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-   -   Can you suggest a starting prop (https://www.rcuniverse.com/forum/questions-answers-154/5448907-can-you-suggest-starting-prop.html)

ntsmith 02-20-2007 06:59 PM

Can you suggest a starting prop
 
Hi.
Have a Minicraft Javelin 90 with an SC91FS (Virtually the same as Magnum or ASP)
the suggested prop to run in on is a 14X6 so not to load the engine too much but what for flying?
Pattern aircraft allways seem to use hihg pitches. why?
Would I be right in think along the lines of 13 X 8?

Any help appreciated.

LANNYBOB 02-20-2007 07:05 PM

RE: Can you suggest a starting prop
 
14 is about max for a 91 four stroke. any bigger and you'll over prop it, resulting in inadequate performance

ntsmith 02-20-2007 07:37 PM

RE: Can you suggest a starting prop
 
I meant 13 X 7 or 13 X 8 but why does everyone use large pitches on pattern props anyway. Surely they will not help maintain a speed both in up lines and downlines. Surely a lower pitch would be better for better acceloeration. although I am digressing here. I revised my original message btw to 13 from 18

carrellh 02-22-2007 11:37 AM

RE: Can you suggest a starting prop
 
I don't know the "why" but they all seem to run 13x8 or 13x9 on the 91 engines. Maybe 'someone' won a championship with it and everyone copied him.

jaka 02-22-2007 12:21 PM

RE: Can you suggest a starting prop
 
Hi!
The reason patternflyers use high pitched props is because of noice!
The rules in F3A which is the international name for pattern flying say you must have an airplane/engine combination that will not sound more than a certain db A at a certain distance.

For your .91 fourstroke a 14x6 is good propsize to start with both for running in and for flying.
There are naturally other propsize that will work good too but it all depends on what model you intend to use it in. If it is in a slow flying model a 15x4 or 15x6 APC might be best. In a faster, smaller model a 13x8 or 14x6 APC might be best.

ntsmith 02-22-2007 05:26 PM

RE: Can you suggest a starting prop
 
I find that answer interesting. Since most pattern flyers fly in areas that are more away from any houses than the standard club flyer and this because the models tend to eat up vast expases of sky, I would have thought that was the least consideration. The higher pitch causes the planes to fly faster but also lack the better vertical climb rates of lower pitches, and perhaps this accounts to a certain extent in the apparent smoothnes of the flight. I have heard comments that the quieter models are perceived to be smoother, but I do not hold with that view.
My plane the SC91 is going in is the Minicraft Javelin 90.
I am currently showing how its progressing on the web at http://fp.ntsmith.plus.com/this_is_t..._of_events.htm
the 14 X 6 was seen to be used in a write up on the very similar ASP91 so thats how I come to choose that for a starter.
The question is then. Would a 13 X 7 or 8 be better. Certainly with my Venus 40 a 6" pitch was jsut right and more pitch meant it flew faster but lacked the power in the up lines. I guess the same is going to apply here.
I will try the prop I have and see how it goes. Just get used to the plane at first for a good few flights any way.
Couple of weeks away I guess with the speed of my building (assembling!)

carrellh 02-22-2007 05:51 PM

RE: Can you suggest a starting prop
 

ORIGINAL: ntsmith
the 14 X 6 was seen to be used in a write up on the very similar ASP91 so thats how I come to choose that for a starter.
The question is then. Would a 13 X 7 or 8 be better. Certainly with my Venus 40 a 6" pitch was jsut right and more pitch meant it flew faster but lacked the power in the up lines. I guess the same is going to apply here.
I will try the prop I have and see how it goes. Just get used to the plane at first for a good few flights any way.
Couple of weeks away I guess with the speed of my building (assembling!)
Most of the people I fly with like to use a prop that keeps a 91 four stroke between 9,000 and 10,000 rpm static. The 14x6 seems to do that on Magnum, OS, and Saito 91s running 15% or less nitro in the fuel. I think a 13x7 would allow excess rpm and a 13x8 would turn numbers similar to the 14x6.

The Javelin is very sleek design. The 13x8 would likely do as you suggest; more speed but less vertical power than the 14x6. You may like the 14x6 so much you never even try a different size, and there's nothing wrong with that.

My brother and I have 91 four strokes on 3 planes each; all running 14x6 props. They pull great, go fast enough for us, and slow down well for landing. We've never tried different sizes because we are happy with what we have.

ntsmith 02-23-2007 04:01 AM

RE: Can you suggest a starting prop
 
Looks like the 14 X 6 should suit well then. I'll post back after its tried. I agree with getting that sort of rpm range as its loading is less in flight.
I also agree the 13 X 8 may be too fast or should I say make the plane too fast with insufficient vertical power.
and the 13 X 7 underprop the engine.
There is a huge difference however between a 14 X 6 Master prop (My preference and the APC. almost everybody elses it seems) the Master prop is said too be louder but I always get a lower idle from them than APC props. APC props give slightly higher RPM figures too.


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