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I read that the wing of a hang glider should be tightened with cables so that it has a "negative washout", but I didn't really find anything about what this means

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    $\begingroup$ Welcome to avitation.se Entaldalpe! @everyone else: if you have an answer, post it as an answer. Don't send people elsewhere, they came to you for help. $\endgroup$
    – Federico
    Oct 1, 2022 at 15:08
  • $\begingroup$ This should answer your question. $\endgroup$
    – sophit
    Oct 1, 2022 at 15:31

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I strongly suspect that what you read was an example of poor writing or word choice, or a mis-translation. Almost certainly what was intended, was to say that the wing should have anhedral-- i.e. when the glider is placed in a position with the mean chord line roughly horizontal, or perhaps with the root chord line horizontal, or the keel tube horizontal (lots of choices here!),1 the leading edges should have some down droop, root to tip. In flight-- but not when the glider is "unloaded" on the ground2-- this geometry is determined by the length of the lower "side wires".

I've never seen an example of a modern flex-wing hang glider lacking this configuration (no matter which of the three ways listed above we choose to define it). Nor have I ever seen an example of a modern flex-wing hang glider lacking (normal, positive) washout.

A key question re your question, is "which cables?". The main wing wires ("lower side wires")? If so, see the above content re anhedral. The "luff lines"? If so, then maybe the source is simply in error about which direction of twist would be implied by (normal, positive) washout versus this so-called "negative washout". The glider needs to have positive washout, and the "luff lines" do have some influence on that,3 at least when the glider is fully unloaded on the ground.

"Washout" means that the chord line of the wing tip airfoil is inclined in a leading-edge-down, trailing-edge-up manner in relation to the chord line of the wing root airfoil. Like if you grabbed hold of the wing tip and twisted it so the trailing edge was raised. (Note that the aerodynamic loads tend to do this naturally.) A hang glider wing that was twisted in the opposite direction ("negative washout"), while actually loaded in actual flight, would be a death trap.

"Geometric" usually means that we are simply measuring things with a ruler or protractor, without knowing the true direction or the airflow at any point on the aircraft, and therefore not accounting for "twists" or changes in the direction of the airflow over the glider's span, length, etc.. In the hang gliding context, if the intention was really to say "geometric anhedral" rather than "negative geometric washout", this might mean that we are measuring the downward "droop" in leading edge tubes with the keel tube positioned to be exactly horizontal, rather than with the glider in some other pitch attitude. This is also often called "airframe anhedral", as it is defined only in relation to the metal airframe, not in relation to the shape of the sail (the fabric part of the wing.)

There is actually one rather tricky nuance at play here. The geometry of the swept, tapered, "billowed", sail (wing) of a flex-wing hang glider is such that when the wing is loaded, as well as when the trailing edge is simply held up by the "luff lines" with no load on the sail, when we trace the trailing edges from root to tip, we see that they rise up as they approach the mid-span area of each wing, and then they descend again as they approach the wing tips. This is a consequence of sail "billow". In a wing with no taper (i.e. with a constant chord width), and linear leading edges, this kind of trailing-edge geometry would actually mean that the outer part of each wing does in fact have negative washout compared to the mid-span area of each wing. But hang glider wings always have enough taper, in relation to the amount of sail "billow", that the washout actually remains positive, with the twist angle steadily increasing all the way out to the wingtip. This is a difficult concept to convey in words-- a video walk-around tour of an assembled hang glider would convey the point much more clearly. Basically what's going on is that in the outboard part of each wing, the trailing edge is descending as we approach the wingtip, but the wing chord is decreasing rapidly enough that this geometry is still compatible with a steady increase in (normal, positive) washout. See also the graphical content on washout due to "conical sail billow" (as opposed to "cylindrical sail billow") in Mark Markowski's vintage (1977) classic "The Hang Glider's Bible"-- this geometry is much is easier to see in the battenless, highly billowed, highly tapered (long wing root chord, short wing span, short or zero wing tip chord) wings of that era than in modern flex-wing hang gliders. Anyway, it's conceivable that your source is using the phrase "negative washout" to refer to the descending geometry of the trailing edge from the mid-span area of each wing to the wingtip, but if so, this is certainly an incorrect usage of the phrase. Lay a straight-edge (ruler) from leading edge to trailing edge on the underside of the wing of any assembled flex-wing hang glider, at various spanwise "stations" from root to mid-span area to wingtip, and you'll see that the washout is in fact in the "positive" direction, steadily increasing all the way from root to mid-span area to wingtip, despite the descending line of the trailing edge in the outboard portion of the wing.

