This chart is a good illustration of the effects of power settings and supercharging on the fuel consumption of 1940s vintage, high output radial engines. This chart is derived from a Specific Engine Flight Charts (SEFC) for the F4u-1 Corsair. This curve is unlikely to be identical to the actual performance curve for these engines published by Pratt and Whitney (which I don’t have). But I am pretty sure the qualitative properties shown in my chart would also appear in the official ones.
What are these curves?
The chart plots the Specific Fuel Consumption (SFC) curves for each supercharger setting (neutral, low and high) on the F4u-1. SFC is measured in American units – lbs of fuel per horsepower per hour.
A general feature of the curves is that they rise with horsepower until WEP is engaged (always the last point of the curve). That occurs because water substitutes for fuel to cool the charge, so the mixture is automatically leaned out when using WEP. Also note that SFC tends to be higher when the supercharger is used and when the supercharger is set to the high blower.
The best SFC for this engine is about 0.44 at about 50 percent of rated power and with the blower set to neutral. Military power generates an SFC that is nearly twice as high.
The SFC associated with the low blower ratio is nearly 10 percentage points higher than the comparable point of the neutral blower curve until military power is reached, where the difference is only about 4 percentage points. The best efficiency is attained at about 50 percent of rated power.
The SFC for the high blower setting is usually, but not always, higher than that for the low blower setting. The exception occurs at about 50 percent of rated power where SFC at the high blower setting is actually lower.
The Double Wasp is also the power plant for the P-47. The engine in that installation (R2800-21) has only a single speed geared supercharger and a General Electric turbosupercharger. If we drew a specific fuel consumption curve for the P-47 (again from a SEFC chart), we’d find that it is nearly identical to the curve drawn here for the neutral blower setting.
Background on the Double Wasp
Development of the Double Wasp began in March of 1937 and the first models were being produced in 1940. In the F4u, the -8 and -8W engine had a two stage, two speed supercharger with an intercooler. It was a B series engines with cast aluminum heads. The compression ratio was 6.65:1.
The -21 model used in the P-47 is also a B series engine. It has identical performance characteristics to the -59 and -63 models that appeared on Thunderbolts until a C series engine was introduced on the P-47N.
Construction of the chart
As noted above, this chart is derived from Specific Engine Flight Charts (SEFC) for the F4u-1 and P-47. Both charts are for engines rate for 100/130 pn fuel.
For the -8 model (F4u-1), each curve is a collection of observations at different power settings at roughly the same critical altitude. The neutral blower curve uses power settings at critical altitudes of 7,000 feet or lower. The low blower curve uses power settings at critical altitudes of 15,000 - 20,000 feet. The high blower curve uses power settings at critical altitudes of 20,000-25,000 feet. For each curve, except for WEP (which is always lower) the critical altitudes selected are very close to each other. The curve plotted for the -21 model (P-47) is less sensitive to altitudes below the critical altitude of the turbosupercharger.
This is not exactly the approach used to draw the SFC curve for an engine independent of the plane it is installed in. The engine maker is free to test all sorts of settings, holding ambient air pressure constant. Friction losses from the induction system, or inadequate cooling of the cylinders in a cowled engine would not enter into those measurements. I must pick from a few settings at various altitudes for a particular engine installation (the Corsair).
Sources:
U.S. Navy, Specific Engine Flight Charts (SEFC) for the F4u-1 and P-47 (undated)
Graham White, Allied Aircraft Piston Engines of World War II
http://www.enginehistory.org
