class: center, middle, gray-background
Text: CC-BY 4.0
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Get a good .emph[practical overview]
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Not focus too much on why it flies but rather .emph[how we can change flying characteristics] -> this is important for safety
- Technical and academic detail
- Recovery techniques in incidents (SIV courses)
- Flying since 2017
- PP4 (400 flights, 100 hours, 3 SIVs) at the time of writing this
- There are most probably mistakes - please point them out to me
class: center, middle, inverse
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.cite[(c) John S. Denker, "See How It Flies"]
- laminar flow: smooth, even, unhindered, non-mixing flow
- wing changes speed of air above and below wing
- velocity and pressure distribution depends on angle of attack ]
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.cite[(c) John S. Denker, "See How It Flies"]
- downward momentum in air column behind the wing -> upward momentum on the wing ]
.cite[Becker, Sarah & Bruce, Paul. (2017). Experimental Study of Paraglider Aerodynamics. https://doi.org/10.13140/RG.2.2.33674.16321]
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.cite[(c) John S. Denker, "See How It Flies"] ]
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Bernoulli's principle
- higher airspeed <-> lower pressure (suction; s)
- lower airspeed <-> higher pressure (p)
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pressure difference: above and below
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airfoil does not have to be curved on top and flat on the bottom
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air passing above and below does not pass in equal time ]
class: center, middle, inverse
- wing is open in front: cell intakes
- arc (wing is not flat but curved)
- stability design
- negative angle of attack makes wing collapse
- there is pressure inside the wing and it varies
- wing is flexible/collapsible: we can change the size of wing during flight (voluntarily or involuntarily)
- different mechanisms to change angle of attack
- tail-less
- no rudder
- steering
- "fuselage" is 8 m below the wing: human is the pendulum
- connection between human and wing is not fixed (slack/tension): inverted flight not possible, nose-dive requires spiraling
- reducing wing loading can make wing collapse
- thrust comes from gravity
- altitude is the fuel
- in straight and level flight, airflow typically comes at an angle from below, not from front
class: center, middle, inverse
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.cite[From "Nailing the basics of active flying" by Greg Hamerton in the Cross Country Magazine (February/March 2023, p. 36)] ]
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- Brakes (left, right, or both): induce drag -> yaw or pitch
- Speed bar: changes angle of attack
- Weight shift: creates a roll but then also some pitch and yaw
- Back risers
- We can pull at some lines and deform the wing (example: "big ears")
- Most pilots only use a small fraction of the entire brake range
- It's not only about brake position, but also about the timing ]
- pulling one brake: asymmetric drag -> yaw (but also roll)
- pulling both brakes slowly: slows the glider down
- pulling both brakes quickly (and then releasing): pitch back
- it's not only about shifting weight but also about displacing the carabiners up and down
- creates a roll but then also some pitch and yaw; loads one side of the wing more and displaces center of lift from center of mass and creates a roll (which restores center of lift above center of mass)
- roll -> AOA is not symmetric anymore on both sides -> yaw
- look where you want to go, then weight shift, then squeeze the brake
- safer (you look before turning), more efficient and coordinated (weight shift first), safer (less risk to spin)
- Chord line: imaginary line connecting the front of leading edge and trailing edge
- On a paraglider, angle between chord line and horizon is generally not the angle of attack
- AOA: Angle between air flow and chord line
.cite[Pilot’s Handbook of Aeronautical Knowledge, Chapter 5, Aerodynamics of Flight, FAA-H-8083-25B]
- Data is for a specific airfoil (not a paraglider but it is still relevant)
- Increasing angle of attack (AOA) -> increasing lift (L) and increasing drag (D)
- At a certain AOA, this airfoil stalls
- There is an AOA with optimal glide (optimal L/D ratio)
- A paraglider is typically trimmed to fly close to optimal glide when hands-up
- On a paraglider we also want to avoid a too low AOA (more about it later)
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Speed: lift increases quadratically with speed
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AOA: lift increases linearly with AOA
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Wing area: lift increases linearly with the area
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Shape (airfoil)
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Aspect (indirectly)
class: center, middle, inverse
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speed bar
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brakes
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rear-riser control
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collapsing wing (voluntarily or involuntarily)
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turbulence
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aging of lines (A and B lines often lengthen with time, C and brake lines often shorten with time)
.cite[Images from https://www.korteldesign.com/en/reflexion-sur-laccelerateur/]
- left: trim speed
- center: 50% speed bar (first step, legs extended)
- right: 100% speed bar (second step, legs extended, pullies almost touch)
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.cite[R. Falquier, T. Lolies, U. Ringertz, Longitudinal Flight Mechanics of Paraglider Systems] ]
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No brake: hands fully up, trailing edge not deflected
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Full brake: hands fully down
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Beginners will typically not need the full brake range ]
- too high AOA: wing stalls
- too low AOA: wing deflates
.cite[Association of Paragliding Pilots and Instructors (APPI) Pilot Manual, Version 1.2]
- Wing-tip vortices (induced drag): they are a side effect of lift and produced by pressure difference (below and above the wing)
.cite[Pilot’s Handbook of Aeronautical Knowledge, Chapter 5, Aerodynamics of Flight, FAA-H-8083-25B]
- There is an airspeed with optimal glide (minimum drag)
- A paraglider is typically trimmed to fly close to minimal drag when hands-up (and no speed bar applied)
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glide ratio = distance / altitude loss
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typically ~ 10
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launching from 1000 m in still air, no lift or sink, how far can you glide?
