What would be better a helicopter with long thick blades or one with small thin blades?

Question

Its a hard question

Answer 1

Surely long and thin blades would be best - they are still light while providing the necessary surface area:volume ratio?

I'm not an expert but that would be my suggestion

Answer 2

I think you mean long&thin vs short&thick.

It's quite complex...

The blades area/rotor disk area ratio is quite comparable to airplanes wingchord/wingspan ratio. High values mean that is more suitable for high speeds and low altitudes and less efficient, low values mean more suitable for high altitudes, slow speeds and a better efficiency.

In helicopters there are 2 big problems, that the advancing blades must not overcome mach1 and  that the returning blades must not stall.

imagine the helicopter in hovering. the rotor turns at  w angular speed, the bladetip distance from axis is R. bladetip speed is w*R. if the helicopter starts moving in the air(horizontally) at speed S,  the resulting bladetips airspeeds will be S+(w*R) for advancing and S-(w*R) for returning blades.

Shorter blades mean that the rotor must turn faster(w increase) to produce the same lift, this mean that with a relatively small fightspeed the advancing bladetips will become supersonic , and this limits the top speed.

Longer blades mean that the rotor can turn slower(w smaller) so wing tips are slower(not too much because the distance is longer) so the speed is less limited by the advancing bladetip, but in this case the returning bladetip will stall at a slower airspeed(S)

other problems is also that sa you fly faster more the lift resultant shifts to the advancing blades side, and that if the rotor has not a quite advanced configuration if you fly faster the main rotor will turn its plane(advancing side goes up and returning side down).

which is better? almost impossible to answer, but maybe you can determine the best configuration for given specifications determined by the project destination.

For example "the best" for a skycrane(required heavy lifts at slow speeds) for example used for ropeway building(usually high altitudes) will be different from a VIP "executive" helicoper(relatively light-lift, required high speed and comfort), and also different from a marittime Patrol/SAR helicopter(low altitudes, requires long-range/long-endurance, stand windy/stormy conditions, medium-lifts).

you must take into consideration that the axial force along the blade ("centrifugal force") increases with the blade mass, blade radius and angular speed..

another thing you can change is the number of blades.

Increasing the number of blades you can make the rotor turning slower without longer blades. In the other hand you obtain more mechanical complexity and maybe more useless mass. and  a loss of efficiency because each blade influences the others....

Answer 3

long thick would be a challenge to material selection, while the small thin would provide not enough lift.

the two means to increase the flow throught the rotor disc are increasing the blade length and increasing the number of blades.

speaking of helicopters, start with maximum allowed diameter due to the compressive effects at the end of blades during the maximum forward flying speed. then take into the consideration general purpose and desired maneuverability of your helicopter.

Most common types of rotors and the easy to built rotor hubs are the those having 4 blades.

Answer 4

I'm assuming that by thin and think, we are talking chord (length from leading edge to trailing edge) In that case, as with all wings, a high aspect ratio will be most efficient, that being a long thin blade. The reason you see 4 and 5 shorter rotor blades, is because it's not practical to have such a long blade due to weight, as well as clearance requirements between things

Answer 5

Remember that if you have a too fast rotation, you may have a blade going supersonic, with all consequences, so I guess small quick blades could be much worse...