Chapter 7. The Holy Grail - A Fixed Wing Drone

Wow for a holy grail, she sure is one ugly pig, right? Well, prototypes just aren't usually pretty. So, why is a fixed wing drone (like the airplane in the following image) considered a holy grail?

The answer is pretty simple, actually. Airplanes need to keep moving in order to stay airborne. Go too slow, and you stall (a fancy word for fall from the sky until you build up speed again except that it's usually vertical speed, and vertical speed is going down).

With multicopters, tuning them is fairly easy: take it off, hover just outside of ground effect (OGE), and let it twitch around a bit. If it's too crazy, just land it, reset the settings, and repeat. Since an airplane is constantly moving tuning can be quite a bit more hazardous to the aircraft.

While tuning fixed wings, I've had them turn into lawn darts, porpoise like dolphins jumping out of the ocean, and (as stated in a previous chapter) blow their wings right off. So, it's important while...

Why this particular airplane kit?

The plane we're using (as shown in the following image) is a Bixler from Hobby King. It's actually named after one of the two Joshes (Josh Bixler) from the popular YouTube channel Flite Test:

As you can see, it's very inexpensive. Well under $100, and comes with a motor, and all the servos necessary to fly it. The dihedral curve in the wing makes it uniquely stable for soaring. It also comes with the two halves of the main fuselage separated (making wire routing and component placement very easy). Finally, it's a powered glider. This means it's very efficient, and has the potential to travel great distances under minimal power. This is exactly what we're looking for!

The assembly

Before we get into the parts and specific placement thereof, let's take a quick look at the sketch for this drone:

I know but what can I say? I like to sketch out my ideas over a bottle of wine.

The peripherals we're using are the same as all our other drones, with the addition of a pitot tube. We talked about pitot tubes before, but as a quick refresher, a pitot tube is simply a device that measures the air-speed of your aircraft. Think about it—speed is relative. Believe it or not, even when you're standing still, you're traveling at 113,000 mph. Yes, that's right... why? Because the Earth rotates at 1,000 mph + the Earth around the sun is 67,000 mph, plus the solar system's speed through the universe is 45,000 mph. But on Earth, your speed is 0 mph because your speed is measured relative to the ground (Earth).

So when flying, you have two speeds—your ground speed (your speed relative to the Earth), and your air speed (your speed through the air). Think about it. If you're traveling...

Tuning a fixed wing aircraft with mission planner

By now, you're an expert at installing the right firmware and running through the initial set up within mission planner. So, we're not going to waste your time with all that nonsense at this point. Let's start with setting up the pitot tube.

Configuring the pitot tube

The first thing we have to do is tell Pixhawk to actually use the pitot tube, as well as what kind it is. The following screenshot shows the menu in mission planner, where you will activate the pitot tube we installed:

Realizing that it has already been stated, we wish to underscore the need for a pitot tube. Some builders claim that pitot tubes are not required for fixed wing aircraft. Although from a software/hardware standpoint, this is true; in reality, this philosophy couldn't be any more wrong. Using GPS alone only measures ground speed. Without measuring the actual airspeed of the aircraft, going downwind can result in stalls, and going into the wind could result in over...

Setting up for tuning

Much like the multirotor version of Ardupilot, fixed wing (Arduplane) has an autotune mode. It works very differently from the autotune of the multicopter. Before we get into the specific details of autotune for fixed wing, let's take a look at how flight modes in Arduplane work.

Fixed wing flight modes

You'll recognize many of the flight modes in the Arduplane interface from Arducopter. However, many of them work quite differently. And that's understandable. After all, a fixed-wing aircraft must keep moving to keep aloft. Here is a summary of the fixed-wing flight modes:

• MANUAL: Consider this making the airplane dumb. In this mode, Pixhawk's features are essentially off. You're flying the airplane as you would any other RC aircraft.
• STABILIZE: This mode offers basic stabilization. You're still flying the plane on manual control, but Pixhawk will make minor corrections for turbulence and wind. Also, upon releasing the sticks, the airplane will level itself out.
• Fly By Wire_A...

The tuning process

Every airframe is different. Sometimes even the same model airframe can have different settings from one airplane to the next. So, it's important to know that any settings you see for PIDs are not the settings you should use for your plane. Rather, this section is about the process of how to arrive at those PID settings. The first step is to make sure your airplane flies right with no Pixhawk input whatsoever.

Your maiden flight

Maidening an aircraft just means taking it into the air for the first time. The big difference between maidening a Pixhawk airplane and any other RC aircraft is that you should never use the trim controls on your radio.

Trim controls on the radio are small buttons or sliders near the sticks that allow a user to adjust the control surfaces in real time for straight flight by offsetting the value (PWMs) of the sticks. The problem is that by offsetting the values of the sticks by using trim controls, the Pixhawk will not know when the sticks are centered...

Summary

WOW! That was a lot, right? Now you can see why fixed wing aircrafts really are the holy grail of drone designers. In this chapter, we've learned about the extra components needed to integrate Pixhawk into a fixed wing drone. We've learned about placement, and thinking through the design. And finally, we went through the very complex and drawn-out process of tuning a fixed wing drone.

In the next chapter, we're not actually going to build a drone. Instead, we're going to walk you through some concepts of creating a VTOL aircraft... FUN!