Hummelbird Engine Vibration Problem - Need Help??

[Plegg]

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.

[Todd copeland]

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.

[vaflier]

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.

[Plegg]

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.

[Plegg]

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.

[Sam Buchanan]

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.

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.

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!

[1600vw]

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.

Harold Sweet

[lasermanro]

I built a Hummel Ultra cruiser and installed a Hummel Cestler 1200cc 1/2 VW engine. There was more vibration than I really liked but was able to fly around 2600 to 2800 rpm. At 43 hrs the crank broke in half, luckly I got on the ground safetly. Scott repaired my engine and returned it to me in a good turn around time and the only charge was shipping to him. Re installed the engine and the vibration is even worse than before. The only rpm I can seem to fly at is around 2000 at about 45 mph. I like around 60 to 65 at 2800 rpm but the engine is shacking so hard I cannot even read my MGL display. I cannot seem to find a cure for the vibration and am really frustrated as I do not care for 2 cycle engines. Any suggestions would be greatly appreciated.

Robert Dodge

[nrpetersen]

Are there any specs on how the propeller should be indexed to the crankshaft & TDC? I'm unfamiliar with the 2 cyl VW but there would be some serious balance issues, and given the crankshaft failure you have had, some serious torsional vibration issues with such an engine.

Where did the crankshaft break? Is there a flywheel on the backside of this engine?

[1600vw]

Makes no difference where the prop is, but it makes a difference where you put the counter weight. Bring her up to TDC and put the weight at the 9 o:clock position. This will be where the key way is on the crank, the weight goes there.

Tony

[1600vw]

I had a starter on my engine. I wanted to save weight and removed it, ring gear and all. I found after removing the starter ring gear I had a vibration I did not like. I replaced the ring gear and she is silk smooth again.

My 1/2 VW has no vibration what so ever. I even have a video to prove my point. Actually a couple video's.

http://www.youtube.com/watch?v=TVl6Afk9iuQ

http://www.youtube.com/watch?v=wN5rz5pM8ZA

Tony

[lasermanro]

MY counter weight is in the correct position.I also tried running up without it but really couldn't feel any difference, also I have removed the spinner to eliminate any balance issues with it as I have had 2 spinners crack from vibration. The prop index is a great suggestion as it is indexed differently than before. I will try all three positions when the flying weather improves. Also I am trying another prop ( with the help of Hummel Aviation ) less pitch will put me in a different RPM range. Have run static, still to much vib but need to fly to see if I can find a quiet spot to fly in. I will post any improvements, thx for your suggestions

Robert37023702

[rwanttaja]

MY counter weight is in the correct position.I also tried running up without it but really couldn't feel any difference, also I have removed the spinner to eliminate any balance issues with it as I have had 2 spinners crack from vibration. The prop index is a great suggestion as it is indexed differently than before. I will try all three positions when the flying weather improves. Also I am trying another prop ( with the help of Hummel Aviation ) less pitch will put me in a different RPM range. Have run static, still to much vib but need to fly to see if I can find a quiet spot to fly in. I will post any improvements, thx for your suggestions

Robert

Nice looking plane, Robert...congratulations!

One question out of curiosity, though... the pictures seem to show a lot of space open behind the pilot. Is this just a function of the camera angle, the slant of the rear bulkhead, or just the position of the seat? Most of the pictures online include the canopy, and it might be that my eye was thrown off your al fresco shot.

I've always like the Hummelbird; it seems like a good basic aircraft using modern materials. Well-suited to the tin bashers among us. Had a couple under construction in locally here, a while back, always got a kick out of poking my head inside.

Ron Wanttaja

[nrpetersen]

Back to that broken crankshaft. Where did it break? These torsional vibrations can create a major fatigue problem with the crankshaft. Or he may just be lucky and not operating at a critical rpm.

From another later post above, if there is any sort of flywheel (or starter ring) on the back of the engine, a nasty torsional vibration issue could be happening, with the max stress probably between the front crank throw and the prop hub. I know the writer of that post said it ran much smoother with the starter ring gear on the back, but the problem with torsional vibration, is that its presence isn't apparent at all to the pilot.

Years ago a local Dragonfly owner did something remotely similar (i. e. inertia on both ends of the crankshaft) & the failure was immediate. There was extensive charring of the wood/prop flange interface and the propeller bolts were breaking with alarming regularity.

[1600vw]

Starter ring gear is mounted to the prop hub. Not to the rear of the engine.

