Spitfire + Sabre: any facts/opinons?

[tomo pauk]

better power to weight ratio where?

for takeoff?
for 10,000ft
for 25,000ft?

Here is a power chart for a Sabre IIa engine.

http://www.wwiiaircraftperformance.org/tempest/sabrecurve.jpg

Note that the almost 2400 hp at 4000ft(?) decreases to 1600hp at just over 22,000ft. Also note that this is with 400mph of ram into the carburetor intake which means that power levels are going to be down several thousand feet for climb performance.

A Griffon 61-69 was rated at about 2000-2010hp at 6750-7000ft in low blower at 21lbs boost and 1810-1820hp at 21000ft in hi blower. At any altitude over about 1900ft it is making more power and it weighs about 83% of what the Sabre does. on a power to weight basis they are often within about 1% of each other until those altitudes are over 20,000ft. Then then the Griffon runs, not walks away, having a better power to weight ratio by 30% or more .

Many thanks for the link; guess next time I should put more effort to find stuff on the 'net by myself
Re. 2-stage engines for Spitfire, I was commenting onto '2-stage engines for Spitfire' - and only 2-stage engine is Spit (operative squadrons) in 1943 was Merlin. Griffon was single stage engine back then, hence my sentence about greater power-to weight ratio of the plane.

Props and radiators/oil coolers have to be sized for the best power condition (worst cooling load, the extra 350-400hp of the Sabre ?. High altitude cooling may be a problem for the Griffon?)

Agreed; so the Sabre would have 'more' coolers, but no intercoler - slightly more drag than XII, but less than IX XIV?

added: the chart you've provided the link is for Tempest (Sabre uses +11 lbs boost under FTHs, with 150 grade fuel) - not available in 1943?

 

[tomo pauk]

1/. No, I don't, but I have a comparison report, for a Tempest, against a Mustang III, Spitfire XIV, Fw190 with BMW801D, and Me109G (no idea of suffix, but probably an early one.) It's far too big to put it here.
Mustang - Tempest 15-20 mph faster up to 15,000', level up to 24,000', 30 mph slower up to 30,000'; Spitfire XIV - Tempest 20mph faster up to 10,000', level to 22,000', 30-40 mph slower up to 30,000' (Tempest's ceiling, but Spitfire capable of 40,000'.) ; Fw190 - Tempest nearly 50 mph faster at all heights, slower in climb at all heights above 5,000'; Me109G - Tempest 40-50 mph faster up to 20,000', but difference rapidly lessens above that height; almost identical in the climb to 5,000', but slower above that height.

Thanks

2/. The intercooler is part of the Griffon engine; do you mean the intercooler radiator? It's a common mistake to think that the radiators create drag, but the designer was another genius; the radiators worked in the same way as that of the Mustang, and actually increased speed, not reduce it.

Did not know that Spits radiators took advantage of Merredith effect; could you point me/us to a something on-line re. that?

3/. If the Sabre was heavier than the Griffon, it's unlikely that extra fuel could have been carried in front of the CoG.

I was under impression that Spit's hull tanks were at CoG, so making them wider wouldn't make any issues?

4

/. The two-stage "blower" cuts in in stages, which enables the aircraft to increase speed at heights at which single-stage engines begin to lose "oomph." It was the second stage which made the Spitfire XIV so much faster than the Tempest above 22,000'.
Edgar

I agree with your words, but only British 2-stage engine in service in 1943 was Merlin...*

IMO that has no bearing on this discussion as the Spitfire was not designed for a radial engine.

People were complaining that a wider engine would've messed up Spits aerodynamics/looks/etc. I was pointing out that back in WW2 other airforces were installing even wider engines, with benefits outweighting shortcomings.
Hence it has a bearing on this discussion.

added: Spit XIV was far lighter, with less drag, and with more power above 22K - all 3 factors contributing for better performance vs. Tempest at hi-alt?

 

[Shortround6]

People were complaining that a wider engine would've messed up Spits aerodynamics/looks/etc. I was pointing out that back in WW2 other airforces were installing even wider engines, with benefits outweighting shortcomings.
Hence it has a bearing on this discussion.

