How The Spitfire Mk XIV Compared to the K4 and Other Questions

[Milosh]

There were 109s that used C3 fuel even from late 1040.

B4 fuel usually was used with GM1 or MW50.

The 109 had 'pitch' trim from the beginning.

Allied testing of C3 fuel gave it a PN of 140 to 165.

 

[Spacefire]

1.98 ata is 57.3 inches of US MAP and 13.5 psi British boost.

Offhand, I'd say the American and Brits were flying that for a LONG time. The fact that the Germans weren't might indicate B-4 fuel use. Certainly most Allisons were rated at 57" and Merlins in P-51's were ,too.

Even merlin 45's were rated at +18 psi (2.30 ata. 66.6 inches). The Merlin 61 was rated at +15 psi (2.09 ata, 60.5 inches) and the Merlin 63 was rated at +18 psi (2.30 ata, 66.6 inches). Late-war Allisons were cleared to 70 inches (2.42 ata, 19.7 psi) and the pilots ran them to 75 inches (2.59 ata, 22.1 psi).

One of the main reasons why the claims of German fuel quality being equal to Allied fuel quality is suspect is covered by the low boost they ran. 1.8 ata is 52.1 inches and 10.9 psi, and that was considered high boost for a Luftwaffe pilot. It was mid-range power for a Merlin.

Hm interesting, so along with the fuel, would the engines have been able to handle higher boost number? The 109 K4 with 1.98ata did ~2,000 hp, which is already impressive. Could they have actually brought that higher?

Also, somewhat unrelated, but from what I can tell the Mk21 and 22 are essentially just XIV's with reinforced wings, so what gives them their better speed and climb? Was it just the thicker wing? I know that increases lift but what does it do for speed?

 

[Milosh]

DB engine used a CR of up to ~8.5. Merlins and Allisons max was 6:1.

What is the BMEP of the engines?

 

[RCAFson]

How many RAF sorties were flown using 1.98ata boost prior to May 1945?

1.98ata = about 12lb boost... (13.5lb as per previous posts).

http://www.ww2aircraft.net/forum/engines/ata-inches-hg-26858-post734728.html#post734728

I guess the question is whether or not Spitfire XIV squadrons used +21 lb boost in combat. The combat encounter records of the various squadrons are all online, for a fee, at the UK National Archives and free to view for those who go in person. I think the evidence presented by Williams is pretty presuasive that the Mk XIV did use 21 boost in combat, but it is easy enough to verify.

 

[davebender]

How many aircraft are we talking about?

 

[GregP]

I only have a good treatise on BMEP for unboosted engines, but it is quite decent for comparison purposes, so:

For the DB 601 at 1,175 HP and 2,500 rpm, BMEP is 180.

For a DB 605 at 1,775 HP and 2,800 rpm, BMEP is 230.

For a Merlin 63 at 1,710 Hp and 3,000 rpm, BMEP is 274.

Of course, all these numbers change if the HP or the rpm changes since they are all related, and the HP numbers were really cv or ps rather than HP, but they are close.

I have the DB 605 with a compression ratio of 7.3 to 7.5 : 1, per German documents, depending on variant. I suppose they could have run it higher, but almost every other engine designer was trying to run lower and I'm not sure why they would boost the CR unless they had a quantum leap in fuel quality. Upping the CR only makes you HAVE to run lower boost or face detonation. The CR of the DB 601 was closer to the Merlin at 6.9 : 1 to the Merlin's 6.0 : 1. So, I'd expect lower boost on the 605. The 601 should run a bit higher boost ... but it's also running at lower rpm.

Running at lower rpm makes sense in that the Germans liked centerline armament and 3-blade, wide-chord props. Slower-turning props made for a better rate of fire for interrupted weapons and better efficiency at high altitudes. The British and Americans weren't as concerned with prop rpm as they normally had wing armament and didn't really care about rate of fire that might be limited by propeller rpm. They looked at the prop as an independent item, unconcerned with armament, for the most part.

Allison CR was generally 6.65 : 1. Merlins were generally 6.0 : 1.

