I'm bored at work and need some info on glide ratio. Does anyone know what that ratio is for a Fox 285 (v-tech) loaded at .75. (42"pc worth of drag also)
K,
You could probably help me with this. I tried calling but it's 11:00am EST and you guys aren't in yet.
Hello Bryan,
I do not have the data for the configuration you are mentioning.
But with my Fox 245 Multi Vtech loaded at 0.75 lb/ft˛ and with a 28" PC (I was skydiving with my Fox :7 ), having my eTrex Vista attached to my helmet, downloaded data into my PC and plotted the vertical/horizontal plot bla bla bla and so on and so on and so on, I got for glide ratio an average of 2.20 (taking for the average everything, both data where I was flying downwind and data where I was flying upwind).
So, when I need to consider flight distance in my BASE jumps, I conservatively consider to have a canopy whose efficiency (glide ratio if you prefer) is 2.00. That is to say: for every single meter I go down, I go forward 2 m.
I hope it helps!!!
Stay safe out there
Blue Skies and Soft Walls
BASE #689
Stay Safe Out There
Blue Skies and Soft Walls
BASE #689
Hi Bryan...
I don't have this information available at the moment...I'll get it from one of the bosses in the next day or so.
K
Dwain,
I disagree with some of your comments and would like to make a few points, less any unsuspecting jumpers stake their safety on this information.
First let me say that "Glide Ratio" is a virtually worthless parameter for comparison as it is highly variable, difficult to quantify and often misrepresented in advertising. This is not to say that different canopies will not exhibit different behaviour with respect to glide efficiency but the simple "distance covered" approach is not a meaningfull point of comparison.
Many factors beyond the design will influence the glide of a parachute.
Wingloading is most certanly one of them as variations in wing loading effect dynamic pressure which in-turn influences wing shaping and efficiency.
Denisity altitude-for the numbers to have any relevance they must be correctd back to standard -day-sea level.
Ambient wind. Down wind/upwind, as pointed out, have a bearing on emperical measurments.
Canopy age and condition also will have a bearing. Both with respect to cloth integrity and also line trim.
PC drag is a factor but bridle attachment style is probably negligable. The principal influence of a PC is related to form drag not compramises in wing shaping.
I could go on with a list or minor factors but the point is that Glide Ratio is only a viable parameter when discussing fixed wing aircraft under controlled tests.
However, for parachutes the term came into the lexicon as a marketing tool. Any claims about a canopy's glide ratio weather valid or not are certainly not relevant to another canopy unless all tests were conducted under the same set of conditions and rules. Even in such a case the actual numbers would only be relative- not absolute.
Adam.
I agree with nearly everything you are saying. Glide ratio is a highly variable and relative figure dependent on many many factors.
However if somebody is asking for a ballpark figure then I believe 2.5 – 2.6 is probably somewhere close to the maximum (not average) achievable glide ratio given average wing loadings, near sea level air density and nil-wind conditions for both Vtec FOX’s and Mojo’s.
However I do disagree with this statement:
>PC drag is a factor but bridle attachment style
>is probably negligable. The principal influence
>of a PC is related to form drag not compramises
>in wing shaping.
In extreme circumstances pc drag will make a noticeable change in wing shaping which in turn will affect glide ratio.
For example when I jump a Mojo220 with a 52”zp pilot chute, in full flight the drag of the p/c will deform the wing so much that the leading edge of the canopy will have a very noticeable V-shape due to the center cell being pulled back by the p/c. If fact the nose of the center cell will become reduced in width and can become almost pinched off, especially during hard turns. I have video evidence of this occurring on numerous jumps.
Glide ratio will significantly decrease as a result of this configuration due to the change in wing shaping. The canopy will still flare and land well, but glide performance is noticeably poorer compared with using a smaller p/c on the same canopy.
(Note: a 46" zp p/c does not create nearly as much visible wing deformation on a Mojo220 as a 52" zp p/c does).
In comparison a multi equipped canopy (such as a FOX225) will not suffer as much noticeable wing deformation when dragging a 52”zp p/c. The drag of the p/c remains the same but the wing deformation is less. This is because the load is spilt between cells 3 and 5. There is still a reduction in glide performance due to wing deformation, but not as much as with a canopy with a single bridle attachment point.
Although a 52” zp p/c is an extreme example, my point is that PC drag can change the wing shape, which can in turn severely effect the glide ratio of the canopy. I believe this was one of the main reasons for the invention of collapsible pilot chutes in the skydiving environments.
Yo !
