Jerry's got it right: the issue with a metal prop--typically aluminum, but it would be the same with steel--is that they have a fatigue limit, and poor damping. The best way to understand this is to go look at what a company like Sensenich goes thru to get a fixed-pitch metal prop certified on a specific engine. You have to look at vibration modes probably up into the 15-20 range. If you hung the prop by the hub and hit it in various locations with a hammer, how would it ring? Those are vibration modes. And there are a bunch. If those modes are excited by the pulsing of the engine at any allowable rpm, then without sufficient damping the amplitudes are large and fatigue sets in quickly. Part of the testing is done on the engine with strain gauges installed and monitored during operation. (I've been thru something like this with a turbine blisk.) It is far worse for a model engine because the higher rpm means a lot more fatigue cycles are accumulated very quickly. It MAY be worse because most of the engines are single cylinder.
If you want to understand the sensitivity of prop geometry to this problem, go read the accounts out there in probably the '60's and '70's on homebuilt aircraft like the Thorp T-18. The plane had plenty of speed potential but needed a shorter prop and more pitch. So a few guys cut down existing metal props and increased the pitch. And then they started shedding blades in flight. You can imagine that it might tear the engine off the mounts? And it might not be recoverable...so the plane goes down?
There are (or were) commercially available metal props for homebuilt aircraft that had a placard warning that it could not be operated between specific rpm values due to excitation of a particular vibration mode. It happened to sit near a cruise-power setting because the prop manufacturer tried, but could not tweak the prop enough to get rid of the problem. Needless to say, I didn't buy and install that prop....
The model prohibition is sensible because: (a) it is a real phenomenon not some bureaucratic dictate; (b) the only way modelers could achieve a workable metal prop is by cut-and-try methods thereby exposing themselves to high risk; (c) the alternatives are easily available. My thought is this: if you want to try it in your garage and have lots of knowledge and safety precautions then it is your experiment. But be sure you have very good health insurance. (Don't think that the AMA insurance will apply.) But don't bring it out to the field and subject bystanders to that risk who may not understand the situation.
But that prop looks like it will make a super-nice letter opener....
Topic: What's Wrong With Metal Props? (Read 2726 times)