Chapter 8. The Principles of VTOL with Pixhawk

Designing and tuning a Vertical Takeoff and Landing (VTOL) aircraft can take a very long time. It may even take years to design and get your VTOL aircraft set up and working with stability. Originally, this chapter was going to be based on something fun—a Battlebot design. But, when the book was announced, one thing was very clear. You (the readers) wanted to know more about VTOL. So, without the time nor budget to create a VTOL prototype aircraft (by the time we have made one work properly, it's likely a new version of Pixhawk would be released); we're going to work on the principle and theory of how someone would create a Pixhawk VTOL; not how someone did create a Pixhawk VTOL.

In this chapter, we will cover the following topics:

• The types of VTOL aircraft
• Limitations and considerations of VTOL
• Fusing a multicopter and fixed wing design
• Tuning a VTOL aircraft

The types of VTOL

There are essentially two types of VTOL: hybrid and vectored. All other VTOLs are a variation of the two. One is far easier to build, and one flies much better (and more efficiently). Unfortunately though, these are not the qualities of the same type of VTOL.

The hybrid VTOL

This is certainly easy to design and build. It's just a matter of taking a very good airplane and attaching a multirotor frame to it (centred around the CG of the airplane). That's a bit of an oversimplification, but accurate. Unfortunately, with this type of design the multicopter propellers themselves can begin spinning during forward flight due to the wind. And even with no spinning they are essentially control surfaces and can cause strange turbulence and vortices around the wings, which alters the way it flies. This, plus having surfaces designed around moving air that are non-functional during forward flight, will reduce your efficiency. And (in general) making a fixed wing drone is about trying...

Designing your first VTOL

VTOLs have all the complexity of an airplane and a multicopter combined. So, you may want to start simple. Remember the Bixler we fitted with Pixhawk in the last chapter? This would be an excellent candidate to start with. Why? Because it's already a fully set up airplane. So, the first half is already done. Now we just have to figure out how to hybridize it.

We need to integrate the functionality of a multicopter without adding too much weight to the frame. So, it's not just a matter of gluing on a multicopter airframe. The multicopter needs to lift the weight of the airplane, and the airplane needs to keep the weight of the multicopter aloft during forward flight.

Not only that, but the energy requirements for our airplane are not very high (using just a 3S 1100 mAh LiPo battery). We could easily lift a drone and the airplane's weight with a 6S LiPo battery. But then we have all the extra weight of the battery, and all the extra weight of the motors and propellers...

Implementing Pixhawk

In previous iterations of this type of hybrid, you would need to use PX4 firmware (rather than Ardupilot or APM) with heavy customization. Luckily, the newer Ardupilot firmware versions allow for exactly this type of VTOL (they call it a Quadplane). Obviously, the channels are going to be a little different.

The following diagram shows how all of the motors/servos are hooked up:

An easy way to remember motor rotational directions is that the leading edge of the propellers always turn in toward the hull on a Quadplane. Nevertheless, we marked each motor direction (CW for clockwise and CCW for counter-clockwise).

Test and tune before making the leap

With all of the quad parts on the plane, make sure that none of the quad motors have power, and connections to Pixhawk are severed. You'll want to test-fly (and probably retune) the airplane with the new parts before attempting to use the quad functionality. Make sure you have a successful tuning with the most stable flight possible...

Transitioning from VTOL to airplane

Transitioning is simple once everything is tuned. All you have to do is change modes (such as going from QLOITER to FBWA). It will happen automatically. Again, avoid MANUAL. This will instantly cut the multicopter motors and your aircraft will drop like a rock until you manage to regain control and lift up (much like a stall).

This is what will happen when you switch to FBWA or STABILIZE from a Q mode (such as QLOITER):

1. The aircraft will attempt to maintain altitude using the quad motors and engage the airplane's tractor motor to provide thrust at user-inputted throttle.
2. Quad motors will continue to provide lift and stability until ARSPD_FBW_MIN airspeed is reached and if there is no pitot tube, this is based on GPS estimate.
3. Q_TRANSITION_MS will engage and shut down the quad motors over the specified time.

The transition back to multicopter mode works similarly, but has a few differences:

1. Airplane tractor motor will immediately stop.
2. Controls switch to multicopter...

Recommended VTOL RTL procedure

There are several ways to handle RTL with a hybrid like this, but here's what I recommend:

1. Head back (RTL) using the airplane RTL mode. This gets you to the vicinity quickly.
2. Switch to QRTL when close to the home point. If Q_RTL_ALT, RTL_RADIUS, and Q_RTL_MODE are set, this happens automatically
3. Switch to QLAND and again, if Q_RTL_ALT, RTL_RADIUS, and Q_RTL_MODE are set, this will happen automatically

I find this to be the most efficient procedure. No wasting battery time on circling, or other maneuvers. Just fly in, switch modes, and land.

Notes before attempting any transitions

I know, after all that effort, you're going to be itching to see it transition. It's a long and painful road to tune your multicopter mode before even thinking of transitioning.

Make sure you adjust all of the Q tuning parameters so that the multicopter mode is flying as stable as possible before doing any hybrid flights. You'll save yourself a lot of expense and headache.

When attempting transitions, make sure you are well above a minimum height of 20 m above ground level (AGL). Until you get all of your Q transition parameters set, you are very likely to lose altitude, or even lose stability on your first tests. Even then, stay above 20 m AGL.

Summary

Well, there you have it. If we made it seem simple, we apologize. This is the absolute most complex configuration you can embark on (for the hybrid we chose the simpler "Quadplane"). If you ever choose to do a VTOL, we'd love to see your results!

In this chapter, you learned about the different types of VTOL aircraft, and how to set up the simplest flavor of VTOLs. You've learned about all of the hook-ups, design concepts, and limitations of different designs. Finally, we walked you through all of the firmware settings needed to get it to work.

In the next chapter, we'll cover some of the basics of actually digging into the firmware and settings of Ardupilot and manipulating it to customize it even further.