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Rear Riser Turns (Aerodynamics)

 


airdolphin  (D 24416)

Aug 19, 2010, 12:07 PM
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Rear Riser Turns (Aerodynamics) Can't Post

Kind of pertaining to the other thread (Parachute Aerodynamics): can someone please help me resolve the following conundrum:

when a rear riser is pulled down (say on the left), that effectively shortens the rear lines (C-D groups) on the left. Such action increases the angle of attack on the left. Therefore, according to the principles of aerodynamics, the parachute should bank to the right (left wing up, right wing low) and begin turning to the right. However, as we all know, pulling on the left rear riser, the canopy turns to the left. Why?

Are the reasons in the fact that the drag is also increased on the left (due to the "step" between C and B lines)?..

I still seem to be unable to wrap my head around it, and find a solid scientific explanation to the fact easily supported by practice.

Thanks,
Pavel.


fcajump  (D 15598)

Aug 19, 2010, 12:42 PM
Post #2 of 23 (2971 views)
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Re: [airdolphin] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

In reply to:
Kind of pertaining to the other thread (Parachute Aerodynamics): can someone please help me resolve the following conundrum:

when a rear riser is pulled down (say on the left), that effectively shortens the rear lines (C-D groups) on the left. Such action increases the angle of attack on the left. Therefore, according to the principles of aerodynamics, the parachute should bank to the right (left wing up, right wing low) and begin turning to the right. However, as we all know, pulling on the left rear riser, the canopy turns to the left. Why?

Are the reasons in the fact that the drag is also increased on the left (due to the "step" between C and B lines)?..

I still seem to be unable to wrap my head around it, and find a solid scientific explanation to the fact easily supported by practice.

Thanks,
Pavel.

Much more drag on the left then lift. Not sure on this, but you would likely find that the wing is technically in a skidding turn (uncoordinated) to a small degree.


JW


JackC1

Aug 19, 2010, 1:16 PM
Post #3 of 23 (2947 views)
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Re: [airdolphin] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

Here's a theory.

1) Pulling down on the left rear riser transfers weight to the left hand side of the canopy. This would tend to produce a left turn by inducing roll in the canopy to the left.

2) Increasing the drag on the left side by deforming the canopy would tend to slow the air speed of the left side which makes the suspended weight swing out to the right also producing left roll and a left turn.

3) The extra lift on the left hand side by altering the angle of attack with try to counteract the roll which may be why rear riser turns can be flatter then toggle turns.

It's an interesting theory to think about but knowing the in depth mechanics of what causes a turn is massively less useful than knowing which input will burn the most altitude to produce a turn and which will burn the least.


brettski74  (C 3197)

Aug 19, 2010, 2:31 PM
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Re: [JackC1] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

The two main factors that can cause turns are asymmetric speed/drag and bank angle on the wing.

Asymmetric speed induces turns not unlike a tank, which also tends to produce a kind of skidding turn through the air. This may be hard to visualize, but think of a snowboard or skis turning and throwing snow off to the side as you turn. You can notice this to some extent by watching how the pilot chute trails behind the canopy.

Bank angle causes a turn by changing the direction of the lift from the wing. When the wing is banked, the lift vector gets a sideways component that causes a change in direction of the wing.

When you pull on a toggle or rear riser, this induces greater drag on that side and it's the asymmetric drag/speed which results which is the dominant phenomena. This asymmetric drag induces a turn as the side that is pulled down slows down. Because the inside of the wing is going slower than the outside of the wing, it also loses lift. The reduced lift on the inside of the wing subsequently induces banking in that direction also.

If you pull both front risers, the canopy tends to speed up, however, if you only pull one front riser, the speed increase this can create on one side of the canopy is negligible. I'm not sure, but it may even slow down that side of the canopy. I haven't really had the chance to see how the airflow over the wing shifts during such a maneauvre. The way I think about the aerodynamics, however, is to assume that the bank angle is the dominant phenomenon causing the turn. I pull down the nose, inducing a bank angle at the nose, which starts the turn. As the turn progresses, the speed on the inside of the wing reduces while it increases on the outside, which further enhances the bank angle and increases the speed of the turn until equilibrium is reached.

Finally, harness turns pull down one entire side of the canopy. This induces a turn by bank angle alone, so the mechanism is somewhat similar to that from front risers, but less aggressive. In a harness turn, the pilot chute should always be straight behind the canopy just as it is in straight flight, regardless of the turn. This is because the airflow over the wing will be straight and clean, rather than skidding sideways a little like it does when using rear risers or toggles to turn.


