2011-2012 Rules

[new horizon]

Oh I see now, thanks a lot!
But that wouldn't help balance out torque correct? So if a helicopter started yawing dihedrals wouldn't help it as torque and lift are unrelated?

[chalker7]

Torque and lift are very much related. The higher the torque, the faster the rotors spin and the more thrust. This is why helicopters climb at the beginning of the flight (when there is more torque on the motor) and fall at the end (when the torque/turns run out).

Dihedral would definitely help, but it wouldn't be sufficient if the imbalance is great enough.

[fleet130]

Torque is dependent solely on the rubber motor and lift is dependent on torque. At a glance, I don't see where dihedral would have much effect on torque, although I suspect one may have some slight influence on the other. It seems to me there are a lot of factors that need to be balanced for a tandem design to work well. For example, if 2 motors are used, their torque curves would need to remain matched throughout the flight.

[chalker7]

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.

[chalker]

In full-scale applications, dual rotors allow for much greater lifting capacity (effectively double that of a single rotor) and also don't require a yaw-adjusting tail rotor to keep from spiraling out of control.

There's another reason as well for coaxial rotors in real life - it allows for faster horizontal speed of the helicopter. With a single rotor, as the helicopter starts going faster, the blades on one side (during the rotation from the back to the front of the helicopter) experience a faster wind speed (and thus generate more lift), while blades on the other side (where the blades are 'receding', or rotating from the front to the back) see less effective airspeed, and less lift. The net effect is a force that tries to roll the helicopter on it's side. Coaxial, counter rotating rotors cancel each other out in this effect.

[chalker]

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.

[chalker7]

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.

[fleet130]

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.

[calgoddard]

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.

[wlsguy]

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).

[aubrey048]

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

[lllazar]

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.

[new horizon]

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.

[illusionist]

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.

[chalker]

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.