From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical analysis of exhaust system performance and momentum effects on various aircraft.
| Identifier | ExFiles\Box 134\1\ scan0047 | |
| Date | 26th November 1937 guessed | |
| -2- relative to standard types of exhaust system. The results are shown plotted on Curve HK.{Col. T. Harker - Sales} 701, and show that in the case of the 'Spitfire', the actual gain conforms almost to the estimated gain, indicating that there is apparently very little interference with airflow over the fuselage when fitted with standard exhausts. The divergence is more marked in the case of Fairey aircraft, both on the P.4/34, and particularly on the 'Battle', indicating that the momentum manifolds have produced a considerable cleaning up of the airflow. The Henley and Heinkel results fall below the estimated line, but conform approximately with the gain resulting from momentum only. We consider that the results obtained so far, are satisfactory in showing that the exhaust momentum manifolds do produce an increase in speed, and they fulfil the claims made for them. In view of the divergence between the test results, however, we wish to experiment further, particularly with constant speed airscrews, in order to establish more adequately the order of the gains. Momentum Effects. The thrust obtained from the momentum of the exhaust jet is exactly calculable if the exhaust temperature and the engine response to back pressure are known. The momentum line on Curve No. HK.{Col. T. Harker - Sales} 701 is calculated assuming an exhaust temperature of 900°C. inside the manifold, and a trials altitude of 15,000 ft. with 6 lbs/sq.in. boost. The reduction in crankshaft power, due to exhaust back pressure, is taken as being :- 14.7 / (14.7 + Pe) x 1.5% per lb. back pressure, where Pe = the boost in lbs/sq.in. The resultant gain in speed is approximately .075 (V / 100)^2.5 MPH., where V.{VIENNA} is the aircraft speed in MPH. This law also holds for a Merlin engined aircraft being flown full throttle at ground level, the actual gain in speed being a function of the supercharger compression ratio and | ||