Aircraft technical Basics: Aircraft Engines - RAF Flying Training Manual - Chapter VII.- Engines: Running Notes
RUNNING NOTES
(See also Chap. II paras. 55-58)
Introduction
54. The pilot of an aeroplane has in his care a complicated and expensive piece of machinery which, being a machine, can only give reliable and efficient service if it is correctly handled. This applies just as much to the engine as to the flying controls. To one who has read the foregoing paragraphs there should be no difficulty in understanding the various operational limitations laid down in Volumes II of the different engine handbooks and appreciating how important it is for him to observe them conscientiously. Without fully automatic controls it is easy and sometimes tempting to overstep the limits imposed, but doing so may cause damage to the engine which, though it is unsuspected at the time, may bring about failure later on, and it is not fair for one pilot to expose another to a risk of which he is ignorant and for which he is not responsible. The following notes summarise various points concerning the running of an engine in an aeroplane.
Starting and running-up
55. (i) Instructions for starting are given in handbooks.
(ii) In order to ensure adequate piston lubrication, running up should not be too slow (see para. 29).
(iii) Engines should not be opened out until the oil is warm (see para. 30), in liquid cooled engines the coolant must also be warm ; otherwise the sudden expansion of the cylinder block may cause damage.
(iv) Magneto switches should be tested at about half to three-quarters maximum r.p.m.—not full out.
(v) Full power should not be maintained longer than necessary to observe r.p.m.
(vi) Sudden movements of engine controls should be avoided at all times, whether on the ground or in the air.
Taking off
56. (i) Take-off boost must only be used for three minutes or until the aeroplane reaches a thousand feet, whichever is the shorter period (see para. 52).
(ii) Prolonged use of take-off power causes overheating (see paras. 33 and 51), detonation, and excessive fuel consumption (see para. 37).
(iii) When using constant speed airscrews, r.p.m. must be above the minimum laid down, as low r.p.m. at take-off boost give rise to detonation (just as a car pinks on a hill although it may run properly on the level).
(iv) With fixed pitch airscrews it is unlikely that the r.p.m. will be incorrect if the boost is kept within limits.
Climbing and level flight
57. (i) Generally speaking, boost and r.p.m. limits are set for each condition of flight in conjunction with one another. The figures for continuous cruising must not be exceeded for more than five minutes in each hour and then only with rich mixture (see para. 33).
(ii) A weak mixture should not be used on the climb.
(iii) Maximum figures must not be exceeded in any circumstances : boost because of over-heating and detonation (see para. 33), and r.p.m. because of inertia forces (see para. 27).
Economical cruising
58. (i) When running well throttled down weak mixture can and should be used (see para. 33). Pilots should realise that flying at high speeds not only requires much greater power and therefore more fuel but the use of rich mixture, and the range of their aeroplane will be very much curtailed if they fail to fly at economical speeds and with weak mixtures.
(ii) When it is necessary to fly with boost greater than that laid down for economical cruising rich mixture should always be used (see para. 331.
(iii) With the automatic mixture control (see para. 39) the pilot's task is easy, but with manual control some skill is needed to get the best results and it is well worth while to take some trouble to acquire it.
Diving
59. When diving the airscrew not only requires less power to drive it but if the dive is steep enough actually turns the engine round faster than the maximum r.p.m. for level flight and this, if the limits were strictly adhered to, would prohibit such operations as dive bombing. As, however, this condition exists only for very short periods, a higher limit is set for 20 seconds only, and then only if the throttle is partly opened. This is very important, in order that the high inertia loads may be balanced by gas pressure (see para. 27). Pilots must remember that it is not enough to observe the r.p.m. at the beginning of a dive, for they are continually increasing, as the air speed increases, and must not at any time exceed the limit set for diving. Diving at higher r.p.m. than this limit ; diving at over maximum level flight r.p.m. for longer than 20 seconds, or even momentarily with the throttle shut, will cause excessive inertia forces to be set up and will quickly cause failure of the bearings. Even in moderate dives and glides the throttle should not be completely shut for prolonged descents as there is then a tendency for fuel to accumulate in the induction system and for the engine to become so cold that it may not open up when required.
Temperature control
60. The temperature of the coolant in liquid cooled engines is usually directly controllable and should as far as possible be kept constant. When gliding or diving the radiator should be retracted or the shutters closed to keep the engine warm. It must not be forgotten that all coolants boil at a lower temperature at high altitudes. Oil temperature is usually not directly within the pilot's control, but so far as operational conditions permit he should refrain from flying at heights and speeds which tend to raise the temperature (see para. 30). This will probably cause no trouble in Great Britain but difficulty may be experienced in hot climates.
Conclusion
61. The would-be pilot has more to learn than merely controlling an aeroplane ; he must learn to control the engine as well and it is better that he should do this through a sympathetic understanding of the mechanism than by the example of disastrous failures. It is hoped that this chapter may have helped the reader towards such an understanding and that it will enable him to make full use of the faithful service of which the modern engine is capable.