Since the aircooled engines also lost the radiator the installed engine weights were closer than they first appear. Radials are shorter so the center of gravity of the engine is closer to the center of gravity (which should be close to the center of lift) of the airplane. Some air forces that installed radials in place of V-12s did so because their '2nd generation' V-12s didn't work. Radial was the only option left. Lets not forget that not only the engines but the installations were constantly evolving. A 1944 radial installation would have lower drag in just about any country than a 1941 installation. P&W managed to get a P-40 airframe equipped with an R-1830 (sort of a reverse P-36) up to 379mph (?) in the late fall of 1942. Which means a production version may have been possible by spring/summer of 1943 but the liquid cooled installations were getting better too.

added: Spit XIV was far lighter, with less drag, and with more power above 22K - all 3 factors contributing for better performance vs. Tempest at hi-alt?

All true, don't forget, it can take months to change a production line from one major version of a plane to another and it can take several years for a major (not minor) revision of an engine. Production planning schedules have to be done months if not 1-2 years in advance.

 

[Edgar Brooks]

1/. Did not know that Spits radiators took advantage of Merredith effect; could you point me/us to a something on-line re. that?
2/. I was under impression that Spit's hull tanks were at CoG, so making them wider wouldn't make any issues?
People were complaining that a wider engine would've messed up Spits aerodynamics/looks/etc. I was pointing out that back in WW2 other airforces were installing even wider engines, with benefits outweighting shortcomings.
3/.Spit XIV was far lighter, with less drag, and with more power above 22K - all 3 factors contributing for better performance vs. Tempest at hi-alt?

1/. No, sorry, the vast majority of my information comes from 50 years of collecting books and magazine articles. The man's name was Frederick Meredith, and he worked for the Royal Aircraft Establishment in Farnborough.
2/. The Spitfire was designed around the Merlin, and frame 5 (the engine bulkhead) has very short stub spars, to which the ends of the wing spars are attached. Widening the fuselage would have, of necessity, increased the wingspan, so any fuel increase would have to go forwards, since taking it backwards would shift the pilot back, as well, and play havoc with the CoG. The answer is fairly simple really, widen the fuselage, and lengthen it, to allow for the extra weight of the engine, then fit elliptical wings, and it would be so different, you'd really need a new name, "Tempest" perhaps? Frame 5 is the "keel" round which the Spitfire is built, so any extra width would have to go in front of it, making a very weird tapered front indeed.
The Sabre was deeper, too, which would have added depth to the fuselage, underneath, since you couldn't add it on top, because the pilot's view was restricted enough, already. Deepen the fuselage, and the ground clearance reduces, which needs a longer oleo leg, which wouldn't fit into the wheel wells, unless you have a wider wingspan.
3/. Definitely, as test reports show.
Rolls-Royce, and Supermarine, expected great things from the Griffon, which (apart from length) could fit into the same available space as the Merlin. The RAF had enough hassle with the extra work involved in use of the Packard Merlin, with the need for a new Mark, and separate stores facilities. Two Merlin Spitfire ranges, the Griffon family, and a Sabre set-up, with two more totally different Sabre airframes already in existence, would have been a nightmare.
You only have to look at how quickly the R-R Merlin was discarded, immediately at the end of the war, leaving just the Packard (to use up surplus stocks,) and the Griffon, to realise the strain on the RAF's resources, especially with the post-war demobilisation, if there had been any other types. Even the Typhoon was being built, trundled across the airfield, and scrapped, immediately after the war. The Sabre was always seen as an expedient, until the Centaurus became available.
Edgar

 

[tomo pauk]

I'd like to return back to this:

better power to weight ratio where?

for takeoff?
for 10,000ft
for 25,000ft?

Here is a power chart for a Sabre IIa engine.

http://www.wwiiaircraftperformance.org/tempest/sabrecurve.jpg

Note that the almost 2400 hp at 4000ft(?) decreases to 1600hp at just over 22,000ft. Also note that this is with 400mph of ram into the carburetor intake which means that power levels are going to be down several thousand feet for climb performance.

