Thursday, December 1, 2011
Levels of Pilot Gain
The many varieties of helicopter models in the world today are flown by an equally large variety of pilots. Although we could spend pages telling stories and giving examples of lots of colorful individuals, for now we’ll limit the conversation to just two types: low-gain and high-gain pilots.
In technical words, “gain” is what engineers call the ratio of response to error. Pilot gain describes the level of aggressiveness in pilot control activity. It is dependent on training, aircraft dynamics, the task at hand, stress level, and also individual temperament.
The concept is much easier to understand intuitively than mathematically. Riding your bike on a wide sidewalk requires a small degree of precision, with little risk of getting waffled by a car, and so your steering inputs are relaxed and minimal, or low-gain. Riding on the street while trying to keep the tires on the solid white line requires a greater level of attention. Add to that the high stress of getting hit by a car if you veer off the line, and your steering inputs become high-gain. They get much more frequent and deliberate, with a tendency to over-control. In fact, your performance in this case might remind you of your earlier days of learning how to ride.
So what does any of this have to do with flying helicopters? As operators of machines seemingly capable of magic, we like it when helicopters to do what we ask without physically or mentally tiring us out too much in the process (making us look like great magicians). Since manufacturers need to appeal to all pilot types and skill levels, a large part of evaluating the flying qualities of a new helicopter design involves test pilots flying repeated closed-loop tasks at various levels of pilot gain and assessing how it affects their workload.
Almost all of operational flying involves closed-loop tasks—the pilot wants the aircraft to do something, he makes a control input; the aircraft responds; the pilot judges response with feedback through his eyes and his body; his brain processes it and decides if it is in error from what is wanted; he makes another input to correct, and so on. The time to process the info, make a control input, and have the aircraft respond creates time lag in the system. The gain level used by the pilot during the task depends on how hard he’s trying to maintain his goal. As it turns out, this can have a direct affect on the stability of the pilot-aircraft system, and therefore affects pilot opinion of how the helicopter handles, especially as gains increase during “tight” tasks.
Tight tasks are those that require an exceptionally high degree of accuracy, like flying an instrument approach in poor weather, holding crosshairs on a target, or landing on a small platform. During flight test, engineers look for instances where the combination of high gains and time lag can cause the pilot to get out of phase with the aircraft and begin an oscillation by doing the wrong thing at the wrong time, driving the system toward instability.
This is what is commonly called a pilot-induced oscillation (PIO), a subject worthy of its own article. Serious PIOs are not common events in certified aircraft, thanks to thorough flight tests, which uncover potential problems. Still, I’m sure I’m not the only one out there who once or twice quietly thought they were the “Ace of the Base” for holding a rock-solid, hour-long OGE hover during a mission, only to come back to the platform and make everyone sea-sick for the last ten seconds of the flight—a direct consequence of my increased gains driving a bit of an oscillation in the hover as I tried to dial in the perfect landing.
The adaptability of the human pilot adds to the difficulty of using cut-and-dry math when evaluating aircraft for flaws. Gain alone cannot be used to judge task difficulty. Many tasks exclusively require higher gain. But there are plenty of pilots who naturally fly all the time using higher gains.
Have you ever observed someone fly a super-smooth precision approach, only to look over and see the stick darting all around the cockpit, so fast that the aircraft doesn’t even seem to respond? Unfortunately there tends to be a negative connotation or inexperience associated with high-gain pilots because newbie pilots tend to use excessive gains as they learn.
However, such pilots do not necessarily display lesser skill, as evidenced by their ability to accomplish their task with accuracy. A former R-22 pilot will most likely exhibit higher gains in all tasks than a former S-92 pilot. For this reason, there exists rating scales for test pilots designed to help standardize their qualitative opinions of handling qualities exhibited by the helicopters they test—but that’s a topic for yet another time.