Advanced Ascent Rate Profile

We fly LOTS of balloons at EOSS. As a result, we gather lots of interesting data. One thing we've discovered is that our large balloons seem to appreciably reduce their ascent rate (our data shows about 30 percent) when they reach an altitude of approximately 45,000 feet. This reduction in ascent rate can lead to a degree of inaccuracy in a flight path prediction.

Nick Hanks N0LP, one of our members, noted the regular and repetitive nature of this effect and decided to take a look at what was going on. After studying the available literature and some kind correspondence from Dr. Jamey Jacobs of Okalahoma State University, Nick found that this phenomena is a result of increased drag on the balloon once it reaches a lower atmospheric density (remember density is temperature related too) when the ascent rate is around 1100-1300 feet/minute. This may seem counter intuitive as lower density should cause lower drag (which is offset of course by the large diameter of the balloon, but that�s another story). Because the balloon is roughly spherical, a phenomena unique to spheres moving through the atmosphere occurs.

Initially during the ascent below about 45,000 feet, the flow around the balloon is turbulent (i.e., not a smooth laminar flow) and is characterized by a high Reynolds Number (Re). When the air becomes thinner, Re decreases, and at one point the flow around the sphere becomes laminar. But for spheres at this point the coefficient of drag (Cd) actually goes up by a factor of two or more. This is because for a sphere the laminar flow creates a low pressure area on the leeward side of the balloon, and this low pressure area increases the drag. At the higher Re seen at lower attitudes, the turbulent flow actually reduces the drag because the flow �fills-in� around the back of the balloon reducing the size and strength of the low pressure region.

This, as we mentioned, is very counter-initiative to folks who have studied drag around such things as airplane wings, car bodies or flow in pipes where a high Re means turbulent flow and more drag.

I've added the ability to incorporate this phenomena into the prediction routine.

You can either manually enter a "High Altitude Ascent Rate" or have the program calculate one as a percentage of the "Initial Ascent Rate" by clicking on one of the radio buttons at the top of this page.

When this page loads, the "Initial Ascent Rate" is filled in with whatever you entered on the main setup screen.

You can enter a percentage of reduction in ascent rate.

You MUST enter the altitude at which you want this new ascent rate to become effective. Predictions below this altitude will use the "Initial Ascent Rate" above will use the "High Altitude Ascent Rate".

If you have selected to manually enter the "High Altitude Ascent Rate" you can type it in, otherwise when you click on the "Click to Activate" button this value will be calculated from the information provided above.

In this screen shot, this option is ACTIVE and so the label of the "Click to Activate" button has changed to indicate you should ""Click to Deactivate" (just to clear up and confusion).