The 150 grade fuel is 1944 stuff; Sabre should with +9 lbs boost attain cca 1850 BHP @ 18kft (full trottle height for high blower?)?

A Griffon 61-69 was rated at about 2000-2010hp at 6750-7000ft in low blower at 21lbs boost and 1810-1820hp at 21000ft in hi blower. At any altitude over about 1900ft it is making more power and it weighs about 83% of what the Sabre does. on a power to weight basis they are often within about 1% of each other until those altitudes are over 20,000ft. Then then the Griffon runs, not walks away, having a better power to weight ratio by 30% or more .

While I'd agree that Griffon was a superb piece of machinery, declaring that it 'runs' vs. Sabre (above 21kft) is hardly true - advantage is 100 HP (give or take), or 5-7%, depending on altitude. The power to weight ratio of entire plane is what counts, and such would've been better by some 12%, but available a year later.

Props and radiators/oil coolers have to be sized for the best power condition (worst cooling load, the extra 350-400hp of the Sabre ?. High altitude cooling may be a problem for the Griffon?)

Sabre in 1943 requires 50 % more cooling (has 50% more HP) [EDIT: at 6-7kft] than Griffon II (1-stage)

A simple chart is attached, feel free to correct me

added: Spit XII was good for 393mph @ 18K, with only 60% of power Sabre was capable at same alt, or with some 1130-1150Hp up there. Sabre was delivering 1200HP at some 27-28K. All in 1943.

NOTE: THE graph for Griff II is valid for static power (no ram effect), while the one for Sabre is with ram effect, hence the whole chart should not be taken as gospel.

 

[tomo pauk]

Hi, Edgar,

Many thanks for the insights
I've read a thing or two about Mr. Meredith, and I'm really interested to find out if 'his' effect was purposely applied at planes other than P-51.

 

[Edgar Brooks]

The 150 grade fuel is 1944 stuff; Sabre should with +9 lbs boost attain cca 1850 BHP @ 18kft (full trottle height for high blower?)?

While I'd agree that Griffon was a superb piece of machinery, declaring that it 'runs' vs. Sabre (above 21kft) is hardly true - advantage is 100 HP (give or take), or 5-7%, depending on altitude. The power to weight ratio of entire plane is what counts, and such would've been better by some 12%, but available a year later.

added: Spit XII was good for 393mph @ 18K, with only 60% of power Sabre was capable at same alt, or with some 1130-1150Hp up there. Sabre was delivering 1200HP at some 27-28K. All in 1943.

Quite simply you're fixating on the wrong thing; talk to any WWII pilot, and his sole interest was speed, not brake horsepower, compared to his opponent. Below 20-22,000' the Sabre had the edge, but above that the Spitfire XIV did run away from it, to the tune of 30-40mph. Those aircraft were designed as fighting machines, at all heights, and the Spitfire won by a country mile, above 25,000'.
Using the Spitfire XII is the proverbial red herring, since it was only ever intended for low-level use, and the Griffon II never saw service in a Spitfire, so I fail to see what that's doing here. Forget all this talk about BHP, since it's meaningless; talk speed at various heights, because that was what mattered.
Edgar

 

[Shortround6]

I'd like to return back to this:


The 150 grade fuel is 1944 stuff; Sabre should with +9 lbs boost attain cca 1850 BHP @ 18kft (full trottle height for high blower?)?

I think it all depends on EXACTLY when you draw your time line and EXACTLY which version of which engine you are talking about. The Sabre II appears to have been limited to 3700rpm and with 9lbs boost had 1880hp at 15,250ft. The Sabre IIA could turn 3850rpm and at 11lbs boost was good for 2045hp at 13,750ft. The discrepancy between this number and the chart you are going by is that the Chart included the 'boost' provided by 400mph worth of ram effect going into the air intake, While this power is available for level flight it is not available for climb. If the power figures for the Griffon II do not include RAM effect then the altitude is going to be several thousand feet too low, As the Griffon IIB was rated at 1495hp at 12lbs boost at 14,500ft with NO ram. As installed in the aircraft And using a climb speed of 240mph true it could hold the full 12lbs to 15,300ft. But it could hold 12lbs of boost (and the full 1495hp) to 17,800ft in level flight when doing 397mph true.

