22

The air intake on a fanjet typically has many small angled blades. A wind pump in an American film might have many broad slatted blades. My desk fan has 3 very large scooping blades and seems quite effective.

It seems to me that if each blade provides thrust, the more blades you have, the more thrust you might expect. It also seems to me that a propeller with more blades could be smaller and turn more slowly, potentially avoiding going supersonic while providing the same level of thrust.

I ask because I'm building a drone and considering using six small multibladed props.

What are the downsides of adding more blades to propellers?

superluminary
  • 323
  • 2
  • 9

1 Answers1

20

Every blade will create its own boundary layer and its own vortex sheet. It is more efficient to use fewer blades with deeper chord, because the forward part of a boundary layer contributes most to friction drag.

To keep the lift coefficient on the propeller blade sections in a reasonable range (0.6 to 1.0) for efficiency means that blade chord will be reduced, which will make them less stiff. Again, it will be better to reduce blade count to arrive at a more viable design. All the thrust the propeller creates is pulling on those skinny blades, and they must be strong enough to withstand this force.

Only when the propeller disc loading increases do more blades begin to make sense:
When engine power increases, the propeller disc area should also grow, but this growth is limited by the resulting speed of the blade tips. Once the flow speed there becomes supersonic, the drag at this section of the blade increases without a corresponding increase in thrust. To avoid that the next best option is to increase the solidity ratio of the propeller, called also the activity ratio.

Make no mistake, this is bad for efficiency. But if there is enough power available, adding more blades is the best way out.

You are right, a lower prop speed allows to increase its diameter, but while tip speed will drop by less than the reduction in prop speed (after all, flight speed should not change), the available thrust from this propeller will drop by the square of the speed reduction, since thrust is proportional to the dynamic pressure on the blades. And thrust you get only from the circumferential fraction of the local speed at the blade; flight speed does not count here and does not help to mitigate the reduction.

Peter Kämpf
  • 231,832
  • 17
  • 588
  • 929
  • If I understand, the fan in a turbo-fan engine works because the nacelle impacts the airflow around the edges of the fan, is that correct? – FreeMan Mar 26 '15 at 17:15
  • 2
    Indeed, the most "efficient" propeller is probably a one-bladed propeller - but efficiency doesn't propel aircraft, thrust does. – voretaq7 Mar 26 '15 at 17:59
  • So are you in effect saying that a blade will provide thrust if it is moving relative to the air aound it. Adding more blades will swirl the air around in the propeller disk, which means that each blade will be moving less quickly relative to the air around it? – superluminary Mar 26 '15 at 18:32
  • @superluminary: No, the speed around each blade will not change. But more blades will operate at a lower lift coefficient, creating more friction (= torque) for the same lift (= thrust). It is like adding a second wing to an airplane which should still weigh as much and fly as fast as before. – Peter Kämpf Mar 26 '15 at 19:40
  • 1
    @FreeMan: The engine fairing improves efficiency for highly loaded props (= fans) and helps to equalize flow speed over the fan's operating conditions. But thrust is created on the surface of the fan blades by the pressure difference between the two sides of the blade. The fan would still work without the nacelle, but not as well. – Peter Kämpf Mar 26 '15 at 20:08
  • @voretaq7: The motivation for this particular prop was the smaller cutout in the glider fuselage for the retractable engine. But you are right in principle, and some designs have been tested, but so far the regular application of single bladed props has not happened. – Peter Kämpf Mar 26 '15 at 20:16
  • @PeterKämpf Yeah that happens to be my go-to single-bladed prop picture. There are a couple of counterweighted prop photos on the front of "normal" airplanes, but those offend my delicate aesthetic sensibilities to look at :-) – voretaq7 Mar 26 '15 at 20:23
  • @voretaq7: Looks like an amputee. Appalling! – Peter Kämpf Mar 26 '15 at 20:39
  • My hunch was that using more blades would mean that each blade could be much less sturdy and therefore lighter. Perhaps the entire prop could be cheap plastic rather than metal or wood. The loss if a single blade would not be catastrophic. Am I way out here? – superluminary Mar 27 '15 at 15:47
  • @superluminary: Don't underestimate the forces pulling on those blades. In a turbofan there is easily a pressure difference of one bar between the forward and aft side of the fan. Newer fans are indeed made of plastic, but the expensive carbon fiber-epoxy sort. Also, all the thrust a propeller creates is pulling at those thin blades. They need to be strong so they don't tear off. – Peter Kämpf Mar 27 '15 at 21:29