Angle of Attack Indicator for Aerotow Tugs

Doug DuBois, Adventure AirSports Tug Pilot writes:

Aerotow tug pilots spend roughly ¾ 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 ⅛" 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 http://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!

Doug DuBois is a GA/UL/HG pilot with a background in aircraft building,
machining, engineering and industrial design. He is a partner in Adventure
AirSports, LLC, and was the group's first tug pilot.