17
$\begingroup$

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?

$\endgroup$
  • 3
    $\begingroup$ each blade also has it's own drag $\endgroup$ – ratchet freak Mar 26 '15 at 16:41
  • 1
    $\begingroup$ Related question about propeller design. $\endgroup$ – fooot Mar 26 '15 at 16:45
  • 4
    $\begingroup$ @ratchetfreak Not only drag, but also weight. And the closer they are to each other, the less thrust they provide. $\endgroup$ – yo' Mar 26 '15 at 16:48
  • 1
    $\begingroup$ Not sure which windmills "on an American farm" you're referring to, but the new ones they're putting up on wind farms here in the midwest have 3 very long, slender blades. $\endgroup$ – FreeMan Mar 26 '15 at 17:13
  • 5
    $\begingroup$ @FreeMan He means a windpump usually placed over a well to pump water $\endgroup$ – ratchet freak Mar 26 '15 at 17:38
15
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ 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? $\endgroup$ – FreeMan Mar 26 '15 at 17:15
  • 2
    $\begingroup$ Indeed, the most "efficient" propeller is probably a one-bladed propeller - but efficiency doesn't propel aircraft, thrust does. $\endgroup$ – voretaq7 Mar 26 '15 at 17:59
  • $\begingroup$ 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? $\endgroup$ – superluminary Mar 26 '15 at 18:32
  • $\begingroup$ @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. $\endgroup$ – Peter Kämpf Mar 26 '15 at 19:40
  • 1
    $\begingroup$ @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. $\endgroup$ – Peter Kämpf Mar 26 '15 at 20:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.