aerodynamics and canopies

[hudsonderek 0]

I was reading the article in response to Brian Germains "no wind landing" article. in it Scott miller describes the toggles as a lift enhancing device, this makes sense because when you flare, you decrease your rate of decent. why then, when I want to turn do I deflect the toggle on the side I want to turn? ailerons on an airplane don't work this way, when the aileron on the wing is deflected downwards it produces more lift on that side, and hence a turn in the OPOSITE direction from the down aileron is created. could we look at the toggles instead, as a drag enhancing device? The critical angle of attack for most airfoils is about 18 degrees. air just can't be bent more than that, next time you pull a toggle look at your tail material (if you can estimate angles from I bet it's bending more than 18 degrees from the rest of your canopy. to explain landings could we say that we create drag pulling the canopy back, with the toggles and, because we are suspended from the canopy like a pendulum, we swing out and inturn change the angle of attack from that? (sort of like pulling on the rear risers) does this make sense? any comments?

[billvon 2,402]

>why then, when I want to turn do I deflect the toggle on the side I want to turn?

Because the adverse yaw created by the drag on the side you pull down overcomes the roll. Parachutes are extremely stable in roll; your weight keeps the parachute above you and it does not want to deviate from that. However, they are not very stable in yaw, and easily change their heading.

When you pull down on a toggle, the lift on that side increases but doesn't do much, because the canopy is so stable in roll. The drag also increases and that DOES do a lot because it's less stable in yaw - so you turn that way.

However, some parachutes (like my Nitro) _will_ turn away from the toggle if you make very small toggle motions. For small deflections, the lift can increase while drag doesn't change much. That leads to a very small change in roll (because of the stability) but no change in yaw (because drag hasn't increased yet) and thus you turn away from the toggle you are pulling. It was a bit annoying until I got used to it.

[murps2000 86]

to explain landings could we say that we create drag pulling the canopy back, with the toggles and, because we are suspended from the canopy like a pendulum, we swing out and inturn change the angle of attack from that? ***

Yes, we could and many of us do. Get Brian's book, "The Parachute and it's Pilot" and he will teach you about how parachutes turn. They don't have ailerons and the center of gravity is way below the wing so even if they did, they probably wouldn't work so well.

[UDSkyJunkie 0]

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could we look at the toggles instead, as a drag enhancing device?

For turns, I'd say yes, and I think Billvon describes exactly why. (for the record, that part about the Nitron is cool as hell! I've never heard anyone say that about a parachute before).

For landings, I think it's truly a lift-enhancing device, as the deflection of the tail changes the the AOA of the camber line, increasing lift (if this wasn't true, canopies wouldn't plane out!) In very deep brakes, it's more like drag-enhancing... I've seen photos of myself landing where the tail is deflected past 90 degrees and the D-lines are slack! Critical AOA is probably higher than 18 deg since it's a very high-lift airfoil at low speed, but at 45+ degrees it's obviously way into stall territory.

"Some people follow their dreams, others hunt them down and beat them mercilessly into submission."

[quade 3]

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Critical AOA is probably higher than 18 deg since it's a very high-lift airfoil at low speed, but at 45+ degrees it's obviously way into stall territory.

Highly doubtful it's as high as +18. That would be quite remarkable for a well designed rigid wing.

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

quade -
The World's Most Boring Skydiver

[pchapman 261]

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Critical AOA is probably higher than 18 deg since it's a very high-lift airfoil at low speed,

Highly doubtful it's as high as +18. That would be quite remarkable for a well designed rigid wing.

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

Without trying to get too far into airfoil discussions, I'd say 18 degrees is not totally unrealistic.

Foilsim (your link) does seem to show a very early stall if the output is set to Surface Pressures. Just about anything seems to show a stall at 10%. But if one selects Lift vs. Angle, more realistic results occur.

E.g., for something vaguely resembling a skydiving canopy, how about 18% thick, 4% camber. Max lift is at 14.75 degrees geometric, but because of the camber, zero lift isn't attained until -3.75 degrees. Thus the total aerodynamic range to the stall is 18.5 degrees.

However, the graphical part of the display still shows separated flow over much of the airfoil. The problem is the graphics are crude and pretty much suddenly switch from attached to separated at, you guessed it, 10 degrees.

For another source, there's the classic book Theory of Wing Sections (by Abbot & von Doenhoff). For a similar NACA 4418 airfoil: 14 deg geometric to maximum lift, zero lift at -3.5, thus a full aerodynamic range of 17.5 degrees to the stall.

