Rotors

[illusionist]

The spinny things that turn.

[thedoctor]

Anyone think they'll be trying designs using more than 2 blades per rotor?

[illusionist]

I've tried 4 blades, construction was a little difficult (balancing all 8 blades). Also, it adds a lot of weight, which may not be the possible benefit of using so many blades. 3 blades might be better, but construcitng them will be difficult

[thedoctor]

Yeah, I thought so when I was brainstorming ideas for this season. But assuming you could keep weight down and such, do you think more blades is better? I'm inclined to think so though I have no real scientific basis for why.

[illusionist]

Same, I'm not sure of the reasoning behind which one works best. However, I may try to rebuild a 4-bladed design this year since my building skills have significantly improved.

[thedoctor]

I think the reason I think more blades would be beneficial is because real helicopters have a lot of blades (although they aren't the best model of efficiency). Is anyone making Larrabee props for their rotors?

[thedoctor]

Oh does anyone have any thoughts on rotors with large surface area vs rotors with lower surface areas (other than the fact that low surface area = lower weight)?

[illusionist]

Oh yes I do. If you have a large surface are, you will need to use a very low pitch. If you have a smaller surface area, then of course you will need to use a higher pitch. I don't know how accurate the following statement is, but I would use the lower surface area with thinner rubberbands.

[thedoctor]

Thinner?

Doesn't the increased pitch require more torque?

[chalker7]

Thinner?

Doesn't the increased pitch require more torque?

Thinner rotor blades don't necessarily have higher pitches than fat rotors.

Just to throw a wrinkle into the conversation, if I were building a helicopter for competition I would build very wide, elliptical blades with relatively low pitch (they would look like short, fat F1D propellers). However, that would be far more difficult to construct than the traditional cross-style rotors that everyone has used for the past couple of years.

[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