Aerotowing

AEROTOW

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.

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.

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.

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.

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.

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.

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