(This post was edited by brettski74 on Aug 19, 2010, 2:36 PM)


Bertt  (D 99999)

Aug 19, 2010, 2:32 PM
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Re: [airdolphin] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

In fact, if you pull the left rear riser a little bit on some big canopies, you will get a right turn initially, then if you pull down more, you'll get a left turn, so your analysis seems correct.


Premier billvon  (D 16479)
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Aug 19, 2010, 3:33 PM
Post #6 of 23 (2889 views)
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Re: [airdolphin] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

>Therefore, according to the principles of aerodynamics, the parachute
>should bank to the right (left wing up, right wing low) and begin turning to
>the right.

This happens both with rear risers and toggles. On both my Nitron and my Crossfire, slight deflections of the toggles result in a turn _away_ from that direction, as lift increases on that side. If you deflect it more, drag begins to dominate.

>However, as we all know, pulling on the left rear riser, the canopy turns to
>the left. Why?

Because a parachute is very, very stable in roll but not very stable in yaw. So the adverse yaw dominates, and you turn in that direction.


airdolphin  (D 24416)

Aug 19, 2010, 5:05 PM
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Re: [billvon] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

Bill: is that because when the canopy tries to yaw it starts pulling on the lines and so the suspended weight (jumper) is being swung to the opposite side causing the parachute to bank?


davelepka  (D 21448)

Aug 19, 2010, 5:42 PM
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Re: [airdolphin] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

 
You have hit the nail on the head. What the other posters left out was the result of the aerodynamic changes to the wing, which is exactly what you pointed out, that being the suspended weight shifting around under the wing.

As long as line tension remians, any weight shift under the wing will casue a change in attitude to the wing. When you apply an input to the wing, asymetrical drag will casue the wing to veer off in one direction or another, but the bigger cause of change in the weight of the jumper trying to continue in a stright line. Apply a left input, and the canopy will veer to the left, and you will try to continue in a straight line ending up off to the right side of the canopy. Now you are in a banked turn.

Your movement below the canopy is the main source of control over the canopy. This is why you can get stuck 'in the corner' when making a low turn. When you make a turn, you end up behind and to one side of the canopy. This is why a turn produces a turn and dive, you are effecting the roll and pitch at the same time.

If you find yourself low, and try to stab out of the dive, you can lower your toggles very quickly, but the canopy will not respond in kind. Before the canopy can pitch up and arrest the dive, you the jumper need to move from behind the canopy back to the center point. Once you are back under the center of the canopy, then the canopy can being to pitch up out of the dive as you move forward of the center.

It's that critical time period between lowering the toggles and you moving back under the center the canopy, where if you are going to hit the ground, you just hit the ground. This is, of course, because you can apply input with the toggles but it is meaningless until your weight effects the lions share of the change in attitude.

Look at flying your canopy more as placing yourself correctly under the wing as opposed to effecting aerodynamic change to the wing itself. Yes, you do effect aerodynamic change, but it's just the precursor to the weight shift really making things happen.


brettski74  (C 3197)

Aug 19, 2010, 6:13 PM
Post #9 of 23 (2805 views)
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Re: [davelepka] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

In reply to:
Apply a left input, and the canopy will veer to the left, and you will try to continue in a straight line ending up off to the right side of the canopy. Now you are in a banked turn.

That's quite a cool observation. I remember talking about the lag time between input and response in Brian Germain's canopy course, but I didn't quite put it together like that until now, or maybe I've just forgotten about it in the 18 months since I did the course. Either way, that's something else I'll have to make a point of remembering.


davelepka  (D 21448)

Aug 19, 2010, 6:28 PM
Post #10 of 23 (2800 views)
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Re: [brettski74] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

 
It's an important distinction to make, especially when low to the ground. Your can flap your arms up and down all you want (with your toggles in your hands), but unless you allow the secondary response of the weight shift to occur, the change to the canopy will be minimal. Understanding this, and being mindful of your position under the wing can go a long way toward keeping you of the corner.

This is why the FJC course teaches no turns greater than 90 degrees below (let's say) 400 ft, and no turns greater than 45 degrees below (let's say) 200 ft. Keeping the degree of the turn limited the lower you get reduces the divergance of the pilot from under the center of the wing, and in turn the lag time between input and response from the canopy. You don't explain it to the students that way, but that's the idea behind that rule.


Premier billvon  (D 16479)
Moderator
Aug 19, 2010, 7:14 PM
Post #11 of 23 (2790 views)
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Re: [airdolphin] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

>Bill: is that because when the canopy tries to yaw it starts pulling on the
>lines and so the suspended weight (jumper) is being swung to the opposite
>side causing the parachute to bank?

