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Collapses and Turbulence: Article

 

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BrianSGermain  (D 11154)

Jan 30, 2006, 10:02 AM
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Collapses and Turbulence: Article Can't Post

There are many variables to consider when looking into a canopy collapse:

1) What was the pilot doing?
2) How fast was the canopy flying when it collapsed?
3) Where was the pilot flying?
4) What is the canopy design?
5) What is the wing-loading?
6) Was there any re-active solution employed?


These are the principle considerations, but not the only ones. I will take each one separately.

1) The way in which a parachute is flown can increase or decrease the "G" loading on the lines. A rapid release of one or both brakes significantly increases the chances that the canopy will collapse. This allows the parachute to surge forward to a lower angle of attack, decreasing the lift of the parachute. This reduces the amount of energy exerted by the parachute away from the suspended load, allowing the "negative" portion of the lift to take over and allow the wing to fly towards the jumper.

2) Airspeed is what creates lift. Lift is what causes the wing to strive to fly up and away from the jumper. This is the formula for line tension and therefore stability. The slower you are flying, the more likely your parachute will collapse due to low internal pressure and low line tension.

3) Was the wing flying in clean air when the collapse occurred? This is an important part of the question. All parachutes can collapse in "bad" air. We must always fly considering the invisible dangers that the sky presents us. If you wouldn't fly a kite there, don't fly or land your parachute there.

4) Certain parachute designs do better in turbulence than others. I must avoid pointing fingers here, as this is a volatile industry that can be taken down by non-skydiving lawyers. Nevertheless, certain wings have an increased propensity to go "negative" when presented with adverse condition, while others bump around a bit and keep on flying. This is a complex issue, and the best way to decide which parachute to buy and fly is to listen to the actual statistics, and to your own experience when flying a particular design. I have not experienced any kind of collapse on the parachutes I fly, ever. If you have on yours, you may want to reconsider what is over your head.

5) Parachutes perform differently at different wingloadings. The lighter the wingloading, the slow it will fly. This means that the internal pressurization of the wing will be less on larger canopies. In general, lightly loaded parachutes experience more small collapses than heavily loaded ones. Not only is there less internal pressure in the wing, but the dynamic forces area also less with decreased airspeed. This means that the average line tension tends to be less on a lightly loaded wing, and the wing tends to have a increased propensity to surge forward in the window when flying at low airspeeds. This is why very small, highly loaded parachutes tend to experience fewer distortions, especially when flown at high speed. Flying at high speed increases the drag of the canopy itself, relative to the jumper, so the relative wind holds the parachute back in the window and at a higher angle of attack. This is why I make carving, high G, high speed turns to final approach heading, especially in turbulence. The speed actually reduced the chances of a collapse by increasing the forces that keep the parachute at the end of the lines. I am literally increasing my wingloading by flying fast and at high Gs, and the increases velocity reduces the amount of time that I fly in bad air. I am not saying that you should downsize just to increase your stability. I am saying that until your skills and knowledge are ready to fly smaller, faster parachutes, you should stay out of the sky until the winds come down. I still havent been hurt by a jump I didnt do.

6) If you are flying a good design with lots of airspeed and significant line tension, and in a reasonable location that has no obvious precursors for collapse, you can only deal with a collapse in a re-active manner, as you have addressed all of the relevant variables up to this point. This is all about "Pitch Control". If your wing tries to aggressively surge forward in the window, you must notice it and quickly stab the brakes to bring it to the back of the window. A collapse always begins by a surge to a low angle of attack, but there is very little time to deal with the problem before I folds under. Here are the signs:

a) The first sign is a change in Pitch. The wing moves forward in the window.
This is the limited flying space over your head. Too far forward and it collapses. Too far back and it stalls.
b) The "G" loading drops dramatically and almost instantly. In other words, your apparent weight in teh harness drops because the wing is producing less lift.

This is the time to jerk on your brakes: quickly, sharply, but not more than about 50% of the total control stroke. This action is to pull the wing back in the window, not to stall the parachute. By putting the wing further back in the window, we are increasing the angle of attack. This increases the lift, and forces the wing to fly away from the suspended load and thereby increase the line tension. This can prevent a collapse entirely, or cause the wing to recover to stable flight before things get really out of control.

