protected with the wing leading edge tanks, around a 37% increase in capacity. The Spitfire VIII went to 124 gallons from 87, a 42.5% increase. A difference being the Spitfire had gained over 50% more power and so more fuel consumption, the P-38 increase in power was less, 1,150 to 1,425 HP take off. The P-47D managed a 21% upgrade in internal fuel, the P-47N an 82% versus the original P-47, the P-51B a 46% increase versus the P-51A but with an upgrade in take off power from 1,200 (V-1710-81) to 1,490 HP (V-1650-7).

When mission expansion was desired, to include much larger tactical footprint, neither the Spitfire nor Bf 109 presented 'easy to find and design' space for extra tankage, nor solutions simple with respect to ease of incorporation - either for contiguous volume or ease of production changes.

Yet by late 1944 and into 1945 the use of wing and rear fuselage tanks meant the Spitfire came with approaching 200 gallons of internal fuel, versus the 87 in the mark I, with much of the design work done earlier, for example to handle the heavier Griffon engine, 1,980 pounds for the Griffon 61 versus 1,640 pounds for the Merlin 61. The wing tanks were around in 1942, the rear fuselage in 1944/45, and earlier than that in a smaller size as a ferry option.

In 1943 the Spitfire VIII coming with the internal fuel capacity able to fully exploit the 90 gallon external tank was considered adequate. The needs of the moment were higher performance, the Griffon, and numbers of the latest Spitfires both for 1943 and the expected major air fight that Overlord would provoke in May 1944. There was a major fight, Fighter Command's 4th and 5th highest kill claims in a month were June and July 1944, but no one, even in mid 1944, was willing to build into the plan the loss of quality and quantity the Luftwaffe had actually suffered by June 1944.

The expected performance and availability of the Merlin Mustang was a factor in decisions about the Spitfire range, why make the effort when an off the shelf solution was available. That there were not as many Mustangs made available to the RAF as initially expected (and when) meant increasing the Spitfire range became more useful. The liberation of France opened up the possibility of Bomber Command day raids on Germany, which would need escorts. In the fourth quarter of 1944 Spitfires based in Britain could escort Bomber Command day raids on Western Germany, though given the RAF bomber armament and formation flying, plus the caution of their commander, those raids were not going deep. With the production pressure easing the chances to introduce more and larger modifications increased.

The Spitfire VIII with 30 to 75 gallons of rear fuselage fuel would be a useful addition to the 8th Air Force operations in the 1943 to early/mid 1944 period, and the 15th Air Force for a couple of months longer, as the headline operations. The ability to carry more internal fuel plus bombs would be a bonus but not make a major difference to the missions most Merlin Spitfires spent 1944/45 doing, fighter bomber operations and sweeps.

 

[drgondog]

The figures given for the V-1650-3 and -7 are puzzling as I don't see how the -7 can have the same performance at 26,500 feet as the -3 with the lower supercharger speed.

Its interesting to note that the USAAF chose to sacrifice high altitude performance for better performance at lower levels during the P-51 B/C production run.

I didn't look at your figures for 1650-3 and -7. IIRC the FTH for Bench (Static) runs was 24K+ and 19K+ respectively for Military Power@3000 RPM.

The difference in P-51B/D Altitude FTH Performance for Speed/level flight runs is Ram Air to attain FTH of ~4500 ft over Bench FTH. Ram air delta is result of iterative solution approach.

Look to Reports NA-5534 and NA-8449 for the respective charts to display both Static and Ram air HP vs alitude.

 

[Thumpalumpacus]

They took a beating because they MkV was outclassed by the FW190 and 109F, they did fit 30G slippers to extend range but the pilots instead use it to cruise at a higher speed, the MkIX re-addressed the balance

So plumbing the thing for more fuel doesn't necessarily improve performance. And I'd imagine that with the deep-penetration sweeps you're envisioning, you're going to need a higher cruising speed anyway unless you want to be caught stooging around, no?

 

[Shortround6]

When planes changed the ability to modify the planes changed.

As an example of this the P-36/Hawk carried a tank behind the pilots seat. On the radial engine airplanes the tank was used as an over load (ferry) tank and most maneuvers were prohibited. French crashed a few trying to dogfight with fuel in the rear tank.
On the Allison P-40s the same tanks (self sealing but same location) allowed for maneuvering with fuel in the rear tank but the rear tank was supposed to be starting after take off.
On the Merlin engined P-40s the rear tank was supposed to taken down to 20 gallons and the other tanks emptied and the 20 gallons in the rear tank were kept as a reserve and used for landing or landed in the tank if not needed.

P-51s with the rear tank varied a bit depending on the time and manual. with around 55-85 gallons the elevators could go into reversal in high G turns. With around 25-55 gallons the plane handled normally and in post war fling manuals the last 20-25 gallons were supposed to kept for reserve and the plane was supposed to handle for landing with that amount of fuel in the rear tank.

