Rotors

[illusionist]

Hmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm

And yes doctor, what I meant was that if keeping the pitch constant, go with the smaller rotors. It's actually pretty much common sense. The hardest part is to find the right rubber for your surface area and pitch.

[illusionist]

Sorry for the double-post.
Chalker, when you say elliptical blades, does that mean that they will also have a helical curve to them (higher pitch closer to the prop shaft, low pitch at the tips), or do you mean just flat? I would assume that helical pitch will be much more efficient.
I know that elliptical blades will be more efficient, but I don't understand why that is. Can someone explain or link me to some sort of information on this topic?

[chalker7]

Sorry for the double-post.
Chalker, when you say elliptical blades, does that mean that they will also have a helical curve to them (higher pitch closer to the prop shaft, low pitch at the tips), or do you mean just flat? I would assume that helical pitch will be much more efficient.
I know that elliptical blades will be more efficient, but I don't understand why that is. Can someone explain or link me to some sort of information on this topic?

They would have a helical pitch twist to them as well.

Elliptical lifting surfaces have much lower drag than any other shape at these low speeds. The exact reasons for that are complicated, but essentially any non-smooth bends (like the two 90 degree ones at the end of every rotor I saw last year) are huge sources of drag. Elliptical surfaces have no sharp bends.

Here are a few links that should occupy your time for a little bit if you want to know more about model airplane propellers (which are essentially the same as rotors for our purposes).

http://indoorduration.com/ftp/GrantPropINAV94.pdf
http://indoorduration.com/F1DPropConstruction.htm

[illusionist]

Okay, thanks for the articles Chalker. Btw, is there a rule limiting the # of rotors? (like last year, I think the maximum was 3)

[jander14indoor]

Yes, three's still the max number of rotors. Meant to give plenty of design freedom. Theoretically, more rotors is better than more blades (all other things staying the same). In other words, three two bladed rotors is more efficient than two three bladed rotors. NO idea how to engineer it, but if someone does, I want to see it!!

Jeff Anderson
Livonia, MI

[illusionist]

How about a tandem design with three rotors instead of two? It'll be heavy, but maybe more efficient?

[chia]

...but heavy. I think that without those boron filaments, the mass would outweigh the benefits of three rotors.
Then again, there is that 3x bonus...

[illusionist]

If you can get a tandem heli to fly for 40 seconds, you're already ahead of the competition (although the advanced teams may also choose to do tandem, in which case it doesn't make that much of a difference)

[VeritasEnVida]

Making the rotors wider so they catch more air, do you think that would make the heli fly longer?

[chalker7]

Making the rotors wider so they catch more air, do you think that would make the heli fly longer?

Probably, but there is a definite trade-off that you'll run into. The more surface area on the blade, including both the width (chord) and length (diameter/radius/span), the more lift you will get (keeping speed constant). So, you can reduce the speed of the rotor to use winds more slowly and get the same amount of lift (and therefore, longer flight times).

However, more surface area also leads to more drag. The more drag, the more initial force required to turn the rotor without contributing any lift. At a certain (unknown) point the negative effects of drag will outweigh the positive effects of wider blades. This drag effect will be especially pronounced with the traditional cross-spar design everyone currently uses for helicopters, the sharp angles at the end of the blade are significant sources of drag.

If I were building a helicopter for Science Olympiad competition, I would make very wide rotor blades with an elliptical (egg shaped) form on a helical block. You can search through older posts to see some links on how I would go about doing that.