Building an FPV Racing Quadcopter, Part 2

By Terry Dunn

Today, let's talk about how to wire up the flight controller to the other quad components and set up its various flight modes in OpenPilot's Ground Control Station.

In the first installment of this series, I assembled most of the frame components of the Strider Mini Quad. I also installed and soldered the motors and ESCs. Although the flight controller was installed in the frame, it still required attachment of the various wires and configuration of the firmware within. Let’s focus on those tasks and keep moving.

Plugging in the CC3D

The flight controller is the nerve center of any multi-rotor. It takes your control inputs and the data from its onboard sensors and translates it all into commands for each of the ESCs. There are several different brands of flight controllers. Considering all that they do, most of these units are incredibly small. The flight controller I chose is the OpenPilot CC3D (CopterControl 3D), which fits perfectly on the Strider’s stock flight controller mount.

THE OPENPILOT CC3D IS A POPULAR FLIGHT CONTROLLER. IT FITS THE STRIDER PERFECTLY.

From a wiring standpoint, the flight controller is situated between the radio receiver and the quad’s ESCs. First I attached the ESCs to the CC3D. The CC3D has a bank of pins that accept the standard receiver plugs found on most consumer RC equipment. The quad’s motors are numbered sequentially as you go clockwise, with the #1 motor being the front left. I attached the plug from this motor’s ESC to the #1 pins on the CC3D and then followed suit with the other ESCs.

To connect the CC3D to my Futaba R617FS receiver, I used the 8-wire harness included with the flight controller. The colors of my wires didn’t match those on the OpenPilot diagram, so I just referenced the pin order. The first two pins are negative and positive power. The remaining pins are signals for channels 1-6 respectively.

THIS VIEW ILLUSTRATES THE BUNDLE OF WIRES FROM THE SPEED CONTROLLERS (ORANGE/RED/BROWN) THAT ARE CONNECTED TO THE RIGHT SIDE OF THE CC3D. THE MULTI-COLORED BUNDLE OF WIRES EMERGING FROM THE LEFT SIDE OF THE BOARD ARE CONNECTED TO THE RADIO RECEIVER.

PPM (Pulse Position Modulation) receivers like the FrSky model shown in the Strider manual, and Sbus receivers like some Futaba models require only one signal wire for all of the channels. The R617FS is a standard PWM (Pulse Width Modulation) receiver. As such, it has three pins for each channel: positive, negative and signal. The positive and negative connections from the CC3D can be connected to any channel on the receiver. The signal wires must be connected to their assigned channels.

In addition to the connections to the CC3D, I made a signal wire connection from the receiver to a pin on the Strider’s frame. This wire allows a 3-position switch on the transmitter to operate the Strider’s built-in LED lights, lost model alarm, and also toggle crosshairs in the OSD function. I used channel 7 for this, since it was already mapped to a 3-position switch on my Futaba 7C transmitter. By going this route, I did not need the channel 6 signal wire from the CC3D.

ALTHOUGH PWM RECEIVERS LIKE THE FUTABA R617FS USE UP TO 8 WIRES TO INTERFACE WITH THE CC3D, THE CONNECTIONS ARE STILL RATHER SIMPLE. THE YELLOW SIGNAL WIRE EMERGING FROM THE CHANNEL 7 SLOT IS ROUTED DIRECTLY TO THE STRIDER FRAME TO CONTROL ACCESSORIES.

Programming the CC3D

The CC3D must be configured to reflect the specifics of your multi-rotor and radio system. This is accomplished thru OpenPilot’s free interface program, GCS (Ground Control Station). You basically open GCS, attach the CC3D via USB, and walk through the steps of the Vehicle Setup Wizard. Before starting with my board, I watched a few online videos of the process to familiarize myself with the different steps.

As luck would have it, my experience with the wizard didn’t go nearly as smoothly as any of the videos I watched. One of the first steps of the process is to confirm/install the latest firmware on the CC3D. I kept getting error messages during this task. After some research, I decided to try updating the firmware manually – no dice. Even though my board had the correct bootloader software installed, I had to remove and reinstall it. Then, I had to revert back to the wizard to update the firmware (it wouldn’t work manually).

I spent the better part of a morning futzing with the board, mumbling obscenities, researching solutions, trying different ideas, and then clearly annunciating obscenities before I successfully loaded the latest firmware. After that, it was mostly smooth sailing to work through the steps of the setup wizard. When I had questions about any of the parameters, I was able to find a suitable answer on the Open Pilot Wiki or good ole’ Google.

DESPITE INITIAL TROUBLES, I FOUND THE GROUND CONTROL SYSTEM INTERFACE SOFTWARE EASY TO USE FOR CONFIGURING THE CC3D.