If what is wanted is power at high altitude The MK IX with the Merlin 61 already provided it. 1340-1390hp at 23,500 at 15lbs depending on your source, again without ram. In a combat climb using 3000rpm and 15lbs boost the MK IX could hold the 15lbs (and the 1390hp) to 25,200ft at a 239mph climb speed and for level flight could get the full 15lbs of boost at 27,400ft. A Merlin 61 is one heck of a lot lighter than a Sabre.

The Sabre couldn't provide anything at high altitude they weren't already getting from the Merlin and the two stage Griffon promised to be that much better, and a whole lot less trouble to stick in a Spitfire.

The Spitfire is claimed to use the Meredith effect but there is a world of difference between some mathematical formulas and some sketches and getting measurable performance in the real world. There is also quite a difference in getting a little bit of "thrust" (say, getting 20% of your cooling drag back) and actually getting "positive thrust" (thrust exceeds cooling drag).

 

[wuzak]

A book I have about the Spitfire says that the radiators on the Spitfire were designed with the Meredith effect in mind, but they weren't as effective as they might have been - all to do with the inlet and exhaust area, and the adjustable flap. IIRC the flap on the Spitfire's radiator ducts was, at least for some versions, a two position device, which would in some instances give too much cooling if it were open, and not enough if it were closed.

I suspect Supermarines were too busy making improvements in other areas of the airframe to have time and resources to investigate the radiator ducts.

 

[Shortround6]

All to true.

To get the maximum results from the Meredith effect requires not only a variable 'nozzle' for different flight conditions but a carefully designed duct giving the right changes in cross section to adjust the speed of the air flow both before and after it goes through the radiator. Too high a speed hitting the radiator front surface will cause excess drag. The actual size of the radiator is also critical. A larger cross section radiator will give less drag as the air flows through it. But hiding large cross section radiators is harder than small section ones (which may be thicker front to back) and they are heavier. The air has to pick up sufficient heat while flowing through the radiator to create the desired expansion and then the duct has to reduce in size to compress the airflow to accelerate it up to the desired exit speed. It is asking a lot of a short duct or radiator housing to handle those changes in air flow speed compared to a much longer duct. Look at a cross section of the duct in the Mustang for an idea of changes in cross section they used.

 

[wuzak]

The other issue I have wondered about with the Spitfire's radiators is the lack of a boundary flow splitter. The P-51 has one. The original XP-46 (and XP-40?) had a ventral radiator, but no boundary layer splitter and it wasn't very good. Same with the Hawker Tornado.

 

[tomo pauk]

Quite simply you're fixating on the wrong thing; talk to any WWII pilot, and his sole interest was speed, not brake horsepower, compared to his opponent. Below 20-22,000' the Sabre had the edge, but above that the Spitfire XIV did run away from it, to the tune of 30-40mph. Those aircraft were designed as fighting machines, at all heights, and the Spitfire won by a country mile, above 25,000'.

Sorry if you feel I'm splitting hairs, but:
-I was under impression that BHPs (among other stuff) provide speed.
-Wouldn't it be fair to say that Spit XIV would run away from Tempest - from it's surplus ton of metal greater drag, and then from Sabre?

Using the Spitfire XII is the proverbial red herring, since it was only ever intended for low-level use, and the Griffon II never saw service in a Spitfire, so I fail to see what that's doing here.

This document says Griffon II was powering Spit XII:
http://www.spitfireperformance.com/spitfireXII-ads.jpg
It's the only Griffon that saw service in 1943 in RAF service?