 

[Spacefire]

I only have a good treatise on BMEP for unboosted engines, but it is quite decent for comparison purposes, so:

For the DB 601 at 1,175 HP and 2,500 rpm, BMEP is 180.

For a DB 605 at 1,775 HP and 2,800 rpm, BMEP is 230.

For a Merlin 63 at 1,710 Hp and 3,000 rpm, BMEP is 274.

Of course, all these numbers change if the HP or the rpm changes since they are all related, and the HP numbers were really cv or ps rather than HP, but they are close.

I have the DB 605 with a compression ratio of 7.3 to 7.5 : 1, per German documents, depending on variant. I suppose they could have run it higher, but almost every other engine designer was trying to run lower and I'm not sure why they would boost the CR unless they had a quantum leap in fuel quality. Upping the CR only makes you HAVE to run lower boost or face detonation. The CR of the DB 601 was closer to that Merlin at 6.9 : 1 to the Merlin's 6.0 : 1. So, I'd expect lower boost on the 605. The 601 should run a bit higher boost ... but it's also running at lower rpm.

Running at lower rpm makes sense in that the Germans liked centerline armament and 3-blade, wide-chord props. Slower-turning props made for a better rate of fire for interrupted weapons and better efficiency at high altitudes. The British and Americans weren't as concerned with prop rpm as they normally had wing armament and didn't really care about rate of fire that might be limited by propeller rpm. They looked at the prop as an independent item, unconcerned with armament, for the most part.

Allison CR was generally 6.65 : 1. Merlins were generally 6.0 : 1.

I'm unfamiliar with some of these terms... care to explain?

Sorry. My knowledge isn't quite as in depth as most of yours, but I appreciate the information!

 

[Shortround6]

How many aircraft are we talking about?

It seems that Supermarine built 957 MK XIVs by June of 1945 which is when the they delivered their first MK XVIII. Castle Bromwich was making MK 21s.

 

[davebender]

When did production of the Spitfire MK XIV begin?

 

[Shortround6]

I'm unfamiliar with some of these terms... care to explain?

Sorry. My knowledge isn't quite as in depth as most of yours, but I appreciate the information!

I believe Greg is tying to equalize the the situation. Because using different compression ratios does affect the amount of boost that can be used with a given type ( octane or performance number) a simple comparison of boost pressures fails to tell the whole story.

The BMEP is the Brake mean effective pressure in the cylinder. It is a calculated value based on the power at the propshaft and is the mean pressure acting on the piston/s per sq in at the quoted rpm to give the listed power. The performance number of fuel is more closely related to the BMEP than to boost because of the variation in compression ratios between engines. It may not be ideal but it comes closer than any other number that can be easily calculated. IMEP (indicated mean effective pressure) is more accurate but requires knowing both the friction loss in the engine and the power needed to drive the supercharger and other accessories.
The company engineers have access to such information but most of us do not.

Gregs use of the BMEP seems a reasonable attempt to find a middle ground rather than flag waving.

 

[Greyman]

When did production of the Spitfire MK XIV begin?

October 1943

EDIT: details

Contract No. B980385/39
Vickers-Armstrong - 8th Order
Ordered as Spitfire XIV
Built as Spitfire F XIV (Griffon 65)
October '43 - March '44
RB140 - RB189

 

[Shortround6]

When did production of the Spitfire MK XIV begin?

From Wiki "When the new fighter entered service with 610 Squadron in January 1944"

 

[GregP]

The USA measure manifold pressure in inches of Mercury absolute. 1 atmosphere is 29.92 inches of Mercury absolute or 14.696 pounds per square inch.

The UK measured it in units of pound per square inch (psi) gauge pressure. Since it is gauge pressure, 29.92 inches of Mercury is zero psi or atmosphereic pressure minus 14.696 psi.

The Japanese and Russians used millimeter of Mercury, but one was absolute pressure and one was gauge pressure. 1 atmosphere is 760 mm Mercury. If you use gauge pressure, it is absolute pressure minus 760 mm.

The Germans used technical atmospheres absolute pressure. 1 technical atmosphere is 28.958 inches of Mercury or 1 standard atmosphere (29.92 inches of Mercury) is 1.033 ata.

Why they didn't adopt some standard is anybody's guess. Probably has to do with not doing it like anyone else to help maintain secrecy.

