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HelicopterMain Discussion › OK, I'm confused now....! Blade balancing question for the engineers out there.....
11-29-2007 12:10 PM  10 years agoPost 161
CK_

rrApprentice

Redondo Beach, CA

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It appears to me that the real blade will produce variable drag as the pitch changes but the metal tube will have fixed drag, therefore it will not be possible to achieve balance and successfully fly the helicopter.
You hit the nail on the head, although you don't need perfect balance to be able to fly. In fact it's physically impossible to build a vibration free helicopter.
A 10 lb. machine requires 10 lb of lift to hover.
Not true. The rotor must hold the fuselage in the drag of the rotor wake in addition to the weight. The rotor typically makes about 5% more thrust than the weight of the helicopter.
.1 grams
That is actually closer to 0.0806967 grams
Okay, so you're not an engineer.

The 3° was a hypothetical just to illustrate a point. Even with a decent coning angle, the vibration from the lift vector would be comparable to a two blade head balanced to within the tolerance of a .1 gram resolution digital scale. In other words, insignificant.
I have not run the numbers. Is 3 degrees expected?
Using some gross assumptions like a triangular lift distribution and a constant linear mass distribution, I get 1.4° for a 180 gram blade.
And I do not see how the vertical vector approx. 1.675 ft out can ever be balanced to keep from trying to roll the aircraft over.
With a freely teetering head you essentially have the fuselage hanging from a universal joint. It's impossible for the rotor to transmit a pitching or rolling moment to the fuselage through a universal joint. Within the rotor itself, the moment from the offset lift is balanced by the centrifugal moment due to coning.

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11-29-2007 01:05 PM  10 years agoPost 162
skydude

rrNovice

Gainesville, Florida, USA

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The 0.0806...gram was supposed to be a light-hearted way of telling you that you were correct...roughly 0.1 gram.

qoute:

the drag of a cylinder is 12 times that of an airfoil

unquote

It is my understanding that a cylinder that is aerodynamicized (point the nose a little. add the tail - tear drop, rain drop) that the aerodynamic shape has approximately 2/3 the drag of the original cylinder, keeping the same thickness.

Lengthening the chord may add some drag but 12 times sounds excessive.

10 pounds of NET lift is required to hover a 10 pound bird.

--

Watch out all you moles!!! (Vae, puto deus fio)

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11-29-2007 01:11 PM  10 years agoPost 163
CK_

rrApprentice

Redondo Beach, CA

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Sorry, I didn't detect the sarcasm. You should've used a smiley.

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11-29-2007 01:15 PM  10 years agoPost 164
skydude

rrNovice

Gainesville, Florida, USA

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It was in no way intended to be sarcastic. Just light-hearted. Truly, as if you were using an abacus or your fingers or something to do your calcs.

Just trying to have a good time.

PS - The original question was posted to engineers. I would not think anyone else would answer?

--

Watch out all you moles!!! (Vae, puto deus fio)

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11-29-2007 03:06 PM  10 years agoPost 165
S Bell

rrApprentice

Nova Scotia Canada

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http://www.avionavtask.com/certific...or_Science2.pdf

While not model related it may offer some insight into the historic BS origins of rotor smoothing people seem to grasp bits and pieces of for blade C/G matching "black magic" Remember models are not held to the same restraints for adding blade weight. We do as we please and static balance is just that, no form of fancy static c/g blade-mass matching will be dynamic.

Some of us like to balance the hub and blade assembly as a unit however poor tools and/or modeler skill can make vibration greater compared to doing nothing at all but bolting on a good matched blade set.

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11-29-2007 03:56 PM  10 years agoPost 166
AirWolfRC

rrProfessor

42½ N, 83½ W

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That's an interesting article.

On page 4 we find,
All rotor blades are designed with adjustable weight packages so that all blades can be shipped from the factory with the exact same span moment.
Notice that says moment, not weight.

Junmping down to the "conclusions" we find,
The United States Army now employs dozens of Universal Static Balance Fixtures within all levels of maintenance, depot, intermediate and unit. The United States Army has saved millions of dollars in reduced rotor blade returns to the manufacturers and depot repair facilities. The United States Army has saved millions of dollars in reduced maintenance test flight expenses by cutting associated activity in half.
It looks like they like static balance also, when done right.

This article from the same source says it better,
http://www.avionavtask.com/certific..._Technology.pdf

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12-05-2007 08:22 AM  10 years agoPost 167
MILNEI

rrApprentice

Buckingham, England

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Update from Guenter Knipprath, the one bladed heli designer

I sent Guenter some questions, they are shown below with his answers

Do you have any more pictures or movies of that helicopter
Unfortunately no more pictures. I have a short video somewhere, but I would have to search for it!
Do you still have it or what happened to it?
No, I don't have it anymore.
Did you have any vibration problems?
Yes, there were vibrations. It may have been possible to do something about them.
If you did, how did you solve them?
No I didn't, I would have had to apply myself further.
Was the rotor head designed by you or was it based on another helicopter design?
I had modified a Heim Rotorkopf.
Did your helicopter have a flybar (the picture looks like it didn't)?
No it didn't, but it would also have functioned with a flybar.
How did the helicopter handle without the flybar?
If you use good blades and optimise the centre of gravity then it will fly fine without.

So, he's not really giving much away there......

I'll ask him if he can find the movie, that would be interesting. Are there any other questions you'd like me to ask?

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12-05-2007 04:43 PM  10 years agoPost 168
AirWolfRC

rrProfessor

42½ N, 83½ W

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I think I can give you the answers you are looking for.

On a one bladed rotor with counter weight, the lift will be only on one side of the rotor at any point in time.

The rotor head has a flexible joint connecting it to the top of the main shaft.

Centrifugal force keeps the rotor disk horizontal with a slight cone angle while producing lift. - - - read that again - - -

The attatchment point of the rotor disk to the main mast is the moment center of the rotating assembly. (my see-saw balance result)

It will fly and will NOT have any lateral vibration due to the lack of a blade (if see-saw balanced right in the first place)

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HelicopterMain Discussion › OK, I'm confused now....! Blade balancing question for the engineers out there.....
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