That might be consistent with a "max performance takeoff/unrestricted climb" profile where the aircraft cleans up, accelerates in ground effect to a predetermined zoom climb speed, than pops up into a climb, slowly bleeding back to Vy. This will give an exaggerated "time to climb" performance value, if someone is trying to prove a point, but is not practical in a combat situation, especially if you're trying to get multiple fighters into the air in close proximity as quickly as possible.
XP-39 II - The Groundhog Day Thread (4 Viewers)
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No one would do such a thing just to post a better result would they? Surely not. British tests are sometimes criticised because of the state of the planes tested, that was the state of the planes pilots were flying. What is the point in testing something hotted up just for the test, another issue that has dogged the P-39 since the P-400 tag was made.
Possibly to justify expenditure on a plane seen as having potential, but in danger of being labeled "a dog"? IAC, hoist on its own petard when the combat ready D1 came along.
IIRC, the unconventional P39 faced opposition from some circles in the Army, an organization not known for its futuristic attitudes.
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
The P-39D is lighter?
With the A6M2 at 3,700 pounds empty (6,160 pounds gross) and the P-39D at 5,450 pounds empty (7,500 pounds gross) I am thinking you have that backwards?
special ed
1st Lieutenant
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- May 13, 2018
Indeed. He is a combat pilot, there to fly combat. Why would he dive away or climb away when he thought his modifications gave him a superior P-39
Hello GregP, et al.
That formula you are using makes pretty good sense as long as speed doesn't change.... but there is one factor that has been nagging at me. Shouldn't the propeller efficiency be taken into account as well? Shouldn't it be Thrust available - Thrust required?
Some folks have already commented that they were estimating propeller efficiency at 81% which I am thinking might be a bit high considering that the Advance Ratio at 153 MPH TAS is only about J=0.78.
- Ivan.
Hello GregP,
The extra .30 cal nose guns on the P-39C were actually between the two .50 cal MG.
See attached photographs.
I have been looking through my copy of Aerodynamics for Naval Aviators and can't seem to find that formula though there is a lot of coverage about Jets in my copy.
Edit: Found it. The "Power" that is specified IS Horsepower but not nearly as simple as first thought and is based on thrust and angle of climb..... So it does take into account the factors, I was asking about and the calculations are a bit more complicated.
- Ivan.
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That depends on whether it uses 2% or 20% of its fuel between engine start and takeoff.
GregP
Major
Hi Ivan1GFP,
Since we know a data point ... say ... 3,720 fpm climb at 1,150 hp at 10,000 feet, weight = 6,689 lbs, we can back into the other factor. You could start with the P-39D numbers, too, and then assume something close to the difference bwteen Pr at 10,000 feet and Pr at 20,000 feet for the P-39C, but the P-39D data are very suspect to me. I'd trust the P-39C data more at this point. But, perhaps I'm wrong.
So, Pa = 1,150 hp, RC = 3,720 fpm, W = 6,689, solve for Pr (power required for level flight at 10,000 and 20,000 feet conditions) ... in the configuration you flew the test, with whatever prop, etc.
As long as you are reasonably close to the staring point (vary ...say ...±25%), you should not be too far off. Now, speed isn't taken into account in the rate of climb formula, and they assume that if you increase or decrease the power, you're still going to climb it at close to the same configuration. The formula probably falls apart if you elect to do something like double the climb airspeed. But, if your test conditions are similar, the formula should get you very close.
Cheers!
Since we know a data point ... say ... 3,720 fpm climb at 1,150 hp at 10,000 feet, weight = 6,689 lbs, we can back into the other factor. You could start with the P-39D numbers, too, and then assume something close to the difference bwteen Pr at 10,000 feet and Pr at 20,000 feet for the P-39C, but the P-39D data are very suspect to me. I'd trust the P-39C data more at this point. But, perhaps I'm wrong.
So, Pa = 1,150 hp, RC = 3,720 fpm, W = 6,689, solve for Pr (power required for level flight at 10,000 and 20,000 feet conditions) ... in the configuration you flew the test, with whatever prop, etc.
As long as you are reasonably close to the staring point (vary ...say ...±25%), you should not be too far off. Now, speed isn't taken into account in the rate of climb formula, and they assume that if you increase or decrease the power, you're still going to climb it at close to the same configuration. The formula probably falls apart if you elect to do something like double the climb airspeed. But, if your test conditions are similar, the formula should get you very close.
Cheers!
Simon Thomas
Senior Airman
Hi ThomasP. I re-ran the calc for P-39D A.C. No. 41-6722, however I only noticed once I finished that the level test were with the oil cooler and prestone shutters flush and the climb data was with the Prestone and oil cooler shutter wide open. The f I got with level flight was 4.3 ft^2 and e of 1.65. My calc gave answers within 1% of the test data. My climb data was wonky until I saw the shutters were different. I'll look at the other P-39D tests tomorrow.
Hello GregP,
The problem is that actually speed IS taken into account in that formula if you go up a page and see that the first examples determine the climb angle work via thrust. This is what was throwing me off because I knew it had to be there but it wasn't reflected in your calculation. The examples were all for Thrust versus Drag as for Jets.
