Spitfire cruising settings

[Jabberwocky]

Hi all,

Its been a while, but I'm back after being insanely busy with work for about 12 months. While I've got a little more spare time, I'm working on a long-ish piece on the realistic(-ish) possibilities for a long-range Spitfire emerging in 1941-1942.

With that in mind, does anyone have the following:

Typical range cruise and combat cruise settings for Merlin 45, Merlin 61 and Merlin 66 used over the ETO?

I've got the Mk V and Mk IX manuals, but they don't actually indicate what cruise settings were typically used. Ideally, I'd like boost, RPM and gallons per hour for all the engines.

At the moment I've got some back of the envelope calculations that indicate the Spitfire could have been successfully and comparatively easily modified into a fighter with a genuine 400 mile combat radius - including 20 minutes for combat at high power settings and a 20% reserve to get home.

 

[Shortround6]

Define long range in regards to what?

The ability to fly 400 miles from base to point in Europe, fight for 20 minutes and return?
The ability to actually escort bombers over that distance, even using several relays?
What altitude?
How big a formation? As in how long to get xx number of planes off the ground, get them to desired altitude and the proper order/tactical formation. Radius is determined by first plane to go wheels up.

 

[Jabberwocky]

Define long range in regards to what?

The ability to fly 400 miles from base to point in Europe, fight for 20 minutes and return?
The ability to actually escort bombers over that distance, even using several relays?
What altitude?
How big a formation? As in how long to get xx number of planes off the ground, get them to desired altitude and the proper order/tactical formation. Radius is determined by first plane to go wheels up.

So far I'm just using a slightly modified version of the RAF's standard calculations for combat range:

20% reserve (as standard)
10 minutes for warm-up and taxi (RAF standard was 5 minutes)
5 minutes high power for take-off and climb (RAF standard was 2-5 minutes)
15 minutes at high speed cruise (as standard)
5 minutes at full combat power (as standard)
Remainder at economical cruise (as standard)

Given the typical operating altitude of the RAF mediums and heavies - altitudes would probably range from 20,000 ft and up.

I'm just doing research and attempting to build up a realistic scenario at present, looking at things like the RAF's daylight escort missions for medium bombers in 1942/1943 and the mid-1943 8th AF mission structures with P-47s with combat radius of under 375 miles.

 

[Greyman]

Nothing comes to mind immediately - but I do have an anecdote from an RCAF Squadron (Spitfire IX, Merlin 61) escorting B-17s to Amiens.

The B-17s were 20 minutes late for the rendezvous and had many diversions along the way. As a result the Spitfires were running very short of fuel on the way back. While over the Somme Estuary a young Sergeant panicked and exclaimed that he only had seven gallons left. The Section Leader (author) instructed him to disregard formation and set his engine to +4 boost, 2650 rpm and lean mixture - as well as descend at 100 feet per minute.

This leads me to believe that at that stage of the mission (essentially over enemy territory) they were using higher engine settings.

I'll keep my eye open for any mentions.

 

[Jabberwocky]

Nothing comes to mind immediately - but I do have an anecdote from an RCAF Squadron (Spitfire IX, Merlin 61) escorting B-17s to Amiens.

The B-17s were 20 minutes late for the rendezvous and had many diversions along the way. As a result the Spitfires were running very short of fuel on the way back. While over the Somme Estuary a young Sergeant panicked and exclaimed that he only had seven gallons left. The Section Leader (author) instructed him to disregard formation and set his engine to +4 boost, 2650 rpm and lean mixture - as well as descend at 100 feet per minute.

This leads me to believe that at that stage of the mission (essentially over enemy territory) they were using higher engine settings.

I'll keep my eye open for any mentions.

Thanks Greyman.

I know there's a document about the Mk V compared to the FW 190 that contains some recommendations about high speed cruising, I just can't find it among my notes anymore.

 

[Greyman]

This might be it:

'How to make best use of the performance of the Spitfire V, VI and IX '
August 1942

1. This memorandum, which is based on Fighter Command Tactical Memo No.18, is intended to bring to the notice of all concerned the necessity of making full use of the power available in our Spitfire aircraft. It applies equally, in principle, to all our fighter aircraft when operating against an enemy whose performance is equal or superior to our own.
   
2. At the present state of the war, the enemy in France is equipped with the Fw 190, a fighter with an excellent rate of climb and good acceleration. To defeat this aircraft and to avoid casualties on our side, our aircraft must fly as fast as possible whenever they are in the combat zone.
   
3. In the past, pilots have been told to fly at low rpm and high boost to economise in petrol. All pilots must know the correct rpm and boost at which to fly to obtain the longest duration of flight or range.
   