If you look up engine performance calculations, you'll find BMEP is Brake Mean Effective Presure.

For a 4-stroke engine, BMEP in psi = (150.8 * torque in ft-lbs) divided by the dispalcement in cubic inches. For a s-stroke, change the 150.8 to 75.4.

The operative princilple here is that engines operating at the same BMEP are about equally stressed and produce similar torque per displacement values. For normally aspirated engines, BMEP values of over 200 are difficult to achieve. For refrence, a normally-aspirated Formula 1 automotive racing engine makes a BMEP of about 220 psi. A normally-aspirated NASCAR V-8 make about 203 psi, and so they are quite comparable in power output and stress levels.

All the aircraft engines above are boosted with supercargers. Since any piston engine running on gasoline is just an air pump, boosting the pressure will run more air through it and make more power, adn their BMEP values can easily go up to 350 psi or more.

An engine at 280 psi BMEP is stressed about 27% more than one making 220 psi BMEP. There WILL BE some limit where exceeding that BMEP will lead to engine destruction.

Another such parameter that almost cannot be exceeded is mean piston speed. Under 3,500 feet per minute is generally good reliability. 3,500 - 4,000 feet per minute is stressful and needs good design. Over 4,000 feet per minute means very short engine life ... NOT what you neeed in an airplane taht cannot pull over and park when the engine fails. You use rpm and stroke to calculate mean piston speed.

Here is a great link to help:

Brake Mean Effective Pressure (BMEP): The Performance Yardstick

This paper explains it better than I did. Shortround is right, BMEP tends to equalize out several other factors.

Forgot to add, CR is compression ratio. To find it, you put the piston at top dead center and measure the volume. The at bottom dead center and measure the volume. The Compression Ratio is the big volume divided by the small volume. So if an engine measure 10 cubic inches at bottom and 1 cubic inch at top, the CR is 10 / 1 or 10 : 1.

You may notice that the compression ratio is NOT in the paper above, but IS important because there is some limit of presure in the cylinder where exceeding it will result in detonation. A higher compression ratio means you have less boost available before the limit presure is reached.

This is fun ... if you like engines and math. It is horrible if you don't.

I like discussing it and sharing the formulas and calculations, but hate to argue about it.

Really, these seemingly unrelated engines and propellers installed in completely different airframes in countries trying to keep the details secret resulted in warplanes that have remarkably similar performance to one another. It just goes to show that all the countries involved had some pretty decent engineers who came up with different and sometimes wildly different solutions that approached a very similar limit, and the jumps in performance almost mirriored one another. The Spitfire and Bf 109 traded the title of "best fighter" back and forth for a long time, each one that earned the title being a new variant of the old airframe/engine/propeller combination.

I'm amazed they came so close to one another.

The British had a genius of a supercharger designer named Sir Stanley Hooker. He designed mechanical 2-stage superchargers that were very efficient. The German used a variable hydraulic drive to achieve almost the same results. The Americans use both mechanical 2-stage superchargers (on some radial) as well as a single-stage supercharger and a turbocharger on the same engine to achieve the same type results. I think the British were a bit ahead there. The Spitfires that used a Merlin 60 series and later were 2-stage mechanical, as were all of the P-51B and later Mustangs. The P-39 and P-47 used super/turbos combos.

If you go to an airshow and see the Horsemen aerobatic act in P-51s, one thing that will stand out is the whistling sound as they come down the back side of a loop or cuban eight. That whistling is the sound of a supercharger impeller and is music to my ears. You'll get the same whistle from a Grumman FM-2 Wildcat, though with the sound of a radial in the foreground, because it also has a 2-stage supercharger.

You'll get the same sound from Spitfires, but you'd probably have to have maybe 2 - 3 of them doing some higher-power loops or cuban eights to hear it, and I haven't seen any Spitfire formation aerobatics. Doesn't mean it hasn't happened; means I haven't seen and heard it.

 

[Shortround6]

Most WW II aircraft engines were running around 3000 fpm piston speed or less. Only a few were over (or over by much). The Jumo 213 was probably the highest with the Bristol Pegasus being second ?

