1.0 Comments on Trials.

            As delivered, this aeroplane was fitted with a bullet proof windscreen, armour plating over the fuel tank, and a domed top on the sliding hood to allow of more headroom for the pilot. None of these modifications had been made to K.9793 fitted with a two-pitch metal airscrew the performance of which is given in Part of Report M.692,b dated 12th July, 1939. Consequently changes in performance, particularly in level speed cannot be attributed solely to the change in airscrew.

2.0 Take-Off.

            The take-off run in zero wind and under standard atmospheric conditions is better than that of the 2-pitch airscrew Spitfire. The accompanying table gives the take-off for the 3-types of airscrews.

3.0 Climb

            It was found that the best climbing speed as determined from partial climbs was not suitable owing to the instability, uncomfortably steep attitude, and poor view obtaining at this speed. This climbing speed was increased by 22 m.p.h. to a more suitable speed which improved the handling qualities of the aeroplane on the climb.

            The following table gives comparative times in minutes to reach various heights for the best climbing speed and the recommended climbing speed. Also included are the times to height for Spitfires with 2-Pitch and fixed pitch airscrews.

            In addition to the routine speed tests on this aeroplane as it was delivered, subsidiary tests were made to determine the effect of the bullet proof windscreen, and engine r.p.m. on the top level speed.

4.1 Windscreen test.  The bullet proof windscreen was replaced by the prototype Spitfire pattern of windscreen.

            As delivered the aeroplane was fitted with a bullet proof windscreen which has a flat front panel protruding about 1½ inches beyond the framework. After level speed tests had been completed this was replaced by the prototype pattern of windscreen. This consists of a windscreen with a curved front panel and is perhaps slightly better aerodynamically than the production pattern which has a flat, though flush-fitting front panel. Photographs are included at the end of this report showing both bullet-proof and prototype windscreens. As shown in Fig.2 the loss of speed resulting from the projecting bullet proof windscreen is 6 m.p.h.

4.2 Controlling R.P.M.  Speed tests were made at three heights below full throttle height with the boost pressure maintained constant at 6¼ lb. per sq.inch whilst the R.P.M. was varied over a range from 2600 to 3000.

            The results show that the maximum level speed is reached with the airscrew controlling at 2800 engine r.p.m. On increasing the r.p.m. to 3000 the speed was reduced, on the average by 4 m.p.h.

            For the particular engine fitted there is a reduction of 17 b.h.p. at constant boost (+6¼ lb) when the r.p.m. are increased from 2800 to 3000. The loss of speed is therefore probably due to the loss of power accompanied by a slight decrease in airscrew efficiency. The matter is being further investigated by Messrs.Rolls-Royce and Messrs.Rotols. It will be noted that reducing the R.P.M. from 3000 to 2800 lowers the full throttle height by 2000 feet.

4.3 Engine Power.  The engine installed in the aeroplane develops slightly less power under test bed conditions than that in K.9793, the aeroplane fitted with the 2-pitch airscrew. This could have the effect of reducing the top level speed by about 2 m.p.h.

Conclusions.

1.           This aeroplane has a much better take-off and climbs faster than other Spitfires fitted with wooden fixed pitch or metal two pitch airscrews.

2.           There is a drop of 13 m.p.h in maximum level speed compared with the 2-pitch airscrew aeroplane but of this, 8 m.p.h. can be attributed to sources other than the airscrew.

3.           Below full throttle height an increase in speed of about 4 m.p.h. can be attained by controlling the engine R.P.M. at 2800 instead of 3000.

4.           The limiting diving speed can be reached much more rapidly with this aeroplane than with Spitfires fitted with fixed pitch wooden and 2-pitch metal airscrews.