Glider pilot Practical Test Standards AREA IV, TASK C, MAINTAINING TOW
POSITIONS, requires pilots to exhibit knowledge of and demonstrate skill in both
high-tow (slightly above the wake) and low-tow (slightly below the wake).
||Visual references needed to maintain vertical position in each case
must be developed by the pilot. For high-tow, placing the tow plane on the
horizon will result in the glider being slightly above the wake because
anything on the horizon is at eye level, and the tow plane wake descends
slightly below its flight path, so it will be below the glider some 200
feet behind the tow plane. The pilot can find the low-tow position by
descending slowly through the wake until its effects can no longer be
detected. For most tow planes, that will place the glider where the bottom
surface of the tow plane's horizontal tail surfaces are clearly visible
and the bottom surface of its wing will be barely visible.
Straight flight in either high-tow or low-tow requires that the pilot
maintain lateral position behind the tow plane. This is easily accomplished by
keeping the glider wings level and its nose pointed at the tow plane tail.
Turning on tow requires some additional effort and can produce some interesting
experiences if not done correctly. It is one of the top ten reasons for failing
a practical test.
Note that the
outside side of the tow plane fuselage can not be seen from the glider if the
bank angle exceeds about 15 degrees.
||Many flight instructors advise students to match the tow plane bank
angle and point the nose of the glider at the tow plane's outer wing tip.
This works well for modest bank angles, but not for those greater than
about 15 degrees. The accompanying illustration is based on a Pawnee
towing a Schweizer SGS 2-33 at 65 MPH with a 200 foot line, but it is
probably representative of most other towing operations. The black line
represents the desired flight path of both aircraft. The red line is
corresponds to the glider longitudinal axis and thus shows the direction
that the glider is pointed. The arc in which both sides of the tow plane
fuselage are visible is shown in light blue. |
||Here is the view from inside the glider for bank angles from 5 to 35
degrees. The black line perpendicular to the glider bank angle indicates
the direction its nose is pointed. Again note that it is necessary to
point the glider outside the tow plane left wing tip as the bank angle
increases beyond about 15 degrees, and we lose sight of the left side of
the tow plane fuselage about the same time. |
In our illustrations we show the desired flight path of both aircraft to be
the same. That is desirable because of the relationship between turn radius,
speed and bank angle (Turn Radius (ft) = True Airspeed (kts) Squared, Divided by
11.26, Divided by the Tangent of the Bank Angle). If the glider is turning at a
radius greater than the tow plane it must also be flying at a higher speed in
order to complete each circle in the same time. Because its turn radius is a
function of its speed squared the bank angle must be greater than that of
the tow plane to compensate for the effect of higher speed. The reverse is true
if the glider is flying inside the tow plane's flight path. Therefore, the only
way that both aircraft can be flying coordinated at the same bank angle is if
they are also flying at the same speed and on the same turn radius. Minor
deviations from this ideal can be tolerated but they necessarily imply some
rudder use that results in less than perfect coordination. It is generally true
that it is better for the glider to be in the correct tow position than to be
coordinated, but if the yaw string is not centered during aerotow, the pilot's
technique is less than perfect.
If the glider is inside the tow plane's flight path, correction is easy. The
glider bank angle should be reduced until it returns to the correct position.
Since the glider is flying at a lower airspeed than the tow plane it will
accelerate as its turn radius increases so there will be no tendency for slack
to develop in the tow line.
If the glider is outside the tow plane's flight path (i.e. flying on a larger
turn radius) correction is more difficult and must be initiated immediately. If
not, the glider will tend to move even farther out because of the relationship
between speed, bank and turn radius.
||The correction involves increasing the glider bank angle enough to
decrease its turn radius. In this case the glider is flying faster than
the tow plane so it will tend to overtake the tow plane as the glider turn
radius decreases unless the glider pilot takes action to reduce the
glider's airspeed. Failure to take preventive action will result in slack
in the tow line. (It is always better to prevent slack than to take it
out.) The best way to do this is to yaw the glider away from the tow
plane, creating enough drag to slow the glider and prevent slack from
developing in the line. This is essentially a slip, and it can be used to
prevent slack in other situations such as recovery from an excessively
high tow position. |
Despite the pilot's best intentions, slack may still occur in the tow line.
Skill in removing slack must be developed to handle these situations, and it
must be demonstrated on all pilot practical tests. Slack is the result of the
glider flying faster than the tow plane. As we have just seen, that can occur
when the glider gets outside in a turn. It can also be caused by turbulence, but
regardless of the cause, it must be corrected promptly. Since the slack was
caused by the glider flying faster than the tow plane, it must be corrected by
the glider flying slower, and when the correction is complete, both aircraft
must again be flying at the same speed.
Pitch control should be used to stabilize the glider's
vertical position relative to the tow plane. Returning to the normal tow
position can wait until slack recovery is complete. Yaw control creates the
necessary drag. It should be applied promptly but not excessively. Bank should
be used to position the glider laterally, slightly to the side of the tow plane
in the same direction as the glider was yawed. Having the glider longitudinal
axis offset from that of the tow plane when the tow line becomes taut allows
both aircraft to pivot slightly, absorbing the shock that could otherwise break
the line. Some instructors advocate accelerating the glider by pitching down
slightly just as the last slack goes out, to match the tow plane speed and avoid
shock to the line. It is difficult to do this well, so utilizing pivoting of the
aircraft is a good backup technique in any case. It should be noted that
offsetting the glider excessively can do more harm than good. If it
results in a large yawing effect on the tow plane, it sets up a condition much
like that when the glider is far outside in a turn, and recovery may be
impossible. The ideal position for the glider relative to the tow plane when the
line becomes taut is about half as far out as when boxing the wake.
||One way to slow the glider is to create additional drag, and yawing it
is a simple and easy way to do that. The pilot should yaw the glider's
nose away from the tow plane so that it and the tow line are clearly
visible during slack line recovery. This action tends to cause the glider
to move away from the tow plane in the direction of the yaw, and it may be
necessary compensate by lowering the opposite glider wing. This is another
case where the pilot can assign specific functions to each of the
controls, recognizing, of course, that they still interact.
© 2000 Jim D. Burch
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