Prop selection redux

Slipstream · #1 03-25-2015, 04:54 PM

Last month, I crashed my -7. Crankshaft sheared at the #3 cylinder, suddenly found myself in a glider. I put her down as gently as I could on a gravel spit in the Hood Canal, Washington. .. And then the little tires sunk deep into the pea gravel, and over onto her back she went. Oh well, I'm alive, she's repairable, and now I get make a prop decision all over again.

I was using a Prince Carbon Fiber P-Tip. Great prop, loved it.. Except: it weighs 12 pounds. Weight and balance is an issue: 2 warm bodies, full fuel, zero room in c.g. envelope for baggage. Clearly, the -7 was designed with more weight up front in mind. As much as I'd love a c/s conversion, I just don't want to spend that kind of money right now.

Looking for some thoughts from Sensenich fixed pitch fliers. How do you like them, how's the c.g., performance, etc. Thanks for your input!

Brooks

rvsxer · #2 03-26-2015, 09:39 AM

If you really want to stick with a fixed-pitch prop the Sensenich might be best for efficiency and CG. Pretty heavy. The Catto props (only from what I've read here) seem more efficient through a wider speed range but they are lighter. I have a Sensenich on my RV-6/O-360 and I like it, as my plane would be tail-heavy without it.

DaleB · #3 03-26-2015, 09:45 AM

Catto 3-blade and a big heavy crush plate, maybe? I don't know, mine's not flying yet.

Mike S · #4 03-26-2015, 09:45 AM

First off, sorry about the crash------glad you are OK

Considering your statement it was a great prop, and you love it, you could always go back to the same prop, and add a heavy crush plate. This would address the C/G issue.

But-------is there any possibility the light weight prop may have had a part in the crankshaft failure? Do you know if any TV testing has been done on this engine/prop combo?

gerrychuck · #5 03-26-2015, 10:40 PM

I have a Sensi prop on my O-360 6A. Pretty much impossible to put the cg out the aft end, even with 400 lbs in the seats, 90 lbs in the baggage compartment and empty tanks. I keep thinking about the possibility of changing to a different, probably lighter, prop, but I really like my cg where it is now. No complaints with the Sensi in terms of performance, although I don't have anything to compare it to with this aircraft.

Slipstream · #6 03-27-2015, 09:04 AM

Thanks for the replies! It's been very helpful. I definitely think the Sensenich prop is the way I'm going on the rebuild..

Slipstream · #7 03-27-2015, 09:17 AM

Quote:

Originally Posted by Mike S

But-------is there any possibility the light weight prop may have had a part in the crankshaft failure? Do you know if any TV testing has been done on this engine/prop combo?

That's an interesting question. I don't know of any specific testing of this sort. The side effects I've noticed having a lightweight prop is much less flywheel effect on starting, making it a bit harder to start than you'd expect, and also it's more prone to kickback.

My personal suspicion is that the problem dates back to the previous owner. The logbook shows an overhaul without any clear indication of why it was overhauled. I didn't question this at the time, but in hindsight, it's possible there was some engine damage that wasn't recorded. It passed all the inspections, but I think there are limits to what those inspections can really uncover.

My new conservative attitude about used engines is to put used crankshafts into the "consumables" category -- Scrap it.