The ram-air wing and lift

How air ramming into the intakes holds a wing with no frame in shape, how that wing makes lift, and why too little angle of attack folds it while too much stalls it.

In this lesson you will

On your first ground-handling day you will watch a heap of cloth become a wing in a couple of seconds. You move forward, the leading edge lifts, the cells fill, and suddenly a firm, curved wing is over your head, pulling. Nothing inside it is rigid. Air holds it in shape, and air holds it up.

That one fact explains most of what can go wrong. Collapses are the most common first event in paragliding accidents, and over-controlling into a stall comes second¹. Both come down to one angle between the wing and the air. This lesson is about that angle.

A wing made of air

Your canopy is two skins joined by ribs into tube-like cells; the anatomy lesson names the parts. As the wing moves forward, oncoming air pushes into the air intakes under the nose and pressurises the cells, like the air scoop on a sports car: that is what makes it a ram-air wing²³. Holes in the ribs, the cross-ports, let air move sideways between cells, which also helps a folded part refill⁴. Flexible rods in the leading edge hold the nose in shape and keep the internal pressure steady⁵.

Once the cells are full, only a trickle of air flows in, replacing what leaks through the cloth³. The air piles up against the intakes, and its pressure, slightly above that of the air flowing round the outside, keeps the wing firm⁶. The trailing edge is closed, so the air stays in.

You can feel that pressure. Firm, even pressure in the brakes means a full, flying wing. Active flying is built on this feel: a light, constant tension on the brakes tells you the moment the pressure drops⁷.

Figure: How the cells fill. Watch the centre cells fill first and the cross-ports pass air out to the tips. Once the wing is full, notice that the oncoming air stops at the intakes and flows round the wing.

How the cells fill. Watch the centre cells fill first and the cross-ports pass air out to the tips. Once the wing is full, notice that the oncoming air stops at the intakes and flows round the wing.

The aerofoil and the angle of attack

Cut the wing from front to back and you see an aerofoil: a rounded nose, a curved top, a flatter underside and a thin trailing edge. The straight line from the leading edge to the trailing edge is the chord line⁸.

The air the wing meets comes from the direction it is flying towards: the relative airflow. The angle of attack is the angle between the chord line and that airflow. It is not the angle the wing seems to point at, nor your glide path: in a steady glide you fly gently downhill, so the airflow comes from slightly below the horizon⁸⁹.

Three things change it:

The brakes pull the trailing edge down, which raises the angle of attack¹⁰.
The speed bar shortens the front risers, which lowers it and speeds the wing up¹¹.
The air itself. If the brake pressure rises while your hands stay still, a gust or a thermal has raised the angle¹²¹³. Sinking air lowers it at once¹³, and a sudden lull makes the wing surge ahead of you, which lowers it too.

With your hands up, the wing settles at the angle of attack its line lengths were built for¹⁴. That is trim, and Forces, glide and stability explains why it is so steady.

Lift: a pressure difference

As the air divides at the nose, some flows over the curved top and some under the wing. Over the top the air speeds up and its pressure drops; underneath it slows a little and its pressure rises. Bernoulli's principle links the two: where moving air speeds up, its pressure falls¹⁶.

Add up the push of the pressure over the whole surface and you get the wing’s total aerodynamic force. The part at right angles to the airflow is lift, the aerodynamic force; the part along the airflow is drag⁹. The low pressure over the top usually gives most of the lift. Seen another way, the wing turns the air flowing past it downwards, and the air pushes the wing up in return. Both pictures describe the same flow¹⁷.

More speed means more lift¹⁸, and so does a larger angle of attack, up to a point¹⁵. That point is the stall.

Myth vs fact

Myth: The air over the top must meet the air underneath at the trailing edge, so it goes faster, and that makes lift.
Fact: Nothing makes the two meet again. The air on top moves much faster than the story needs and reaches the trailing edge well before the air underneath¹⁹.

At the “equal transit” speed the lift would be far too small, and flat or symmetrical wings, with both surfaces the same length, make lift too¹⁹. Bernoulli’s principle is sound; only the story’s reason for the faster air is false.

Where the air divides

The point where the oncoming air divides is the stagnation point. The air almost stops there, so the pressure is highest²⁰. In normal flight it sits close to the intakes under the nose, so that highest pressure faces straight into the cells²¹.

The stagnation point moves with the angle of attack. Raise the angle and it moves a little further back under the nose; lower it and it creeps up the nose, above the intakes²¹.

Figure: Move the slider and watch the magnified nose: the stagnation point slides round the nose past the intakes. Low angles end in a frontal collapse, high angles in a stall. Switch on the pressure view to see where the lift comes from. The angle scale is illustrative and the flow is two-dimensional.

Move the slider and watch the magnified nose: the stagnation point slides round the nose past the intakes. Low angles end in a frontal collapse, high angles in a stall. Switch on the pressure view to see where the lift comes from. The angle scale is illustrative and the flow is two-dimensional.

Too little angle of attack: collapse

Lower the angle far enough and the stagnation point climbs above the intakes onto the top of the nose. Now the oncoming air presses the top of the nose down, and the intakes no longer face the ram pressure²¹. The leading edge loses its pressure and folds under: a frontal collapse across the span, or an asymmetric collapse when only one side folds²².

Turbulent and thermic air is where most serious collapses happen²³. The speed bar lowers the angle of attack too, so an accelerated wing collapses more easily²²: manuals say to avoid the bar near the ground and in turbulence, and to release it first if the wing collapses²⁴²⁵.

