FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1936
1936 - 2344.PDF
AUGUST 27, 1936 9 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT 2 23t Operating characteristics of constant-speed airscrews reauire a departure from the conventional method of manual mixture adjustment. Any increase or decrease of rower obtained by adjusting the carburetter fuel/air ratio is accompanied by a change of airscrew pitch, such that the r.prn. is not affected. Tachometer indicat.ons, there fore cannot be depended on as with fixed-pitch airscrews, and instead it is customary to use automatic mixture con trols or fuel-air ratio indicators, such as exhaust-gas analysers and fuel-flow meters. Provision is made so that the pilot can discontinue the „overning action whenever desired and shift the airscrew into full high pitch, but except for checking the pitch- changing action or the engine operation there is no point in discontinuing the governing action with power on. However, positive high pitch does have one very important use, namely, in case of engine failure on a multi-engined aeroplane, wherein the performance of the plane with one engine out can be materially improved by shifting the dead engine's airscrew to full high pitch where its wind- milling drag is much less than in low pitch. Without this feature the dead engine's airscrew would automatically be shifted to full low pitch by the constant-speed control trying to maintain r.p.m. The benefits derived from constant-speed airscrews are most apparent in the case of high-performance aeroplanes, especially when equipped with supercharged engines. For hi"h speed, especially when reduction gearing is employed, comparatively high pitch settings are required. On the other hand, low pitch settings are required during take-off. Therefore the range between low and high pitch is large for high-performance planes, and it becomes increasingly difficult for the two-position controllable airscrew to provide satisfactory performance at all times. Advantages of C.-S. Airscrews To illustrate the advantages pro vided by a constant-speed airscrew the accompanying curves, Fig. 4, show the thrust horse-power available m level flight at cruising manifold pressure with a fixed-pitch airscrew, a two-position controllable airscrew, and a constant-speed airscrew. The curves have been plotted from actual calculations based on a representative high - performance aeroplane and engine. The lowest of the three curves shows the thrust horse-power available with a fixed-pitch airscrew set to give cruising-engine r.p.m. at 17,100ft. altitude with full throttle in level flight. It would, of course, be diffi cult, if not impossible, for the aero plane to take off with a fixed-pitch airscrew under these conditions, and therefore a two-position controllable would probably be necessary, in which case this curve would apply to the controllable in high pitch set for 17.100ft. altitude. The low pitch would be set to permit satisfactory take-off and climb at low altitude. This curve shows that with the air screw in high pitch only slightly more than 300 thrust horse-power is avail able _at sea level, and the maximum cruising horse-power cannot be utilised until an altitude of 17,100ft. » reached. The intermediate curve, bl K 4. shows the thrust horse-power available with a fixed-pitch airscrew adjusted to give cruising r.p.m. at 10,oooft. altitude with cruising manifold pressure in level flight. Under these conditions considerably more power is available at sea level than in the case of the airscrew adjusted for 17,100ft. altitude, and therefore it would probably be possible to take off with a fixed pitch. However, a two-position controllable, with the high pitch adjusted to 10,000ft. and the low pitch for take-off and climb, would be prefer able. Whereas the two-position controllable would make possible better aeroplane performance from sea level to 10,000ft., it would be no different from the fixed-pitch airscrew above 10,oooft. The third of these curves indicates the thrust horse- pow'er available with a constant-speed airscrew installed on the same plane and engine. This shows that the maxi mum allowable cruising power can be utilised at all altitudes from sea level up to 17,100ft. With the constant-speed airscrew 18.6 per cent, more power is avail able at sea level than with the airscrew set for io,ocoit , i.e., a fixed-pitch airscrew or a two-position controllable airscrew in high pitch Likewise, with the constant-speed airscrew, 24.4 per cent, more power is available at sea level than with the airscrew set for 17,100ft. altitude, i.e., the high-pitch setting of a two-position controllable airscrew. It is obvious that at i7,ioolt. (critical engine altitude for cruising power-) the constant-speed airscrew cannot make available more power than the high-pitch setting of the two-position controllable airscrew, but as compared with the fixed-pitch airscrew set for 10,000ft. (or the high pitch of a two-position controllable airscrew set for 10,oooft.) there is 43.5 per cent, more power. Fig. 3- Cut-away demonstration model of airscrew, showing control unit and its oil passage.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
