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Vibration Problems Caused by Gas Power

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Old 02-17-2008, 05:11 PM
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hopkimf
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Default Vibration Problems Caused by Gas Power

I've been reading of vibration induced servo problems in gas powered planes. I have no gas experience yet, but am finishing a number of planes for the season. A Lanier T Craft and a IKON Stinson Reliant, both powered by G-38s. A BUSA Cub and a GP Super Chipmunk, both with G-26s. And an old M-5 Stinson Voyager powered with a G-62. All have resilient engine mounts with the exception of the G-26 Cub. I'm using a mixture of servos, most with plastic gears. A few are metal. Most of these planes will be just cruising around. Should I anticipate problems here? Is the vibration issue likely to cause catastrophic failures in the servos early in the plane's life, or is it one of premature wear?

Another question on resilient mounts. I used the GP Large Engine Mount on all of them. Three different sizes of engine, different operating speeds, masses, etc. The mounts all use the same rubber biscuits with the same stiffness. How do I know whether the soft mounts are better, worse, or the same as hard mounting the engine?

Thanks.
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Old 02-17-2008, 07:36 PM
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Default RE: Vibration Problems Caused by Gas Power

There are lots and lots of gas airplanes flying around without soft mounts and little servo problems.
If you would like to know if your soft mounts work get an accelerometer and make some measurements. Each case will be different. As a general case soft mounts will be harder on the engine and easier on the aircraft structure.
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Old 02-17-2008, 08:03 PM
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Default RE: Vibration Problems Caused by Gas Power

I have found by balancing the prop at 12, 1:30, 3, 4:30, 6, 7:30, 9, 10:30 fixed my vibration problems.
Had one prop off a quick spray of lacquer and it made the cowl shake ~1/8. After balancing as above, now smooth and no shakes.
This was on a DA100, 36% AW Katana.
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Old 02-18-2008, 01:53 PM
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Default RE: Vibration Problems Caused by Gas Power

Properly balanced, a prop will balance at any position "on the clock".
If it won't, it isn't fully balanced.
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Old 02-19-2008, 08:02 AM
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Default RE: Vibration Problems Caused by Gas Power

Onewasp- I've been wondering about prop balance too. If a prop is balanced with the blades horizontal. I call that balanced. Then move the prop to vertical. If it rotates away from vertical, then the center of balance is offset from the hub. Since hubs are not wide, the location of imbalance must be fairly close to the hub. Is it small enough to ignore?
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Old 02-19-2008, 08:53 AM
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Default RE: Vibration Problems Caused by Gas Power


ORIGINAL: onewasp

Properly balanced, a prop will balance at any position "on the clock".
If it won't, it isn't fully balanced.
Not the entire story. The concept of 'overbalancing' can reduce the effect of the prop on the shaking forces. The position around the clock relative to TDC does make a difference in the shaking forces.

Engineering textbooks describe the shaking forces in great detail.

Google search for 'overbalance' will produce additional information. Below is a link that may help describe.

http://books.google.com/books?id=LLW...XZjeU8cUFHjjHw

Bill
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Old 02-19-2008, 09:24 AM
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Default RE: Vibration Problems Caused by Gas Power

ORIGINAL: BillS


ORIGINAL: onewasp

Properly balanced, a prop will balance at any position "on the clock".
If it won't, it isn't fully balanced.
Not the entire story. The concept of 'overbalancing' can reduce the effect of the prop on the shaking forces. The position around the clock relative to TDC does make a difference in the shaking forces.

Engineering textbooks describe the shaking forces in great detail.

Google search for 'overbalance' will produce additional information. Below is a link that may help describe.

http://books.google.com/books?id=LLW...XZjeU8cUFHjjHw

Bill
For our uses -- the engineering handbook approach - of trying to adapt an accelerometer is simply not plausible.
Let's be serious .
For our use the best approach is to learn to perfectly balance a prop .
Then learn to properly setup an engine mount which best transfers the vibes to the strongest parts of the fuselage.
These two tasks are hard enough for 90% of the flyers- and getting em right solves 90% of the problems
In some cases the props are simply not stiff enough (typically wood ones) and these MAY waver at various rpm .
I see more and more crummy spinner setups being supplied in ARFs and BARFS and RTFS. thes also cause problems the new guys don't recognize..
In short; learn and DO the basics first before going off on some theoritical flyer
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Old 02-19-2008, 09:58 AM
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Default RE: Vibration Problems Caused by Gas Power


ORIGINAL: dick Hanson

ORIGINAL: BillS


ORIGINAL: onewasp

Properly balanced, a prop will balance at any position "on the clock".
If it won't, it isn't fully balanced.
Not the entire story. The concept of 'overbalancing' can reduce the effect of the prop on the shaking forces. The position around the clock relative to TDC does make a difference in the shaking forces.

Engineering textbooks describe the shaking forces in great detail.

