Last week we discussed (yaw) string theory in general terms, peeking into arcane questions like whether a string has more ends than one, and if so, which end is where we get the business. Elemental stuff for sure, but there’s more to keeping a string straight than knowing how it works.
Some pilots, even a few old salts, think yaw strings are unnecessary. That may be okay for them, but I say it ranks with keeping last week’s bugs on your canopy for the next flight — permissible, yet functionally dubious. Especially when anyone else wants to fly the plane. If those folk ever do clean their canopy, they’re apt to find the old tape peeling up at one corner because it’s been ignored so long, throwing in a ten degree error that makes their string useless anyway.
Crooked yaw strings can result from several kinds of aerodynamic flaw, such as an aileron (or two) out of rig, wings being of unequal weight, or not entirely symmetrical. Any glider with one wing replaced from a sister ship, for example, is a candidate for control harmony issues. And let’s not even mention angle of incidence! Problems like these may not make you call your shrink, but your mechanic might…
Yaw strings reveal other factors too, aside from the aerodynamic, that require us to become our own shrinks. How do I know? Ninety-nine point eight percent of my logged flight time over the current millennium has been in aft cockpits, studying from three feet behind how countless individuals fly. So yes I may be nuts, but I’ve seen enough to bid for a Dissociate Degree in Aerial Psychotomy, and can vouch that factors of the ‘human’ variety are usually what make yaw strings misbehave.
One very common error I’ve never heard anyone but myself discuss is flying supposedly straight, but not exactly level, and correcting for that with a toe’s weight of opposite rudder to stay on course. Its evil twin holds a smidge of rudder (routinely nosing into a crosswind) and one degree of involuntary bank as a fix. Same difference? Maybe, though maybe for different reasons… Either way it’s a slip. Imagine how much energy that wastes on a long flight! For the following discussion, let’s refer to any kind of inadvertent slipping in ‘straight and level’ flight as One Wing Low Syndrome, or OWLS. Got it?
Because all but the very youngest pilots drive cars, every one of us transitions from ground to sky with a heavy right foot, and/or a lazy left. Mental clubfoot from the getgo. To that, single-engine pilots add the same malady, brought from their habitual countering of P factor, which in gliders doesn’t exist. Given these and perhaps other reasons (could right- or left handedness also play a role?) it’s more often the port wing held low by most OWLS sufferers.
Launching in a crosswind also can skrew a pilot’s sense of level, well up into the tow or even throughout a flight. Seen it many times! After those initial seconds of holding windward wing low during takeoff, they seem determined to keep it there for the whole day. Even when called out on this, it’s amazing how soon many resume the error, completely unawares. Unchecked, it becomes a bad habit.
Others will hold one wing low for a while (or a whole flight), and then inexplicably switch sides for a comparable period, offsetting the whole time with that same reflexive toe of rudder to get where they’re going. They too will dutifully straighten up when reprimanded, but also relapse the moment something else crosses their mind. Almost funny to watch, but no.
This next example is not one infection site for OWLS, it’s an inbred family of them. No listing these in clerical order; each one’s effect is another’s cause, all overlapped and symbiotic. Let’s start with the subtlest and probe its roots, down to where some of the kin are much less benign.
Picture a broad panorama, nearly flat, with the horizon a few degrees higher on one side. That difference can lure an unthinking pilot into squaring with the mean horizon instead of true level, even miles from any hill. Reintroduce a pesky crosswind, less visually impressive at altitude but probably much stronger, and you have two deceptive inducements that can each lead to OWLS. Depending on wind direction, they might compound each other or partially cancel out, raising even more uncertainty.
Symptoms intensify exponentially as we come closer to high ground. Running ridge exposes a visceral urge to shy away from hills and/or lean into the wind, again necessitating opposite rudder to stabilize. Add a load of plain old fear, and soaring near high ground can seem overwhelming for a beginner. Problematic though all this be, ridge lift is nevertheless an ideal laboratory for identifying those bastards from the OWLS family and sorting them out, and there’s no better place to begin than the standard confusion between crabbing and slipping!
I catch myself committing these sins too, naturally, and the remedy is to always dedicate a certain portion of awareness and control, every moment of every glide, to enforcing wings really level and string really straight. An under-appreciated key here is the cultivation of quiet hands and itchy feet (truth I must no longer formally endorse, but can still whisper in an empty theater). While this veritable garbage bag of ‘human’ factors may reflect poorly on our intelligence as a species, the solutions are primarily up to us, and in this struggle the yaw string remains a formidable ally. Borrow a line from the ballers: string don’t lie!
Now before we spend this coin, let’s flip it. Say you’re one of the few with truly solid stick and rudder skills, and a finely tuned derriere to boot (pun intended). You know who you are — and you still have yaw string problems? Oh good. This is where you learn which of those mechanical issues we mentioned earlier apply to your bird, and look to resolve them. Yet another silent benefit from our humble little scrap of yarn!