Prop tip critical Mach number?

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

TXFlyGuy

Well-Known Member
Joined
Apr 25, 2012
Messages
1,585
Location
Republic of Texas
Is there a critical Mach number for prop tip speed? For most efficient cruise rpm?
Might it be Mach .65?
And if you cruise at Mach .645, this would be getting the most from your propeller?
 

Turd Ferguson

Well-Known Member
Joined
Mar 13, 2008
Messages
4,942
Location
Upper midwest in a house
Is there a critical Mach number for prop tip speed? For most efficient cruise rpm?
Might it be Mach .65?
And if you cruise at Mach .645, this would be getting the most from your propeller?
Yes. Yes. It depends on the airfoil used. Mcrit or Mcr is the speed at which airflow reaches the speed of sound but does not exceed it. Takes a lot of power to maintain that speed so it is definitely not the most "efficient"
If the Mcr speed of the propeller is .70, .65 might be the sweet spot. But then again, might not.
 

lr27

Well-Known Member
Joined
Nov 3, 2007
Messages
3,698
If you're spinning a large prop using a small motor and a redrive, my guess is that your most efficient tip Mach number could be much less than Mach .65. OTOH, if your rpm's are higher, your prop is carbon fiber or metal, and the tip airfoil is very thin, maybe even with a bit of sweep at the tip, it might make sense to go higher. It's not just about the thickness of the airfoil. There are ways to raise an airfoil's critical Mach at least some without making it thinner. I've read that some "laminar" airfoils have higher critical Mach numbers. This seems plausible, but I haven't seen hard numbers or looked into it. I wonder if anyone makes props with supercritical airfoils at the tips? https://en.wikipedia.org/wiki/Supercritical_airfoil
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
10,463
Location
97FL, Florida, USA

Voidhawk9

Well-Known Member
HBA Supporter
Joined
Mar 26, 2012
Messages
297
Location
Timaru, NZ
That would be true, there are several variables in prop design that could affect this.
 
Group Builder
Top