Factors Affecting Glider Performance (Part Two) – Wind

Wind

Wind affects glider performance in many ways. Headwind during launch results in shorter ground roll, while tailwind causes longer ground roll before takeoff. [Figure 5-2] Crosswinds during launch require proper crosswind procedures or control input to track along the runway.

Figure 5-2. Apparent wind effect on takeoff distance and climb-out angle.

Figure 5-2. Apparent wind effect on takeoff distance and climb-out angle.

During cruising flight, headwinds reduce the groundspeed of the glider. A glider flying at 60 knots true airspeed into a headwind of 25 knots has a groundspeed of only 35 knots. Tailwinds, on the other hand, increase the groundspeed of the glider. A glider flying at 60 knots true airspeed with a tailwind of 25 knots has a groundspeed of 85 knots.

Crosswinds during cruising flight cause glider heading (the direction in which the glider nose is pointed) and glider track (the path of the glider over the ground) to diverge. In a glider, it must be remembered that crosswinds have some head or tailwind component that results in a lower or higher true groundspeed. When planning the landing, the wind effects must be factored into the landing pattern sight picture and allowances must be made for the winds, indicated or expected. It is a lot easier to lose altitude than it is to make up altitude when the glider is down low. When gliding toward an object on the ground in the presence of crosswind, such as on final glide at the end of a cross-country flight, the glider pilot should keep the nose of the glider pointed somewhat upwind of the target on the ground. For instance, if the crosswind is from the right during final glide, the nose of the glider is pointed a bit to the right of the target on the ground. The glider’s heading is upwind (to the right, in this case) of the target, but if the angle of crab is correct, the glider’s track is straight toward the target on the ground. [Figure 5-3]

Figure 5-3. Crosswind effect on final glide.

Figure 5-3. Crosswind effect on final glide. [click image to enlarge]

Headwind during landing results in a shortened ground roll, and tailwind results in a longer ground roll. Crosswind landings require the pilot to compensate for drift with the proper flight control input, such as a sideslip or a crab. Glider pilots must be aware of the apparent angle versus the rate of descent. The glider descends at a constant rate, but the different ground speeds will result in different approach angles. These different approach angles require specific techniques to ensure safe touchdowns in the landing zone. For example, if landing in a strong headwind, the glider pilot should plan for a closer base leg to allow for the apparent steeper approach due to the slower ground speed. Another technique would be the delayed extension of spoilers or drag brakes and accepting the faster airspeed to counter the headwind component. In the case of a tailwind and an apparent lower angle of approach, the glider pilot can use more spoiler or drag brake extension or slipping or a combination as allowed by the GFM to touchdown in the landing zone. In any condition, the glider pilot must make allowances for the current conditions of wind and density altitude to ensure a safe landing in the landing area. A glider pilot must respond to the current conditions and amend the traffic pattern and/or modify their procedures to compensate for the conditions and land safely. The different ground speeds also result in different touchdown points unless the pilot takes some kind of action. In any case, the pilot should aim for the touchdown zone markers and not for the end of the runway. [Figure 5-4]

Figure 5-4. Wind effect on final approach and landing distance.

Figure 5-4. Wind effect on final approach and landing distance.

During landing in windy/gusty conditions, there is a tendency to lose airspeed (flying speed) and increase sink. The friction between the ground and the air mass reduces the wind strength. The glider may be flying into a strong headwind at one moment; a few seconds later, the windspeed diminishes to nearly zero. The pilot landing into a headwind can usually expect to lose some of that headwind while approaching the surface due to surface friction slowing the wind. When the change is abrupt, the pilot experiences a loss of airspeed because it requires some small time and loss of altitude to accelerate the inertia of the glider up to the airspeed previously displayed when into the stronger headwind. This takes on the appearance of having to dive at the ground to maintain flying speed. Fly a faster approach to ensure staying above stalling speed. Depending on the wind change, a longer ground roll may be the result. If this occurs near the ground, the glider loses speed, and there may be insufficient altitude to recover the lost speed. This is called “wind gradient.” Consideration of wind gradient during a ground launch is important, as a sudden increase in windspeed could result in exceeding the designed launch speed. [Figure 5-5]

Figure 5-5. During gusting conditions, the pilot must monitor the pitch during the tow.

Figure 5-5. During gusting conditions, the pilot must monitor the pitch during the tow. [click image to enlarge]

The pilot landing with a tail wind has a higher groundspeed for an indicated airspeed. As the surface friction slows the winds, the pilot may see an increase in airspeed before the higher inertia-induced airspeed is dissipated, which may increase the ground roll distance to touchdown. This may be experienced with a downdraft in the vicinity of obstructions upwind of the runway as the winds curl down and wrap under the obstructions. This effect can lead to major undershoots of the approach path and landing short if the winds are strong enough. Local pilots can be a rich source of information about local wind currents and hazards.

Glider pilots must understand that wind near the ground behaves differently higher up. Atmospheric conditions, such as thermal formation, turbulence, and gust and lulls, change the in-flight behavior of the glider significantly. As the wind flows over the ground, ground obstructions, such as buildings, trees, hills, and irregular formations along the ground, interfere with the flow of the wind, decreasing its velocity and breaking up its smooth flow as occurs in wave and ridge flying.

Wind gradient affects a pilot turning too steeply on final approach at a very low airspeed and at low altitudes near to the surface. There is less wind across the lower wing than across the higher wing. The rolling force created by the wind gradient affects the entire wing area. This can prevent the pilot from controlling the bank with the ailerons and may roll the glider past a vertical bank. [Figure 5-6] Be cautious with any bank angle and at any airspeed while close to the ground when transitioning a wind gradient.

Figure 5-6. Effect of wind velocity gradient on a glider turning into the wind. Stronger airflow over higher wing causes bank to steepen when close to the surface where surface friction slows winds.

Figure 5-6. Effect of wind velocity gradient on a glider turning into the wind. Stronger airflow over higher wing causes bank to steepen when close to the surface where surface friction slows winds.

When approaching to land during windy and gusty conditions, add half of the wind velocity to the approach speed to ensure adequate speed for a possible encounter with a wind gradient. During landing under these conditions, it is acceptable to allow the glider to touch down a little faster than normal instead of holding the glider off the ground for a low kinetic energy landing. Upon touchdown during these landings, extending the air brakes fully prevents the glider from becoming airborne through a wind gust during the landing roll.

Some self-launching gliders are designed for extended periods of powered cruising flight. For these self-launching gliders, maximum range (distance) for powered flight and maximum duration (elapsed time aloft) for powered flight are primarily limited by the self-launching glider’s fuel capacity. Wind has no effect on flight duration but does have a significant effect on range. During powered cruising flight, a headwind reduces range, and a tailwind increases range. The Glider Flight Manual/Pilot’s Operating Handbook (GFM/POH) provides recommended airspeeds and power settings to maximize range when flying in no-wind, headwind, or tailwind conditions.