How I started writing a DIY Autopilot

My intention still is to write my own c# based autopilot. It is not trivial as I found out. Lots and lots of details are needed. The first thing that needs to be done is to determine the instruments that you will need to measure the quantities that you need to know to control flight. Even this seems to be a matter of opinion. For a reference, I found every group that I could that was writing their own version.

From my perspective the quantities that we need to know to control flight are:

• air speed, ft/s or m/s
• altitude, ft or m
• orientation, roll, but pitch is good

These are intrinsic quantities that are really easy to do yourself. Air speed is the model's forward velocity. Altitude is the vehicle's position relative to the surface of the earth. Pitch and roll are the angles of the vehicle relative to the ground. Let me rephrase that, roll is the angle between the right wing and a plane parallel to the ground. Pitch is also known as angle of attack. Pitch is the angle between the plane at the center of the vehicle located on vectors from the center of gravity to the right wing tip and the center of gravity to the tip of the nose and the relative wind.

At the beginning you will need to measure the following quantities:

• air pressure
• acceleration in at least two directions
  

Pressure taps are the easiest ways to measure air pressure in a flying vehicle. Piezo-electric accelerometers are cheap these days and are really accurate. In the next post we will talk about issues with these instruments and the physical quantities that you are actually measuring and how to use those as a basis for a control system.

For all of those inch haters, a quick note about units. You can do this in any unit system. It is unimportant. I will write a quick post on the conversions between the US Customary System and the metric (mks) system. Remember metric is actually several unit systems in one and you have to keep them consistent. Oh, and for those trying to buy nuts and bolts, Japanese manufacturers use odd metric sizes, European use even metric sizes, and the US Customary System/SAE is in units of 1/64th of an inch. So SAE/SAME will have three bolt sizes for every bolt size in the two metric systems. Remember that the thread counts are different though.

In my opinion, if the units are managed consistently then the issue is one of presentation to the user. Part of the display system of my code will show you how to do this with a simple object oriented approach.