Fine tuning wood props?

I'm about to start fine tuning some wood props for use.

What is the importance of the sharp leading and trailing edges as mentioned in a prior post?

If a prop is balanced as far as weight, does any small difference in pitch between the two blades matter?

Cy

>
>I'm about to start fine tuning some wood props for use.
>
>What is the importance of the sharp leading and trailing
>edges as mentioned in a prior post?
>

The prop is a wing with an airfoil. The shape of that airfoil is as important as if it were on a plane traveling at 400 miles per hour (tip speed). If you wouldn't have a square leading or trailing edge on your airplane wing, you can see the importance of not having it on your prop. However, if you start your engine by flipping the prop forward (against the back edge of the prop) you will either want to keep this a little "round" or wear kevlar gloves (Matt's trick) when starting the engine

>If a prop is balanced as far as weight, does any small
>difference in pitch between the two blades matter?
>
>Cy

Yup. It is like balancing the tires on your car. There is the old static method of balance as on the buble balancer (which helps a lot) and then there is the almost universal (today) dynamic balancing that is done while spinning to balance them from side to side. A prop that has differences in pitch from plade to blade will do "funny" things while rotating in addition to the static balance being off. The real question is, however, how "small" of a difference is there between the two blades? Fortunately most props today are made on machinery that keeps the blades fairly uniform. Be aware, however, that a wood prop can warp. We had a 12 x 6 (old) BY&O prop that had one blade almost 8 pitch. Since all the rest in the batch were correct, and the thickness of the hub was correct, I am assuming that the wood in the prop warped after manufacture. (Just a little something to keep you on your toes.) Oh, and that prop, it was a disaster. But we sanded the pitch back, rebalanced the prop (statically) and it worked fine.

I have done a bit of re-tuning of the Zingers, they are great prop "kits". Good wood and lots of it. First I undercamber it about .010" by filing with a round file at 45 degrees to the long axis (outward at the leading edge). This automatically gives a hint of undercamber.

Next, I thin the blades to have a nice taper, whatever I think I can get away with on the particular size of prop and planned engine (Haven't lost one yet to anything but the ground). For the 10x4, this means about .120 at the root, and .070 at the tip. Hog away the flat sections back to where the prop will be shielded by your spinner or hub, but make a filleted transition from blade to hub.

Finally, file the top to give you a real airfoil with the high-point at about 30% of the chord, and a nice ROUND leading edge (less tendancy to stall, thus wider lift range over varying loads). I find that the wood is so hard, the trailing edge can be thinned to about .005 without shredding under finger starting. Haven't cut myself with one yet, either.

Balance carefully on a magnetic balancer (Top Flite). Be sure it will stop at any position, not just horizontal. I have had to cut away a bit of the hub on some, and also add CA to the other side of the hub on others to achieve this.

Paint with spray outdoor clear Urethane or Black Baron Clear Epoxy, 2 coats, sanding between, then balance again.

Keith T and Jim Armor have both used these props, and voted them much improved over stock.

As Larry notes, a round leading edge will give greater range and less tendency to stall. A sharp leading edge (I've discovered) gives a better, more predictable turn and it seems that there is less stick pressure, but can stall. I haven't had a problem yet, but I'm told it can be ugly.

>A sharp leading edge (I've
>discovered) gives a better, more predictable turn and it
>seems that there is less stick pressure, but can stall.

Zow! You've got sensitive hands.

I've been timing lots of my props recently,
using Supercool's Doppler approach to get speed and RPM.
I find the measurements are often at odds with my
(probably ill-trained) perceptions.

For future tests, I'm planning to do some blind testing
(another Supercool idea) so my preconceptions don't get
in the way of my perceptions.