Calculating glide ratio

[ChrisD 0]

ChrisD

***Lee, You should read:http://en.wikipedia.org/wiki/Polar_curve_(aviation)

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.

John

BTW: Your cutaway video is comming. Thing have been busy here in Deland.

JS

I wanted to flesh out some numbers to lend support and also to add support to knowing a little bit more about how our canopies fly and especially the idea of penetration, such as when you are caught downwind and attempt to make it back by using ½ brakes. Which for some canopies this might be a good idea and for others this is the wrong approach.

Using your data, for a FB 200 at the .94 wing loading, you have in free flight (toggles full up) and please help me here if I make a boo boo, FF: 22fps forward speed and 14 fps downward.

½ Brakes: 11 fps forward and 9 fps downward?

Let’s say we are at 1000 feet altitude.

In the ½ brake configuration this gives us 111 seconds from 1000 feet until we hit the ground? (Straight line, no density altitude stuff, no wind, etc…) Basically 1000 divided by the time 9fps will give us 111 seconds? During this time of 111 seconds we will travel >>> 111seconds times 11fps = 1222 feet.

In the full flight configuration: 1000 / 14 = 71.5 seconds 71.5 x 22 fps we will travel 1571 feet.

So in effect by using ½ brakes we lose 350 feet?

This makes the argument for penetration on this canopy, all other things being equal. Fly fast for distance.

Now using another’s data for a different canopy, (astute readers will recognize these numbers.)

Full flight: 67 fps forward, 29 fps downward.
½ Brakes: 36 fps forward, 12 fps downward.
From 1000 feet you will travel about 689 feet farther in brakes than in full flight!

This makes the penetration argument and technique wrong! You clearly fly father in brakes than full flight!

So making the statement that to make it back isn’t soo much flying in full flight as it is you should know yor canopie!


True or not???

C

Thanks again
Does any of this make any sense?

Velocity made good....


Something to know before hand before you just accept that all canopies fly farther in 1/2 brakes. ???


Can we try to stay on point here?


Students are taught that going to half brakes will lengthen their glide, is this the case with all lightly loaded canopies?

Cause this data shows there is some variability to this genereally accepted way of thinking,

In other words not all canopies go further when in half brakes???

C

But what do I know, "I only have one tandem jump."

[pchapman 261]

ChrisD

Students are taught that going to half brakes will lengthen their glide, is this the case with all lightly loaded canopies?

Cause this data shows there is some variability to this genereally accepted way of thinking,

One of course has to be careful in conversations whether someone means their "glide" as the true glide ratio through the air, or the glide ratio over the ground with the current winds.

Half brakes is a lot. I'd rather teach that "in general" 1/4 brakes as usually increases the glide ratio, for typical larger canopies.

Once you get into 1/2 brakes, really slowing the canopy but probably not reducing descent rate much more, then it gets trickier to predict without data.

When you get into small canopies that are built to be ground hungry, built with nose low trim, there things change a bit. They drop out of the sky with such a poor glide ratio, that when applying 1/4 brakes or 1/2 brakes or even 3/4 brakes (compared to zero brakes), it may improve the glide ratio.

(I know that sometimes someone under a small crossbrace, in deep brakes, can almost stay next to a student flying at full flight under some big boaty canopy. As soon as the small canopy's brakes are let up, it drops away steeply.)

At least all this is impression from the numbers I've seen and my experience.

[ChrisD 0]

pchapman

***
Students are taught that going to half brakes will lengthen their glide, is this the case with all lightly loaded canopies?

Cause this data shows there is some variability to this genereally accepted way of thinking,

One of course has to be careful in conversations whether someone means their "glide" as the true glide ratio through the air, or the glide ratio over the ground with the current winds.

Half brakes is a lot. I'd rather teach that "in general" 1/4 brakes as usually increases the glide ratio, for typical larger canopies.

Once you get into 1/2 brakes, really slowing the canopy but probably not reducing descent rate much more, then it gets trickier to predict without data.

