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Dec
01

Speedwing Design Basics - sneak peek

Here is a little preview of the upcoming feature on Speedwing Mechanics and Speedwing Design Basics....

By understanding the basics mechanics of a speedwing you will be able to better predict the positive or negative effects any input / change in parameters has.

Be it the carabiner distance of your harness or be it a very heavy or a very light pilot, a heavy backpack or freeride boots and skis... all has an effect on how your wing flies, and the smaller the wing and the higher the wingload the more substancial and more noticable (and sometimes even dangerous) these effects will be.

 

 

 

Let it roll :-) ... and lets have a look at the main parameters:

 

(a) distance between the resulting force input point of any brake induced roll and the wing/pilot systems virtual balance point

Virtual Balance Point = the point marked [v] in the first graph (not an axis, a POINT) where all lift vectors (force vektors) of the wing-halves (red and green lines) meet

Dont loose it here, the brake input goes to the OTHER side and pulls that side up - so e.g. right brake (and the lift it will generate on the right wing halve) will induce an upward lift momentum to the LEFT of the wings center line at the force input point (distance "a") and turn the wing/pilot system via the roll axis (marked with [x] in the first graph) to the right - otherwise a Speedwing (or Paraglider) would roll to the "wrong" side when you pull one "brake" to initiate a roll-turn.

Attention: the roll axis ("x") is ABOVE the virtual balance point ("v") aproximately 1/3rd of the line canopy center - pilot center in the upper third towards the canopy, this, together with "a" generates a resulting vector that represents the total force ("the leverage arm + applied force") rolling the canopy.

In the (by purpose) very simplified model we use here (only one main lift vector per side is shown, in reality its a bunch of vektors going through all line attachment points and lines with the different oriented forces meeting in the virtual balance point) any change of the canopy curve has a nicely visible very direct effect on the "a" parameter (all other parameters beeing theoretically unchanged) this means:

flatter canopy curve (more "turned up" wingtips) = less distance between force input point and virtual balance point (and true roll axis which is above it) = shorter leverage arm = less responsive on roll input

more curved (more "bent" down wingtips) = more distance between force input point and virtual balance point = longer leverage arm = more responsive on roll input

Important remark (special thanks to Philipp (Pipo) for pointing that possible source of misunderstanding out):

The "real" or "true" Roll Axis [x] (the axis the wing pilot system visually rolls around e.g. in a spiral) is NOT in that same spot where the lift vectors meet (we named that spot where the lift vectors meet "virtual balance point [v]"  ) and is much closer to the wing (usually in the upper third of the distance pilot / canopy). The location given in our graph is just a rough estimate, its exact location is depending on various design parameters and the pilots weight and can be best observed while filming a specific wing / pilot combo fly barrel rolls or spirals at a specific speed.

 

(b) distance that the wing/pilot system virtual balance point [v] gets shifted by unequeal carabiner height (e.g. caused by skiing on an inclined surface)

The bigger this distance gets the more the wing will respond to changes in carabiner height.

For speedRIDING you want this distance to be as small as possible in order to have a wing that stays as neutral as possible with any possible differences in carabiner height due to leg / hip inputs while skiing.

 

(c) distance the Pilots COG gets shifted/moved to the side in a "true" wheightshift turn

The COG (center of gravity, the theoretical single point where the pilots mass is concentrated in a vector based physics model) of the wing/pilot system is directly altered, the weight distribution relative to the wing is changed, while the lift forces on the wing stay the same / act in the same direction as before, so the wing will roll around its true roll axis and will start to bank.

Full weightshift input is (of corse) only possible while flying, any input caused by the pilots COG change is not fully effective as long as the pilots wheight is suspended partially by the ground and not 100% by the wing. Same while skiing at slow speeds / with little canopy lift.

