why resieze a prop ??

getback Sat Nov 15, 2014 5:46 pm

I have heard a lot I talk about resizing perfectly good props ? to a smaller state of being for more RPMs ? And I am wondering what is wrong with using a smaller prop with more or less pitch //// What is the advantage !?!? Huh...

Ken Cook Sat Nov 15, 2014 6:56 pm

Eric, many times your faced with a prop that just loads the engine a little more than you would like. I clip props to trade off speed for rpm's. Seeing that one of my events utilizes a criteria in which I can't exceed a certain mph adjusting prop sizes is the way to do it. If I chose to drop in inches or in pitch, my fuel economy suffers as a result of doing so and it's imperative that I have enough fuel to make it to the 5 minute mark. The same can be said in regards to flying stunt. A square tip Master Airscrew is less effective due to the tip vs a curved APC. Reworking a Master Airscrew prop to the shape of a APC will increase the effectiveness of the prop. Cutting down a prop can slow a plane down as I mentioned above or it can also increase speed due to the engine being allowed to run in the best part of its torque curve.

The only way to know what your doing is beneficial is by timing it and documenting it. I've done just that and I know what works. For example on my Norvel's I use on my 1/2A's, I could use a stock Cox 5x3 but it has a fair amount of blade area and it loads the rpm's in the maneuvers when pulling a streamer. I start cutting the prop down 1/4" at a time until the engine doesn't load and still utilizes a good amount of drive. I usually run a 3 3/4"- 4 1/4" prop on my Norvel's. Think of the engine as if it's running in 2nd gear opposed to lugging uphill in 4th.

Mark Boesen Sat Nov 15, 2014 10:47 pm

I guy told me years ago, or maybe I read it someplace: Diameter for RPM, pitch for speed. Keep that in the back of your head when picking a prop for your engine/airframe.
Also remember pitch is an average of the prop, if you cut down a 7"x3" prop and inch, you do not have a 6"x3", but a higher pitch, a 1/2" on a 10" prop is not going to change much, but 1/2" on a 6" would.

Paulgibeault Sun Nov 16, 2014 4:56 am

Hi Eric,

The main reason for cutting down prop diameter, is because an "ideal size" prop doesn't exist. There is nothing wrong with using a smaller prop with more pitch ...IF you can find one. In many cases, us modelers do not know what the ideal pitch to diameter ratio is supposed to be for a given application. I know I don't. I only have a best guess estimate. I have seen speed prop calculations done by an aeronautical professional. For the most part, it is beyond me, so the 'cut & try' method, based on best guess is all we have left... The major factor 'best guess' is based on is at what RPM your engine sounds happiest.
Shown is an array of props for Vintage Speed. All turn in a a similar RPM range, but have different pitch-diameter & blade areas. Only my stopwatch can tell for sure...
-

Cheers, Paul

getback wrote:I have heard a lot I talk about resizing perfectly good props ? to a smaller state of being for more RPMs ? And I am wondering what is wrong with using a smaller prop with more or less pitch //// What is the advantage !?!?

Paulgibeault Sun Nov 16, 2014 5:18 am

Hi Mark,

I believe you are correct. " Diameter for RPM, pitch for speed". I am convinced that you can't really go fast without enough pitch. Of course in order to swing the big(er) pitch, you need to have a small(er) diameter, small(er) blade area & thinner blade(s). Your statement "pitch is an average of the prop" assumes the pitch is helical...which it may not be. A pitch gauge is the final arbiter in cases like this, & often shows up surprises.

Then there is the engine power curve side of things. Some engines can swing a heavy prop & TORQUE around the circle. Others need to swing a light prop & REV their way around the circle. (Some have an in between combination) I have seen both combinations work well & so it all comes down to prop matching for a given use. The record holders & podium finishers are generally the ones who get it close. When you see somebody being lent a prop & he goes WAY FASTER & his engine sounds COMPLETELY DIFFERENT, then you know right away he was using the wrong prop all along...

Famous (true) saying: " It's ALL in the prop & half in the engine."

Cheers, Paul

Mark Boesen wrote:I guy told me years ago, or maybe I read it someplace: Diameter for RPM, pitch for speed. Keep that in the back of your head when picking a prop for your engine/airframe.
Also remember pitch is an average of the prop, if you cut down a 7"x3" prop and inch, you do not have a 6"x3", but a higher pitch, a 1/2" on a 10" prop is not going to change much, but 1/2" on a 6" would.

getback Sun Nov 16, 2014 8:10 am

Answers from the pros That is great stuff , Thanks Ken, Paul and Mark for you input . I understand better the purpose now , ben wanting to try some resizing and comparing but did not know what I was looking for out of it . ( to make a more happy running engine for the application ) and get the most from that engine is what I am gathering . Eric Cold

**Cribbs74** Sun Nov 16, 2014 8:45 am

All the things mentioned above are very true, one thing left out was sometimes you break your last prop right before you go out to the field, cutting one down on the fly may keep you going.

Keep in mind when you trim a prop it must be re-balanced. There are ways of taking of equal amounts off the blade, but it still needs to be balanced.

Ron

Paulgibeault Sun Nov 16, 2014 4:16 pm

Hi Ron,
Absolutely correct. At the field I've been known to trim Cox props with a toe nail clipper! My carbon props seem to have a short life & the higher the prop's performance, the more likely it is to get damaged or broken!! ARG!
Cheers, Paul

Cribbs74 wrote:All the things mentioned above are very true, one thing left out was sometimes you break your last prop right before you go out to the field, cutting one down on the fly may keep you going.
Keep in mind when you trim a prop it must be re-balanced. There are ways of taking of equal amounts off the blade, but it still needs to be balanced.
Ron

Mark Boesen Sun Nov 16, 2014 6:23 pm

Lol, I'll second the toenail clipper, I've had one in my flightbox for years!

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