In normal flight, with your hands near the top, you can often feel it coming: the brake pressure goes soft on one side or both. Active flying answers that softness with a brief, measured brake input until the pressure returns⁷. Learn it with your instructor in active flying.

In certification tests an EN-A wing must reopen from a frontal collapse on its own within 3 s²⁶, and certified wings normally reopen by themselves after a brief pitch forward²⁷. The first response is in the frontal collapse lesson. A collapse makes the brake pressure vanish, and many pilots react by braking deeply, which can stall the wing²⁷.

Too much angle of attack: stall

Raise the angle too far and the air can no longer follow the curved top. It breaks away from the back of the upper surface first, then further forward, and tumbles in eddies. Lift drops sharply, drag soars, and the wing has stalled²⁸.

Deep brake is the usual cause; big ears, a B-line stall and tight turns also raise the angle of attack²⁹. The warnings are clear: the wind noise fades, the wing slows and the brake pressure rises. If the pressure keeps rising while your hands stay still, a gust has raised the angle for you, so ease the brakes up¹²³⁰¹³.

At the stall itself the feel reverses. The pressure that was building suddenly drops, the brakes go light and the wing drops back¹²³⁰. Heavy and quiet means close; suddenly light after deep brake means it has happened. The answer is hands up, never more brake. Stall, spin and parachutal stall explains why, and stall and parachutal stall covers the first response.

A wing can also stop flying while it stays open: a parachutal stall. Porous or aged cloth and out-of-trim lines make it more likely³¹³². Stall, spin and parachutal stall takes this further.

Flying between the two edges

Normal flight lives between the collapse edge and the stall edge²². With your hands up, a school wing sits comfortably in the middle; class-A wings are built for maximum passive safety and forgiving behaviour³³. The speed bar moves you towards the collapse edge, the brakes towards the stall edge, and rough air nudges you both ways¹⁰¹³. Your hands tell you where you are.

Reading the angle of attack through your hands

Hold the brakes with a light, constant tension.
Cue About the weight of your arms; firm, even pressure on both sides.
Why You feel the wing’s internal pressure through the brakes⁷.
In normal flight, with your hands near the top, notice when the pressure softens on one or both sides.
Cue The wing feels soft, or surges ahead of you.
Why The angle of attack has dropped and a collapse is more likely²². Your instructor will teach the active-flying response⁷.
If the pressure keeps rising and the wind goes quiet, raise your hands.
Cue Heavy brakes, fading wind noise, a slowing wing.
Why The angle of attack is climbing towards the stall¹²¹³.
If the pressure suddenly goes light while you are deep on the brakes, put your hands up.
Cue Heavy, then suddenly light; the wing drops back.
Why The wing has stalled; more brake makes it worse¹².

Common mistake Answering a soft wing with a big handful of brake

Symptom: The wing suddenly feels soft, you pull hard on both brakes, and it goes quiet and drops back.
Cause: The loss of pressure feels like losing the wing, so the hands go down by reflex.
Fix: If you are on the bar, release it first; then use only the light input your instructor has taught you. Deep brake can turn a collapse into a stall²⁷.

Common mistake Speed bar in rough air

Symptom: The wing collapses on the bar, or feels nervous and twitchy.
Cause: Pushing the bar in turbulence or close to the ground, where the lower angle of attack leaves less margin²⁴²².
Fix: Use the bar only where your instructor has trained you, in smooth air, away from the ground. Release it at the first sign of rough air.

Instructor's note The stall edge moves

Students learn the stall as one fixed hand position. The angle at which the wing stalls stays about the same³⁴, but how close you are to it changes:

After a collapse the wing that is still flying is effectively smaller, so it stalls at a higher speed and too much brake on the open side can stall or spin it³⁵.
With big ears the airspeed drops and the margin to the stall shrinks³⁶.

A wet wing is a special case: water on the cloth makes a parachutal stall much more likely even at normal hand positions, which is why rain is outside the operating limits²⁹.

Questions students ask

If the wing is only cloth, why doesn’t it fold all the time?

Ram pressure inside, the rods in the nose and a normal angle of attack keep it firm⁵³²¹. Rough air can still fold it, which is why you learn to feel the pressure²².

Is Bernoulli wrong, then?

No. Faster air over the top really does have lower pressure¹⁶. What is wrong is the reason often given for the faster air, the idea that the two streams must meet at the trailing edge¹⁹.

If the speed bar makes the wing faster, why does it collapse more easily?

The bar lowers the angle of attack¹¹. The stagnation point moves up the nose, above the intakes, so the pressure inside drops²¹ and a smaller gust is enough to fold it²²²⁴.

How will I know my angle of attack? There is no instrument for it.

By feel and sound. In normal flight, with your hands near the top, soft pressure means the angle is dropping. Rising pressure and fading wind noise mean it is climbing towards the stall, and heavy, then suddenly light, means the wing has stalled⁷¹².

Key points

Air ramming into the intakes pressurises the cells, and that pressure, with the lines and ribs, holds the wing in shape.
The angle of attack is the angle between the chord line and the oncoming air. Brakes raise it, the speed bar lowers it, gusts change it.
Lift comes from lower pressure on top and higher pressure below. The “equal transit time” story is a myth.
Too little angle of attack: the stagnation point climbs over the nose and the leading edge folds under (collapse).
Too much: the airflow breaks away from the upper surface and the wing stalls.
Feel it through the brakes: in normal flight soft pressure means the angle is dropping, heavy pressure and fading wind noise mean it is rising, and heavy then suddenly light means a stall: hands up.

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.