4. Wings must still fly at the most economical rpm when they are flying under the enemy RDF screens, but it is essential, as soon as they are liable to be detected, that they open up to maximum power for formation flying.
   
5. The acceleration of the Spitfire is relatively poor. It is therefore dangerous to cruise at, say, +2 boost and 1900 rpm when the Hun is about, because the time taken in accelerating to maximum speed will allow him quickly to draw into firing range.
   
6. It is fully realised that the speed of formations depends on the ability of the worst pilots to keep up. This is only a question of training and practice. At present, +5 boost and 2650 rpm are the maximum boost and rpm settings known to be used successfully by a wing. On this occasion, the pilots said that they could have gone faster, and this is definitely a step in the right direction.
   
7. It is recommended that when planning operations it should be decided at what speed the aircraft should fly and at what point in the operation wings should open up to maximum speed. After opening up to maximum speed, they should not throttle back to economical cruising speed until they are well clear of the area in which they may be attacked.
   
8. Spitfires are now modified to give +16 emergency boost. It bust be impressed on pilots that this gives a great increase of speed under 21,500 ft and 18,500 feet for the merlin 46 and 45 engines respectively, and that if used for combat only, there is no risk of engine failure.

 

[Shortround6]

There is a further appendix to that note. It lists speeds (both ASI and true) at different heights with the boost/RPM and fuel consumption. It has been reprinted in at least one of Alfred Prices books.

It goes some thing like this for 20,000ft

ASI.............TAS...............Boost..............RPM.............GPH.
283.............368................+9.................3000............88
268.............350................+6.................2650............70
258.............300..............+3 3/4..............2650.............65
240.............310...............+3 3/4.............2400............50
230.............300...............+1 1/2.............2400............46*
" ".............." ".................+1..................2650...........48
200.............263...............-1 3/4..............2200............36*
" ".............." ".................-2 1/4..............2650............40

It is for both Merlin 45s and 46s.

The 300 TAS I made red was listed that way in Price's book (300TAS not red) , Typo in the book or original document I have no idea so take any other numbers with a grain of salt if you wish.

Now please note that a P-51D could cruise at 20,000ft at 316mph true using 57 US gallons (47.5 Imp gal ?) of fuel per hour according to the charts or 346mph true using 67 US gallons (55.8 Imp gal).

Also try to figure out out fuel used to climb to 20-25,000ft after take off. A Clean Mustang using 2700rpm and 46 in (8lbs boost ?) needed 7 minutes to climb form 15,000ft to 25,000ft, used 12 US gallons doing it and covered 21 miles of ground.

A MK V Spitfire could fly at 200ASI/225TAS at 10,000ft and burn only 29IMP gallons an hour but that is a suicidal speed and altitude to fly over German held territory even if perfectly reasonable over the last part of the Channel on the homeward flight or while looking for the home airfield.

 

[Greyman]

Thanks Shortround,

I plotted a quick graph and all the other numbers are in line except the '300 TAS' of course.

336 mph TAS would fit the graph for 258 mph IAS.

 

[Jabberwocky]

There is a further appendix to that note. It lists speeds (both ASI and true) at different heights with the boost/RPM and fuel consumption. It has been reprinted in at least one of Alfred Prices books.

It goes some thing like this for 20,000ft

ASI.............TAS...............Boost..............RPM.............GPH.
283.............368................+9.................3000............88
268.............350................+6.................2650............70
258.............300..............+3 3/4..............2650.............65
240.............310...............+3 3/4.............2400............50
230.............300...............+1 1/2.............2400............46*
" ".............." ".................+1..................2650...........48
200.............263...............-1 3/4..............2200............36*
" ".............." ".................-2 1/4..............2650............40

It is for both Merlin 45s and 46s.

The 300 TAS I made red was listed that way in Price's book (300TAS not red) , Typo in the book or original document I have no idea so take any other numbers with a grain of salt if you wish.

Now please note that a P-51D could cruise at 20,000ft at 316mph true using 57 US gallons (47.5 Imp gal ?) of fuel per hour according to the charts or 346mph true using 67 US gallons (55.8 Imp gal).

Also try to figure out out fuel used to climb to 20-25,000ft after take off. A Clean Mustang using 2700rpm and 46 in (8lbs boost ?) needed 7 minutes to climb form 15,000ft to 25,000ft, used 12 US gallons doing it and covered 21 miles of ground.

A MK V Spitfire could fly at 200ASI/225TAS at 10,000ft and burn only 29IMP gallons an hour but that is a suicidal speed and altitude to fly over German held territory even if perfectly reasonable over the last part of the Channel on the homeward flight or while looking for the home airfield.

Thanks for those Shortround, much appreciated.

