Forces, glide and stability

The three forces on a gliding paraglider, how the glide ratio turns height into distance, why the wing settles at trim speed, and why hanging far below it makes you so stable.

In this lesson you will

Standing at the top of the training hill with the wing laid out behind you, you will look down at the landing field and wonder: will I get there? Your instructor already knows the answer, because it comes down to one number, the glide ratio, and to three forces in balance.

The same balance explains why a paraglider feels so steady. You hang far below the wing like the weight on a pendulum, and the wing keeps settling back to the speed it was built to fly. Knowing why helps you trust that steadiness, and respect the few moments when it works against you.

Three forces in balance

Three forces act on a gliding paraglider¹:

Weight: gravity pulling on everything that flies, you, your harness and the wing, straight down. It acts at the centre of gravity, which sits low, close to you².
Lift, the aerodynamic force: the wing’s push at right angles to the oncoming air³.
Drag: the air resisting your movement, acting straight back along the airflow³.

There is no engine. In still air you fly gently downhill along a straight glide path, and gravity provides the push. Because the path slopes down, the airflow meets you from slightly below the horizon, so lift tilts slightly forward and drag points slightly upward³. Together they point straight up and exactly balance your weight¹. That is what a steady glide is: forces in balance, with height steadily traded for distance.

Figure: Lift and drag together point straight up and balance the weight. Joined head to tail, the three forces close into a triangle that contains the glide angle, the same angle as the glide path. Angles are drawn much steeper than real.

Lift and drag together point straight up and balance the weight. Joined head to tail, the three forces close into a triangle that contains the glide angle, the same angle as the glide path. Angles are drawn much steeper than real.

Look at the triangle in the figure. The angle between lift and the vertical is the glide angle, the same angle as your path down to the ground. So the ratio of lift to drag is also the ratio of distance flown to height lost⁴. A wing with a lot of lift for little drag glides flat.

Glide ratio

The glide ratio is how far you travel forward for each metre of height you lose: 8:1 means eight metres forward for every metre down. It equals lift divided by drag (L/D), and in still air it also equals your forward speed divided by your sink rate⁴; on a glide this shallow, your airspeed is close enough.

At trim, about 37 km/h23 mph is roughly 10 m/s1,960 ft/min forward; with about 1.2 m/s240 ft/min of sink that is a glide of about 8.5:1, one maker’s best glide figure for its school wing⁵. Its minimum sink, about 1 m/s200 ft/min, comes at a slower speed⁶. A glide of 8.5:1 is a glide angle of about 7°⁴, much flatter than it looks from launch. Makers’ figures are not directly comparable, though: one maker publishes none, because the harness and the way you sit alone can change the glide by up to a whole point⁷.

The arithmetic is simple: distance = height × glide ratio⁴. From 100 m330 ft at 8:1 you glide about 800 m2,600 ft in still air; from 300 m985 ft, about 2,400 m7,850 ft. Try it.

Figure: Change the height, the way you fly and your weight. Notice that hands up goes furthest, and that a heavier pilot lands in the same place, only sooner: the dots mark equal times. Still air; a typical school wing. Numbers are approximate; deep brake and full bar are estimates.

Change the height, the way you fly and your weight. Notice that hands up goes furthest, and that a heavier pilot lands in the same place, only sooner: the dots mark equal times. Still air; a typical school wing. Numbers are approximate; deep brake and full bar are estimates.

In flight you judge your glide by eye. Watch the ground ahead: the one point that stays still in your view is where your present glide will arrive. Points that rise in your view are out of reach; points that sink below it you will overfly⁸.

Trim: where the wing settles

Let go of the brakes and the wing settles at its trim speed. The trim comes from the relative lengths of the lines and risers, which set the angle of attack at which the canopy meets the air⁹. Push the speed bar and you shorten the front risers against the rear ones: the angle of attack falls and the wing speeds up¹⁰. Where speeds have been measured, school wings trim at about 37–38 km/h23–24 mph¹¹, and certification requires more than 30 km/h19 mph¹².

Trim is steady because the balance corrects itself². If a gust slows the wing, it drops its nose a little and picks up speed again; if it speeds up, it rises a little and slows back down. With your hands up you are back at trim within moments, and on school wings that is also about the best glide in still air¹³.

Trim can drift. Lines stretch and shrink with use, which can leave a wing flying slower and handling less nimbly¹⁴; shrunken brake lines can even leave it slightly braked with your hands fully up¹⁵. That is why wings get trim checks: see care, inspection and packing.

Pendulum stability

On a school wing you hang about 6.6–7.15 m20–25 ft below the canopy¹⁶, and almost all the weight is you. The system’s centre of gravity therefore sits far below the wing that holds you up.

That is the secret of the paraglider’s steadiness. If turbulence pitches or rolls the wing, your weight swings back under it like a pendulum, and the whole system returns to steady flight by itself: pendulum stability². One preliminary measurement found the pilot and wing swinging with a period of about 5 s¹⁷.

A pendulum also swings, though: it overshoots and swings back and forth until the swings die away². After a disturbance the wing can surge ahead of you or drop behind, and in a turn you swing outward. Steep turns or quick turn reversals near the ground set up a big pendulum swing and lose height fast¹⁸. Near the top of its weight range a wing is more dynamic, with stronger pendulum swings¹⁹. Damping those swings is part of pitch and roll.

