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Old Oct 10, 2012, 07:38 AM
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Flying = Falling (Slowly)
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Tulsa, OK
Joined May 2004
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Quote:
Originally Posted by Larry Jolly View Post
Don,
I actually am not worried about the motor or mount integrity, I know how to mount a motor. But I am worried about an undetected cracked propellor lug.
I witnessed an accident at Visalia that could have been very serious. I am also concerned that a competitor may attempt to fly a damaged prop, gambling that it is not so bad that he can't get through that last round. Please remember I am blue skying here and trying to get an idea of the direction this event will trend. What is obvious to me is what I have said all along. 200 meters is too high it takes the soaring skills out of the equation. But it would be useful to get the 2 function class high enough to make a contest out of it. My best Regards Larry
Larry,

I share your concern about propellers. My observations concerning hard landings, as I mentioned, were not an advocacy of them, just a prediction that we will see more of them as we get more familiar with this stuff. And to be sure, I don't foresee F3J type landings ever. But landings which come in fairly slowly at a foot and a half or so and "gently dump" at the end are adequate to stop inside an 80 inch circle.

I would be interested in knowing more of the particulars about the Visalia incident. In the course of my testing, I have had a number of occasions where props have come off and one particularly frightening occasion where a relatively new prop blew up on my test stand, embedding one blade deeply into the solid core door that was my bench top. I keep harping on propeller safety and hope some of this catches on before something bad happens.

At the simplest level, props which are attached to shafts with set screws should never, never, never use two diametrically opposed setscrews. These WILL come loose. Secondly, hubs which use setscrews should ALWAYS be attached to shafts which have a matching flat ground on to shaft. This should be done precisely and not with a hand file. In addition, I check the tightness of the hub each flying session (and after any hard landing) and change the set screw about once a month.

A little more insidious is the problem of ignoring the RPM ratings on our props. The prop I blew up on the bench was a fairly large Aeronaut and I was operating it well within the manufacturer's rating -- I thought. Aeronaut publishes its ratings on its website and it is important to note that they decrease very significantly as the propeller diameter increases. The prop I was testing was mounted on one of Vladimir's offset (not twisted) hubs. In corresponding with some of my friends in Europe, we determined that you have to derate the prop max RPM by about 25 percent when you use an offset hub. And it is likely that I was operating that prop in the danger zone. It turns out that even on regular hubs it is very easy to put together combinations, especially as Aeronaut props get to 17 inches and up which exceed the prop rating. In addition, the material used in the Aeronauts is a molded carbon infused composite which simply does not have the properties of laid up props like the RF. RF does not publish its max RPM ratings like Aeronaut, but they have furnished me, on occasion, ratings for the specific propellers that I have used. And, as one might expect, these are much more robust than the Aeronaut ratings. Empirically, I had a piece of paper get sucked into an RF on the test stand which promptly destroyed the prop. But comparing the stub that remained at the hub with the Aeronaut, and taking into account the much higher rating of similarly sized RF props, I doubt that landings short of full on F3J landings would do much damage to the RF's. I do not have the same trust for the Aeronauts.

Your point with respect to propeller damage is very well taken. But even more importantly, our guys need to become educated on the proper application of props to their power combinations.

Happy Landings,

Don
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