The Agile Aircraft Project

Background

Variable pitch propellers and rotors are widely used in aircraft. In winged aircraft, variable pitch helps match propulsion to the requirements of each different stage of flight. In helicopters blade pitch is varied over each rotation cycle to provide both lift and flight control.

Variable pitch propellers have been used in multi-rotor electric Vertical Take Off and Landing (eVTOL) aircraft in which the design requires propellers to provide both vertical lift for takeoff and efficient thrust for horizontal cruising at a high advance ratio. The goal to optimise performance over a wide but slowly transitioning range of advance-ratios does not require a quick-acting collective pitch control mechanism. Other designs for low advance-ratios have often used fixed-pitch propellers, perhaps to reduce complexity and expense.

However, a simple, quick, variable-pitch rotor capability in electric VTOL aircraft could help achieve aircraft design objectives 1,2,3 & 4

In helicopters, collective and cyclic pitch control is typically provided via a swash-plate with links to the rotor and connection to a control mechanism. But this is a critical and complex system that is inevitably expensive to manufacture and maintain to adequate safety standards.

In winged aircraft propeller-propulsion systems, variable-pitch is often provided, in the propeller spinner using an actuator operated hydraulically, electrically, or mechanically by a controller that maintains a desired rotation rate. This collective-only pitch control is much simpler than the full cyclic control necessary in a helicopter.

But such collective-only pitch controls are designed to achieve power-system efficiency rather than very quick pitch change, and are still an expensive level of complexity for a small drone, or even a bigger vehicle, if they have multiple lift-rotors.

Simpler variable-pitch designs like [1 – 8] appear not to have been widely used to date. One reason is that simpler designs almost inevitably entail behaviour limitations. Therefore it is crucial that a simpler design be matched to the intended use-case.

The following designs are simple, requiring no mechanism outside the rotor blades and hub, but are intended to optimize performance over a wide range of advance-rates. The objectives 1,2,3,4  are very different. Consequently, none of the following designs can provide very quick adjustment of lift at slow advance-ratios, such as when hovering, in order to compensate for buffeting of the vehicle by wind gusts:

1. Centrifugal-force actuated mechanisms like [6], [7] do not satisfy objective 1A, since when hovering; an increase in rotation rate is needed to change collective pitch and increase thrust, which will therefore be delayed by rotational inertia of the rotor.
   
2. Aerodynamically tailored propellers like [1],[3],[7]  have the same limitation as (a), although different elastic tailoring might allow a solution.
   
3. The constant-torque design [2] is simple, but when in steady hover, suddenly applying more power will result in a sharp decrease in lift as the lift-rotor immediately sets it’s blades to a finer pitch. Although this transient loss of lift will subsequently at least partially recover, recovery will be delayed by rotational inertia as the rotor accelerates to a new steady state. And data in [2] shows states where the final lift is less than the original. These behaviours are not what are needed to compensate for buffeting by wind-gusts, and do not satisfy objective 1A.
    
4. The “Passive Variable-Pitch Propeller” [1] exploits the aerodynamic pitching moment of the blades to optimise efficiency over a wide range of advance-rates. This requires special blade aerofoils, which may not be optimal in other ways, and careful mass-balancing to allow the relatively small aerodynamic pitching moment to dominate.
   
   This design does not satisfy objective 1A, since the only way to increase lift at a fixed advance-rate, such as when hovering, is to force faster rotation. Although faster rotation should result in a coarser pitch, the consequent lift increase is nevertheless delayed by rotational inertia of the propeller.
   
5. The cyclic pitch control designs [4] and [5] are a clever use of torque-modulation to provide very simple means of control of vehicle attitude, as for a helicopter. But neither [4] nor [5] provide control of collective pitch, so neither satisfies objective 1A: An increase in rotation rate is needed to increase thrust, which will therefore be delayed by rotational inertia of the rotor. And neither design satisfies objective 2A, since loss of power implies loss of control of the vehicle ([5], page 75).

But the objectives 1A and 2A can be achieved with a novel, simple, and very quick lift-rotor collective pitch control.

References

• [1] S Heinzen, C. Hall, A. Gopalarathnam (2015) Development and Testing of a Passive Variable-Pitch Propeller.   Journal of Aircraft 52(3): 1-16
  
• [2] C. Burger, R. Hartfield (2007) Design, Testing and Optimization of a Constant Torque Propeller. 25th AIAA Applied Aerodynamics Conference 25 – 28 Jun 2007, Miami, FL
  
• [3] Dwyer, W.J. and Rogers, J.B. (1977) Aeroelastically tailored propellers. Society of Automotive Engineering Meeting, SAE Paper No 770455
  
• [4] Y. Fujihira, R. Sasaki, M. Ando (1993 ) Propeller blade tip path plane inclining device. US patent  5259729 A
  
• [5] J. Paulos (2017) Rotorcraft blade pitch control through torque modulation. Publicly Accessible Penn Dissertations. 2901. https://repository.upenn.edu/edissertations/2901
  
• [6] O. Morelock (1945) Automatic pitch change propeller. US patent 2547037
  
• [7] H.Reissner, C.Manfried (1938) Variable pitch propeller for aircraft. US patent 2275053A
  
• [8] B.Gemeny  (1934) Automatic variable pitch propeller. US patent 2030953