Modify engine re-test the PROP
Have you ever seen a shaker table with two different length rods with a weight at the end? The table shakes back and forth at a certain speed and the rods kind of swing back and forth (oscillate) gently; now you change the frequency the table moves and one of the rods starts going wild and the amplitude of the swing is huge, while the other rod is just bouncing back and forth like nothing has happened. As you change the freq the table moves back and forth again, the wild rod calms down and its back to normal for both. Now you change the freq again and the other rod goes wild and the first rod to go crazy is happy. That is harmonic or natural frequency. This is what happens with metal props. They are tuning forks. If you vibrate them just right they will oscillate wildly. This oscillation can get bigger and bigger and the bending and twisting can fatigue the prop very fast, like bending a paper clip back and forth. If you bend a clip just a little, a small deflection, you can go forever; however bend the clip just a little further back and forth, it will last only a short time. That is fatigue.
Brantel said:
After talking with a Sensenich rep, he was informed that this combo was not tested with this prop and he was on his own as to its reliability using this combo.
The Rep is right and the post above are right on. Just talking to one that flys that combo who is "still alive" will not give you the"truth". You really need to do flight test and take a "vibration survey".
WE SHOULD PRESSURE SENSENICH TO DO TEST ON RV's WITH 360's AND EI.
Its all about fatigue. These blades have some economic life, say 30,000 hours (just an example). In GA terms 30k hrs is infinite life right, because the prop will be worn down way before 30,000 hours. If fatigue life is more, than the part is not life limited. However a small change like EI or high compression could cut that down to 15,000 hours or even a 1000 hours, who KNOWS? You have to test.
Going 8.5:1 to 9.0:1 with standard mags you are probably going to be OK (at your own risk). However add EI you are taking more risk. Again who knows 1,000 hours or 30,000 hours.
Sensenich should do what Hartzell did, pony up and test at least one RV with EI. I say EI because that's a much more likely modification than HC pistons and may be as critical or more critical? Again who knows without test.
Sensenich makes a prop with performance and value. If you're going fixed pitch it would be my choice. It is after all a design specific RV prop, not just a generic prop. It also is fast, low maintenance and very reasonably priced. I am a constant speed prop guy, but I admire the Sensenich. I love wood props for their beauty, light wight and low cost.
The down side of ANY metal blade/ prop is it has to be tested for max safety. Metal props could be made to be fatigue or harmonic free if they made them thicker and heavier, but that would be a double whammy. It takes performance away (thin is good) and adds weight. So the engineers balance the performance, weight and fatigue.
The up side of wood or wood/glass props is we know wood is a good natural damper of vibration and fatigue is not really a concern with wood and composites. However wood or wood/composite should be tested as well, but often they are not tested even on a stock engine, much less a modified engine. Do you think Catto did testing? Its still a question what MT has tested or not. Unlike Hartzell who has written documents, MT has not really said what they have tested, engine mod wise. I hear they have but have not seen it in writing.
Most wood glass props will die due to impact damage, erosion or material degradation before fatigue. However if I was going with a radical engine, I would consider a composite prop. Since I don't have oodles of money I went with a stock 360/EI and a Hartzell that has been tested. At least I know the critical RPM's. On the plus side of wood, at least when the blade lets loose, the gyroscopic forces of a blade being spit off will be less.
However I personally know of a Thorp T18 that was lost after the wood prop came undone. It happens. I don't think it was fatigue however, it was just a failure.
When you modify ANY engine with a metal prop all bets are off. Hartzell has done the most of any prop manufacture with their (EI) electronic ignition test. However Hartzell has not done any high compression test either. Reading all the info from Hartzell, they seem to have indicated HC pistons alone has the same or similar effect as EI, may be a little less, in their theoretical opinion, since they have not tested it. Combined HC pistons with EI is going it could be worse than just one or the other logically. However vibration does not follow logic, it follows absolute rules of physics.
Hartzell found the old standard HC-C2YK/F7666 prop (not the BA prop) went from no life limit to about 8,000 hours with EI or high compression pistons. It's totally unscientific to make correlations to Sensenich, but for grins and giggles. You can assume you have less prop fatigue life than you would with a stock engine. Helicopters are full of life limited parts. Again like a broken record, who knows if it's not tested. I think Sensenich should do an EI test for their reputation.
As an individual you can have a prop vibration survey done by Hartzell for about $10,000 to $25,000, rough numbers. The Prop company may be willing to do it free or discounted if there is some interest in it for them, like a new engine, e.g., IO340 or IO390. If its a new new market or product for them than they will want to do it.
As a group of Sensenich's customers, Vansairforce, we should ASK them to do it, free. Some one can volunteer the use of their plane, time and gas to do the test and they would do the technical testing and data collection. I think it takes three days or a full weekend if the weather is good. It would be great if Sensenich did this EI test, since this is what their customers are doing with their prop.
A Sensenich prop failure and lawsuit would cost more than doing a free vibration test.
