Juker008

Iwould like to know how much the diameter of a prop can be shortned. Can a 22x be shortned to an 18x, or even 16x?

Thank you

1320Fastback

As long as it's balanced you can go as short as you want.

There is a point when the thrust will go down dramatically as the airfoil gets shorter and shorter.

I tend to buy a new prop if it is over 1 inch shorter.

combatpigg

My Feedback: (3)

Unless you recontour the entire blade, shortening the prop too much will make it a dog. That's been my experience for the most part.....but this is the Speed Forum, not the 3D forum.
You might end up with a nice weed wacker tool, though.
I don't know of a hard fast rule of thumb, but if you end up with a prop that has really wide, blunt tips it won't do much good in speed applications in the .5 c.i. and under set ups I play with.
You're playing with bigger sized stuff, so hold on for better advice due shortly from someone else. I've seen full scale warbird props that contradict my advice.

Juker008

That was my thinking. I guess Iwould need to recontour the blade. My problem is that the only available prop in the pitch that I would like is larger than Ineed, or can support.

combatpigg

My Feedback: (3)

It isn't that hard to do. I do it on carbon props that I rework to be compatable with an engine that has a "pre-tuned" pipe.
I came up with my own method to do recontouring out of neccessity. Once the blades have been rough cut to the new "overhead view" contour, then the airfoil of the blades are reworked by eye. This might seem hit & miss...but if you [re]work the prop in small increments and test your work as you go, you should arrive at a workable pattern in due time.

Juker008

I've never worked on CF, but can only assume that working on the props, wouldn't the CFfray?

combatpigg

My Feedback: (3)

If it's well impregnated with resin, it should be very easy to work with. The strands are supposed to be wetted out before they are stuffed into the mold. The biggest hazard is the dust and possible splinters that aren't compatable with our health. Rig a vacuum system that traps the dust and work with protective gear.
I don't know how hazardous it is compare to coal dust or fireplace soot...but it seems to be high on everyone's radar as a hazardous material to grind on.

KC36330

back in the days we cut down 18x8 props to 10" to break in ducted fan engines, it's the only way you could get the correct RPM and proper load on the engine and not sling the hub apart.

the way i cut mine was simply cut them about 1/8" long on both blades, then insert a dowel the correct distance away from my oscillating drum sander and spin the prop into the sander, it'll curve the tips and trim it to the desired size, toss it on a balancer and that was it, nothing more had to be done.

Juker008

Got to say, having a hard time imagining the process. Can you explain further or maybe show some pictures?

KC36330

take the prop, cut it 1/8" longer on each blade then desired (say you want an 8" prop, it would be 8.25" after cutting). insert a dowel/bolt (size of prop hub hole just as CP has the drill bit pictured above) with it's center 4" away from the edge of the sander, stick the prop down over the dowel, turn on the sander and slowly rotate. the sander will sand the prop down to a perfect size on both sides as the prop is turned on the dowel. (always feed against the rotation of the sander) after it's sanded to size, simply balance the prop.

Juker008

Guess Ihave to see it to understand. I don't know if you are discribing now to recontour the pitch, or recontour just the tips. If it is just the tips then I do understand.

KC36330

just the tips, it wasn't necessary to do anything else.