When you get into small canopies that are built to be ground hungry, built with nose low trim, there things change a bit. They drop out of the sky with such a poor glide ratio, that when applying 1/4 brakes or 1/2 brakes or even 3/4 brakes (compared to zero brakes), it may improve the glide ratio.

(I know that sometimes someone under a small crossbrace, in deep brakes, can almost stay next to a student flying at full flight under some big boaty canopy. As soon as the small canopy's brakes are let up, it drops away steeply.)

At least all this is impression from the numbers I've seen and my experience.

I appreciate the reply, I used your stuff from a couple of years ago. Mr. Sherman calls em lawn darts, but some of these darts can increase their glide thru the air by about 900 feet per thousand by going to some intermediate toggle position.

I say thru the air because so many get confused with the ground speed confusion which is the stuff that this penetration issue arises in the first place. It's not penitratation as much as it is knowing your particular canopy.

I was on a canopy a couple of days ago, that did precisely what your indicating, it went down, in half toggles, and it went forward at the same speed as full flight. When on the ground I heard the usual stuff about being downwind and the penetration would have been better if that canopy was flown in full flight etc, etc.

This is why I was trying to start gathering data in units of 1000 foot altitude. It's a gravity powered device, that will move at a certain speed, depending upon how the toggles are held. Trying to simplify some of the little understood concepts about how our kites fly. All to often, Mr. Shermans vidios are a prime example of this, anyone starts speaking about glide and all kinds of crap about wind speed and ground speed come up, etc...

I'm not talking about the inertia at ground level in gusts. Now there's a subject that could use some additional interpretation. My point being, in most respects if your landing in a gust, and the gust stops, you stop flying and drop. This is beyond anyon's control and I wish this was more of an independent point than any particular skill that many hold....in high and variable wind conditions!

C

Thanks!

But what do I know, "I only have one tandem jump."

[pchapman 261]

format

Am I the only one who enjoyed this amusing discussion?

I starts at 2:25 with:
"Air is a non-compressible liquid". - John Sherman

Oh god. John Sherman really has to stick to what he knows, and know what he doesn't know enough about.

- He's totally wrong on air being non-compressible. (And keeps on arguing the point.) For low speed aerodynamics though, say under 200 mph, one can assume air to be incompressible, allowing simpler equations that are still very accurate.
- He's wrong on air being a liquid, and equating liquid with a fluid. Technically there is a difference, even if in daily life we might tend to mix the terms up.
- He's also effectively wrong when saying that in parachuting, Reynolds numbers can be ignored, because we're operating at 50,000 Reynolds or below. Yet much general aerodynamic data is for much higher Reynolds numbers, where aerodynamic results can be somewhat different, so one wants to be aware of whether some bit of data applies to higher or lower Reynolds numbers. (And besides, without doing the math again, I thought skydiving canopies are often up in the 10^6 or greater Re number. Whatever.)

Still, it is nice to see someone showing us outsiders their data acquisition methods and data collected!

[ChrisD 0]

What I would rather spend productive time on is answereing the question: What's the point?

And by that I mean what can we do for the average skydiver?

Perhaps if someone could point out factors that can help understand this penetration issue, like some kind of table, half toggles, full toggles, etc..

The comparison to sailplanes is nice but what does that have to do with understanding how my canopy flies?

I think there is a part 2 and we will have to wait till next year?

Put it this way,...45 min talk about glide ratios, and not a word about how this affects me on a long spot....and or how I can learn about my particular canopy....

C

I speak with so many that have it in their heads that by flying at full flight this increases their "Penitration" when all it is going to do is going to get them on the ground quicker and very short!

The polar curve is a great idea, but until you intimately understand how the "delta" T factors into things most are left with a incomplete understanding of what it means to fly fast. A DG 2000 sailplane is not a Katana, and making statements like getting out of sink by flying fast is not a safe comparison to make to a canopy. That's my only real objection to the Wiki site that was referenced...

So when so many D lic divers start speaking that penetration is "different" than glide and in high winds you need to penetrate to make it back, this is like holding up a sign

Best glide is best glide, has nothing to do with which way the wind is blowing. And is dependent and varies upon toggle position; this was missing....,

But what do I know, "I only have one tandem jump."