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line loading in flight

 


yoink

Oct 16, 2012, 7:43 PM
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line loading in flight Can't Post

So I have a quick question I'm hoping someone knows the answer to...

In level flight (no control inputs and assuming a symetrically balanced load), is the load EQUALLY distributed between line groups?

Ie. If there is 100 pounds distributed between a single A, B, C and D set of lines, do they each assume 25lbs of load or is it unequal, or does it depend on the canopy and trim?

Speculation is welcome, but please say if you're making a guess! My gut feeling is that it should be divided into 2 equal sets of loadings dependant upon the lines / canopy... One set of loading per riser group.

I'm really hoping I don't need to build some sort of simulation to work out the answer!

Anyone?


(This post was edited by yoink on Oct 16, 2012, 7:46 PM)


Squeak  (E 1313)

Oct 16, 2012, 8:43 PM
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Re: [yoink] line loading in flight [In reply to] Can't Post

from a construction rigging point of view, i would say that the lines would share equal load per riser. and if the load on the risers is ballanced then, equal load all around


yoink

Oct 16, 2012, 9:10 PM
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Re: [Squeak] line loading in flight [In reply to] Can't Post

Cheers Squeak. That's my intuition too.


pchapman  (D 1014)

Oct 16, 2012, 9:25 PM
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Re: [yoink] line loading in flight [In reply to] Can't Post

The loading on an airfoil is normally going to be weighted much more towards the front. That's a matter of basic aerodynamics of airfoils, but the difference between canopies will depend a lot on how the lines are arranged (eg, trim angle and location of payload relative to the canopy).

You'll notice differences just from testing the forces on front vs rear risers on different canopies. Or reach up and pluck at individual lines, and you'll feel a big difference in how much force is involved.

The weight the canopy supports will also be something like slightly less than the total weight of the jumper & gear (maybe 5-8%), because the weight is supported by a combination of lift and drag because the canopy is on a descending flight path (not level flight of course).


JerryBaumchen  (D 1543)

Oct 17, 2012, 11:12 AM
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Re: [yoink] line loading in flight [In reply to] Can't Post

Hi yoink,

Once you include this req'ment: a symetrically balanced load

Then the lines have to take an equal loading.

Just my 2 cents,

JerryBaumchen


sundevil777  (D License)

Oct 17, 2012, 12:16 PM
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Re: [JerryBaumchen] line loading in flight [In reply to] Can't Post

In reply to:
Hi yoink,

Once you include this req'ment: a symetrically balanced load

Then the lines have to take an equal loading.

Just my 2 cents,

JerryBaumchen

Quite right!

Quote:
In level flight (no control inputs and assuming a symetrically balanced load), is the load EQUALLY distributed between line groups?

If the assumption is a symmetric load, by definition it is equally distributed.

1=1


RiggerLee

Oct 17, 2012, 12:34 PM
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Re: [JerryBaumchen] line loading in flight [In reply to] Can't Post

I refranced this here once before but I couldnt find it.

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

It's not a bad example and makes pritty pritty pictures that make it fairly understandable.

The answer is that in stable flight the load is well to the front of the canopy. This is of course dependent on the angle of attack, camber, thickness, etc. But generally as a rule of thumb the aero dynamic center is around the .25 cord point. So when we model it we talk about a lift, drag, and moment around that point.n so the center of lift changes slightly with the angle of attack.

But I'll do you one better. Look at it spanwise. A finite wing has a circulation atround the wing tip. It wants to flow from the high pressure on the bottom to the top. In fact lift it self is a circulation around the wing and you can kind of think of this as that vortex droping off the tip and trailing behind the wing. It turns 90 deg and goes backwards. Here's the thing, that circulation affects the angle of attack that the wing sees all across the span of the canopy. The air actually does not hit it at the angle you would expect. In fact that angle changes all along the wing. This generaly affects you in a negative context. It sort of tilts the lift vector backwards from being perpendicular to the flight path. In other wards it turns some of your lift into drag. I'm not nit picking. This is a lot of drag and for low AR canopies it's a big deal. Low aspect Ratio, AR, sucks. So the lift across the A lines for example even on a rectangular canopy were you would think that they are all suporting the same weight is not the same. It's not an even distrobution from side to side. It's not even a smooth elips, which would be ideal to minimise induiced drag.

In short the answer is no. Even in stable gliding flight, other then center line symitry, you would be hard pressed to find two lines that have the same load on them. Nothing is eaven unless it's just a querk of the spread that some c happions to equal some d further out on the span.

Lee


councilman24  (D 8631)

Oct 17, 2012, 1:07 PM
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Re: [sundevil777] line loading in flight [In reply to] Can't Post

What makes this hard to imagine is that that line lengths are different, so this would normally indicate different load. But the suspension platform is not fixed in relation to the load so it can equalize. Into what we call angle of attack or trim.

