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Finishing a Carbon Fiber Prop

by Bill Lee

Our carbon fiber props are all hand-made. They start from an original prop from which we make a mold. If we desire to make changes to the original, we try to make the mold to accommodate the changes. Each prop is hand laid up in the mold using continuous strands of carbon fiber and epoxy.

As a new prop comes out of the mold, it will have flashing around the edges. It will contain all of the flaws that were in the original prop, plus those we added as we made the mold. The majority of the work will be on the "face" of the prop. The "back" of our props are very accurate as the come from the mold and you should not have any work there. We try to make the mold as good as possible, but perfection is not possible. (The "face" is the side of the prop that faces forward when mounted on the model.) All this means that the raw, unfinished prop needs to be "worked up" before it can be used. There are several steps to finishing a CF prop.

Deflashing:
The thin flashing of epoxy and carbon around the edges of the prop must be carefully removed. Use a sanding block and some 80-100 grit paper. Carefully sand the "outside" of the flash, bending it back over the surface of the prop as you sand. As you thin the flash, it will fall off the edge of the prop when you have sanded it through.Be careful when deflashing. It is easy to sand too hard and eat into the edges of the prop or leave big sanding grooves.

The area around the hub needs special attention. There is usually flash around the edges of the hub which should be sanded off, leaving the back of the hub clean and flat. Be careful when you sand the back of the hub since that surface controls the pitch and tracking of the blades. The flash that leads from the leading edge of the blade along the side of the hub needs to be carefully sanded off. (A round stick wrapped in sand paper or a round rat-tail file is useful here.)The front of the hub should be sanded flat and clean.

Finishing the tips:
The tips of our props are purposely "crude" as they come from the mold. This allow you to make the prop that suits your needs. The tips must be finished, i.e., sanded to shape and thickness.

Cut the tips to give you the length of prop you desire. Use a small needle file and file through the prop about 1/16" beyond the desired diameter, then sand the tips to the exact length. Be careful since you can split the tips. (A little CA will usually handle minor problems if you do.)

Shape the tip to your desired shape by sanding the leading and trailing edges. Typically, the general outline of the rest of the blade is carried to the tip, with the actual tip shape (e.g., square, raked, etc.) as you desire.

Sand the face of the tips to blend the blade surface to the tip. The front surface of the tips will usually be somewhat rough, reflecting the "working" of the mold as we adjust it to give the desired raw propeller. The tips come from the mold pretty close, but will purposely have extra thickness: you need to make sure they are thin and continuous with the remainder of the blade. Make sure that the leading edge of the tip is properly radiused to match the rest of the blade and that the trailing edge is thin and fairly sharp.

Balancing:
Use your favorite balancer. We typically use the TopFlite Magnetic balancer, although it is a bit of a pain to use on the larger props. Sand the face of the heavy blade making sure that you maintain the airfoil. Remember that a small amount of material taken off a long way from the hub changes the balance more than that taken off close to the hub.

Once you have the blade-to-blade balance correct, you may still see some hub imbalance. Typically, this is so small as to be ignored, but you can sand on the hub some to try and get rid of it. An alternative is to add material on the light side of the hub. I have seen some use coats of CA on the light side of the hub. I also have seen suggestions of drilling small holes on the light side and adding small lead pellets, but I worry about potentially weakening the hub by drilling holes in it.

Finishing:
Once balanced, sand the prop lightly on all surfaces with progressively finer paper. Don't get to aggressive here since you are just trying to smooth the surface, not take off large quantities of material. Start with 100 grit and end up with 400 grit paper. Re- check the balance to make sure you haven't changed anything. The prop is quite useable at this point but you can make it cleaner by using a small amount of rubbing compound and a coat of wax, or by adding a finish of some sort. In any case, keep an eye on the balance after each step.

 

How do you finish a carbon fiber propeller

These notes lifted from the CL racing forum. http://groups.yahoo.com/group/clracing/

Bill Lee started the discussion by asking how people finished carbon props.


