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Engine / Prop Vibration

Old 04-08-2006, 05:30 AM
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PlaneHeli
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Default Engine / Prop Vibration


I have a new OS46AX. No damage or prang.

At idle RPM I noticed a vibration of the engine. At higher RPMs it seems to dissapear.

I changed the prop and it appears the same.

Im using master Airscrew and APC.

The APC came with the package. Its only 10x6. Seems a bit small for this engine.
After reading recommendations on the internet ive tried a 11x5 and 10x7.

I checked the balance roughly by putting a srew through the centre and seeing if one end dipped down. It seemed ok. Does it need to be measured more accurately than that.

Is this vibration normal for these engines / props at low RPM due to the prop or spinner been slightly out of balance.
Should I be concerned about it.
Old 04-08-2006, 06:37 AM
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APIA
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Default RE: Engine / Prop Vibration

Yes the vibration is normal, enjoy
Old 04-09-2006, 06:22 AM
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PlaneHeli
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Default RE: Engine / Prop Vibration


Looks like your the only reply Apia, so guess everyone agrees with your comment.

Thanks, Will enjoy.
Old 04-09-2006, 09:25 AM
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da Rock
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Default RE: Engine / Prop Vibration

While you're not doing anything much, take a moment and check to see if the bolts that hold the motor mount to the firewall are tight. Then check to see if the bolts holding the engine to the mount are tight. In the last three outings, I've suggested to two fliers that their engines were loose. Both were flying at the time and both not only had engine bolts that were loose but had bolts that'd had their heads broken off by the vibration that loose bolts allow.

There are a bunch of "expert" fliers who suggest that balancing props is a waste of time. It is not for one simple reason. If you do check the balance of every prop you buy, you'll spot the odd prop that is WAY out of balance. There are more than you'd expect.

You don't have to balance a prop perfectly, but that's not a waste of time either. And you don't have to buy the most expensive balancer either. There was a "button on a string" balancer sold for some time. It was dead cheap and worked well. There is also a Dubro "field" prop balancer that works for props with holes that're in most medium size props and it's cheap too. They work good for rejecting lousy props. And are well worth the time, effort, and money.

I take the string/button one with me on every LHS trip. I simply don't buy badly props any more. And I put a checkmark on the hubs of every prop that fails my test and leave it for somebody else to buy. I figure the experts who think balancing props is a waste of time will be the ones who buy those marked props, and there will be a little justice in the world.
Old 04-09-2006, 09:29 AM
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da Rock
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Default RE: Engine / Prop Vibration

I checked the balance roughly by putting a srew through the centre and seeing if one end dipped down. It seemed ok. Does it need to be measured more accurately than that.
Yes, if you're going to go to any trouble to check, then use something that will do the job with some accuracy. What size screw actually fit into a 10-6 prop's hole? That had to be one heck of a screw. ..... hmmmm?????
Old 04-10-2006, 01:14 AM
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PlaneHeli
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Default RE: Engine / Prop Vibration

Thanks darock.

I have checked the engine bolts, but not the firewall bolts closely. Will check in more detail.

What is this string and button balancer, do you have a link to a photo of it. Not a bad idea checking the props at the LHS.

Perhaps my "screw" is not accurate enough. Also I may be arrested for carrying an offensive weapon into my LHS. LOL

Ok it was a small bolt.

Cheers
Old 04-10-2006, 05:14 AM
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Default RE: Engine / Prop Vibration

Hi!
At idle a .40-46 engine should not be seen vibrate much (the crankshaft should not be seen go from side to side at all) .
If it does most likely you have a too weak motor mount or a very unbalanced prop.
For your engine, in a sport model a 11x6 APC is the best prop.

Regards!
Jan K
Sweden
Old 04-10-2006, 06:08 AM
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PlaneHeli
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Default RE: Engine / Prop Vibration

Thanks jaka, will look into it.

Do you think the APC (gray shaped props) make much of a difference over the master airscrew (more traditional shaped props).

If so whats your findings.

PS hows the snow there, are you flying nowadays.

Cheers
Old 04-10-2006, 08:13 AM
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da Rock
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Default RE: Engine / Prop Vibration

The shape of a prop does have an effect, but not a very predictable one for us modelers. What we really have to do is simple. Test different props on each model to see which works best. You won't find out a thing comparing rpm figures from static test runs, well, except for the rpm figures.

Most of the new scimitar shaped props will usually turn more rpms on the ground. They will most likely turn more in the air as well, but there's nothing of real value to that unless you're overpropping in the first place. The problem is that we have little way to insure that the stated pitch on the prop means what it says. We simply have to slap 'em on and fly 'em to see if they give the results we are looking for.

After some years experience with a prop pitch gauge, I've come to the conclusion that the promised pitch is just that, a promise. And it's a useful generalization that helps us get a halfway decent start with our tests.

There are way too many variables that have major effects on how a prop will work on your engine, on your model, on the specific day you have chosen to fly. And you're going to also throw in whatever fuel you're using with your engine, how old and how decent that engine is, and how well you've tuned it. And that's just the obvious details.

