Asif

My Feedback: (30)

Hi Guys,
I am building Topflite Piper arrow and Thunder tiger Rare Bear, and would like to know what size 3 blade prop should I use. I am installing OS 91 in Piper Arrow and OS 120 AX in Rare Bear. Please suggest appropriate prop size for both.
Thanks
Asif

NikolayTT

Isn't it OS.91FX a bit too high RPM engine for a 3-blade ?

Asif

My Feedback: (30)

Maybe you are right, but people have used OS 91 FX on topflite Piper Arrow and it flying just fine. I am not sure what size prop to use.... 12x8, 13x6 or 13x8?!!!!
Asif

NikolayTT

Well, nobody can tell very precisely. Check the Propeller calculator of Pe Reivers and also
make some experiments with Fuels and Glow plugs so you can reach near 12000 RPM of
OS.91FX where the engine is giving most of its performance; here is the link:
http://www.mvvs.nl/
Click 'Propellers' on the left and then click the
'Propeller power calculation sheet' link. You can use
it on-line, or download it.

Would be nice to know later on what did you select and what RPMs you got.
Of course do not go much abouve 13000 for .91FX, it might be too much
for that engine.
Cheers,
Nick

Asif

My Feedback: (30)

many thanks for your "prop calculator"
Regards
Asif

NikolayTT

Nice, but this should be Mailed to Pe Reivers, he is nicely "guilty" for our
pleasure of having such nice tool.

Nick

Don41

I read this tidbit somewhere. I don't know how accurate it is.

[[With three blade props, take the numbers from the 2blade that's working for you and reduce the diameter by 1" and increase the pitch by 1.]]

Asif

My Feedback: (30)

HI Din41,
well I have read that too on tower hobbies's master airscrew site. But its confusing there too. They have conflicting comments:
If you browse master airscrew 14x8 prop the comment says: The general rule for converting a 2 blade prop to a 3 blade is subtract 1" from Diameter OR subtract 1 from the Pitch.

If you browse master airscrew 12x8 prop the comment says:
The general rule for converting from two blades to three is to decrease diameter by 1" and increase pitch by 1"

Now which one is true!!!!!

Asif

Don41

Another Snip

Three-blade propellers are useful
when you have a scale airplane that’s
modeled after an aircraft that uses
them. However, since the engine has
more mass to turn, the maximum rpm
is lower. The general rule is to use a
three-blade propeller one inch smaller
in diameter than the two-blade you
would typically use. This will allow the
same maximum rpm. You also may
increase the pitch by one inch, but
experiment and see what works best
with your engine and airplane. Your
model also will have a different sound
with a three-blade propeller.
from Barf Rag

Asif

My Feedback: (30)

Don41,
Thanks for ur reply
Asif

Mustang Fever

My Feedback: (2)

For what's it worth, I have a QF-4N Phantom (prop in the nose) with an OS 91 FX. I'm using an 11x8 Master Airscrew three blade because of ground clearance issues. It turns around 12,500 on the ground and goes like a rape ape.