So I'm just saying that 18 degrees, isn't totally out of whack when talking ideally about an airfoil. Although that is without the limitations of a 3 dimensional fabric wing, where the performance wouldn't be nearly as good -- and that you're right about.

(One could still argue about what exactly defines the stall, and whether FoilSim is really doing a good job at predicting stall behaviour...I'm skeptical... it is a notoriously tough thing to do well even without the most advanced aerodynamic codes.)

[kallend 1,632]

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Critical AOA is probably higher than 18 deg since it's a very high-lift airfoil at low speed, but at 45+ degrees it's obviously way into stall territory.

Highly doubtful it's as high as +18. That would be quite remarkable for a well designed rigid wing.

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

Airfoil data as quoted in data books is generally for infinite aspect ratio. Finite aspect ratio lowers the effective AoA on account of downwash (and hence raises the stall angle). Canopies are very low AR wings. It wouldn't surprise me at all if the stall AoA were 18 degrees. Planform would have an effect too (rectangular vs tapered vs elliptical) because it affects the flow at the tips.

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The only sure way to survive a canopy collision is not to have one.

[hudsonderek 0]

I'm thinking toggles don't produce any praticle lift (at least in the manner we use them on canopies) and are moreover drag devices, I believe we're producing more parasitic form drag than we are induced drag. When the canopy slows we swing out because of our momentum, and hence change the AOA of the main canopy. our mean camber line then extends from the nose of the canopy to where the tail material begins to bend. how can, in turns, the toggle and tail material be a drag device, and in flaring a lift enhancing one? I agree that with very little toggle pressure, you are increasing lift, but as I said in the manner we use them on landing we are deflecting them to the point where they are creating more form drag than anything else. am I totally off here?

[kallend 1,632]

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I'm thinking toggles don't produce any praticle lift (at least in the manner we use them on canopies) and are moreover drag devices, I believe we're producing more parasitic form drag than we are induced drag. When the canopy slows we swing out because of our momentum, and hence change the AOA of the main canopy. our mean camber line then extends from the nose of the canopy to where the tail material begins to bend. how can, in turns, the toggle and tail material be a drag device, and in flaring a lift enhancing one? I agree that with very little toggle pressure, you are increasing lift, but as I said in the manner we use them on landing we are deflecting them to the point where they are creating more form drag than anything else. am I totally off here?

Not sure I understand exactly what you mean here. I'll simply state that all of my canopies fly slower (steady state) in brakes than in full flight. Since they have to support the same load (me) at the slower speed, the lift coefficient must be higher. Whether this is due to the trim change, or due to the changing camber of the airfoil, or some combination of the two, would need to be determined by experiment.

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The only sure way to survive a canopy collision is not to have one.

[quade 3]

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One could still argue about what exactly defines the stall, and whether FoilSim is really doing a good job at predicting stall behaviour...I'm skeptical... it is a notoriously tough thing to do well even without the most advanced aerodynamic codes.

On the other hand, NASA generally knows what it's talking about, which is where the program comes from.

quade -
The World's Most Boring Skydiver

[billvon 2,402]

>When the canopy slows we swing out because of our momentum,
>and hence change the AOA of the main canopy.

Definitely true! The lift produced by a canopy can be changed many ways; the two ways we care about are:

1) Holding toggles down to some specific position. This changes the shape of the canopy and changes the "trim speed" such that the canopy is stable at a lower speed.

2) Penduluming forward by using the brakes to increase drag. This increases AOA and transiently increases lift; helpful during the flare.

Note that we are pretty much always generating enough lift to counteract our weight; changes we make by flaring/turning are transient and small.

[kallend 1,632]

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One could still argue about what exactly defines the stall, and whether FoilSim is really doing a good job at predicting stall behaviour...I'm skeptical... it is a notoriously tough thing to do well even without the most advanced aerodynamic codes.

On the other hand, NASA generally knows what it's talking about, which is where the program comes from.

Uh oh, appeal to authority.

Would this be the same NASA that lost a Mariner spacecraft to Venus on acount of a programming error, and then lost a Mars Climate Orbiter on account of confusion over units?

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The only sure way to survive a canopy collision is not to have one.

[quade 3]

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Would this be the same NASA that lost a Mariner spacecraft to Venus on acount of a programming error, and then lost a Mars Climate Orbiter on account of confusion over units?

In any sufficiently complex thing, errors are bound to creep in. It's the nature of programming and engineering. When you talk about engineering failures in something as complex as a space craft, it's a miracle they work at all.

