Propeller Sense

PROPELLER SENSE But, of course, the propeller blade does not simply follow the line of least resistance, like the knife in the cheese or the corkscrew in the cork. The blade is in fact awing, obeys the same aero­dynamic laws as the wings of the aircraft, and must strike the air at an angle of attack if it is to give a thrust to balance the drag of the aircraft. (On the climb it must further balance a ‘component’ of the force of gravity in order to pull the aircraft upwards.) So the angle CAB is the angle of advance, but not the angle at which the blade is set to the plane of rotation, which must abe greater angle DAB, greater by the angle of attack DAC. The bit of blade rotates in the direction AB, moves forward indie direction BC, widi resultant corkscrew path indirection AC, and with its nose pointing indirection AD. With a fixed propeller, the blade angle DAB is fixed and the angle of attack DAC is directiy tied up with the angle of advance, CAB—diat is, with the advance per revolution, BC. We get small angles of attack at high advance per revolution, (BC big), and large angles at low advances (BC small). So at given r.p.m. we have small attack- angles at high airspeed and large attack-angles at low airspeed But we can also get small angles of attack at low air speeds if the r.p.m. are low enough for BC, being the advance per revolution, i.e. airspeed divided by r.p.m., will go up as r.p.m. go down. Arise of airspeed, or a drop of r.p.m., reduces the angle of attack and conversely. The propeller at zero airspeed At zero airspeed—for example, when the engine is being ground tested—it would seem that the angle of attack will bethe whole blade angle DAB but in fact it is not so. For CAB is the angle of advance of the bit of blade through the air immedi­ately around it, and it happens that this air will be flowing quite fast—at say 50 m.p.h.—under the action of the propeller. You are familiar enough with the slipsteam behind the propeller, but per­haps you had not realized that the flow starts ahead of the propeller and half die 100 m.p.h. or so gale at the tail has got going by the time the stream reaches the propeller. This fact makes quite a difference to the angle of attack we get at zero m.p.h. of the aircraft. The correction we must make for this flow towards the propeller—the induced flow—gets less and less as we fly faster and faster, or as we reduce the work that we ask the propeller to do at a given speed, as ingoing from a climb to a level cruise. The graph below shows how the difference between propeller speed and aircraft speed gets less and less as air­craft speed increases. In passing, you maybe interested to note that this flowing of the air in which the C air-screw’ is working involves a loss of efficiency—just as it is inefficient togo up by the down escalator and hard work climbing a heap of loose shingle. This accounts for the efficiency of a propeller being higher in level flight than on the climb. 2
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