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Building techniques

Posted: August 26th, 2010, 10:16 am
by smartkid222
creating new topics

Re: Building techniques

Posted: December 4th, 2010, 7:18 am
by ironchef48
I am wondering how should I attach the freespinning rotor to the motor stick... I've looked at tons of designs and can't quite figure out how they do it.

Re: Building techniques

Posted: December 4th, 2010, 2:09 pm
by eta150
ironchef48 wrote:I am wondering how should I attach the freespinning rotor to the motor stick... I've looked at tons of designs and can't quite figure out how they do it.
with glue.

No, seriously, I'm pretty sure that "free spinning" is a misnomer.
That prop should spin with the motor stick, as opposed to being powered by the rubber band. This is, of course, depending on the rotor type. I'm not sure of the technical names, but if it is this kind (not my heli, just an example):

http://freedomflightmodels.com/images/c ... 0Large.jpg

You could do it that way (the way the bottom prop is attached).
If it is more like a custom-built Wright Stuff prop, you could just glue it to the motor stick.

Re: Building techniques

Posted: December 4th, 2010, 6:13 pm
by ironchef48
I was thinking more of the rotor that IS propelled by the motor... I should have been more specific

The link helps- thanks!

Re: Building techniques

Posted: December 5th, 2010, 10:50 pm
by lllazar
Yes, i was wondering the same thing - im having trouble understanding how we should attach the rubber motor to a hook in such a way that it causes the rotor to spin. What is the other end of the hook attached to? What is the hook made off?

Re: Building techniques

Posted: December 6th, 2010, 7:28 am
by jander14indoor
Let me take a stab at responding to the questions on hooking rotors and motors.

Basic assumption. We're talking about a simple two rotor, counter rotating, helicopter similar to the recent videos posted in another thread. If that doesn't make sense, see those videos FIRST, and then come back to read this.

Terms. At least what I mean when I say them.
Free rotor. A rotor which is ONLY a rotor with a shaft and hook. It is mounted to the motor stick with some sort of bearing or prop hanger so it can turn freely from the motor stick. Just like a propeller in Wright Stuff if you are familiar with that event.
Fixed rotor. This rotor is attached to the motor stick so that it can only turn if the motor stick turns.

NOTE, BOTH rotors are powered by the rubber band if set up correctly. How's this happen? Remember, equal and opposite reactions, Newton's laws are correct until we get in extreme situations where relativity or quantum effects become important. Even then he's right, the results just need to be adjusted a little. None of the exceptions or adjustments to Newton apply to this event. So, if the motor is attached at one end to the free rotor with a hook, and at the other end to the motor stick with a hook, BOTH are being twisted to spin, but in opposite directions.

This means if you hook the rubber motor to the free rotor and the motor stick, both are being twisted, and when released, BOTH will spin, but in opposite directions. This means the free rotor will spin one way, and the motor stick in the opposite direction, TAKING THE FIXED ROTOR GLUED TO IT ALONG!!!

So, how do you mount the free rotor and the fixed rotor? ONE way the free rotor can be mounted is just like the propellers on Wright Stuff planes. A prop hanger is attached to the motor stick on the side, bearings are added on the rotor shaft and it is slipped into the hanger. The fixed rotor is just glued to the motor stick. On the end of the motor stick opposite from the prop hanger a fixed hook is attached. The wound motor ends are attached to the motor stick hook (which is attached to the fixed rotor) and the free rotor hook.

It really is that simple. Yes the helicopter is a little unbalanced. But not enough (unless you build crazy) to degrade performance much. Last years winning national copter was built this way. It just wobbled a little in flight.

There are of course other ways to build. Like the twin stick or hollow tube designs. These allow both rotors to spin on a balanced, common axis. The bearings for the free rotor will be different. Yes they fly smoother. But the description of the rotors and how they react to the motor above DOES NOT CHANGE! One turns by itself, the other turns with the motor stick.

There is an alternative, make BOTH rotors free, one on each end of a motor stick. The axes can be aligned. The motor stick will not spin much, its only driven by the friction in the bearings now and that's working in opposite directions. Which way the motor stick spins will depend on the balance of friction between the two hangers.

Hope that's clear, if not, restate your question and I'll try again.

Thanks,

Jeff Anderson
Livonia, MI

Re: Building techniques

Posted: December 6th, 2010, 2:50 pm
by lllazar
Ok, where can i get prop hangers and bearings?

Re: Building techniques

Posted: December 6th, 2010, 4:50 pm
by illusionist
Would there be any other benefits to having both rotors be "free rotors" other than the fact that you can align the axis? Such as efficiency or improved flight times?

Re: Building techniques

Posted: December 7th, 2010, 5:07 am
by jander14indoor
lllazar wrote:Ok, where can i get prop hangers and bearings?
Start with the NSO event page, resources listed there. http://www.soinc.org/helicopters_c
illusionist wrote:Would there be any other benefits to having both rotors be "free rotors" other than the fact that you can align the axis? Such as efficiency or improved flight times?
Excercise left to the student. In other words, I don't know.

Jeff Anderson
Livonia, MI

Re: Building techniques

Posted: December 7th, 2010, 1:25 pm
by illusionist
Haha, okay I'll try it out and post my findings here.