Spitfire Combat Radius (range) evolution, limitations?

PAT303 said:

What front line fighter didn't have it's fuel capacity increased throughout the war?.

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F4U-4 from F4U-1 removed the wing fuel tanks, as an example. P-39Q from P-39D reduced internal fuel. F6F-3 same as F6F-6.

Perhaps the over-riding reason nobody in authority was willing to compromise scarce design resources and disrupted production to introduce a version of a Spitfire for which there was no compelling national interest. There was no RAF mission for long range daylight bombing escort. There was no RAF mission for long range fighter bomber (short range compare to bomber escort) role that US didn't fill nicely with P-47 and P-51 and P-38. The long range low level recon/interdiction role was filled by Mustang, Ditto for RN ops for which USN types fit nicely.

The RAF/RN didn't have the clout to demand types for aircraft that the US designed, produced and delivered that met roles that RAF/RN would 'like to have', but didn't fill 'must have' niche.

PAT303 said:

Spitfires had 85G because they started with two bladed props, then two speed props with less than 1000hp, by 1939 they had constant speed props, 100 octane fuel and 1200hp, the reason for weight saving was no longer there as it's performance increased, and kept on improving.

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The reason Spits had 85 gal is that the intended fuel fraction, combined with other design attributes such as low drag wing, design gross weight, landing gear design and armament led to all fuselage internal fuel decision. The technology improvements led to better performance, but the specification under which it was designed controlled the decision for original fuel location and future changes were limited by CG and space considerations.

The Mustang and Lightning were originally designed to specifications that emphasized range as an attribute and had the happy circumstance of relaxed GW parameters based on initial projections of the original available Allison/prop capability of the time. In the airframe business, 'do overs' on original design generally cause extended design and production insertion schedules. The P-47D-25 was a good example. When the P-51B 85 gallon tank was ordered by Materiel Command in early July, and the P-38J 110 gal LE tanks, the two fighters were able to increase internal fuel by 46% and 36% respectively and deliver both into combat ops within 6 mo. The P-47D, however only increased internal fuel by 23% and took nearly 11 months to enter combat ops due to the very significant structural changes in the middle airframe.

As to 'no longer having a 'reason for weight saving', any increase in GW is detrimental to all the other performance features of the design, ranging from landing speed, top speed, climb rate and acceleration - in comparison with the adversary - in order to push the combat radius. The AAF-HQ (and 8th, 15th, 5th AF) had the missions and determination to accept the consequences and order the change

PAT303 said:

I think you should read and understand what I am saying, more fuel allows more flexibility, flying feints and bogus raids is part of warfare, flexibility gives you options to deal with it instead of guessing your enemy's intensions.

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Guessing the intentions, or at least the target, is important, stern chases are often a waste of time. In 1940 the RAF was not worried about fighter range. Scrambling according to the estimates of distances to a target and time for the interceptors to take off and climb. Which references report delayed take offs due to range limitations in 1940? Versus the known uncertainties, radar could do range quite well, size was an estimate, height was a problem and it took a visual sighting to confirm composition, behind the coast the raids were plotted visually and slight errors in the measurements made the raids appear to zig zag. The system was meant to reduce flying hours.

The Luftwaffe fighters defending Germany began using drop tanks, but they had more warning, longer tracks of the raids and a bigger area to defend.

RAF contract cards say 174 Spitfires from the first order had Merlin II, with Merlin III from K9961, aircraft number 175 of the order, which would mean a change over in May 1939, the online Spitfire histories say the first 194 had Merlin II, which would mean a change over at K9980 in June. Contract card notes against individual aircraft at times have the words wooden airscrew while the first variable pitch notation is K9855 (aircraft 69) but that is over written by wood airscrew, K9860 has the note V.P., it would appear for a time wooden airscrews were fitted to aircraft meant as replacements for squadrons still using wooden propeller versions, until those units were upgraded to variable pitch. While the notation Rotol Airscrew starts appearing against some aircraft starting with N3096 and N3097 in October 1939 (these two under contract B10783/39 or B10983/39). The June 1939 publicity photographs at Hornchurch all show 3 bladed two pitch propellers, with the two identified serials delivered in March 1939, N9910 and N9912, aircraft 126 and 128 of the order.