Tony

[av-mech]

I highly suggest that you find someone with either a MicroVib II or ACES system and have your rotating mass balanced. The rotating mass consists of the crank, prop, spinner and anything else connected to that which spins. I do prop balancing several times a month at my shop on all kinds of aircraft and I can tell you that it will make a big difference in most cases. The A&P/engineer that does the balancing should be able to work you down to a value of at least .030" ips (inches per second). Values lower than that are possible but as you get closer to zero the need for accurate placement and the amount of weight needed goes up exponentially. If you get a lower value great, but don't let them spend to much time as it just ends up being not worth the time invested. Weight will be added to your spinner screws during several runs. When the value is as low as the operator can get it, he will move that weight combo from the outer screws to the spinner backplate. He should check it after moving to assure that the value did not change to much.

Broken crankshafts, cracked oil coolers/baffling/brackets/exhaust etc....are many times attributable to a poor distribution of mass about the centerline of the crankshaft. Prop balancing will go a long way to improving this issue.

Remember, with a wooden prop always leave the aircraft with the prop in a horizontal position. If left vertical, moisture will tend to migrate towards the bottom blade and can significantly effect your balance.

Arnold Holmes
AV-Mech LLC
EAA 519850

[1600vw]

av-mech: great info. I always keep my prop horizontal as you say.

By the way, My grandmother was a Holmes and I live at Holmes airfield owned by Dave Holmes.

Small world.

Tony

[Sam Buchanan]

I highly suggest that you find someone with either a MicroVib II or ACES system and have your rotating mass balanced. The rotating mass consists of the crank, prop, spinner and anything else connected to that which spins. I do prop balancing several times a month at my shop on all kinds of aircraft and I can tell you that it will make a big difference in most cases. The A&P/engineer that does the balancing should be able to work you down to a value of at least .030" ips (inches per second). Values lower than that are possible but as you get closer to zero the need for accurate placement and the amount of weight needed goes up exponentially. If you get a lower value great, but don't let them spend to much time as it just ends up being not worth the time invested. Weight will be added to your spinner screws during several runs. When the value is as low as the operator can get it, he will move that weight combo from the outer screws to the spinner backplate. He should check it after moving to assure that the value did not change to much.

Broken crankshafts, cracked oil coolers/baffling/brackets/exhaust etc....are many times attributable to a poor distribution of mass about the centerline of the crankshaft. Prop balancing will go a long way to improving this issue.

Remember, with a wooden prop always leave the aircraft with the prop in a horizontal position. If left vertical, moisture will tend to migrate towards the bottom blade and can significantly effect your balance.

Arnold Holmes
AV-Mech LLC
EAA 519850

The prop on my RV-6 has been dynamically balanced and I agree it can make a significant difference in smoothness.

But I have a question. Where would weights be placed on the prop of my Legal Eagle?

3703

There is no prop spinner and the prop bolts are very close to the prop shaft. I suppose a prop tip could be trimmed but that sounds a bit extreme. Suggestions?

[av-mech]

Sam,

Engine configurations like yours are difficult to do for the stated reason that you have nowhere to hang weights. It is possible to add heavy washers under the prop bolts. A number of problems arise doing that including: needing a longer bolt(which changes weight on it's own), the weight added not being very far from the center line of the crank, which in turns requires more weight that if you could add at a location farther out (think lever arm). You most certainly DO NOT want to trim the tips. Blade length being equal on both sides is critical, as is blade tracking. That is something I did not mention to the original poster and will make a difference as well.

To: 1600VW, make sure that each blade tip passes in the exact path of the other blade tip. An easy way to do this is to remove spark plug from cylinder so engine is easy to turn over. place prop in vertical position and place a cardboard box (or anything else you like) under the lower tip. Try to get it within 1/4" or so. Using a good sharpie marker, place it on the face (back side if looking at the aircraft from front) and gently lower it down until you make a mark on the box top. Next, gently rotate prop so opposite blade is in the lower position, use your marker again to make a mark on the box top. the two marks should be within 1/16" of each other. If one is displaced forward or aft of the other, retorque your prop bolts until the tips are marking at the same point. You may need to loosen one side of the bolts while torquing the other side. Go slowly and don't get aggressive, just use baby steps.

[1600vw]

Its not a tracking issue. Don't get me wrong you need to check tracking but this is not the problem with my setup.

The weight goes on the prop bolts in the front of the prop, you will need two longer bolts for this. Bring the piston, does not matter what one they both are at TDC together. Bring the piston to TDC and mount the weight at the 9: o:clock position. This is standing in front of the airplane looking at the engine.

If you have never seen one of these weights its a piece of steal about 3inch by 3 inch by 1/4 inch.

I left mine on my engine and Scott at Hummel has it. Call Scott tell him what you need and he will help you. Washers is not the way to do this, they will do nothing.