 

(d) Distance of the Pilots COG (the pilots "balance" point) to the the virtual balance point [v] of the whole wing / pilot system

The bigger "d" gets, the more "uprighting" moment is available to counter a wheightshift input by the pilot (or external forces on the wing !), the smaller you make "d" the more "twitchy" a wing will feel (also to external input), up to the point were it will get unstable (which will happen as soon as you move the pilots COG ABOVE the pilot /wing system virtual balance point [v] )

In a real world (speedwing) application the wing / pilot system virtual balance point [v] usually will be somewhere around between 1m above the head and the solar plexus region of an average sized pilot and can only be determined by detail measuring - contrary to that - if you want to know the true roll axis [x] for a given wing / pilot size / wheight combination, just have someone video you as you bank the wing on a more or less straight line from turn to turn, or while you fly a spiral dive - in both cases the true roll axis in that specific flight situation will be relatively clearly visible.

 

Important remark: the illustration (c) shows the COG shift exactly at the moment when the COG change is applied, but the wing still flies straight and level.

As soon as the wing starts to bank and centrifugal forces build up (pilot & gear gets "heavier"), the wheightshift effect increases by itself via the centrifugal force effect on the pilots total (body + gear etc.) mass, thus further increasing the COG offset effect vs. the inside (lower) winghalve's lift and as a result the rollrate also increases.

In case a (very small) wing is either constructed to be very sensitive to wheightshift and/or with little uprighting moment against wheightshift inputs (see Point "d"), and / or a high enough pilot & gear mass is applied (and thereby generating a very high flying speed for the given construction parameters), this can lead to a spiral (or in some cases turn-) behavior that dynamicly increases on itself.

A lot of very small highly loaded (over 10 kg/sqm flat area calculated wingload) speedwings with their extremely short lines can enter such stable or even (for a short amount of time in the initiation phase) increasing spirals, sometimes only a few kg of extra wheight (e.g. a wide backpack full of climbing gear or a very heavy pair of freeride skis & boots) can change the behaviour from stable to dynamic increasing as a result of the higher flying speed (and resulting changes in aerodynamic forces on the wing - mainly very high rollrates and small spiral diameters causing outside wing lift effects vs inside wing partial stall effects).

This will materialize itself usually as a "snappy" feeling roll behavior where the wing suddenly increases its rollrate at a certain rollrate / speed / banking angle. Usually this will occur  during hard, wheightshift "supported" roll maneuvers (e.g. initialising barrel rolls or hard / steep spiraldives) and almost any experienced speedflyer / rider has already experienced those effects which usually are "corrected" almost automaticly by an experienced pilot via very small and correctly timed brake and counter wheightshift inputs.

The good thing is (compared to paragliders) that the lines are so short that the amount of centrifugal force caused wheight increase is limited by aerodynamic forces, so the risk of the pilot loosing his /her conciousness from G-forces is more or less zero.

On the other hand the heightloss can be very dramatic in only a few seconds as straight into ground speeds in excess of 160 kph can occur.

In most cases (but not in all) the best way to quickly (= as early as possible) exit such an "increasing spiral dive" is by applying outside brake as early as possible in order to keep things (rollrate) under control.

As the outside wing is faster and has way more lift then the inside ("lower") wing, a strong enough counterforce to overcome the "banking" force of the wheightshift and to slow and stop the "spiral dive" can be (usually, otherwise i would not be writing this ;-) generated that way.

But be prepared that the force required to pull the brake deep enough can be higher compared to what you might be used to during normal flight.

This phenomenom is another good reason why flying barrel rolls whith a lot of wheightshift is not a too good idea with some speedwings - if you are interested in that topic see also: Barrel Roll Secrets

WARNING: dont confuse this kind of stable or increasing spiral with a purely aerodynamicly stable spiral where the pilots body air resistance (aerodynamic drag) gets momentarily bigger than the air resistance of the wing and so the wing "overtakes" the pilot while flying in a spiral dive. In this case you need to INCREASE the air resistance of the wing as much as possbile which means you need to pull BOTH brakes.


This is just a short teaser, more 2 come in the next days / weeks....

If you have comments, questions, further inputs etc.: pls use the button below :-)