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Educate me

GyroRon

Active Member
What is the ins and outs of wood verses metal props on our planes?

I have a RV-4 with a 160hp O-320 and a Metal Sensenich prop and it has a vibration that bothers me. It is not all that bad, but it is certainly not as smooth as I would like it to be. The builder of my plane noted a vibration on the 1st flight of the plane and the only fix I believe he did to fix this was to rotate the prop 180 degrees, which did not make it any better. I asked him his thoughts on this and he said he thought it was the engines carb causing this - ??? - :confused:

I had the plane in a shop earlier this year to do a dynamic balance but after each run the computer asked for more and more weights to be added in various places on the spinner and the vibration at high rpms got no better each time, and the low rpm vibrations got worse. So we put everything like it was when I came into the shop - no added weights, just prop and spinner.

I was thinking of possibly ordering a new prop for it - the prop on it now was brand new when installed.... 70cm7s9-0-81..... and now has about 240 hobbs hours on it. Was thinking of trying a wood prop and don't know why one would be better than the other etc.... I have heard other pilots tire kicking about planes on trade-a-plane and Barnstormers.com and when the plane has a metal prop it is as if that was a bonus or something ??? :confused:

The only limitation my prop has that I am aware of is to avoid operations above 2600rpm and for the most part I do not run it that fast - I may exceed 2600 rpm in a dive but only for a few seconds. I do run the engine real close to 2600 rpms in cruise if I am in a hurry to get somewhere, wonder if this is okay....?

Anyhow just educate me please. I got a chance to fly a friends RV-4 with a O-360 on it with a wood prop and it was very smooth, almost night and day comparied to mine.
 
This could be totally un-related, involved an RV6A, which might differ in construction for this particular area,

but I know of a 6A, that did not have the brackets installed, that tied the forward floor stiffiners to the front of the two main spars. It was quite a source of vibration until the brackets were installed. Just mentioning this, since you didn't build the plane.
 
I have no idea if something like that is even supposed to be in my plane..... Anyone else know?

Also about props in general, does it matter - other than for hand starting a engine - if the prop is in a specific spot on the crankshaft? I mean there is 6 bolts so I could take the prop off and rotate it around one bolt hole at a time to see if it gets smoother - this was something another pilot advised me to try - Would this be considered good or bad or ????
 
There is no problem with rotating the prop one bolt at a time to try to improve vibrations. The "standard clocking" is usually the smoothest on a 4 cylinder opposed engine, but not always. I have a friend with a Cherokee 180 with the prop clocked vertical.
A wood prop will always be smoother than a metal one because wood absorbs it's own vibrations.
Mel...DAR
 
RV-4 shakes

I had the same thing in my -4, but I had conical mounts and an extended hub constant speed hub Hatrtzell.

First not all prop balances are the same. Equipment, technician are all factors.

I went to two mechanics and went back for a rebalanced balance twice with each guy. The forth time was the charm. The guy that got the best results was a both a Helicopter/airplane guy. His equipment was more "Laboratory" like, less automatic. The other guy was good and his newer looking device was self-prompting with self-diagnostic. The first guy way a good AI/A&P with his own business. The step by step tester unit that tells the operator what to do is fine I am sure. The chopper guy seemed to understand the physics more and knew where vibration came from and the typical frequency they are found at. Like you can vibration from the accessory case of the engine. The prop is part of a system and you have to balance the whole system (engine, crank, accessory case). If the engine has some out of balanced reciprocating mass (pistons, rods) you can only do so much with weight on the prop. Get a guy who knows what the theory is. Many test equipment companies sell their stuff as equipment anyone can use to make revenue. The first guy was good and did a good job, but the second guy was a balancing artist.

The helicopter guy also he had me tie the plane down and run it up to very high power (full), higher than the the other guy did. He also had me run it at cruise power and exercise the prop. His test equipment has a paper graph tape and I guess he was determining the best compromise for balance. All I know is when I started the process of getting the vibes down to when I declared victory, there was small improvements each time and the last time was noticeable. New engine mount isolators where a big plus.

I had another issue, comical mounts. I switched from the 50's rubber bushing style donuts to the LORD Corp. vulcanized rubber steel vibration isolators. They cost an arm and a leg but this also made an improvement. I made the change to the better rubber engine isolators after my first prop balance, so I had to rebalanced the prop. The mount change may account for at least 1 of the 4 iterations. Even if you have Dyna-focal consider LORD corp engine mounts. They are the best. Call LORD and talk to a customer service engineer. They have different isolator models and they know homebuilts. Vibrations is a bit of art, magic and science. (I have an engineering degree and vibration is a complex dynamic; they know what they are doing and have been doing it the longest.) Consider new LORD mounts if the ones you have are old or possibly not matched for the JOB. If you have conical and are using the old rubber bushing washer set up you MIST replace them you will never get it smooth.

IN the RV-4 I found my plane was smoothest at max high RPM 2750 (Gov was adjusted a little over 2700). I mean it was sowing machine smooth. There where lower RPMs that where smoother also (don't recall the exact numbers) like 2400, 2100 rpm. In a word the smooth RPMs where in between RPMs that where noticeably less smooth. 50-100RPM could make a difference. With a C/S prop I had the option the adjust RPM more than a fixed of course. Found I would be at 2750 or 2400 RPM most of the time. The O320 does not have the RPM limit like a O360, but I observed it and never cruised below 2300 RPM, which was an OK RPM for smoothness.

The moral of the story is the RV-4 is a compact plane with the engine a foot from your FEET. The RV-6/7/8/9 are all bigger planes and have more structure to isolate the pilot. RV-6/7/9's have big flat cockpit floors that hum from exhaust. The upper engine mount on the RV-4 supports tie to the longerons that run next to your shoulders, so vibration is OK and normal. Just make sure you don't have cables, wries , baffles or any thing that can transfer Vibes to the airplane. Some RPMs may be annoying; just avoid them. The LORD mounts will help; I would stick with the Sensenich, but would check the tracking and blade angles are the same. A good technician can tell where the vibration is coming from with two or more accelerometers. It will take work but you can improve what you have, but it will never be smooth like a Japanese motorcycles since it is the Harley of the air. Like the Beach Boys sang, I'm feeling good vibrations...

George
 
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Okay to be clear on one matter..... It IS ok to put the prop in any position on the crank? One or two pilots have been telling me it is ok, a few others say the prop is supposed to be in a specific spot to act as a counter balance for the engine.... This is one thing I think I can easily try to see if I can find a difference with.
 
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