On the other hand, a simple airfoil . . . yes, I'm pretty confident in NASA's ability to model that with pretty good fidelity. Certainly well beyond what most of us can do with our eyes and in our heads, which is where this branch of the thread started. Again, I go back to that post and I'm pretty confident the +18 AoA figure is out of whack with reality.

quade -
The World's Most Boring Skydiver

[kallend 1,632]

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Again, I go back to that post and I'm pretty confident the +18 AoA figure is out of whack with reality.

And I still think you are using data uncorrected for aspect ratio.

To quote an aerodynamics text book that I happen to have on my bookshelf:

"Another implication [of downwash in finite wings]is that when the finite wing reaches the geometric incidence at which the theoretical wing stalls its effective incidence is less than the stalling value. Thus the geometric incidence can continue to increase until the lift coefficient reaches the same maximum value as the theoretical case.

"The downwash angle increases as the aspect ratio is reduced. Thus at still lower aspect ratios, the tendencies described above are enhanced, i.e., the lift-curve slope is further reduced and the stalling angle further increased..."

The equation given suggests that an aspect ratio of 2.4 would DOUBLE the stall angle (measured from the zero-lift angle). My Spectre has an AR of 2.14.

Now, our canopies are very low AR wings, are they not?

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The only sure way to survive a canopy collision is not to have one.

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[quade 3]

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The equation given suggests that an aspect ratio of 2.4 would DOUBLE the stall angle (measured from the zero-lift angle). My Spectre has an AR of 2.14.

Now, our canopies are very low AR wings, are they not?

Take that to it's "logical" conclusion though and see if what you're suggesting makes sense.

Would an infinitely low aspect ratio produce an infinitely high AoA?

I seriously doubt it.

quade -
The World's Most Boring Skydiver

[kallend 1,632]

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The equation given suggests that an aspect ratio of 2.4 would DOUBLE the stall angle (measured from the zero-lift angle). My Spectre has an AR of 2.14.

Now, our canopies are very low AR wings, are they not?

Take that to it's "logical" conclusion though and see if what you're suggesting makes sense.

Would an infinitely low aspect ratio produce an infinitely high AoA?

I seriously doubt it.

Here's a low AR wing.
www.aeroflight.co.uk/types/international/aerospat-bac/concorde/Comcorde.jpg

I bet that's over 18 degrees.

The F16 stalls at approx 30 degrees AoA.

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The only sure way to survive a canopy collision is not to have one.

[kallend 1,632]

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The only sure way to survive a canopy collision is not to have one.

[kallend 1,632]

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The equation given suggests that an aspect ratio of 2.4 would DOUBLE the stall angle (measured from the zero-lift angle). My Spectre has an AR of 2.14.

Now, our canopies are very low AR wings, are they not?

Take that to it's "logical" conclusion though and see if what you're suggesting makes sense.

Would an infinitely low aspect ratio produce an infinitely high AoA?

I seriously doubt it.

OK here's a NASA study on "ram air fabric wings" (sound familiar).
ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670027396_1967027396.pdf

Take a look at the lift coeff vs AoA curves. Stall is around 30 degrees.

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The only sure way to survive a canopy collision is not to have one.

[pchapman 261]

This discussion has helped refresh my memory on low aspect ratio wings.

As general comments for everyone:

The Jalbert ram air wings in the NASA report Kallend referred to, were very low aspect ratio, about 0.65 to 1.0. That's a particularly low aspect ratio, which can explain the maximum lift not occurring until 30 degrees, well beyond the "normal" stall angle of an airfoil.

When aspect ratios get very low there just isn't much of a "stall" in a conventional sense anyway. This is especially true of swept back delta wings where vortex lift is added.

Either way, there is no longer a sudden point (when angle of attack is being increased) where lift suddenly drops off a large amount, drag continues to shoot up, and the pitching moment maybe even tries to pitch the airfoil nose down more than usual.

The "stall" for a very low aspect ratio wing is a more gradual affair. As the angle of attack goes up, the lift just peaks and slowly starts to decrease. The vehicle, whatever it is, is less likely to suddenly drop and pitch nose down, but just start mushing, slowly becoming less and less efficient at flight, and descending if there isn't enough power to keep it flying level.

For those who know something about aerodynamics:

I agree that for low aspect ratios the lift curve slope is less steep (more angle of attack needed for the same lift), and the stall angle is higher, and the maximum lift at the stall angle tends to be a bit lower.

