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chalker7 wrote:Dihedral wouldn't affect torque or lift. What I was implying is that dihedral would probably help some stability issues coming from torque/thrust imbalances, but definitely wouldn't be enough if the problems are significant enough.

Yes it would affect lift. The rotors are generating a net lifting force that is non-vertical, meaning that the lift is a bit less than if they were generating a perfectly vertical force.

That said, I don't think the dihedral 'correction' force would be enough to overcome the difference we'd likely find in most tandem rotor systems - it's too hard to perfectly equalize the design of the rotors and the motors.

We also haven't even mentioned the difficulty in winding or launching such a craft - which would be significant as well.

This is true, dihedral certainly affects life in a non-meaningless way.
However, what I meant was that dihedral doesn't necessarily affect the torque/lift relationship on a individual helicopters/flights (assuming that dihedral is constant throughout the flight).
Those other challenges are certainly true as well, but I imagine creative solutions are available. I've seen a number of dual-motor/propeller rubber powered airplanes (primarily scale models) flown in the past and the winding/launching process for tandem helicopters would be similar.

Dihedral doesn't affect thrust (the acceleration and mass of air moved by the rotors remain the same) but it does affect lift as the thrust vector is not vertical. The main purpose of dihedral is to provide some degree of positive stability, not balance. Just as in airplanes, the craft would need to be designed with the necessary factors balanced.

My suggested bonus for a tandem rubber powered helicopter is probably not practical for this event.

I doubt the lift of the two rotors could be equalized sufficiently to maintain stability throughout the flight, even with dihedral, but I could be wrong.

The Chinook helicopter has an active flight control system that maintains stability. By the way, in a tandem helicopter design the
rotors must spin in opposite directions in order to maintain stability.

The standard co-axial rotor design widely used in the 2011 SciOly Helicopters event accommodates any lift differenential between the rotors sufficiently so that
stability can be maintained throughout the flight. Sometimes a co-axial helicopter will fly sideways and/or dive at the end of the flight, but generally not if the correct rubber is used and the rotors are well balanced.

I hope I have not wasted anyone's time with my suggested bonus for a tandem rubber powered helicopter. I appreciate everyone's comments.

In regard to the 2012 Helicopters event, slight changes to the 2011 rules are probably the best approach, as I previously noted.

calgoddard wrote:My suggested bonus for a tandem rubber powered helicopter is probably not practical for this event.....

In regard to the 2012 Helicopters event, slight changes to the 2011 rules are probably the best approach, as I previously noted.

I agree to both statements but, a tandem helicopter would be very interesting to watch. Maybe I can find the time to put one together in time for Ohio State. (to demonstrate, not for competition).

If you can do that, I think we all would love to see a video...

I drew a theoretical design for one in physics today but i think it's be too heavy to fly well, or even fly at all. The issue is i can't think of how to do it without using two different motors. However, im going to build one after state and if i can get it to work well enough that it'd be worth watching, i'll post a vid.

That's awesome, I want to build one too. The only idea I had for making it work with one motor is if you pinch it or make a fixed end in the middle, you wind one rotor CW and another CCW... I don't think it'd work though.

Maybe a V-shaped fuesalge, so that it only requires two motor sticks, and no other middle piece? It wouldn't be efficient for thrust, but it is a more lightweight design. You would still have to use two motors.

illusionist wrote:Maybe a V-shaped fuesalge, so that it only requires two motor sticks, and no other middle piece? It wouldn't be efficient for thrust, but it is a more lightweight design. You would still have to use two motors.

Thinking outside the box, how about 1 motor directly connected to one prop (like current designs), but also incorporate a small plastic 'pulley' and use a drive belt to turn the other prop. You'd have to use some ballast to balance it, but that would help solve the torque issue.