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 Post subject: So.... whats the formula for glide?Posted: Tue Jun 05, 2012 10:36 pm
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Joined: Sat Aug 29, 2009 11:25 pm
Posts: 1558
Location: The Naki
well we had mutliple pages explaining the maximum height that can be achieved in a jump.

Whats the formula for glide?

I know aircraft and paragliders have a glider ratio.

What makes the best gliding kite?

My practical experience tells me the higher aspect and the flatter the kite the better.

My speed 3 is the gliding king!. but why?... i also have an ozone edge which is also a glide monster. but it doesn't glide aswell as the speed.... why.???

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 Post subject: Re: So.... whats the formula for glide?Posted: Tue Jun 05, 2012 10:47 pm
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Joined: Mon Feb 16, 2004 5:36 am
Posts: 7842
Location: Oahu
plummet wrote:
well we had mutliple pages explaining the maximum height that can be achieved in a jump.

Whats the formula for glide?

I know aircraft and paragliders have a glider ratio.

What makes the best gliding kite?

My practical experience tells me the higher aspect and the flatter the kite the better.

My speed 3 is the gliding king!. but why?... i also have an ozone edge which is also a glide monster. but it doesn't glide aswell as the speed.... why.???

You mean hangtime? I'd hangtime and glide are related but not the same.

If you mean glide as in the area you cover in a jump:
speed of kite, size of kite relative to windspeed....bigger kites will glide because of the hangtime...
if you send the kite from low you'll cover a lot more ground...

of two kites the same size aspect ration will generally hang longer.

You'd have to get JS or PF to answer about the math behind it.

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 Post subject: Re: So.... whats the formula for glide?Posted: Tue Jun 05, 2012 11:56 pm
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Joined: Sat Aug 29, 2009 11:25 pm
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Location: The Naki
nope i don't mean hang time. I mean glide. but i agree they are different.

I want lots of glide. i want to jump. glide above the earth for many many horizontal meters and then touch down super soft.

hang time in my book is popping straight up and hanging there for a bit and coming back down closeish to were you start.

So....... technique for glide is different to max height. for max glide you don't want to convert all your energy to upwards motion like a jump with max height. You want to retain some forward energy. jump lower but longer.

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 Post subject: Re: So.... whats the formula for glide?Posted: Wed Jun 06, 2012 12:06 am
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Joined: Mon Feb 16, 2004 5:36 am
Posts: 7842
Location: Oahu
plummet wrote:
nope i don't mean hang time. I mean glide. but i agree they are different.

I want lots of glide. i want to jump. glide above the earth for many many horizontal meters and then touch down super soft.

hang time in my book is popping straight up and hanging there for a bit and coming back down closeish to were you start.

So....... technique for glide is different to max height. for max glide you don't want to convert all your energy to upwards motion like a jump with max height. You want to retain some forward energy. jump lower but longer.

Yup makes sense. I do think they are different but related. Hangtime will definitely have you glide longer as the wind hits the LE or drag of the kite, it will go downwind and lead to glide until the speed is so high you loose pressure right? I guess that is the layman's version of it.

I am guessing the when you send the kite from low the reason you cover a bigger distance is that the speed of the kite lifts you off earlier and so the energy is not converted into upward speed, but you retain the speed you already have?

Goodness this makes me want to read physics.

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 Post subject: Re: So.... whats the formula for glide?Posted: Wed Jun 06, 2012 12:34 am
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Joined: Sun Sep 04, 2011 5:37 pm
Posts: 960
plummet wrote:
well we had mutliple pages explaining the maximum height that can be achieved in a jump.

Whats the formula for glide?

I know aircraft and paragliders have a glider ratio.

What makes the best gliding kite?

My practical experience tells me the higher aspect and the flatter the kite the better.

My speed 3 is the gliding king!. but why?... i also have an ozone edge which is also a glide monster. but it doesn't glide aswell as the speed.... why.???

GLIDE -- that's L / D or Lift divided by Drag

The Lift will affect the amount of hang-time or sink-rate Ya get but it's the
Drag that determines how far you can go with that Lift.

The Heavier kiter won't fly further, but will cover the same amount of
ground at a faster speed.

The aspect ratio or span squared divided by the aria affects the L/D a Lot
but if two kites have the same AR and same lift, it will be the one with the lesser
amount of Drag that will get the better glide. So the Ozone is single surfaced
and the speed-3 is double surfaced so the twin-skin should make less drag
and thus go further.

Here are a few good reference :
http://www.grc.nasa.gov/WWW/k-12/airplane/ldrat.html

also

http://en.wikipedia.org/wiki/Lift-to-drag_ratio

Bille

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 Post subject: Re: So.... whats the formula for glide?Posted: Wed Jun 06, 2012 12:37 am
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Joined: Sun Oct 26, 2008 5:29 pm
Posts: 466

I think a lot has to do this kite flying skills and timing. The last jump here you see the rider send the kite to the right to jump high, then he sends it left, right , then loops it three times to the left, to almost land a wild jump of almost 10 seconds.

Here glide/hangtime is created. Without those movements he would have decended
more rapidly.

I personally and working on three loops in the air. I have snuck two in a couple times, but its been challenging.

