December 2013

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

Issue link: http://experimenter.epubxp.com/i/234576

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Page 34 of 40

deflected elevator. As he relaxes his pull, nothing will happen until his pull decreases below one pound. This is because the friction is still "applying" 2 pounds of resistance (in the opposite direction now) against the restoring force caused by the air load on the deflected elevator. The friction can prevent the stick and elevator from returning to their pre-displaced positions, leaving the airplane out of trim. Beginning to see how friction complicates precision and pilot workload? We could talk for pages about the ramifications of friction while maneuvering, but for now let's focus on friction's effect during the initial control displacement. Breakout Control system breakout is the cockpit control force needed to deflect the control surface from its trimmed position. It's the initial force necessary to just get the surface moving. Because friction is present whenever the control system is exercised, any measurement of breakout always includes friction. Hence the phrase "breakout plus friction." While friction alone can be measured while flying off-trim, such as during static stability testing, breakout plus friction can't be separated. It's measured simply by slowly applying a force—let's say a pull force to the stick—and noting how much force is applied when the airplane first responds; that is, pitches nose-up in this case. Do pilots care about the numerical value of breakout plus friction? Probably not, but the effect on airplane handling qualities can be huge. Analogy time. Go to the fridge, get a gallon jug of milk, and place it on a table. Using one hand, move it exactly one-quarter inch. Too much breakout plus friction makes small, precise inputs difficult. Try the same thing with a marble. Too little breakout plus friction can lead to overcontrolling. Breakout can be added to a control system in a variety of ways. Preloaded springs is one example. No control motion will occur until the force of the spring preload is overcome. Mechanical cam arrangements can provide breakout using springs without the penalty of increasing the force requirement as stick displacement increases. A little breakout is usually desirable. Without it, the pilot might have trouble finding the trimmed stick position, especially with friction present. Having a tactile reference for the trimmed stick position is handy (okay, bad pun). Imagine if you had to visually check stick position after every input. Too much breakout makes small control inputs difficult, particularly if the control force gradient beyond the breakout is shallow. For example, let's say this airplane has a breakout (breakout plus friction, actually) of 5 pounds, so it takes 5 pounds of stick pull to move the elevator. If that same airplane requires only 6 pounds of stick pull to maintain level flight in a 30degree bank, altitude control could be a challenge as the pilot attempts to target that 6-pound pull when nothing happens for the first 5 pounds of exertion. Of course, we don't think in terms of pounds of this and that, but this situation will likely result in the pilot pulling and pulling, then getting too much airplane response. That's when we creative adapters start reverting to workarounds, such as trimming for small temporary changes instead. The right amount of breakout plus friction is enough to preclude making inadvertent control inputs but not so much as to make small control inputs difficult in any axis. The relative magnitudes of stick forces in the pitch and roll axes is called control stick harmony. For the past 70 years or so, it's been generally accepted that the best ratio between roll and pitch is 1:2. Breakout plus friction in pitch that's about twice of what it is in roll feels right to most pilots. Ever watch a plane come into the flare and exhibit a wing rock for no apparent reason? Could be the too-low roll breakout plus friction resulted in the pilot making an inadvertent lateral stick input as he pulled back to flare. Then the rocking occurred because he then had to find wings-level while overcontrolling the tiny roll forces while applying increasing pitch control force to complete the flare. Okay, I've wandered a bit from pure control system breakout and friction, but this mechanical characteristic is just one ingredient of the control system stew. And that stew is just one course in the flying banquet. So the point to be made is even something as seemingly minor as breakout plus friction can have a profound effect on the airplane's apparent stability and the pilot's opinion of its handling qualities. Ed Kolano, EAA 336809, is a former Marine who's been flying since 1975 and testing airplanes since 1985. He considers himself extremely fortunate to have performed flight tests in a variety of airplanes ranging from ultralights to 787s. EAA Experimenter 35

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