How elliptical must it be to be elliptical?

[tdog 0]

So, I was glancing thru the gear reviews on this site and found the Pilot to be listed as "elliptical" - while the Sabre2 as "square" and the Spectre as "elliptical", and of course the velocity to be "elliptical".

This got me thinking, what is “elliptical”? What is “semi elliptical?” What is “square”? What do these words really mean…

Then I search this site and find some people saying that the Sabre2 is more elliptical than the Pilot... Ok, so I became confused….

So, worthy of at least half a brownie point, I opened a perfectly good packjob on my Pilot (which I even paid for because I was on a quick debrief-turn training day that got weathered out) and whipped out my tape measure in the name of learning about my gear... The center bottom seam is 98"... The last seam on the end is 76". That means my canopy is 28% less wide at the end. If I could make 28% in the stock market, I would be happy, so to me, 28% is a pretty big number...

So then I went to the Aerodyne website and found the chord measurements… They read 9.41 and 8.01 as max and min… That is a 17% change… They must not measure the bottom seams…

So I went to the PD website and found the Sabre 2, in the same size as my Pilot has a chord of 8.61 ft root, 7.92 ft tip… So, the Sabre 2 is only 8% narrower at the ends, while the pilot is 17% narrower - nearly twice as elliptical…The Specre in the same size is 10.10 ft - 9.21 ft… So about 10% narrower…The biggest Velocity measures 7.12 ft - 5.44 ft…. That is a 30% change. The Stiletto measures 9.03 ft and 6.31 ft – a 43% change in chord… (Note, obviously wing loading on the Velocity and Stiletto is going to be much higher, but I eliminated that variable from this discussion.)

So, I guess what I am getting at is, anyone want to speak their mind as to what the word “elliptical” truly means to them??? For an example when a canopy coach advises, “don’t intentionally stall an elliptical canopy as they will not recover nicely” – who is he really directing that to, by definition of elliptical? How elliptical does it have to be to be elliptical???? There was a thread a while back that was titled, “50 jumps and jumping an elliptical.” Well, technically the Navigator by PD, a student canopy is not square – so where do you draw the line? If tomorrow a new canopy came out half way between the Pilot and the Velocity in terms of shape, what would you call it and how would you expect it to perform? Do you think the Pilot performs much differently than the Sabre2 because it is twice as elliptical?

This is a very open ended question for discussion, knowing the answer is not going to be a cut and dry number... Kind of a spring board for learning here...

[dragon2 0]

How elliptical a canopy is does not say all about it's performance. We just updated our Dutch canopy charts, to give you an idea about which canopies we consider more or less hp:
http://www.parachute.nl/uploads/media/bvrbb.pdf
We have 5 categories, based on the number of jumps you have, how current you are, the wingload, which model canopy and the size of the canopy. This is the list of which MODELS you can jump

Surpisingly (to me, but I don't make the rules LOL), the pilot ended up as a canopy you can jump after 25 jumps but for the sabre2 and safire 2 you need at least 100 jumps....

ciel bleu,
Saskia

[ScottMcC 0]

Take a loop of string, a pencil, and two punchpins.

Put the two punchpins some distance apart, and hook the loop of string around them. There should be a fair amount of slack.

Using the pencil, pull the loop taught and trace around the figure, keeping it taught as you move.

You have now drawn an ellipse. If you move the two punchpins (foci) closer or further, you will note that the ellipse becomes more or less circular vs broad. If you remove one of the pins, you will notice that you draw a circle.

If a canopy is to be elliptical, it is elliptical or not elliptical. Semi-elliptical refers to the planform being described by some other equation than the ellipse equation I described above. For instance, the spectre is tapered, ie straight line but swept back, not curved as in an elliptical.

But I agree with the poster above. Elliptical alone does not dictate performance. Maybe we should compare focal lengths or something (how far apart the punchpins are in my example).

[tdog 0]

Quote

But I agree with the poster above. Elliptical alone does not dictate performance

That is kind of where I was going with this post when I first posted it... How does one know how a canopy will perform when “elliptical” often is used to describe high performance canopies while many elliptically shaped canopies are not high performance???

