Helicopters B

[powderking1]

Thanks so much for your replies! Thinking super glue could work now, much easier to acquire. Again, thanks!

[chalker7]

The FAQ responses for many of the questions about single bladed rotors have been posted, you can see them at http://www.soinc.org/node/828

[SOCoach]

My students have built several of the Freedom Flight models with decent success - 1:20 minutes. The models have 3 blades on the top and bottom rotors, weight just over 3 grams (3.2), they are using 125 rubber with about 1100 winds. Aside from working to lower weight, which direction should I steer them in to improve times? Which do you think would give us the most bang for our buck . . .

- Adjusting pitch
- Adjusting cord
- Maybe going with 2 blades on the bottom (which would greatly assist with the weight issue)?

Any help would be appreciated - Thanks!

[powderking1]

What length should the rubber motor be? The motor stick is 12 inches, a little more than 30 cm

[jander14indoor]

The rubber motor should be the length needed to provide maximum flight time, has NOTHING to do with stick length. It will most likely be significantly longer than the stick.

Matching motor length, width and winds to the rotor system is the KEY to success in this event (once you can build helicopters to minimum weight). We can't tell you the exact answer because it depends on the design of your helicopter.

Dig back through the archives on helicopter discussions for more on how to get the right match of rubber to helicopter. But it will depend on hard work. You have to lay out a plan to vary the key parameters in an organized fashion, fly LOTS of flights, take data, analyze data.

Regards,


Jeff Anderson
Livonia, MI

[SOCoach]

The freedom flight designs this year have three rotors, I assume to offset the reduction in length from 30 to 25 cm. Would a wider cord length help counter the shorter blade length? I am thinking two blades will be easier for my 7th graders to build plus help them build to weight. They have a really hard time getting under 3 grams . . . 3.3 to 3.2 is their best so far.

[NASA123]

I recommend reducing the motor stick to a thiner balsa stick, maybe 3/16"x 3/16"x 12" with the cable truss from the "Above and Beyond" section installed as an alternative to cutting down on rotors.

[AznPr0d1gy]

The freedom flight designs this year have three rotors, I assume to offset the reduction in length from 30 to 25 cm. Would a wider cord length help counter the shorter blade length? I am thinking two blades will be easier for my 7th graders to build plus help them build to weight. They have a really hard time getting under 3 grams . . . 3.3 to 3.2 is their best so far.

Everything works in harmony up until a point where it actually hinders. If you have a wider chord length, then it means more surface are, but then you'd need more power to spin the rotor.

I have figured out, the best helicopters are the ones that are most efficient, the ones that can fly on a lower torque, so they can utilize the thinner rubber bands specialty of more winds to an advantage.

[jander14indoor]

There's another issue on increasing chord. High aspect ratio wings (long skinny ones) are more efficient than short fat ones. Compare to full size helicopters you've seen, note how long and skinny the rotor blades are.

So, as you increase chord you increase area, increasing lift, BUT you also increase drag (inefficiency) reducing lift. As with any relation like this, there is an ideal point for chord where these balance.

Alternatively you can add area by increasing blades, but that has an effiency penalty too.

The students job in playing with these factors is to find the right combination that gives you the most efficient overall helicopter. That's what makes this event so good as a model to do test and evaluation.

Regards,

Jeff Anderson
Livonia, MI

[SOCoach]

What should we be looking at in terms of a competitive motor stick weight-wise? Our best is .9 with most in the 1.1 to 1.2 gram range.