2011-2012 Rules

[new horizon]

Oh they weren't quadrotors, must have misread, sorry. Yeah I've seen the video of the quadrotor UAV built by UPenn, it's really amazing.
Is it the differences in lift/thrust generated that accounts for the instability in two free rotors (tandem or coaxial)? I always thought it was the torque of the rotors that caused such a thing, you'd need them to be exactly perfect to cancel each other out, or it wouldn't be balanced. It is really difficult to do that on a power system like rubber bands so maybe that would account for the lack of experimentation on two free rotors, coaxial or tandem.

[chalker7]

The actual forces acting on tandem helicopters would end up being very complicated. I believe (but am not certain) that if the torque on the two rotors was different, but somehow the thrust remained the same (unlikely), that the helicopter would experience a rotational force that would cause it to yaw (spin around like a top). The more likely scenario is a difference in torque causes a difference in thrust. If one rotor is pulling harder than the other and they are side by side, it will cause the helicopter to tumble end over end (rotate on the pitch axis).

FYI, there seems to be some confusion about coaxial vs tandem. Coaxial means sharing the same center of rotation, which describes pretty much every SO helicopter. In this case, tandem means side by side (http://en.wikipedia.org/wiki/CH-47_Chinook)

[fleet130]

Stability could be improved by adding "dihedral" to the rotors at the cost of some lift. Not sure, but I believe some real-world helicopters use this principle for greater stability.


In today's world, stability is often less, since computer "reaction" times can make up for it.
More info.

[new horizon]

Thanks for explaining dihedrals to me, I never understood them from wright stuff last year nor do I really understand how on a physics standpoint that increases stability.
Though computer control does make these practical for real world applications, it's difficult to keep these constant with a rubber band model so perhaps tandem rotors is a bit of a stretch but I'll definitely try to build a model or two next year

[fleet130]

When the aircraft/heli tilts the vertical component of the lift vector decreases on one wing/rotor and increases on the other, causing the aircraft to tilt in the opposite direction and return to "level".

[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.