Forums: Skydiving Disciplines: Swooping and Canopy Control:
glide ratio and speed at same wingloading different sized canopy

 


donscott26

Apr 6, 2011, 11:43 PM
Post #1 of 7 (931 views)
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glide ratio and speed at same wingloading different sized canopy Can't Post

Hello, just wondering if either the glide ratio or forward/ground speed (in no wind) of a canopy changes if the wingloading is the same but the canopy size is different

e.g
-if a safire 170 has a speed of 15 knots and glide ratio of 2.9 when loaded at 1.2, would a safire 120 also have a speed of 15 knots and a glide ratio of 2.9 if it is loaded at 1.2?

thanks for any feedback


Premier quade  (D 22635)
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Apr 7, 2011, 12:17 AM
Post #2 of 7 (916 views)
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Re: [donscott26] glide ratio and speed at same wingloading different sized canopy [In reply to] Can't Post

They will absolutely be (technically) different.

With the exact same canopy if you change the wing loading (let's say you wore lead on one jump and not on another), the glide ratio remains constant, but the speed you would travel down that slope would be higher with the higher wing loading.

So, does that mean if you changed the canopy to a smaller size but kept the wing loading exactly the same you'd get the same result? Not quite.

It would be a different speed because you'd also changed the coefficient of drag because the lines are slightly different as well. This is known as a scaling issue.

Would it really matter in the example you've given? Honestly, I don't think it would be a perceptible difference. Maybe some fraction of a mile per hour while going straight down a glide slope.

That said, the two canopies WILL behave significantly different in turns also because of the line length with the smaller canopy being noticeably more "twitchy."


indyz  (D 28525)

Apr 7, 2011, 12:21 AM
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Re: [donscott26] glide ratio and speed at same wingloading different sized canopy [In reply to] Can't Post

I know from experience that between two differently sized CRW canopies at the same wingloading, the larger canopy will out drive (horizontal speed) and out float (vertical speed) the smaller canopy. As the size gap increases the difference in performance can become quite pronounced. Larger formations, and even small formations with a wide range of sizes, have to take this into account when designing the formation.


donscott26

Apr 7, 2011, 1:12 AM
Post #4 of 7 (903 views)
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Re: [quade] glide ratio and speed at same wingloading different sized canopy [In reply to] Can't Post

thanks for the reply guys

In reply to:
That said, the two canopies WILL behave significantly different in turns also because of the line length with the smaller canopy being noticeably more "twitchy."

what about when the same canopy size is used with different wingloadings e.g one with lead, one without? would the canopy behave the same in turns? e.g a tandem master takes 1 customer that weighs 60 kg, he then takes a customer that weighs 100 kg, both jumps were under the same canopy in no wind, will the landing pattern be exactly the same to hit the center of the target? (I understand drag of a big customer compared to a small customer will make a very slight difference)


Premier quade  (D 22635)
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Apr 7, 2011, 1:36 AM
Post #5 of 7 (898 views)
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Re: [donscott26] glide ratio and speed at same wingloading different sized canopy [In reply to] Can't Post

In reply to:
what about when the same canopy size is used with different wingloadings e.g one with lead, one without? would the canopy behave the same in turns?

They will not behave the same. With more weight, you'll have a slightly increased lag between the time you pull the toggle and the time you actually begin the turn, but because of the increased mass swinging out from the canopy, the turn will end up being more dramatic.


morris

Apr 7, 2011, 4:18 AM
Post #6 of 7 (877 views)
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Re: [donscott26] glide ratio and speed at same wingloading different sized canopy [In reply to] Can't Post

In addition to the difference in linelength, the smaller canopy will - while having the same aspect ratio (at least very likely but not for granted, sometimes youll find minor differences in the aspect ratios of different seizes of the same type of canopy but those differences are usually after the second decimal point) - have less span.
A theoretical wing with an infinite span would not create any induced drag at all (as there wouldnt be no possibility for the high pressure air under the wing to move towards the low pressure air on top). So less span results in a higher induced drag for the smaller canopy.
On the other hand less span will reduce profile drag. Which one is more important?
There isnt just one answer to this question as this depends on the airspeed you are flying at a given moment, at high speeds profile drag "takes over" and at lower speeds induced drag is your "enemy".
Dont want to go into details, just wanted to add that its not just about the difference in linelength.
In addition parasite drag will not be that much lower on the smaller chute to compensate for the loss of liftcreating surface area...


DrewEckhardt  (D 28461)

Apr 8, 2011, 12:29 PM
Post #7 of 7 (761 views)
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Re: [donscott26] glide ratio and speed at same wingloading different sized canopy [In reply to] Can't Post

In reply to:
Hello, just wondering if either the glide ratio or forward/ground speed (in no wind) of a canopy changes if the wingloading is the same but the canopy size is different

e.g
-if a safire 170 has a speed of 15 knots and glide ratio of 2.9 when loaded at 1.2, would a safire 120 also have a speed of 15 knots and a glide ratio of 2.9 if it is loaded at 1.2?

thanks for any feedback

Close, but

1) Mass is a cube of height while surface area is a square function so pilot sectional density is higher under larger wings at the same wing loading.

2) Lines get shorter with smaller canopies but don't get any skinnier.

which means pilot and line drag is higher relative to the wing's drag on smaller canopies at the same wing loading for a worse lift/drag ratio. Increased drag below the canopy can also push the pilot farther back pointing the canopy more nose down.

The bigger canopy will have a flatter glide and more horizontal speed; the smaller one will be more ground hungry with higher air speed and less drive.


(This post was edited by DrewEckhardt on Apr 8, 2011, 12:32 PM)



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