wuzak
Captain
448mph @ 25k with 78inHg MAP, the critical altitude for 90inHg was 21,200ft.
Max Speed of P-51H : 487mph or 471mph ?
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448mph @ 25k with 78inHg MAP, the critical altitude for 90inHg was 21,200ft.
drgondog
Major
Not according to the NA-8284A Report "Performance Calculations for P-51 (NA-126) Airplane, dated 9-2-45.
The P-51D and H Cdp as f(CL) for bomb racks alone (OR stubs for 10 5" HVAR) are ~.0008. Speed reduction for this pressure drag has to be calcuated for Mach. I would estimate that the drag contribution for both racks and stubs to be in 6+mph range.
That doesn't seen enough to explain the speed
From that specific flight test
At 25,000 ft, P-51H reach 448 MPH, so assuming the assumption is correct then without racks, the aircraft can reach 454 MPH
At 21,200 ft, P-51H reach 451 MPH so assuming the assumption is correct then without racks, the aircraft can reac without racks, it can reach 457 MPH
But according to this chart below
At 25000 ft P-51H can reach 482 MPH
At 21000 ft P-51H can reach 467.5 MPH
From the F-51H chart, we get another result
At 25000 ft, F-51H can reach 406 knots = 467 MPH
At 21000 ft, F-51H can reach 404 Knots = 464 MPH
please forgive my lack of knowledge in piston aircraft, what exactly is critical altitude?
wuzak
Captain
It is the highest altitude at which the given amount of boost/MAP can be maintained. Also known as Full Throttle Height in the UK, the height at which the throttles are fully open, the throttles being used to control boost.
You can see it in the table you posted in post #43.
In low blower the throttle is partially closed at sea level to give 90inHg MAP, and fully open at 6,800ft.
In high blower the throttle is partially closed at 13,400ft to give 90inHg MAP. 13,400ft is the gear change point, so between 6,800ft and 13,400ft the supercharger will be in low gear, the throttle is wide open and the MAP will be lower than 90inHg.
In high gear the throttle is fully open at 21,200ft the MAP is 90inHg. Above that altitude the throttle remains open, but the MAP reduces, so that at 25,000ft the MAP is 78inHg.
6,800ft and 21,200ft are the critical altitudes for low and high gear at 90inHg. There are different altitudes for critical altitude based on the boost/MAP.
wuzak
Captain
Another factor is weight.
This test was done with bomb and rocket racks, no ordnance on the racks at a take-off weight of 9,544lb.
This test was done clean (without racks) at a take-off weight of 8,445lb, nearly 1,000lb lighter.
The speed results were at 9,430lb. No indication of whether racks were involved.
drgondog
would be able to explain why weight makes a difference to speed better than I.But this one is done at 9530 lbs thoughAnother factor is weight.
This test was done with bomb and rocket racks, no ordnance on the racks at a take-off weight of 9,544lb.
This test was done clean (without racks) at a take-off weight of 8,445lb, nearly 1,000lb lighter.
The speed results were at 9,430lb. No indication of whether racks were involved.
The speed still higher than F-51H chart and the test mentioned
drgondog
Major
Induced Drag is a f(CL^2) where Lift Coefficient CL=W/(Q*S)). Q= dynamic Pressure = (1/2*Rho*V^2). W= Gross Weight. S+Wing Area.Another factor is weight.
This test was done with bomb and rocket racks, no ordnance on the racks at a take-off weight of 9,544lb.
This test was done clean (without racks) at a take-off weight of 8,445lb, nearly 1,000lb lighter.
The speed results were at 9,430lb. No indication of whether racks were involved.
The Lift Coefficient is a function W but also a function of relative angle of attack. For the same airplane, configured the same, the angle of attack is slightly higher for level flight for the higher W and therefore higher CL and therefore higher Induced drag.
Total Drag = (sum of all parasite/pressure drag CDp)*CDm + CL^2/(Pi*AR) where Pi=3.14 and AR = Aspect Ratio of the wing. CDm = Mach correction (increase) factor which is only near 1 at ~M=0.3.
At equilibrium in level flight Total Drag = Total Thrust and Thrust is a function of Thrust Horsepower of engine and (in case of P-51) exhaust gas thrust - Thrust HP pressure recovery losses (i.e. carb intake, pressure drag of prop vortex). Cooling drag is a factor at lower speeds but not at high speed for a P-51B/D/H.
I debated whether to spend the time (one time only) to walk through Drag calculations to explain how Induced Drag is influential to achievement of top speed differences between lighter and heavier (in this case strictly internal fuel load) P-51H with no stubs/pylon in clean Fighter condition. That said, Induced drag for a P-51H is a much smaller factor than Parasite drag when M> 0.5.
Pardon me if iam bring too thick, but i still don't quite understand what make the P-51H speed so low in that specific test. The rack have minimal impact as you said, and the weight is similar in this chart
On a side note, one thing i don't understand, if the engine RPM is the same at 3000 RPM, then what the different between 90 hg and 78 hg?