If the wing is allowed to collapse, it may recover quickly on its own. This is why the more modern airfoils have the fat point (Center of Lift) so far forward. It causes the wing to pitch nose-up when it begins to fly again, bringing it back to the end of the lines. Nevertheless, parachutes can still collapse fully, which often involves significant loss of altitude and possibly a loss of heading.

If your wing goes into a spin because of a collapse, your job is to stop the turn first, as you increase the angle of attack. If it is spinning, there is less chance of recovery until the flight path is coordinated and the heading stable.

Conclusions:
1) Don't fly an unstable parachute. If it is prone to collapse, ground the parachute. Do not sell it to an unsuspecting jumper at another drop zone. These people are your brothers and sisters.

2) Don't fly in crappy air. Land in wide open spaces, in light winds, and never directly behind another canopy.

3) Practice stabbing your brakes in response to forward surges on the pitch axis. This must become a "learned instinct" that requires no thought at all. Like pulling emergency handles, pulling the wing to the back of the window when the lines get slack is essential for safe skydiving.

4) Keep flying the parachute. If your parachute does something funny near the ground, don't give up. If you keep your eyes on YOUR ORIGINAL HEADING, you will unconsciously do things that will aid your stability and keep you from getting hurt. Looking toward what you don't want is how you make it occur.

I hope this little article helps you understand the phenomenon of collapses a bit better. I know as well as anyone how painful a collapse can be. I do not want to go back to that wheelchair, and I don't want anyone else to have to experience that either. You morons are my family, and if information can help protect you, I will give it until my lungs are out of air.

Blue Skies, Sky People.

+
Bri


(This post was edited by BrianSGermain on Jan 30, 2006, 10:03 AM)


tr027  (D License)

Jan 30, 2006, 10:11 AM
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Thank you very much for that gift of information, Brian. Smile


BigSky  (B 28752)

Jan 30, 2006, 10:17 AM
Post #3 of 38 (9547 views)
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Thanks for the info! It was well written and easy to understand. Smile


micro  (A 49033)

Jan 30, 2006, 10:43 AM
Post #4 of 38 (9510 views)
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You, sir, have a gift. Many thanks.


Dolph  (D License)

Jan 30, 2006, 12:42 PM
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Thanks. This is really good stuff.


FrogNog  (C 34484)

Jan 30, 2006, 3:00 PM
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Have you tested your "stab" theory to keep the wing in the window?

Now, I'm sure all of us (including me) think you _have_ tested this, but I thought I'd ask because you mention that you've never experienced any kind of collapse on any canopy you've ever flown before. That could certainly be due to superior pilot skill, but since you tend to sew your own parachutes it could just as well be superior design and manufacturing. Or maybe it's superior flight planner judgement, and you just stay out of crappy air really well*.

But I would like to know if you've tested the "stab theory" and how that worked out for you. Smile

* when skydiving. Tongue


BrianSGermain  (D 11154)

Jan 30, 2006, 3:43 PM
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In reply to:
Have you tested your "stab" theory to keep the wing in the window?

Now, I'm sure all of us (including me) think you _have_ tested this, but I thought I'd ask because you mention that you've never experienced any kind of collapse on any canopy you've ever flown before. That could certainly be due to superior pilot skill, but since you tend to sew your own parachutes it could just as well be superior design and manufacturing. Or maybe it's superior flight planner judgement, and you just stay out of crappy air really well*.

But I would like to know if you've tested the "stab theory" and how that worked out for you. Smile

* when skydiving. Tongue

Yes, I have tested the "stab theory" on many occasions. I would even go on to say that it is much more than a theory. I try not to post things that are just guess-work.

As for not having experienced a collapse, nothing could be further from the truth. I have had countless bad ideas over the years. I have found myself under many parachutes that did not want to stay at the end of the lines, and I managed to land them safely.
I do this by flying fast and staying away from inputs that would decrease my angle of attack or cause uncoordinated flight. Tight lines are the secret to stability, and the stab method or "Flex the System" as I call it in the course, seems to be the best way to momentarily increase your line tension to get you through bad air.

I appreciate the question. There is a good deal of conjecture out there, and the only way to pick through the information is to find out what is based on empirical evidence through experimentation and what is coming from guessing. I design parachutes so I can find my way to the truth. Well designed experiments and conscious manipulation of one variable at a time is the best way to do that.

