Programmable mixers

Model Airplane News, Feb 2001 by Edberg, Don

This month's column will focus on one of the handiest-and most commonly misunderstood-features of computer radios: mixing (or programmable mixing). Mixers come as standard equipment on many of today's computer radios, but they often aren't used. Why not? A lot of folks don't know what to use them for! In this column, I'll show you how to use them to make flying your models better and more fun by compensating for pitch and roll tendencies during knife-edge flight. But first, let's go over a few definitions and explanations.

WHAT IS A PROGRAMMABLE MIXER?

Let's start with the definition of a mixer. A mixer is something that makes a control input device on the transmitter affect the response of a normally unrelated channel on the receiver (in addition, of course, to the function that it's supposed to control!). Depending on the type of radio, the mixer can be made to respond to one of the control sticks, a toggle switch, a rotary knob, or a sliding lever. Mixers are commonly used to:

1. Correct the flight characteristics of models that don't behave properly, such as those that are warped or misaligned or have layouts that affect their flying qualities.

2. "Build your own" functions to make your models do what you want them to do. For example, Property 1 is used to automatically add correcting aileron or elevator motion to get rid of unwanted rolling and pitching during knife-edge flight. Another example would be to apply rudder to correct for an engine thrust line that's off. In this column, we're going to concentrate on Property 1.

Property 2 could be employed to create a second throttle control for a twin or to make preset drop positions for wing flaps. Of course, there are unlimited uses for both properties.

Note that many computer radios-particularly the high-end models-have builtin functions to do some of these things. It's difficult, however, to design a system that will do all the things that everyone might want, so the designers provide us with programmable mixers to accommodate all of the functions that they could not anticipate.

HOW DOES A MIXER WORK?

For those who aren't familiar with the principle of mixing from one function to another, I like to use the analogy of a car with brakes that are badly in need of adjustment. The brakes cause the car to "pull" to one side, so when you press the brake pedal, you have to turn the steering wheel at the same time to keep the car going straight. The steering you apply compensates for the brakes' pulling to one side.

If the car was an RC model, you could simply turn on a mixer and mix from brake to steering. That is, whenever you applied the model's brakes, the radio would automatically apply steering in the way you told it to so that it would continue driving in a straight line. So, we can make the following definitions for this example: the master control-brakes-causes the action to happen. The slave control-- steering-moves because of the programmable mixer. This concept is illustrated in Figure 1. Now, let's look at an airplane example.

FIX UNWANTED ROLLING DURING KNIFE-EDGE

Knife-edge flight occurs when you roll the model 90 degrees to one side or the other and hold lots of rudder to keep it flying more or less horizontal. You need a model with lots of power to accomplish this. Ideally, the plane will just cruise along sideways for as long as you hold a lot of rudder.

In reality, it doesn't usually work this way. Your model may roll during knifeedge flight, forcing you to hold a bit of aileron stick in addition to a lot of rudder. The unwanted rolling is caused by a bunch of factors, including the position of the fin and rudder on the fuselage, the thrust angle, stabilizer/elevator position, airspeed and more ... too complex to go into here. Depending on its geometry, the model may roll the same way as the rudder is applied, i.e., with full left rudder, it will roll to the left, but it may also do the opposite. If that's not enough, because of the direction of rotation of the propeller, the amount of aileron needed may vary depending on which side the model is flying! Models respond differently, and you have to find what you need by trial and error during test flying.

Having to hold the aileron stick a bit is uncomfortable, not to mention that it's difficult to consistently hold just the right amount. Solving this problem is a perfect use of a programmable mixer. We will set up the mixer so that you won't have to worry about inputting the right amount of aileron during knife-edge flight with lots of rudder; it will happen automatically.

You will do this by setting up a mixer to mix rudder into ailerons. In this case, the master control is rudder, and the slave control is aileron. This is usually illustrated in the abbreviated form: RUD -> AlL; the direction of the arrow indicates the master and slave controls.

The instructions for setting up mixing depend on the make and model of radio, but they generally follow the steps given in Table 1. Note that I will abbreviate programmable mixer as "PMIX" from here on.

You usually have to input separate mixing values for each side of the control function. If you forget, mixing will occur only on one side of knife-edge! I recommend that you start with smaller values and work up to larger ones so you aren't surprised the first time! (Note that the percentages given here are examples; you'll need to determine your model's actual needs by trial-and-error flight tests, in which you continue to adjust the percentages until the model flies just the way you want it to.)