We know that the rear fuel tanks in the Spitfire were taken out of service after the end of the war. Granted different air forces have different standards and have different standards during wartime and peace time (RAF accident rates with Meteor were horrific even in the 19590s though).

Now what was the max gross weight the plane was "supposed" to operate at without any restrictions in flight envelope or maneuvers.

Could the Spitfires operated with the 90 gallon drop tank installed? and it was close to the center of gravity.
Sticking 60-70 Imp gallons in the rear fuselage may be much like the Mustang (25-30 gallon need to be burned off before combat)

For some reason the MK VIIIs didn't seem to get the rear tanks ? or only got one tank instead of two?
MK VIIIs had 120-123 gallons of internal.
MK XIVs went back to 85 gallons in fuselage tanks instead of the 96 in the MK VIII.

How many MK XIVs got rear fuselage tanks?

Spits could carry a bigger load than they could maneuver with, so could most other fighters, drop tanks and bombs could overload the structure.

For the Long range Spitfire what was the load limit it had when maneuvering and then what would the load limit have been with 450-600lbs in the rear fuselage?

 

[nuuumannn]

Nuuumannn thanks for posting that, I'm not home so can't cite my books, did he have a drop tank also?.

No worries Pat, yes it did have a drop tank during that flight. What it does prove is that the idea of improving the Spitfire's range was taken seriously by the manufacturer at least and at high level within the RAF the idea of a long range escort fighter was discussed and seen as a potential fix against bomber losses during daylight ops. A pity Portal was so dogged in his determination not to pursue the idea, but it could have been done, as you've stated.

 

[PAT303]

So plumbing the thing for more fuel doesn't necessarily improve performance. And I'd imagine that with the deep-penetration sweeps you're envisioning, you're going to need a higher cruising speed anyway unless you want to be caught stooging around, no?

They used the extra fuel to cruise at a higher speed making it harder for them to be bounced and needing less time to accelerate. I would not use the MkV for deep penetration missions.

 

[PAT303]

For some reason the MK VIIIs didn't seem to get the rear tanks ? or only got one tank instead of two?

All MkXVI's got rear tanks, the flight restrictions on the Spit were no different to any other aircraft with extra tanks or drop tanks, the only Spit that reached it's max take off weight was the Wright field modified MkIX's from what I can find.

 

[PAT303]

A pity Portal was so dogged in his determination not to pursue the idea, but it could have been done, as you've stated.

I supposed there's our answer to why it wasn't done.

 

[nuuumannn]

I supposed there's our answer to why it wasn't done.

As I mentioned earlier, even Churchill saw the limitations of not choosing that option.

 

[PAT303]

As I mentioned earlier, even Churchill saw the limitations of not choosing that option.

Maybe they should have left it to the people who built the thing on whether it could have been done before making a decision.

 

[Thumpalumpacus]

They used the extra fuel to cruise at a higher speed making it harder for them to be bounced and needing less time to accelerate. I would not use the MkV for deep penetration missions.

My point was that if you're planning deeper missions into occupied Europe, you'll probably want a faster cruising speed in order to minimize enemy response times where at all possible. After all, those hypothetical deeper missions by Mk IXs are going to be exposed to much more potential enemy response, don't you think?

 

[nuuumannn]

Maybe they should have left it to the people who built the thing on whether it could have been done before making a decision

Quite possibly, but aircraft manufacturers don't establish policy, they produce equipment to enact policy. Of course, manufacturers can offer advice, but it's up to the policy makers to take it or not.

 

[PAT303]

My point was that if you're planning deeper missions into occupied Europe, you'll probably want a faster cruising speed in order to minimize enemy response times where at all possible. After all, those hypothetical deeper missions by Mk IXs are going to be exposed to much more potential enemy response, don't you think?

Your arguing against reality, the British did do 300 mile rodeo's in the Spit XIV with just a 90G drop tank, the MkVIII/IX has an extra 100G of internal fuel, that gives them the flexibility to fly faster or further or both, I don't understand the fierce objection to giving the Spit more fuel.

 

[Shortround6]

I would not use the MkV for deep penetration missions.

as I have noted before (repeatedly) the MK V has a least 300lbs less weight forward of the firewall compared to a MK IX and would have more trouble with weight in the rear fuselage than the MK IX.

"the only Spit that reached it's max take off weight was the Wright field modified MkIX's from what I can find."

Not quite the same thing. Max take-off weight is not max maneuver weight.

Max take-off weight for the Spits in the Manual was 8700lbs but that required a hard smooth runway and only gentile maneuvers.
all forms of flying had a limit of 7,800lbs.
A MK IX with 85 gal of fuel weighed 7445lbs. ?
The MK VIII was limited to 7900lbs.


