Calculating Freefall Drift from the Winds Aloft Forecast

Last post was all about reading the winds aloft report.  Now, we get to estimate how far we will drift in freefall. Notice the word estimate…this is not an exact calculation as we use generalizations in our number crunching. But, it will give you a good starting point when you are deciding where to exit on the first load of the day.

Ahhh…the winds aloft report for Feb. 4, 2020.  If we look at the predictions for the Jacksonville airport (the row for JAX), we can see that from 3000′ to 12000′ the winds are coming from 220, 230, 230, and 230 degrees, respectively…meaning from the southwest. The winds at 3000′ are predicted to be 13 knots, at 6000′ they are 23 knots, at 9000′ they are 28 knots, and at 12000′ they are 36 knots.

So, what do we do with the winds aloft information?

First we average the wind speed and then we will average the wind direction.  For our example, the average wind speed will be the sum of the wind speeds from 3000’to 12000′ divided by the total number of numbers, or in our case, (13+23+28+36)/4.  And our average wind speed is 25 knots. The conversion of knots to the more well-known miles per hour is 1.15 mph per knot. So 25 knots is equivalent to 28.8 mph.

Once we have figured out our average wind speed we can now think about how far we will drift.  In general we are in freefall for approximately 1 minute. Since our average wind speed has been converted to miles per hour we can translate that into miles per minute.  28.8 (miles/hour) * 1 hour/60 min = 0.48 miles/min During our freefall we will drift approximately half a mile.

But that is not the end of the story.

What is forward throw?

When we exit the aircraft we do not immediately begin falling straight down.  Forward throw is due to your body wanting to continue moving at the same speed and direction as the plane you just left. It takes several seconds before the relative wind slows you down and you begin to just fall.  All this means is that in light to moderate winds (illustrated as the blue line) you will be thrown forward about 0.2 miles upon exit before being subjected to drift. In heavier winds you will experience little to no forward throw (the red line).

So, in our example, our drift will be 0.5 miles-0.2 miles=0.3 miles.

In which direction am I drifting?

To figure out which direction we are drifting we calculate the average of the wind directions.  In our example we would find the average with this equation: (220+230+230+230)/4=228.

Since we know that the average wind direction is from 228 degrees (coming from the southwest) then we will drift in freefall to the northeast.  The wind will be blowing us from the southwest to the northeast.

Where should I exit the aircraft?

The example that I pulled up from Feb. 4th shows that you should select an exit point roughly 0.3 miles to the southwest of your intended opening point (where you will deploy your parachute).