Untested Prop/Eng - How do I know if I need a limit?

My partners and I are building a 7A, and want to use one of the Hartzell BA props. We've already talked to Hartzell, and read a bunch of posts on a bunch of forums, but nobody seems to address a very basic question: If we go with an untested engine/prop/etc combination, we are wondering how we would know if we had a vibration problem? Will it be obvious, or something that we won't discover until that first overhaul, or worse, that first blade flying off due to metal fatigue?

At present, we are leaning most strongly toward a Superior XIO-360-B1CD3 engine, or possibly something similar from Aerosport or one of the other shops. This engine has 9.0:1 compression ratio pistons, reportedly delivering 185 hp. It also has dual Lightspeed ignition. Superior's website suggests that there may be RPM restrictions on using this engine with the Hartzell props.

We are also intrigued by the ECi XIO-340 "Stroker," which is an IO-320 engine with 9.0:1 pistons and an extended stroke, reportedly capable of 177 hp, as most of you probably know already.

Probably be a good idea to fly according to other NON counterweighted parallel valve 180HP lycomings Limits. Thats the nature of experimentals, can't test everything.

Prop testing

I too looked into this several months ago. I called all the prop shops listed in Trade-a-plane. All said that only the prop OEM would have the motivation and equipment to do such testing. The number of engine prop combinations that have actually been tested is really very small. Obviously the ignition and fuel system have an impact on the engine pulses. These days there are many different variants. Too many for the OEM to test. The prop OEM's have only tested the basic popular engine set-ups like an off the shelf Lyco. The very nice people at Sensenich said that destructive vibration profiles will not be detectable from inside your cockpit or with typical prop balancing/vibration equipment.

Bottom line -- I was not able to locate a place where such testing for hire could be done.

That is about all I know. Could be I'm way off on some aspects but I did talk to a good many prop shops and people in the business of servicing them. I would be very interested in any additional information you uncover.

Very sincerely, Brian Vickers, RV4 finishing

If you use popular engine and prop models (e.g. O-360-A & one of the Hartzells commonly used on RVs), there is already a quite large successful service history with higher compression, electronic ignition, etc. If there is a problem with harmonic vibrations, eventually some of the "early adopters" will have a prop blade failure and we will read the report of the fatal accidents. You should be OK, as long as your hours in service are quite a bit less than several of the early adopters.

If you go with a new, rare engine like the XIO-340, then there is no service history with these props. You need to decide whether you want to be one of the "canaries in the cold mine", or whether you should use a proven prop with composite blades, like some of the MTs.

At the very least, if you use a prop with metal blades on an XIO-340, you should consider installing a cable from the engine to the airframe, as required by some classes of air racing. The idea is that if a prop blade fails, and the resulting imbalance causes the engine mount to fail, the engine will hang from the cable below the firewall. If the engine were to fall free, the CG would be so far aft that the aircraft would almost certainly be uncontrollable, leading to a fatal accident. If the engine is hanging below the firewall, the CG won't go as far aft, and the aircraft should still be controllable, albeit with a very steep descent, and a loss of control once the engine hits the ground. You might live.

I'm not sure where the airframe end of the cable would be attached to though. I don't think there is any point attaching it to the engine mount, as that is the part that is expected to fail.

I, too, have been wondering how best to safety cable the engine to the airframe.

If the engine mount breaks at the point where it's bolted to the airframe, then that's obviously not a workable attachment point for a safety cable. But, if the engine mount is almost certain to break somewhere else -- perhaps near the engine vibration mounts -- then it may be appropriate to attach safety cables between the engine and airframe at the point where the engine mount is bolted to the airframe.

Is there a recognized engine mount failure point following a prop blade separation? If not, is there enough material in the firewall stiffeners to drill holes for attaching safety cables? Is there some other more appropriate attachment point?

Surely, someone must have tackled this problem already.