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Meeting will be at 6:30 pm at the field on Thursday 12 June 2008.

                                                                                                                                                                                                  

ON THE SAFE SIDE

From the Temple Aero Modeler's Newsletter, Temple, Texas

Propeller Sense
Never use or try to repair a damaged propeller. You may get by with it a time or two, but is the cost of a propeller worth risking injury to yourself or a friend?

If the propeller is visibly damaged, then whatever force did that could also have caused other damage that remains invisible to the naked eye. So, please when you have a damaged propeller, either use it strictly for static display purposes only, or better yet, break it clean in half before discarding to keep anyone else from using it. Don’t even think about using it as a back-up spare.

There are some solid black propellers on the market, which become invisible to the naked eye once they’re spinning. This is a dangerous hazard which can be remedied by simply painting the propeller tips with a bright color. You can even use the paint to help balance the propeller. You do balance your propellers don’t you?

Why bother balancing a propeller?

It won’t hurt the engine any. This may be true, but the vibration and shaking caused by an out-of-balance propeller tends to loosen nuts, bolts, and screws, both on your engine and throughout the model. Here again, it’s a simple matter of spending five to ten minutes to balance a propeller, or risk spending ten hours or more repairing or rebuilding your model. Just consider the few minutes that it takes as a sort of insurance.

When installing a propeller, always use a hard metal washer that’s flat on the surface facing the propeller, in between the propeller and the propeller nut. This washer should be larger than the propeller nut too. The washer is there to give additional surface area to be tightened against. The smaller the washer area, the greater the chance of the propeller being crushed under the pressure of the tightened propeller nut.

When the propeller is crushed at the hub, it can be damaged to the point of being dangerous to use or it can become loose to such an extent that it becomes dangerous. This “crushing” action is also why it is important to recheck the tightness of the propeller nut every so often, especially with new wood propellers. In most cases, the propeller washer supplied with the engine is adequate, so don’t use anything smaller. But again, never tighten the propeller nut directly against the propeller itself. You need more surface area to secure the propeller safely, plus there’s a good chance that the action of twisting the nut tightly into place will tear into the propeller hub.

Propeller Markings
Nearly all propellers have some sort of identification marked on them, be it brand name, propeller size, something else, or all of the above. In addition to noting the size of the propeller, the marking also denotes the front of the propeller, and the front of the propeller always faces toward the front of the airplane. Don’t make the mistake of installing a propeller backwards. You’ll probably get lots of RPM from the engine, but very little thrust from the propeller.

Propeller sizes are almost always marked with at least two numbers such as 10x6. Sometimes there will be three numbers, such as 10x6-12. The first number represents the length of the propeller, or the diameter of the “disk” formed by the spinning propeller. Propellers are usually pretty accurately marked when it comes to their length/diameter.

The second number represents the pitch of the propeller, which is theoretically the distance the propeller moves forward in one complete revolution, disregarding slippage. One might think at first that the angle of the blade would be constant from hub to tip for a constant pitch propeller (one having the same pitch all along its length), but it isn’t so. Remember, the farther out from the hub a given point on the propeller is, the father it travels to complete one revolution. So, the farther out from the hub a given point is on a constant pitch propeller, the smaller its angle will be.

When a propeller has a third number, such as the example of 14x6-12, it means that the pitch progresses from 6 inches near the hub, to 12 inches near the tip. This is called a progressive pitch propeller, and in this case, the angle of the blade might actually be constant from hub to tip, since the progressive pitch has more pitch near the tip than at the hub. Progressive pitch propellers, however, are commonly seen only in sizes appropriated for 1.20 size engines and larger. And, as far as I know, the verdict isn’t in yet on whether they have any advantages over constant pitch propellers.

Some manufacturers of propellers are very precise. There are propellers marked with their pitch out to the second decimal point, as in 8x3.8. Don’t mistake this “second number” as described above. In this example, the second number is a fraction of the first, and has in fact a pitch of 3.8.

Regretfully, the number shown on the propeller representing the pitch is not universally accurate. Some manufacturers are very good in this aspect, while others are downright terrible. In a series of tests conducted by R/C Report, it was found that in most cases, propellers have less true pitch then indicated by their markings.

Not all propellers are created equal. Much of the variations in the way they perform have to do with their shape, airfoils, and the material it’s made from. If you’re tweaking every last bit of power out of your engine, it’s worth experimenting and finding the propeller that works best for your engine/airplane application.

Play it safe, and keep your propellers clean, tight, and balanced.Q

Hope to see you at the meeting