3 Blade Prop For Cox Engine

What are you powering? My 4" prop three bld worked great for the heavy PT - 19 i scratched some time back,, better than the 2 bld 5in. had on i t. baby bee!!

I have read somewhere on CEF that - in our 049 prop size range - adding a 3rd blade has the same impact on the rpm as increasing either the 2 bladed blade Dia by 1 inch or the 2 bladed blade pitch by 1 inch..so a 3 bladed 4x4,5 props loads the engine like a 5x4.5 2 bladed or 4x5.5 2 bladed. I know these are approximations but make some sense.

Yep, that's been a "rule of thumb" ever since i can remember, for about any 5" to 12" prop, another one is: diameter for RPM, pitch for speed.

The comparison of the three blade to two blase is more to the diameter (RPM) then for the given pitch.

I don't think the three blade prop is much of a performance prop, unless ground clearance is an issue i'd stay with a two blade, but for general sport flying and you already have them, why not. The 5x4.5 is a bit big, maybe add a head gasket and go for it.

Ok. Thank you. I have not yet ordered these and now probably will not. I do plan on doing a small test on the 5 x 4.5 two blade props that I did order and are on the way. I was reffered to try these 'drone' props out. Someone else has been using them with apparent good success so I orderd some to try as well. They do definately appear to have a heavy hub area, and I will be running them in about a month or so when they arrive. As for performance, I have run and still have a few MA 5.5 x 4.5 props on the same engines.

NEW,

Why are you set on using a 4.5 pitch? I believe most folks flying 1/2A CL planes use a 3" pitch. To effectively use a 4.5 pitch I think you would need a very light, well streamlined (low drag) airplane and light lines to let the prop make use of the pitch.

It is OK to over-prop an engine and let it unload in the air, just don't over-prop it so much it can't unload and attain the RPM it was designed for.

Either way, good luck.

George

I am not set on the 4.5 pitch, it is just what is available in this style. There is also a 6 x 4 which is also interesting me to try out as well. I had also forgot to mention that even the 5" prop was cut down to 4.5" to help it unload, and by what I have heard, it did unload well in the air. And if I did also order the 6 x 4 they would be cut down as well to the same. I have 5 x 3 on hand, I just like to play with and try new things. I will be trying out the props when it warms up a bit, and also when they arrive on the slow boat....

I do this for my .20 and larger engine, fuel, prop, break in, flight fiddling:

For the .049s or .061 I have quit fussing with the variable pitch* props and use Aero Hobby supplied very rigid combat glass props

But for the bigger engines---

For each engine series of size and type... iron vs ABC ..start with a Base line fuel mix and base line prop for bench running/break in

For flight testing I research same type planes with same engine and steal known good fuel, prop recommendations.

I BUY various props ranging from larger lower pitch or smaller higher pitch range...I am talking low cost ( usually APC) props---- not some high zoot, hollow, carbon fiber, hand made, pre balanced, made just for me $35~$48 each propeller

For cox, Norvel and other .049 to .061 engines there is actually a broad range of usable propellers...BUT they are almost all two blade designs

In MY mind the only reason for a three blade prop on these engines is-- the LOOKS cool factor--- and also, unfortunately, NOBODY makes a broad enough range of 3.75 to 6.5 inch diameter by 2 to 7 pitch triple bladed propellers

Also--- for these typically plastic small propellers about all you can do is cut down diameter... heat to re-pitch is a fools errand

Back to some retaliative base line (without using any cobbled together small engine dyno)

For Cox Reedies and TeeDee engines there is well published data for fuel and prop RPMs

Pick your fave fuel and cheap base line prop... I say 25%N Sig and a Yellow Rubber Ducky 5x3 and average three to 5 runs as your base line RPM data...for THAT engine

Test again with any other blade and note any increase or decrease in peaked** out RPM

** peaked out in this case means backed off a few clicks rich to pretend we know how lean it will go in the air at flight speed

At most ALL, two bladed props are going to unload in the air and be MORE efficient ----but that is NOT true for the three bladed props ---and I tend to believe much of this is that, each brand and prop type changes shape a LOT at FULL RPM...

MY not scientific experience is that the typical black plastic three blade designs tend to be MORE pitch variable than the typical yellow rubber ducky...I do wish they molded these 3 blades with the harder Cox Grey formula

Of curse all this is just mental pontificating... long ago I let loose all the PRECISION intrigue and decided I was fussing more on the bench and computer than Flying..

Today I have 9 different 1/2a planes I fly-- and when I nose plant and break a prop...the replacement is more and more likely to be a one of the out of the packaged yellow 5x3 rubber ducky that I got free from Bernie a few years back

Of course I nail clip a bit off the tips of a virgin yellow prop for the Norvel Aero .049 short nosed combat engine--- and usually happy that the clipped down rubber ducky ( free) does a good enough job as the $4.95 Purple Aero Hobby combat prop

BTW...back a year or two I had 5 Three bladed props and I wanted one for a particular bird...cool factor
The engine was a well worn TeeDee .049 and fuel was 40%N stuff

I set one 5x4 Tripple prop and started the engine...peaked out at 12,500 some thing RPM
Grabbed a 150 grit sanding bar and slowly reduced diameter as I listened...peaked at 14,800 but I kept going and as it got smaller the RPM did not change ---so I knew the best for that fuel needle was 14,800 rpm

Threw that prop way and put another new prop on...sanded down to 14,800 RPM and stopped

The Brodak Baby Clown racer with anemic TeeDee and cut down three blade looks cool and is pretty fast ( Ok faster than I can keep up) lap times

I have dorked this plane at least 4 times and the stupid soft Cox prop will NOT break

OK OK ...

Final thought.. FRED sez.... if dorking...use Cox rubber props
If you have a clue...Use finger slicing APC props
If Combating....do not bother with Cox engines

Old Phart out

Try-out a two-blade Cox L/H 6 x 2 if you can find one. A L/H prop will torque-roll the model to the right, helping to increase line-tension, especially on take-off. A lot depends on your airframe too. Build as lightweight as possible.. and balance it carefully. No prop will do a good job if the model is too heavy, out of alignment and/or out of balance.