Because of the complexity of aspect ratio effects, sometimes I think it is simplest when talking about airfoils just to talk about the ideal 2-dimensional airfoil (the same as an infinitely long airfoil). So one might talk about an airfoil that stalls at 15 or 18 degrees or something like that. Use that for one's basic discussions of how parachutes fly. But then put a disclaimer on the end that for a 3 dimensional airfoil, especially of a low aspect ratio like for parachutes, the actual angles and stall point will differ... (like all the stuff in the arguments in this thread).

[billvon 2,402]

>The F16 stalls at approx 30 degrees AoA.

Well, to be fair, it "stalls" much earlier than that - but the F16 has enough power to keep the thing in the air anyway.

"Stall" is not a sudden loss of lift so much as it's a sudden increase in drag (= sudden decrease in L/D.) The engine can no longer maintain sufficient airspeed with the additional drag, the plane slows, lift decreases due to lower airspeed, the apparent AOA changes due to the changed trajectory - and it feels like you're falling out of the air.

However, if you had sufficient thrust, you could keep flying at that AOA. Some vectored-thrust experimental aircraft can fly at an AOA of 60 degrees. At that point you are generating way more drag than lift (and your control surfaces don't work very well) but with enough power/vectoring authority that doesn't matter.

[denete 2]

If you like doing airfoil simulations (especially to see how your favorites work at low Reynolds numbers), you should try xfoil from Dr. Mark Drela at MIT. http://web.mit.edu/drela/Public/web/xfoil/

[relyon 0]

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... My Spectre has an AR of 2.14.

Now, our canopies are very low AR wings, are they not?

With A/Rs generally between 2 and 3, they're not very high or very low.

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Here's a low AR wing.
www.aeroflight.co.uk/types/international/aerospat-bac/concorde/Comcorde.jpg

I bet that's over 18 degrees.

The F16 stalls at approx 30 degrees AoA.

Similar results would be seen if observing an SR-71 or the Shuttle during landing. In every case the subject aircraft has an very low A/R (less than 1.0), is experiencing significant ground effect, has power on engine thrust (except the Shuttle - and it shows), and is under dynamic conditions. The same aircraft gliding steady state power off appears to drop out of the sky.

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OK here's a NASA study on "ram air fabric wings" (sound familiar).
ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670027396_1967027396.pdf

Take a look at the lift coeff vs AoA curves. Stall is around 30 degrees.

Those do have very low A/Rs (below 1.0). Take a look at the L/D ratios at 30 degrees - they're around 1.3. I don't about you, but I'm not a big fan of my steady state flight path on final being around 40 degrees to the surface ...

Bob

PS - I'm guessing the NASA ram air wing study was not using ZP, either. I'm fairly certain that would have affected the results.

[kallend 1,632]

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... My Spectre has an AR of 2.14.

Now, our canopies are very low AR wings, are they not?

With A/Rs generally between 2 and 3, they're not very high or very low.

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Here's a low AR wing.
www.aeroflight.co.uk/types/international/aerospat-bac/concorde/Comcorde.jpg

I bet that's over 18 degrees.

The F16 stalls at approx 30 degrees AoA.

Similar results would be seen if observing an SR-71 or the Shuttle during landing. In every case the subject aircraft has an very low A/R (less than 1.0), is experiencing significant ground effect, has power on engine thrust (except the Shuttle - and it shows), and is under dynamic conditions. The same aircraft gliding steady state power off appears to drop out of the sky.

And the glide ratio of a sport canopy is what? 3:1 maybe? That's dropping out of the sky too. (My Mooney has a 10:1 glide ratio).

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OK here's a NASA study on "ram air fabric wings" (sound familiar).
ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670027396_1967027396.pdf

Take a look at the lift coeff vs AoA curves. Stall is around 30 degrees.

Those do have very low A/Rs (below 1.0). Take a look at the L/D ratios at 30 degrees - they're around 1.3. I don't about you, but I'm not a big fan of my steady state flight path on final being around 40 degrees to the surface ...

Bob

PS - I'm guessing the NASA ram air wing study was not using ZP, either. I'm fairly certain that would have affected the results.

Maybe it would, maybe not. That's just speculation.

Regardless of how low is "low", a typical ram air canopy has an AR < 3. This is definitely in the regime where AR has a strong effect on stall AoA, and is way far away from the mythical 2d-wing.

The empirical formula for the effect of AoA on lift suggests that the stall AoA is DOUBLED at an aspect ratio of 2.4 compared to the 2-d wing.

I still find it quite plausible that a normal sport canopy can reach 18 degrees before stalling.

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The only sure way to survive a canopy collision is not to have one.