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Wind Penetration and Turbolence

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Thank you for the corroboration and explanation.

By no means I meant to portray negatively Sabre canopies. To date I have only owned used Sabres (190, 170, and 135) and I have been completely satisfied with their opening, flying, and landing characteristics.

I am at the point where I could see myself exploring a different planform (at my current windloading) as well as happily flying my Sabre for few hundred more jumps.

I will try to demo what is available when I go to boogies.

Cheers,
Riccardo

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if the Katana has a steeper trim built-in (i.e. a "permanent double fronts" without the distortion), does that make it more vulnerable in turbulence?



I think in general the less the angle of attack of a ram-air canopy in sustained flight, the less resistance it has to sudden collapse.

One way of arriving to this conclusion is to take it to the extreme: imagine a canopy trimmed so steep that the angle of attack (relative to the line of zero lift) is nearly zero. Then even a slight decrease of AoA (for example, due to powerful downdraft) will cause the catastrophic collapse of the top skin and total loss of lift.

Another way involves some illustrative calculations. Suppose we have 2 canopies of the same area and same airfoil trimmed to have angle of attack (measured from the line of zero lift) A1=10 degrees and A2=5 degrees in full flight, respectively. Also, the canopy 1 has the total speed of 26.9mph (25mph horizontally, 10mph vertically), canopy 2 flies at total speed of 33.5mph (30mph horizontally, 15mph vertically) due to more aggressive trim.

The canopy 1 sees relative wind hitting it at 26.9mph which has 26.9*cos(10)=26.5mph longitudinal (parallel to the line of zero lift) component and 26.9*sin(10)=4.7mph tangential (perpendicular to the line of zero lift) component hitting it from below.

The canopy 2 sees relative wind hitting it at 33.5mph which has 33.5*cos(5)=33.4mph longitudinal component and 33.5*sin(5)=2.9mph tangential component.

Now, both canopies fly into a 3mph downdraft.

If you draw a diagram, it's easy to find out that this 3mph downdraft has a tangential component to the line of zero lift equal to 3mph*cos(G-A), where G is canopy's glide angle. For canopy 1, G1=atan(10mph/25mph)=21.8 degrees, for canopy 2, G2=atan(15mph/30mph)=26.6 degrees, so for canopy 1 this tangential component of downdraft will be 3mph*cos(21.8-10)=2.9mph, while for canopy 2 this tangential component of downdraft will be 3mph*cos(26.6-5)=2.8mph.

Now, canopy 1 will see the relative wind hitting it from below at 4.7-2.9=1.8mph, a 62% decrease, while canopy 2 will see the relative wind hitting it from below at a mere 2.9-2.8=0.1mph, a catastrophic 97% decrease! For canopy 1, the relative wind is now hitting at approximately asin(1.8/26.9)=3.8 degrees (instead of initial 10), while for canopy 2, the relative wind is now hitting at approximately asin(0.1/33.5)=0.2 degrees (instead of initial 5).

Thus, canopy 2 will have much more dramatic reaction to downdraft and will most likely fully collapse.

At small angles of attack (~12 degrees or less), wings generally have a linear dependence of coefficient of lift vs. AoA: Cl ~ A. Therefore, canopy 1 will lose (10-3.8)/10*100%=62% of lift, while canopy 2 will lose (5-0.2)/5*100%=96% of lift (if not collapse completely and instantly).

According to this theory, just a bit of rear riser or brake input (=increased AoA) should make the canopy more resistant to [at least] downdrafts.
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if the Katana has a steeper trim built-in (i.e. a "permanent double fronts" without the distortion), does that make it more vulnerable in turbulence?



[...] Suppose we have 2 canopies of the same area and same airfoil trimmed to have angle of attack (measured from the line of zero lift) A1=10 degrees and A2=5 degrees in full flight, respectively. Also, the canopy 1 has the total speed of 26.9mph (25mph horizontally, 10mph vertically), canopy 2 flies at total speed of 33.5mph (30mph horizontally, 15mph vertically) due to more aggressive trim. [...]


