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Another way in which a wind shear can increase the rate of descent is by diminishing the capacity of the envelope. For example, when descending, below is a low level wind shear with the air near the surface moving much faster or slower than the air mass in which the balloon is traveling. As the balloon enters the lower air mass, the side of the balloon is pushed in, decreasing the capacity of the envelope and pushing the air out the mouth. The larger the difference between the speed of the two air masses, the greater the effect. The lift created by buoyancy is decreased and the balloon starts to descend. If this happens at a low altitude, and the pilot has not responded in a timely manner, this may result in a hard landing.
If a balloon descends abruptly from a 30 mph wind into a 15 mph wind, it experiences an effective abrupt increase in wind across its surface from no wind to a 15 mph wind. This removes the boundary layer on its surface and greatly increases heat lost by conduction, while causing distortion in the form of a “dish.” This condition is very dangerous to the low level flight of a fully loaded balloon. It should be noted that as the envelope lowers into the slower wind and begins to distort and slow-up, the effective wind speed over the top begins to increase. Air moving over the top of an envelope produces false lift. Combined with diminished capacity, this again presents a hazardous condition which may result in an extremely hard landing.
“Dishing” usually does not affect the flight path of a lightly loaded balloon as much as a more heavily loaded balloon because no internal lifting heat is dumped; it is only redistributed inside the distorted envelope. To understand what a “dish” can do to lift, the balloon pilot needs to understand how the “heat line” fluctuates under different loading conditions. A balloon “floats” in the air because the hot air inside it weighs less than the volume of air it displaces. Usually the bottom of the heated air is down close to its mouth. Notice that a normal two-second single burn on a heavily loaded balloon adds only a small percentage of heat compared to its total required hot air volume. The same burn in a lightly loaded balloon adds a much larger percentage of lifting heat compared to its total required hot air volume. This is a simple way to visualize responsiveness.
If a heavily loaded envelope experiences a major “dish,” important lifting heat can be squeezed out. This condition is extremely serious if on a final approach to landing because there is not enough time and altitude to add enough heat. Remember, not only must the heat loss be replaced to make the balloon neutrally buoyant, but more must be added to stop any downward momentum.
Heat loss can change the slope of the approach and, pilot unaware, make it steeper. The stronger the shear, the greater the change in slope. Knowledge of this and the importance of adding heat quickly could prevent an excessively hard landing or an accident.
Some signs of shear to watch for are any movement of crown lines, handling lines, throat ropes, skirt, or even basket movement. It is important to realize that as the balloon lowers
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into the slower moving air, distorts, and starts slowing, the effect of heat loss can be masked by false lift. Even if the shear is a mild one, false lift momentarily exists. When the balloon slows enough and exits the faster moving air, the hidden false lift and dishing disappear and the balloon descends out of control.
Some pilots intentionally create a situation of diminished capacity when making a high performance descent. In low or stable wind conditions, this can be successfully executed. However, in the presence of strong wind shears, this technique can prove disastrous.
If a pilot is in a false lift scenario, the first action should be to continue to fly the balloon. If the situation is encountered during lift-off, or is believed to exist, the pilot should maintain a positive rate of climb until the false lift dissipates. If the false heavy scenario exists during the landing, the pilot must be prepared for an acceleration in the descent rate. An appropriate action would be to add heat to slow the descent, unless a steep approach to landing is desired.
In reality, there is nothing false about any of these situations. They are real and may create hazardous flight dynamics. In many cases, more than one of these elements is at work. It is important for the pilot to be aware of them, their effects, and consider what actions are necessary when they are encountered. The best prevention is anticipating these conditions and maintaining situational awareness. If the conditions are extreme, it may be said that the best and first consideration is staying on the ground. Landowner Relations
An otherwise perfect flight can be marred without the use of the proper relationship skills needed to foster good landowner relations. Often neglected, these skills provide the balloon pilot the locations necessary to inflate, launch, and land. Without these properties, ballooning would be severely limited. Taking the time to explain one’s actions to a landowner, or dealing with a farmer whose livestock have been spooked by an ill-timed contour flight, can create lasting impressions that have tremendous long-term negative impact on the continued evolution of the sport.
　
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本文鏈接地址：Balloon Flying Handbook(83)