>However if somebody is asking for a ballpark figure
>then I believe 2.5 – 2.6 is probably somewhere
>close to the maximum (not average) achievable
>glide ratio given average wing loadings, near sea
>level air density and nil-wind conditions for both
>Vtec FOX’s and Mojo’s.
Another obvious point is a particular flight mode: glide ratios will be different for full flight, half-brakes, rear risers or a combination of them and brakes etc (with some rear risers providing the best ratio, in my experience). Were your numbers measured in full flight ?
In addition to what Adam said about comparing different canopies: L/D depends on a trim, and some canopies will fly steeper than others in full flight. However some brake or riser input changes the flight angle quite a bit, and more so for steeper-trimmed canopies, so the end result can be the same max L/D.
A side note: we have already stepped over L/D of 2 for the wingsuits, nearly or exactly matching L/D of a somewhat beat up base canopy. This means that in order to reach a distant LZ, soon we will have to be pulling low - as opposed to pulling high in the old days of base jumping... What a concept! ;-)
bsbd!
Yuri.
> Another obvious point is a particular flight mode: glide ratios will be different for full flight, half-brakes, rear risers or a combination of them and brakes etc
Not exactly, Yuri. Glide ratio is glide ratio. Full stop.
Glide ratio for an aircraft, for a canopy, for a paraglider, etc., is by definition "the best flight capability of that particular wing" which depends only by its geometry (which consequently affects its aerodynamc properties) and is a unique number.
"Glide ratio" of a "wing" is the maximum possible ratio between distance flown horizontally and distance flown vertically.
If you put yourself in the same conditions prescribed by the manufacturer you are going to achieve a flight path corresponding to the "glide ratio", that is, you can reach the maximum horizontal distance possible for that particular wing, that is again, the "flattest" trajectory possible with that wing.
Anything different from the "particular conditions" given by the manufacturer of the wing, will yield a flight path that is worse the "thoeretical one".
Example: the glide ratio of an aircraft is 12. But in the manual of that aircraft it is also written that you can achieve the "theoretical" horizontal distance given by the glide ratio (for the height at which you are) only if you "put" in the wind speed meter THE speed for maximum horizontal distance flown. And obviously you don't have to put flaps on your wing!!!!! That, ONE SINGLE speed for that particular aircraft. Having the engine at minumum, any speed different from the "maximum efficiency speed" (i.e., either slower or faster) and any other configuration of the wings (i.e., flaps on), will cause your aircraft to fly "worse" than the "best possible glide ratio flght path". You don't reach the "target" if don't fly AT that particular speed.
Coming back to out parachutes. Of course, each parachute has its own "glide ratio", that is the ratio between horizontal and vertical distance flown, in the best conditions, that is, full flight, canopy fully pressurized, best wing load for that canopy etc etc.
Now, from the flight corresponding to the "glide ratio" trajectory (=zero brakes), you can move on and apply brakes, make turns, and so on. Of course, doing so you "worsen" your "theoretical" glide ratio flight path (i.e., straight in approach on a tight landing area, you fly straight applying half or 3/4 brakes and so on, so "worsening" your flight path with respect to "glide ratio" flight, but doing so you land wherever you want!!!).
To summarize: "Glide ratio" is a geometrical/aerodynamical property for each wing such as you can reach the maximum horizontal distance possibile for that wing ONLY IF you fly that wing in the best possible conditions (stated by manufacturer). I.e.: if you are good, you can approach the "glide ratio" flight path (=you apply zero brakes), anything different from the "best conditions" will make you fly worse than the "glide ratio" flight path.
And, obviously, given for granted all the important features required for a BASE canopy (opening fast and on heading, with and without slider, etc), a BASE jumper would prefer to have a canopy whose glide ratio is the highest possible, so that, if you want/have to fly long distances, you can, if you want/have to fly short distances (=straight in approach), you simply "worsen" the "glide ratio" flight path applying the right amount of brakes.
My 0.02 €.
Stay safe out there
Blue Skies and Soft Walls
BASE #689
P.S.: My numbers (i.e., glide ratio of 2.20) were taken from a whole flight where I flew at full flight (zero brakes) and only applying brakes to make turns. So it is an average experimental glide ratio for a standard flight (where you apply brakes from time to time to do turns/correction of trajectory once in a while!!!).
Stay Safe Out There
Blue Skies and Soft Walls
BASE #689
I have the Etrex summit what software are you guys using to down load this and plat the ratio, this is because I want to get hold of some.
Greeny
Yo !
>Glide ratio for an aircraft, for a canopy, for a
>paraglider, etc., is by definition "the best flight
>capability of that particular wing" which depends
>only by its geometry (which consequently affects
>its aerodynamc properties) and is a unique number.
