Maximum Performance of Spitfire MKii (12.5lbs boost), and questions military vs "WEP"

[Ottobon]

Something i have been wondering is how does one gauge a planes full potential performance if it was only ever tested at say "Military" power, but had access, say 5 minutes, to increased boost.

In the case of the Spitfire MKii there is lots of good data for 9lbs boost, but the aircraft was capable of 12.5lbs boost in 5 minute intervals if i have read correctly. Similarly i have recently found out that NO Japanese aircraft were ever tested on "War Emergency Power" in Japan, even in late 1945. Luckily on those aircraft, for example the Ki-84 Hayate or J2Ms there are TIAC reports which did test the aircraft at "War Emergency Power" so their actual performance is known. Although rare or prototype Japanese Aircraft (Say A7M, N1K2, or if we want to be extreme Ki-83, Ki-64, J7W1) you simply have to guess what sort of maximum performance they were capable of.

The Ki-83 being a interesting case because it did 706kph according to Japanese sources, with obscure and probably false claims of it doing 760kph in American hands. Apparently on some old thread on J-Aircraft people who were much better at math then me calculated with WEP its true top speed at high altitude was more likely around 730kph.


Anyways on more common aircraft like Spitfires, was there ever a time when say the old Spitfire Mkii running at 12.5 lbs boost ever got tested for its maximum performance on "War Emergency Power", and if not what can we presume about it given what we know about its 9lbs boost performance? If nothing else what type of power did a Merlin XII make at 12.5lbs boost, i believe at 9lbs its 1135hp?

Obligatory WWII Aircraft performance link for reference: http://www.spitfireperformance.com/spitfire-II.html

 

[GregP]

There are standard aerodynamic formulas. If you know a current top speed, you can calculate a constant k: k = p/(V^3). The exisitng power is P.

Next you need the new power level and % increase in frontal area. If you are just using more boost to get more power, then the % increase in frontal area is zero and the new speed is the cube root of the new power over k.

So if you go 317 mph on 1,500 HP and have a WER of 1,800 HP, the k = 4.709 *10^-5. The new speed is the cube root of (1,800/4.709 * 10^-5) = 336.86 mph .... for 5 minutes or so anyway.

Speed will increase as the cube root of power with no change in drag.

 

[fastmongrel]

A service MkII was about 6 mph faster than a MkI and was probably good for about 340-345 mph iirc 360mph could be had with the throttle rammed forward but there was a wire that got cut by the throttle lever that showed when overboost had been used and a pilot had to make a report as each time the overboost was used it cut into the engine life. Too much use and you ran the risk of a piston and con rod popping out of the crankcase, it would be a brave or desperate pilot that ran an early model Merlin at max emergency power for 5 minutes. From reading memoirs of Battle of Britain pilots max emergency power was used for about a minute maybe two.

 

[BiffF15]

Fastmongrel,
If a situation boils down to loss of life or possible loss of engine, I would pick the engine and roll the proverbial bones.
Cheers,
Biff

 

[Greyman]

... it would be a brave or desperate pilot that ran an early model Merlin at max emergency power for 5 minutes. From reading memoirs of Battle of Britain pilots max emergency power was used for about a minute maybe two.

In my reading it appears to be the opposite - and British pilots used emergency boost often and for long periods.

There's a lot of good information at Spitfire Mk I Performance Testing

Fastmongrel,
If a situation boils down to loss of life or possible loss of engine, I would pick the engine and roll the proverbial bones.

Supermarine's test pilot Jeffery Quill, after getting some operational experience in the Battle of Britain reported back to the firm:

"It should be understood that in the stress of battle pilots do not adhere to 'max. permissible R.P.M. and Boost' figures, but take all available power from the engines. On patrols and normal operational work they treat their engines with the utmost care and respect, but when an engagement is in progress they tax them to the full, regardless of any limitations; this fact should be taken into consideration when drawing up cooling suitability requirements."

 

[GregP]

When I was with an Allison shop, we tlked with some former Soviet WWII fyers who flew P-39s. They said they ran them at 75" when in a dogfight with a German and only followed the US-recommended limits when not in direct combat. I supposed if you were fyling borrowed airplanes there was little incentive to make them last, but the Britsh guys in early SPitfires did it too.

When your life is on the line, the redline becomes transparent and you do what you need to do.

 

[fastmongrel]

Fastmongrel,
If a situation boils down to loss of life or possible loss of engine, I would pick the engine and roll the proverbial bones.
Cheers,
Biff

Not saying they didnt hammer the engine Biff as they certainly did and a Merlin could take it but 5 minutes at max on an early series Merlin would be really pushing your luck I bet the temp guage needle would be bouncing off the stop pin

 

[Edgar Brooks]

12.5 boost was only permitted, on the Mk.II, for take-off to 1000'.
The maximum Combat boost was 12, with a 5 minutes limit, though, during the Battle of Britain, if the C.O. called "Buster," it was a signal to push the throttle right forward, and "bust" the retaining wire.
The 12 12.5 figures only pertained to 100 octane; with 87 octane, the figures were correspondingly lower (+7 in both cases.)