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glide ratio changes with speed (brake application or speed bar)
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we always glide "down", even when thermalling (surrounding air then rises faster than we sink)
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sail planes can reach glide ratio of 50-60
- polar curve: relation between horizontal speed and vertical speed
- horizontal speed (brake application or speed bar) and vertical speed are not independent
- slower than min speed: stall
- reducing A lines beyond max speed: frontal collapse
- trim speed ("hands up"): close to best glide, around 10 m/s (36 km/h)
- beginners on modern gliders: do not try to optimize sink with brake application
- min sink is not the same as optimal glide (max distance)
- note how we can read off the glide ratio when looking at the -1.0 m/s vertical speed line
- polar curve depends on how still the air is, altitude, harness, weight, position
- great video: Andre Bandarra: Polar Curves - Basics
.cite[inspired by the article "Staying in touch" by Bastienne Wentzel in the Cross Country Magazine (October 2022, p. 34)]
- pushing speed bar increases speed (increases lift) but also reduces AOA (reduces lift)
- modern gliders and higher performance gliders compensate the two better
- high performance gliders are faster and less sinky at higher speed
- there has been a lot of progress in the last 10 years
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polar moves to the left
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push speed bar for best glide
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we like a little bit of head wind to start
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shows why we don't want to fly in 10 m/s wind ]
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polar moves to the right
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hands up for best glide
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shows why we don't want to land in tailwind ]
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polar moves up
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hands up for best glide ]
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polar moves down
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push speed bar for best glide ]
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polar moves tangentially along the best glide
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glide is unchanged but speeds goes up (also the stall speed)
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adding a lot of weight might change shape which would change glide and flight characteristics ]
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.quote[[ ] Your fly slower]
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.quote[[ ] Your fly faster]
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.quote[[ ] Glide becomes shorter (you land too short)]
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.quote[[ ] Stall speed goes up]
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.quote[[ ] In weak lift you climb less well]
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.quote[[ ] The certification of your glider is still the same]
.cite[Solution: 2, 4, 5]
- Upward forces balance downward forces
- Vertical component of lift is smaller
- Higher sink rate
- More Gs (you feel heavier)
- More wing loading
- The pendulum weight (pilot) swings faster around the turn
- Avoid steep turns close to ground
- Avoid turns on final approach before landing (sometimes a tree landing is better than a steep turn to force the glider onto the nice grass field)
- Less speed -> less lift -> less flare authority
- More speed -> more lift -> more flare authority
- Landing at sea level -> more air density -> more flare authority
- Landing at high altitude -> less air density -> less flare authority
- .emph[You want speed on approach] to have more control and possibly a softer landing
- When landing at high altitude, you need to be very precise with your flare timing
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.cite[DLR, CC-BY 3.0] ]
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- Airflow does not follow upper side of wing anymore
- Wing stops producing lift
- Always at same AOA
- Not always at the same speed: it is possible to stall a wing at any speed
- Not always at the same hand position
- On a paraglider a stall often deforms the wing
- Often wing tips "peel" first
- It can take surprisingly much brake to stall a wing from normal flight
- It takes surprisingly little brake to (re-)stall a wing which is not in normal flight ]
- Video: J. Sanderson, "Stalls: SIV Paragliding Safety Training"
- Wing rocks back
- Brakes become soft
- Peels back from wingtips
- Highly sensitive and unstable
- Risks: twist, spiral, canopy contact
- Video: J. Sanderson, "Spins: Paragliding Safety"
- One brake becomes soft
- One side of the wing stalls
- Half wing flies forward, half wing backward
- Can be accidentally induced by very heavy brake with no or wrong weight shift
- Risks: twist, spiral
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.cite[From Paraglider Control: Stall, Spin, Collapse!] ]
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- Wing looks open but you experience huge sink (7 m/s)
- Very yaw-unstable
- Waffle-grid pattern: Lower surface sucked up against upper surface
- Line attachments look like being pulled out
- Wing becomes parachute
- Air flows from below ]
- Stall/spin can be a useful tool to reset/ fix problems (learn during SIV)
- Acro: stall/spin are basic "every-day" elements
- Observe how wing falls back
- However, it is not the same point and not the same pressure/feeling as in flight (on the ground the wing is not loaded)
.quote[Quiz: what is the advantage of wing falling back after landing instead of falling forward?]
