Hummelbird Engine Vibration Problem - Need Help??
I have just completed building (from plans) a tri-gear Hummelbird with a 2-cylinder 40 HP Mosler engine. Her first test flight was completed successfully; however the engine vibration seemed somewhat excessive. Has anyone else had a similar experience with a 2-cylinder engine (Mosler or half-VW) on a Hummelbird? Any comments, suggestions, input etc. would be MOST appreciated.
Last edited by Plegg; 10-30-2012 at 03:37 PM.
It's a half vw. It won't be as smooth as a four cylinder engine. That said, you should have concern if it is excessive vibration but its hard to tell what that is if you don't already have experience with that engine. Another builder on this forum might not be able to help either because description is subjective. First hand experience would seem to be the only way to judge this. I would look for someone close by with experience to come and experience the vibration and they can better judge if its excessive. My two cents anyway.
Have the prop balanced on the engine with the engine running. This will balance all of the rotating parts at the same time. It may help a lot. Just be sure that if you remove the prop it goes back on in the exact same position to maintain the balance. Usually costs about $150 for this work. Find a local aircraft mechanic and he will probably know some one who can help.
To Todd Copeland: Thank you very much for your input. So far I have not flown it -- I had a very experienced pilot carry out the initial flight test. He commented on "the vibration as being excessive". This was his first experience of flying a Hummelbird however. Your comments on being subjective are right on the ball. Cheers.
Last edited by Plegg; 11-01-2012 at 04:36 PM.
To: Vaflier: Thank you very much for your comments. I believe you are absolutely right in suggesting that the prop should be balanced with the engine running. As luck would have it, the new occupant of the hangar next door to mine is a professional aircraft engineer. He has seen the H'Bird running & I'll approach him regarding help on the prop balancing. Cheers.
I built and fly a Legal Eagle XL with the Hummel (Casler) 45 hp (1200cc) 1/2 VW engine. Total time on the engine is ~40hrs over the course of the past two summers.
Originally Posted by Plegg
The 45 is a shaker....period. I can see where it would be distracting to a pilot who has never flown a two-cylinder engine. My engine has two distinct rpm bands where the heads are a blur. Fortunately these bands are narrow and not at the rpm I fly during cruise. This vibration is due to the unique harmonics present in a two-cylinder, opposed engine. Casler uses an external counterweight at the prop flange to balance the rotating parts, but nothing can be done about the rocking coupling mode of the two pistons. But you might experiment with the engine mount bushings. Softer mounts might make the vibes more tolerable.
The smaller 1/2 VWs run smoother, probably because of the smaller bore and stroke. But the 45 is a hotrod, bored and stroked as far as the VW case can accommodate. I like my engine, it pulls like a tractor, but I realize it will shake at certain rpm.
If your prop has been statically balanced, you have probably done about all you can do with the prop. If you can get someone experienced with large-displacement 1/2 VWs to listen to your engine run, you will be able to determine if your engine is typical. I don't know the specs on the Mosler 40 and how it compares to the Hummel 45, but I suspect the bore and stroke is similar. Keep a very close eye on the engine mount and firewall during pre-flights to prevent any unseen cracks from turning into a surprise.
Best wishes and enjoy your Hummelbird!
Last edited by Sam Buchanan; 11-01-2012 at 09:05 PM.
I fly with this same engine. Use the counter wieght on the prop and she runs smooth. My does not vibrate. When I removed the prop I put the counter wieght back on in the wrong spot and she had a vibration. I put it back like it was and she runs smooth again.
That wieght makes a big differance in the vibration. Myslef I would not run the engine without this counter wieght. I have pics of it if you need them, its just a piece of steel. When I remove this, for maint. I could weigh it for you.
Let me know if I can help in any way.