I would not feel comfortable teaching anything I haven't tested fully. Further, you should not fully accept a new piece of information without testing it yourself. Pull high and play with your toy. There really is no better way to find the truth.
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srsand1960  (A 49234)

Jan 30, 2006, 6:44 PM
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The slower you are flying, the more likely your parachute will collapse due to low internal pressure and low line tension.
Im a 510 200lb guy (God (or my eating habits:) gave me a great natural arch). Im a new student and the last DZ I was at had me flying (I believe) a 290 which is way over the 1:1 load ratio. What I understand from #2 and #5 is that my chances of canopy collapse is greater then someone at the same internal pressure with a closer wing load, correct?
At my first DZ on my 3 static line jump, I experienced a partial right side collapse. DZ owner and my instructor said it was due to a turbulence pocket at about 8 to 10 ft off the ground. As I was reading your article it played over again in my mind and I do recall that the canopy was pitching down (like a nose dive). It was forward to the point I could see way past the leading edge. However I remember being in a full flare. That was a hard landing and I was in long shorts and got skinned up pretty good. After spending the last 4 months learning about this sport I realize that I was really lucky.
You have a great style of writing and your use of metaphors is definitely pronounced. Do you have any books on the market for a beginner? If you have a book that would be good for a beginner to read, email me and Ill get a copy. Im currently reading the USPA SIM but Im finding that Poynter & Turoffs Handbook was better reading and seems to go into more depth then the SIM (Im on page 31).
Bigun turned me on to your article. Yesterday, I emailed him a question about riding to breaks as a combat method for turbulence.
Thanks for the lesson!
Smile


bob.dino  (E 2185)

Jan 30, 2006, 6:58 PM
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In reply to:
Im a ... 200lb guy ... flying a 290 which is way over the 1:1 load ratio.

Way under. Wingloading = Suspended Weight (lb) / Area of parachute (sq ft).

So, a 200lb guy + 25lb of gear = 225lb suspended weight. If the parachute's 290sq ft, then your wingloading is 225:290 or 0.78:1, or just 0.78.

In reply to:
That was a hard landing and I was in long shorts and got skinned up pretty good.

It'd be a good idea to wear a jumpsuit until you've got your landings down pat. It'll save you from lots of scratches!

In reply to:
Do you have any books on the market for a beginner?

Brian's books are for sale here. You definitely want The Parachute & Its Pilot, and if you're interested in psychology, pick up a copy of Transcending Fear as well.


srsand1960  (A 49234)

Jan 30, 2006, 7:49 PM
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Thanks for the formula and the advice Ill pick up both books Bob. Thanks for the link!
I went and got measured at Tony Suits in Dec and just got it two weekends agLaugh


freeryde13  (Student)

Jan 30, 2006, 9:39 PM
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thanks...i'll buy the book.... for real, never got a better answer than that before.


darkwing  (D 4164)

Jan 31, 2006, 6:10 AM
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In reply to:
The slower you are flying, the more likely your parachute will collapse due to low internal pressure and low line tension.

Slow speed doesn't make line tension less. In straight flight your line tensions must add up to your suspended weight.


BrianSGermain  (D 11154)

Jan 31, 2006, 7:08 AM
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In reply to:
In reply to:
The slower you are flying, the more likely your parachute will collapse due to low internal pressure and low line tension.

Slow speed doesn't make line tension less. In straight flight your line tensions must add up to your suspended weight.

This is true. Your "G" loading in full flight is a tiny bit less than One G (because you are going with gravity). Nevertheless, I aim to fly at more than One G in turbulence. FLying in smooth, continuous turns allows me to fly faster than full flight, and increases my G Loading. I can increase my wingloading by adding a bit of brakes at the end of the turns to create a high speed positive carve.
+


tnscorcoran  (Student)

Jan 31, 2006, 7:42 AM
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Thanks for the very enlightening article Brian. I have The Parachute and its Pilot and heard your recent interview on skydive radio, both of which are compelling.
In conclusion 1) you say, Don't fly an unstable parachute. If it is prone to collapse, ground the parachute.
How does one find out what parachutes are good and which ones are bad in this regard? I guess you can't name names but is there anywhere on line to find such data?


rehmwa  (D 12816)

Jan 31, 2006, 7:49 AM
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In reply to:
Your "G" loading in full flight is a tiny bit less than One G (because you are going with gravity).+

Can you explain this? Steady state is steady state - weight equals lift, exactly (no 'tiny bit' one way or the other). But maybe I'm not understanding some dynamic input you intuitively know about.

