P-39 vs P-40
- Thread starter gjs238
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CORSNING
Tech Sergeant
Trying to figure out how to make graphs,
...
..Good luck and God speed sir. I haven't had any luck in that area either.
Shortround6
Major General
That 1700hp at sea level is a theoretical (or calculated ?) number but it does come from an Allison chart for the -33 engine.
We are told that the Flying Tigers overboosted their engines but we are not told by how much. We are told the Russians over boosted their Tomahawk engines but not by how much, as did the British (and commonwealth) in the desert. This is possible limits on the engine if/when a squadron or pilot did use more than the factory limit.
I don't believe Allison or the US military ever approved over boosting the -33 engine, please remember that Alison had to rebuild 277 of the very early -33 including new crankshafts just to get them cleared for the 1090hp rating at 13,200ft. Nobody in the US was in any hurry to push the engine any harder until they accumulated thousands of hours in service to see how well they standing up. Once they found they could extend the overhaul several times over the original rated life then perhaps they could seriously consider pushing the engine.
On the -33 the reduction gear was a weak point and would not stand up very well to higher than normal powers. Strangely enough it was not usually the reduction gear itself that failed but the crankshaft right behind the front bearing.
The Chart says 1700hp at 61in but it is dated 12-5-39, actual power due to charge heating or other problems may be the reason behind the later engines needing more inches of pressure to get similar power.
We are told that the Flying Tigers overboosted their engines but we are not told by how much. We are told the Russians over boosted their Tomahawk engines but not by how much, as did the British (and commonwealth) in the desert. This is possible limits on the engine if/when a squadron or pilot did use more than the factory limit.
I don't believe Allison or the US military ever approved over boosting the -33 engine, please remember that Alison had to rebuild 277 of the very early -33 including new crankshafts just to get them cleared for the 1090hp rating at 13,200ft. Nobody in the US was in any hurry to push the engine any harder until they accumulated thousands of hours in service to see how well they standing up. Once they found they could extend the overhaul several times over the original rated life then perhaps they could seriously consider pushing the engine.
On the -33 the reduction gear was a weak point and would not stand up very well to higher than normal powers. Strangely enough it was not usually the reduction gear itself that failed but the crankshaft right behind the front bearing.
The Chart says 1700hp at 61in but it is dated 12-5-39, actual power due to charge heating or other problems may be the reason behind the later engines needing more inches of pressure to get similar power.
CORSNING
Tech Sergeant
I've brought this up before --- but with no automatic boost control -- the point is there is no real limit. It's whatever the pilot wants at that moment (depending on altitude). Inattentive or undisciplined throttle control is going to result in an 'overboosted' Tomahawk.
Now this could be intentional as well. Sort of a "Well boys, we all know the book says we're limited to 5 minutes at 40 inches, but we're gunna do what we gotta do out there to get home."
DarrenW
Staff Sergeant
Grumman also said the very same thing about the F6F-3.
Shortround6
Major General
Thank you Tomo.
I would note that the graph itself uses smaller boxes on the right side than on the left so it turns a curve into a straight line. This was common for a number of engine charts of the time.
I would also note that some engine companies (or book writers) would try to compare theoretical engine power at sea level as a way of comparing engines, The Merlin II for instance was supposed to make the equivalent of 1500hp at sea level at 2600rpm. Of course with 87 octane fuel it had zero chance of coming anywhere near that power level.
Also please note this test was done using US 100 octane fuel which was pretty much 100/100 which is another reason the US did not going in for over boosting engines in 1940-41.
FWIW, the -81 and similar engines were not always rated for 1125 HP at 15500 ft. Sometimes the rated altitude is stated as low as 14500 (14600?) ft, or even at 14200 ft (all without ram effect): link (scroll down a bit)
Why would loaded weight effect HP?
