Rotors

[chia]

By using two free rotors on the same axis in my new copter, I'd hoped to have a more balanced copter - instead, it is much less! I think the explanation, though, is that the top rotor weighs significantly more than the bottom rotor (.25 g more) and neither rotor is balanced (each one has a heavier side). When in the air, it has a tendency to go around in circles with the motor stick getting close to horizontal. Also, the whole copter is underweight. Could I kill two birds with one stone by adding .5 grams of clay at the bottom of the motor stick? I'm afraid I'll be having balance issues either way, though.

[illusionist]

Since you're underweight anyway, definitely go for it. Let me know how that works, because I too want to try two free rotors for my next helicopter.

[chia]

Bleargh. I think I made the pitch of the rotors on the copter too high. After reinforcing the hangers to keep them from coming off and adding clay at the bottom to bring it up to 4 g and fix the center of gravity, it still only stayed in the air for around 15 seconds on 650 winds of 1/8 rubber, with most of the winds left over. I don't think the rotors (both free) can turn fast enough to hold the copter's weight.
The other one I built from a kit with a 30 cm wingspan, however, is consistently getting times of 1:15 to 1:30 in my foyer, with virtually no winds left when it hits the ground. Bah.

[lllazar]

Are you guys generally getting higher times with 40 cm rotors or less?

[kjhsscioly]

Oddly, we were getting much higher times on a rotor that was 30 cm, but it could just have been construction.

[chia]

Oddly, we were getting much higher times on a rotor that was 30 cm, but it could just have been construction.

The helicopter with the 39-cm rotors had loads of issues... weak joints, pitch too high, topheaviness, etc. I did get it to fly 30 seconds or so, though.... so if I made a similar copter with a lower pitch, I think it could fly longer than my other one.

[kjhsscioly]

Oddly, we were getting much higher times on a rotor that was 30 cm, but it could just have been construction.

The helicopter with the 39-cm rotors had loads of issues... weak joints, pitch too high, topheaviness, etc. I did get it to fly 30 seconds or so, though.... so if I made a similar copter with a lower pitch, I think it could fly longer than my other one.

Which is probably why it was flipping horizontally at the middle of the flight

[devin159]

Has anyone tried to make a variable pitch propellor? That is what ours is. Its a tad on the fat side at 5.7 grams, but it did get 47 sec. in one of our test flights. Unfortunatley we only got 7th at our Regionals because it got caught on a hanging overhead light twice , but I think if I can get the weight down It will probably work much better.

[illusionist]

Variable pitch?? The rules state "up to three fixed pitch rotors".
I am curious as to how your design works... If you don't mind, could you please explain it?
By the way, does it involve circuit boards by any chance? (your Location:P)

[devin159]

Variable pitch?? The rules state "up to three fixed pitch rotors".
I am curious as to how your design works... If you don't mind, could you please explain it?
By the way, does it involve circuit boards by any chance? (your Location:P)

Fortunately for me, there are no circuit boards. Our blades aren't variable pitch in the sense that we can change it in mid-flight, they are fixed in place, but at higher speed we've managed to get the blades to warp significantly enough to that the pitch is in effect raised to make it climb faster, then as the winds on the band run out, the rpm also drops, and the pitch goes back to its original posistion.