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 Post subject: Re: So.... whats the formula for glide?Posted: Wed Jun 06, 2012 5:13 am
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Joined: Sun Jul 06, 2003 12:50 pm
Posts: 645
Location: Darwin, Northern Territory, Australia
You want glide,,, get a Dyno 18m

cheers,

Robbie

ps not very good contribution ey, sorry!

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 Post subject: Re: So.... whats the formula for glide?Posted: Wed Jun 06, 2012 6:02 pm
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Joined: Wed Oct 16, 2002 1:00 am
Posts: 4789
Location: Denmark
The glide, typically expressed as glide "ratio" is simply L/D (Lift/Drag), very simple.

So glide ratio is how many meters you fly forward, when you drop one meter (or feet or whatever unit - does not matter).

(Hangtime is a VERY different thing, not happening at max L/D but that was not the question...)

If we take a look at how glide works, without relating to practical issues :

The glide of a kite or any aircraft, is independent of kitesize and rider weight, so constant for a given kite/wing at a specific or optimum AOA, just as Bille wrote

The MAX glide does not happen when the kite is sheeted to max hangtime, but max glide happens at a lower angle - kite depowered a bit

But when the wind comes into play, things changes a lot !

Full scale gliders and model gliders, very often carry water ballast or similar, in order to increase the wing loading (how much weight per wing area).
This is done for two reasons
1. The target for most glider competitions, is to get through turning points very far away and back, as fast as possible.
2. And/or to be able to penetrate heavy headwinds and find thermals/lift far windward.

If you double the weight, you get only half the hangtime, true
But you get twice the forward speed, and the very same glide ratio (speed is directly proportional to wing/kite loading, so if you weigh twice as much, or have half the wing/kite area - you will fly twice as fast, very simple )

What does this mean ?

When we jump, we will usually fly with the kite slightly into the wind IMO, during "normal" glides.

And as we often kite in winds over 10m/s, a heavier rider will actually be able to glide further than a light one IMO (or a smaller kite will glide longer than a big one).

If you glide WITH the wind, it changes to the opposite though... The light rider (lowest sink rate) will glide further than the heavy one.

Taken overall, for a glider going out and home - you will always be able to travel the longest distance with the heaviest glider and the same glide ratio, if there is wind, even it you have to go with headwind and tailwind back.
One can calculate on this to confirm if in doubt.
Thus many gliders, carry weight because of this

As the object for "good" pilots, is not to have the lowest sink rate (= max hangtime), but to be able to cover a huge distance to find the maybe sparse thermals around - where you can glide "up" and stay as long as you want - when you are skilled

Of course in kitesurfing - the higher you jump - the longer you can glide, so a too small kite or heavy rider will not give max distance in real life.
Also - there will be more loss when converting energy if too small a kite - so also a difference.

But in general - the stable glide ratio is NOT dependant on kite size nor rider weight, if there is no wind.

Very flat kites, with low drag foils and high AR, will have the best glide, no doubt.

But kites with max L/D profiles will NOT have the best hangtime - as max hangtime requires really deep profiles and drag not as important, whereas max glide happens with faster foils with less drag (and lift)

Hope this gives a very basic idea about how things work regarding glide

Peter

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 Post subject: Re: So.... whats the formula for glide?Posted: Wed Jun 06, 2012 9:43 pm
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Joined: Sat Aug 29, 2009 11:25 pm
Posts: 1558
Location: The Naki
That was a good read pf. my pratical experience emulated your theory..... so the theory must be correct!.

With my speed 3 there is a magical point of letting the bar out... about 1/3 when jumping. When i find that magical spot i glide A LOT longer!.

On the inflatables that point is far less obvious.

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 Post subject: Re: So.... whats the formula for glide?Posted: Thu Jun 07, 2012 7:41 am
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Joined: Wed Nov 24, 2010 11:03 am
Posts: 121
We come back to speed polar (what Peter refers to but do not directly mention) which has handled at maximum hang time thread.

(for more text http://flysafe.raa.asn.au/emergencies/aircraft.html)

So every wing/system has a speed polar. An-225, Space shuttle, A320, Cessna, glider, kite. Shape of polar changes according to characteristics of the wing but in general they look like attached image.

X-axis is airspeed, again with kiter it is a bit harder because of the pendulum and changed AoA (3d apparent wind) because of it. Y-axis is sink rate like how fast you go down. We must assume that center of gravity and kite lift are on same Z-axis of gravity with same airspeed and pointed to same vector.

No looking at the polar you see top of the curve (Vmp) which is the minimum sink rate of the system. Fly (airspeed) any slower than that and your sink rate increases. Fly faster and your sink rate increases. Point is that for maximum glide or distance, do not sheet in too much to slow airspeed down to go left side of the Vmp. At Vmp or right side (faster) of it you are happy and ok. You hang and cover distance. Vbg is velocity best glide so in "still" air (meaning headwind, tailwind, sinking air) you glide the best to cover greatest distance.

Now to wing loading, speed polar stays the same but it's position on airspeed/sinkrate coodinates change a bit. In general heavier the system more right and down the polar curve is transferred. So heavier riders need to fly even faster to be at minimum sink rate of their system at their wing loading.

*edit* changed quoted speed polar image and text according to it.

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