What changes your glide

Speed. Hands up gives about your best still-air glide, so flying slower or faster glides less far¹³²⁰. Fly slower and the drag that comes with making lift grows²¹; fly faster and the drag of you, your harness, the lines and the cloth grows with the square of your speed²⁰. Light brake for minimum sink keeps you up longer but glides slightly less far⁶. Deep brake or full speed bar cost clearly more, although no measured figures are published for school wings¹¹. Into a headwind or through sinking air the best speed rises²²; the polar shows the speed-to-fly in the polar lesson.
Weight. Within the weight range, a heavier pilot flies the same wing faster and sinks faster, but at the same best glide ratio²³. In still air you land in the same place, sooner⁹; into wind the faster wing penetrates better¹⁹. Fly within the certified range²⁴; certification and sizing explains where to sit in it.
Wind and moving air. Your groundspeed is your airspeed minus the headwind²⁵, so into wind you cover less ground per metre of height and downwind more²⁶; sinking air steepens your glide and rising air flattens it. The air carries you; your airspeed and the wing’s own sink rate stay the same²⁶. Airspeed, groundspeed and wind covers this.
Altitude. In thin, high or hot air the wing flies faster through the air²⁷, with about the same glide ratio²³²⁸.
Your own drag. The wing itself makes the largest share of the drag, but studies suggest the thin lines add about a fifth or more, and you and your harness add to that²⁹. That is why your harness and how you sit can change the glide by up to a point⁷.

Judging your glide on a training-hill flight

Fly hands up on a steady heading towards the landing field.
Cue Steady wind noise, light and even brake pressure.
Why On a school wing, hands up is about your best still-air glide¹³.
Find the point on the ground that stays still in your view.
Cue Points beyond it rise in your view; points before it sink.
Why The still point is where your present glide will arrive⁸.
If the landing field keeps rising in your view, follow your instructor’s call early.
Cue The field creeps up your view and the still point stops short of it.
Why You will not reach it. Braking does not stretch a glide; an early, unhurried choice of another field does.
Keep any turns gentle and well away from the ground.
Cue Small heading changes, no hard swings under the wing.
Why Steep turns low down set up a pendulum swing and lose height fast¹⁸.

Common mistake Braking to make the field

Symptom: The landing field looks far away, you add brake to stretch the glide, and you land shorter than ever.
Cause: Thinking that slower means further.
Fix: Hands up is your best still-air glide, and into wind even more speed helps, on the speed bar only if your instructor has trained you¹³²². Decide early, and choose another field if the still point falls short.

Common mistake A steep turn to lose height low down

Symptom: Arriving high, you crank a steep turn near the ground; the wing dives, you swing out and lose far more height than you meant to.
Cause: Forgetting that the pendulum swings back and that the wing has to dive to regain speed after the turn.
Fix: Lose height early with a planned approach, and keep turns near the ground small and gentle¹⁸.

Common mistake Forgetting the wind in your estimate

Symptom: You were sure you had the glide, but into wind the field stopped coming closer.
Cause: Judging the glide as if the air were still.
Fix: Watch the still point, not the map in your head. Into wind it moves closer to you⁸.

Instructor's note Real glides are worse than brochure glides

Published glide ratios are best-case numbers in smooth air. On a training day your real glide over the ground is usually worse: a headwind, a bit of brake, an upright sitting position, some sink. That is why your instructor plans first flights with generous margins, and why you should never plan a glide that only just works on paper⁷.

Instructor's note Stable is not unbreakable

Pendulum stability puts the wing back over your head after a bump, so students often feel the wing is unshakeable. It is not. The same pendulum that steadies you can swing you hard after a steep turn, an abrupt brake input or a surge¹⁸³⁰. Smooth hands, no steep turns low down, and the pendulum stays your friend.

Standards differ Glide and pendulum control in early training

Common ground: Every system starts with short, planned flights under instruction; the USHPA tasks ask for smooth speed control towards a chosen target³¹³².

USHPA (USA): P1 asks for launches with pendulum and directional control, two flights at constant airspeed towards a pre-selected target and two with smooth, slight airspeed changes³¹.
BHPA (UK): Club Pilot training starts with first hops, then at least 12 flight exercises and 4 pre-soaring flights before soaring; the detailed task book is not public³³.
APPI (international): Students learn to judge their glide by watching a fixed point on the ground, and to assess lift or sink by watching two points⁸.

Summarised from each body’s published material, which changes over time. Check the current official documents and follow your school.

Questions students ask

If lift and drag point up, what pulls the wing forward?

Gravity. You fly gently downhill, and because the airflow meets you from slightly below, lift tilts slightly forward¹³. You trade height for forward movement all the time.

Does a bigger wing glide further?

Larger sizes of the same wing perform slightly better⁷, but you choose the size by your weight, not by glide. See certification and sizing.

I am light for my wing. Will I glide less far?

Not in still air. You fly a little slower and sink a little less, at about the same glide ratio²³⁹. You will penetrate a headwind less well¹⁹.

What is my glide into a headwind?

Worse over the ground, because the wind takes some of your forward speed away²⁵²⁶. Watch the still point to see your real glide.

Key points

Weight, lift and drag balance in a steady glide: lift and drag together point straight up.
Glide ratio is distance per metre of height lost, and equals lift ÷ drag. Distance = height × glide ratio.
School wings glide at roughly 8-9:1 in still air (one maker gives 8.5:1), a glide angle of about 6–7°; real glides are usually worse.
With hands up the wing settles at trim, set by its line lengths, which is about best glide in still air.
You hang about 7 m25 ft below the wing, so the system is pendulum-stable, but steep turns near the ground swing you hard.
Weight changes speed, not glide ratio; wind and sinking air change the glide over the ground.

Ground to Solo is a study aid. Paragliding is learned in person with a qualified instructor at a recognised school; nothing here certifies you to fly.