jwalsh1

Hello,

What are the effects of changing only the propeller diameter, not the pitch? Say going from a 12x5 to a 13x5, on the same engine and same plane. Thanks!

Jason

britbrat

In terms of power load, increases in diameter are more significant than increases in pitch. The change that you have suggested (12-5 to 13-5) is about the same as increasing the pitch by 2" (more or less equivalent to a 12-7). The increase in diameter will increase the total thrust & improve the efficiency of the powertrain/airframe combo, provided that the engine has the grunt to swing it.

jwalsh1

Ah, ok. Im currently playing around with props to gain performance. First I tried a 13x7, and the performance was bad. I am now thinking of a 14x8; increasing both pitch and diameter thinking that the engine was underpropped before. I havent yet flown it, so I dont know what the results will be.

Say I went from the 13x7 to a 14x7, that would be equivalant to say a 13x9 prop in terms of load and performance change?

Thanks!

Jason

jaka

HI!
What engine do you have in mind?
Choosing the right prop is not easy.
But some general rules to follow is : Flying fast, slim airplanes: use a small diameter prop so that the engine can rew.
Flying a slow, with lots of drag, airplane: use a prop with large diameter and less pitch.

Lets take an example with a 6,5cc (.40) two stroke sport engine like an OS FX .40, TT PRO, or Webra GT) in a low winged airplane like a Q-500 pylonracer with a span of around 132cm and a weight of 1,6-1,8kg fully loaded...a 9x6-9x7 APC prop might work best here...using the same engine on a high winged trainer airplane like a Kyosho "Calmato" a 11x5-12x4 APC is the propeller to use.
This rule of thumb i valid with all engines, two strokes as well as four strokes.


Regards!
Jan K
Sweden

RCKen

My Feedback: (9)

When you say "more performance" what do you mean? More speed? More power? There are different props for each. If you want more speed then you want a higher pitched prop. If you want more power (basically means more pull going upwards) then you want a lower pitched prop. Going with a larger diameter prop isn't always good. You will get to a point where the prop is overloading the motor. You will want a prop that still lets you get enough RPM's to be within the manufacturers specs for the engine.

Unfortunately (as said above) props vary depending on the plane and motor. What works well with a motor on one plane may not be good on another plane. The best way to find out is to get several different props in your choice range and try each of them to see what suits you best. Ask other pilots at your field and see if you can borrow a few props to test with, that way you won't be out the money just to test a prop.

Hope this helps

Ken

britbrat

Tell us what the airplane & the engine are and we can be more specific about the prop selection. Tell us what performance parameters you are trying to enhance (level speed, acceleration, climb, reduced landing speed, etc) & tell us where & how you fly ( airfield altitude & average air temp, plus --- 3D, pattern, sport flying, float flying, scale warbirds, etc).

jwalsh1

Great!

Its a widebody 60 pattern plane, Im a beginner in pattern and will be flying the Sportsman routine. Plane is between 7.5 and 8 lbs, and it has an OS .91 four stroke on there now. Balance is correct and trim seems not out of the ordinary. Straight and level flight was all checked out as good.

The first couple flights with the 13x7 showed a very slow flight speed with little verticle power. The plane fell off of 45 degree up lines without rpms decreasing, which was quite confusing, and the reason behind a few of us think its actually underpropped.

I am looking not necessarily for flight speed, but for verticle pull power. I had the 14x8 recommended to me and I figured that might be a good start to see whether or not the plane was in fact underpropped. I wouldnt mind more speed if possible, but its very important to have that verticle. I dont need it unlimited vert, but enough to have fun doing the pattern routine.

Thanks again! I cant fly this prop combo till Sunday night at the earliest, and Im getting antsy as to what to expect. I was thinking of picking up a 13x8 and maybe even a 13x9 to bring with me, and I have a 14x6 I could bring out there to try too.

I loko forward to your comments!

Jason

jwalsh1

Oops,

I fly in the chicago area, and currently the temps are in the upper 80s/ lower 90's.

I was turing the 13x7 at 9300 rpm on the flight days, and have recently had help tuning to turn the following:
13x7 = 10300
14x8 = 9000

I have not had a chance to fly at the new "tuned" levels, for the crosswind has been awful AND we have an event all weekend at our field.

britbrat

Now we're getting somewhere. An OS 91 FS should easily pull 7 1/2 lb straight up --- you were seriously underpropped with the 13-7 & I'm sure that the 14-8 will show a noticeable improvement. At 580', your elevation is not affecting your engine output to any significant degree, so the engine should develop pretty close to its max HP -- it will probably pull OK with the 14-8 -- on a cool day it might even pull a bit more prop than that. With the 14-8 you can expect a little more level speed than you get since you re-tuned the engine, & a big chunk more vertical.

jwalsh1

Man you made my day, thanks.

Jason