This is true for a fixed wing aircraft. On a ram-air parachute flying straing you have to change the wing shape and/or trim in order to change the airspeed and the angle of attack. We change the wing shape with brakes input and both the shape and the trim with risers. Therefore, you could define "the best flight capability of that particular canopy", but not the wing - it will be a different wing for the different flight modes.
>Now, from the flight corresponding to the "glide
>ratio" trajectory (=zero brakes), you can move on
>and apply brakes, make turns, and so on. Of
>course, doing so you "worsen" your "theoretical"
>glide ratio flight path (i.e., straight in approach on
A major disagreement: full flight mode does not give you the best glide ratio on a majority of the canopies.
For a fixed wing, there is a minimum descent speed and a maximum glide ratio speed. To achieve and keep them you use controls outside of the wing itself.
For a given ram-air canopy, you still have these 2 speed values but they are associated with different wing shapes and trims. They are generally slower than a full-flight speed, and achieved by applying some brakes and/or rear risers. Ram-air in a full flight can be compared to a fixed-wing glider in a dive ;-) The best glide ratio for most base canopies i have flown is achieved with a bit of rear riser input, or a combination of rear risers and some brakes.
bsbd!
Yuri.
Thanks for all of the responses!. I basically wanted to get a rough Idea, so that I can evalute landing area options at some new sites.
I think 689 touched on what I was trying to get at the best. I'm looking for the best case scenario, based on (since I own their gear) BR's tests.
What is the greatest amount of horizontal flight I can get for every ft. of vertical descent, under perfect conditions. With that # I can estimate (based on the numerous variables at hand) what kind of performance I can expect.
Just to comment: I estimate that with the a generous tail wind and the gear config of the original post, that a glide of 3:1 (or more if the wind is really humming) is possible.
Disclaimer--this is totally my opinion and comes from limited experience.
Adam,
you've pointed out some of the variations but how much does this really affect the glide ratio. I mean, if someone gives 2.5 as the glide ratio near sea level for some typical wing loading, not using rear risers to obtain lift, does this mean 2.5+-0.5 ? I mean, what is the quantifiable error on the glide ratio? thanks, Chris
Reply to Yuri:
> ...Therefore, you could define "the best flight capability of that particular canopy", but not the wing - it will be a different wing for the different flight modes...
Yes, what I meant is "wing" in a general term. Obviously at each amount of brake applied it varies slightly (or greatly!) the shape of the canopy. Still it remains our only "wing"!!!
> A major disagreement: full flight mode does not give you the best glide ratio on a majority of the canopies.
Yes, you are right indeed, Yuri. I just said so to semplify. It is true that the best glide ratio is achieved with a little bit of brakes applied, or, better, with a little bit of risers pulled down. Again, what I meant, simplifying, is that there is ONE SINGLE "CONFIGURATION" for a given "WING" that achieves the best glide ratio. For a rigid wing, it is THE pitch that gives you THE maximum efficiency. For a "soft" wing like our parachute, it is THE amount of brakes (or, better, THE amount of pull on rear risers) that gives you the best glide ratio. Anything different from what above, yields a "worsening" of the "best glide ratio".
You are very precise, Yuri. What I meant with my long post above, is that any "wing", rigid or soft that could be, has its OWN UNIQUE "GLIDE RATIO", that can be reached ONLY in a certain "configuration". For our parachutes, we can say that we have the "glide ratio1" if we are using a little bit of brakes and we have "glide ratio2" if we are pulling down a little bit the risers. For sure, it holds true that: "glide ratio2" > "glide ratio1".
Any other "configuration of brakes" (or risers that could be) gives you a worsening of the best glide ratio. I.e.: to use the glide ratio of your canopy, you must stick your hand in a given position (=amount of brakes) and not moving from there (apart from correcting the route or turning).
And to end up this story, I would simply like to say that what we would want in a BASE canopy is the highest possible glide ratio!!!! And finally, we SHOULD know at which "brake amount" we get this famous "best glide ratio"!!! In fact, when I am in doubt about reaching the designated landing area of our terminal wall (low pull, but, you know, tracking is so fun!!!), I simply attach myself on rear risers and look carefully if I am going to make it...!!!
To Bryan:
I would like to say in advance (without any willingness to offend one of the best BASE jumpers in the world) that Dwain's numbers (around 2.6) of glide ratio for Fox and Mojo seems slightly high, if we consider that PD gives for a Sabre a glide ratio of 2.7, being a Sabre "thin" as profile (=less drag) and ZP made (=less surface induced drag).
When you say:
> ...I estimate that with the a generous tail wind and the gear config of the original post, that a glide of 3:1 ...
