Campgems

I;m new to the gas/ 33% world and I now have two engines that are cranking 20 to 22 inch props. Both are second hand, even though the Zenoha has yet to run. Both came with some new props as well as a couple that have been opened up to fit. The other engine is an O.S 300 FT and the props that came with it are 20" APC props. Three of them were bored out to fit the crank, but it looks like a sloppy job and it also looks off center.

My quesion is two fold. First, can I use the cast in ring on the back of the APCprops to "find center" so I can bore and bush the prop to a known good center. Second. The O.S. comes wiht a drilled and tapped prop hub and washer for 6 4-40 screws in adation to the prop nut. The previous owner didn't use the 6 screws, but relied on just the nut. It this a problem.

Thanks,

Don

Tired Old Man

My Feedback: (1)

APC props aren't really designed to be drilled for a multi bolt hub, but it is frequently done. Safety IS compromised when doing so. Not much side material around the pilot hole and what is there is not solid. You might want to check the balance of the props before you consider altering the pilot hole size. If the blades are not in balance you can ream the pilot hole off center to correct the imbalance and run safely.

A single center bolt works just fine for prop retention and always has, so if you have the option on the OS of only using the center bolt you might want to consider doing so.

Truckracer

My Feedback: (19)

I would not use an APC prop on that OS 300 FT! That engine requires a huge hole to mount the prop .... somethig like 7/16" or something similar to that. Drilling an APC to that size would remove considerable hub material. More than I would feel comfortable being around. Lots of good wood alternatives out there.

With that said, seems like many APC props are not drilled very accurately and the hole is often off center. The molded recess in the back of the prop is the true center and some APC users make drill jugs that key off that molded hole. Also, many racers that use the smaller props drill the center hole oversize and use bushings that pilot in the molded section to accurately center the prop on the crankshaft.

As for drilling wood props, B&B use to make special piloting drill bits that centered in the stock prop hole. They were expensive though. I just use a versabit, drill from both sides just enough to establish the new hole diameter then use a correct sized drill bit to finish drilling through the center of the hub. Harder to describe than to do but the versabits center themselves very well and keep the hole well centered.

rcdude7

My Feedback: (264)

I put my new APC props on a dubro balancer to see where the hole center should really be. I will then use my bench drill press to ream the hole approx where it should be. I find that the final balancing is much easier this way. After you do a few props this way you get a pretty good idea of how far off center to ream the "new" hole.

Campgems

Thanks for the replys. I did some measurements yesterday and it appears that there is some distortion of the molded ring in the back of the 20" props I have. I have three of the 20" APC's and t found one to be DOA as it looked like the engine backfired on a chicken stick. The TE of one blade is cracked mid way to the tip. All the rings are out of round by as much as 0.005" One prop that I would like to salvage had had the center hole drilled 0.046" off center to the ring.

I'm not concerned about the center hub being weakened by opening up the hole to the 1/2" required for the 300FT.The hub diameter is 1.750" so there is plenty of meat around the hole. The two props that were drilled, one being the damaged one, still have quite a bit of thecenter ring left. It's just that the holes are off centere visibally. I'm thinking though that the 1/2 Pilot "and" the six smaller screws in combination may not be a great idea.

Truckracer, you have given me some good ideas. Ilike the bushing in the back of the prop. I handn't thought of going that route, My idea was to bush theentire depth of the prop. I think I like your idea better.

Balancing the props that have been drilled is going to be interesting. The bushing in the back of the prop would be away to center the prop that has been drilled for balanceing.

Last question, Any tips on balancing a prop with the 1/2" pilot or even the10mm hole for the Zenoah?

Thanksagain for your help. Lots and lots to learn. Infact, with the size of the two planes, andthe dificulity in getting them to the field, I'm wondering is I mightbe better sticking to my 1.20 to 1.80 size planes. Maybe when I get oneof these big guys up, I'll be hooked, so I've at least got to try it.

Don

arobatx

My experience with APC props led me to avoid choosing APC for props longer than 17". Their weight at 18" and up seemed excessive for their size, and the performance compared to alternatives rarely held up...that is, except for specific racing props that perform rather well.

Truckracer

My Feedback: (19)

Chad, you beat me to this response. I was going to suggest the same thing.

Campgems

I did notice quite a difference in heaft between the 20x10 APCand the 22x10 Zinger. I just weighted them up and the APCcomes in at 203 gms vs 106 for the zinger. On a size for size comparison, the wood will be half the weight of the APC. That 3.4 oz difference that far forward will make a bit of a CG change, if nothing is changed but the prop. Lots more to consider I guess.

Don

Truckracer

My Feedback: (19)

On larger engines, heavy props tend to shake the airframe at slow idle speeds. You'll really see the effects show up with bouncing control surfaces, etc.