In any case, I appreciate the advice about keeping a curve in your flight to add extra tension for bad air - keep the lines taught = holds the canopy shape (that's why the lines are cut to those lengths) = keeps the foil pressurized = less oddball trouble. That's different than busy/jerky hands, it must stay smooth.

What is your opinion on the normally trained advice to newbies of "slight brakes" through turbulence? Is it just something that feels right but is technically wrong? Or does it depend on your platform?

Canopy design and development must be greatly fun and satisfying.


(This post was edited by rehmwa on Jan 31, 2006, 7:50 AM)


BrianSGermain  (D 11154)

Jan 31, 2006, 9:11 AM
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In reply to:
In reply to:
Your "G" loading in full flight is a tiny bit less than One G (because you are going with gravity).+

Can you explain this? Steady state is steady state - weight equals lift, exactly (no 'tiny bit' one way or the other). But maybe I'm not understanding some dynamic input you intuitively know about.

In any case, I appreciate the advice about keeping a curve in your flight to add extra tension for bad air - keep the lines taught = holds the canopy shape (that's why the lines are cut to those lengths) = keeps the foil pressurized = less oddball trouble. That's different than busy/jerky hands, it must stay smooth.

What is your opinion on the normally trained advice to newbies of "slight brakes" through turbulence? Is it just something that feels right but is technically wrong? Or does it depend on your platform?

Canopy design and development must be greatly fun and satisfying.

When we go with gravity without resistance, we sit at less than one "G". In the case of orbital dynamics, we are falling without resistance, and the "micro-gravity" environment feels like Zero-G's. Notice the feeling in the harness when you are under a big canopy versus a tiny one. You actually weigh a bit less on the smaller canopy. None of this matters, however, it is just an interesting point of discussion.

As for training students to fly in brakes, I am against it. Especially under a huge parachute, we must maintain what little airspeed we have. This pressurizes the airfoil more and gives us the ability to spike the G's when we need the canopy to fly away from us to increase the line tension. When it comes to flying, airpseed is our friend.

We need to teach students to do what we would do, or at least a more conservative version thereof...


HydroGuy

Jan 31, 2006, 9:24 AM
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Quote:
Nevertheless, I aim to fly at more than One G in turbulence. FLying in smooth, continuous turns allows me to fly faster than full flight, and increases my G Loading. I can increase my wingloading by adding a bit of brakes at the end of the turns to create a high speed positive carve.

Damn...now there is a piece of advice, especially for those hot afternoons.

Thanks Brian


rehmwa  (D 12816)

Jan 31, 2006, 11:07 AM
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In reply to:
As for training students to fly in brakes, I am against it. ..... airpseed is our friend.

Thanks, this is what's always made more sense to me and the old teaching of holding quarter brakes in turbulence always seemed counterintuitive and a bit distressful to me.

As for the orbital mechanics effect (in low orbit) on the actual weight of a hanging load on various wing sizes (read orbit velocity) in low altitude semi-degraded orbital flight......(sounds like a graduate student paper). It would be fun to argue just how tangible a true loading difference you'd really be able to measure. But it's not for this thread which is to educate practical stuff.

{thanks for the insight}


PapaSmurf  (D 5824)

Jan 31, 2006, 11:27 AM
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"You morons are my family, and if information can help protect you, I will give it until my lungs are out of air."

Tanx! Brian Smile

See you at "the top of the hill"


The111  (D 29246)

Jan 31, 2006, 1:43 PM
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In reply to:
2) How fast was the canopy flying when it collapsed?

Brian, I've read your reasons for recommending a high airspeed to combat turbulence. I'm curious what you think of this idea (another justification for high airspeed).

One day at a large boogie I watched a helicopter take off over and over again and fly right over a treeline where I knew there was a lot of windshear (it was a windy day). The aircraft did not even wobble. The conclusion I came to is that because of the extremely high airspeed of the blades, a +/- 20mph variation in wind (for example) simply wouldn't be very noticeable, in a case where the average airspeed is so much higher.

Example:

A wing flying at 5mph loses 5mph of its headwind. Big deal.

A wing flying at 50mph loses 5mph of its headwind. Not a big deal.

Does that theory make sense to you? Just to satisfy my own curiosity. Smile (having a degree in aeronautics I should know this, but college, drinking.... you get the picture)


BrianSGermain  (D 11154)

Jan 31, 2006, 3:18 PM
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That is an interesting brain-teaser. Yes, the loss of groundspeed is a lesser precent of the overall airspeed when you are flying faster. But the issue is not groundspeed at all when we are talking about turbulence.

