Fan balance

[pgkevet]

I'd noticed the fan had thrown a magnet..not that i had a governor on there.. but obviosuly it was going to be out of balance so i tidied up the fan hole and glued one in.
I've been trying to be less slapdash about stuff so thought I'd check the balance afterwards.
The clutch was still on there but it's no bad thing to check them together and my little old 450 blade balancer worked by reversing the knurled nuts so they coned in each side. Boy, was the thing way out!
So I pulled the clutch off and checked that seperately..perfect.
To check the fan itself I found i could wedge a bearing in it's centre and teeter that on a spare shaft. Way, way out. I stuck different size bolts to the fan magnets to get an idea of how much out.. it took an M3x16 caphead to balance it. You can't drill that much out of a plastic fan!

Invetsigating further and the fan is lots thinner one side than the other. Almost certainly this happened wearing against the fan shroud. I changed the shroud a couple of years ago (blitz was secondhand) but never noticed fan issues back then.
There are no wear marks on this new shroud. With the blitz belt drive any vibes would have been more hidden

Total replacement parts ordered are the skid bow, belt and a new fan which are unlikely to get here until after the hols so jobs left to do are to clean up and replace bearings in 2 engines and if I can find a dry enough bit of land (or at least thin enough snow to stamp down enough) then start flying and tuning 600/700 and the 500...

[KevDavies]

You could use that heavy spot on the fan to cancel out the engines inherent lack of balance. You need the heavy spot exactly OPPOSITE the big end pin and youll find that the engine is silky smooth!

[Andy from Sandy]

[QUOTE=KevDavies;998545]You could use that heavy spot on the fan to cancel out the engines inherent lack of balance. You need the heavy spot exactly OPPOSITE the big end pin and youll find that the engine is silky smooth![/QUOTE]

That used to be a recommendation from Colin Mill as I recall about only fitting one magnet in the fan.

[KevDavies]

[QUOTE=Andy from Sandy;998547]That used to be a recommendation from Colin Mill as I recall about only fitting one magnet in the fan.[/QUOTE]

Bingo!! Thats how I always do fans with twin cones. I cant do it with the Rappy unless I spend one arm a lower leg and a right testicle for a QUK fan

[pgkevet]

[QUOTE=KevDavies;998545]You could use that heavy spot on the fan to cancel out the engines inherent lack of balance. You need the heavy spot exactly OPPOSITE the big end pin and youll find that the engine is silky smooth![/QUOTE]


So why don't folk add weight there normally? Wouldn't be hard to do - just need a witness mark on the crank or cut a woodruff type slot on the crank to counter it? Doesn't the mixture chamber on the crank achieve the same? Intrigues but wondering if being wound up too..could start balancing my cranks..
And when the snow melts I've still got that tree to shift..(and a new woodstore to build)

[KevDavies]

the crank will be balanced already. the extra weight on the fan dynamically balances out the weight of the piston assy. and the easiest way to do it is take the backplate off and line up a mark at the heavy spot of the fan and have it opposite the conrod pin

A lot of people do do it but its one of them black art things no one really talks about

[pgkevet]

..so the advice is to re-use this fan thus and keep the new one in the spares box when it arrives? Remember it's the weight of an M3x16 out of balance..

[KevDavies]

how heavy is a 90 piston n conrod??

Its completely up to you buddy on a screw on fan youve no choice BUT to balance the fan and just fit it on. But on double cone fittings, you can dynamically balance the whole assembly AND dti the fan mount so its 100% true aswell.

And screw on fan helis work fine, but if it was me with such a boutique heli as the Avro i would dynamically balance everything and set the run out so it was as smooth as possible

[pgkevet]

One of the features of the blitz is that the fan/clutch mounts between dual collets and states doesn't need to be dialled whereas it was a real PITA on my spectra getting the run out under a thou but there the zenoah crank is tapered.
The chewed edges of this fan wouldn't allow dialling either.