In the 1946 manual for the Spitfire IX, XI and XVI acrobatics are prohibited when carrying any external stores except the 30 gal "blister" drop tank nor when the rear fuselage tanks contain more than 30 gallons of fuel. and are not recommended when the rear fuselage tanks contain any fuel.

Italics are in the manual. Yes it is a peacetime manual.

Tests of a MK IX using experimental metal covered elevators and an inertia weight in Jan/Feb 1945 showed an improvement with the metal elevators but the inertia made no difference.
With 74 gallons of fuel in the rear tanks the CG was 12.2in aft of the datum point. With 34 gallons used the CG was 9.9in aft of the datum point and with the tanks empty the CG was 7.4in aft of the datum point.

Basically it seems that the Spitfire VIII/XI could hold about an extra 35 ? gallons of fuel for use in combat. 120-123 for the MK VIII with larger fuselage tanks and wing tanks.
MK IX with 85 gallon forward fuselage and 30-40 gallons in the rear tank/s = 115-125 gal.

Spits with 40 gallons in the rear tanks were tightening up turns even at two Gs but could be held. Mustangs at 55 US gallons in the rear tank were pretty much handling normally.


With 34 gallons used the fuel load is about 10 gallons more than the post war manual wants but then many of the left over Spits had fabric covered elevators.

 

[Shortround6]

From a test dated Oct 1942.

Spitfire F Mk IX BF274 Test

www.spitfireperformance.com

Difference in speed and climb with and without a 30 gallon drop tank.
Putting 30 gallons inside the plane is going to improve the speed performance substantially.

the climb performance isn't going to change a whole lot from the drop tank. If the plane is climbing at 203-240mph (best climb speed from 15,000 to 30,000ft) the change in drag by getting rid of the drop tank is nothing like the change in drag when trying to fly at 350MPH +.

The loss of about 200fpm in climb stays pretty constant no matter what the altitude.
You want 150 Imp gal in the Spitfire for combat you loose another 200fpm climb over the loss from 120 gallons.

The Spit is about 30-40mph slower than a P-51B, it does climb better but now you want to cut some the Spits climb rate ( and ability to turn).

 

[Thumpalumpacus]

Your arguing against reality, the British did do 300 mile rodeo's in the Spit XIV with just a 90G drop tank, the MkVIII/IX has an extra 100G of internal fuel, that gives them the flexibility to fly faster or further or both, I don't understand the fierce objection to giving the Spit more fuel.

I don't object to giving the Spit more fuel, I'm wondering what the objections were and trying to understand why your suggestion wasn't taken up. Apparently Portal and Churchill were against it, but did they actually have grounds?

Apparently

 

[33k in the air]

Your arguing against reality, the British did do 300 mile rodeo's in the Spit XIV with just a 90G drop tank, the MkVIII/IX has an extra 100G of internal fuel, that gives them the flexibility to fly faster or further or both, I don't understand the fierce objection to giving the Spit more fuel.

You are operating with the benefit of hindsight.

To those at the time, with no RAF bombers making deep penetration raids in daylight, there is no reason to greatly extend the Spitfire's range. Indeed, the expectation was the American insistence to do deep raids in daylight would result in disaster. Which they did, enough so that there was an effort made to convince the USAAF to switch over to nighttime bombing.

A long-range fighter is not necessary to win air superiority. You do not need to fly into the enemy's airspace; you can just control the air over your own forces.

 

[PAT303]

In the 1946 manual for the Spitfire IX, XI and XVI acrobatics are prohibited when carrying any external stores except the 30 gal "blister" drop tank nor when the rear fuselage tanks contain more than 30 gallons of fuel. and are not recommended when the rear fuselage tanks contain any fuel.

I'm not suggesting the Spit goes into combat with any external stores such as a drop tank, no plane could except the MkXIV with the 90G combat tank so I don't understand what you are saying here, the Spit had the same restrictions put on it as the Mustang in regards to maneuvers with rear tanks and drop tanks, RAF mustangs had their rear tanks wired shut post war also, is there a report on their flight restrictions in 1946?.

 

[PAT303]

A long-range fighter is not necessary to win air superiority. You do not need to fly into the enemy's airspace; you can just control the air over your own forces.

That's like being in a car race with the fastest car but with not enough fuel to make it to the end.

 

[PAT303]

The loss of about 200fpm in climb stays pretty constant no matter what the altitude.
You want 150 Imp gal in the Spitfire for combat you loose another 200fpm climb over the loss from 120 gallons.

The Spit is about 30-40mph slower than a P-51B, it does climb better but now you want to cut some the Spits climb rate ( and ability to turn).

So if that's your logic they shouldn't have put rear tanks or drop tanks on the P51 because it couldn't climb turn or fight with full rear tanks and drop tanks either.