Simple Variable Pitch for Brushless Motor

With a brushless motor and a TRex tail rotor mechanism I could assemble a simple variable pitch system. I measured and it generates 700g with the TRex 500 tail rotor blades.

I tried the Trex 250 blades from the main rotor but although they are longer the thrust is smaller, just 400g. This is probably because the motor doesn't have enough torque.

This could be used for a variable pitch quadcopter or with an airplane.

Motor: Hextronik DT750
ESC: Plush 25A
Battery: Turnigy 2200mah 25C
Tail rotor: HK-500GT
Wood to support the tail mechanism: 38mm tall

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  • Matt,an

    Autorotation works on two principles actually - trading potential energy for kinetic energy and the intertia of the rotor system.  In an autorotation, the upward flow of air through the rotor system drives the blades.  The angle of attack of the blade's airfoil (the pitch of the blades) can slow the rotor system and arrest the descent rate.  There are two mechanical actions that need to happen for this to happen - a decoupling of the rotor system (the rotors are allowed to spin independent of the motor) and pitch control during the auto.In helicopters, there are two phases to an autorotation, the entry/descent and the flare.  The entry/descent is simply a controlled descent utilizing the principles of changing the blade pitch in order to arrest the descent rate and maintain maximum rotor efficiency.  Rotors become less efficient if they get too fast.  The flare is the final phase, where the pilot introduces a nose up attitude to slow his airspeed to near zero.  At the same time, the pitch of the rotors is decreased to gain a higher rotor speed and mainitain a rate of descent instead of ballooning to a higher altitude.  This serves to increase the intertia of the blades.  In the final seconds before ground impact, the pilot levels the nose attitude and increases the blade angle, which slows the rotor system, and arrests the descent to cushion the landing.  The goal of this flare maneuver to impact the ground as near to zero groundspeed and zero ft/min descent as possible.    

  • Ok, so we are getting to the point of getting four helicopters and connecting them together with sticks.  Starting to see why some people have claimed this as busted.

  • Matthew, the main rotor would be better for using larger blades, however it would be more difficult to change the pitch angle (collective) since it has a swashplate. It would need some modification ;)

  • Sorry, auto-rotation not authorisation.
  • I don't know if a flightgear or xplane model would you experiment. Not sure if the authorisation functionality is there.

    Can't help feeling that you are better off using main rotor heads instead of tail rotor mechanics since they are designed to take the load. Just a hunch. What do you think?
  • Thanks for the tips Joly! So may be autorotation with 4 motors and 4 variable pitch mechanisms wouldn't be possible since they may not turn at the same speed

  • Autorotation? Use a one-way bearing like we do in helis :) cheap, light and effective. 

    The next issue is keeping all 4 props spinning at the same rate without power. If you used a central motor and torque-tubes/belts to mechanically connect them it wont be an issue.

    Also, If you have bigger blades then you will have more kinetic energy to use on the way down -- but you know that. Nice work :)

  • Hhhauhauah i thought it would be as dangerous as a 12" prop rotating, but may be it's sharper?

  • hands down:  one of the most dangerous videos I've ever seen.  

  • Yes a clutch would help but would be very complex to add. I think that may be better to use larger blades and maintatin the motor always attached to the blades ;)

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