You may recall from part one of this series that I oriented the CC3D board 90-degrees off of the suggested direction so that the mini-USB plug would be exposed to the side of the Strider. I suggest you do the same. It is very helpful to have the plug easily accessible during configuration of the board. It is also quite likely that you will need to make updates later on to tune your quad’s flying characteristics.

To accommodate the change in the board’s orientation, you must tell the GCS what position the CC3D is in via the Attitude menu. Since I had my board turned 90-degrees counter clockwise from the default attitude, I entered “90” in the Yaw field. If I had oriented the board 90-degrees clockwise of normal, I would have needed to enter “-90”.

Flight Modes 101

Some of the most important settings that you will input to the flight controller deal with flight modes. Your flight mode configurations define how the quad will respond to control inputs. These settings alone can determine whether a multi-rotor is a docile pussycat or an untamed tiger. So it’s important to understand the basic options.

There are two fundamental flight modes, Attitude Mode and Rate Mode. In Attitude Mode, you define a maximum bank angle for pitch and roll. Where you position the right control stick will determine the bank angle of the quad. For instance, pushing fully forward on the stick will make the quad tilt forward to the maximum bank angle and maintain that angle for as long as you keep the control stick in that position. Returning the stick to neutral will make the quad level off. Moving the stick 50% forward will yield 50% of the maximum bank angle.

UNDERSTANDING THE DIFFERENT FLIGHT MODES IS AN IMPORTANT STEP IN CONFIGURING A RACING QUAD. THERE ARE MANY OPTIONS WHICH CAN PRODUCE VASTLY DIFFERENT FLYING TRAITS.

Attitude Mode is generally preferred by less experienced pilots because of the self-leveling it provides. When you get disoriented, you can just let go of the right control stick to level off and catch your breath. The flip side of Attitude Mode is that you forfeit aerobatics such as flips, rolls and loops. Any maneuver that would exceed your maximum bank angle is out of reach.

Attitude Mode is generally preferred by less experienced pilots because of the self-leveling it provides.

In Rate Mode, you define a maximum rotation rate (degrees per second) for pitch and roll. When you provide a control input, the quad maintains that rotation rate until you tell it to stop (or it hits the ground). Let’s say that you move the right control stick full left, the quad will rotate to the left around the roll axis at the maximum rate until you return the stick to neutral. Since there is no self-leveling in Rate Mode, the quad will stay in whatever orientation it was in when you released the controls. You must provide inputs to return the quad to level flight.

Obviously, Rate Mode is the more challenging flight mode. Yet, it also provides experienced pilots with the control authority necessary to perform aerobatics. I sense that there is an informal rite of passage among quad racers when graduating from Attitude Mode to Rate Mode.

You may have noticed that I didn’t talk about controlling the yaw axis yet. That is pretty much Rate Mode all the time. Open Pilot has a variation of Rate Mode called “Axis Lock” that can be helpful. Basically, Axis Lock attempts to hold the current heading of the quad when outside forces such as wind try to alter it. But your yaw control inputs behave like Rate Mode.

SINCE I MOUNTED THE CC3D 90-DEGREES OFF-AXIS TO MAKE THE USB PORT ACCESSIBLE, I HAD TO DEFINE THE BOARD’S POSITION USING GCS.

There are other types of flight modes available in GCS. Perhaps the only other one that would be appealing to flyers who are new to quad racers is “Rattitude”. Below 50% of control stick movement, Rattitude acts like Attitude Mode, with maximum bank angles and self-leveling. Beyond that, Rattitude behaves like Rate Mode. You essentially get the best of both worlds. You can cruise around with the safety net of Attitude Mode and occasionally cut loose with aerobatic moves. Rattitude is a good choice to help you transition from Attitude Mode to Rate Mode.

As long as your transmitter supports it, you can define up to six different flight modes on the CC3D that can be selected on-the-fly via switches. You can also define a unique flight mode for each axis. You may want Attitude Mode for pitch and Rate Mode for roll. There is a lot of adjustability.

I initially configured just two flight modes on the Strider: Attitude and Rate. I subsequently made several changes, and continue to do so as I tweak the quad to my liking. I’ll cover several of the available tuning options and how they affect the quad’s flying characteristics when I discuss flight tuning in the final installment of this series.

Up Next: Video Gear

In the next part of this series, I will outline the equipment I used for the Strider’s FPV and video recording systems. I will also cover final assembly of the quad’s frame. After that it’s time to fly!

Terry spent 15 years as an engineer at the Johnson Space Center. He is now a freelance writer living in Lubbock, Texas. Follow Terry on Twitter: @weirdflight