Why are canopies not flat?

[gus 1]

Imagine you're flying your canopy directly behind someone else, see how the wingtips are noticeably lower than the middle of the wing (spanwise)? Why is that? Aren't aircraft wings pretty flat? Couldn't you just trim a canopy to be flat? Why wouldn't you do that?

Gus

OutpatientsOnline.com

[Sonic 0]

I've wondered this as well. Most canopies have longer lines on the outside, but they still "curve"

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It's like something out of that twilighty show about that zone

[nacmacfeegle 0]

Dayumm, somebody go wake up Billvon, or at least take him off 'autorecharge'....

Okay I'm no aerodynamics specialist, but if the canopy were trimmed flat, the forces acting on the risers and suspension lines would cause the canopy to collapse in the centre. The lift must act more or less in a straight line from the under surface of the canopy to the suspension attachment. Hence the 'arc' design

If the canopy were trimmed flat, the suspension lines would have to be attached to some sort of spreader bar of a similar dimension to the canopy span. This would make for a more efficient wing, in terms of lift, but exits would be awkward.

If you notice serious swoopers almost always loosen their chest straps, this allows the canopy to spread/flatten more, giving a much more efficent wing by flattening the canopy.

BTW most aircraft have a degree of angle to their wings, known as a dihedral (sp?). This angle may not always be obvious on the ground, but it is noticable when the aircraft is flying.

I'm pretty sure Quade, Billvon, Prof Kallend etc will have more explanataions to add.....Probably better worded too.

Editted to add.....
http://www.afn.org/skydive/sta/highperf.pdf
Page 40 shows the forces acting on the canopy that are pertinent here. You need acrobat reader (www.adobe.com), the whole document is exceptionally good reading for anyone desiring to understand their canopy better......Print it out, it looks better on paper than on screen.

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He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me. Thomas Jefferson

[cobaltdan 0]

bacically because they are more efficient, and also to provides additional spreading force to the flexible structure.

airplane wings would look similar if the fuselage had to be located significantly lower than the wing.

sincerely,

dan<><>

Daniel Preston <><>
atairaerodynamics.com (sport)
atairaerospace.com (military)

[Sonic 0]

Like a cessna you mean? Those wings look very flat in comparison to some other planes.

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It's like something out of that twilighty show about that zone

[AggieDave 6]

Not quite, Dan is saying if the body of the plane was something like 20ft below the wing, *then* you would start having a curved (vertically curved) wing.

--"When I die, may I be surrounded by scattered chrome and burning gasoline."

[gus 1]

Quote

and also to provides additional spreading force to the flexible structure

That seems to backup what I thought nacmacfeegle's funky diagram was showing: that the horizontal lift (Dan's 'spreading force'?) generated by having a curved wing helps it keep its shape.

Gus

OutpatientsOnline.com

[nacmacfeegle 0]

Ah Gus, its not my diagram, that document was written by someone far more versed in the science of aerodynamics than I. Credit where credit is due and all that, it was Mr Sobieski's work.
You more or less have the idea though.

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He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me. Thomas Jefferson

[riggerrob 558]

This concept of parachute end cells being lower than their center cells (anhedral or negative dihedral) began when parachutes were square and manufacturers cut all the A lines the same length. Equal length A lines worked well, so it became a tradition.
Now that we have fancy elliptically tapered canopies with different cell sizes and different line lengths for every cell, some manufacturers have tried flattening the center cells, but they still retain a bit of anhedral in the end cells.
Oh, and if you look at the best souped-up STOL airplanes, many of them have been retrofitted with drooped wingtips because it helps with aileron control at low airspeeds.
I believe that this anhedral helps with spreading force. Internal pressurization also helps with spreading force.
As for anhedral, any flying machine with the center of gravity significantly below the wing can benefit from anhedral. Just look at any of the large military transports with high wings. C-130 wings are pretty much flat, lacking dihedral (raised wing tips) or anhedral (drooped wing tips). All the C-17, C5, C-141 and all the big Antonovs and Ilyushins have anhedral. Without anhedral they would be too stable and "Dutch Roll" across the sky.

[pilotdave 0]

Drooped tips are a completely different thing from anhedral. They're for improving takeoff performance mostly. Anhedral is used mainly on large, high mounted, swept wing aircraft. It's a little hard to explain without diagrams, but high wing planes kind of trap air under the low wing (in the "armpit") when they turn. This creates a high pressure under the low wing which brings it back to level. Swept wings also have another "dihedral effect" in turns (or sideslips actually). The low wing sees a slightly faster airflow normal to the leading edge. (really need a diagram to explain that... but its true). This gives the low wing more lift.

Together, those 2 factors give a high swept wing too much dihedral so they must be angled down to counter it. As you said, too much dihedral causes dutch roll, where the plane kinda wanders in roll and yaw.

Dave

[darkwing 4]

that is a great article, but I don't think it makes a compelling case for cathedral being a design necessity. Certainly there have been ram air canopies that were essentially flat. If the cathedral were desirable or necessary, then wouldn't it be a mistake to go out of your way to minimize it by using wider sliders, bringing the slider down to neck level, using split sliders, etc?

I'll go for an argument that says anhedral is a performance advantage, but not one that says it is to contribute any useful spreading force.

-- Jeff
My Skydiving History

[nacmacfeegle 0]

"I'll go for an argument that says anhedral is a performance advantage, but not one that says it is to contribute any useful spreading force. "

Not arguing here, but I'll go with the explanation from a guy who actually builds flexible wings (for landing only, as opposed to fixed wings designed for take off as well)....Mr Preston (Cobaltdan)and his text above....
My original suggestion was that the anhedral (my bad in the original post) design of the canopy was to minimise unbalanced forces. IE by aligning the suspension and lift angles, the spanwise compression of the canopy could be managed. Without this alignment of forces, the only thing to prevent this spanwise collapse would be cell pressurisation. The cell pressurisation is at a minimum when you need a stable canopy the most, ie at low speeds on landing (when the forward motion of the canopy is at a minimum). Ignoring air locked canopies of course...
I was looking a it from a purely static point of view (very simplistic), if you draw up a vector diagram of the forces described within the referenced document, and the discussion here, I'm sure you'll see what I mean.
If the spanwise compressive force caused by out of alignment suspension and lift forces equals or exceeds the cell pressurisation force, the canopy would become structurally unstable, and 'concertina' at low air speed, or when flying through turbulence.

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He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me. Thomas Jefferson