Then you compare load pulling down/up on front risers, individual lines, and brake lines (essentially non fixed length suspension lines.) We know these can be different between canopies with the same load. But these also involve distorting the flexible suspension point.

And we know the load is different during inflation but the suspension points are developing.

So gut tells me load the same, and load different.Crazy


I HATE PHYSICS!MadCrazy


wolfriverjoe  (A 50013)

Oct 17, 2012, 3:30 PM
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Re: [pchapman] line loading in flight [In reply to] Can't Post

In reply to:
The loading on an airfoil is normally going to be weighted much more towards the front. That's a matter of basic aerodynamics of airfoils, but the difference between canopies will depend a lot on how the lines are arranged (eg, trim angle and location of payload relative to the canopy).

You'll notice differences just from testing the forces on front vs rear risers on different canopies. Or reach up and pluck at individual lines, and you'll feel a big difference in how much force is involved.

The weight the canopy supports will also be something like slightly less than the total weight of the jumper & gear (maybe 5-8%), because the weight is supported by a combination of lift and drag because the canopy is on a descending flight path (not level flight of course).

Yes on the uneven loading. There is a significant difference in the tension on different lines during steady state flight. That tells me that the loading on those lines is different.

No on the weight. In steady state flight, the weight is the weight. The only time the weight is different is if there is acceleration in any direction.

Even in a fairly steep descent, you are at one G. You can prove this in an airplane with a spring scale if you want to make the effort. During any change in the vertical speed, the G-load will change. But once the vertical speed stabilizes (climb, descent or level flight) the G load goes back to one.


yoink

Oct 17, 2012, 3:41 PM
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Re: [councilman24] line loading in flight [In reply to] Can't Post

Thanks for the info guys. Some fantastic stuff here! Smile

My 'symmetrically balanced load' was trying to indicate a pilot simply sitting square in the harness - maybe I could have worded it better.

I'm trying to get to an answer in my head of exactly what happens to individual line loadings as we shift around in the harness - in particular fore and aft - for example as a swooper leans forward in the harness when planing out, discounting any input they're having by pushing / pulling on risers to do so.

As the pilot leans forward does the point of load move in relation to the system, and if so, what happens to the line loads s it does so?
I've heard lots of speculation - "You're putting more load on the front lines. No! You're putting more load on the rears!" etc etc, and it's got me thinking.


pchapman  (D 1014)

Oct 17, 2012, 4:10 PM
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Re: [wolfriverjoe] line loading in flight [In reply to] Can't Post

In reply to:
No on the weight. In steady state flight, the weight is the weight. The only time the weight is different is if there is acceleration in any direction.

Sorry, I'll have to call you on that one -- it's a piece of aerodynamics people aren't often taught properly.

In straight level flight, like for an airplane with an engine, the Lift is vertical and is matched exactly by the Weight. (Without getting into tail vs. wing lift details)

But we're talking parachutes, gliding downwards at a fairly steep angle. The Weight is matched by a combination of the Lift (perpendicular to the descending flight path) and the Drag (parallel to the flight path).

So it is 1 g unaccelerated flight, just that both Lift and Drag hold us up. The amount of Lift isn't typically a whole lot less than the Weight, but it is important to understand how Lift & Drag are defined when one is trying to understand even the basic aerodynamics of parachutes.

If you used a parachute that descended on a 45 degree angle, a 200 lb jumper & gear would be supported by 141 lbs of Lift (angled 45 degrees forwards & upwards) and 141 lbs of Drag ( angled 45 degrees backwards & upwards)

Sorry I don't have a diagram handy.


RiggerLee

Oct 17, 2012, 4:28 PM
Post #12 of 20 (1199 views)
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Re: [yoink] line loading in flight [In reply to] Can't Post

Leaning forward and back in the harness doesn't do any thing for you. The risers come togather at a confluance. For the most part the canopie acts as if it were loaded at that point. As far as trim goes you can't shift the load relitively to the canopy. However the distance of the cg downwards from that point can afect the dynamics of the canopy. That length can afect how the canopy pitches. If you could signifagently lower the cg, weights on your ankles or pulling up your legs, you could probable do more to effect the flight of the canopy then leaning forwards or back.

Lee


pchapman  (D 1014)

Oct 17, 2012, 4:55 PM
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Re: [yoink] line loading in flight [In reply to] Can't Post

OK, RiggerLee beat me to it but I'll reinforce his point by what I've been typing up:

In reply to:
I'm trying to get to an answer in my head of exactly what happens to individual line loadings as we shift around in the harness -

There has been some debate about that. I don't know what people are saying these days but here's my take:

While a swooper can change their drag and thus the overall dynamics of the flight & swoop, we are hanging by the 3 rings. So you could climb around your harness using it as a trapeze, and it would be like playing around while standing on a kid's swing -- in the steady state condition, your center of mass will remain directly below where it is hanging from (Ignoring drag effects for the swooper at speed.)