It is:
1) How long are you in the bad air
(downdraft, updraft or choas)
And:
2) Is the variance of the fluid going to effect the flight path?

Number one is clear. The faster you are flying, the sooner you will pop out of the nasty stuff into clean air. The second issue, partially predicated on the first, is a more complex question that requires all of the relevant details.

The example you presented is a helicopter, which is a very different set of dynamics. Not to say that helicopters do not suffer in turbulence, but the effects will be somewhat different, and I do not pretend to be an expert in that field. Actually, I do not pretend to know everthing about parachutes either. I just seem to be saddled with this responsibility of the "go-to" guy for this kind of question. So I will try...

The flight path of a parachute is based on
1) Lift
2) Drag
3) Flying Environment

The first two partner up to give us our efficiency, calculated by L/D resulting in glide ratio. The flying environment is a variable that includes vertical air movement (+ and -) as well as chaos which reduces the efficiency of the airfoil. If the flow of air over the wing is disrupted and becomes less "laminar", the drag will increase and the lift will decrease. Increasing the airspeed will increase both forces, Lift and Drag. This does not necessarily result in a linear correlation between the variables as they are altered by the conditions, but it stands to reason that the higher the airspeed, the less the percentage of the whole the loss of both figures will represent.

If airplanes did not weigh as much, we might choose to increase the airspeed in bad air due to this assumption, but we do not. We slow down. This is beacuse of the limited ability of the airframe to handle the stress. If you increase the airspeed, you increase the forces on the aircraft as the loading on the wings varies positive and negative. This is why there is "maneuvering speed" on your airspeed indicator. If you fly too fast in turbulent air, your wings may fall off. That sucks...

Parachutes are quite different. The mass of the suspended load is far less than an airplane, and parachutes are designed to handle quite a bit of stress for opening shock. This means that there is pretty much no chance of you flying so fast that the positive "g's" actually bust your parachute. Good thing. Not to mention, negative "g's" are not an issue at all, as all that does is release the line tension and collapse the parachute. Not good for descent rate, but you parachute will fare well from the encounter.

This all suggestes that the first issue is the most important. Flying fast will keep you from lingering in the oatmeal air, and will super-pressurize the parachute to handle the hits. As far as we are concerned, the faster the better, as long as the maneuvers utilzed to create the airspeed maintain significant line tension to keep the skeleton of your system intact.

OK, that was the most verbose answer I could have given. Sorry about that. On cold winter days, a little mental masterbation can be a good thing.

Anybody got a towel?
;)


(This post was edited by BrianSGermain on Jan 31, 2006, 3:24 PM)


The111  (D 29246)

Jan 31, 2006, 3:47 PM
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Thanks for the answer. Makes sense. Just for clarification, I was not referring to groundspeed anywhere in my post.


DuckDodger  (D 27017)

Jan 31, 2006, 4:21 PM
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Great thread. There is a lot to learn here.

I see you mention manuevering speed and this got me thinking about a question I have had for years. I was a pilot long before a skydiver, maybe something to do with going to Rhinebeck Aerodrome back in '75. That was cool.

Anyway, manuevering speed increases as you increase weight/loading, which to me seems absolutely backwards on the surface. I thought that maybe this had something to do with inertia, that generally, increasing weight made the aircraft more stable, or less likely to move to a critical angle of attack and overstress the wing.

On the other hand increasing weight, should actually increase loading on the wing, so not wanting to stress my little mind, like we seemed to be doing with the wing, I left it there.

A few years later, a good friend of mine said the reason manuevering speed increased was totally dependent on the testing of aircraft. In respect to my friend, I didn't tell him this sounded like crock to me. Then I thought about our dear Gov and thought, maybe so.

Doesn't seem to be much parachute application, other than the thought about inertia, like a more heavily loaded boat crushing through heavier seas. The parachute translates weight to the lines so that sort of makes sense to me, but it might just be acid flashbacks. What do you think?


BrianSGermain  (D 11154)

Feb 1, 2006, 6:21 AM
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In reply to:
Great thread. There is a lot to learn here.

I see you mention manuevering speed and this got me thinking about a question I have had for years. I was a pilot long before a skydiver, maybe something to do with going to Rhinebeck Aerodrome back in '75. That was cool.