Btw, do anyone have this instead of the usual F-51H manual?
90 and 78 in HG are boost pressures. The engine may be turning at the same speed but at a higher pressure it is processing more air/fuel so produces more power, the prop turns this into more thrust.
Wouldn't the propeller speed is entirely depend on the RPM of the engine?
The speed (RPM) is, but the pitch of the blades changes to compensate and produce more thrust.
wuzak
Captain
To clarify what is shown there:
At 30,000ft the engine is past its static critical altitude for those MAP, except for 61inHg. The ADI must be making the supercharger more efficient at those altitudes, thus making more power. More efficient = less power to drive supercharger.
That is, at 30,000ft the engine is not making 90inHg, 80inHg or 70inHg. It is only making 61inHg.
This is the same situation, but for low gear.
Except that the MAP doesn't completely fall to 61inHg before changing into highgear.
At 30,000ft the engine is past its static critical altitude for those MAP, except for 61inHg. The ADI must be making the supercharger more efficient at those altitudes, thus making more power. More efficient = less power to drive supercharger.
That is, at 30,000ft the engine is not making 90inHg, 80inHg or 70inHg. It is only making 61inHg.
This is the same situation, but for low gear.
Except that the MAP doesn't completely fall to 61inHg before changing into highgear.
drgondog
Major
Not Ram when discussing radiator exhaust plenum acceleration of heated air in a shrinking area exhaust plenum to exit gate. This is the Jet Effect due to Meridith Effect design. So the 'automatic' shutter adjustments were made solely on Radiator/Aftercooler fluid temps through the radiator.
True. I have to pull the 1945 NA Report on design and flight test results on P-51H cooling system. My report was pulled from microfilm and I am attempting to recast the 22 page report. If Lowell and I 'do' a follow on to P-51B Bastard Stepchild book ,the D/K will be important but the primary focus will be on 'The Lightweights". The Mustang Meridith Effect story was an evolving one from semi important drag reduction to major factor in B/D, culminating in the H.
I agreed with much of what you stated. That said the cooling system was independent of Ram Effect - which was intake carb scoop forcing high velocity air into the carb system. Two separate discussions, two separate drag discussions.
In Drag calculations, there are three distinct blocks of calcualtions to obtain total drag of the airframe.
Parasite Drag of the airfarme immersed in the freestream - independent of Propeller system - (includes AoA, Friction, 'normal parasite drag components', form (exhaust stack, bomb racks)) - all corrected for Mach. - These will be summed and calculated as THp losses.
Then there is Induced drag as factor of Aspect Ratio and CL^2. Included in sum for THp losses
Last are the various calculations dealing with Hp gains and losses - Thrust from Propeller system plus Thrust from exhaust and Meridith Effect, minus HP losses from pressure drag in carb duct and radiator duct, pressure drag losses due to immersion within prop vortex - Summed in THp losses.
Note - for flight regime below max V for a specific altitude and THP available as f(RPM/Boost), Cooling Drag vs Thust is NOT = Zero. That is usually accmpanied by the exhaust shutter opened to various stages beyond Flush.
Note also - that Temp at exahust shutter outlet must be equal or greater than 180F when Coolant Temp in 200-210F range
NAA assumed net zero for Meredith effect thrust balanced by internal pressure drag Radiator/Oil Cooler duct.
The P-51H was perhaps the first Meridith Effect design to obtain test results showing true positive thrust, but also shown to be unable to sustain without overheating quickly at low to medium altitude. Flush is the position for optimal 'thrust' but must be carefully regulated for range of 200-210F (EDIT) in the intercooler radiator.
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We've gotten the 'P-51B Bastard Stepchild', the 'Secret horsepower race', the follow-on book about the automatic weapons by T. Williams should arrive in 2022, plus that book - excellent.
Metrallaroja
Airman
- 97
- Nov 21, 2020
In level flight P51H was noted to not overheat at flush even at 90'' at all alts:
And in climb it was opened a bit more after flush, but you can see that it doesnt leave the most optimal range of opening at 90'':
So it would be around this area all climb:
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So i looked very careful, but i can't find any chart showing that for the P-51H, it seem that value is either for the prototype of just estimated value for P-51H
While you are indeed correct that F-51H in service use the AN 01-60JF-1 fuel, which limit the MP to 80 inches Hg instead of 90 inches Hg likeAN-F-33 fuel
The performance in the SAC actually follow the NA-8284-A report which show the P-51H use 90Hp MP
Top speed shown in this chart is 475 MPH which is roughly equal to 412 knots as indicated in the SAC
The only question now is whether this fighter has rocket rack and how much speed that reduce
drgondog
Major
You accidentally pulled the comment from the May 1945 Flight Test report, under which 67"MP was maximum flown and in the same time frame as the NA P-51H Cooling System fight tests in which overheating quickly occurred at 80 and 90"MP (w/WI). NAA made some improvements to the exit plenum/gate controls to partially solve the problem
Metrallaroja
Airman
- 97
- Nov 21, 2020
