Cut Down Propellers

LAST EDITED ON Nov-29-04 AT 04:27 PM (CST)
 
What is the reasoning behind Cutting Down Propellers?
Ex. 10 x 6 cut down to 9 x 6.
I see it all the time, but don’t understand. Why not just buy a shorter prop?
Does it have to do with differences in total mass?

LAST EDITED ON Nov-29-04 AT 04:37 PM (CST)
 
It's propeller trim. Also, the optimum diameter may be a half size that's not readily available. It's easier to cut bigger props down than size smaller ones up (just kidding!)

For example, my Fox Combat Specials perform best with 9x7's cut down to 81/2x7. That size was either not made, or not readily available (you tended to bust a lot of them).

I do not know why someone would cut one readily available size down to another readily available size, if that's what they're really doing.

Usually to get a wider tip or more blade area. Sometimes, just to see what happens.

Bigiron

My last plane (a Saito 56 powered job) had a pretty severe yaw problem. One of the trim attempts was to use a big prop (14x5 3 blade) and cut it down to about 11.5 inches. The idea was to keep the blade area (and transmission of power) while stuffing it into a smaller diameter in order to decrease factors adding to the yaw.

As noted, it's a trim device.

Many years ago Frank McMillan showed me what a Super Tigre .46 would do on a 13/6 rev up cut down to a 12" prop. I still have many left. The difference was so profound that I simply could not believe the effect of a prop on proformance, (until that day). Thanks Frank for your wisdom.

I have a hard time noticing any difference when a prop diameter is reduced by 1/2". I have noticed a speed difference of about 10 mph when going UP or DOWN one pitch size. With my OS 25 LA it flies 75 mph with an 8x6 prop, about 70 mph with 9x5 prop, about 60 mph with 10x4 prop. I broke the tip off on a 10x4 so I cut it down to 9x4 the airplane still flies about the same even in the manovers.

With my OS 40 LA I broke the tips on the 11x6 prop so I cut it down to 10x6 and I can not tell any difference in the way the engine runs or the way the airplane flies.

If the prop is too big, diameter or pitch the engine RPM will not come up. That is very noticable.

>I have a hard time noticing any difference when a prop
>diameter is reduced by 1/2".

Depends a lot on the engine and prop. I trimmed 1 cm off a F2D prop the other day and saw RPM jump from about 34,500 to 38,400.

Ken Burdick told me that when he was experimenting with VA .049 engines, as he removed the last 1/8" from the prop diameter (from 4" to 3 7/8"), RPM would jump several hundred RPM every time he shaved off a tiny bit.

Sometimes it's just to cut down the load on the engine so it doesn't overheat and wear out early.

Once I hesitated on the reverse wingover pullout and lost about half an inch on blade diameter. It just happened. Lost a little off the tail too.

I agre with the others, I do it to get a size not offered. My Fox 35 likes a 9 1/2 X 5 on my All Amer. Sr. I cut down a Rev Up 12-5 drone prop to get the same blade as an old paddle blade Top Flite. Works real good, and you can get the Rev ups real cheap a lot of times. They have enough blade in the 12" size to make good Top Flite PB clones in the 9" to 11" sizes. It's some work, but if I don't have what I need, I can make some pretty quick.
Bill <><

I dunno, one day in 1954 I started sanding down a chipped one, to fix it. Then I went to work on the other end, to match the first. Then to the first side, then the second. Mike Shannon asked what I was doin', and told Joe Cronin. Sweeney found out, and someone made a prop balancer, and then the rest of youse guys found out, and.....

In order to get the best performance out of your engine you must mate the prop to the engine. There are some variables you can control of wich is pitch and diameter. If diameter is too big then the engine strugles and does not want to brake. This is when you cut doun the blade dia. Do it in small increments until the engine is happy. Pitch will control the speed. Too much plane is fast too small and the plane flies slow. Also a biger pitch will load the engine more than less pitch. You do this to mate te prop to the engine. a Wider blade will load the motor more than a narrower blade at same diameter. If you lack power a biger ventury will also help. If you have too much power than add an air filter (same as making the ventury smaller). These are all variables to consider when you choose a prop for your plane. Good luck. Hope this will help

Sometimes the prop you want is inside another prop.

