The Aerodrome Forum - View Single Post - "Gross" and "Net" rotaries power outputs.

Greybeard · 3 January 2008, 12:57 AM

From when Bletchley and other mates on this forum let me discover windage losses were an important factor about aircraft powered by that kind of engine, I got more and more "jammed" when reading about performances.

I try to explain by an example:

Air Data Board reports Sopwith Camel performances with various engine models, included an A.R.1 (Bentley rotary) and Clerget (not better specified). Actual respective horsepowers are 150 HP at 1250 RPM and 126 HP at 1250 RPM. Speed are almost the same, with a little advantage for Bentley powered one. Anyway, those 24 more HP are absolutely incoherent with difference in speed, which is negligible; also weight, that is higher by only 50 lbs for Bentley powered, can't explain its almost equal speed.

Only explanation could be Clerget horsepower was expressed "net" of windage losses (on the contrary of Bentley, on same document), so very close to 126.5 HP developed by A.R.1 in same circumstances. This implies "gross" Clerget power output should have been about 150 HP, but I never read such a datum.

If so, the question is: how to orientate into this labyrinth?

GB

3 January 2008, 12:57 AM	#1 (permalink)
Greybeard Forum Ace Join Date: Jul 2005 Location: Italy Posts: 698	"Gross" and "Net" rotaries power outputs. From when Bletchley and other mates on this forum let me discover windage losses were an important factor about aircraft powered by that kind of engine, I got more and more "jammed" when reading about performances. I try to explain by an example: Air Data Board reports Sopwith Camel performances with various engine models, included an A.R.1 (Bentley rotary) and Clerget (not better specified). Actual respective horsepowers are 150 HP at 1250 RPM and 126 HP at 1250 RPM. Speed are almost the same, with a little advantage for Bentley powered one. Anyway, those 24 more HP are absolutely incoherent with difference in speed, which is negligible; also weight, that is higher by only 50 lbs for Bentley powered, can't explain its almost equal speed. Only explanation could be Clerget horsepower was expressed "net" of windage losses (on the contrary of Bentley, on same document), so very close to 126.5 HP developed by A.R.1 in same circumstances. This implies "gross" Clerget power output should have been about 150 HP, but I never read such a datum. If so, the question is: how to orientate into this labyrinth? GB __________________ ------------------------------------------------------ It doesn't matter what we do but in what relationship we put each other while doing what we do.