Forget all this talk about BHP, since it's meaningless; talk speed at various heights, because that was what mattered.
Edgar

Covered above

I think it all depends on EXACTLY when you draw your time line and EXACTLY which version of which engine you are talking about. The Sabre II appears to have been limited to 3700rpm and with 9lbs boost had 1880hp at 15,250ft. The Sabre IIA could turn 3850rpm and at 11lbs boost was good for 2045hp at 13,750ft. The discrepancy between this number and the chart you are going by is that the Chart included the 'boost' provided by 400mph worth of ram effect going into the air intake, While this power is available for level flight it is not available for climb. If the power figures for the Griffon II do not include RAM effect then the altitude is going to be several thousand feet too low, As the Griffon IIB was rated at 1495hp at 12lbs boost at 14,500ft with NO ram. As installed in the aircraft And using a climb speed of 240mph true it could hold the full 12lbs to 15,300ft. But it could hold 12lbs of boost (and the full 1495hp) to 17,800ft in level flight when doing 397mph true.

Thanks for setting my numbers straight; the doc linked above doesn't mention it's 'static' power (no ram).

If what is wanted is power at high altitude The MK IX with the Merlin 61 already provided it. 1340-1390hp at 23,500 at 15lbs depending on your source, again without ram. In a combat climb using 3000rpm and 15lbs boost the MK IX could hold the 15lbs (and the 1390hp) to 25,200ft at a 239mph climb speed and for level flight could get the full 15lbs of boost at 27,400ft. A Merlin 61 is one heck of a lot lighter than a Sabre.

The Sabre couldn't provide anything at high altitude they weren't already getting from the Merlin and the two stage Griffon promised to be that much better, and a whole lot less trouble to stick in a Spitfire.

Again, thanks for the data

The Spitfire is claimed to use the Meredith effect but there is a world of difference between some mathematical formulas and some sketches and getting measurable performance in the real world. There is also quite a difference in getting a little bit of "thrust" (say, getting 20% of your cooling drag back) and actually getting "positive thrust" (thrust exceeds cooling drag).

Well put; for a radiator to put Merdith's effect to use, it's about narrow inlet, well widening towards the radiator core itself, and again narrowing towards exit?

 

[Ratsel]

Thrust means air leaving the radiator is faster the air entering it. Very hard to do with a big block of copper tubes and cooling fins in the way. The 'Merdith effect' is moot at this point. Exhaust thrust is much more prevelant, yet nowhere near effective on prop driven a/c during WWII. I read something like 2-5mph at full boogy with a DB 605D in a Me 109G-10.

 

[Shortround6]

It was possible and it was done. The Mustang used both. and it is not the speed of the air entering and leaving the radiator but entering and leaving the radiator duct. Sort of like a ramjet but using the radiator as a heat source. The problem with figuring effect for exhaust "thrust" ( and 2-5mph at full boogy is pretty poor) is that it varies with altitude (back pressure on the exhaust outlets), quantity of exhaust ( full throttle height gives most exhaust but not the most thrust) and speed of the exhaust compared to the speed of the plane. A Spitfire would get more benefit than a Hurricane at the same altitude and throttle setting because the Spitfire is moving faster and the exhaust speed and aircraft speed are a better match. Exhaust thrust doesn't do much for low level climb.

 

[Ratsel]

yah, and theres a big ole' radiator in the way. so the air stream has to go either up and over / down and under / around the radiator or all three. slowing the airflow even further. In front of the radiator housing there is a high pressure buildup, caused by the interuption of the airflow ( ie: the radiator ), out back after the air passes will be hot tubulant votecies, negating any sort of thrust. P-40's had that problem which caused latteral instability.

 

[Shortround6]

and that is why it took a while, and good engineering, to get the theory to work. Any air in the duct that isn't going though the radiator and being heated is a dead loss. It isn't helping with thrust and if it is not going though the radiator it isn't helping with cooling. It is only adding to the drag.

 

[Edgar Brooks]

Sorry if you feel I'm splitting hairs, but:
1/.-I was under impression that BHPs (among other stuff) provide speed.
-Wouldn't it be fair to say that Spit XIV would run away from Tempest - from it's surplus ton of metal greater drag, and then from Sabre?