I found that the Zinger 7-5 had the pitch distribution I wanted on the face. I carved down the back, cut the diameter and re-cut the blade shape to make a 6-1/2 x 5. I made a mold and now can reproduce a great fiberglass prop for the diesel on my pdq flying clown.

I then cut it down some more which made a 6-1/4 x 4-3/4. With a second mold I can now produce a CF scale race prop with nice beefy hub for my screamin' glow engine.

If I remember right, George Aldrich used to cut down bigger blades to get the paddle shape that he claimed pulled better. Sometimes I think the new props do not have enough area at the tip. Later, -DOC

>If I remember right, George Aldrich used to cut down bigger
>blades to get the paddle shape that he claimed pulled
>better. Sometimes I think the new props do not have enough
>area at the tip. Later, -DOC

Speaking of which - I remember he was selling those strange looking, petal-bladed carbon props on his web site (I think Joe Supercool was making them). What were those for?

>What is the reasoning behind Cutting Down Propellers?
>Ex. 10 x 6 cut down to 9 x 6.
>I see it all the time, but don’t understand. Why not just
>buy a shorter prop?

I think for most, it's just a matter of practicality. Of you want slightly less load, cutting off the diameter reduces it without changing anything else. If you get a smaller prop, you have changed much more than just the diameter. Even in a perfect world, you change the blade shape and almost certainly the airfoil. And in the real world, you also change the pitch and the pitch distribution. Plus, you can't buy props in 1/4" diameter increments.

My opinion is that there is a bit less pitch on shortened blades, as well as a significantly thicker section, which would result in less flexing.....

See Ya

Friends. My thinking has always been practicality, availability, and the most important factor-(What works!)Sometimes this hobby is far more art than science. If I am at the field trimming a model, and the prop I am using is a fraction away from working, I then have the option of cutting and trying..Maybe it will, maybe not..If I have 2-12-4 Bolly props, and my friend says try a 11 1/4- 4 Bolly it will work!! I am going to cut my 12-4 as oppossed to waiting and purchasing a new 11 1/4 4 from Randy etc.....That is the practicality aspect..Based on the success of the prop, I then have the option of purchasing a new prop of the diameter I just cut to. To do what?? Once again, try and see if it "WORKS"--This hobby is all about what works!!!The "art" is in the function of the "process"..The science is left to guys like Brett co...Just a thought...David....

>My opinion is that there is a bit less pitch on shortened
>blades, as well as a significantly thicker section, which
>would result in less flexing.....

Maybe. I'm sure it will flex less if it's stiffer. But while I sometimes hear knowledgable people complain about "flexing", I can't say I have ever seen anything in my own experiements that have suggested this was a problem. Maybe I'm not perceptive enough to tell.

I use a lot of shortened props. My reason is that the prop makers have not kept pace with engine progress. Modern engines have the power and RPM to throw blades off many props. The answer is to buy a bigger prop and shorten it.

In FAI combat, we used 6/4 props. If you bought a stock 6/4, it was a 1/2 A prop. We could buy a 7/4 (made for a .15) and cut it to 6/4.

Same story in Slow combat. A cut-down 9-inch is stronger and has more blade area than a stock 8-inch.

I’ve also been told with the last two engines I had to break-in, (Enya 19, Fox 35) to use a cut down prop. Since the engines are not in the air flying, why would a cut down prop make engine break-in better?

>I’ve also been told with the last two engines I had to
>break-in, (Enya 19, Fox 35) to use a cut down prop. Since
>the engines are not in the air flying, why would a cut down
>prop make engine break-in better?
>
>Chris

Simple Chris. Higher revs at less load results in a richer setting for the same number of piston/sleeve cycles. Cooler run and more lubrication enhance the wearing in procedure.

gotta be a little careful when dealing with modern ABC/AAC piston sleeve sets. These are designed to control piston/sleeve clearance based on different expansion rates of the disparate materials between the piston and sleeve. Thus you don't want to try to break them in real rich like we got used to doing in the iron piston age.

In these engines you need to let them rev up in a clean two stroke. Doing so with a prop smaller than intended for normal use reduces the load and the heat while allowing the mixture to still be lean enough to limit the amount of friction from viscous lubricants.

Generally speaking running prop you expect to use for flying but cut down an inch or so in diameter achieves the desired load reduction.