 

[Greyman]

I've never read anything on a throttle wire, +12 boost was obtained by pulling the boost control cut out knob on the instrument panel.

Could the Merlin XII use 87 octane? As far as I understood it was 100 only.

 

[RCAFson]

Not saying they didnt hammer the engine Biff as they certainly did and a Merlin could take it but 5 minutes at max on an early series Merlin would be really pushing your luck I bet the temp guage needle would be bouncing off the stop pin

The Merlin could take 12lb boost repeatedly:

http://www.spitfireperformance.com/spit1-12lbs.jpg

 

[Edgar Brooks]

On early Marks, the boost cut-out control was a small lever on the forward corner of the throttle quadrant; it was held in place by a small cross-piece of wire. The Pilot's Notes give the settings for 100 and 87 octane fuel, so it seems fairly safe to assume the engine could cope with both.

 

[Greyman]

Ah yes I got the Hurricane's control position mixed up with the Spitfire's.

Strange about the fuel. I have a poor quality Pilot's Notes copy downloaded from somewhere (maybe this forum!) and no mention is made of 87 octane. It's possible it was amended and due to the low quality I can't tell - but it doesn't appear so.

But looking this image here http://www.spitfireperformance.com/spit2pnfs3.jpg it's right there; 87 octane. My document's AP number and volume are the very same.

Either way, there it is.


EDIT: forgot to mention the wire business.

I can find no mention of this breaking wire setup. Any more information?

Is it possible you're confusing it with another aircraft? Here is what I have for the Mustang I pilot's notes:

(ii) Mustang IA aircraft.- On these aircraft Delco Remy automatic boost control is fitted.

...

A stop, which consists of a plunger held in position by a wire (not visible to the pilot) is provided at the take-off position. To obtain 56 inches boost for combat, the throttle lever is pushed hard against the stop, thus operating the plunger and breaking the wire; the plunger will then remain in the war emergency position until reset by the ground staff; the boost control will, however, still regulate at any boost to which the throttle lever is set.

 

[Greyman]

I can find no mention of this breaking wire setup. Any more information?

All right, replying to myself here.

I must admit to being baffled by Air Ministry Pilot's Notes naming conventions, as my copy of Spitfire I notes - named exactly as those linked below - are worded in such a way as to not mention the wire seal whatsoever. Anyway, as usual, wwiiaircraftperformance to the rescue:

http://www.wwiiaircraftperformance.org/100-octane/AP1565A_June_1940-Sec8-7.jpg

 

[kool kitty89]

When I was with an Allison shop, we tlked with some former Soviet WWII fyers who flew P-39s. They said they ran them at 75" when in a dogfight with a German and only followed the US-recommended limits when not in direct combat. I supposed if you were fyling borrowed airplanes there was little incentive to make them last, but the Britsh guys in early SPitfires did it too.

When your life is on the line, the redline becomes transparent and you do what you need to do.

That's not all that far off from the overboosting reports from some US and Commonealth P-40s, with the exception that 70" Hg was the highest pressure possible with their testing (with ram at sea level). With the P-39's poorer ramming capabilities this seems unlikely as well unless the colder temperatures on the Eastern front had an impact on that. (I'd imagine it would help with overheating and carb air intake temperature and density, but not sure it'd affect pressure)
The official WEP ratings for those 8.8:1 supercharged engines was either 56" or 60" depending on the model and limited due to structural integrity and wear concerns. (detonation was not an issue at 70")

It also seems unlikely that the 9.6:1 ratio supercharged engines on the P-39M, N, and Q would tolerate those pressures without detonation, even with colder air, given the 57" limit set due to detonation risks (and that was using 100/130 octane fuel, so assuming lend-lease supplies on the Soviet end). I could perhaps see them pushing those engines to 60" but not much beyond that, even with 100/130 fuel. (maybe a bit more with 100/150)

 

[thedab]

hello mate,right to give you some idea of the speed we have this http://www.spitfireperformance.com/spitfire-I-rae-12lbs.jpg which give a top sea level speed of 314mph for the mkI on 12 boost and we this one for the mkV http://www.wwiiaircraftperformance.org/Spitfire_V_Level_Speed_RAE.jpg which give a sea level speed of 302mph,I think the mkII sea level speed would between the two

the Merlin XII was about 1280hp at 10500ft on 12lb boost, the Merlin III was 1305hp at about 8500ft on 12lb boost and the merlin 45 was 1280hp at about 15.000ft 12lb boost