I don't understand how to dynamically balance it - static yes. Fancy an omelette :-)?

either way I'll decide when new fan arrives.

Just got it cleaned up and time to cook some bearings...

[coolice]

Hey.

I see the logic with this to counteract the the piston at top dead centre, but surely then the fan would be out of balance for the rest of the engines rotation cycle and cause vibration? Essentially you'll still be sticking weight out on the end of the fan and spinning it at high rpm with no counter balance weight to offset.
Also isn't the weight of the piston offset by the crankshaft design anyway? With engine designers paying attention to this for helicopter applications.

CSM went on to sell a dedicated counter balance weight from memory.

Would a flybarred model be more tolerant to this procedure than an FBL model that can be effected by vibration? Speaking from recent experience of a twitchy Raptor 50 & vbar combination, for which I've got the fan/hub/clutch assembly to balance.

I must admit it's an interesting subject, I've never come across it in all my years, balance the fan and dial indicate it as required. I wonder if Mark Christy has heard about it in the top F3C circles?
.

[KevDavies]

Ian, you need to think in a slightly different way than static balancing. Like with wheels on your car, where they are spun up and the heavy spots are found. but if you were to use a static balancer you would have the weights in a different place.

With the engine being a separate entity to the fan hub you cant really static balance the whole assembly, and thus have to dynamically balance it. It is generally hit and miss unless you have a dedicated vibration attenuation unit. But the way I do my fans that are on double collets is I static balance the fan to find the heavy spot and mark it, and then line that heavy spot up 180* out to the crank pin.

And from my own personal results the helis where ive done this they have been smoother running than screw on fans.

My understanding is you have the weight of the piston firing out in a linear motion and if you a weight similar in kinetic energy to that piston in the opposite direction then that cancels out the vibrations. Pretty much how a balancer shaft works in car/motorbike engines

[coolice]

Hey buddy.

It's an interesting subject and open to many variables
Effectively you're balancing the engine to counteract the piston being thrown up and down, but then putting the fan out of balance when it is not offsetting the piston weight. Being crude if the out of balance fan is 90 degrees to the crankcase its going to want to pull the motor towards that side.
Perhaps this method just helps the engine run smoother rather than reduce airframe vibrations?

In an ideal world we need as you say either a seperate balancer shaft or another piston on the back side of the engine (longitudinal twin?), which then balances the engine internals. Then balance the fan to avoid vibration induced by this going into the airframe.
As you say some cars and bikes have dedicated balancer shafts, but their flywheel is balanced, which doesn't generate cooling air but mimics the rotational movement of our fan.

Food for thought though, be interesting to take airframe vibration readings from identical models balanced using each method. Unfortunately I can only base my theory's on simple physics of rotating weights.
.

[dillwhacker]

I think the crank on its own is miles out of balance - it has that great lump opposite the crank pin as an attempt to counteract the mass of the piston and rod. Piston up, lump down etc.
But it doesnt work perfectly because the crank weight rotates, but the piston goes linearly.
Also the piston doesn't move sinusoidally because of the angle of the conrod - it rapidly reverses at TDC but slowly reverses direction at BDC. This is like adding some second harmonic into its motion, which is why a lot of balance shafts turn at twice crank speed e.g. some vertical twin bike engines.
Probably...

[Andy from Sandy]

In a helicopter we are looking for a constant speed so I would think the best thing to do is make a nice heavy flywheel and fit that to smooth out the banging up and down of the single piston.

[KevDavies]

Like Ive already said in another post somewhere, probably about Haydn's big 1000 jobber. Best engine for a helicopter is a flat twin/flat four. And a lot of light private helis run flat fours for that reason. Vibrations.

Indeed its a fantastic subject and we could talk for hours on it. Like Ian id be interested to get hold of a heli and try two different balancing methods and see what the difference was if any. I can only go from my previous experience in where the heli "seemed" to be smoother