I've seen a guy swap in a lesser pitch prop and have his model GO FASTER instead of going slower. And seen a guy throw on a new prop of the same type and have to reset his needle valve and after the flight check his prop to see if he'd put on "the wrong size" because that same dia/pitch prop flew the model so very differently.

There's also often a bunch of differences between props of the same dia/pitch by the same mfg. and they're all mass produced, right?

But one beauty is that no matter what you believe, if you think something helps you, it did.
Old 04-10-2006, 08:21 AM
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Default RE: Engine / Prop Vibration

Hi!
APC perform better in the air compared to MA black props and they are more quiet.
APC's are not the best prop for every purpose though, like here in Sweden where we race Q-500 with standard 6,5cc engines, 80/20 fuel and 10x6 plastic props, then RAM and Graupner props seems to have the edge, but for most sport and scale flying APC are the props of choice.
(Picture shows a RAM 10x6 prop)

Regards!
Jan K
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Old 04-10-2006, 08:34 AM
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mcarleno
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Default RE: Engine / Prop Vibration

I use a small finger balancer that works great for balancing. It cost about $5 and I always balance my props. Don't usually find that they need to much but you wouldn't want to drive 70 Mph on a set of new tires with having the balanced.

I have also found that sometimes plastic spinners can really be out of balance. We were trying to break in my Dad's new engine with a spinner that came with an ARF. We had a lot of vibration and found by looking at the engine during idle that the spinner was not true.
Old 04-10-2006, 09:36 PM
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da Rock
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Default RE: Engine / Prop Vibration

The button/string balancer is dead simple.

The button is a "button" that fits into the hole in the hub. It has a flange that keeps it from passing through the hole. It fits the hole snug. It has a small hole in it's center with a string threaded to come out the side opposite the flange. You slip the thread through the hole in the hub and pull the button into the hole where the flange "locks" it. When you hold the string and let go of the prop, the heavy side hangs down.

I've made myself a couple. One fits my lawnmower blade! I sharpen that sucker every spring (past time to do that as I've already cut the grass 4 or 5 times) and balance it after sharpening. They work excellently.
Old 04-11-2006, 03:44 AM
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PlaneHeli
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Default RE: Engine / Prop Vibration

Thanks all for the advice.

If you have an out of balance prop, how do you balance it. Sand it.

I cant imagine trying to glue small weights on it, or even just put glue on it, as they could fly off.

darock, when using your string and button method, are you checking it as it would be mounted on the plane (pulling both ends of the string horizontally, ---or by hanging it just from one end of the string (orientated as if it were a heli blade)

I think ill try a couple of new 11x6 APC props and see what happens.

Cheers
Old 04-11-2006, 06:32 AM
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da Rock
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Default RE: Engine / Prop Vibration

The string is knotted on one "end" (obviously not a very long "end") and the long end is pulled until the knot stops at the button. So you have a button on the end of the string. You hold the long end and the prop hangs down. The button locates the string to be in the very center of the prop hole.

These buttons are very easy to make and work well. I have one that stays in the car just in case I find myself near a new hobby shop. grin
Old 04-11-2006, 06:43 AM
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da Rock
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Default RE: Engine / Prop Vibration

Sand or scrape the top of the heavy blade. Don't scrape or sand the flat side as you may disrupt the pitch.

A couple of suggestions from one who has balanced a TON of props over the years........
...When you have a prop that's very obviously way out of balance, put it aside. Next time you're in the LHS, ask if you can swap an unused prop for "a different one".
...When it's obvious there is a heavy blade, look at it. Good possibility that you'll see that it's thicker toward the TE than the light blade(s). If it's also thicker at the LE then look carefully at the width of the light blade. Sometimes it's obvious how the heavy blade wound up so much heavier.
...You can fuelproof with CA or paint. The faster a paint dries, the less it's going to add weight. I've got some clear lacquer that I keep thinned out that works great for me. (You don't have to fuelproof composite props (Yes, I knew a newbie once who was painting the sanded blade of his plastic props) but you can paint the light blades if you wish.)
Old 04-11-2006, 07:02 AM
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da Rock
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Default RE: Engine / Prop Vibration

It's worthwhile to make yourself a prop length gauge. They're dead simple but worth the very small effort many times over.

Find a wooden dowel that snugly fits the prop hole. Take a piece of 1/4" or thicker plywood that's slightly longer than blade of your longest props. Drill a hole for the dowel at one end of the plywood plate. Make the hole a tight fit for the dowel.
Old 04-11-2006, 07:08 AM
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da Rock
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Default RE: Engine / Prop Vibration

Here are a couple of pictures that're worth a thousand words.

I made this jig in about an hour with about 5 minutes work. It's made from scrap plywood and scrap wood that was lying around. I pieced it to avoid having to route the slot and to give the sanding block locating flanges.

It's the jig I use for the size props that're marked at the holes. It can also be used to make accurate "flared" prop tips. I slap a strip of masking tape on it and pencil in aligning marks for one blades angle and can easily and accurately align the other blade.

