RANDOM THOUGHTS FROM THE BACK SEAT... By Jeff Orchard
This one is going to raise an eyebrow or two. The subject is turns. All students eventually come to the realization that turns happen when the wing is banked, the steeper the bank, the faster the rate of the turn in the direction of the low wing. Most of the time, because of outside influences or the inherent stability (or instability, as the case may be) of the glider, some control pressures may be necessary to maintain a constant angle of bank resulting in a constant, smooth turn. The student also finally begins to understand that the rudder (the moving part that sticks up in the back) provides correction for a force called adverse yaw, which wants to make the nose swing in the opposite direction from which the stick is moved, and that a proper "feel good" turn requires that the rudder and stick be moved in the same direction at the same time.
After time, these students begin to make really nice coordinated turns, then the instructor casually asks the student "What makes an airplane turn?". This question is generally best asked on the ground, because the answers and arm movements that ensue can be quite elaborate. After explanations ranging in complexity from something that would put Wolfgang Langeweisch (ask an old-timer) to shame, to the simpler "a little stick and a little rudder", the instructor pops the second part of the question. "Can you turn using just the stick or just the rudder?" Almost without fail, the answer is "Of course not!" Privately, the student is thinking "Does this guy think I'm stupid? He just spent ten flights getting me to coordinate the controls, and practically yelled at me on the last one to use the rudder every time, even when I'm making little tiny corrections on tow." At this point, the instructor is trying to hide a mischievous smile, and the student is beginning to sense that something is awry. Hopefully, an instructional flight is in the offing, it would be cruel and unusual punishment to make the student wait until next time to discover the truth.
The flight instructor should take the controls, and with the student following through, be very careful to not let the rudder move a millimeter as he moves the stick to one side. The glider begins a very uncoordinated bank in the direction of the stick movement, the nose swings wildly to the outside of the turn, and as the bank develops, the instructor relaxes the sideways pressure on the stick and applies a bit of back pressure to keep the nose from dropping. And voila, the glider settles into a coordinated turn. Recovery is accomplished in the same way, and after another swing of the nose the wrong way, stick pressure is again relaxed as the wings come level, and straight and level flight occurs once again. A continuation of the demonstration has the stick being held firmly and rudder pressure being applied in the desired direction. The nose first moves in the direction of the turn, and then slowly (depending upon how far the rudder is moved) the wings begin to bank into a turn. As the rudder pressure is released, a stable turn develops and the student is asked if the glider is in fact turning. Now that he is properly humbled, the instructor can explain the mechanics of the exercise.
In the first instance, the bank of the wing overcomes the desire of the airframe to continue in a straight line, and the turn finally develops in spite of adverse yaw. The mechanism of the rudder only turn is somewhat different. By moving the rudder, say to the left, the first reaction of the glider is to swing its nose to the left and relatively speaking, the right wing travels faster through the air than the left wing. Again relatively speaking, the faster wing produces more lift than the slower wing and is lifted upward. This unevenness is technically described as a bank, and as we agreed before, a bank of the wings produces a turn in the direction of the low wing. I'm not asking you to believe me as you read this, in fact I insist that you try it for yourself the next time you get the chance.
Ever since the Wright brothers, aircraft designers have tried to develop ways of making an airplane turn without a rudder. Adverse yaw, as you might imagine, creates a lot of drag, because every time the control surfaces are moved out of the relatively streamlined airflow surrounding them, turbulence happens. Turbulence creates drag (Aviation 101) which lowers the efficiency of the flying machine. In a powered aircraft, this means more fuel is expended; in a glider it means gravity wins sooner. After decades of research, the most practical method of making a turn happen without "adverse" yaw is used on the Mitsubishi MU-2 and a couple of the newer airliners. Spoilers. These spoilers, however, are able to operate independently of one another, allowing the lift of one wing to be reduced, and removing some of the lift generated by that wing causes it to drop. But that isn't all it does. It also creates drag on the "correct" side which makes the nose of the airplane turn into the turn without a rudder. The next time you are flying commercially, try for a window seat and watch all the moving parts on the wing, especially during the final approach. If it doesn't scare you, it will most certainly fascinate you.
Now that we know that drag is the enemy, how do we know when we have some, and what do we do about it? Going sideways through the air produces lots of drag. Slips prove that. But even subtle slips and skids which are produced by poorly coordinated turns create drag, and if you are climbing in a 32 foot per minute thermal at the end of the day, trying desperately to get back to the airport, the last thing you want is drag. Thank heavens you have a yaw string. There it is, mercifully right in front of your nose, pasted firmly to the canopy with some high tech, low-coefficient-of-drag, magic mylar tape. You watch that string through every turn, and you are so proud of yourself and your ability to keep it centered. Why, it might as well be painted on the canopy. It hasn't even deviated a silly millimeter from the centerline of the glider. And you land out. What happened? There was lift, not much admittedly, but some. The vario proved it, you were going up or holding your own part of the time. Eyebrows twitching just a little? Are you afraid that I'm going to tell you that the yaw string lied? Get out the bumper jack and prepare to jack those brows back down. The string led you astray, but it didn't lie. It was telling you all along that you were subtly skidding around your turns. Making drag. Falling slowly out of the sky.
Surely, you say, I jest. All the contest ships have yaw strings, some even have neat little flagpoles on the nose to keep the string up out of all the turbulence and parasitic drag near the fuselage. Even F-16's have yaw strings. Imagine, a 46 million dollar airplane with a piece of yarn to tell the worlds best trained pilots whether they're doing it right or not. What the heck are they teaching those guys, anyway. OK, OK, I'll admit that at 1400 knots the yaw string probably works. Give you any hints as to what I'm getting at? Some of you aeronautical engineers are already way ahead of me, and I'll bet that the championship sailplane pilots know what's coming, too.
Aviation 101 teaches us that an aircraft pitches, rolls and yaws around it's CENTER OF GRAVITY. The center of gravity is generally about a third of the way back from the leading edge of the wing and right on the centerline of the fuselage (unless you only drained one of your water ballast tanks). It would be right where you would tie the string if you were going to hang a 2-33 in level flight attitude from your den ceiling. In a right turn, the nose of the aircraft is moving to the right relative to the C.G. If it wasn't, you wouldn't be turning. The C.G. is tangential to the turn, not the nose. The nose, by definition, has to have a slight sideways component relative to the air during the turn, and at the speeds that a glider flies, especially when thermalling, that sideways component can be substantial. Watch how fast the horizon goes by during the turn. If you want to keep the string in the center, you have to hold inside rudder. Holding inside rudder makes you skid around the turn. A skidding turn creates DRAG, and the Earth rises up to meet you sooner than it would if you were making coordinated turns. Once again, do not take my word for it. In an aircraft with a needle and ball instrument and a yaw string, establish a coordinated turn with the gyro instrument, then look at the yaw string. I will let you figure the rest of it out for yourself. I will tell you that the successful competition pilot knows where the yaw string sits in various turns, and it is not in the center unless they are either flying very fast or in a straight line. Some pilots know enough to put their yaw strings directly over or a little behind their heads to keep it closer to the C.G., and some continue to chase theirs right to the ground, never understanding what it's all about. The latter should tape theirs to the inside of the canopy with a 1/2 inch nut tied to it. At least it would tell them whether they were right side up or not.
Keep having FUN. More later........
Copyright 1995 by Jeff Orchard