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One consideration that must be made at the outset of any balloon flight is the possibility of experiencing phenomena variously referred to as “false lift,” “false heavy,” or “uncommanded buoyancy.” These terms all describe an onset of various factors and conditions, which, despite the differences in terminology, all relate to the result of air moving over or under the balloon. The most important thing to remember is a balloon encountering one or more of these factors is not under the full and complete control of the pilot, and is therefore a hazard. Pilots should be aware of these conditions, avoid them if possible, and be aware of procedures and practices to minimize their effect on the balloon’s flight.
Three areas of focus warrant discussion:
• False lift
• False heavy (air flow under the lower portion of balloon, creating downward lift)
• Envelope distortion causing diminished capacity
It is important to understand the total physics involved. While the balloon is at neutral buoyancy on the ground, there are two lift forces at work. The first is from the heating of the air, creating buoyancy inside the envelope. The second is the flow of air over the top. The lift from these two elements combine to create the lift necessary to be at equilibrium. The addition of a small amount of heat, through a short burn, increases the total lift and allows the balloon to rise.False Lift
During initial flight training, pilots are taught about the effects of air flowing over the top of the envelope. While the balloon is static on the ground, the shape of the top forces the flow of air to compress over the top creating a low pressure area. [Figure 6-16] This low pressure area creates lift in much the same way an airplane wing does. There are two components of lift: heated air inside the envelope and the lift created by the air passing over the top. As the balloon takes off and accelerates to the speed of the air mass, the flow of air over the top diminishes, thus any lift created by it is no longer available. If the balloon is at equilibrium at launch, and there is not some response by the pilot to add more heat, there will not be sufficient lift to stay in the air, as a portion of the total lift has diminished.
Pilots are usually taught that the lift created by air flow over the top is to be considered “false lift,” because it was not created by applying heat to the envelope. The lift is real; as long as the speed of the air flow and the balloon remain sufficiently different, the lift continues to be generated. As the balloon accelerates, the lift created by the air flow is lost and more heat must be added to maintain the same rate of ascent.
The same condition can exist when descending through a low level wind shear or jet. As the balloon penetrates the lower boundary of the wind shear, the top of the balloon is momentarily in a faster moving layer of air which increases the air flow lift component of the total lift generated. For a short period, there may be equilibrium with the two components of lift, heat and air flow. As the descent continues, the top of the balloon moves out of the faster moving air (the value of one of the lift components), and air flow is diminished. The rate of descent increases, unless the pilot takes action to increase the buoyancy portion of the total lift by making a burn. Some pilots, when experiencing this, believe it to be a “false heavy” situation, an incorrect perception.False Heavy (Downward Lift)
False heavy is a condition which is the exact opposite of the false lift scenario described above, except that it is happening at another part of the balloon and the force generated has a downward component.
6-16
Wind
Lift Low pressure area
Figure 6-17. False heavy dynamics.
This phenomenon occurs when descending into a faster moving air mass or wind shear. The lower half of the balloon enters the shear and the surface of the balloon allows the air flow to generate lift. This lift is tangent to the surface of the balloon. Because it is below the equator, where the tangent line points in a downward direction, the lift has a downward component. This downward component of lift pulls the balloon down. [Figure 6-17]
This effect is more severe on a fully loaded balloon than a lightly loaded one, by reasons of skin tension. A lightly loaded balloon has more slack in the fabric on the lower portion of the balloon. On a heavily loaded balloon, the fabric below the equator has greater tension providing a surface where a low pressure can develop.
A scenario having much less impact is fast moving air across the mouth of the balloon. This creates a dynamic low pressure, similar to a venturi, which may cause the air to be pulled out of the envelope. As the air moves over the mouth of the balloon, it creates the dynamic low pressure, which pulls the static air inside the envelope out. Of all the possibilities discussed here, this point has the least impact on the lifting capability of the balloon, but is important when standing on the ground following the initial inflation to equilibrium.Diminished Capacity
　
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本文鏈接地址：Balloon Flying Handbook(82)