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 21
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.
I just want to know my glide ratio from full up toggles to half.
Never mind the wind, fix the wing loading at some Fixed value, how muuch further am I going to go at half toggles????
I think the only way to determine that is empirically. Even for identical twins flying the same size and type of canopy, things like how in-trim the lines are will still make a difference, so there is no grand formula you can just stick a bunch of variables into.
(This post was edited by -Joey- on May 26, 2013, 4:36 PM)
You can't "never mind the wind". If you're flying downwind and add a little brake you may go further than flying hands up. Using the same amount of brake in a headwind may shorten your glide.
Even if you quantify the extra glide distance in still air on half brakes you'll find the data useless. Gliding is too complicated for such simple assumptions. How much is half brakes anyway? Is your brake pull calibrated so that it is exactly repeatable? Did the trim on your canopy change in the last 50 jumps? Are you gliding up/downwind and at what velocity? Air temp? Altitude? Humidity? Different jumpsuit=different drag? Gain/lose some weight? Is there some rising or sinking air that will have an effect? And a bunch of other influences.
Just so you know, my instincts are to want the same information that you are looking for, I finally gave up trying to get it. Pick a method that suits you, get some data and understand that it will never be any more than an approximation. And a pretty useless one at that. I have a standing offer for anyone who wants to ground launch a skydiving canopy on a cliff above my house. You'll only need a glide of 2.5:1 to make a "safe" landing area. The offer is that I'll show you the exit point and I call 911 when you launch. No takers so far.
May 27, 2013, 12:40 PM
Post #17 of 21
Re: [JohnSherman] Calculating glide ratio
[In reply to]
Hmmm. There's some stuff I could really pick you apart on. It's tempting because not only are you wrong but you're up there screaming these things from the roof tops and I'm not sure every one here can edit out your misconseptions. But I'll make you a deal. I wont nag you about your math if you give me a pass on my spelling.
There are some things I would like for you to clarify. How exactly are you getting your points across speed ranges? The way this would normally be done is by changing the elevator angle so that the plane is trimmed at different speeds. It's a stable steady state airspeed and you get a very nice range of sink rates vs airspeed. How were you chosing your data points? You seemed to be paying a lot of attention to how it dived in a turn. That isn't really a representation of it's glide across airspeeds. He needs to just fly in a straight line for thirty sec at a time at different break settings. With ten points like that you should get much smoother data then what you seemed to have there. I'd like to see glide plotted relative to break setting.
I'm not sure what conditions you did this under but I don't see how your addressing thermals which could be as hard to filter out of your data as wind speed from gps data. Unless you have a nice inversion layer that you can play above you can probably only do a jump like this on the first load of the day at dawn. It makes me wonder if some of the variation you see in between jumps is from this. If you catch a thermal, even a small one, with that tandom or one of the larger canopies it could make your glide look really good. You really need multible jumps on each canopy.
In theory glide should not vary with wing loading. If I put on a forty lb weight vest my canopy should maintain the same glide just at a higher airspeed. I mention this because I've done this and it does. There could be some small losses in efficentcy do to seam leakage at higher wing loadings etc but these are minor unless you're Bruno. It seem that what you really compairing there is the size of canopies. If the same guy jumps two different sizes of canopies of the same make. that is not a compairison of two different wing loadings. There are a lot of things that don't scale as you change the size of the canopy. Large canopies fly much better then small canopies at equivalent wing loadings. Apples and oranges.
Another nice graph that people might like to see would be the results including wind. First you'd have to correct the data down to a standard altitude. Then you can do a circular plot for best distance vs crab angle for a given wind, wing loading, and break setting. A set of graphs like this is what you are really facing in a real world setting.
I appoligize for making back to back posts but, one more thing:
I had to make a choice several years ago. Should I spend my available cash on a new web site or should I buy the expensive/extensive test equipment I needed to continue to grow my knowledge of parachutes. I bought the test equipment and and therefore am able to convey to the sport test results they other wise would not have as no one else has done it.
Sorry 'bout the web site.
BTW: I have "Polar Curves" for the FB110, FB119 & FB200 which I previously posted. Additionally, I have tested the Katana120 and the JVX84. Anyone who wants to see them may E-mail me as they are too big and unnecessary to post here.
Here is the source data logger graphs for you to pick apart: See Attached.
All gilde data is aquired within the air mass of residence. ROD and Forward spped is relative to the air mass. Therminals don't matter, neither does the wind. Data is taken is straignt lines with stready brakes held for the length of the run. The runs are identified and seperated by video record.
Bring on the math nag!
I don't know whoses theorie you are quoting but you can quote me. Glide slope does vary with wing loading. After all these are just parachutes. If glide slope didn't change with wing loading then we would have 300 pounders swooping JVX84's.
I would not dissagree that different sized canopies might perform differently with the same wing loading. Initial data from 3 compariable jumps indicate this might be true. For now it is just theorie, sound thought it may be. Emperically there doesn't seem to make much of a difference, none that we can notice over the 17 sizes of Firebolts over the last 10 years.
BTW: Your cutaway video is comming. Thing have been busy here in Deland.
May 27, 2013, 2:02 PM
Post #20 of 21
Re: [JohnSherman] Calculating glide ratio
[In reply to]
How are you gathering your rate of decent data? If you are using a static pressure port and differentiating it in the data or a VSI then the altitude and rod are not relative to the air mass. The only way to measure is relative to the air mass you are traveling through would be with an actual vain calibrated rrelitive to the horizon. This would actually give you glide directly and some things like this have actually been done. but if your doing what I think you're doing with just a reading from a pressure altimeter then your rod data will be affected by thermals and this could quear your data if you don't have enough repetition of each test to compare them and filter out the anomallys.
And what break settings on the graph do each of the points you have marked refer to? How many inches of break at each one?
(This post was edited by RiggerLee on May 27, 2013, 2:14 PM)