Tony

[av-mech]

Washers is not the way to do this, they will do nothing.

Tony

Technically that is not a true statement. Adding weights under the prop bolt will have an effect, just not a big one. It is not likely that adding weight under the prop bolts would be enough to get you where you need to be but adding weight will change the force vector and thus the distribution of mass about the crankshaft.

Just saying.

[Sam Buchanan]

There is confusion due to two different topics being merged together. 1600VW is referring to the crankshaft balance weight that Hummel Engines uses to balance the rotating mass of their large engines. The other topic, the one I was participating in with av-mech, has to do with dynamically balancing the prop using weight (usually bolts and/or washers) added to either the prop spinner or hub.

The two topics are entirely different and don't overlap--maybe this will clear things up a bit.

[1600vw]

Sam with all do respect you are not helping this man. From the pics he posted he is missing a part. That part is the weight I speak of. Until he gets this weight he is chasing ghosts. He will never get this to run like it suppose to.

Now I do like your post for this could be my problem. I have everything setup just as Hummel states but have a vibration that was driving me nuts without my starter ring mounted to the prop. When I removed this starter gear or ring when I removed my starter then flew my airplane, this vibration although very small drove me nuts. I reinstalled the starter ring and she is smooth as silk, no vibration at all. None.

I called Scott at Hummel and talked with him about this. He told me his thought was the mass of the ring gear was taking away some torsional vibration I might be having. But I always wondered about an out of balance somewhere. I checked tracking. I would call it perfect. Maybe off by 1/32 tip to tip.I always keep my prop horizontal not sure what else to check.

Tony

[Sam Buchanan]

Sam with all do respect you are not helping this man. From the pics he posted he is missing a part. That part is the weight I speak of. Until he gets this weight he is chasing ghosts. He will never get this to run like it suppose to.............
Tony

Tony,

I have no idea what photo you are referring to unless it is this one of my Legal Eagle that I attached to my earlier post concerning how to balance a prop that has no spinner:

3707

The crankshaft weight is indeed attached properly in this photo, it is at the 9:00 position but behind the prop so it isn't visible in this photo. Is this the photo you are referring to?

Here is another image showing the weight on the backside of the prop hub:

3708

The other photos in the thread are of a complete Hummelbird with a spinner so it would be impossible to tell whether or not the weight is installed.

I hope this helps to clear your confusion of which threads are referring to the Hummel weight and the others that are referring to dynamically balancing the prop when no spinner is installed (posts #16-19).

[1600vw]

Yes, sorry I was referring to those pics. I like how you did that or put the weight on the back side. I might have to try this. Not sure but my cowl might get in the way.

Tony

[nrpetersen]

I'm surprised the Hummel 1/2 VW engine can be balanced with only only a single predetermined offset weight. Given the amount of engine modification (i. e. cutting away 1/2 of it!) theoretically I would think there would have to be a second offsetting weight on the rear of the engine to get reasonable dynamic balance as well as static balance.

There are two components to every balance situation, in-plane (static) balance and - moment (dynamic) balance. (In addition, there can be 1/ rev propeller blade pitch errors, which would have to be handled by installing tapered shims behind the prop.)

True dynamic balance is not obtained with simply rotating everything and measuring at a single point. It cannot be achieved by having the correcting weights in a single plane. Dynamic balance involved having two motion sensing transducers, one near the prop and one near the rear of the engine so that yawing and pitching of the engine can be sensed.

Also something is funny about an unbalance situation in which adding a starter ring gear (which should be balanced by itself) makes it run smoother. And pure torsional vibration affects the cyclic stress in the crankshaft, and may even be acoustically sensed, but is generally is not reflected in the pilot's seat.

This doesn't address 2/rev vibration, which is a big contributor to torsional vibration. However, there isn't much that can be done about that.

[Sam Buchanan]

I'm surprised the Hummel 1/2 VW engine can be balanced with only only a single predetermined offset weight. Given the amount of engine modification (i. e. cutting away 1/2 of it!) theoretically I would think there would have to be a second offsetting weight on the rear of the engine to get reasonable dynamic balance as well as static balance.

I don't have the answers to all your questions but can tell you the weight is customized for each individual crankshaft. Mine has one corner ground down where the tech fine-tuned the balance and it is marked so it will be installed exactly as it was on the bench.

[1600vw]

If I was to redo this or build one myself I would leave the full case or as it is with 4 cylinders. Just remove the back two pistons and rods then block off the jug holes. From my research really no weight penalty in keeping a full case and not cutting it in half. Keeping a full crankshaft should help with the balance.

