Getting started in Multi-Rotor models can be a bit overwhelming for someone new to this segment of the hobby. There are dozens of models available today, in a wide range of configurations, with more being added every month. Different models such as Quad-Copters, Hex-Copters, Octo-Copters and Tri-Copters, as well as configurations such as X-8, Y-6, and V-tail Quads are now available. With all of these different frame styles, one might ask, “Where do I start, and which configuration is best?” In this month’s Multi-Rotor Flight column we will take a look at several of the most common configurations, and consider the pros and cons of each type.
By definition, any hovering type airframe with 2 or more rotor blades can be called a Multi-Rotor aircraft, and there are many different configurations available. A quick glance at any of the multi-rotor forums will turn up models with 2, 3, 4, 6 and 8 motors, in a variety of different sizes and styles. Some models offer better maneuverability, while others offer more lifting capacity and power system redundancy. In most cases, any frame that has an even number of motors does not require any servos or other auxiliary controls to provide yaw control. For models with an odd number of motors, such as a Tri-Copter, a servo needs to be used to tilt one of the motors left and right to provide yaw control.
The most common configuration available today is the four-motor type, which is typically called a Quad-Copter or Quad-Rotor, or simply Quad for short. This configuration uses four motors, four speed controllers, four props along with a flight controller board to run everything. The Quad-Copter frame is available in two variations: The Quad-X as seen in Figure 1 and the Quad-Plus that is shown in Figure 2. In all of the figures shown throughout this column, the Red or Blue arrows denote the direction of propeller rotation and the black arrow indicates the forward direction of flight for each frame style. The propeller directions shown are typical, but can vary depending on the brand of flight controller board used. Always check the owner’s manual for your specific flight controller board to get the proper motor rotation directions.
The Quad-X frame is very popular, and can be seen in dozens of different commercially available models. This configuration allows the positioning of a camera facing forward in flight, without having the propellers show up in the camera’s field of view, so it is a natural choice for FPV type flying. On the down side, it can be difficult maintaining orientation on this type of configuration without the use some colored markings or LED position lights on the frame.
The Quad-Plus frame is often used by airplane pilots transitioning from fixed wing flying because it offers a more familiar outline in flight. The motor in front can be thought of as the “Nose” while the motor in back can be seen as the “Tail.” The two motors and arms that extend left and right appear like a “Wing” in flight, and this can be easier to maintain visual orientation. This configuration does have a couple drawbacks however. First, if you want to add a camera, you will almost always have one of the motors and props directly in your field of view, and second, at higher forward flight speeds, having the two side motors spin in the same direction can cause an unwanted roll coupling due to changes in lift. The outer blade is spinning into the on-coming wind on one side and with the wind on the other, so the lift created shifts left and right as the forward speed changes.
All Quad configurations have the disadvantage of not having any power system redundancy. If a motor or speed controller fails in flight, and power is lost on one motor, the machine can no longer be controlled and will flip over onto its back and crash. The good news is that with today’s reliable electronics, it extremely rare for this to occur under normal circumstances, and the lower cost and simplicity of the Quad style outweighs the chance of power system failure.
The next most popular Multi-Rotor configuration would be the six-motor type, normally called Hex-Copters. Just like the Quad configurations we just looked at, the Hex-Copters are available in a Hex-X configuration as shown in Figure 3, and a Hex-Plus style as seen in Figure 4.
The six-motor configuration does add a couple important benefits over the Quad style frames. First is the fact that you get 50% more available thrust due to the two extra motors, but only have a weight increase of around 15 to 20%, because the frame has two more arms, motors and speed controllers. This gives the ability to carry heavier loads in basically the same size machine. On the down side, 6 motors will draw more power from the battery than 4 motors will, so there will be a slight reduction in flight time, and if you are using the full power of all the motors you will see around a 35% reduction in flight time, but a larger battery will take care of that.
