P51D climb rate

There's lots of discussion on the P51D level TAS and the graph of p51d-15342 level speed test is shown numerous times.

There's even a video with a very good explanation for the engine power switching from low to high blower for the p51d-15342 level speed test.

I haven't seen the same level of discussion on the p51d-15342 climb rate tests where the engine power and climb rates when switching low to high blower are actually more puzzling.

The p51d-15342 climb rate test shows the following:

Alt MP (") BHP ROC (fpm) Blower Throttle

15000 47.2 1232 2275 Low WO (it's above crit alt for blower)

16000 67 1580 3200 Hi Part (auto switched to high blower)

19000 67 1500 3200 Hi WO (crit alt for high blower)

20000 65.2 1455 3050 Hi WO

Questions:

1. How come BHP on climb, with lower RAM, is exceeding BHP on level speed where there's more RAM effect?

2. How come BHP curve, at high blower and 67" MP, decreases on the climb test (between 16K and 19K)? The vid explains the level speed situation very well - why doesn't the same explanation work for the climb power?

3. How come 1580 BHP at 16000 on the climb test - it's about 80 BHP more than the Packard V-1650-7 engine power curves show.

And lastly, how well does this simulated P51D's climb rate fit with the p51d-15342 climb graph?

1. The answer is BHP curves themselves as they vary with speed. The power at the constant and limited MP lowers with RAM because of excessive adiabatic ram heating.

2. Could not understand the question.

3. It's a different chart - see the chart for the climb test.

 

Generally, the power depends on both altitude and speed, but the in opposite direction as the MP is limited (throttled).

Thanks for the reply.

1. Ok. Although I'd understood the main effect of RAM was to increase critical alt of that supercharger stage. The other Merlin engine power curves I've looked at typically show power for both a min and a max RAM on the same line until crit alt at the min RAM is reached when that curve starts to decay with alt whilst max RAM curve continue upwards. Not seen one where the min RAM power exceeded the max RAM power.

2. I'll try to restate the question. On the P-51D_15342 level speed test graph, engine power is 1410 BHP from whatever height it switches to high blower (say 16K) until 26K. There is a video that does a great job of explaining that - power is constant as the engine is constrained to 67" MP.

Now look at the P-51D_15342 climb data, specifically the data points I posted. The power is given as:1580 BHP @ 16000 ft and 1500 BHP @ 19000 ft. Yet the same constraint of 67" MP applies to both these data points.

So, on level speed we have the explanation of the constraint that does not appear to apply to the climb power. That's why I'm puzzled...

3. Ref your comment "see chart for the climb test" are you referring to P-51D_15342 climb test or a DCS chart. I've not found a climb test chart for the DCS P51D.

I do have a Packard engine chart for V1650-7 (a bad quality scan) that shows high blower power for 72", no RAM. Drawing a line on that chart for 67" the no RAM power would be around 1375 BHP @ SL, rising parallel with but below the 72" curve to around 1500 BHP @ 19k, then decaying with alt. Well, the 1500 BHP at 19K agrees perfectly with the P-51D_15342 climb data at 19k. However, I don't really understand how they got 1580 BHP at 16K. (Unless MP>67".)

Be great if you could help me understand all this.

1. It's not a "main effect". It is only one side of the whole effect. If you want to plot the power diagram for the engine working with ram pressure the simplified procedure would be:

 

1. Obtain he new critical alt by calculating new altitude at which the total pressure - static and dynamic is equal to the pressure at the static critical altitude.

2. Shift the maximum power point from the static curve to the new altitude (to be accurate, the new point will be a bit lower, but we can neglect it - anyway, intercoooler system coolant temperature has more effect on the power.)

3. PLOT A NEW ascending LINE from the auxiliary point where the ascending static line crosses the horisontal axis (sea level) to the new critical alt.

4. Plot a new line from the new critical point parallel to the descending part.

 

2. The power, generally, as I wrote above, rises slightly at constant MP, but it is right only for constant TAS (M, to be accurate, bu at he TAS and altitudes we operate it is no a big difference).

 

The main question is - what parameters was used to obtain this power... looking at the points on the fig 3 and 6 I'd rather say that i looks like a kind of experimental or calculated points regarding the dispersion around the straight lines.

 

So, the power dependance on the altitude looks sometimes abnormal (Fig 6 - descending with altitude), and I can only suggest that not only ram pressure was used but intercooler temperatures, for example, because the engine power is very sensitive to this parameter.

I think what you are outlining in item 1 would produce something like this . That's for the V1650-9 engine so the -7 will obviously have different critical altitudes, boosts, gradients. But that's the general sort of curves that would appear?

For item 2... yes, according to the test report those BHP values in Fig 6 "All power figures are based on power curves prepared by the Wright Field Power Plant Laboratory on December 3, 1943 according to Eng. Spec. AC-1070 and AC-1170." Not read from on board instruments.

I take your point - other factors like aftercooler temp may be in play. Looking at the low blower below crit alt (SL to 5000) there's a similar power drop with alt...So maybe.

Although that jump in power to 1580 BHP seems an outlier result compared to all the other Packard and RR Merlin engined tests. Some of the P51Bs were also fitted with -7 engines when tested.

Thanks for your help.