Interpretation

[quade 4]

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I've imagined that the input distance gets smaller, when moving from toggles to rears input but the ratios remains the same. My assumption was not true, wasn't it?

This will vary from canopy to canopy with some being wildly more sensitive than others.

Next time you skydive, if you're in a safe position and at a safe altitude to do so (let's call that above 3000ft AGL), release the brakes so the canopy flies "normally" and while keeping your hands in the toggles, experiment a little with just pulling the rear risers. Slowly increase the force applied until you feel the canopy start to rock...that's the edge of the stall. To recover, just let up on the force. If you still have altitude to play with, try it again faster. You should notice a more dramatic stall. Experimenting this way is a -very- good way to learn how your canopy flies.

I wouldn't recommend trying to land on rear risers unless you've experimented a bit or have to because of a gear issue. (See next responce.)

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In the next season I was planing of doing some rear risers landings. Now I'm thinking that this manoeuvre is for experienced pilots only. Is it safe for someone like me to try a rear-riser landing? If is not recomanding to do so with my limited experience, in your oppinion is it better to cut away when I have a steering line broken?

Generally speaking, I would consider it to be a poor choice to cut away just because of a broken steering line. That doesn't mean the landing is necessarily going to be pretty, but probably one you will walk away from.

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Does canopy control lessons covers in practice those aspects?

They certainly should. If you're planning on getting a few canopy coach lessons, you might wanna broach the subject.

quade -
The World's Most Boring Skydiver

[pilotdave 0]

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In the next season I was planing of doing some rear risers landings. Now I'm thinking that this manoeuvre is for experienced pilots only. Is it safe for someone like me to try a rear-riser landing? If is not recomanding to do so with my limited experience, in your oppinion is it better to cut away when I have a steering line broken?

Generally speaking, I would consider it to be a poor choice to cut away just because of a broken steering line. That doesn't mean the landing is necessarily going to be pretty, but probably one you will walk away from.

I'd just add that rear riser landings are something to treat with a lot of respect. It's very easy to stall suddenly on rear risers (the point of this thread) and slam into the ground before you even see it coming. Our DZ had an experienced jumper (500+ jumps) break his back on a "routine" rear riser landing after a steering line broke. My only skydiving injury was a twisted knee (minor injury, never went to a doctor or anything) on a rear riser landing off the DZ when I had a stuck toggle. I had ~200 jumps. Not sure what happened, but I either stalled it and slammed in or flared too late. I wasn't expecting such a hard landing, so I wasn't ready for it. Prepare to PLF!

Dave

[KrisFlyZ 0]

First thing is to appreciate that the for a glider, the relative wind is always coming at the glide angle. Then rest of this post will make sense.

Let us look at this

AoA = ARCTAN(1/(L/D)) -

ARCTAN(1/(L/D)) is the glide angle.

For a canopy, the is what is usualy called the trim angle.

This angle can also be thought of as the angle of incidence.

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We measure this angle between an axis running the length of the aircraft, called the longitudinal or x-axis, and the chord line of the wing airfoil and call this value the angle of incidence.

is the definition of angle of incidence and unless I am making a blunder(please point it out if you can spot it) the longitudinal axis of the 'aircraft'(that is the jumper under canopy) is the horizon.

Are angle of attack and angle of incidence directly related? No, as I have stated in my first post to this thread.

Changing the angle of incidence (we can do this for canopies by pulling down on both front or both rear risers by the same amount) will indirectly change the angle of attack. Angle of incidence is the input we can change to change the Angle of Attack. Is there a simple relationship between the AoA and AoI? No. Is there a relationship at all? Yes, the equation above.

For powered aircraft that can fly at any L/D they want


I may have oversimplified when I said. Again, if we are talking about the wing only, I still think pitch == AoI when we are talking about the canopy, lines and jumper.

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Decrease AoA = increase pitch and reduce L/D == pull on front risers

Increase AoA = Decrease Pitch (and possibly increase L/D) == pull on rear risers

Pulling down on front risers increases the AoI but does it reduce AoA? Can't really tell because we know that the L/D of our canopy has reduced when we pull down on the front risers. This means that the value of glide angle(the arctan bit) is higher. Whether AoA has increased or decreased depends on how much we pull down the risers by and the resultant L/D.

Pulling down the rear risers decreases the AoI but also increases L/D. This means that the glide angle is also lesser and we cannot directly guess the if the resultant AoA is higher or lower.

Kris.

[pchapman 279]

I figure the most important reason why it is easier to stall on rears than on brakes is because using brakes, the maximum angle of attack before the wing stalls is higher. It isn't really about the speed at which some angle of attack is changed.

With brakes, the curvature at the back of the wing is like putting flaps down on an aircraft, which allows the wing to keep generating more and more lift by going to higher angles of attack where the wing would otherwise stall. (Of course brakes don't physically affect the whole span of the wing.)

With rear risers, the back of the wing is pulled downwards, distorting the wing in an odd way. This does increase lift, adds some drag (though not as much as for brakes), but who knows how the angle of attack at which it would stall will change. In any case, it won't increase greatly as when brakes are used.

So during a landing, one can get a lot slower for touchdown with brakes, than with rear risers.

The original post mentioned how Germain's book said that the angle of attack changed quickly when using rear risers, and slowly when using brakes (as brakes took time to drag the canopy back and swing the jumper forward).

I don't think he uses a "proper" definition of angle of attack. In aerospace engineering terms, the reference line is from the nose to the tail of the original, undistorted airfoil. (Leaving aside some details). This reference line is not changed when flaps or brakes are moved downwards, even though this changes where the tail of the airfoil is.

So using the aerospace engineering definition, distorting the airfoil suddenly, whether by rear risers or brakes, technically doesn't immediately change the angle of attack at all.

But, as the shape of the airfoil changes, the lift produced by it will change -- and as I described earlier, the stall point may also change.

Germain said that the angle of attack changed quickly with rear risers, perhaps because I think he uses a different definition of angle of attack, a layman's definition where it is defined as the line between nose and tail, wherever the tail may happen to move. So pulling rear risers clearly moves the back end of the wing downwards, including the tail, so according to this definition, the angle of attack has increased -- it's almost like trying to rotate the whole wing suddenly. Call this definition the 'visual angle of attack' for want of a better term.

But that definition still causes problems. Pulling the tail down with brakes by say 18" is deflecting the tail down a lot more than when pulling the rear risers down 6", and thus brakes would change the visual angle of attack a lot more. The words in the book just don't make logical sense to me.

I find Germain's explanations generally to be a bit mixed up when it comes to understanding detailed aerodynamics, or explaining aerodynamics clearly. But that being said, he does have a very good feel for what is actually happening to the parachute. So I'd generally trust his conclusions, even if the aerodynamic background isn't always clear. And his book still puts a lot of good stuff together in one place, something nobody else has done.

In the end, when it comes to rears vs. brakes for reaching the stall, I think the main reason brakes don't stall you as easily is that one can pull brakes a lot further without causing a stall, as brakes deform the wing in a way that is quite efficient adding lift yet avoiding a stall, compared to pulling down on rear risers. Rear risers may not be able to add as much lift as brakes can, but they can add a moderate amount of extra lift more efficiently (with less added drag) than can brakes.

(Among prior answers, I like what quade and mr2mk1g wrote. They are using the visual angle of attack definition I think. That's fine, as long as we can all sort out what definition each person is using.)

Phew, how's that?