Rotors

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Re: Rotors

Post by obstinate » August 29th, 2010, 10:24 am

my rotors definitely aren't balanced perfectly, but i don't know how to go about fixing this. Do i add weight to the other side?

also, i bent my own thrust bearing out of .015 music wire. not only does it bend a little when the rubber is attached but also, i couldn't
get the top part (the side that the top rotor contacts) of it to be perfectly flat. could that be contributing to the wobble?

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Re: Rotors

Post by illusionist » August 29th, 2010, 12:57 pm

Just curious, which is heavier, a Harlan aluminum pig tail prop hanger, or an Ikara plastic prop hanger?
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Re: Rotors

Post by obstinate » August 30th, 2010, 6:34 pm

i saw that many helicopters had the two rotors closer together, including the parlor copter kit.
is this a better design than spacing them out?

i built some both ways, and the ones with the rotors further apart seemed less wobbly. that may just have
been due to craftsmanship issues though...

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Re: Rotors

Post by smartkid222 » August 30th, 2010, 7:34 pm

It is said that generally farther apart=more stable, but you also need to consider the center of gravity when making such a change.
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Re: Rotors

Post by franklinknights » September 28th, 2010, 6:09 pm

we plan on using heli rotors made out of taco bell cups. w ewill use same design from plastic cups of last year
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Re: Rotors

Post by blue cobra » October 11th, 2010, 6:47 am

I don't understand how to build on a prop block. I'm looking to build this. That cuts prop blocks, so once I have that, what do I do with it? Do I make the surface of the block the flat underside of the prop? What will stop the wood from springing back to straight once it is taken off of the block? Thanks.
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Re: Rotors

Post by calgoddard » October 11th, 2010, 9:56 pm

Prop "forms" are normally used by modelers who want to build near perfect helical pitch props for indoor duration rubber powered airplanes like Penny Plane, EZB, F1M and F1D airplanes.

The prop forms are sometimes carved from a solid block of wood. Other times they are made with planks, spaced apart balsa sheet pitch stations or strips that extend between the leading and trailing edges of a prop form frame.

A prop blade can be made of a thin sheet of balsa. After soaking in hot water the sheet balsa blade is wrapped with cloth over the prop form and baked at low temperature. When dry, the sheet balsa blade keeps its helical shape.

Alternatively a built-up prop blade can be fabricated from very thin balsa strips glued together to create an oval-shaped outline that is crisscrossed by ribs. The built-up prop blade can be constructed directly on a prop form and can be sprayed with water to ensure it keeps its helical shape when dried. The finished built-up frame is covered with plastic film.

Either way, the finished blades are glued to opposite ends of a prop spar using a pitch gauge. A pitch-to-diameter (P/D) ratio of between 1.8 and 2.2 is preferred for the airplanes listed above. I am not sure if this range of P/D ratios is optimum for helicopters.

I have seen pictures of rubber powered helicopters with film-covered blades apparently formed on prop blocks. The rotors made with these blades are used on helicopters seeking world record class flying times.

The article in your link is very interesting, but it looks like it would take a tremendous amount of effort to set up the tooling to fashion solid prop blocks in this fashion. All this effort is unnecessary unless you are an expert indoor modeler intent on flying competitively in national and international competitions.

Most students competing in the 2011 Helicopters event will probably be perfecting the flight profile of helicopters embodying the Parlor Copter design. Better rotors would no doubt improve the performance of its twin co-axial counter rotating rotor configuration but such rotors, if made on custom prop forms, would require tremendous effort and expertise in construction techniques beyond the skill set of most high school students.

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Re: Rotors

Post by jander14indoor » October 12th, 2010, 4:05 am

franklinknights wrote:we plan on using heli rotors made out of taco bell cups. w ewill use same design from plastic cups of last year
Might want to go back and read the rules, see para 2.b. "The functional components must not be constructed from rigid plastic" probably makes such rotors illegal.
blue cobra wrote:I don't understand how to build on a prop block. I'm looking to build this. That cuts prop blocks, so once I have that, what do I do with it? Do I make the surface of the block the flat underside of the prop? What will stop the wood from springing back to straight once it is taken off of the block? Thanks.
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calgoddard wrote:<SNIP>
Better rotors would no doubt improve the performance of its twin co-axial counter rotating rotor configuration but such rotors, if made on custom prop forms, would require tremendous effort and expertise in construction techniques beyond the skill set of most high school students.
While calgoddard correctly discusses the use of a prop block, I wouldn't be so quick to dismiss their use in this event. Partly because I've found SO students are capable of darn near anything. However, you don't need that jig to make prop blocks. Its not beyond HS students who have the tools, but more useful if you make many blocks. Its overkill to make a few prop blocks.

A simple way to make a accurately helical prop block is to cut a proper size isoceles wedge triangle. You'll need to to some calculations to figure out the base, height and thickness to make a particular pitch helix, but its not very complicated geometry. I'll leave that to the student for now, there's descriptions on the web I think, and certainly in some indoor flight books. Anyway, once you have the wedge, you just cut away all the material from one side of the diagonal drawn on the base square and following the edges. Much harder to describe than do.

Note, you need at least two blocks for a dual rotor copter, one in each direction.

If you want to but some airfoil on that helical surface, the common practice is to cut out a piece of balsa sheet the shape of your blade and the thickness of the highest point of the curve. Then sand down the edges till the top give the curve you want. Now soak the piece good and strap it onto your block at the proper location. That will give a curved underside to your helical blade made on the block.

If you make frame rotors covered with plastic, you don't need that underform. The outline lies on the block, and the ribs give the curve, just like a WS wing.

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Re: Rotors

Post by franklinknights » October 13th, 2010, 4:44 pm

jander14indoor wrote:
franklinknights wrote:we plan on using heli rotors made out of taco bell cups. w ewill use same design from plastic cups of last year
Might want to go back and read the rules, see para 2.b. "The functional components must not be constructed from rigid plastic" probably makes such rotors illegal.


what if we use the tiny cups? that are made out of cardboard/paper ? they are usall the "small" size cups
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Re: Rotors

Post by jander14indoor » October 13th, 2010, 5:14 pm

As a sometimes event supervisor, not sure I could disqualify paper cup blades.

As a coach, not sure I'd advise it. Heavy, probably too much curve.

Here's a suggestion. Instead of using paper or plastic, instead consider molding thin balsa onto a curved surface and cutting your blades from that. Cut at an appropriate angle, they make pretty accurate approximations of helical props.

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