how to calculate balance weight for single blade setup - RC Groups
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Nov 10, 2012, 10:34 AM
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how to calculate balance weight for single blade setup

Is there a rule on the required contra weight for single blade setups?
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Nov 10, 2012, 02:53 PM
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I think that in first approximation you can just consider it as a lever with the fulcrum on the prop hub, so the formula here would apply:
the problem is that you would at least need to know where the CG of that single blade is in order to calculate the length of its lever arm
Nov 10, 2012, 03:41 PM
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For the above formula to work at varied rpm, the CG of the blade, and counter balance would have to be the same distance from center.
Here are some formula's
To simplify it is balanced like any other rotor. The CG, and weight on each side must match to balance over a varied rpm. To acheive that with having the cg close to center usually requires, adding weight inboard on the rotor blade side.
Last edited by Roto Rob; Nov 10, 2012 at 04:02 PM.
Nov 10, 2012, 06:01 PM
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richard hanson's Avatar
This can be EZ or impossible to get right
IF you live in a magic kingdom where ONLY the balance is concerned - it's ez.
In the cold real world, the single blade tries to deflect the motorshaft from center .
IF the prop is slow turning balsa type - - the forces are low and just balancing it statically may be just fine
If it's a screamer - -make sure the motorshaft is solidly positioned. you may get some harmonics.

Years back we worked out some fuselage changes for dealing with four stroke IC engines-
Basically it was case by case -depending on the engine and fuselage AND wing construction
Resonances from dynamic loads which are not equal, can produce bad results
Nov 11, 2012, 08:54 AM
An itch?. Scratch build.
eflightray's Avatar
If it help at all -

But it was 1937
Nov 11, 2012, 09:05 AM
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richard hanson's Avatar
Originally Posted by eflightray
If it help at all -

But it was 1937
Paragraph 3 is patent BS

There is no way there is not a fore/ft load which can be cancelled by centrifugal force
W T H does the work of moving the craft forward?
The idea never caught on because it is simply a bad (stupid) idea for this application..
Nov 11, 2012, 06:20 PM
B for Bruce
BMatthews's Avatar
Dick, it's not intended to cancel out the thrust. But when the weight is behind the blade it does produce a dynamic balancing effect. This was well understood by the rubber model fliers back when single blade props were popular. It's why on the Korda Wakefield winner and many other models that the balance weight is behind the blade and is swept somewhat forward of where a second blade would be positioned.

In one of the Zaic Yearbooks there's a great writeup on single blade props for rubber models where all this dynamic as well as static balancing is discussed.

In the end though of the three single bladers I've built two out of the three spent much of the flight trying to clap their landing gear wheels together at some points in the flight. The other didn't simply because it had no landing gear....

I tried to bend the wire arm around to eliminate the "dancing" but while some improvement was made in some positions there was always some point in the motor run where the landing gear started trying to clap and the model visibly wobbled around. Now I simply don't build designs that need single blade props anymore. It's not worth watching the poor thing shake like it has palsy during the motor run.
Nov 11, 2012, 07:37 PM
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richard hanson's Avatar
I saw what they "designers" were trying to do -
the rub is - something has to move the plane and that something is an assymetric loading on the hub.
In my past life - one of my jobs was removing harmonics in industrial machines . Thankfully on none of my designs -
When designing pattern models for the then new OS/YS four strokers - it was a repeat of earlier work

I would feel along the fuselages and find the point where the shaking setup a resonance - then had to figure aout how to cancel it
I did an adjustable motor mount (Rotormount) which allowed the engine to rotate on axis -in a cradle- with adjustable rubber stops
it worked but the real problem - long interval in power pulses- remained
The single blade setup -I never liked simply because no matter what was done - the load on the prop hub awas not balanced
If the prop never pulled the plane - it would have been a great idea.
I lump these ideas with the flying car, as well as other impractical dreams .
Last edited by richard hanson; Nov 11, 2012 at 07:50 PM.
Nov 12, 2012, 07:40 AM
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It might be possible to eliminate the asymmetric load on the shaft using a teetering blade, I guess. But then you end up introducing complexity, and with odd gyroscopic precession behaviors to handle. If the scope is that of simplifying the prop then it defies the purpose.

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