The biggest benefit from a six-motor configuration is the fact that the machine gains a level of power system redundancy. If a motor loses power, and the flight controller board is set up correctly, the craft can continue to fly. The motor that is opposite the dead one will reduce power to even out the thrust imbalance, and the craft will still continue to fly on the four remaining motors in what is effectively a long skinny Quad configuration. This gives a Hex-Copter the ability to limp home safely if a motor fails in flight instead of falling out of the sky.
Another popular frame style is the eight-motor configuration, commonly referred to as an Octo-Copter. This type of frame is typically used where lifting heavy payloads are required, or when higher levels of power system redundancy are desired. Once again, there are two common variations to this frame type, the Octo-X shown in Figure 5 and the Octo-Plus shown in Figure 6. With eight motors, this configuration can lose power in 2 motors at the same time, as long as they are not right next to one another, and still be able to fly in most cases. This configuration is commonly used for aerial photography, where the camera is carried underneath the frame in a stabilized gimbal assembly. When you carry a camera that may weigh upwards of 10 pounds, and cost $20,000 or more, you need a lot of lifting power AND a high degree of power system redundancy to ensure that the expensive camera does not come plummeting to the earth in the event of a motor failure.
There are a few more frame types that are not as common, but still deserve to be mentioned. The three-motor Tri-Copter configuration is commonly seen on the multi-rotor forums, and it also has some advantages and disadvantages associated with it. A typical Tri-Copter can be seen in Figure 7. Because there are only 3 motors, there is a very wide spread between the front motors, and this gives an unobstructed view forward for good camera visibility. Because of their low weight and reduced moment of inertia, the Tri-Copter frames are usually very aerobatic and can perform aggressive maneuvers with ease. The disadvantage to a Tri-Copter is that since there are an odd number of motors, you cannot cancel out all of the rotor torque, and it is difficult to get yaw control the way you would with a Quad or Hex configuration. To create Yaw control in a Tri-Copter, the entire rear motor needs to be rotated left and right to provide thrust vectoring in the direction you wish the tail to move.
Some Tri-Copter designs use a servo in the center body of the craft and rotate the entire arm to provide this movement, while others will use a servo out at the end of the arm and rotate just the motor mount. In either case, there is the complexity of some type of pivot mechanism connected to a servo with pushrods, ball links and other support structures. This assembly can produce a weak spot in the frame, and is usually one of the first things to break in a crash.
Another variation to the Tri-Copter type frame is the Y-6 configuration, which can be seen in Figure 8. In this frame, you still have three arms, but each arm has two motors attached, one on the top and another on the bottom. By having the top motors rotate one direction and the bottom motors rotate in the opposite direction, the motor torques cancel out, and this eliminates the need for the pivot mechanism that is required for a Tri-Copter frame to provide Yaw control. This system also provides a degree of redundancy, since if one motor stops running, the other motor on the same arm can speed up to compensate for the loss of power, and the craft can continue flying. The Y-6 frame also has the same wide open field of view to the front, and this makes for a good camera platform.
The one downside to the Y-6 airframe is the fact that the lower prop on each arm is running in swirled turbulent air from the propeller above it. This can create inefficiencies in the lower motors, so a Y-6 platform will usually require a bit more power than a similar size Hex frame of similar size and weight.
Another frame style that is occasionally used is the X-8 configuration. This frame is similar to a Quad style, but uses two motors per arm, just like the Y-6 that was just shown. Again, this configuration provides higher power in the same size airframe, and give the ability to continue flying if one or more motors loses power, but also suffers the inefficiencies caused by one prop being directly below the other.
This covers the most common frame configurations that are used in Multi-Rotors, but is in no way a complete list. Other configurations have been tried, with varying degrees of success, and some offer very interesting flight characteristics. The only real limit to different frame styles is the availability of flight controllers that can be re-programmed with the new configurations that allow the aircraft can be flown. The open-source type boards allow for user changes to the firmware, so if you, or someone you know, can write the code to make your new design fly, then virtually anything is possible.
That wraps up another installment of Multi-Rotor Flight. Next time we will take a look at several of the different flight controller boards that are currently available, and look at the options and limitations of each one. Until then, enjoy flying your Multi-Rotor aircraft!