A few months ago, I read the Wright Brothers biography by David McCullough. It inspired me to create. I have always been interested in flight and have enjoyed RC airplanes in the past. The widespread popularity of the quad-copter has sparked a new interest for me.
I am a mechanical engineer, and work in the process control industry. Part of my job involves understanding the inter-relationships between process variables in order to better design a control strategy for the process. Along those lines, one day I got to thinking about the equations governing flight of a helicopter.
At it's most basic level, lift is produced by transferring momentum to the surrounding air. The lift is proportional to the rate of momentum transfer which is proportional to (p)(A)(v^2) where p is the density of air, A is the area of the rotor disc, and v is the velocity of the air downdraft produced by the rotor. Additionally, I learned that the power required to produce this lift is proportional to (v^3). What this meant to me, is that theoretically, I should be able to build a very low power helicopter if I made the rotor disc large enough. Of course, reality intervenes, as it often does, and practical limitations (such as weight, strength, of rotors) steer us to some reasonable compromise between all of the competing design parameters.
With that said, my goal is to design a full scale, low power, quad copter, and then with any luck, build it, and fly it. One of the drivers for this idea was the prospect of eliminating the complication, and tremendous vibration and fatigue associated with the cyclic pitch control of a "traditional" single rotor helicopter. I want to make something more simple, like the small quads. I believe that simple somehow inherently associates with "reliable". Not always true, but a worthy goal.
The basic parameters of this project are (currently) as follows:
Rotor Diameter = 4 meters
Number of Rotors = 4
Empty Weight = 96 kg
Gross Weight = 200 kg
Rate of Climb = 2.5 m/s
Initially, after finding that the theoretical power required to hover 50 kg with a 4m rotor was around 2.7 HP, I was thinking that I could just put a compact power pod consisting of a small 2 cycle engine, gearbox and rotor at the end of each of the 4 arms, and away I would go. Then I started reading about "why we don't have large quad-copters", and realized the inertia of the large rotor makes control an issue as the individual rotors cannot respond with the quickness that a small plastic propeller can. This lead me to the conclusion that I would have to use variable pitch (collective pitch) control rotors in order to get quick response from the aircraft. That gets messy, and is anything but simple, but since my hope is to one day hop aboard this creation, safety is a significant concern, and without control nothing else much matters. So after concluding I need variable pitch rotors,
I thought some more about the 4 engine design, with the 4 engines located at the rotor hubs. I did a quick estimation of the aircrafts moment of inertia in the various axis (roll, pitch, yaw). NOT PRETTY. If I lost an engine, I would be upside-down, and whirling around in a fraction of a second. No time to cut power to the other engines, just a big airborne tilt-a whirl. YIKES! So that revelation let me to a single engine design. OK, not as radical as the 4 engine design, but certainly more sensible.