wuzak
Captain
448mph @ 25k with 78inHg MAP, the critical altitude for 90inHg was 21,200ft.
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Seems strange to me. Charts show a speed of 448 mph at 25k, 90" boost for the P-51H. Other charts show 444 mph at 25k, 75" boost for the P-51B. This doesn't make since. Tis a puzzlement.
Not according to the NA-8284A Report "Performance Calculations for P-51 (NA-126) Airplane, dated 9-2-45.I thought bomb and rocket rack on P-51H only reduce speed by 3 mph?
That doesn't seen enough to explain the speedNot 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.
please forgive my lack of knowledge in piston aircraft, what exactly is critical altitude?448mph @ 25k with 78inHg MAP, the critical altitude for 90inHg was 21,200ft.
please forgive my lack of knowledge in piston aircraft, what exactly is critical altitude?
That doesn't seen enough to explain the speed
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
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.
drgondog would be able to explain why weight makes a difference to speed better than I.
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.
drgondog would be able to explain why weight makes a difference to speed better than I.
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 chartInduced 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.
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.
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.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?
Wouldn't the propeller speed is entirely depend on the RPM of the engine?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.
The speed (RPM) is, but the pitch of the blades changes to compensate and produce more thrust.Wouldn't the propeller speed is entirely depend on the RPM of the engine?
http://www.wwiiaircraftperformance.org/mustang/p-51h-booklet-pg10.jpg
You can see that the static power (red graph) at 30000 ft is ...
3000 rpm, 90" Hg, water injection: 1450 HP
3000 rpm, 80" Hg, water injection: 1450 HP
3000 rpm, 70" Hg, no water injection: 1220 HP
3000 rpm, 61" Hg, no water injection: 1220 HP
And at 13000 ft ...
3000 rpm, 90" Hg, water injection: 1880 HP
3000 rpm, 80" Hg, water injection: 1880 HP
3000 rpm, 70" Hg, no water injection: 1620 HP
So the assumption apparently is that the addition of water injection at unchanged boost pressure increases horse power by 230 HP in high supercharger gear, and by 260 HP in lower supercharger gear.
As this is a gain in the region of 16 - 19% of brake horse power at the respective altitudes, this seems to be far more than British report which expected a 4% power increase from MW50 on German engines above full throttle height, and that would translated into something like a 1% speed increase
What's more, the power curves in the booklet are not identical to the one used for the revised edition of the NAA report, which due to the substitution of the Bendix speed density pump with a carburetter have a reduced full throttle height.
Original curve with the Bendix speed density pump:
http://www.wwiiaircraftperformance.org/mustang/p-51h-powercurve.jpg
Revised curve with carburettor:
http://www.wwiiaircraftperformance.org/mustang/p-51h-na-8284-pg12.jpg
The report (referred to as NA-8284-A, I believe) states:
"These calculations were necessary to provide complete performance data for airplane equipped with an engine incorporating a carburetor for fuel metering instead of the speed density pump originally anticipated in preparing report NA-8284 dates September 25, 1944. "
This leads to the following conclusion:
- The coloured "booklet" figures were prepared on the basis of an estimate of engine power for the expected production configuration, which is shown in p-51h-booklet-pg10.jpg, which is basically identical to that in p-51-powercurve.jpg.
- Engine power had to be revised a bit towards lower altitudes due to the (expected) introduction of a carburettor, and that power curve is depicted in p-51h-na-8284-pg12.jpg.
- The revision of the power curve means that the coloured "booklet" performance figures are purely theoretical as they represent an interim state of planning that was invalidated by the course of engine development.
- The calculated performance resulting from the revision is depicted in this graph: http://www.wwiiaircraftperformance.org/mustang/p-51h-na-8284-pg5.jpg
You can see that the static power (red graph) at 30000 ft is ...
3000 rpm, 90" Hg, water injection: 1450 HP
3000 rpm, 80" Hg, water injection: 1450 HP
3000 rpm, 70" Hg, no water injection: 1220 HP
3000 rpm, 61" Hg, no water injection: 1220 HP
And at 13000 ft …
3000 rpm, 90" Hg, water injection: 1880 HP
3000 rpm, 80" Hg, water injection: 1880 HP
3000 rpm, 70" Hg, no water injection: 1620 HP
The reason for having an automatic shutter was to ensure optimum airflow through the radiator consistent with keeping the engine at its optimum temperature; the shutter's optimum position would be somewhere between fully open and flush - fully open contributes drag and reduces ram effect - fully closed and the airflow is less effective. The aircraft was tested with the radiator shutter flush to determine how this configuration would affect performance; "Max" simply means the maximum speed achieved in this configuration, as opposed to the performance with the shutter in its optimum position.
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.The coolant flap emergency release was added to later production P-51Hs to ensure that the exit flap would not close to a position where the engine could overheat, probably as a result of these flight tests; not forgetting the P-51H was modified in the eight years between 1946 and 1954.
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.The P-51H's radiator scoop and exit shutters were of a completely different design to that of the B/C/D/K so the test results of the H with the shutters in different positions would be different to those of the earlier models.
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".
In level flight P51H was noted to not overheat at flush even at 90'' at all alts: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 200-210C in the intercooler radiator.
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-51HNAA achieved 487mph in Fighter Light (interceptor) with 100 gal internal fuel, full ammo and guns, no wing racks for either combat tanks or rocket stubs.
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 fuelIIRC 150 octane fuel was discontinued after 1945, reducing max potential even with WI. I'm search my files but not very well organized and I am consumed by indifference at the moment.
So, the SAC values represent the 1650-9 with 130 octane fuel - which I don't believe permitted 90" Mp. (Memory)
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 problemIn level flight P51H was noted to not overheat at flush even at 90'' at all alts:
View attachment 651000
All is from the same October 1946 document: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