Real world experience.......
Van's engineering department was as interested as anyone, in what the actual fuel economy benefit of the iS engine is.
Well now we know.
The red prototype (N412RV) piloted by Proto shop guy Tony Kirk and the new iS airplane (N912VA) Piloted by Mitch Lock made the trip to OSH together.
They flew at close to the same weights and flew the same distance and flight profiles (together).
Total fuel for the trip east....
N412RV with ULS engine - 60 Gal
N912VA with iS engine - 45 Gal (A 25% reduction in fuel burn)
The prop pitch is set on both airplanes to values that we feel provides an equal level of speed performance (both airplanes achieve the same TAS at any given altitude when operated within the max. continuous RPM limitation of 5500 RPM)
What is interesting is that to get equal speed performance, the iS engine has its prop pitch set .4 degrees courser than that of the ULS airplane, but when both airplanes are flown side by side in a climb test at exactly the same weight, the iS airplane climbs 100 FPM faster.
We attribute this to the peak torque output of the engine being right at the RPM value that is typical in a Vy climb with the fixed pitch prop on an RV-12. In fact, even with .4 degrees more pitch, the iS engine is turning a higher RPM climbing at Vy than the ULS RV-12 does. Another factor likely having some influence is the iS having cold air induction fed via a NACA scoop on the right side of the cowl, vs the ULS ingesting ambient engine compartment air.
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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