 

[GregP]

You are spot on as usual, Shortround. The 3,500 I quoted above is more for modern engines.

We don't do the math any better than they did back then, but computers make it a LOT faster and easier than in the 1930s, huh?

 

[Spacefire]

The USA measure manifold pressure in inches of Mercury absolute. 1 atmosphere is 29.92 inches of Mercury absolute or 14.696 pounds per square inch.

The UK measured it in units of pound per square inch (psi) gauge pressure. Since it is gauge pressure, 29.92 inches of Mercury is zero psi or atmosphereic pressure minus 14.696 psi.

The Japanese and Russians used millimeter of Mercury, but one was absolute pressure and one was gauge pressure. 1 atmosphere is 760 mm Mercury. If you use gauge pressure, it is absolute pressure minus 760 mm.

The Germans used technical atmospheres absolute pressure. 1 technical atmosphere is 28.958 inches of Mercury or 1 standard atmosphere (29.92 inches of Mercury) is 1.033 ata.

Why they didn't adopt some standard is anybody's guess. Probably has to do with not doing it like anyone else to help maintain secrecy.

If you look up engine performance calculations, you'll find BMEP is Brake Mean Effective Presure.

For a 4-stroke engine, BMEP in psi = (150.8 * torque in ft-lbs) divided by the dispalcement in cubic inches. For a s-stroke, change the 150.8 to 75.4.

The operative princilple here is that engines operating at the same BMEP are about equally stressed and produce similar torque per displacement values. For normally aspirated engines, BMEP values of over 200 are difficult to achieve. For refrence, a normally-aspirated Formula 1 automotive racing engine makes a BMEP of about 220 psi. A normally-aspirated NASCAR V-8 make about 203 psi, and so they are quite comparable in power output and stress levels.

All the aircraft engines above are boosted with supercargers. Since any piston engine running on gasoline is just an air pump, boosting the pressure will run more air through it and make more power, adn their BMEP values can easily go up to 350 psi or more.

An engine at 280 psi BMEP is stressed about 27% more than one making 220 psi BMEP. There WILL BE some limit where exceeding that BMEP will lead to engine destruction.

Another such parameter that almost cannot be exceeded is mean piston speed. Under 3,500 feet per minute is generally good reliability. 3,500 - 4,000 feet per minute is stressful and needs good design. Over 4,000 feet per minute means very short engine life ... NOT what you neeed in an airplane taht cannot pull over and park when the engine fails. You use rpm and stroke to calculate mean piston speed.

Here is a great link to help:

Brake Mean Effective Pressure (BMEP): The Performance Yardstick

This paper explains it better than I did. Shortround is right, BMEP tends to equalize out several other factors.

Forgot to add, CR is compression ratio. To find it, you put the piston at top dead center and measure the volume. The at bottom dead center and measure the volume. The Compression Ratio is the big volume divided by the small volume. So if an engine measure 10 cubic inches at bottom and 1 cubic inch at top, the CR is 10 / 1 or 10 : 1.

You may notice that the compression ratio is NOT in the paper above, but IS important because there is some limit of presure in the cylinder where exceeding it will result in detonation. A higher compression ratio means you have less boost available before the limit presure is reached.

This is fun ... if you like engines and math. It is horrible if you don't.

I like discussing it and sharing the formulas and calculations, but hate to argue about it.

Really, these seemingly unrelated engines and propellers installed in completely different airframes in countries trying to keep the details secret resulted in warplanes that have remarkably similar performance to one another. It just goes to show that all the countries involved had some pretty decent engineers who came up with different and sometimes wildly different solutions that approached a very similar limit, and the jumps in performance almost mirriored one another. The Spitfire and Bf 109 traded the title of "best fighter" back and forth for a long time, each one that earned the title being a new variant of the old airframe/engine/propeller combination.

I'm amazed they came so close to one another.

The British had a genius of a supercharger designer named Sir Stanley Hooker. He designed mechanical 2-stage superchargers that were very efficient. The German used a variable hydraulic drive to achieve almost the same results. The Americans use both mechanical 2-stage superchargers (on some radial) as well as a single-stage supercharger and a turbocharger on the same engine to achieve the same type results. I think the British were a bit ahead there. The Spitfires that used a Merlin 60 series and later were 2-stage mechanical, as were all of the P-51B and later Mustangs. The P-39 and P-47 used super/turbos combos.