Anyway, manuevering speed increases as you increase weight/loading, which to me seems absolutely backwards on the surface. I thought that maybe this had something to do with inertia, that generally, increasing weight made the aircraft more stable, or less likely to move to a critical angle of attack and overstress the wing.

On the other hand increasing weight, should actually increase loading on the wing, so not wanting to stress my little mind, like we seemed to be doing with the wing, I left it there.

A few years later, a good friend of mine said the reason manuevering speed increased was totally dependent on the testing of aircraft. In respect to my friend, I didn't tell him this sounded like crock to me. Then I thought about our dear Gov and thought, maybe so.

Doesn't seem to be much parachute application, other than the thought about inertia, like a more heavily loaded boat crushing through heavier seas. The parachute translates weight to the lines so that sort of makes sense to me, but it might just be acid flashbacks. What do you think?

Although there clearly are some discrepencies in government testing standards, weight tends to decrease maneuvering speed because, as you said, inertia plays into the equation. Yes, the larger the mass, the less it wants to change direction. However, the larger the mass, the more force will be exerted on the vehicle itself when it is changed in flight path.

The magnitude of positive and negative "g's" that an airplane can handle without risk of structural failure is, in essence proportional to the mass of the vehicle as a whole. You would think that a larger airplane would be able to handle more loading on the wings, and you would be correct. Nevertheless, the percentage of the overall mass is what is in question here, and this figure decreases with the mass of the vehicle.

Size does matter. Small parachutes, for instance, experience less damage from opening than large ones. This is due in part to the fact that there is a larger amount of volume in the wing, and the increased amount of air moving in at opening time exerts a greater amount of force on the fabric. Further, the increased surface area collects more energy in the decelleration from terminal velocity, so the stresses on the fabric are greater. Yes, the force is distributed over a larger area, but the force localized on any given part of the fabric are still greater. So big parachutes tend to blow up with a much greater frequency. That is why tandems are so heavily reinforced.

Likewise, forces do not scale in a linear fashion when it comes to the ability to handle weight on the airframe. Big things just don't like to change direction. They are slow to alter their path, but they also resist the change with great lethargy. Like a fat person trying to get off the couch, it requires a great amount of effort, and they are more likely to break a hip in trying to do so.

Does that work for you?
+


(This post was edited by BrianSGermain on Feb 1, 2006, 6:28 AM)


pchapman  (D 1014)

Feb 1, 2006, 9:37 AM
Post #25 of 38 (8817 views)
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Comments on all of this thread:

-- Brian wrote:
Quote:
This is why I make carving, high G, high speed turns to final approach heading, especially in turbulence.

Excellent idea. This both gets the speed up AND keeps the angle of attack up*. (See footnotes for the asterisked items.) When flying faster rather than slower, the effect on the wing's angle of attack by a given sized gust is reduced. Having a higher angle of attack is beneficial when worrying about a collapse resulting from the front of the wing being pressed down rather than lifted up.

One caveat for others would be that the entry into the diving turn should be smooth. I wouldn't want to either toggle whip or front riser too suddenly into the turn, which could for a moment reduce the angle of attack of the canopy to a low level, making it especially vulnerable to turbulence induced collapse at that time.

-- Brian wrote:
Quote:
a) The first sign is a change in Pitch. The wing moves forward in the window.
This is the limited flying space over your head. Too far forward and it collapses. Too far back and it stalls.

Also a good idea. This matches the idea in paragliding of "flying actively" -- keeping the wing over one's head by small, quick inputs, rather than letting it bounce around in turbulence, doing nothing until there's an actual collapse.

-- Someone else wrote:
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Can you explain this? Steady state is steady state - weight equals lift, exactly

Note that this is untrue for a parachute, which is in a steady state descending glide. It only works for a powered airplane that is flying level. When the flight vehicle is descending, the Weight is exactly countered by the vector sum of Lift and Drag. Lift is defined as acting perpendicular to the direction of flight. So if the parachute is descending at an angle 20 degrees below the horizon, Lift is tilted 20 degrees forward from the vertical, and drag is elevated 20 degrees from straight back horizontally.

With gliding parachutes, we're actually held up in the air by a combination of Lift and Drag.

-- Brian agreed with someone's comment that "in straight flight your line tensions must add up to your suspended weight".

In a general sense, yeah, they'll typically add up pretty close. But the statement is wrong, even according to the most basic balance of forces calculations, because 'straight flight' for a parachute is 'descending straight flight' not 'level straight flight'. See the Weight = Lift + Drag explanation above.