Interesting set of assumptions. Neat little model :)
Here's what I worked out as the "critical downdraft speed" (defined as the speed that would trigger a collapse in your model):

critical downdraft speed = airspeed x sin[ AoA ] / cos[ AoI ]

where AoI is angle of incidence (effectively your G-A). Of course, one would want the largest possible critical downdraft speed for one's canopy.

Now, AoA and airspeed both depend on AoI, so we should rewrite as:

critical downdraft speed( AoI ) = airspeed( AoI ) x sin[ AoA( AoI ) ] / cos[ AoI ]

By assuming airspeed/glide ratios, you effectively arbitrarily defined these functions in a way that may or may not be reasonable (double the assumed airspeed on your steeply trimmed canopy, suddenly there's no problem and your conclusion flips)

Clearly, an increase in AoI will always lead to an increase in airspeed which on the margin makes my critical downdraft speed higher (speed is good).

At the same time, that would likely make the AoA smaller which would have an offsetting effect.

Moreover, cos[ AoI ] would get smaller which would contribute positively to downdraft resilience.

Because these effects are offsetting each other, one would have to actually work out the equilibrium AoA and airspeeds as a function of AoI. I'm sure you're up for it ;)

I think that how a canopy recovers once the downdraft is gone is more important than its critical downdraft speed. If the canopy loads mostly the rears as it regains lift (i.e. light front riser pressure), it would stand to reason than the nose could fold under (collapse) more promptly than if the fronts are heavily loaded.

Thoughts?

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Actually, on a no wind day I cannot come close to a horizontal stop under my Sabre 135 and when I touch down I have to run. ]



It is certainly possible to come to a zero horizontal speed under a Sabre 135 on a nil wind day. You might want to get a rigger to check the length of your brake lines, or get some video of your landings - maybe you're not using all your flare?

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Actually, on a no wind day I cannot come close to a horizontal stop under my Sabre 135 and when I touch down I have to run. ]



It is certainly possible to come to a zero horizontal speed under a Sabre 135 on a nil wind day. You might want to get a rigger to check the length of your brake lines, or get some video of your landings - maybe you're not using all your flare?



The most likely cause of this is incorrect flare technique and spending too long in the 'plane out' phase leaving the pilot with not enough airspeed (energy) to pitch their body in front of the wing when they punch out the last bit of their flare.

It's extremely unlikely that spectra lines are going to be too long (since they shrink over time and come with a good setting from the factory).

Blues,
Ian
Performance Designs Factory Team

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Lines. You have a point there. I bought my Sabre 135 used when it had between 1000 and 2000 jumps. It is actually 16 years old.

I had my rigger lengthen the brake lines since the canopy was effectively flying in brakes. A, B, C, D lines look old. I don't know how old. What I see though is that in full flight there are no wrinkles on the bottom skin of the canopy suggesting out of trim. Moreover, the canopy has always opened predictably and on heading.

Do you think that a slight out of trim of A, B, C, or D line could actually be the cause of decreased fare?

I have been hesitant due to $ in replacing the lines, but I will check with my rigger and follow his recomendation.

Thanks,
Riccardo

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It is possible that my flare technique is not refined enough for no wind days. I will try to get video and criticism.

You have an interesting point on pitching your body forward at the end. I see people doing. I will practice with it and see how that changes things.

Thanks,
Riccardo

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About your lines: Spectra should be replaced anywhere from 400-500 jumps. Yes, I know you CAN go longer, but if you're serious about performance you shouldn't.

Also see my post above about the most likely cause of your 'flare' problems.

Blues,
Ian
Performance Designs Factory Team

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It is possible that my flare technique is not refined enough for no wind days. I will try to get video and criticism.

You have an interesting point on pitching your body forward at the end. I see people doing. I will practice with it and see how that changes things.

Thanks,
Riccardo



That's not quite what I was driving at (close though) :)
What I mean is, imagine your body suspended under the wing in full flight - typically you are slightly behind the nose of the wing (side view). When you flare, what happens is that the wing pitches up, swinging your body in front of the nose of the canopy (if you imagine a side view).. Most people get right under the nose of the wing during the plane out phase - this is normal and expected. Where things typically go wrong is the pilot waits too long before giving more input and the wing doesnt have enough energy left in the system to swing your body even further forward - causing a slight ascent and then a touchdown. Conversly if you do it too early you pop up too high and have a hard landing anyway.