Yes, it can happen. Consider that when I got "my" average on my Fox of 2.2, I got "legs" of my flight path of 2.4-2.5 when I was flying downwind (moderate wind) and "legs" of 1.8-1.9 when I was flying upwind.
What I don't understand, though, is what you do with a glide ratio of 3 under a "generous tail wind"?!?!?!?
You cannot plan a flight path down to the landing given for granted a glide ratio of 3 under a generous tail wind" simply because you cannot have a guarantee that the wind remains "generous" and with no change in direction all the way from canopy opening to landing. What if the wind reduces its intensity (generally, it happens approaching ground) or if it changes its direction (and so it is not any more a "tail wind")? You cannot make it any more to the designated landing area?
I think that the usefullness of glide ratio of your canopy, that you would prefer to be as much "accurate" as possible, is to give you an idea if in a "no wind" condition, you can make it to the landing with a good safety margin, or not. Then, from there, you must keep into consideration that for any "quantity" of wind you must reconsider your "glide ratio under that wind" (to semplify: tail wind or front wind) and from there move on to consider another safety margin for your flight down to the landing.
Stay safe out there
Blue Skies and Soft Walls
BASE #689
Stay Safe Out There
Blue Skies and Soft Walls
BASE #689
> Dwain's numbers (around 2.6) of glide ratio for
> Fox and Mojo seems slightly high, if we consider
> that PD gives for a Sabre a glide ratio of 2.7
I totally agree that the numbers I gave seem slightly high and I was somewhat surprised at these numbers when I downloaded and averaged them. The figures I gave were not supposed to be attributed with any sort of accuracy, but simply the average value of what I had obtained from a small number of jumps.
My figures came from only 5 jumps on Vtec FOX's (245 and 205) and 3 jumps on Mojo's (280 and 220) by three different jumpers so this isn't even close to being a large enough sample size. The one thing I did noticed however was that differences in wing loading didn't create significant variances in glide ratio to the extent that has been theorized by some people.
All flights were from BASE jumps and involed a straight line of flight descending about 350 feet (so this distance is small enough for GPS inaccuracies to effect the figures as well). On the jumps I personally did I was working the canopies to get the best glide possible (combination of rear riser and very light toggle input).
All jumps were from the same object in what I perceived to be nil wind at the time (nothing at exit, and nothing at landing). However I can't guarantee that no wind was blowing during the mid point of flight (weird wind conditions are known to exist at this object). I do suspect that there may have been a slight tail wind push on some of those jumps (the only clue I have is that these figures seem a touch too high). In retrospect I should have had the pilots drop a streamer during flight and had someone on the ground observe it. It the streamer was seen to drift then I would have discarded the GPS data for that flight.
While BASE #689 averaged a glide of 2.2 when flying both into and with the wind, wind speeds can vary at different altitudes so this figure (like mine) is only indicative in nature. The only way to get more accurate figures is to obtain a much, much large data pool.
In regards to the term glide ratio I was referring to the best possible glide attainable on a particular brand of canopy in nil wind. The fact that there are variables that will create differences in glide ratio is obvious.
However the fact that some brands of canopies out glide others (eg. Blackjack vs Mojo) becomes very obvious when you've done just a handful of jumps on both of them, without a need for detailed scientific data.
>What I don't understand, though, is what you do
>with a glide ratio of 3 under a "generous tail
>wind"?!?!?!?
>You cannot plan a flight path down to the
>landing given for granted a glide ratio of 3
>under a generous tail wind" simply because you
>cannot have a guarantee that the wind remains
>"generous" and with no change in direction all
>the way from canopy opening to landing. What if
>the wind reduces its intensity (generally, it
>happens approaching ground) or if it changes its
>direction (and so it is not any more a "tail
>wind")? You cannot make it any more to the
>designated landing area?
>
689,
Thanks again. To Clarify: My comment about a glide of 3 was just my opinion of what the maximum glide might be in a "generous" wind. I don't use it as a standard--ever.
It would be interesting to get a glide ratio number for a specific canopy from the manufacturer and to learn exactly how they conduct an experiment/s to come up with that number.
These are the average GPS figures I have from a pretty small sample size (in other words these figures are only indicative in nature):
Vtec FOX: 2.59
Mojo: 2.55
Wing loading shouldn't have a huge affect on glide ratio as long as it remains within the recommended canopy specifications.
The only thing wing loading should have a major effect on is the airspeed (both vertically and horizontally).
P/C drag can make a noticeable difference as well (e.g. 32" vs. 48"). This is because with a single bridle attachment point a large p/c will distort the wing shape significantly.
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