The first 363 Hurricanes had Merlin II, change over in April 1939 at L1909, the first variable pitch propeller production aircraft was number 435, L1980, in June 1939.

The crash program in mid 1940 was the change from variable pitch to constant speed.

According to the RAF the Spitfire I with Merlin III engine rated at 1,030 HP at 16,250 feet could reach 15,000 feet in 6.2 minutes, tare weight 4,795 pounds, 84 gallons of fuel weight 630 pounds. Spitfire II with Merlin XII engine rated at 1,150 HP at 14,000 feet could reach 15,000 feet in 4.9 minutes, tare weight 4,738 pounds, 84 gallons of fuel weight 605 pounds.

The Hurricane I fitted with a DH propeller could reach 15,000 feet in 7.25 minutes, with a Rotol propeller 6.3 minutes.

PAT303 said:

No idea, apparently if a fighter can't carry enough fuel to fly to Berlin and back it's not worth perusing.

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YOU miss the point. There was no reason, doctrine or mission important enough to greatly extend combat radius - based on an unfilled need.

PAT303 said:

No idea, apparently if a fighter can't carry enough fuel to fly to Berlin and back it's not worth perusing.

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And that requirement would be known in 1936?

drgondog said:

As to 'no longer having a 'reason for weight saving', any increase in GW is detrimental to all the other performance features of the design, ranging from landing speed, top speed, climb rate and acceleration - in comparison with the adversary - in order to push the combat radius

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In fact the increase in weight may have the least effect on top speed.

Many of us have been indoctrinated of many years (around 60 for me) of reading about aircraft were the overly simplistic explanation of using increased weight to show a decrease in speed from prototype or early version to later versions was used.
It was never explained that a lot of the weight increases were accompanied by changes to the external envelope that caused drag (extra machine gun ports, extra cartridge case ejector slots, BP glass wind screens, wear view mirrors, radio antennas. etc) and the loss of performance was all reduced to a single sentence. And that sentence usually made no reference to
" landing speed, turning rate, climb rate and acceleration"

The people that worked in aviation knew the difference but many of us amateur historians took quite a while to pick up the difference.

Shortround6 said:

In fact the increase in weight may have the least effect on top speed.

Many of us have been indoctrinated of many years (around 60 for me) of reading about aircraft were the overly simplistic explanation of using increased weight to show a decrease in speed from prototype or early version to later versions was used.
It was never explained that a lot of the weight increases were accompanied by changes to the external envelope that caused drag (extra machine gun ports, extra cartridge case ejector slots, BP glass wind screens, wear view mirrors, radio antennas. etc) and the loss of performance was all reduced to a single sentence. And that sentence usually made no reference to
" landing speed, turning rate, climb rate and acceleration"

The people that worked in aviation knew the difference but many of us amateur historians took quite a while to pick up the difference.

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SR, while the loss in top speed is in low % due strictly to increased weight, Induced drag IS a detrimental factor in the entire performance envelope and increases with the square of lift coefficient CL. CL grows in direct proportion to increase in GW.

Further, Induced Drag dominates total drag from takeoff into cruise speed envelope.

drgondog said:

SR, while the loss in top speed is in low % due strictly to increased weight, Induced drag IS a detrimental factor in the entire performance envelope and increases with the square of lift coefficient CL. CL grows in direct proportion to increase in GW.

Further, Induced Drag dominates total drag from takeoff into cruise speed envelope.

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It does but most popular books over simplified things a lot.
Like the Curtiss XP-46 that went from a claimed 400 (or 403) mph to 355mph when operational equipment was fitted and the difference was usually simply attributed to weight.
Probably unknown but the climb rate was never mention?

In some cases the author would write something like "Bomber XX had it's top speed drop from 313mph to 287mph due to increased weight in the later versions"
While the increased weight covered new, higher drag gun mounts, or external gun pods or some other rather large lumps and bumps on the aircraft (new air intakes with bulky air filters or larger oil cooler intakes or??????)

