I always wondered why there are no "real life" sized quadcopters in use for manned flight.
The explanation is simple: quadrotor physics do not scale up. The rotational inertia of the rotors is proportional to the length of the rotor blade to the fifth power. Double the rotor blade length and the inertia of that rotor goes up by a factor of 32.
High inertia rotors are not suitable for quadrocopter because fast changes in the rotation of the rotors required for maneuvering them.
Having one big rotor for lift and three (or more) small ones for control is a very good idea. Another idea that is being researched is having multiple quadrotor copters flying in co-operation.
Mass is not linearly proportional to length, because a longer rotor needs to be a lot heavier to be structurally sound. A uniform rod is not a good analog for a rotor blade. It's not a far fetched idea to think that the mass is cubically or at least quadratically proportional to the length of the rotor.
Unfortunately I can't remember the source where I saw the 5th power figure.
In addition, the aerodynamics of a rotor don't scale up linearly either.
Your assumption is way too simple. Mass increases with the cube of the scale, while cross section area increases with the square of the scale. The cross section will need to increase to cope with the extra mass, and that in turn will increase the mass itself, and this mass is specifically the rotating mass.
Comments
The explanation is simple: quadrotor physics do not scale up. The rotational inertia of the rotors is proportional to the length of the rotor blade to the fifth power. Double the rotor blade length and the inertia of that rotor goes up by a factor of 32.
High inertia rotors are not suitable for quadrocopter because fast changes in the rotation of the rotors required for maneuvering them.
Having one big rotor for lift and three (or more) small ones for control is a very good idea. Another idea that is being researched is having multiple quadrotor copters flying in co-operation.
The moment of inertia for a uniform rod pivoting on end is (1/3)ML^2 where M is mass and L is length.
Assuming mass scales with length, inertia should be proportional to the third power. Where am I wrong?
Mass is not linearly proportional to length, because a longer rotor needs to be a lot heavier to be structurally sound. A uniform rod is not a good analog for a rotor blade. It's not a far fetched idea to think that the mass is cubically or at least quadratically proportional to the length of the rotor.
Unfortunately I can't remember the source where I saw the 5th power figure.
In addition, the aerodynamics of a rotor don't scale up linearly either.
Your assumption is way too simple. Mass increases with the cube of the scale, while cross section area increases with the square of the scale. The cross section will need to increase to cope with the extra mass, and that in turn will increase the mass itself, and this mass is specifically the rotating mass.