.left-column50[
.cite[From "Nailing the basics of active flying" by Greg Hamerton in the Cross Country Magazine (February/March 2023, p. 36)] ]
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- if wing is pitched/rolled/yawed away from equilibrium, it has the tendency to return to equilibrium (above your head)
- pilot is the weight on a long pendulum -> tendency to restore
- wing design supports stability (especially beginner wings)
- reduced stability if you reduce wing span (e.g. during "big ears" or "big big ears" maneuver)
- roll/pitch/yaw movements are typically coupled (it is however possible to do them separately: dolphining, spiral/looping, heli) ]
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Sweep back: yaw stability
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Center of drag is behind center of lift: yaw stability
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Arc: stretches the front open
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Aspect ratio
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Line setup
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Increased drag when pitching: pitch stability
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Front collapse when over-pitching: avoid canopy contact
- Internal pressure
- Line attachments
- Line tension
- Positive AOA
- Wing has cell openings
- Pressure decreases progressively from front to back and from center to wingtips
- Brake input can briefly increase/restore internal pressure (like squeezing the end of toothpaste tube; "pumping out the deflation")
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.cite[(c) 2018 JackieLou DL]
.cite[(c) 2020 Sebastian Schmied, CC-BY-SA-4.0] ]
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- aspect ratio = span*span / area
- you will never have to compute it, you can look it up for any wing (example 1, example 2)
- some example values for orientation:
- school wings: 4.5 - 5
- beginner: 5 - 5.5
- intermediate: 5.5 - 6
- advanced/sport: 6.5
- competition: 7.5
- lower aspect: rounder, less performance, more stability/rigidity
- higher aspect: thinner, less induced drag, more performance (speed, better glide, better climb); less stability: more piloting needed to prevent deflations and cravats; more dynamic deflations ]
- more speed
- more performance
- deflations more dynamic
- less stability
- more piloting needed in lively air and to fix problems
- On https://para-test.com/ find and read the test report for your glider and find out why it was rated A or B or C or ...
Weight range refers to "all up" weight (take-off weight)
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- Flying outside the weight range? You might be outside of certification.
- Glider has been test flown on the lower and higher end of the weight range.
- Homework: Measure your "all up" weight (you and all the gear and clothes and everything) and compare with the weight range of your glider.
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Slower
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Easier to climb in weak lift
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Less wing loading
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Deflations more likely
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Deflations less dynamic ]
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Faster
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Worse climb in weak lift
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Higher wing loading and tension
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Deflations less likely
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Deflations more dynamic ]
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Defined as weight / wing area
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Typically around 3.5 kg / m^2 (homework: figure out the wing loading for you on your wing)
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Acro wings: high wing loading (6 kg / m^2, or even more)
- More tension
- Higher speed
- Higher stall speed
- Same glide (unless wing distorts)
- More dynamic in turns
- More dynamic after deflations
.quote[More weight or less wing]
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More weight (gear, body, different harness, extra ballast)
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Smaller wing
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Collapsed wing (can behave like a smaller wing)
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Broken lines
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More Gs during maneuvers (tight turn, spiral, acro maneuvers)
.quote[This is about the aerodynamics, not about SIV or recovery techniques!]
- Broken line: less maneuvering possibilities, wing structure can change
- Knot in lines: drag, wing turns towards the knot
- Line-over: maneuvering very limited, might not even fly
- Twisted start: less maneuvering possibilities, brakes are "opposite"
- Twisted brake lines: less maneuvering possibilities
- Aging lines: AOA changes, brake range changes, difficult to launch, closer to stall/spin
- Too much porosity: bad glide, deep stall
- Cravat (cloth caught in-between lines): huge drag, wing turns towards the cravat, rotation
- Deflation (symmetric, asymmetric): depends but often leads to rotation towards the deflation
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Line attachments and line lengths determine the shape/camber/trim.
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Trim speed: speed when "hands up" (close to minimal drag)
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When a wing is out of trim:
- "hands up" AOA changes
- A and B lines often lengthen, C and brake lines often shorten over time
- Wing can deform/stretch/shrink over time
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To trim a wing: bringing it back to trim by adjusting lines (often done by expert). Check the trim when buying a second hand wing.
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Check your wing at least every 2 years. It may not be enough to check line lengths alone (wing can deform/stretch/shrink).