wsmalley

My Feedback: (6)

Do I get more 'power' with a bigger prop turning fewer rpm's, or a smaller prop turning higher rpm's? So, I hopped up my old Saito 45 MK2 with a 50/56 size cam. Ran a new ring in with a 12/4 prop. I could get down to 17-1800 rpm's and up to 12k + using the Master Airscrew prop. I seemed to have problems getting it started and had to use the electric starter. I looked around for a 12 APC thinking a heavier prop might help. Didn't have one, but found a big 13" 'Dynathrust', thought I'll give it a try. I can back flip that thing once or twice, and off she goes. Will sit there and tick over so quiet! But, I can only get her up to about 7500 rpm's. BTW, I have it running inverted in a Bird Dog, so speed is not the issue here. What am I gaining or losing with the prop size/rpm factor, is there a mathematical answer to this? I'll add, I run a big 22" Dynathrust prop on my ST G4500 and it's the same, easy starting with a back flip-it just seems to like a big ole heavy timber! Bill

warhwk

My Feedback: (3)

"Power" is not definable unless you have a dyno. Generally larger diameter/Low pitched props that have the same load factor will pull harder but at the cost of reduced airspeed.
The mathematical formula for thrust is: Thrust = 2.83E-12 x RPM^2 x D^4

What is the pitch on your 13" prop. Someone can then calculate approximate thrust and speed calculations.

For example...ThrustHP puts out these speed figures:
Prop Selector from http://www.gylesaero.com/_frames/f_propcalc.shtml]Gyles AeroDesign Propeller Calculator puts out the thrust measurements:

12-4 @ 12,000rpm = 45.46 MPH and 6.1 lbs of static thrust
13-4 @ 12,000rpm = 45.46 MPH and 8.03 lbs of static thrust
13-4 @ 7500rpm = 28.41 MPH and 3.14 lbs of static thrust
13-6 @ 7500rpm = 42.61 MPH and 4.02 lbs of static thrust
13-8 @ 7500rpm = 56.82 MPH and 4.87 lbs of static thrust

Actual Prop brand and type will change these figures substantially.

wsmalley

My Feedback: (6)

Thanks, very interesting data. The static thrust variation between the pitch values is especially noteworthy. I'll have to pull the prop off tomorrow and check the pitch. I will probably experiment with a few more props. Normally, I would just overpower something, but here I'm trying to save a scale appearance without a head cut through the cowl. The engine is marginal for the plane so i want the most 'power' to get it off the ground, but want slower, scale like flight.

warhwk

My Feedback: (3)

Here is a bit more info on your engine. http://www.rcuniverse.com/forum/m_42665/tm.htm

It looks like a 12-4 is probably your best prop. However, I would replace the Master Airscrew with wood or an APC. The MS props are loaded pretty lightly for their pitch.