2/.This document says Griffon II was powering Spit XII:
It's the only Griffon that saw service in 1943 in RAF service?
3/.for a radiator to put Merdith's effect to use, it's about narrow inlet, well widening towards the radiator core itself, and again narrowing towards exit?

1/. For initial power, that's true, but the Griffon, in the XIV, had a two-stage blower, so it continued to give increased power to the engine as it climbed, unlike the Sabre, which gave out in the mid-20s. If the Sabre had had some extra blower(s) behind it, who knows what it could have done at height? Without knowing drag coefficients, it's impossible to say how the airframes would have behaved, given equal power.
2/. I suspect that form is for DP845, the XII prototype, which was fitted with a Griffon IIB, and began testing in September 1942. The Griffon II was the Firefly engine; the Griffon III was redesigned. Some XIIs did have a Griffon II, but the designated engine was thye Griffon III, with some getting the Griffon IV. Those Griffons were the only engines to see service use in Spitfires, in 1943, though six Mk.VIIIs were converted, that year, into prototype XIVs, with some using Griffon 61s, and one a Griffon 65.
3/. That's precisely how the Spitfire radiator(s) work; the housing widens to left and right, and upwards, but remains straight aft of the matrix, all of which means that the air, when heated, expands, and leaves faster than it came in. However small the effect might be, it means that, contrary to popular belief, the radiators were not a source of drag.
I don't feel that you're "splitting hairs," but there was a lot more to squeezing extra speed out of an airframe than just the raw power of the engine. Mr. Ratsel is dismissive of an extra 2-5 mph, but, to a pilot, that can mean life or death. Whether you choose to believe it, or not, is immaterial, since it's well documented that the radiators made the Spitfire go faster, and the radiator matrices are a honeycomb, through which air readily passes (check a car's radiator,) not "a mass of tubes."
In 1942, tests were carried out on a Vb, to see what effect various alterations made. The multi ejector exhausts, in place of the three ejectors (airily dismissed by Mr. Ratsel) actually increased the speed by 7 mph. Removing the ice and stone guard, in front of the carbuettor intake, added 8 mph (standard Mk.VIIIs, IXs XIVs used a lengthened intake, with a "tropical" filter; on take-off the air was directed through a filter, then, when clear of the ground, it was redirected straight into the carburettor. A new style of mirror added 3 mph.
In late 1942, Supermarine persuaded the Air Ministry to go over to synthetic, rather than cellulose, paints, since they'd found that they could get a genuine smooth finish. They also began to fill panel lines, and rivet divots, on the wing l/e, plus primer, and smoothing down, to give a perfectly smooth surface, which gave a further 6 mph increase. By the time all of these changes had been made, the aircraft was over 28mph faster.
Edgar

 

[Ratsel]

not dismissed, the overall design of the rad housing contributed to less drag. not any sort of thrust produced. 5mph gain from exhaust thrust is outstanding.

 

[Edgar Brooks]

yah, and theres a big ole' radiator in the way. so the air stream has to go either up and over / down and under / around the radiator or all three. slowing the airflow even further. In front of the radiator housing there is a high pressure buildup, caused by the interuption of the airflow ( ie: the radiator ), out back after the air passes will be hot tubulant votecies, negating any sort of thrust.

Yah, and that big ole radiator had big hexagonal holes through it, through which the air easily flowed, and the matrices were tightly fitted, in box-like structures, so that the air couldn't go up/over/down/under/round, but had to go through. The radiator installation was the only component which was tested in a wind tunnel, so it wasn't a hit-and-miss system (the British don't use knotted string for everything.) Supermarine/Farnborough's tests showed a gain in air speed, and therefore thrust; do you have figures to disprove that? There were no turbulent vortices aft of the radiator(s) either, otherwise the airflow, over the flaps, would have been disrupted, causing havoc.
Edgar

 

[Ratsel]

yah some NASA windtunnels use the same honeycomb setup for directional flow.. for curbing those unstable vortecies.. nothing for thrust. guess I'll have to agree to disagree. thanks for your explanations.