Why don't propeller designers use multiple profiles?

I'm designing a ducted fan, and I'm deciding on the airfoil profile. NACA 16-series profiles are well-suited to M > .5 or .6, but they're not particularly efficient at lower Mach numbers. On the other hand, older profiles, like Clark YM, work quite well at lower speeds.

My question is, why do propeller designers generally only use one profile (with scaled chord and thickness) throughout the propeller? Is there some reason not to use, for example, a Clark YM for 2/3 of the blade and a NACA 16-709 for the outer 1/3?

Note: this being a ducted fan, laminar profiles will not work, so they aren't under consideration.

• You can't have a sudden "step change" between two profiles, because the step would create a stress concentration that would cause the blade to break. Simple "straight line ruling" between a small number of airfoil sections (root, tip, and possibly mid-height) was easy to manufacture with "pre-computer" machine tools. Remember that the inner "1/3 of the blade" only accounts for about 1/9 of the swept area and therefore 1/9 of the total mass flow - is it really worth complicating the mechanical design to try to get just a small percentage of aerodynamic efficiency? – alephzero Aug 6 '18 at 7:15
• Obviously it would be a smooth blend from one profile to another, just like a wing with multiple airfoils along its span. "Remember that the inner "1/3 of the blade" only accounts for about 1/9 of the swept area" I think this is actually the answer to the question. Even if every station were ideal, the inner stations just aren't all that important. – user1493362 Aug 6 '18 at 7:44

1 Answer

There are propellers with different aerofoil sections. For example, the propeller attached to the Rolls Royce engine kept in my graduate college had symmetric aerofoil sections roughly about 25 % and smoothly varying to bottom flat airfoil sections at the rest.

The aerofoil profile selections are based on the performance requirement, structural rigidity, and modes of operation. You can go for any aerofoil sections with high ($C_l / C_d$) but, in the end, it should be structurally rigid to produce high thrust at high RPM. When you have variable pitch propellers you don't need different aerofoil sections since the required performance achieved by adequately rotating the blade.

In my point of view having the same aerofoil sections will be advantageous in the below-given ways.

• Matching the aerodynamic centres of the same aerofoils sections would be easier

for example, NACA 0006 have the $a.c$ at $\sim c/4$ and NACA 2412 have at 0.3$c$, then locations of $a.c$'s will not be smooth along the span of the propeller. This would be critical in structural stress distribution on the propeller.

• Downwash of the propeller sections will have smooth variations.

For similar sections, the downwash induced velocity component will be predictable and the twist of the blades could be compensated easily while designing. And the twist will be smooth for similar profiles from the structural point of view.

Hope this helps.