The example of how to draw an ellipse was a great reminder of my high school geometry... In the computer age I am so used to drawing ellipses on CAD software programs, ah technology… So what factors on the ellipse makes it more aggressive or sporty when on a canopy, I wonder??? In other words, if you see a canopy flying thru the air that, say is a prototype that has no marketing hype or name yet, and you look at it carefully – what things do you see that are tell tale signs to how it will perform – other than size? How do you tell how it will behave in, say, a stall or line twists (I.E. spin up on ya). Without flying it to see, how would you know where to put it on that chart that Dragon2 posted?

I hope Brian Germain does not mind me quoting page 101 of his book, "the Canopy and it's Pilot":

Quote

The distinction between "Tapered" and "Elliptical" is not terribly significant in ram air canopies. The difference is that the word "Tapered" refers to a linear reduction of the chord, while elliptical refers to a curved or incremental reduction in the same parameter. Although the linear-ness of the chord reduction is not hugely significant, the degree and shape of the wing's taper is very consequential.

It can be safely stated that the amount of taper is significant determining factor with regards to the speed at which the canopy turns. We must remember however, that there are other significant variables to consider as well....

So, the words I currently am pondering are in bold above.

I know this is very open ended, just thinking and learning out loud here about the science of canopy shapes and performance characteristics…

[Hooknswoop 19]

Most ‘elliptical’ canopies are actually just tapered. Some are tapered on the trailing edge only, ‘semi elliptical’, and some are tapered on the leading and trailing edge, ‘elliptical’.

Two elliptical canopies, tapered on the leading and trailing edges, can be very different by the amount of taper.

Generally speaking a fully elliptical turns faster and is more efficient than a semi-elliptical or square canopy. There are exceptions to that though.

Given the lack of other terms to replace elliptical, semi-elliptical, and square, they seem to wrk OK.

Aerodyne international has what they cal the planform factor, http://www.aerodyne-int.com/download/planformfactor.pdf, in an attempt to compare how elliptical canopies are.

Derek

[tdog 0]

Thanks for posting that link to the Aerodyne PDF... It is titled, "'The Planform Factor' How elliptical is elliptical?" - so it really answers a lot of the questions I had.

• Quote

[Samurai136 0]

Ask Scott Miller about elliptical.

Elliptical is a sales/ marketing term. Scott will tell you that no parachute is elliptical. Draw an ellipse and compare it to the canopy planform. They are not the same.
What the marketing guys are refering to by elliptical is the taper of the canopy. No parachute is tapered in the same way. Look at two "semi-elliptical" parachutes or "fully-elliptical" parachutes, their tapering will not be the same. Designers use different amounts of tapering specific to each design.

From a certain point of view a Sabre or Diablo is more tapered than a Xaos or Velocity.
It's all relative.

Ken

"Buttons aren't toys." - Trillian
Ken

[crazydiver 0]

Quote

From a certain point of view a Sabre or Diablo is more tapered than a Xaos or Velocity.
It's all relative.

Ken

Explain that one please...

Cheers,
Travis

[Samurai136 0]

The degree of tapering is not the single defining characteristic of how a canopy will fly. The Sabre and Diablo respectively have more leading or trailing edge taper than a Velocity.

'Elliptical' is a relative term to sell parachutes. Tapering is one of many characteristics in a canopy's design. Every design is unique and uses different amounts of tapering for different purposes.

Ken

"Buttons aren't toys." - Trillian
Ken

[crazydiver 0]

I would say that a diablo possibly has more of a leading OR trailing edge than a velocity, but there is no way you can argue that a sabre has more taper than a velocity. Sabres have no taper at all.

Cheers,
Travis

[chayes12 0]

If you go to PD's website and click "education" at the bottom you will see a drop down menu with all the line trims for PD canopies in PDF format.

http://www.performancedesigns.com/education.asp

At the top of each line trim chart is a drawing showing the shape or planform of the individual parachute. Notice the trailing and leading edges. A Sabre has no taper at all. If you look at the Stiletto, the center five cells are straight and the taper starts at the two endcells whereas the Katana and say the Vengence or Velocity have more gradual tapers that start from the center cell with each cell tapering more from the center outwards. You can compare these shapes to determine which one is more, or less, "elliptical"