Prop balance ART and Science
jclark said:
Soooooo......Any additional "tips" on getting that ultimate smoothness?? ... rather be called "smooth" than "vibes" ;-) James
Oh yea, I hear you. I spent lots of time on this. This topic is not an exact science, but it sounds like your at 90% and trying for the last illusive 10%, I'll tell you what I SWAG (Scientific Wild Ass Guess) about the conical mount.
First, there will always be ranges where the engine goes thru harmonics and mine would also go thru it reducing RPM in the pattern. Since it has been a few years since I sold her, I recall it was in a very small band somewhere in the 2100-2300 rpm range and was not an issue since it was only for a second or two when slowing in the pattern. I also recall 2400 was OK, smooth but at 2700 rpm it was supper smooth. Since I did not cruise at 2700 RPM I only got to experience this smoothest RPM when I was racing or taking off. The Lycoming has these harmonics every 300-400 RPM.
As I said getting the Lord Corp conical mount Vibration isolators made the most difference. I speak engineering and have called Lord's engineering a few times. You might want to call them and use some anytime minutes. They are experts on vibration isolation and can give you tips in simple terms. If anything it will be interesting. Get the phone number off their web site.
The second thing that made the most difference was Pop balancing (several times). Not all prop balance jobs are the same. Part art and part science:(Although my prop was a Hartzell C/S prop it is relevant to all props.)
I had my prop balance 3-4 times from two different mechanics , both good but using different equipment and technique. The first guy, an A&I with his own shop who worked on general aviation aircraft had a new style digital prop balance unit, which told him what to do step by step via a LCD display. I had him balance my prop twice. I was not real satisfied the first go, but the second balance was a little better. The second guy, also an A&I, worked on both fixed and rotary wing aircraft. His main business was helicopter maintenance, repair, overhauled and rebuilding. He balanced helicopter blades with the same equipment he used on my RV. His equipment was more manual, had paper tape trace output. It gave no step by step instructions to the operator and looked like a piece of lab equipment. Also I recall he used two accelerometers not one, one on the front of the engine and one on he back. I also recalled he wanted the lower cowl on but had some issues and elected to leave it off.
The big difference is the second Guy had me do checks at various power settings, including full power, cruise power RPM (2300-2500) with the prop off the low pitch stops. (Yes we had it tied down.) I don't recall all details, but the idea is to load up the engine mount and configure the engines power as close to flight settings and loads as possible on the ground.
The second guy produced a noticeable improvement from the first mechanic. I don't recall the ips, before and after, but I could feel it. The key was to balance it at the RPM I flew at. Also mechanic #2 understood the physics behind it. With different accelerometers he was able to estimate the where the vibrations came from: engine accessory case gears, engine recipicating mass or the prop off the front of the engine. Also doing so many he knew what freqs where from where and what was normal. We ended up taking weight off the prop and used less total balance weights. We also checked and rechecked, making iterations until we got the best solution. He had me flight test it and check vibrations at various cruise settings and come back for a check-up and adjustment if needed. None was needed and victory was declared.
The bottom line was the final prop balance made a big difference from the previous 2 or 3 attempts. I am not putting down the automatic prop balance equipment or the first mechanics skill, which I am sure is excellent. However the operator?s skill (art) in recording measurements and applying weights is the key. Many of these automatic prop balance kits are sold to FBO's to make money. The idea of the equipment is any mechanic can balance a prop because the machine tells them what to do. At least that is what the people who make and sell the equipment say. I am sure 90% of the time the average mechanic with one of these devices can get good results, but if you want that 10%, I would shop around, skill and judgment still count. The other part is experiment with ground checks and flight test, so if you have access to the equipment you can play with it by adding and removing small amounts of weight or changing weight location.
The interesting thing is finding and isolating where the vibration is coming from.
If your accessory case (on the back of the engine) is vibrating, prop balancing will not help. There is only so much prop balancing can do. If your engine recipicating mass is out of balance the prop may be ample to compensate. Also make sure nothing is touching the airframe HARD. Everything touching the engine is a path for vibrations: baffles, airbox, cowl inlet seals, control cables, hoses, wires and brackets. Look for things rubbing (too much) or that can transfer the shaking to the cowl, firewall or engine mount.
Again it is a little physics and a lot of ART. If you want to take it to the next level you could trade out a borrowed prop and see if it makes a difference. I think you said your prop was wood. Could it be out of track? My guess is you are close to as good as you can get by making sure the ignition is correct and you have even power pulses from each cylinder (EGT/CHT). With the Lord engine mounts and prop ballance there is not much else to do but go flying. You might go fly other simular RVs right after flying your RV and see if there is a huge differnce. Other wise may be your friends are right and you are crazy.
Cheers George
PS The helicopter shop I went to (4 years ago) was at Snohomish, WA - Harvey Field. The FBO next to the helicopter service is called Snohomish Flying service:
http://www.snohomishflying.com/sfscpatf.htm