-- Brian wrote:
Quote:
Your "G" loading in full flight is a tiny bit less than One G (because you are going with gravity).
and later
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When we go with gravity without resistance, we sit at less than one "G". [] Notice the feeling in the harness when you are under a big canopy versus a tiny one. You actually weigh a bit less on the smaller canopy.

I'll have to argue that that's simply wrong from a physics point of view. Whether the canopy is big or small, if one is descending at a constant rate (not accelerating) then one experiences exactly 1 g of force.

But I think what Brian was talking about was not that we experence less than 1 g, but that there's less than 1 g of line tension. Here's my explanation of what I think he means: (Correct me if I misunderstood, Brian.)

One can feel slighty lighter in the harness with a steeply descending parachute: The worse the glide ratio of a canopy, the more that drag contributes to carrying the weight, rather than the lift. This goes back to the Lift + Drag = Weight in vector terms.

An extreme example would be someone flying a 25 sq. foot canopy, as used to fly formation with a wingsuit flyer. The guy under the small canopy would be in a steep dive (compared to under a regular parachute), going very fast, with a lot of wind pressure on him. As both Drag and Lift are counteracting Weight, the Lift is less than the suspended Weight, so there's less line tension. The jumper doesn't have nearly his full weight supported by his leg straps, but is also supported by the air pressure on his body.

This is an effect of the angle that the canopy is pulling on the jumper, relative to the vertical, rather than an effect of canopy size in itself.

-- Brian wrote how turbulence effects on a canopy depend on:
Quote:
1) How long are you in the bad air
(downdraft, updraft or choas)
And:
2) Is the variance of the fluid going to effect the flight path?

It's a good point, that reduced time of exposure to turbulent air will reduce the total risk (As long as increasing speed does not increase the risk from a given turbulent gust. Which it doesn't according to the arguments in this thread.)

Brian had a good response on the difference between an airplane and a parachute, where we under parachutes are concerned about too low an angle of attack, collapsing the parachute, but are not concerned about overstressing our flight vehicle.

Brian had been responding to:
Quote:
A wing flying at 5mph loses 5mph of its headwind. Big deal.
A wing flying at 50mph loses 5mph of its headwind. Not a big deal.

That's a large part of the issue of why flying faster can help. A given sudden change in what the air is doing, will have less of an effect on a wing flying faster than one flying slower.**

It's good to remember that turbulence can change the motion of the air in any direction. So we could have the sudden change in air motion be horizontal, or vertical. Looking at the case of a vertical gust:

When flying 25 mph forward, the effect of a 5 mph down gust has a larger effect on the angle that the airflow is hitting the canopy, than if the canopy were flying 50 mph and encountered the same 5 mph down gust. The effect is about halved. (A change of 5.7 degrees angle of attack, vs. 11.3 degrees)

A major complicating factor is what the angle of attack of the wing started out as, before the gust. If the wing starts at a low angle of attack, it takes only a few degrees reduction in that angle before the canopy collapses from a negative angle of attack.*** If the wing was flying at a higher angle to start with, there's a greater reduction of angle possible, before the canopy collapses.

So if comparing the slow and fast canopy, it would be more dangerous to go fast, if the angle of attack of the fast canopy started out a lot lower, despite the change in angle of attack from the gust being reduced due to speed.

Figuring out that trade off requires a more full understanding of the way the whole parachute system flies -- lift, drag, trim angle, line lengths, pitch coefficients, etc. Without trying to crunch numbers, I'm guessing that parachutes are usually flying at a reasonably high angle of attack, even in normal flight, brakes off. Here I don't have any simple answer from theory.

We also could consider two different slow vs. fast situations here, that shouldn't be confused:
1) Should one fly faster versus slower on a given canopy (by using no brakes or partial brakes); or
2) should one fly under a big slow canopy, versus under a small fast canopy.

These are four different angle of attack situations. I don't have proof or clear evidence even for myself, but my feelings are that for #1, the faster speed should have a lower angle of attack, as would be true for a regular aircraft. For #2, both the large and small canopy could well be designed to fly at the same angle of attack in normal flight. Then that really is in favour of going fast by using a higher loaded canopy.

Whatever the situation, if one starts with a reasonably high angle of attack, then that would fit well with what others believe to have observed: Then it is beneficial to stay fast to reduce the effects of a gust.