I don't think so, because it does the same thing steady state, and it does the same thing with both toggles and risers (which transmit a very different amount of load to the canopy.)

Of course any roll at all causes the load to shift beneath the canopy, but in this case that seems like a minor effect compared to the rolling forces applied to the system.


jurgencamps  (D License)

Aug 19, 2010, 10:38 PM
Post #12 of 23 (2753 views)
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Re: [airdolphin] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

When you pull the left toggle or left riser down, 2 things happens. The left side lifts up and slows down.
The lifting up at the lefthandside would generate a righthand turn.
But the slowing down makes your body swing to the right, pulls the left side down (against the generated lift) and makes your canopy turn to the left.
There is/was an article about this effect. Don't know anymore where I have read it.

Gr
Jurgen


kallend  (D 23151)

Aug 20, 2010, 8:04 AM
Post #13 of 23 (2676 views)
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Re: [fcajump] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

In reply to:
In reply to:
Kind of pertaining to the other thread (Parachute Aerodynamics): can someone please help me resolve the following conundrum:

when a rear riser is pulled down (say on the left), that effectively shortens the rear lines (C-D groups) on the left. Such action increases the angle of attack on the left. Therefore, according to the principles of aerodynamics, the parachute should bank to the right (left wing up, right wing low) and begin turning to the right. However, as we all know, pulling on the left rear riser, the canopy turns to the left. Why?

Are the reasons in the fact that the drag is also increased on the left (due to the "step" between C and B lines)?..

I still seem to be unable to wrap my head around it, and find a solid scientific explanation to the fact easily supported by practice.

Thanks,
Pavel.

Much more drag on the left then lift. Not sure on this, but you would likely find that the wing is technically in a skidding turn (uncoordinated) to a small degree.


JW

In order to turn effectively the lift vector needs to be tilted - the yaw forces by themselves are very small compared to the magnitude of the lift vector. Because the CG is far below the center of lift and drag, the yaw WILL induce a rolling moment in the correct direction, thus tilting the lift vector.


MrDree  (Student)

Aug 20, 2010, 8:14 AM
Post #14 of 23 (2668 views)
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Re: [billvon] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

In reply to:
This happens both with rear risers and toggles. On both my Nitron and my Crossfire, slight deflections of the toggles result in a turn _away_ from that direction, as lift increases on that side. If you deflect it more, drag begins to dominate.

Is that the reason why elliptical canopies turn faster? The input will still produce drag at the wing tip, but a lot less "adverse lift" (which counters the effect of the input) than rectangular ones?


Premier billvon  (D 16479)
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Aug 23, 2010, 11:57 AM
Post #15 of 23 (2512 views)
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Re: [MrDree] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

>Is that the reason why elliptical canopies turn faster?

I don't know. I guess it's a possibility, but I don't think it's a major effect. A Sabre1 97 (completely square) still turns a lot faster than, say, a Pilot 124.


MrDree  (Student)

Aug 23, 2010, 2:51 PM
Post #16 of 23 (2491 views)
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Re: [billvon] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

Of course. I meant all things being equal.


airdolphin  (D 24416)

Aug 25, 2010, 3:44 PM
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Re: [billvon] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

I found an interesting article re wing planform (elliptical, tapered) by the late Ian Bellis:

http://www.flyaerodyne.com/download/planformfactor.pdf

Also, googling "elliptical planform" yields lots of articles:

http://en.wikipedia.org/wiki/Elliptical_wing

http://www.faatest.com/books/FLT/Chapter17/WingPlanform.htm

http://www.onemetre.net/design/downwash/liftline/liftline.htm

The main point seems to be that "An elliptical wing is a wing planform shape that minimizes induced drag" and "An elliptical wing planform has the interesting property of yielding tip vortices which are the least "concentrated", that is, the downwash they yield is spread most evenly along the wingspan"


shropshire  (C License)

Aug 25, 2010, 4:26 PM
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Re: [davelepka] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

Dave - that all sounds good ..... BUT have you tried Ground Handling a canopy (Paraglider)?

I don't weight shift whilst on the ground but still have enormous control authority... The canopy responds (or appears to) just as quickly to Brake input on the ground as it does in the air from control input alone.

OR ... maybe I'm just used to the difference between the control authority on the ground .vs. in the air and mentally ignore it (That would be difficult if the difference was large).

Interesting .....


davelepka  (D 21448)

Aug 25, 2010, 5:57 PM
Post #19 of 23 (2328 views)
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Re: [shropshire] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

Quote:
Dave - that all sounds good ..... BUT have you tried Ground Handling a canopy (Paraglider)?

