RCU Forums - Prop Mounting

Quote:

ORIGINAL: jester_s1

Prop installation is one of those little internet myth depositories that just refuses to die. Here are the facts:

1. The kind of hole doesn't matter. The prop is held in place by friction on the backplate or thrust washer by design. All the hole does is center the prop so that the hub will be in balance while the engine runs. You could get a prop centered properly and then Dremel out all around the engine shaft so that it doesn't touch the prop at all and, assuming the prop was balanced right, you'd never know the difference in flying.

jester_s1 (- 0 years building - 6 years operating - Intermediate pilot) You seem to have a few myths all your own. The size of the hole does matter. If a prop does not touch the engine shaft, high RPM and centrifugal load will gradually move the prop to one side over time. This causes an unbalance which will become noticable and further operation can cause a prop to depart the aircraft. BTDT but not in the last 20 years. :eek:

Quote:

2. You should never have to hold on to the thrust washer to tighten down a prop. If you are doing that you don't have enough backplate friction. I can put a prop nut on finger tight which the engine compression is enough to be able to tighten it down, and I can then turn the engine over with the prop. You'll notice the thrust washer has texturing in it to bite into the prop hub. When you use a plastic spinner, you need to add some kind of material to replace that friction. The best thing is a piece of open mesh plumber's sandcloth or either the drywall sandcloth that looks like window screen material. If you use a spinner with a metal backplate, it will have some texture on it that is adequate to hold your prop.

Wet/dry sandpaper with a 2-400 grit works fine and definitely needed with any spinner especially a metal backplate. The thrust washer has the rough on that side and the grit of the home-made washer should be against the backplate. Tighten the prop with a wrench, NOT pliers.

Quote:

3. (not discussed yet but important) You have to use enough torque, especially with a 4 stroke. The little 4 way wrench you bought for glow plugs probably fits your prop nut. It's fine for snugging down the nut, but break out the box end wrench and really crank down on that thing if you want it to stay on. If you ever use a wood prop, retighten occasionally as the wood will compress a little. You don't need any kind of threadlocker on any engine if you tighten the nut hard enough to begin with.

Excellent 100%

Quote:

4. Do not under any circumstances sand off the sharp trailing edge on your props. They are made that way for a good reason- drag reduction. Even rounding it off a little bit can make a significant difference at the 12-13k rpm you'll be running at most of the time. Many people will take off the casting line at the leading edge and maybe sand away the burr at the trailing edge, but you want the back of that prop to be razor sharp.

Absolute ZERO! There is no reason to NOT sand the back of props especially the fibre ones. Sport fliers don't know the difference nor even need to know for a hundred or so RPMs. There is definitely no need for "razor sharp". YOU can slice all your own fingers as you wish. Please don't try to get newbies to do same. I sanded mine even when I went pylon racing. They are made that way because the mfgers don't have to take any trash off the back since the molds are very tight.

Quote:

5. Balancing is worthwhile. I know beginners don't want to spend money on tools, but $20 for a good prop balancer will save you much more than that in repairs caused by vibration and even engine reliability issues cause by fuel foaming. You'll occasionally see someone tell you they have been flying unpteen years and don't bother balancing props on smaller planes. But once you get a few balanced right and get used to see how your plane runs with them, you'll know the difference instantly when you forget and put a new one on that isn't.

Again 100% Excellent. Not all bad but you do generate some myths of your own. :eek: