Quadcopter design

A torque free architecture

Whilst the main frame (hub) houses all the electronics, four radiating arms (booms) provide stable mounting positions for 4 rotors. Each rotor consists of a propeller driven by a motor controlled by an ESC (electronic speed controller) which is usually located on a ventilated location near the main hub. Applying Newton’s third law of physics – for every action force there is an equal and opposite reaction – a quadcopter’s clockwise and counter clockwise rotors results in a torque free system. In contrast, a conventional helicopter’s single clockwise rotor forces its fuselage to rotate in a counter-clockwise direction which is then countered by a very complex and vulnerable tail rotor system. With 4 rotors doing the work of one, quadcopter rotors also don’t have to rotate as fast for less twitchy controls.

Motors and maneuvers

Quadcopter motor#1 and motor#3 are counter-clockwise and motor#2 & motor#4 are clockwise. Increasing and decreasing rotor speeds on any side creates the pitch (tilting forward or backward) and roll (banking right or left) responsible for a quadcopter’s lateral directions. The synchronized speed of all motors provides throttle (climbing & descending) to control the quadcopter’s altitude. As opposed to using the tail rotor for the yaw (turning right or left), a quadcopter utilize the torque differential between the clockwise and counter clockwise propellers. For example, by speeding up the clockwise rotors and/or slowing down the counter-clockwise rotors, the quadcopter gathers more torque towards the counter-clockwise direction thus turning the quadcopter’s heading to the left.

The importance of center of gravity

Having a well-balanced CG (Center of Gravity) is crucial to any aircraft operations. In the case of quadcopters, CG is usually located in the midpoint between the four rotors. Any payload, such as camera and gimbal not located at the quadcopter’s CG are usually compensated by moving the battery in the opposite direction. Without proper CG, one or more of the quadcopter rotors will constantly strain to maintain balance. Due to a lack of knowledge, experience and planning, CG is often the Achilles’ heel of small integrators & DIY quadcopters. A flight controller with associated gyros should be located at the CG. To adjust and verify proper CG, find 2 opposing points next to the flight controller and lift the quadcopter with your left and right index fingers. The quadcopter should represent a well leveled seesaw. If the quadcopter is not properly balanced, the operator can adjust CG by moving either the payload or the battery until the seesaw levels.

Aerodynamics

A quadcopter’s aerodynamics is quite different than that of an airplane. Unlike wings providing lift on an airplane, extensive quadcopter surfaces represent a significant liability in wind resistance. This is especially critical in windy conditions and when a quadcopter is descending. More advanced flight controllers and electronic flight controller firmware offer algorithms that address both issues.

Structural integrity without excess weight

Inadequate infrastructure can negatively impact larger quadcopters more than smaller systems. Since a smaller quadcopter possesses neither high torque nor long arms, rigidity on smaller toy quadcopters doesn’t have the same relevance compared to larger structures. The lifting power of rotors provides a quadcopter’s budget as in the overall takeoff weight. If you exhaust this budget with a heavy structure, you will have nothing left over for payload. Therefore, larger quadcopter with more powerful rotors require the integration of stronger, but lighter materials. Strong and rigid integration of carbon fiber or aluminum booms, brackets and propellers on a well-structured central hub are hallmarks of an uncompromising design. Cheaper quadcopters usually resort to mass-produced plastic injection arms and brackets that result in excessive flex and mass.

Simplicity eases maintenance and repairs

If you haven’t notice already, there are no gears or long shafts on quadcopter designs. Fast spinning gears on long vulnerable shafts are a pain in the neck to align and maintain on helicopters. Ever try mounting pinion gears on helicopter motors? Never mind, that’s another nightmare you don’t have to deal with on a quadcopter. In fact I’m dumbfounded that commercial helicopters aren’t quadcopters.  One possible factor is the cost of four sets of motors and controls (ESCs) versus one. On smaller cheaper quadcopters, a $10 set of motors and ESC multiplied by 4 is only $40. But on a much larger quadcopter, with a $200 set of motors and ESC the total cost for 4 motors and ESCs is now escalates to $800.