Rovers usually either have treads (like a tank) or wheels (like a car). The Mars Rovers are great examples of USV Rovers. They move along the surface of Mars taking various readings and measurements in an effort to explore the surface of another planet without the risk to human life. The following picture shows one of these Mars Rovers:

Not every endeavor for a rover needs to be quite so lofty as to explore another planet. Rovers are currently used in aspects of our daily lives (the iRobot Roomba is a daily life example of a USV Rover) all the way through exploring our own planet (rovers designed to explore lava flows—areas far too dangerous and inhospitable to humans).

However, rovers don't have to simply be driven across the ground. Some rovers are actually boats designed to skid across the surface of water for various purposes. An example of boat-drone USV is the one designed by the US Navy to escort ships through pirate-ridden waters, and protect naval vessels from other hostile vessels in dangerous waters (eliminating, or at least reducing, the need for bloated battle-groups).

In this book, we will be designing three surface rovers:

Submersibles are vehicles designed to go beneath the surface of a liquid. Traditionally, this liquid is water. However, submersible vehicles could potentially be used to monitor other liquids, such as fermentation tanks at breweries, mixing tanks at chemical plants, and diving into raw sewage tanks. Other than diving into a vat of beer, I'm sure you can see how using drones is preferable to diving in yourself.

We won't be attempting to create a submersible in this book. However, the techniques employed are very similar to the subject matter of this book. Submersibles should only be attempted by highly experienced drone designers, as they'll have to contend with reduced ability to utilize GPS, reduced radio transmission distance, and highly reduced visibility in water (or other liquids). Therefore, technologies such as forward looking infrared (FLIR), signal repeaters, and collision avoidance (via SONAR) should be considered. However, as you can see from the following image, the similarities between underwater drones and fixed-wing aerial drones (airplanes) are fairly obvious:

The similarities between the Navy's "Ocean Glider" submersible drone and the original X-1 airplane (the airplane which first broke the sound-barrier) are rather astounding.

Sometimes, people refer to these as quadcopters or quadrocopters. Technically, this isn't accurate unless the multirotor has four-propellers. There are also hexacopters (six-propellers), octacopters, (eight-propellers), and even all the way up to thirty-two-propellers (although I have no idea what that word would even be). So, you can see how multirotor simplifies it all. After all, they are essentially the same type of aircraft, just with more or less motors and propellers. (It's like calling a monofoil and a biplane an airplane, but calling a monofoil a biplane is incorrect).

How do multicopters fly?

The principle is simple. So simple (in fact) that you may wonder why these weren't a thing until recently. Propellers blow air down to provide lift and the airframe (the whole body) tilts to direct that thrust behind (to go forward), in front (to go backward), or to either side (to slide left or right). You can see this in the following illustration:

But then there's controlling it.

It requires thousands of calculations per second to decide exactly how much power should go to each motor to keep the aircraft level and stable in the air. No two motors come out exactly the same. So, it's not just a matter of providing equal voltage to each motor. It's a matter of adjusting that voltage to each motor based on what's currently happening to the aircraft. So, in essence the aircraft doesn't fly it reacts to its inability to fly and compensates for it. And then there's turning (yaw).

You may notice there's no tail rotor on a multicopter. Tail rotors on traditional helicopters serve two functions:

So, how does a multirotor achieve yaw? The answer is in the preceding first reason. There are (usually) even numbers of rotors on a multicopter. Half of these rotors spin in one direction (clockwise) and the other half in the other direction (anti-clockwise). When you want to turn left, the blades spinning to the right speed up and the blades spinning to the left slow down. This way, the effective lift remains the same but the torque from the motors yaws the aircraft to the left. The opposite happens when turning to the right. The following illustration shows this in action:

This is also why tricopters (three rotors) are largely not used. They were notoriously unstable and battery-inefficient. They looked super cool though. Rather like the drop ship from the movie Aliens. However, a variation of the hexacopter (six-rotors) uses that motif (which brings us to yet another variation on blade-configuration). A flat-hexacopter has all-six blades stationed on a single plane. The over-under version of a hexacopter looks like a tricopter with three basic lift points, but counter-rotating blades stacked atop one another so one plane of blades moves in one direction; while the one following moves opposite.

The following picture shows a standard quadcopter (upper left), an over-under hexacopter (upper-right), and a flat-hexacopter (bottom). Most configurations for multicopters are now flat, as it's more efficient for battery time and lift capabilities:

So, as you can see, multicopters are a platform based on a simple principle that is extremely complicated to make work. You (the pilot) don't actually fly a multicopter. It flies itself. Instead, you really just tell it where to go and what to do. You don't pilot a multicopter, you wrangle it.

For the guidance system (in our case we'll be using Pixhawk 2.1—an Ardupilot-based system) to accurately calculate what each motor should be doing there is a plethora of sensors it needs:

So, accuracy of sensors is extremely important (as you can see) with regard to flying multicopters. If it's truly flying itself and you're just telling it where to go, you want that robot brain to have as much information (which is as accurate) as possible.

The myth with multicopters is that they're extremely simple. This is probably because they are fairly easy to fly. That myth should be dispelled right here and now. Of all the drone-types, they are the most difficult to design and build. You'll find that parts should work together, but don't. You'll find that balancing lift to flight-time is extremely difficult. And you'll find that when things go wrong, they go very wrong. Trust me I have the scars to prove it. So, when we get into multicopters, please pay the utmost attention to safety. They are essentially flying Cuisinarts.