Accurate work is often the result of a special tool.
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Old 04-11-2006, 09:26 PM
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Default RE: Engine / Prop Vibration

Why not just buy a Dubro Prop Balancer or some other brand. It will last you a lifetime. It's one of those really good investments. If you go to larger models and props, it will work for them too!
Old 04-11-2006, 09:50 PM
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Default RE: Engine / Prop Vibration

A lot of clubs that have noise restrictions shy away from the APC props. The tip noise is bad compared to other props. Darock is right, you just have to try different props. The 11x7 is normally where people start out with .46 engines. Generally speaking, the APC will outperform the MA. Not always. Never the same on different airframes and engines. Also, you may like a prop a lot only to change your mind one day when your flying styles change....as in to discovering pattern flying or 3D.

I've used a finger balancer set between 2 identical drink glasses. Thought that was pretty good until I got a Dubro balancer. I went back and found that all of my props were out of balance by a lot. You should have a good balancer. I once landed and found 3 engine mount bolts loose from the firewall. 2 were missing and one was trapped by the engine. My 3 bladed prop imbalance was the extreme. Now I know why the gear mounting bolts backed out a few weeks prior to that.

I balance the blade, spinner, and backplate. I make marks so that I can line each up the right way. Unbalanced props will vibrate your glue joints, and they don't need that. They also rattle your servos and receiver. They are hell on engine mounts. Plus, they rob you of power.
Old 04-12-2006, 01:40 AM
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PlaneHeli
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Default RE: Engine / Prop Vibration

Thanks all.

darock-thanks for the photos, your right a picture is worth a thousand words. Also thanks for the explaination of the button.

blw - Thanks also. I didnt realise an 11x7 would be ok for a .46.

Im off to the LHS to get a 11x6 and 11x7 apc prop.

PS darock - perhaps you shouldnt carry your string balancer in the car, maybe you wouldnt end up at the LHS so often LOL... OK OK ill go wash my mouth out with soap.

Cheers
Old 04-12-2006, 07:00 AM
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da Rock
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Default RE: Engine / Prop Vibration

PS darock - perhaps you shouldnt carry your string balancer in the car, maybe you wouldnt end up at the LHS so often LOL... OK OK ill go wash my mouth out with soap.
chuckle, chuckle..........

I also carry a Triner scale in the car. It's a very small hand-held beam balance style scale with a weight range from 0 to a couple of ounces. It's excellent for weighing hobby shop balsa. I used to live in Minneapolis and the place has a couple of EXCELLENT hobby shops. They all knew me and would smile like cheshire cats when I walked in and would instantly tell me if any new balsa had arrived. They knew I was going to weigh every piece and mark the weights.... and buy the best (not always the lightest, but always the best).

I've got a stockpile of excellent balsa in the shop. Props..... not so many excellent ones. The LHS here didn't like my checking their props. Go figure.
Old 04-12-2006, 07:08 AM
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Default RE: Engine / Prop Vibration

BTW, the button balancer is good for rejecting props. I use a better one down in the shop, one that'd be a pita to carry in the car.

Also, btw........
The LHS that didn't want me to check their props with my button deal............
They have a display with a magnetic balancer that sits over the prop bins. I use that.... grin...... when they're not looking of course.

And about that display they have..........
They have a prop in it 24/7. The prop they chose was "perfectly balanced" (their words). The reason they think it's perfectly balanced is because it will INSTANTLY whip around to horizontal as soon as you let it go. The sucker is so far out of balance that when the hobby shop door opens and a breeze whips in and turns the prop, the prop almost won't turn. And will return to horizontal in a heartbeat. chuckle...... some people really don't have a talent for their chosen profession.
Old 04-12-2006, 02:57 PM
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Default RE: Engine / Prop Vibration

I know that this is not a thread on sizing but since it was brought up, there are a few general recommendations:

APCs are generally more effecient than MA (the MA G/F is a better prop than their standard). This means that a MA 11x7 is about as efficient as an APC 11x6 (not scientifically analyzed just MO).

A good ABC 45/46/50 can be prop'd with varying sizes dependent on what you are flying.

-- A fast airframe (such as a super sportster) an 11x7 is a great choide
-- A not-so fast airframe (trainer or pattern) an 11x6 or 11x5 may be better
-- A speed plane (quickie, pylon) an 10x8 or 10x9
-- A Scale may use a 12x5
-- A 3D would be better with a 12x4 or my preference 12.25x3.75 APC (this prop is huge -- must weigh as much as two 12x4 MAs but when coupled with a strong 46 will give your 5 lbs plane unlimitted vertical and great prop hanging capabilities).

I have a SK50 in my Yak-3. This is a powerful engine but doesn't produce high RPMs. I have found that the best prop for the airframe and speed is a Master Airscrew 11x7 cut down (ala Top-flite power point style) to a 10.5 x 7. It gives me more thrust than a 10x7 and more speed than the 11x6. I fly APC on my Uproar (46 w/12.25x3.75), an APC on my Trainer for the kids (K&B 40 with a 10x5) and on my Super Sportster (Tower 75) I run a 12x6 MA Scimitar.
Old 04-12-2006, 08:52 PM
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Default RE: Engine / Prop Vibration

yep, a lot of people think that if the prop always lands horizontal that it is balanced.

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