The formula with just power was at the very tail end after a bunch of preceding formulas about thrust, so I didn't see it at first.
At the very end, the Thrust is translated to Power but THAT power if we are discussing Piston Engines isn't really the HP from the engine itself but the Engine HP * Propeller Efficiency at whatever J the TAS translates to and the Drag also changes because that would be determined by the IAS (which probably hasn't changed much).... Though the induced drag has probably increased a bit?
The bottom line is that at some point, the Engine Power has to be reduced by the propeller efficiency for the numbers to work out and it is highly unlikely that the propeller efficiency will be the same at 10,000 feet as at 20,000 feet because at least the speed is different.
- Ivan.
The problem is that actually speed IS taken into account in that formula if you go up a page and see that the first examples determine the climb angle work via thrust. This is what was throwing me off because I knew it had to be there but it wasn't reflected in your calculation. The examples were all for Thrust versus Drag as for Jets.
The formula with just power was at the very tail end after a bunch of preceding formulas about thrust, so I didn't see it at first.
At the very end, the Thrust is translated to Power but THAT power if we are discussing Piston Engines isn't really the HP from the engine itself but the Engine HP * Propeller Efficiency at whatever J the TAS translates to and the Drag also changes because that would be determined by the IAS (which probably hasn't changed much).... Though the induced drag has probably increased a bit?
The bottom line is that at some point, the Engine Power has to be reduced by the propeller efficiency for the numbers to work out and it is highly unlikely that the propeller efficiency will be the same at 10,000 feet as at 20,000 feet because at least the speed is different.
- Ivan.
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
They can have a fuel truck waiting there to top it off after it's run-up
ThomasP
Chief Master Sergeant
Hey Simon Thomas,
If it is any help, NACA War Time report No.8843 FLIGHT INVESTIGATION OF THE VARIATION OF DRAG COEFFICIENT WITH MACH NUMBER FOR THE BELL P-39N-1 AIRPLANE gives a minimum CD of .022 with CL of .06 (so flat plate area of just under 4.7 ft2) between M0.2 and M0.6. The AC was standard except for instrumentation. with normal armament fitted (I think, the photo of the test airframe clearly shows the 37mm barrel, but I cannot make out if the MG ports are taped over or not and it does not say in the text) with ballast for ammunition. Weight as flown was ~7630 lbs with CG at .285 MAC.
If it is any help, NACA War Time report No.8843 FLIGHT INVESTIGATION OF THE VARIATION OF DRAG COEFFICIENT WITH MACH NUMBER FOR THE BELL P-39N-1 AIRPLANE gives a minimum CD of .022 with CL of .06 (so flat plate area of just under 4.7 ft2) between M0.2 and M0.6. The AC was standard except for instrumentation. with normal armament fitted (I think, the photo of the test airframe clearly shows the 37mm barrel, but I cannot make out if the MG ports are taped over or not and it does not say in the text) with ballast for ammunition. Weight as flown was ~7630 lbs with CG at .285 MAC.
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Hot fueling avgas with a gravity nozzle? Not me, friend!They can have a fuel truck waiting there to top it off after it's run-up
That USAF Historical Study #101 would indicate that they might have been doing a fair bit of head scratching 78 years ago as well!
Now let's take a little walk back to Peter Gunn's post #1381 on page 70 for an apples-to-apples comparison of three different blade sets on the same model hub, same engine, same airplane (in this case, a P40F, same s/n for all tests), and same test conditions for all trials. The parameter being tested is rate of climb at 18,000 feet. The hub is a C-532D, which, in a version with a cannon channel, is also used on some P39s.
Blade #89306-19S climbs at 1460 ft/min.
Blade #89301-03 climbs at 1970 ft/min.
Blade #89303-24W climbs at 2010 ft/min.
I'm no math whiz, but that looks like a 550 ft/min difference to me between best and worst. The fact that the engine is a Merlin, not an Allison is irrelevant, as this is merely a test of different prop blades, with all other factors kept equal. Admittedly the Allison will have a little less horsepower available at this altitude, but not enough to erase that large a difference between prop blades. This, plus the weight difference, plus added drag from additional antennas that weren't on the C and other small incremental drag increases (wing guns for example) can easily add up to the performance deficit of the P39D. The best thing you could do for the P39D, IMHO, would be to get the carburetor air intake out of that dead zone behind the canopy so it could get some ram effect.
836 lbs alone ain't gonna do it.
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GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
Amen, mister - but that was a reference stolen from the "Lancaster as a potential nuclear bomber" thread, which seems to parallel this thread quite closely.
Its a parallel universe.
Peter Gunn
Master Sergeant
Don't worry, we'll wrap you in asbestos and you'll be fine...
Peter Gunn
Master Sergeant
Hello Ivan,
Any info as to why they didn't go with the two .30's in the nose like the photo you posted? Looks like a rather lethal setup ala P-38 to me. Also, could they have ditched the .30's for 4 .50's? Or even 5 by replacing the 37mm hub cannon?
Thanks
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