If you go to an airshow and see the Horsemen aerobatic act in P-51s, one thing that will stand out is the whistling sound as they come down the back side of a loop or cuban eight. That whistling is the sound of a supercharger impeller and is music to my ears. You'll get the same whistle from a Grumman FM-2 Wildcat, though with the sound of a radial in the foreground, because it also has a 2-stage supercharger.

You'll get the same sound from Spitfires, but you'd probably have to have maybe 2 - 3 of them doing some higher-power loops or cuban eights to hear it, and I haven't seen any Spitfire formation aerobatics. Doesn't mean it hasn't happened; means I haven't seen and heard it.

Thank you! Some really helpful information there.

And yes, I've heard the whistle. It is beautiful.

 

[mhuxt]

K calling someone else biased is rather like IS claiming Coalition air strikes are brutal. Of all the wilful ignorance and outright BS-as-fact he spewed, that particular claim, having had the longest legs, is the worst.

 

[Spacefire]

K calling someone else biased is rather like IS claiming Coalition air strikes are brutal. Of all the wilful ignorance and outright BS-as-fact he spewed, that particular claim, having had the longest legs, is the worst.

Uh, who are we talking about here?

 

[GregP]

I've never seen a report which rated C3 fuel at 165. Doesn't mean they didn't make such a batch. The reports I've read compare C3 with US 100/130.

The gist I get is British 80/87 was 7 - 8 points below B4 wich was 6 - 7 points below C3 which was comparable to 100/130 which was 8 - 10 points below US 145/150 rich.

There IS a pic of a Bf 109K with a B4 fuel decal on it. That makes me wonder since the Bf 109K series production didn't even start until August 1944. If C3 was available, why placard a Bf 109K with B4?

Hhmmmmm .... can't say. Maybe there were two different boost settings depending on which fuel you were running? And whether or not MW50 was avialable? I KNOW they had C3, so there has to be some explanation why they'd run on B4, but it isn't something I'll dig into much unless I have the time and curiosity later. I'd undestand in the last month or two as the war was winding down, and the picture isn't dated that I noticed.

Maybe it's April 1945 and B4 was all that field had. Flying on B4 might be better than not flying at all.

 

[Shortround6]

When rating or measuring the PN number of fuel there was a limit as to what the testers could do. ALL of these tests were comparison tests. The fuel being rated/tested was run in an engine and it's "performance" was compared to one or more reference fuels run in the same engine on the same day (or withing a few hours) to eliminate changes in the air (temp/humidity/etc). 100 octane fuel acts like the reference fuel that was composed of 100% pure iso-octane. 96 octane fuel acts like a mixture of 96% iso-octane and 4% heptane. Over 100 octane was measured (initially) by using reference fuels of 100% iso-octane plus a certain amount of lead.

Some British reports on German fuel samples show a rating like >130 PN. which means the sample performs better than 100/130 when run rich but if the British lab doesn't have a fuel available with a PN number higher than 130 they can't say how much better the German fuel is, like saying it was 98/140 or something. They have nothing to compare it to over 130.
BTW during WW II aviation fuels (most particularly the higher PN ones) were tested in supercharged test engines. Modern tests of motor car fuel are done (mostly) with unsupercharged engines.

While the German fuel was, at times, close to the allied fuel (maybe better on occasion?) the Germans don't seem to have taken as much advantage as they might have. It is a question of what exactly constitutes lean and rich mixtures. There can be quite a span of either mixture. Some allied engines when running at full power trailed enough black smoke to make a coal fired steam engine jealous. Some reports of ground running the engines for tests say that 'neat' fuel (liquid) was running out the exhaust pipes. Not only was the engine running rich (the highest fuel to air ratio that would still burn and make power) but the extra fuel was being used as a coolant. When fitted with a water/alcohol system the carb/s were set up to flow much less fuel when the water/alcohol system was in operation.
Perhaps the German injection systems were not capable of delivering the wide range of mixtures the carbs could?