## WIND SHEAR 

Wind shear is a phenomena that resembles. false lift in many ways. The principle difference is that the•balloon is airborne and may be accelerating or decelerating. In a practical sense, there is no difference between acceleration oi’ deceleration since the balloon is symmetrical and responds uniformly to air floW in any direction. The real difference from a pilot’s view› point is the location of the occurrence. False lift occurs from a known position selected by the pilot. Wind shear locations are usually unknown to the pilot which can increase the hazard. 1 recall a windy landing which was unfortunately expedited by a strong decelerating wind shear. At that time my balloon was not fitted With a skirt. The wind had increased unexpectedly to a force of about 25 to 30 mph. On a previous approach 1 ac› cidently struck a bank and tree causing the deflation valve to open over3 panels. Frankly,! was scared, nervous arld excited and probably not thinking too clearly.! was flying down a river valley and the landing sites were typically small and surrounded by trees. I decided to descend to tree top eiev~tion so that 1 would be ready when a suitable landing site appeared. 1 wasn’t experienced enough to realize that strong shears are typically present just above tree lines. The shear destroyed the balloon’s lift and I descended through the trees and into a small clearing. We hit hard but there was no damage to oc› cupants or balloon. I learned 3 big lessons that day. First, that velcro tops are not rated for the life of the envelope. Second, wind shears can deflate the •balloon sufficiently to lose con› trol. Third, that skirts are a necessity because they allow you to heat during a shear. 

Let’s examine what is happening dynamically and ther› modynamically to the envelope in a wind shear. The initial reaction of the envelope to a wind shear Is to distort and gain false lift. False lift occurs instantaneously and is momentarily beneficial. Distortion occurs a little slower and is proportional to the gross weight of the balloon, envelope volume, and the strength of the shear. There Is no evidence to suggest that one envelope design is more resistant to wind shear forces• than another. The effect of distortion is to reduce the envelope volume causing exhalation through the throat which Is follow› -ed--by cold a_i{ inhalatlon.-The-distortion--or breathing of-the envelope may, and typically does, cycle several times. Each distortion causes the average envelope temperature to decrease because of the inhalation of cold air. The heat transfer rate of the balloon envelope is also gre~tly increased. The wind shear wipes away the dead air layer which is laying on the surface of the envelope. This dead air layer, or bound?ry lay~r. represents significant insulation to the envelope. When the boundary layer Is wiped away by a gust the envelope heat loss nearly doubles. To view this in perspective the ballool) will 

. act like you doubled the load insofar as the operation and feel› ing of the balloon is concerned. The net result is a significant decrease in lift. 

I mentioned that the reaction of the envelope to shea~:] also dependent upon, the internal pressure, the volume of\ . envelope and its gross weight. The internal pressure of’ ... _ balloon envelope of a given volume is directly proportional to its lift. The limiting pressure is a function of e•nvelope height. Given an identical load, a small envelope will have a higher in› ternal pressure than a larger envelope. A large envelope, loaded to its rated gross weight, will have a higher internal pressure than a small envelope loaded to its rated grosS weight because of its greater height. What does all .thi.s mean to a pilot? Simply that a lightly loaded balloon will distort more than a heavily loaded balloon when it encounters a shear or gust because the internal pressure offers the only resistance to. those wind forces. 

There is a serious hazard to flight associated with wind shear which could be easily overlooked. A heavy shear gust can displace the vertical axis of the balloon. This displace› ment from vertical can be extreme, perhaps as much as 30 to 40 degrees: in severe turbulence. If the burner is operating at the moment the envelope displacf;!S, severe structural damage to the throat can occur. Structural damage is not loss of fabric; fabric Is not significant In the throat area because there Is no Internal pressure. Structural damage is loss of load tapes or ropes. The gravity of this problem is not easily visualized. The scenario involves the Joss of lift and thus pressure in the envelope and the loss of two or mo(e load tapes or ropes. When the load elements are burned free the fabric of that por› tion of the envelope is no longer supported. If the throat pressure Is low or negative the fabric repositions Itself Into the balloon throat thus blocking the burner from the envelope. As the balloon begins to descend the velocity pressure created bY. the downward flight of the balloon presses on this unsl’/) po~ed fabric causing it to close the throat further. At sol pomt, probably over 1500 feet/min., the force on fabr1 ... becomes so great that the fabric tears. This enormous internal flap of fabric effectively splits the balloon from the inside". This _ i~ -~-~n_<?wn __ fa_i!u~ _which _has _contributed to at least _c:me fatal balloon accident. AdViCe? DOn’t use Ccir1S018 nloLinted -bi..irner controls In turbulent air or high winds because they have too much afterburn. In turbulence and shears be ready to shut off the burner Immediately. When at altitude It is sei’dom urgent to maintain eqUilibrium thus there is no particular reason to operat_e the burner when heavy envelope distortion is present. Be aware that high burner pressures can cause the flame to reach to distorted fabric on the side of envelope and result in major damage. 