They didn't have to spell everything out but just adding the words ".....and drag....." after the word "weight" might have encouraged some of us to try and find out more.

Shortround6 said:

It does but most popular books over simplified things a lot.
Like the Curtiss XP-46 that went from a claimed 400 (or 403) mph to 355mph when operational equipment was fitted and the difference was usually simply attributed to weight.
Probably unknown but the climb rate was never mention?

In some cases the author would write something like "Bomber XX had it's top speed drop from 313mph to 287mph due to increased weight in the later versions"
While the increased weight covered new, higher drag gun mounts, or external gun pods or some other rather large lumps and bumps on the aircraft (new air intakes with bulky air filters or larger oil cooler intakes or??????)

They didn't have to spell everything out but just adding the words ".....and drag....." after the word "weight" might have encouraged some of us to try and find out more.

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Ah, yes. You might note that very few Aero engineers write books. Kit Carson is one that comes to mind and he injected several technical pearls of wisdom regarding the P-51. I duly noted the lack of serious discussion regarding the technical features and devoted most of the Appendices toward explaining the "Why and "How' of superb qualities of the Mustang design - some unintelligible to the math/aero 'challenged' but I felt important enough to try.

Your point regarding post prototype changes is dead on when pointing finger at most performance changes implemented (good and bad) after the airframe entered service. Most every physical change resulted in Parasite drag, often overcome with more horsepower. But even high performance aircraft like P-51, Spitfire (and bomber/cargo aircraft) live in low speed (land), climb, and medium speeds cruise range where Gross Weight and Drag (like take off and climb) duke it out - .

At high speed for a given altitude and air density, the CL will be at minimum (and CDi will be minimum). The drag reduction methodology is always focused on reducing parasite drag (turrets/enclosed gun positions, complete landing gear cover, retracting tail wheel, better butt joints, flush rivets, flap/aileron seal, etc) influencing better airflow properties.

What many technical but non-aero types do not understand going into a drag discussion surrounding Induced Drag, is that there are TWO components of Lift Coefficient related phenomena. The commonly understood component is Induced drag due to Lift (and GW). The second component, and not insignificant, is the increased Lift Coefficient (and increased Pressure/Parasite drag) on the wing when flying at increased altitudes. Due to decreased density, the aircraft has to Increase CL for same GW and airspeed in order to maintain level flight. At high altitude/high speed, that CL contribution to parasite drag on a Mustang is about the same magnitude as the exhaust stacks or gun ports.

drgondog said:

YOU miss the point. There was no reason, doctrine or mission important enough to greatly extend combat radius - based on an unfilled need.

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"Leaning into France"?

Though I am not sure that longer range/endurance would be a good thing.

It may lead to more Spitfire pilots being lost, as it gives the Luftwaffe more opportunity.

wuzak said:

"Leaning into France"?

Though I am not sure that longer range/endurance would be a good thing.

It may lead to more Spitfire pilots being lost, as it gives the Luftwaffe more opportunity.

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What were the numbers' odds during the 'leaning into France', RAF Spitfires vs. LW Fighters?

It rather depends on how far you want to lean into France.

As 45-60 gallon drop tank get you well past the coast.

Around a 90 gallon drop tank is where you run into trouble, Will 90 gallons outside get you further into France than you can get back out with 85 gallons inside?

Merlin 45 burns 2.5 gallons an hour so 15 minutes of combat power costs 37 gallons. You need about 15 gallons for 30 minute reserve once you are back in England leaving you with about 33 gallons for the trip home. A Spitfire V will do 281mph true at 42 gallon an hour so just over 90 miles will cost you 14 gallons (over the channel) and at 331mph the plane burns about 70 gallons an hour. At the 331mph speed you are good for about 100 miles in 18.5 minutes.

So you are good for about 100 miles inland with a 90 mile crossing with 30 minutes once you are over land with a 15 minute combat duration.
With a XX gallon drop tank and 85 gallons internal.
A lot of things can wrong but again, how far do you want to push into France?
Every minute in combat is worth over 10 miles of getting home at 331mph. and 5 minutes at an economical speed of just over 200mph when trying to find the home airfield.

