This Page is dedicated to RC experiments we are currently conducting or will be doing soon.
Quadcopter (Quadrotor) helicopters are emerging as a popular platform for unmanned aerial vehicle (UAV) research, due to the simplicity of their construction and maintenance, their ability to hover, and their vertical take off and landing (VTOL) capability. Current designs have often considered only nominal operating conditions for vehicle control design. This work seeks to address issues that arise when deviating significantly from the hover flight regime. Aided by well established research for helicopter flight control, three separate aerodynamic effects are investigated as they pertain to quadrotor flight, due to vehicular velocity, angle of attack, and airframe design. They cause moments that affect attitude control, and thrust variation that affects altitude control. Where possible, a theoretical development is first presented, and is then validated through both thrust test stand measurements and vehicle flight tests using the Stanford Testbed of Autonomous Rotorcraft for Multi-Agent Control (STARMAC) quadrotor helicopter. The results enabled improved controller performance.
As the collective pitch changes, the amount of power required to keep the main rotor spinning at the same RPM changes, and must be compensated for by changing the throttle. In model helicopters, the collective pitch and throttle are controlled by the same control via the pitch curve and throttle curve, which specify how much pitch or throttle to use at each position of the collective control. Different pitch and throttle curves can be selected for different types of flight using the flight mode switch. The pitch of the blades can be measured and so adjusted using a pitch gauge. On quadcopters the blades are fixed or also knows as fixed pitch.
Fixed Pitch (FP) is exactly that. The pitch of the main rotor blades are held at a fixed or constant angle. You control the amount of lift to your quadcopter (multirotor) by simply varying the speed of the engine/motor. If you increase the speed of the motor, the rotor blades turn faster and produce more lift. The reverse happens if you lower the speed of the motor.
This is a very simple means of controlling the amount of lift to your RC quadcopter and if you just want a very simple and inexpensive, this method of controlling lift does work. This is one of the main reasons fixed pitch is generally what most people who get into the hobby first start out on. Fixed pitch quadcopters and helicopters are also generally more stable in a hover (in no wind) due to the shape of the high lift rotors and the lower speed in which they spin. There are several draw backs to this type of lift control however.
First off, motors and/or engines, can’t just speed up and slow down instantaneously when you give them a command to do so. The mass of the rotor blades and rotor mechanism compound this problem as the engine or electric motor has to overcome the inertia to get the rotor spinning faster or the kinetic energy stored in the rotor assembly to slow it down.
So here is where our experiment comes into place..
Most model collective pitch helicopters can be configured so that in addition to ‘positive’ collective pitch for climbing and normal flying, they also have equal ‘negative’ collective pitch, allowing very rapid descents, autorotation and inverted flight. Nowadays it is normal to set up a helicopter to have equal amounts of positive and negative pitch, and limit the available collective pitch via the pitch curve. Quadcopters in the other hand do not have collective pitch, control is done through the speed of the motors, thus having a very slow and sometimes uncontrollable decent if the wind is too strong.
A helicopter pilot manipulates the helicopter flight controls in order to achieve controlled aerodynamic flight. The changes made to the flight controls are transmitted mechanically to the rotor, producing aerodynamic effects on the helicopter’s rotor blades which allow the helicopter to be controlled. For tilting forward and back (pitch), or tilting sideways (roll), the angle of attack of the main rotor blades is altered cyclically during rotation, creating differing amounts of lift at different points in the cycle. For increasing or decreasing overall lift, the angle of attack for all blades is collectively altered by equal amounts at the same time resulting in ascents, descents, acceleration and deceleration.
A typical helicopter has three separate flight control inputs. These are the cyclic stick, the collective lever, and the anti-torque pedals. Depending on the complexity of the helicopter, the cyclic and collective may be linked together by a mixing unit, a mechanical or hydraulic device that combines the inputs from both and then sends along the “mixed” input to the control surfaces to achieve the desired result. The manual throttle may also be considered a flight control because it is needed to maintain rotor speed on smaller helicopters without governors. The governors also help the pilot control the collective pitch on the helicopters main rotors, to keep a stable, more accurate flight. To be continued…
As we gather enough funds and parts for these experiments, we will post the progress, photos and video clips in this section. If you would like to help, donate or simply send us any spare parts that you think might help us we will be very grateful for it!