Item 1716

 OTHER ~ Flight Dynamics - Rotor Hub - Cyclic Control for a Simple Ultralight Helicopter

Objective:

  1. To evaluate various methods of providing enhanced cyclic rotor control for a simple ultralight helicopter.
  2. Then evaluate the preferred method in one or more twin-rotor configurations.

Methods:

Weight-Shifting Hub:

Pros:

  • Gives a fast response to pilot inputs.
  • The gap between a pair of coaxial rotors can be small.

Cons:

  • Is good for extremely small rotors (propellers). Conventionally sized rotors may require an excessive force by the pilot.
  • Autorotation is impossible due to the high disk loading on the small disk area.

 Existing Coaxial Craft:

More Detail:

  • The response to directional control inputs is primarily related to the ratio of rotor-assembly tipping rate to fuselage tipping rate.
  • This implies that it is advantageous;
    • To put the weight in the fuselage when possible.
    • Have the weight moment arm of the rotor-assembly about the gimbal as short as possible.
    • To minimize the out-of-plane profile drag of the rotor.

_____________________

Direct Control Hub:

The above link is to a conventional gyrocopter rotorhead. This proposed method would be slightly different due to its application in a coaxial rotor head.

 

Pros:

  • Allows control over conventionally size rotors with minimal piloting force.

Cons:

  • Requires a large gap between coaxial rotors due to their independent teetering.

Note:

  • Is the method used by gyrocopters. (un-powered & single rotor) 

Existing Coaxial Craft:

Idea:

Direct Control Hub with Hub Spring:

The feathering hinges in the sketch below will not be needed.

The springs operate between the hub-bar and the mast.

Pros:

  • Gives greater control than the Direct Control and requires much less force by the pilot than a Weight Shift control.
  • The required force increases exponentially with the angle of cyclic input.
  • It should be possible to put a 'trim tab' in the cockpit to minimize the required force during a prolonged flight attitude.
  • The springs can be adjusted for desired control assist.

Cons:

  • The springs will create a 2/rev vibration when the cyclic stick is moved off of its mean position.

Notes:

  • A conventional Direct Control hub, as per the above, but with the inclusion of teetering restraint springs and a double gimbal joint for constant velocity. The control arm is connected to the post by a scissors mechanism and to the rotor by a thin ring bearing.
  • The compression springs (die springs) are located between the hub-bar and the mast.
  • This link is to a rotorhead that is similar except for its 3rd blade and feathering bearings. DESIGN: SynchroLite ~ Rotor - Hub - 3-blade - CVJ & HS - Layout

 

The following 2 methods must be reconsidered in respect to how they are applied to a Direct Control Hub.

The following layouts will align the tip-path axis with the control axis, whereas the objective is to force the shaft axis to align with the tip-path axis.

Bi-Teetering Hub:

 

Notes:

  • This is an alternative to the Hub Spring. The characteristics of the two should be quite similar.

 

Pros:

  • Advantages approaching those of the Weight Shifting and the Direct Control.

Cons:

  • Disadvantages approaching those of the Weight Shifting and the Direct Control.

Tri-teetering Hub:

The above link is to a rotor head c/w with full flight controls. This proposed method would be simplified version of that in the link.

 

Pros:

  • Will reduce the vibration over that of all the 2-blade rotors above except for the Direct Control.

Cons:

  • Additional weight, cost and complexity; due to the addition of a third blade and its hub components.

Notes:

  • For an example of a Tri-teetering Hinge see Dragonfly.

Notes:

Related Pages:

Introduction Page | SynchroLite Home Page | Electrotor Home Page | UniCopter Home Page | Nemesis Home Page | AeroVantage Home Page:

Last Revised: August 13, 2011