My current 'back of the envelope' calculations weren't that accurate. Looks like I'd underestimated both the TAS cruising speed AND necessary boost/rpm settings/fuel consumption, so it might be all swings and roundabouts though.

My current thinking is that a theoretical combat radius for a Merlin 45 powered Spitfire with a total of 200 gallons might be a little under 400 miles, and the practical radius might be 350-375 mile.

 

[Jabberwocky]

These are my current 'back of the envelope calculations' for a Mk V with 200 gal – 150 internal and 50 in an external DT - are

20% reserve: -40 gal. 160 gal left
Warm-up, taxi and take-off: -10 gal. 150 gal left
10 minutes full climb power (+9/2850/84 gal hr) to 20,000 ft: -14 gal and 35 miles covered. 136 gal left.
15 minutes at high speed cruise (+7/2650/67 gal hr): -17 gal. Leaves 119 left.
5 minutes at full power (+12/3000/130 gal per hr): -11 gal. 108 gal left.

108 gal at cruise, divided by two gives 54 gal for cruise to target and then cruise back home

54 gal at +3 ¾ lb boost, 2400 rpm gives 50 gallons per hour at a TAS of 310 mph at 20,000 ft – cruise range of 335 miles + 35 miles for climb

Theoretical combat radius of 370 miles

Alternatively

54 gal at +3 ¾ lb boost, 2650 rpm gives 65 gallons per hour at a TAS of 336 mph at 20,000 ft – cruise range of 260 miles + 35 miles for climb

Theoretical combat radius of 295 miles

24 gal for warm up, take off and climb is probably a little high. In addition, the RAF added in high-speed cruise to range, which I'm not doing, and reserve was 20% of the cruising reserve, not total tankage. So, i could probably add 10-15 gal extra fuel to my allocation for cruise.

However, I'll leave it as is for the moment, as I'm making greater allowances for consumption for time to form-up and rendezvous with the bombers, as well as potential adverse conditions on any long range mission, so I think the greater reserve is probably necessary.

370 miles isn't too far from historical norms though. In August 1944, Mk VIIs escorted some Lancasters all the way to La Pallice, which was a round trip for them of 690 miles, covered in 3 hours 50 minutes. This was very slow, but the fighters were throttled right back and weaving to stay with the bomber.

Mk VIIIs did round a round trip escort mission of 850 miles, escorting B-24s against a Japanese radar station at Cape Lore on Portuguese Timor. Flying time was 4.5 hours – again, rather slow.

On both occasions, fuel was about 210 gallons.

 

[Shortround6]

And there you have the difference. Flying over water in the Pacific could and was done at lower speeds and altitudes until the target area was reached and throttling back for the cruise home was also possible.

The raid on La Pallice was was done around two months after D-Day and depending on starting point, could have flown either over or close to the Normandy beaches. In fact a fairly large portion of the flight could be done over allied controlled parts of France and German air strength in the area might be described as "light".

Trying to fly a 690 mile round trip in 1943 into Germany might have been a whole different story.

Your actual combat radius is determined by how much fuel is in the internal tank/s at the furthest point from home, after the drop tanks are gone.
Using US standards you would have:
5 minutes at 16lbs boost was 12.5 gallons gone.
15 minutes at 9lbs and 3000rpm was 22 gallons gone.
Using a 30 minute reserve would be around 13-14 gallons.

47 gallons accounted for leaving ??? fuel in the internal tanks?

Egress over the channel might not have to be done at at the same speed as egress over land. 12 gallons will see you almost 90 miles at 200ASI at 20,000ft. 59 gallons used?

Now the problem is IF you are escorting the bombers back or simply leaving them after combat and depending on a second relay of fighters to do the return escort? If you are weaving over/around them you are burning the fuel at the 300TAS rate but only covering ground at around a 200 TAS rate. Or say around 50 imp gallons to "escort" 200 miles. If you want to keep the speed up it gets worse.

Sticking 50-60 IMP gallons inside a MK V gets a little tricky. You can find room but what does it do to the climb, the sustained turn rate and the allowable "G" loading? This ignores any change in the handling of the aircraft due to change in the CG. Please remember that the MK IX had several hundred pounds of engine and propeller stuffed in the nose to help balance those rear fuselage tank/s.
Photo recon Spitfires weren't expected to dog fight but pretty much run in a straight line (or gentle curve) at high speed for evasion when loaded.

 

[Jabberwocky]

And there you have the difference. Flying over water in the Pacific could and was done at lower speeds and altitudes until the target area was reached and throttling back for the cruise home was also possible.

The raid on La Pallice was was done around two months after D-Day and depending on starting point, could have flown either over or close to the Normandy beaches. In fact a fairly large portion of the flight could be done over allied controlled parts of France and German air strength in the area might be described as "light".