Google search for 'overbalance' will produce additional information. Below is a link that may help describe.

http://books.google.com/books?id=LLW...XZjeU8cUFHjjHw

Bill
For our uses -- the engineering handbook approach - of trying to adapt an accelerometer is simply not plausible.
Let's be serious .
For our use the best approach is to learn to perfectly balance a prop .
Then learn to properly setup an engine mount which best transfers the vibes to the strongest parts of the fuselage.
These two tasks are hard enough for 90% of the flyers- and getting em right solves 90% of the problems
In some cases the props are simply not stiff enough (typically wood ones) and these MAY waver at various rpm .
I see more and more crummy spinner setups being supplied in ARFs and BARFS and RTFS. thes also cause problems the new guys don't recognize..
In short; learn and DO the basics first before going off on some theoritical flyer
Hockey puck.

Not just theoretical. The shaking forces can easily be observed on the airframe by placing the prop in different positions relative to TDC.

Bill
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Old 02-19-2008, 11:40 AM
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Default RE: Vibration Problems Caused by Gas Power

Maybe on your stuff-------
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Old 02-19-2008, 01:16 PM
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Default RE: Vibration Problems Caused by Gas Power

Maybe one of the engineers in your club can explain the theoretical and show you how to observe the results.

Bill
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Old 02-19-2008, 01:47 PM
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Default RE: Vibration Problems Caused by Gas Power

ORIGINAL: BillS

Maybe one of the engineers in your club can explain the theoretical and show you how to observe the results.

Bill
Dream on --
The current, well designed singles have two bearings forward the crankpin
these are typically balanced as well as possible (can't be perfect as the single cyl design won't allow it.)
Do you actually believe the designers don't take into account the engine use and "overbalance /balance" as well as possible??
the better ones use an ignition curve which smoothes the impulses in a pretty competent manner -
A prop -if it is perfectly balanced and without flex will not change the smoothness of the engine. I put mine in any position and it all ways works well and runs smoothly-

On some singles - the prop position is of some importance - these "designs"typically have one bearing fore and one aft the crankpin the crank actually bends each time around
and to continue - the WORST offenders are the budget rehased industrials with a long machined prop shaft - one sharp smack on th prop/shaft and the friken "crank pressed together assy " is all skittywampus.
add to that a wood prop with grain of varying angles and you have a made to order shaker.
One more time - a perfectly balanced totally stiff prop really does not care where it is positioned on a engine with double front bearings - the other types - I really don't bother with 99% of em flex easily so any dynamic change can cause a problem

I read all that external balance stuff years ago -- that's single cyl 101
here is something to ponder ----- or confuse
most of the model singles have very short con rods -- to make the package of the engine more acceptable -most weedies use this setup
only a very few specific purpose designers have used the long rod to stroke ratio approach as it is a bugger to do in a decent space- the piston pin ends up in the ring pack.
However it allows for longer burn time at top of stroke as the piston slows earlier at TOP of stroke and permits earlier ignition point with far less "kick back" which occurs in short rod designs --which have a shorter "dwell " time at TDC and require a bit later ignition point before TDC to get max smoothness . The long rod has a lower rod angle..
This technique is not peculiar to model engines - far from it--
On performance car engines - , changing to a long rod / stroke ratio can permit use of lower octane fuel -and keep the same power -simply because the piston stops it's upward path earlier in the crank rotation- and greatly lessens detonation -caused bthe fuel charge being compressed additionally as the piston continues upward
The bore and stroke will be the same in this example
anyway - just something to read ---------------
or get an engine engineer to read it to you --or draw a diagram you can follow



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Old 02-19-2008, 08:57 PM
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Default RE: Vibration Problems Caused by Gas Power

Thanks for all the comments, but I'm still looking for an answer to my questions. What is it about gas engines that creates undesirable vibrations? Most glow engines are also single cylinder, prone to the same shaking forces.

The second part related to iso mounts. My question was how do I know if I'm helping if an iso mount is used. Placing accelerometers on the plane is not the answer I want. As a hobbyist, I'm looking for simple solutions. I work with a crew that makes a career assignment of measuring accelerations, etc. That's not acceptable here. The collective comments of what makes good sense, and has worked would be welcome.
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Old 02-19-2008, 09:35 PM
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Default RE: Vibration Problems Caused by Gas Power

low rpm ( larger interval between power pulses" and a "sharper power impulse many times --makes for more reaction - that's one problem - another is the aprox at 3000rpm out of balance point many of these engines have --which is simply the best place to have th out of balance shake occur -if it has to happen.
Some engines are very good at smoothing this - some are a POS notably those I mentioned early on
A good solid wide mount (mass balance) helps a LOT- using spindly extension mounts is not a good plan on most setups--
Very well made props and spinners make a difference -sometimes a huge difference . heavier props sometimes help stabilize things
I prefer very thick hub props of wood or - cf props. solid and non wavering.
learning to set low sped adjustments can smooth low speed running
My own singl cyl engines rune very smoothly but I did shuck a couple of real bad designs, early on . as noted above her is a pic of a solid mount which passes all forces down the fuselage sides with no oil canning on the "frigen motor box"
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