Microlighters

How on earth do I get rid of vibration caused by a wooden prop?

And how do I go aboute to check the balance?

Bennie
Bel vir Hennie Malan in Pretoria.Hy maak hout proppe,en sal jou sekerlik kan help.Sy sel nr is:nul agt 2 955vier17vyf.
nicow
ZU-AWA

Morph described an excellent method he used for balancing the wooden two blade on his Challenger. Probably about two years back.

Thanks Griff, I tried to find it but can't so here it is again

Bennie,

I balanced it myself. The secret is to get the two blades exactly the same weight.

My prop had a 25mm hole through the centre. I found a Gaviscon Tablet tube had exactly the same dimension. I drilled a 1.5mm hole dead centre in the plastic top and bottom of the tube. I tied a knot in a piece of string and fed it though the plastic top, then into the tube and out the small hole in the bottom. I then inserted the tube into the hole in the centre of the prop hub from the front, i.e. the side of the blade that is curved faces the platic proppie(top) of the tube. This is important.

Now find a place where there is no wind draft and hang the prop from a light, or the ceiling, or a ladder etc. (the back of the prop blades i.e. the flat end) should be facing up.

Leave it to settle down and one of the two blades will hang lower (it's heavier) than the other. Using a simple aerosol can of clear lacquer(fast drying), spray a bit of on the back tip of the lighter (i.e. higher) blade and allow to dry. Work slowly and allow the prop the settle down between spraying. Eventually the prop will hang perfectly level. Use something like a window sill as a indicator as to what level is. I managed to iron out over 90 % of the prop vibrations like this.

if the gaviscon tube doesn't fit measure the hole, go to the pharmacy and see if they have any tablet tubes that fit.

BTW this will work for any prop

Oh yes, vitally important, when you put the prop back on, make sure it is tracking perfectly. Make sure the prop bolts are torqued correctly. Then use a broomstick or a dowel stick and attach it to a ladder or chair etc or to some point on the airframe. Turn the blade being measured horisontal and position the stick against the tip of the prop. Now rotate the prop without moving the stick and compare the next blade. They must all be exactly the same. If one is out use a shim (I found a small piece of broken hacksaw blade placed between the prop and the hub works perfectly, the teeth bite into the wood and it will not come out). Otherwise you will have to remove the bolts and put a washer on the relevant bolt. Tighten all the bolts up to the correct torque settings and check again.

Would the shim upset the balance or is it too close to the centre of rotation to have any real effect? Reason I ask is that they guys who do dynamic balancing add washers to fine tune the balance.

I am surly going to try this, thank you Morph.

Hey Bennie........

If the prop guys like P.deNecker, George an' others read this thread 'about putting a shim behind the prop, they will go ballistic You can't do this man

A propellor is driven mainly by the friction betwen the backing plate and the surface of the propellor which touches against it. It is not driven by the bolts. The bolts are only there to create the bond, or the surface friction between the backing plate and the prop.

So, putting a shim between the backing plate and the prop hub is a no-brainer Aish, I always maintained that every morning before sunrise, God sends more than half his angels to work at microlight airfields By reducing the surface area of this friction zone, you are thereby increasing the drive load on the bolts. It is not designed to work this way.

You track a wooden prop by varying the torque on the various bolts. Phone Pieter de Necker and ask him to sell you his propellor care handbook. He explains perfectly well how to track a wooden prop. It works.

Cheers.

Later,

My advice would be to turn off the engine before you start working on the prop

ey! I enjoy the sense of humour on this forum. Gets me grinning many times

The ultimate example of friction between two smooth surfaces is a train wheel. There the friction is created purly by the immense mass of the engine.

But seriously, the friction between even two composite materials is great if the correct torque is applied.

Hey, isn't there an engineer out there who can work out the torque value over the surface area and the type of material and give us a friction value and compare this to the power developed by the motor and give this power a value over that same surface area. I have no doubt you weeeel geet ze answer to be Friction exceeds Drive power.

And here I went and deleted my response because in hindsight I thought it was a bit harsh and I didn't want to stir.

Lamercyfly is right, don't use shims, just torque to adjust the tracking. I would still like to know how to set the tracking if the hub was aluminium?

Lamercyfly is responding to a statement I made saying I doubt that the friction and not the bolts stop the prop from rotating. If that were the case then the prop would only require 1 bolt to hold it in place. Anyway debate is healthy

I suppose it's a case of 6 of one or 1/2 dozen of the other i.e. the same thing

How much should a wooden prop be talked or do they differ?

Check with your prop manufacturer but I have the following based on prop bolts. I think we normally use 8mm bolts

Bolt size
6mm 11Ft lbs, 132Inch lbs, 15Newton Meters, 1.5213Kg Force meters

8mm 12ft lbs, 144inch lbs, 16 Newton meters, 1.6596 kg force meters

10mm 16ft pounds, 192inch lbs, 22 Newton meters, 2.2128 kg force meters

Hey Bennie,

You can talk to your prop all day, or you can juz swear it to moer 'n gone

But if you want to torque the bolts, then 14 NM is plenty for a wooden prop. More than this and you WILL start crushing the timber hub.

Later,

lamercyfly wrote:Hey Bennie,

You can talk to your prop all day, or you can juz swear it to moer 'n gone

But if you want to torque the bolts, then 14 NM is plenty for a wooden prop. More than this and you WILL start crushing the timber hub.

Later,

Thank's you all. I also have it as 13 NM, but it seems to be a bit loose.