4-blade Pawnee Performance?

On Nov 20, 8:51*am, bildan wrote:
On Nov 20, 9:09*am, wrote:



On Nov 19, 5:59*pm, "noel.wade" wrote:

Can anyone corroborate those numbers?

Numbers? *No...

A four blade Pawnee going low overhead at full power is still very
loud. *The prop does nothing for the engine noise and the Pawnee has
quite a lot of that. *What it eliminates is the 'snarl' from high
speed prop tips that is most noticeable to an observer in the plane of
the prop. If 'snarl' is your problem, maybe this is your solution.

The performance loss is noticeable and annoying, but not a deal
breaker in itself.

Clubs I know who've bought them have not been real impressed. *For the
price, I'd like to be impressed. *Ymmv.

-T8

It's interesting to do some highly oversimplified "back of the
envelope" numbers on Pawnee tugs. *(The following ignores some
important stuff.)

Assume a 40:1 glider weighing 1000 pounds being towed at its best L/D
airspeed of 60KTS and climbing at 500FPM. *1000/40 = 25 pounds of
aerodynamic drag. *60x25/325 = 4.6HP to overcome that drag.

Of course the tug is lifting the glider at 500FPM or 8.33 FPS so
8.33x1000/550 = 15.14HP to lift the weight of the glider.

15.14+4.6 = 19.74HP which is the rate at which work being done on the
glider by the Pawnee. *In other words, only approximately 20HP is used
to tow the glider.

If the tug engine is actually producing 230HP (highly doubtful), 210HP
is being wasted somewhere - most of which is probably just the Pawnee
hauling itself through the air. *That wasted energy is where most of
the noise is coming from.

The above suggests that a tug with a highly aerodynamic airframe could
use far less HP which is why 80HP motorglider tugs seem to work so
quietly.

Or lets guess around 30 hp assuming prop (in)efficiency, ~80% best
case?, but who knows in practice. And the wasted energy is where *all*
the noise comes from :-) And as a data point, the ~50 shp engine in my
ASH-26E give up to 700 fpm climb... on a good day.

A more efficient tug could reduce the amount of prop thrust needed and
therefore noise that needs to be made in the first place. If prop
noise is a significant factor, which it seems to be, adding blades
definitely seems to help.

BTW I agree with your earlier post about the apparent differences in
towing behind a Pawnee with a Hoffman 4 bladed prop. Soar Truckee
switched to a four bladed prop and did some good community PR work
around reducing noise, including a positive article in the local
newspaper. See -

www.soartruckee.com/pdfs/Pickle_part_1_0612.pdf and
www.soartruckee.com/pdfs/Pickle_part_2_0612.pdf

The rate of climb with a heavy glider (e.g. DG-1000S with two
occupants) at ~7k'+ density altitude seemed a little lower than a two
bladed prop, but that's not a scientific comparison. Mile High may be
able to provide data on that as well. Hopefully a tow pilot with
experience with these will weigh in.

Darryl