solid fixed pitch rotor blade.

[douglas4]

Guys im no engineer but i posted to this thread to find a diffinitive answer. im working on a rotor craft idea i really need advice on. im using an r/c helicopter as a basis for my experiment. This is what i plan to do. Now think bout the blade on the stick toy you spin in your hands and let go and it flys. well i want to design a solid rotor blade 38" long with blade cord of 2" tip to tip and rounded tips like the toy heli stick thingy. now ill be making it out of maple and will have a 10 degree fixed pitch. i have to lift a 3lb helicopter and use the tail rotor to steer by varying left right tail movements using an speed control with a electronic gyro combonation. i will use weight in front to produce forward flight which will shift the CG slightly forward to have slight nose down atittude hoping it will still be stable enough to fly. what am i missing....?

 

[MikeHalloran]

With fixed main rotor pitch, your only control of lift thrust is by varying the rotor speed. You'll need an engine with a wide powerband, and considerable excess torque available in order to accelerate the rotor to 'flare' and not crash when landing.

If the tail rotor is geared to the main rotor, then its thrust will also be affected by changing engine speed, so you'll have a yaw moment strongly coupled into the lift control.

The moving weight for pitch control might work, but it doesn't give you roll control, so you won't be able to fly sideways or bank or counteract any uncommanded roll.

Natural stability to a limited degree may require that you add dihedral to the main rotor.
















Mike Halloran
Pembroke Pines, FL, USA

 

[douglas4]

thanks for applying Micheal. I see it this way as well. because of the weight of the heli being 3lbs im hoping the heli body will hang under the rotor like a pendulum affect just like the plastic stick of the hand twisting heli toy idea. moving the CG forward by adding weight ever so slightly to give forward momentum nominally will work i suppose. i dont think there would be any unwanted roll characteristics if the whole system is balanced dynamically. any yaw with the tail rotor would be controlled with the use of an electronic gyro and speed control combo. hooked up in unison the gyro will tell the speed controller to speed up or slow down the rear tail motor to correct the yaw and keep the nose straight. the model doesnt have a gear drive or belt drive for the tail. the main motor is electric as well and designed for the model. it will have its own speed controller to vary the rotor rpm. i have to have the main shaft extended above the fuselage so the heli will hang under it when the rotor is spinning. as i mentioned before the chord on the blade will be 2" end to end with rounded tips like the toy thingy and fixed pitch of 10 to 15 degrees which i feel should be sufficient for lift. the more pitch as i see it the less rpm needed the less the motor has to work to gain lift i think.

 

[Gruntphuttock]

The more pitch the greater the torque and power required. Also the blades will "cone" upwards if the rotor speed is insufficient unless you make the blades stronger and, hence, heavier. They do have to be spinning fast enough to generate the required lift.

I suspect you will find that speeding up or slowing the tail rotor for directional and torque reaction control is hopelessly slow. A constant speed with pitch control of the blades or "fenestron" will achieve the necessary reaction time