I don't weight shift whilst on the ground but still have enormous control authority... The canopy responds (or appears to) just as quickly to Brake input on the ground as it does in the air from control input alone.

I never said that the canopy does not respond quickly to control input, in fact I said that the aerodynamic reposnse to control input was the primary response to an input. It's the first thing that happens, and it happens right away.

What I said was the the weight shift of the pilot, the secondary response, effects a bigger degree of change in both amplitude and time. The toggles get the party started, but your fat ass swinging around under the canopy is what really gives the input some 'teeth'.

All of that aside, there are some fundamental problems with calling ground handling 'flying'. In some attitudes, yes the canopy is flying like a proper wing, such as when the canopy is facing directly in to the wind and is roughly directly overhead the pilot. In that case, you could make some direct comparisons to a canopy in flight aloft, but if you vary too far off of that orientation, the principals of 'flight' effecting the canopy are not the same as a canopy aloft.

If you think of the old standard that, 'The canopy doesn't know if you're upwind or downwind, it flies the same in any direction', you know that the reasoning is that the air moving across the canopy is always coming from the same direction, that being the direction of travel. This is why it's called the 'relative wind', becuase it's direction is relative to your direction of travel.

In ground handling, there is no 'relative wind', there is just 'wind'. If you want to face your canopy into the 'wind' and keep it primarily level, it would be a similar circumstance to a canopy aloft. Anytime you deviate off of that attitude, you are exposing your canopy to a wind compnent which it could never experience aloft, and therefore the principals governing it's 'flight' will differ.

For example, say I am attempting to kite my canopy in light winds, and while I cannot generate enough lift to get it up over my head, I can manage to inflate it and get it to wash around behind me three feet off the ground. The direction of the wind relative to the canopy is roughly perpendicular to the direction of 'flight' of the canopy. The nose is pointed straight up, but the wind is hitting the bottom skin square-on. This configuration would never be possible with a canopy aloft, so you cannot compare the two situations apples to apples. While this example is an extreme version of attitudes you cannot attain while aloft, it doesn't take much deviation from striaght-over-your-head and into-the-wind while ground handling to go beyond what would be possible aloft.


shropshire  (C License)

Aug 25, 2010, 7:20 PM
Post #20 of 23 (2308 views)
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Re: [davelepka] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

Sorry . I misunderstood you .. we are now on the same page -thanks


kallend  (D 23151)

Aug 25, 2010, 8:04 PM
Post #21 of 23 (2304 views)
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Re: [airdolphin] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

In reply to:
I found an interesting article re wing planform (elliptical, tapered) by the late Ian Bellis:

http://www.flyaerodyne.com/download/planformfactor.pdf

Also, googling "elliptical planform" yields lots of articles:

http://en.wikipedia.org/wiki/Elliptical_wing

http://www.faatest.com/books/FLT/Chapter17/WingPlanform.htm

http://www.onemetre.net/design/downwash/liftline/liftline.htm

The main point seems to be that "An elliptical wing is a wing planform shape that minimizes induced drag" and "An elliptical wing planform has the interesting property of yielding tip vortices which are the least "concentrated", that is, the downwash they yield is spread most evenly along the wingspan"

That's hardly news. Why do you think the Spitfire of WWII had elliptical wings?


airdolphin  (D 24416)

Aug 26, 2010, 7:52 AM
Post #22 of 23 (2244 views)
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Re: [kallend] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

In reply to:
That's hardly news. Why do you think the Spitfire of WWII had elliptical wings?

My post was in response to other posts which questioned whether elliptical planform makes the canopy turn faster. I think that elliptical canopies are more efficient wings, capable of generating greater lift. The reason why they respond to control inputs so much more readily is because they are usually smaller and tend to have shorter lines and be flown at a higher wing loading.

In fact, the length of lines will probably be the greatest factor affecting maneuverability of a ram-air parachute.

Thus a famous PD question - having the same two canopies 210sq.ft. and 120sq.ft. and loading them 1:1, will they be flown similarly? The answer is no. Even at the same design and wing loading the smaller canopy will be more agile (mostly due to shorter lines and to the fact that air molecules don't scale, as parachutes do).

Thus another dilemma - what parachute to put small light people (small women) under? Putting them under large canopies will create a very lightly loaded system, putting them on smaller canopies creates a hazard for them...


airdolphin  (D 24416)

Aug 26, 2010, 10:50 AM
Post #23 of 23 (2215 views)
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Re: [airdolphin] Rear Riser Turns (Aerodynamics) [In reply to] Can't Post

In the other thread, this article was also offered as a reference:
http://www.bpa.org.uk/...llipticalleblanc.pdf



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