Propeller efficiency and energy loss

[mheslep]

Googling for information on the topic leads me to graphs such as this, which suggest that if Beta and advance are optimal efficiency tops out at 85% or so. Some questions:

1) Is the ~15% lost energy converted to heat, or is there some stray kinetic energy in airflow that is somehow unusable as thrust? And to verify then, these prop inefficiencies are not including some losses inherent in the prime mover when measuring 'power' in?

2) Do fans or ducted fans improve on prop inefficiencies (assume sub sonic tip speeds for both)?

 

[AlephZero]

There are (at least) two sources of energy loss in an unducted prop. One is the flow around the blade tips, where in a viscous fluid you can't have a "clean" separation between the air accelerated by the prop and the air "outside" it. The other is the whirl component of the velocity created by the prop, which gives a helical motion to the air but produces no net thrust.

A ducted fan reduces the tip losses by providing a physical barrier between the two airstreams flowing at different speeds. That allows higher tip velocites (up to transonic flow) without a rapid decrease of efficiency. If there are stator vanes behind the prop, they can also partially straighten out the whirl component of the velocity.

 

[mheslep]

There are (at least) two sources of energy loss in an unducted prop. One is the flow around the blade tips, where in a viscous fluid you can't have a "clean" separation between the air accelerated by the prop and the air "outside" it. The other is the whirl component of the velocity created by the prop, which gives a helical motion to the air but produces no net thrust.

Yes, thanks.

A ducted fan reduces the tip losses by providing a physical barrier between the two airstreams flowing at different speeds. That allows higher tip velocites (up to transonic flow) without a rapid decrease of efficiency. If there are stator vanes behind the prop, they can also partially straighten out the whirl component of the velocity.

Any guess or experience as to the efficiency gain by adding the duct and vanes? That is, if a prop can hit 85%, would (say) a 95% efficient duct-ed and trailing vane fan be typical, without a drag penalty that negates the effort?

 

[AlephZero]

I'm not a "prop expert" but I think the biggest benefit would be maintaining efficiency at higher blade tip speeds (either from faster RPM or faster aircraft) rather than pushing the peak efficiency much about 85%. For example the blades on turbofan jet engines run with slightly transonic flow without losing efficiency, but that would completely kill an unducted prop. Since power output is proportional to RPM cubed (aside from efficiency considerations), higher power output without larger fan diameters is a big win.