Hard to explain in text - I'll see if I can find a picture depicting what I'm trying to explain.

Blues,
Ian
Performance Designs Factory Team

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This is a good point, about increased speed of more aggressive trim offsetting the effect of downdrafts on the canopy. I think that while it diminishes the effect, it is still there: the first effect (change in AoA when entering downdraft) is of the first order of AoA, while the offset due to increased speed is of the half of order of AoA (since the speed changes approximately proportionally to the square root of AoA inversed). The net result is approximately of the half order of AoA and thus lower AoA still leads to greater susceptibility to collapse in downdrafts.

I agree with your thought that lower front riser pressure (given fixed wingloading) probably makes canopy more susceptible to collapses. Lower tension on the lines means lower difference between pressures on the bottom and top skins of the canopy. The lower this difference, the more chance that some rotor will have pressure gradients higher than this difference and cancel it out, making the nose fold under.

Does our genius theory mean that making a front riser turn on a lightly loaded canopy in turbulence is not a good idea?
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Thanks Ian,

Your explanation was very appropriate and clear.

The point you made I much appreciate is that the two main inputs of the two stage flare (plane out and final flare) must be timed and applied precisely. I can see that, especially during no wind condition in order to kill as much horizontal speed possible. With winds as low as 5mph my landings are to the point I don't need to run, so I will try to refine things through video now.

I called my rigger that currently has my rig for cypers2 maintenance and re-pack and ask him to order and install new lines. All in all, $250 for new lines is better spent that on physical therapy for ankle chronic injuries.

Thanks to all who contributed to this thread.

Cheers,
Riccardo

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Here you go - diagram compliments of the USPA sim



Ian,

If you want to discuss the fine points of no-wind flaring - lets do it in a different thread B|

Jokes aside, yoink said

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It is certainly possible to come to a zero horizontal speed under a Sabre 135 on a nil wind day.



I'm curious to hear how you explain the magic ability of the canopy to produce lift at zero airspeed? In no wind conditions, I'm pretty sure I've always had to take a few steps, slide on my feet, or even run a bit ;)

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(Note to Ian: I'm replying in that thread to keep the posts together. I'm assuming you will move us into a different thread as you see fit)

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[...] (since the speed changes approximately proportionally to the square root of AoA inversed). [...]



I'm getting to similar results. I will work on this a bit more and try to plug some numbers to see if I can get anything realistic.

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[...] The lower this difference, the more chance that some rotor will have pressure gradients higher than this difference and cancel it out, making the nose fold under.



Agreed.

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Does our genius theory mean that making a front riser turn on a lightly loaded canopy in turbulence is not a good idea?



Hmmm.. I don't see a direct link as everything we discussed was mostly steady state, full flight, level wing. Front riser turns are much harder to analyze. Besides, during the recovery of a front riser turn, 1) glide ratio is flattened, 2) airspeed is increased, 3) front riser pressure is increased. All these are good things in your model. Now during the initiation of the turn, you might be vulnerable..

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If you want to discuss the fine points of no-wind flaring - lets do it in a different thread



Heh - Iwas thinking that when I came in here. Whoops.

Now, that said, we're still talking canopy control and swooping - you guys are talking Gear and Rigging.....if you want to be technical that is ;)

Blues,
Ian
Performance Designs Factory Team

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Jokes aside, yoink said

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It is certainly possible to come to a zero horizontal speed under a Sabre 135 on a nil wind day.



I'm curious to hear how you explain the magic ability of the canopy to produce lift at zero airspeed? In no wind conditions, I'm pretty sure I've always had to take a few steps, slide on my feet, or even run a bit ;)


I don't remember stating that a canopy produces lift at zero airspeed, but then I have been known have an occasional beer, so my memory isn't what it used to be!

I said that it's possible to get zero horizontal speed on a nil wind day... try it. Go up, take 6 wraps and bury both toggles. I suspect you'll have no forward speed at the end of it! :ph34r:

I can't explain the pysics of it, and I'll just end up looking a tool if I try. I only know that I've occasionally managed to do it. Maybe I've just got really sticky feet? :D

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