Airplanes are considered by many as the Holy Grail of UAVs. They can generally fly for a lot longer than multicopters (as they have wings and don't have to use up battery power on lift). They are much easier to fly (as they can actually glide and, if the motor cuts out, they don't drop like a rock). However, the difficulty with airplanes is takeoff and landing.

Since airplanes don't hover, they must be moving at speed to take off and land. Landing an airplane isn't too easy either. They must touch the ground softly, or they may bounce up again, stall, and crash. So, how do you design a drone to land at speed softly? It's not easy. But Pixhawk has been known to do this pretty well.

Also, since an airplane needs to maintain a certain speed in the air to keep wind flowing over the wings in order to provide lift (called air speed); the guidance system needs to know how fast it's moving through the air (not just in relation to the ground). In addition to all of the sensors employed by the Pixhawk for a multicopter; one more sensor is needed for an airplane a pitot tube.

You'll find a pitot tube on any real airplane to measure airspeed. This device is the little sticks you see protruding from the sides or nose of the airplane. The following image shows how a pitot tube works:

Image courtesy of Wikimedia commons

The following image shows a pitot tube on a piloted aircraft:

The Pixhawk does not have this sensor, but one can be purchased to use with it.

Fixed-wing drones can also be launched in a variety of ways. Some are thrown; others are launched on catapult systems, while still others are launched by taking off like traditional aircraft (rolling takeoff with wheels).

The following image shows one of my drones (2012) being launched via a catapult system known as a Jetapult:

Since this drone had a seven-foot wingspan, it was far too large to launch by throwing it. The system was very simple. It used PVC pipe to angle the aircraft and slide it up at a slight angle while a bungie cord pulled the aircraft forward at a high rate of acceleration. If you look closely, you can see the black bungie cord wadded up on the ground just behind the (now flying) drone. The release was a simple foot-pedal that slid a ring off a peg.

Although this is a simple mechanism, even military drones are sometimes launched from catapults (as shown in the following image):

You may wonder why we spent so much time on takeoff and landing. Vertical Takeoff and/or Landing (VTOL) drones solve a lot of issues, but present a high degree of complexity. These drones can take off like a helicopter (or multicopter), fly like an airplane, and land again like a helicopter/multicopter.

The XplusOne (by Xcraft) takes off and can even fly like a quadcopter, but then it can transition into forward flight like an airplane. As you can see though, designing such an aircraft is incredibly difficult. Wind resistance from the wing (when vertical) can create instability during multirotor flight and transitioning to airplane flight can be equally difficult. However, the most difficult proposition of all is coming back to quad-flight for landing. How do you keep the aircraft from ballooning (gaining altitude) and then stalling into a crash? You should only attempt this when you have a high degree of funding and experience, because it's virtually guaranteed that you'll crash it several times before tuning your design to be just right.

Traditional helicopters do have some advantages. They can fly faster than multicopters (usually) and are more stable during forward flight (due to a tail fin and horizontal fins). They can also (sometimes) stay in the air longer, as they do not require the quick responsiveness of electric motors. Therefore, they can fly using combustion engines or even turbo-shaft engines, meaning they can carry more fuel and fly for longer. The following picture shows a US Navy helicopter drone:

However, traditional helicopters shift the pitch of their blades through the use of a swash plate. They also (usually) shift the pitch of their tail rotor blades using a similar method (albeit less complicated). A swash plate is basically two plates that rotate and move up and down the rotor shaft to alter the pitch of the blades. How does a helicopter tilt forward to start moving? The swash plate tilts and shafts going down to the swash plate make the blades bite into the air more at the back of the helicopter, and bite less toward the front (producing more downforce at the rear and less at the front).

All of these moving mechanical parts require lubing and maintenance between every flight. The more complex a machine is, the more opportunity there is for failure. So, although there are advantages to traditional helicopters the disadvantages (in terms of cost and maintenance schedule) make them a nonstarter for many.

The following image shows how a swash plate works. The red disk remains stationary and rotates or moves up and down. Meanwhile, the beige parts turn with the rotor head and the aqua control rods shift the pitch of the blades depending on where they are positioned during their rotation cycle:

That's the end of helicopters for the purposes of this book. As there really is little difference between helicopters and multicopters for the purposes of our Pixhawk, we have chosen to cover multicopters in this book.

Blimps are actually pretty great. They can stay aloft for extremely extended periods of time. They are very stable as well. However, they can be a bit of an eyesore to landscapes as they have to be quite large to carry any sort of payload. The following image shows a Navy dirigible drone:

There are some other drawbacks to dirigible drones. They must be inflated, costing large amounts of money each time you fly in whatever lighter-than-air gas you choose to use (helium, hydrogen, and so on). They are also highly susceptible to strong wind currents (due to their lighter-than-air nature). So, strong engines must be employed to counteract those currents. This adds more weight and therefore more size to the blimp. Due to these limitations and costs, dirigibles are not widely used.

It's a shame, because mechanically, they are the simplest drones to design. It is just that in practice, they are expensive and largely impractical.