So in that way, shifting in the harness will do squat.

Now, maybe you can do something dynamically -- the physical act of swinging your legs forward could do something, just like under a swing set.

And another exception is that in addition to hanging under our 3 rings, we will have brakes and sometimes risers in our hands. That can provide a little leverage for moving one's center of gravity. Even then, often all one would tend to do is change the angle that one swings under the 3 rings. So to oversimplify, if you are in your loose swooping harness and you push back on your rear risers while trying to lean forward, you'll just rotate your body, and the C of G will stay under the 3 rings. I haven't thought about it enough, but maybe you can find an angle to pull or push to pivot the C of G away from the vertical - but it seems hard to figure out any decent leverage to do very much of that. If you tried to do chinups from the top of the front risers, OK, but that's not what you need to do with the risers during the swoop.

So if someone is leaned forward in their loosened harness, they are NOT "shifting their center of gravity forward", unless they also somehow are pushing on the risers in a way to hold themselves forward and not just rotate forward.

As for what the line loadings and flight path and all such things are, that's going to be a complex interaction of the lift & drag & torque of the canopy vs. the drag and location of the jumper beneath.

A complex subject in any case.
Attachments: swooper C of G.jpg (31.9 KB)


wolfriverjoe  (A 50013)

Oct 17, 2012, 7:54 PM
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Re: [pchapman] line loading in flight [In reply to] Can't Post

Ok, I get you. The drag is actually supporting some of the weight.
Thanks. I got to learn something today.

I'm just used to people thinking that there is a lower G load due to the descent.


riggerpaul  (D 28098)

Oct 18, 2012, 7:16 AM
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Re: [wolfriverjoe] line loading in flight [In reply to] Can't Post

Regarding the statement that nothing in the harness does anything, it is also true that side loading of the harness will make the canopy turn.

Now, maybe you are only asking about forward and back, and not side to side. As previously said, shifting forward and back won't change the loading on the canopy.

But, for completeness, harness loading can induce turns.

So, when somebody starts messing about with their harness loading, they should not be surprised that they can make the canopy point somewhere else.


DrewEckhardt  (D 28461)

Oct 18, 2012, 1:37 PM
Post #16 of 20 (1002 views)
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Re: [yoink] line loading in flight [In reply to] Can't Post

In reply to:
So I have a quick question I'm hoping someone knows the answer to...

In level flight (no control inputs and assuming a symetrically balanced load), is the load EQUALLY distributed between line groups?

No. Some canopies carry more weight on the front risers and brake lines than others.


Squeak  (E 1313)

Oct 18, 2012, 6:18 PM
Post #17 of 20 (970 views)
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Re: [pchapman] line loading in flight [In reply to] Can't Post

Given that the harness load point is the leg straps, wont the CoG remain above that point, (unless pilot is inverted). so the downward shift of CoG is relatively small (unless you're built like TallGuy).


JerryBaumchen  (D 1543)

Oct 18, 2012, 9:18 PM
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Re: [Squeak] line loading in flight [In reply to] Can't Post

Hi Squeak,

Quote:
Given that the harness load point is the leg straps

The load point between the canopy and the harness is where the riser wraps around the 3-ring harness ring. A single point load ( one on both sides Tongue ).

As mentioned, unless you do something strange, the CG will always be directly below that load point(s).

JerryBaumchen


Squeak  (E 1313)

Oct 19, 2012, 2:12 AM
Post #19 of 20 (929 views)
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Re: [JerryBaumchen] line loading in flight [In reply to] Can't Post

In reply to:
Hi Squeak,

Quote:
Given that the harness load point is the leg straps

The load point between the canopy and the harness is where the riser wraps around the 3-ring harness ring. A single point load ( one on both sides Tongue ).

As mentioned, unless you do something strange, the CG will always be directly below that load point(s).

JerryBaumchen
Yup i know that, but i was referring to Pchaps picture about leaning forward IN the harness. where leaning fwd actuallt does not move the CoG fwd, but lower.

Harness load point not canopy load pointWink


(This post was edited by Squeak on Oct 19, 2012, 2:13 AM)


pchapman  (D 1014)

Oct 19, 2012, 6:21 AM
Post #20 of 20 (902 views)
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Re: [Squeak] line loading in flight [In reply to] Can't Post

In reply to:
i was referring to Pchaps picture about leaning forward IN the harness. where leaning fwd actuallt does not move the CoG fwd, but lower.

I was only trying to address the forward/aft C of G, not trying to address anything vertical. It does make sense that if the chest strap is loosened a lot and a jumper leans way forward, yes that should lower the C of G. (As Jerry says, fore and aft the C of G will stay below the 3 rings, so the leg straps will kick back as the jumper leans forward onto the loose chest strap.)



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