Balancing a Quadcopter

After a short break, I am back to working on the Deadcat SK450 quad.  One thing I am learning is that balancing the quadcopter is important.

Model aircraft are very sensitive to their point of balance, but I did not realize quadcopters had a similar tendency.

If your quadcopter, properly calibrated with the trims set at 0, tends to go forward or backward slowly after takeoff in Stabilize mode, you are likely nose (forward movement) or tail (backward) heavy.  If your quadcopter is not symmetrically built (left to right across the centerline), you could also see a consistent movement left or right.

The solution is to move equipment to counteract the imbalance.  The battery is usually the easiest thing to move.  The process is like this:

1. Take a short test hop inside or in very calm winds and note which way the quad wanders.
2. Move the battery back if you wander forward, or move the battery forward if you wander backward.  Mark the battery's new position with masking tape.  See figure 2 for battery installation (farthest to rear) on my quadcopter when camera is installed.
3. Try another test hop, see if the quad will hover in one place.  If yes, the balance is OK.
4. If it still wanders, move the battery as in step 2. and also move the tape.   Take a test hop, see if it is fixed.  Repeat until you get a nice stable hover, something like you see in the video below.  My problem was the quad moving forward consistently, which is now fixed, it still moves from side to side due to my clumsy control inputs.
5. Once you have the battery in the right spot, leave the tape in place and mark it so you know where to put the battery next time.  See figure 1.

Figure 1 Tape markings for different quad configurations

Figure 2 Battery installed in camera configuration

You may have to do this for different configurations, like with and without camera, so you would have two markings for where to put the battery.

Don't add weight to get a balance unless you really have to.  Adding weight just decreases flight time and dynamic stability.

Lots of Little Things to Complete the Quadcopter

I am making progress but am now at the stage where there are lots of little things to do to make the quad work.

1. Install the flight controller with special vibration reducing foam (see photo 1 below).
2. Install the power monitor and battery velcro mounts (see photo 2)
3. Calibrate the ESC's.  This is always a pain as you have this "one man band" performance where you are plugging the main battery in and out, watching lights on the controller, listening for musical tones and beeps, changing throttle settings, listening for more musical tones.  I usually end up doing it 5 or 6 times before it works.
4. Calibrate the flight controller - Mission Planner complained a lot until I saw in the fine print that I had to recalibrate the compass due to a special new firmware load.  Lots of waving the quad around when you do this calibration.
5. Installed camera mount and new camera - looks really good except the new camera wants a special power connector that I do not have.  More waiting for parts from Amazon.com.
6. Items that remain include installing the final Spektrum receiver and PPM converter, wiring the camera, putting some mechanical pieces on the dead cat frame, installing the new professional looking cover, and then some flight testing in the basement.

Photo 1 Battery Mount and power monitor

Photo 2 Note Green Kyosho vibration absorbing foam

Photo 3 New camera that includes a 5.8 GHz video transmitter

Photo 4 Overall view of Quadcopter

Lessons from First Quadcopter Build

I learned a few things from building my first quadcopter drone.

1. The USB port is very fragile, attach a permanent short extender cable to connect to your PC.  Otherwise, you will be buying a new controller.
2. You need to take time to think as you assemble the drone.  There are lots of ways of attaching components and taking some time to think will yield a better result.  Spending time with the Mission Controller application will help you set up your drone, understand how it will work, and allow you to test some of the functions before flying.
3. The best places to get pieces are online stores like Amazon and Hobbyking, Home Depot, and your local hobby shop.
4. Nylon cable ties are a great way to attach things to the quadcopter frame.  Why use bolts when a simple tie will do?
5. Learn how the controller works before flying.  There are some modes like "althold" that sound useful but should only be triggered after flying.
6. Make a tethered test setup to test fly your quad.
7. Use the Wiki, google, youtube, and manufacturers sites to find solutions to problems or helpful tutorials.
8. Get a good soldering iron and learn to solder.

Building and Flying Quadcopter Part 12

Well I went back and tried to fix the problems in the last test flight, which were:

1. ESC's not properly calibrated - I was able to get them to calibrate once, but there is something weird about how these speed controls interface with the controller.
2. Poor throttle control - It turns out that you should land and take off in "stabilize mode" and I was in "Althold" mode.  It took a lot of google searching to stumble across this.
3. The receiver refused to work for a while and I don't know why.  After plugging and unplugging wires, disassembling the receiver, testing and testing, it started to work again for some unknown reason.

After fixing these issues, I tried the quadcopter with no props and it seemed better.  I then lengthened the tethers and ran another test flight shown above, which seems much better.

Building Quadcopter Part 9

The build continues.  I temporarily wired up the controller, receiver, speed controls, GPS and connected to Mission Planner using my desktop PC.  It is not easy to figure out exactly how to wire it as the new controller had no documentation!  I found some useful information on the Ardupilot Wiki and found a basic pinout here.  Hooked it up by USB and it seemed to work, but...I had the controls all wrong.  After playing around with the radio calibration screens, I got the rudder, throttle, ailerons, elevator mapped correctly to the controller.  I could then calibrate the compass, accelerometers, radio, etc.

The I had a brilliant idea (uh-oh).  Since I would be programming things in the field, I should use my laptop with Mission Planner to set up the drone.  Well, for some reason the laptop did not like to calibrate the compasses so after a few hours of frustration, it was back to the desktop where everything worked.  There was also the issue of the compass being off by 90 degrees due to no markings on how to mount it, but this was easily fixed.  The compass is mounted with the GPS on the wooden tower in the photos above.

Next steps are to tidy up the wiring and mounting, calibrate speed controls, attach props, and do some testing with the unit tethered to make sure it is set up properly.  Stay tuned.