Really basic question

[Sir Real]

How much extra drag is created by longer rotor blades, versus what kind of improvement in lift? Is there any sort of generic break-even point? How about a significant drop-off point in lift with shorter blades, or in speed with longer blades? And how much does horsepower and/or prop configuration change things?

Okay, maybe not such a basic question....

Thanks;

Kurt Franz

[scottessex]

I think the biggest factor is blade loading, a larger blade will give more lift, but only up to a certain point, if they are too large they will turn too slow and you are then at risk of other problems. This also depends on what type/MFG of the blade.

[CLS447]

Scott, what about blades that are pitch adjustable for proper speed ?

[Al_Hammer]

Surreal,
The relationship between rotor diameter and drag, rpm, power required and lift is fairly complicated.
Lift actually is the least complicated to figure, because it stays the same as you change rotor size. Lift is equal to weight.

A smaller rotor moves less air faster to make the same lift, so is less efficient and requires more power to push it through the air.

I think that rotor speed increases as the ratio of (old diameter/new diameter) to the 1.5 power.
Lets say you replace 25 ft blades with 23 ft blades and the original rpm is 300:
(25/23)^1.5 =1.13
1.3 X 300= 340 rpm
So , the 23 ft blades will turn 340 rpm, which sounds about right to me.


Blade loading and tip speed need to be kept within certain bounds, so manufacturers provide tables telling you which blade diameter to use for a given all up weight range.

Chuck Beaty recently posted about the rotor profile drag and induced drag if you care to do a search.

And yes, blade pitch will affect rpm, but there is an optimum blade pitch setting which gives the best overall lift to drag ratio for all the blade sections at cruise speed.