Cox Propeller Modification Tip

Yes, I also estimated this as about a little more than 2 ins.

How 'bout 2 5/8"?  That would work out to the recommended 5 1/4".  It's this "New Math" that has me all confused.  

It says 5 1/4 through 6 " prop So depends on the size ur cutting down .

Yes, size changes with propeller diameter. A photocopier can do this. I just measured template by placing a 5" propeller on template, and template is full length of half blade.

This should and will be interesting. I also wonder about the use of this on other stiff propellers such as the MAS?

Geez.. I don't know about taking a knife to one of my Cox grey props.. I have 6 x 3's and 6 x 4's that I sourced before I understood that they were better suited to the higher-performance Cox engines; of which I had none at that time. I now have a few Tee Dees to experiment with..

I agree Mr. Roddie. I would tread lightly. This modification may cause a out of round crankcase and or crankshaft!!! Due to prop pitch variance. Could Be Bad Vibes Bro. Mark

Mark Diedrichs wrote:I agree Mr. Roddie. I would tread lightly. This modification may cause a out of round crankcase and or crankshaft!!! Due to prop pitch variance. Could Be Bad Vibes Bro. Mark

Agreed.. I'm not experienced with modifying propellers.. and am not about to start learning now. I have an aero-prop balancer that I built.. and feel that it works quite well. It uses a "Kavan" shaft and hubs.. magnetically-suspended between Neo-magnets.

For 1/2a pylon (Tee Dee) we use to cut down Master Airscrews (6x4) to 4.75" anything less just wouldn't pull, Unless they were pretty loose, Tee Dees had a pretty short life at those RPM's.

When I flew 1/2A Proto (redundant/cancelled class now) and speed in the 1970s, we used the Cox grey 5 1/2" or 5 1/4? -4 and cut it down to a bit less than 4 1/2", reshaped it and thinned it out. I would have been so happy then to have some of the APC props that you can get now. They were a lot of time down the tube when you nicked the ground on takeoff. Nice idea for the template with the locating notch though.

here's an article from late 80's on (Tee Dee) pylon set-ups..

I suppose we can agree to disagree. I personally have modified several props. I don't alter the leading edges as suggested in Mark's instructions. In addition, if I'm seeing the template correctly, this leaves the tips squared off. Having square tips is about the most inefficient way to gain thrust. We use it as a tool to slow the engine down. Our speed limit rules in combat is not to exceed 75 MPH, this equates to around 7.2 -7.3 seconds for two laps. For every 1/8" we cut back leaving the tips square, we gain a little in time. I don't recall the actual speed we lose or time gained just due to knowing the exact size prop I need.

Thinning the blades and lessening the weight of the prop blades gains rpm's. There's a point however where this will just beat air and the engine goes into high rpm's with little speed. The props of choice back when these articles were written were made by Kelly props. More common to the speed crowd of the day and probably not recognized by the average modeler. The Grish black Magnum Tornado props in my opinion worked better than any props on the market that were sold in hobby shops.

A larger problem with props is that the prop hole isn't square to the mating side and or either sides of the prop are parallel to each other. Therefore you can have two identical props where one works and the other doesn't work as well. The same goes for pitch. All props regardless if they're the same, one might have slightly more of less pitch to one of the blades. This is a must to check which very few do.

This doesn't mean they have to be identically the same either. The only way to test is by time. It's quite possible that a prop with a heavier pitched blade on one side can work effectively. This is what makes a single propeller effective is that it grabs clean air as it's coming around whereas a two bladed propeller can have it's opposing blade grabbing turbulent air.

As Alan mentioned, we currently have a small variety of APC propellers nowadays to choose from which work exceptionally well for under $3. Others have also experimented with the quad props in carbon and glass varieties.

I agree with Ken there is no need to mod the leading edge the factory is more consistent there.
These days I prefer APC E props it is very simple to scissor cut the trailing edge the airfoil tapper
is so thin with E props there is nothing more to do other than check balance. A good eye balling
can normally tell you which blade may need a bit of sanding to match the width of the other.
Calipers confirm your work. I try and keep the dia. for thrust a narrow blade picks up rpm
while thrust picks up due to higher static rpm. Back cut at a angle for a more narrow tip
as shown produces lower weight prop narrow tips for higher rpm cooler running engine.

E props the most easy to redo the more you snip the better you get at it.
In other words less sanding to match blades as your eye ball skills improve.
You can cut one blade then use some tape with color stick it on the cut blade
and trim off the excess then use it as a template for the other blade. Goo way
to go when your first learning to modify.


IC prop requires a extra step you must redo the airfoil's trialing edge I use fine files and a sanding bar.
Followed by fine grit sanding paper to polish the prop to factory sheen. I rarely fly stock props anymore.
The effort happens fast once you have done a few.



With stock internals you will get performance that no other can obtain without prop mods.
After a turbo plug that is.