A better way to start would be with an airspeed indicator and a rate of descent meter. That would give you airspeed readings rather than groundspeed numbers, which are sensitive to thermal activity and upper winds. To calculate the horizontal component of the airspeed, simply take away the square of the descent rate from the square of the total airspeed, and then square root the answer. That's a lot easier than it sounds. Once you have the horizontal component of airspeed, you can divide it by the rate of descent to give you a glide ratio.
If going the airspeed indicator route: And then you have to understand that most airspeed indicators will read an IAS that does not change with altitude (despite changes in True Airspeed), while the rate of descent (whether barometric or gps based) will change with the air density at altitude, so one has to know how to correct the data for air density (using pressure altitudes and approximations of average airmass temperature) so that one is either comparing calibrated speeds for ISA conditions or uncalibrated speeds for local density altitude.
If that isn't crystal clear already... then yes it is messy to calculate properly.
If you have access to a map with scale like Google maps type, check your spot on the DZ when in freefall, check again at opening, when your parachute is fully open, then get an alignment to the target while noting mentally your altitude. Go straight over the target or any landmark of your choice then note the altitude again when passing over.
The glide ratio is the distance travelled versus the loss of altitude to cover that distance a) on the map, evaluate the distance between the spot (where you started your ride under parachute) and the landmark of your choice (eg. 3000 ft) b) substraction to get the difference in altitude between the altitude where you started your ride and the altitude you had over the landmark (eg. 3500 ft - 2500 ft = 1000 ft)
c) do the ratio (using figures in exemples given: 3000 ft / 1000 ft = 3 to 1 ratio Is it clear enough ? Note 1 : repeat this procedures few times to check if you get about the same results. Do an average of the results. Note 2 : to do this, you need to have no wind since going downwind increases the glide ratio and vice versa.
We have actually set up a data acquisition system for measuring Glide Slope. It consists of a data logger with an altitude sensor and an anemometer. Data is collected at 20 points per second. We plot the two curves against a common time line and the software has a Rate of Descent indicator. We obtain ROD and Forward Speed in FPS and plot them into an Excel spread sheet. Once we get five or six points we apply a Second order polynomial trend line from the graphing function. This is what is known as a Polar Curve. To get L/D from the Polar Curve you draw a straight line from the 0,0 point of the graph and align it tangentially to the Polar Curve. This line is the Glide Slope of the device. I have measured some sport canopies, little fast ones, and I can tell you they glide like a rock. Slope soaring canopies manufacturers are reluctant to publish Polar Curves for their products but some of their organizations have released some data. Their secret to getting any kind of glide is wing loading. They load at about .4 to .5 to 1. We load at .8 to 1 and much higher. I have scheduled some additional evaluation jumps with low wing loading. A 396 Tandem main canopy with a 150 pound jumper. I will let you know what we get.
Oct 31, 2012, 5:12 PM
Post #7 of 9
Re: [JohnSherman] Calculating glide ratio
[In reply to]
I'd say glide ratio also has a lot to do with how the canopy is trimmed. It can be trimmed nose low for speed, or nose up for floating around more like a paraglider.
John S.'s comment about wing loading being a key to glide is kind of confusing. It is true in one way and isn't in another. Technically, if everything else is the same, increasing the wing loading on a canopy or other gliding vehicle won't change the glide angle; it'll just go faster along that flight path. But in skydiving if you jump the same canopy model in a big version vs. a little version, the little one at higher loading will have a much worse glide since some things don't scale down (eg, the lines may be the same thickness), and especially, the suspended load (the jumper) is going to stay the same and is a big source of drag when flying faster. Small, highly loaded canopies do have a low glide ratio (e.g, 2.3) when brakes are off, more so for those designed to be ground hungry, trimmed nose low.
Back to the real question. No, there is not an easy way to calculate"any given" canpy's glide ratio. Unless you use monitoring equipment, and you use it for yourself only, ie your weight, and build etc. Then you will be able to pin point the glide ratio, but that will then only be for yourself.