To develop the student’s shill in launching a hot air balloon over an obstacle.
This type of launch will allow you to comfortable setup your system before a launch to assure that you will safety launch over obstacles.
The appropriateness of any launch site involves much more than its physical size and absence of obstructions, even though these are important considerations. Of equal or greater importance is the direction the balloon will track following the launch. Any site selection made should include consideration of local winds as determined by pibal, smoke, trees, or other physical indications; forecast and reported winds aloft; and local phenomena peculiar to the specific site determined by personal knowledge or by consulting with local balloonists.
Pilots should take careful consideration about obstacle clearance during lift off and climb out from a launch site. The rule of thumb for computing the minimum distance between the launch point and obstacles downwind is as follows: multiply the wind speed expressed in miles per hour by 100, (a balloon will travel 88 feet per minute if the wind is blowing at 1 MPH. The 100 are 88 feet rounded up). The results of this calculation are the suggested minimum horizontal distance in feet from the obstacle that the balloon should be positioned for launch. The product of 100 times the wind speed is the distance that the balloon is expected to travel during the first minute of flight.
It is suggested establishing an initial rate of climb by adding the height of the obstacle plus a minimum desired clearance to arrive at an initial rate of climb. Thus, a 100-foot obstacle and 200 feet of desired clearance over the obstacle totals 300 feet. Upon takeoff, establish an initial rate of climb of 300 feet per minute.
In is also suggested an allowance for errors in distance estimation and changes in surface wind or other unforeseen occurrences. It is suggested that a landmark be selected halfway to the obstruction. If it should appear that the balloon would not achieve a minimum of half the clearance altitude by that halfway point, immediately terminate the flight by activating the deflation port (Smart Vent) before reaching a critical altitude (usually about 20 feet above the surface.
Example: “The wind speed is 4 mph and there is a 75- foot line of trees downwind at the end of the field. Using the formula as described above, multiply the wind speed by 100 (4 X 100 = 400). In order to pass 100 feet over the trees after takeoff, add 100 feet of the tree height of 75 feet, resulting in a total of 175 feet. Lay the basket at least 400 feet upwind of the treelike. Upon takeoff, establish a minimum initial rate of climb of 175 fpm. The balloon should pass 100 feet over the trees.”
Review the formula using different wind speed and obstacle heights and discuss the various variations that can be used.
Applying this formula during your setup and launch sequence will assist you in avoiding obstacles downwind during launch. But consideration also must be accounted for false lift during takeoff.
While this formula might work well for low obstacles, those under 100 feet, additional considerations in wind speed and direction must be considered for taller obstacles. For a more detailed discussion of launch consideration read Box Canyon (avoiding radio/TV towers) in the February, 1991 issue of Balloon Life.