Don't entrust your life to poorly-written manuals! Hang gliding has reached a high "state of the art", and the optimal configurations are well known. Speak to a manufacturer or dealer for your specific wing.

Footnotes:

  1. It's a function of simple geometry that when we vary the pitch attitude of a swept-wing aircraft, we also vary the amount of apparent dihedral or anhedral that we'll "see" if we view, photograph, or measure the leading edges of the aircraft, as we stand somewhere along the extended centerline. Hence the several different suggested ways to assess dihedral or anhedral. Ultimately what matters is whether or not the existing geometry creates a difference in angle-of-attack between the left and right wings in the presence of sideslip, and this may be relatively independent of the overall angle-of-attack of the entire wing system. It appears there may be merit in assessing the geometry for dihedral or anhedral when the wing system is in the pitch attitude that yields a net lift force of zero when the relative wind is horizontal. All these considerations are completely separate from the issue of the roll torque directly created by sweep itself, independent of dihedral or anhedral, during a sideslip-- the roll torque created by sweep itself is highly dependent on overall lift coefficient, which is highly dependent on the overall angle-of-attack of the wing system as a whole.

  2. On a calm day, one way to make the airframe and sail of a flex-wing glider adopt a position slightly closer to the actual in-flight configuration, is to simply stand the glider on the kingpost and hold it there with helpers (or ropes). The empty weight of the airframe and sail will load the lower wires, unload the upper wires, and hold the sail in a shape that somewhat resembles the actual in-flight shape, though with significantly less "billow" than would be seen in actual flight.

  3. Because when the glider is unloaded on the ground, the "luff lines" pull the sail (wing) up into the intended "billowed" shape. And see comments above re illustrations in Markowski's book showing how "cylindrical sail billow" contributes to washout.

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  • $\begingroup$ PS - may be able to find some links to photos that illustrate these concepts more clearly- and maybe even a couple scans from Markowski's book-- but that's way on the "back burner" for right now. $\endgroup$ Oct 2, 2022 at 19:05
  • $\begingroup$ @Entaldalpe - answer has been over-edited already, but would have good to emphasize that - unlike anhedral/dihedral- washout is not really much affected by the adjustment (length) of any particular cable, at least under normal flight loads. "Luff line" comments only apply when wing is unloaded on ground. Hence my strong suspicion that your source really was intending to discuss anhedral, not washout. Exactly what does control the washout angle, in a flex-wing hang glider in actual, normal flight, where sprogs and/or luff lines are inactive, is a complex question. Batten tension plays a role. $\endgroup$ Oct 3, 2022 at 17:22
  • $\begingroup$ PS by "mid-span" I mean about halfway out from root to tip-- not at the aircraft centerline. $\endgroup$ Oct 3, 2022 at 21:37
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Washout is a term for twisting a wing. Washout is measured such that the tip of the wing's leading edge has rotated 'down' as compared to the root.

Washout serves to un-load the wing tip.

Geometric twist is exactly that -- changing the angle of the airfoil along the wing section.

Aerodynamic twist is when you use changes in camber to achieve the same kind of un-loading of the wing. For example, a root section with substantial camber and a tip section that is symmetrical.

Negative washout would simply go the other way -- increasing the local angle of attack of the wing as you move from root to tip.

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    $\begingroup$ "Negative washout would simply go the other way -- increasing the local angle of attack of the wing as you move from root to tip."-- this is a true statement-- but no hang glider would ever have this feature. $\endgroup$ Mar 1, 2023 at 23:45
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    $\begingroup$ @quietflyer I agree it smells as fishy as can be. I was just trying to answer the question as concisely as possible. In particular, the emphasis of geometric twist vs. aerodynamic twist. $\endgroup$ Mar 2, 2023 at 7:07

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