Bill Lee
I sand off the flash and lightly sand all of the surfaces. Then trim to length and balance. Finish is a light coat of CA spread with my finger tip, then sanded with 400 paper. Followed by a bit of rubbing compound. I'm interested in seeing if there is a better way.


Noam Weismnan

I do it a bit different. I finish the prop and use 600 and even 800-1000 water paper.
When the prop is ready and balanced I finish it with diluted clear polyurethane.
I use a two component polyurethane. The same that we use to finish wings etc...
After the first cote is hard I use 800 water paper to sand it and balance again.
After it is nice and free from holes etc... I give it another polyurethane coat.

Walt Perkins
WHEN we were racing (note we're not CURRENTLY racing) we finished our carbon/epoxy props:
Deflash- 180 grit sandpaper
Balance chord lengths side-to-side- 180 grit (finish profile)
Balance radii- 180 grit?(finish diameter)
Balance tracking side-to-side and blades pitch- 180 grit?(sand off drive surface)
Sand front hub face parallel to driving surface- 180 grit
Balance chord thicknesses side-to-side- 320 and 400 grit
Static balance- 400 grit
Surface finish- 600 grit wet all over
Check static balance- 600 grit wet
Seal with two coats bowling alley wax all over
Polish with cotton cloth and final check static balance
Of course this is a lot of work but it's the prop that turns power into thrust
so we felt it deserved a lot of detail attention.? Prop adjustments on the field
(diameter and tip pitch were done with 320 or so grit (not critical).

Scot Newkirk
My method is the same basic method that I've been using for 20 or so years (with a bit of a hole in the middle)
1) deflash with diamond files
2) trim rough diameter + .02/.05
3) true hub back then front
4) pitch blade
5) profile trailing edge
6) match chords within .005
7) match thickness/airfoils within .002 using fine diamond files then 240/320/400 sandpaper
8) finish tip shape and diameter within .005
9) balance blade 400/600 sandpaper
10) 1 coat 2 part polyurethane clear thinned for very light coat but with full flow
11) check balance mark light blade if there is one (they should be *very*close).
12) if one blade is light, scuff blade with 600 grit and give a second coat of poly and add a touch more on the light blade.
13) re-check balance finalize with polishing compound.

 
Chris Wee
I am not sure whether anyone else made side by side comparison of a prop just finished with:
(1) A fine diamond file, then sealing the end of TE with a quick smear of cyno, diamond filing any excess off the rear top/bottom blade surface, and
(2) Fine finish the surface with #320 grit then #800 grit, and
(3) Sealing with a thin coat of cyno and then fine polishing with #1200 grit paper, then polishing with metal polishing compound till a mirror finish I used to do method (3), then (2) unless the blade is too thin and one blade is heavy so some cyno on the light blade and finishing to help balance. Side by side comparison were done from time to time and I could not convince myself that there was any measurable gain that did not come from variation in the airfoil shape, so have been doing method (1) for the past 3 years.
Attention to details is however critical, especially maintaining the correct airfoil shape. It is notuncommon for me to spend up to 2.5-3 hours to complete a propeller to satisfaction. When I have a prop that stands out of 10 that is similarly finished, I make a template of top and bottom airfoil with 0.20 mm carbon sheet at R=20, 30, 40, 50, 60, 70 and then proceed to re-finish the other 9 props to be as similar as possible to the good one. Even then, you'll have 0.1-0.2/10 differences.  As far as balance is concerned, it does not have to be absolutely perfect. For the 2 pointed end balancer that you hold between your fingers, as long as the heavy blade takes more than 5 seconds to arrive at the vertical position, it is good enough. The fastest that we have ever flown the same model is on a prop that is slightly inbalanced (l~3 seconds in that test and one blade being noticeably heavy). You can hear the engine resonate and then hit a sweet spot in syn with the frequency of the vibration. But added vibration can only reduce the life of the model so ................ ~0.10/10 slower after we balanced it!