Shortround6 said:

It rather depends on how far you want to lean into France.

As 45-60 gallon drop tank get you well past the coast.

Around a 90 gallon drop tank is where you run into trouble, Will 90 gallons outside get you further into France than you can get back out with 85 gallons inside?

Merlin 45 burns 2.5 gallons an hour so 15 minutes of combat power costs 37 gallons. You need about 15 gallons for 30 minute reserve once you are back in England leaving you with about 33 gallons for the trip home. A Spitfire V will do 281mph true at 42 gallon an hour so just over 90 miles will cost you 14 gallons (over the channel) and at 331mph the plane burns about 70 gallons an hour. At the 331mph speed you are good for about 100 miles in 18.5 minutes.

So you are good for about 100 miles inland with a 90 mile crossing with 30 minutes once you are over land with a 15 minute combat duration.
With a XX gallon drop tank and 85 gallons internal.
A lot of things can wrong but again, how far do you want to push into France?
Every minute in combat is worth over 10 miles of getting home at 331mph. and 5 minutes at an economical speed of just over 200mph when trying to find the home airfield.

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Hi Shortround, I'd change that 2.5 gph to 25 gph and, are you sure. That seems might low compared with real-life Merlins today. Most of them run 50 - 55 gph at cruise. You can cruise at less fuel flow, but the spark plugs just don't like it much, and the engine stays much cooler at slightly higher fuel flow.

tomo pauk said:

What were the numbers' odds during the 'leaning into France', RAF Spitfires vs. LW Fighters?

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As I remember the loss ratio was about 5 to 1 in the LW favour at worst and operations were cut back and suspended or curtailed because of that. The Spitfire had nothing to escort and no spare Spitfires were available in 1940. In 1941 to 42 the Spitfire was not better than the f 109 and Fw 190 so increasing range would only increase losses. When the Mk IX appeared in mid 1942 what could be done with it, more than what was done they escorted USA bombers at Dieppe, but there were only 6 squadrons of them at Dieppe.. In Big Week early 1944, the USA had 800 P-47s 100 P-38s and 100 P-51Bs that is 1000 aircraft and pilots and they were hard pressed to protect their bomber force AND they were assisted by the RAF with some Spitfires. 1,000 pilots and aircraft is pretty much the normal RAF front line strength, so to run regular daylight bomber operations you need another "RAF fighter Command"

pbehn said:

As I remember the loss ratio was about 5 to 1 in the LW favour at worst and operations were cut back and suspended or curtailed because of that. The Spitfire had nothing to escort and no spare Spitfires were available in 1940. In 1941 to 42 the Spitfire was not better than the f 109 and Fw 190 so increasing range would only increase losses. When the Mk IX appeared in mid 1942 what could be done with it, more than what was done they escorted USA bombers at Dieppe, but there were only 6 squadrons of them at Dieppe.. In Big Week early 1944, the USA had 800 P-47s 100 P-38s and 100 P-51Bs that is 1000 aircraft and pilots and they were hard pressed to protect their bomber force AND they were assisted by the RAF with some Spitfires. 1,000 pilots and aircraft is pretty much the normal RAF front line strength, so to run regular daylight bomber operations you need another "RAF fighter Command"

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5:1 odds?? Luftwaffe had 5000 fighters to call during the 'leaning into France' vs. 1000 Spitfires?

700-odd RAF BC bombers in mid-1941 would be called by someone as 'substantial number', not 'nothing to escort'.

tomo pauk said:

5:1 odds?? Luftwaffe had 5000 fighters to call during the 'leaning into France' vs. 1000 Spitfires?

700-odd RAF BC bombers in mid-1941 would be called by someone as 'substantial number', not 'nothing to escort'.

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Sorry that is the loss rate.

Didn't I read in this thread that a 90gal dt would give the Spitfire a 500 mile combat range?

Milosh said:

Didn't I read in this thread that a 90gal dt would give the Spitfire a 500 mile combat range?

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Range or radius?

Shortround6 said:

Range or radius?

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The only way a spitfire reaches Berlin in escort role is to have ~ 300 gal internal coupled with 90gal combat tank

or take off from Hamburg