Trying to fly a 690 mile round trip in 1943 into Germany might have been a whole different story.

I agree with you. That's one of the reason's I'm doing this, to work out what was possible beyond theoretically just cramming as much fuel into a Spitfire as possible.

Your actual combat radius is determined by how much fuel is in the internal tank/s at the furthest point from home, after the drop tanks are gone.
Using US standards you would have:
5 minutes at 16lbs boost was 12.5 gallons gone.
15 minutes at 9lbs and 3000rpm was 22 gallons gone.
Using a 30 minute reserve would be around 13-14 gallons.

47 gallons accounted for leaving ??? fuel in the internal tanks?

Egress over the channel might not have to be done at at the same speed as egress over land. 12 gallons will see you almost 90 miles at 200ASI at 20,000ft. 59 gallons used?

Theoretically, you can get 150 gallons into a Mk Vc without too much difficulty. All the modifications for this were done during the war. 96 gal in nose tanks (Mk III, Mks VII, VIII and most Griffon Spitfires), 29 gal in rear/under seat tank (PR Type G and Mk Vs for Malta transfer flights) and 2 wing tanks of 12-13 gallons each (Mk VII, VIII).

I agree that there are chances to wind back fuel consumption. A read a few accounts of Spitfires doing combat patrols to Paris and some do mention lower throttle settings over the Channel and English airspace.

Assumption would be that rear-tank is used on warm up and for the climb, which would burn around 20-24 gallons (based off Mk VIII and Mk IX climb data). Form up with the last of the rear tank and then use the 50 gal D/T for cruise. This is pickled off prior to entering the expected combat area. Basically, this leaves about 125-130 gallons to fight with and get home again.

Now the problem is IF you are escorting the bombers back or simply leaving them after combat and depending on a second relay of fighters to do the return escort? If you are weaving over/around them you are burning the fuel at the 300TAS rate but only covering ground at around a 200 TAS rate. Or say around 50 imp gallons to "escort" 200 miles. If you want to keep the speed up it gets worse.

From the little reading on Spitfire escorts of Ramrod operations, RAF escorts were actually a pretty complex effort. Squadrons were variously tasked with close escort, high or low escort or as a forward/roving escort. The relay system was used for some of them, but not until 1943, as far as I can see.

Sticking 50-60 IMP gallons inside a MK V gets a little tricky. You can find room but what does it do to the climb, the sustained turn rate and the allowable "G" loading? This ignores any change in the handling of the aircraft due to change in the CG. Please remember that the MK IX had several hundred pounds of engine and propeller stuffed in the nose to help balance those rear fuselage tank/s.

Sidney Cotton found a way with the PR Type G somehow (fitted and operated with armament and the 29 gal under seat tank). Plus the larger nose tanks will probably help somewhat. The Mk IX had to have 75 lb of rear ballast all the way out in the tail, so the rear fuselage tanks probably wouldn't of hurt it that much.

The Mk Vb wasn't fitted with ballast (ostensibly) but there are accounts of Mk Vc with 40 lb of ballast in the tail.

Photo recon Spitfires weren't expected to dog fight but pretty much run in a straight line (or gentle curve) at high speed for evasion when loaded.

Somewhat agree, except that the PR Type G was armed for the express purpose of being able to "defend" itself.

The weight is obviously going to have an adverse effect on performance, particularly climb and turn.

 

[Jabberwocky]

Just doing some idle distance calculations, and it looks like a 350 mile combat radius would let you hit all of the major Rhur-area targets (roughly 250-275 miles) and possibly out as far as Frankfurt, Hannover and Bremen with an escort from bases in Norfolk and Suffolk.

The more I look at this, the more I think that something around 350 miles would be about the absolute limit for a Merlin 45 powered Spifire, given the fighter's limitations and the contested airspace over Europe.

Proper mission calculations would probably include low power settings over the Channel - anywhere from 100-150 miles of the flight - and then higher power settings over occupied Europe.

 

[Zipper730]

Wait, I thought piston engines always produced the same boost at a certain RPM altitude with a given supercharger setting (other than turbos)?

 

[Greyman]

Throttle movement will vary the boost.

 

[Zipper730]

Greyman,

So to vary boost RPM you'd adjust fuel/air ratio and altitude?

 

[Greyman]

Sorry I'm not 100% sure what you're asking.

 

[Zipper730]

Greyman,

I'm confused

 

[gumbyk]

Throttle adjusts the boost (power) of the engine vi the throttle butterfly.
Propeller (RPM) is changed by a separate prop control, and is governed to a selected speed by a governor.
Mixture (Fuel:Air Ratio) is adjusted by another lever (Mixture), and compensates for varying air densities at altitude.