Angle of Attack Indicator for Aerotow Tugs
by Doug DuBois, Adventure AirSports Tug Pilot
Aerotow tug pilots spend roughly 3/4 of their airtime in very close proximity to their aircraft's "stall speed," and much of this time is done in windy/gusty/turbulent conditions. Hang gliding's remarkably good aerotowing safety record speaks very highly of the skill of our tug pilots, as ultra-light aircraft combined with "stall speed" proximity in gusty conditions could easily amount to a recipe for disaster, especially when you add to the mix a glider on tow! Please note the quotes used around the term "stall speed," as it is somewhat of a misnomer for reasons discussed below.
I'm still a rather new tug driver myself, having started my ATP training less than 3 years ago and having conducted fewer than 300 tows so far. When I first began towing in rowdy midday conditions, I was uneasy with the narrow airspeed margin and also noticed that the normal airspeed cues (airspeed indicator, the feel and sound of the wind, etc.) didn't always seem to give an accurate sense of the tow plane's proximity to a stalled condition. Since we're generally trying to keep the airspeed very low (within manageable limits for the glider and pilot), and a stall/spin under tow could spell disaster, I decided to investigate the use of an angle of attack indicator on our Dragonfly tug.
"A wing can stall at any airspeed or attitude, but it will always stall at the same angle of attack." This axiom is drilled into the heads of new pilots, and with good reason. If we let complacency lull us into the practice of avoiding stalls simply by attempting to maintain a minimum airspeed, that practice can fail us someday. Maintaining an appropriate angle of attack (AOA) for the current conditions is the true key to stall-free flight, and this observance is relevant to any flying machine that creates lift by pushing an airfoil through the air. I can think of no flying activity in which AOA management is more critical to safety than in our low-speed aerotowing.
In theory, AOA is easy to understand. It is simply the angle between the chord of your wing's airfoil and the relative wind. In practice, the path of the relative wind is easy to observe by attaching a piece of yarn to the aircraft in the passing airstream, taking care to locate it in air that is undisturbed by the aircraft. Go one step further and install a reference line for the yarn to register against (relative to the aircraft) and voila, you have an AOA indicator.
This photo shows the first AOA indicator I made for the Adventure AirSports tug. A piece of 1/8" music wire has been attached to the tug's mirror, holding the yarn about 12" above the mirror. This separation is important to keep the yarn out of the disturbed air around the mirror. A metal scale was fitted behind the yarn and calibrated with black tape on the bottom and red tape on top. In flight, when the yarn is in the black, you have a low angle of attack or a "lift reserve." When the yarn is above the black, the critical angle of attack has been reached or exceeded (the wing is "stalled"). For someone getting used to the AOA indicator, the mnemonic "red is dead" might be helpful. In this photo the yarn is in the upper half of the black, indicating a low lift reserve. This is as close to a stall as I care to get while on tow. Having your lift reserve status always available at a glance is very instructional and comforting, especially in turbulence.
The yarn is also quite useful as a traditional yaw indicator. If you look at the above photo carefully you'll see that the yarn has drifted right of the scale, indicating a subtle, uncoordinated yaw to the right. In normal 3-axis flying (no towed glider in the mix), it would be appropriate to add some left rudder or right aileron (or a little of both) to get the aircraft re-coordinated with the relative wind and your desired attitude and/or path. With a glider on tow, however, safe and appropriate technique will not always result in coordinated flight.
Under tow, we primarily use the rudder for yaw/roll control and try to minimize aileron excursions to avoid stalling a wing while flying so close to the critical AOA. In this case "stalling a wing" means asymmetrically exceeding the critical AOA, which can drop either the left or right wing panel and possibly result in a spin. When an aileron is deflected downward, it effectively increases its corresponding wing panel's AOA. When you're flying at an already high AOA especially in turbulence an inappropriate aileron input has the potential to ruin your whole day.
During those high "pucker factor" moments when turbulence pops a wing up hard, or you feel the tug "sliding off the edge" into a steep bank, a yaw string can save your bacon by preventing you from reflexively applying opposite aileron and "crossing the controls." This is an unfortunately common cause of stall/spin accidents in light aviation, and something you definitely want to avoid during a tow.
A yaw string will also point out the normal effects of a towed glider on the tug's yaw attitude. Obviously, an out-of-position glider will tend to pull the tug's tail out of place, both in yaw and pitch. But even during turns with the glider tracking properly on the same arc as the tug, the glider tends to induce a bit of "adverse yaw" into the tug's path by pulling its tail into the turn. To maintain a turn under tow, it may be necessary for the tug pilot to hold rudder input throughout the turn to counter this influence from the glider. But outside of the realm of towing, holding rudder throughout a turn is considered bad technique. My ATP instructor yelled at me for holding rudder in turns, but it wasn't until after I started flying with the string that I was able to make the distinction between "good" rudder holding during a tow and "bad" rudder holding when off tow.
In this photo, the glider has released and the tug has begun its descent. Notice that the yarn is well below the black now, indicating a very low AOA or a large lift reserve. Time to chill out and enjoy a beautiful sunset glide to the LZ.
This is the new, deceptively simple version of the AOA/yaw indicator that we're using now. The scale has been removed, leaving only a simple length of bent wire with the upper/forward end flattened where it is drilled for the attachment of the yarn. The horizontal portion of the wire is the key to its function as an AOA indicator. When the yarn is below this horizontal portion, your wing is flying (as shown in the photo). When the yarn is parallel to the wire or higher, your wing is stalled (if you haven't already noticed!). Obviously it is important to calibrate the instrument by bending the wire until it is parallel to the yarn at the break of a stall.
This new version also features a length of miniature streamline tubing over the vertical portion of the wire support, to both streamline and stiffen the device. One weakness of this prototype is that it shakes, rattles and rolls badly when the tug is taxied over rough terrain. Sometimes I'll take off and discover that the yarn has tied itself into a knot from the previous taxiing. Perhaps a carbon fiber arrow shaft or some other rigid but light tube would make a better upright for future iterations, although that will increase the design and fabrication complexity. (If anyone out there comes up with a better version, please contact me through adventureairsports.com to share your improvements.)
I recommend the use of an AOA indicator for beginning and seasoned tug pilots alike. Sometimes when the tug flies out of strong lift and into sink, your airspeed will seem to plummet. If you're managing your AOA by airspeed alone, the natural and safe response is to shove the nose down quick and hard to prevent a stall. It's very reassuring in this kind of situation to have the yarn to glance at to see what's really happening AOA-wise. In my experience with this indicator, that sudden sinking feeling rarely requires aggressive elevator correction, and by avoiding that I'm not diving away from the glider and slacking the rope as much as I might otherwise. Although I prefer to have the indicator when I'm towing, when I do fly without it my senses, instincts and reactions will be much better informed from all this experience with it. Seasoned ATP pilots will also probably learn some subtle lessons about lift reserve and coordinated flight in a Dragonfly, I'll wager!
© 2002 Adventure AirSports Flying Club