main menu

Pages

Tip speed

Tip speed

  • The power developed by a piston engine depends upon the pressures attained during combustion in the cylinders and on the revolutions per minute.
  • The greatest power in most engines is developed at a fairly high number of revolutions per minute, and if the propeller rotates at the same speed as the engine crankshaft, the tip speed of the propeller blades is liable to approach or exceed the speed of sound (about 340 m/s in the air at ground level, and less at higher altitudes).
  • This causes compressibility effects, which, in turn, mean an increase in torque and a decrease in thrust; in other words, a loss of efficiency.
  •  It is clearly of little purpose to design an engine to give high power, if at such power the propeller is to become less efficient and so transfer a lower proportion of the engine power to the aircraft.  
  • In the early stages of compressibility, some improvement can be affected by changing the blade section near the tip to a thin laminar-flow type and by washing out the blade angle slightly; if this is done the loss is not serious so long as the actual speed of the tip does not exceed the speed of sound.
  • As a further help, a reduction gear is often introduced between the engine crankshaft and the propeller; the reduction is not usually very large, perhaps 0.7 or 0.8 to 1, but is just sufficient to reduce the tip speed to a reasonable margin below the speed of sound.
  • The tip speed, of course, depends not only on the revolutions per minute, but also on the forward speed of the airplane and the diameter of the propeller.
  • The high forward speed of modern airplanes is such that it is becoming very difficult to keep the tip speed down below the speed of sound, and it would seem that at forwarding speeds of 350 knots or more some loss in efficiency must be accepted.
  •  At 430 knots the loss inefficiency is serious and has spread to a larger proportion of the propeller blades so that it affects not only the tips but what should be the most efficient sections.
  • At this stage, there is nothing for it but for the propeller to retire gracefully and hand over supremacy to jet propulsion.
  • A further objection to high tip speed is that the noise caused by the propeller (incidentally a large proportion of the total noise) is much intensified, especially in the plane in which the propeller is rotating. This can be annoying both outside and inside the aircraft, and in severe cases, structural damage can result.

 

reactions :

Comments