One example of where the trade off likely isn't worth it, is front risering on approach. While that also involves the complicating effects of a massive distortion of the airfoil, it may significantly reduce the canopy's angle of attack. The gain in speed isn't worth the reduction in angle of attack.****

I am still vaguely uneasy about the tradeoffs for normal canopy flight. Old canopies seemed to be trimmed to fly fairly slow, with a high angle of attack, while some fast modern ones must be trimmed fairly nose low to make them "ground hungry", to give them extra speed for the swoop and flare. (E.g., one early example was the floaty Stiletto vs. the divier Jedei. Brian could probably comment here...) The ground hungry trim would imply a lower angle of attack in normal flight. How much, I don't know. A lower angle of attack in normal flight should make collapses more likely. In practice, it doesn't seem to be a problem, but where's the limit? Is this something canopy designers have had to deal with?

Flying fast becomes better when one adds in the idea of flying at above 1 g in a descending turn on approach, because increased g loading requires a higher angle of attack.


-- A point of my own: There's the whole discussion of whether to hold a little brakes in turbulence or not, which has come up in other threads over the years. While Brian's line of thought suggests not to use brakes, there is one very particular case where "pulling a little brake" can be beneficial.

That's when one is only removing slack from the brakes. So one is "pulling brakes" from the point of view of the control toggles, but not from the point of view of actually deflecting the tail of the canopy. Taking the slack out of the system allows two things:
1) It removes the slack that would make quick, precise control applications more difficult.
2) It allows the pilot to feel the pressure on the brake lines, which can be a guide to how well pressurized the canopy is, providing better feedback to the pilot.

-- DuckDodger wrote:
Quote:
Anyway, maneuvering speed increases as you increase weight/loading, which to me seems absolutely backwards on the surface. I thought that maybe this had something to do with inertia, that generally, increasing weight made the aircraft more stable, or less likely to move to a critical angle of attack and overstress the wing.

Yeah, the standard answer is inertia. A given gust will exert a given force on the wings of the plane, but if the plane is loaded up to a higher weight, the acceleration produced on the aircraft will be lower. (Acceleration = Force divided by Mass) So the g loading on the plane is less at higher weight. (And it is common, but not universal, for the allowed g loading to be the same for the aircraft at any allowed weight.)


-- A follow on issue to Collapses & Turbulence is how to best react to a collapse. Brian touched on it, although it could be a topic for another thread later. One type of collapse is where one side of the canopy folds under, another is general deflations (whether due to a stall at one end or a negative angle of attack at the other).



Brian obviously has plenty of practical parachute flight and design experience, but I sometimes think the physics of his technical explanations occasionally aren't quite correct, or at least aren't quite clear. But I'm sure others agree that clarity, comprehension, and correctness are also problems I struggle with when tackling these kind of issues!




Footnotes:

* The angle of attack is the angle at which the airfoil meets the oncoming airflow. It is sometimes incorrectly thought of as some form of trim angle (angle of the airfoil vs. the suspended jumper) or some sort of glide angle relative to the horizon (where someone says, "that ground hungry parachute has a really steep angle of attack"). Usually one can just talk about angle of attack without getting into the detail of what exactly zero angle would be. That can either be defined aerodynamically (where the airfoil has zero lift) or geometrically (such as by conveniently saying the canopy would be at zero angle of attack if the airflow is approaching parallel to the bottom skin of the canopy).
Lift is pretty much proportional to angle of attack. Double the angle of attack, double the lift. This works up to the point where the canopy begins to get close to stalling.

** Regarding the way that a wing reacts to a sudden change in airflow: There are also inertia effects, and stability issues, which affect how quickly a flight vehicle can react to the change in airflow. Sometimes more inertia is better, sometimes less is better. Sometimes you want the vehicle to 'punch through', sometimes you want it to 'adjust faster' to the air.

***I'm using "negative angle of attack" to signify that the nose of the parachute is being 'pushed down' and the nose of the canopy collapses. That's a useful simplification, although technically with a ram air parachute, one has to consider more than just overall lift on the airfoil, but also pressure coefficients relative to the openings on the nose of the canopy, the airflow stagnation point, etc. Canopy collapse does not need to happen exactly at zero angle of attack.

**** Supporting this argument is that paragling pilots are well known to be much more susceptible to canopy collapses from low angle of attack, when using their speedbar, which is a sort of efficient front risering technique.


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