Hartzell has done their homework
Low n Slow said:
Can anyone shed some light on this? I've seen several threads talk about various engine types and CS prop combinations that shouldn?t be matched.
Yes you can use a 320 with C/S prop and high compression pistons but its likely not tested, so there are unknowns. Here is some generic info on HC website.
http://www.hartzellprop.com/kitplane/index_kitplane.htm
If you use a different model prop that was never on a certified plane than its an unknown. Does not mean its unsafe, just unknown. Take a stock approved engine/prop combo and modify the engine, you are back in unknown land. However to Hartzell's credit in supporting experimental homebuilders has tested electronic ignition. However the air frame even is a factor. Again Hartzell has tested RV's.
Here's a spread sheet summarizing a partial list of engine/prop limits with different mods: high compression pistons, electronic ignition, exhaust and FADEC.
This is all around the 360 for a reason, I explain below. Of interest to all engines is the CYA note on the bottom.
Low n Slow said:
Can you mate a Hartzell CS to a 0-320 with high compression pistons? And do all 0-320?s advertised as 160hp have them?
I am going to make a bold statement, the 360's are MUCH worse on props than 320's (not real bold its true). The power pulse of the 360 is much more powerful and its not just the extra 20 hp. It has to do with vibration and harmonics, which has to with bore, stroke and the whole power section, combustion chamber, piston, rods and crankshaft. They have different natural frequencies.
Low n Slow said:
What about electronic ignitions and Hartzell CS props?
As mentioned above. If you are talking about the new BA prop and blade than you can run electronic ignition. If you have an older Hartzell prop you may have some restrictions. Hartzell has tested electronic ignition with the 360 engine and older popular prop (C2YK/F7666). They recently documented this. You can go to their web site and look it up.
HOWEVER most effort has been on the more critical 360's, not the 320's, which as I mentioned are not as critical. If you have a specific prop and engine combo call Hartzell.
If your engine is modified you may be out of luck for a definitive scientific answer. If you go with high compression pistons I am sure they have not tested it yet, or at least published that data. Hartzell does not test HC pistons very much (I asked). For one thing they don't have to. Also as a mod, high compression modifications is not as popular as electronic ignition for homebuilders, so Hartzell is less likely to test it.
Remember we have experimental planes, but they do support the us and do know the effects in general of what engine mods do. Again the key word is its unknown until tested. If you go away from one of the approved combos or engine mods (aka electronic ignition) you are a "test pilot".
Both HC pistons and EI (especially on the 360's) really amplify the power pulse and reduce the fatigue margins on the prop. So if you go with a IO340, or a 320 with high compression pistons and/or electronic ignition there is an UNKNOWN. However 320's in stock form are pretty mild on the props comparatively to 360's.
Hartzell can do a vibration survey on any plane with any engine, modified or not, but its going to cost $8,000-$20,000? They have to instrument the plane and do flight test. If there is a popular engine or mod, they will be more likely to test it, since there is a market. If you have a one off modified engine /prop combo than you will have to pay for it. For example the new IO390 has been tested since it's likely to be a popular engine. I am sure they will get to the IO340. They need a homebuilder with that engine to volunteer to test.
Low n Slow said:
There are a lot of 320's with constant speed props, but when you add HC and EI the waters get muddy. To give you and idea I have a O360 with EI. The prop I have has restrictions in a low rpm range, which was a know limitation even on certified planes (aka Piper Arrow). However with EI an additional limit of blade life, to around 8,000 hours (which is a long time for a GA sport plane). When Hartzell designs a prop they basically design for double the economic life, which is essentially an infinite fatigue life. Props are not usually life limited, like many time helicopters have life limited parts and rotor blades. Most GA plane props are retired because they just wear out, the rock dings that get dressed out and erosion, which makes the dimensions get smaller (slowly). At some point the blades are to narrow or thin. How many hours? Depends on where, when and how you fly. Props can last 10's of thousands of hours. Therefore that is why Hartzells normal goal is to make the fatigue life not a factor. All these engine mods work on reducing the fatigue life. If the engine and prop interact and cause harmonic vibration the blades life could be cut by half or by a factor of 10? Don't know unless you keep a certified or tested configuration or test it. Of course a spiting off a blade due to a fatigue failure or any failure for any reason is bad news day, so conservatism is the usual approach.
Its possible HC may not have much negative effect on fatigue life? But you just don't know for sure unless its tested. Frankly with 100LL possibly going away I would not go with high compression pistons.
I did not bring
composite props up, but for the discussion composite props should be more resilient to fatigue (not immune). By definition they don't have metal blades so no metal fatigue. However the hubs are metal. The composite props are also lighter, which is considered a plus, however I am not really a fan of them. For one these compsite props come from small boutique prop companies, semi-exotic, cost more to buy / maintain and less resistance to some damage and repair of that damage is more complicated. However if I had a radical engine high with high compression, EI, cold induction and fancy exhaust, I'd consider a composite prop, if there was not a metal BA Hartzell tested.
I would be very, very, leery of going over stock or at least 9.0:1. That is my opinion. You start to get into the 9.5:1 range you have an engine that needs very careful set-up and operation.
