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Rotors

Posted: August 26th, 2010, 10:14 am
by smartkid222
Creating new topics so we can have more order.

Re: Rotors

Posted: August 26th, 2010, 12:10 pm
by illusionist
For attaching the rotating rotor, can I use a propeller shaft holder, such as the ones used for WS planes, except with my own rotor replacing the blades?

Re: Rotors

Posted: August 26th, 2010, 2:00 pm
by smartkid222
illusionist wrote:For attaching the rotating rotor, can I use a propeller shaft holder, such as the ones used for WS planes, except with my own rotor replacing the blades?
Since you've asked this 3 times now i might as well answer it lol. Persistence pays off i guess.

If you are talking about using an ikara prop and the hook(or thrust bearing not sure what it's properly called) then probably not. The rules will probably say not to use commercially made rotors and you might still be keeping parts of the origional. Also solid plastic (excluding covering) is a no-no on the rotor.
You can probably use the the hook in the picture and make someone yourself but I suggest you use instead something like a Harlan pennyplane thrust bearing http://www.indoorspecialties.com/index1.html also sold from a2z corp.

Re: Rotors

Posted: August 26th, 2010, 2:27 pm
by illusionist
Haha, sorry for posting it so many times, I wasn't sure which thread to post it in until you made this one :P
Ok, thanks for the clarification.

Re: Rotors

Posted: August 28th, 2010, 10:42 am
by illusionist
What is the easiest method for shaping a helical blade? I made a cardboard template to curve the balsa around to make the circular shape (similar to a Wright Stuff propellor shape), and want to make it helical (greater vertical angle near the middle, and flattest with least angle near the tip). How do I make a mold, or template to get the desired twist?

Re: Rotors

Posted: August 28th, 2010, 11:25 am
by smartkid222
Thats a good question but part of the event since it is new is to figure stuff like that out.
One alternate to making a helical blade is probably easier and not too much worse.
I'm not sure of the terminology, but you can make a cylindrical style blade by wrapping the balsa around a cylinder.

Re: Rotors

Posted: August 28th, 2010, 1:57 pm
by jander14indoor
See my discussion on how to make rotors in last years discussions at http://www.scioly.org/phpBB3/viewtopic. ... &start=180

Alternatively you can make them on a dedicated jig. Here's how you make it. Skipping the theoretical stuff about why this works.

Cut out a long equilateral triangle wedge of wood. Height and angle will dictate the pitch of the resulting blades. Takes a little math to figure it out. I'll come back to that. The height of the equilateral triangle is the radius of the finished blade.

Now, trim half the wedge away along opposite corners. Which opposite corners depends on whether you want clockwise or counterclockwise props. When I say opposite corners, look at the square or rectangle base of the block. Draw a diagonal from one corner to the next. Cut away all wood on one side of the diagonal. Follow down the edges to the point of the triangle. The angle will change, eventually becoming vertical and no wood is cut away at the point of the wedge. Oh, the point of the wedge is the center of the blade, the axis of the prop is along the edge of the tip of the wedge. The outer tip of the prop is at the starting diagonal.

That twisting diagonal surface is the surface of a helix. Build your blade on that.

Hmm, the math part is a pain for me to explain, but you might try this: http://www.keveney.com/prop2.html which is how to carve a prop. Cut that in half and stop when you have one face carved of the simple wedge prop.
Just found this, good explanation. http://www.indoornewsandviews.com/files ... _props.pdf
A fancy jig to make the prop block: http://www.indoorduration.com/ftp/Preci ... Blocks.pdf
And maybe also: http://www.indoorduration.com/ftp/GrantPropINAV94.pdf

Hope that helps some, or generates useful questions.

Jeff Anderson
Livonia, MI

Re: Rotors

Posted: August 28th, 2010, 2:12 pm
by illusionist
Ok, thanks, it's going to take me sometime to work through all of the information.

Re: Rotors

Posted: August 28th, 2010, 8:57 pm
by obstinate
My helicopter seems a bit wobbly, and i'm not quite sure what is causing this.
i'm guessing its because my rotors aren't exactly in a straight line, but i also noticed
that my thrust bearing bends when the rubber is attached...

is there any way to fix the problem?
thanks

Re: Rotors

Posted: August 29th, 2010, 4:36 am
by jander14indoor
Some wobble is normal, but excess is inefficient.

Things to try or check.

Are your rotors balanced?
- For the 'free' rotor hold the motor stick horizontal, no rubber band. Does one blade always hang down, no matter where they start? If so, its heavy. Fix.
- For the fixed rotor, see if you can balance the horizontal motor stick with both blades level. If not, fix the side that always drops. Oh, take off the free rotor to do this.

Is the pitch even from one side to the other?
- Look along the rotor towards its axis, axis held vertical. The tip up should look symetrical. If not, fix.

Is the offset of the axis of the two rotors TOO great (not sure how much is too much, but try to keep it relatively small)? If so, redesign and rebuild.

Not sure motorstick or rotor hanger bending will make it wobble, but it does introduce other problems. So, evaluate, is it bending? Where, stick or hanger?
Bending motor stick. If you are under weight, brace it by laminating a small piece of hard stiff balsa on the side away from the motor stick. If you are at weight, you'll have to make another. How much weight did you budget for the stick? I'm guessing around 1.5 to 1.8 gm is about right. Cut a bunch of sticks, sort all around the weight target, select the stiffest. Shortening the stick will make it stiffer AND lighter. Note, you may NOT want hard, stiff balsa for the motor stick. To hit weight it has to be too skinny. Instead, select a very large cross section but low density piece of balsa. See the Wright Stuff historical threads for more discussion on why this works.
Bending prop hanger. Are you using the plastic Ikara hanger? I've always found those too flimsy. Try an aluminum one like those Harlan sells. You can also bend your own.

Note, someone thought the plastic Ikara hanger would not be legal, didn't think the rules were written that way, but read them yourselves when available in a week or so.

Hope that helps,
Jeff Anderson
Livonia, MI