Tony

[Sam Buchanan]

If I was to redo this or build one myself I would leave the full case or as it is with 4 cylinders. Just remove the back two pistons and rods then block off the jug holes. From my research really no weight penalty in keeping a full case and not cutting it in half. Keeping a full crankshaft should help with the balance.

Tony

Not cutting the crank will make the engine impossible to balance. The crank weights are designed to balance in concert with the rods and pistons. Remove the rods and pistons for the two unused cylinders and leave the crank untouched and you have an unusable engine.

The full-case 1/2 VW is about five lbs heavier than the cut case, but this difference is absorbed to a degree by the need to build an engine mount for the cut-case. Biggest advantage of the full-case version is not needing the extensive machining required by the cut-case. But magneto installation is a bit simpler and lighter on the cut-case.

[nrpetersen]

I don't have the answers to all your questions but can tell you the weight is customized for each individual crankshaft. Mine has one corner ground down where the tech fine-tuned the balance and it is marked so it will be installed exactly as it was on the bench.OK that passes the theoretical test. A single offset weight will work for both static and dynamic balance - providing it is properly located along the axis of rotation. I'm still surprised (amazed?) that just happens to be immediately behind the propeller.

IOW there may still be a first order unbalance.

[1600vw]

I'm surprised the Hummel 1/2 VW engine can be balanced with only only a single predetermined offset weight. Given the amount of engine modification (i. e. cutting away 1/2 of it!) theoretically I would think there would have to be a second offsetting weight on the rear of the engine to get reasonable dynamic balance as well as static balance.

There are two components to every balance situation, in-plane (static) balance and - moment (dynamic) balance. (In addition, there can be 1/ rev propeller blade pitch errors, which would have to be handled by installing tapered shims behind the prop.)

True dynamic balance is not obtained with simply rotating everything and measuring at a single point. It cannot be achieved by having the correcting weights in a single plane. Dynamic balance involved having two motion sensing transducers, one near the prop and one near the rear of the engine so that yawing and pitching of the engine can be sensed.

Also something is funny about an unbalance situation in which adding a starter ring gear (which should be balanced by itself) makes it run smoother. And pure torsional vibration affects the cyclic stress in the crankshaft, and may even be acoustically sensed, but is generally is not reflected in the pilot's seat.

This doesn't address 2/rev vibration, which is a big contributor to torsional vibration. However, there isn't much that can be done about that.

I believe you need to speak with this man..

http://www.betterhalfvw.com/engine.htm

[Sam Buchanan]

OK that passes the theoretical test. A single offset weight will work for both static and dynamic balance - providing it is properly located along the axis of rotation. I'm still surprised (amazed?) that just happens to be immediately behind the propeller.

IOW there may still be a first order unbalance.

Field history has proven this method to be reliable and yields an acceptably smooth engine. Keep in mind we are talking about a very short crankshaft.

3716

The Hummel version of the 1200cc engine uses an additional bearing on the prop hub (not shown in the image above, that is a different crank) that beefs up the front bearing considerably.

[1600vw]

Yes my engine is a Hummel. But nothing special about that crank. That is a standard crank cut in half. If you look at the end without the taper it should be machined to except a impulse coupler but nothing special about this crank.

Tony

[Sam Buchanan]

Yes my engine is a Hummel. But nothing special about that crank. That is a standard crank cut in half. If you look at the end without the taper it should be machined to except a impulse coupler but nothing special about this crank.

Tony

Tony,

Reread my post, I specifically stated the image was NOT a Hummel crankshaft. I used the image only to show the length of a 1/2 VW crank.

[1600vw]

That is the same crank. Nothing special about a Hummel crank. Its a standard crank cut in half. Here is a pic of the bearing you speak of. Along with the other end machined for a Impulse Coupler

37183719

[Sam Buchanan]

Sigh......I hate to belabor the point but the image I used is a 78mm Scat crank:

http://www.vermilyea.com/engine.htm

The 1200cc Hummel/Casler engine (45 hp) has an 86mm crank, and I'm pretty sure it uses Chevy journals in order to achieve that huge stroke within the confines of the VW case.

Once again I apologize to readers for continuing this thought, but having incorrect conclusions drawn from my posts is a pet peeve. I hope this clears up the confusion.....again.... :eek:

[1600vw]

If you really want the scoop on these cranks and any other part of this engine do not ask here. But do what I did..Call Scott at Hummel, he will set you straight. No guess work. As to the crank, Scott did not tell me any of the things Sam mentioned. Does it mean he is wrong. No. Just that Scott did not tell me any of this when I spoke with him about these cranks.

I would send you more pics but Scott has my engine.

Tony