Basics on airfoils and lift generation

Chordline

Camberline

Highspeed airfoil that will generate low Cl and low Cd.Rather sharp stall at low angle of attack

Airfoil for medium speeds that will produce more lift, but also more drag”Nice ” stall

Low speed airfoil, will produce much lift at low angles of attack, much drag and smooth stall at medium angle of attack

”Paraglider airfoil” A lot of volume gives much lift and smooth stalling at high angle of attackAlso much drag as airspeed increases.

Angle of attackThe angle between the chordline and flightpath

Smooth stall characteristics depends largely on :• Large nose radius• High camber• Medium to high profile thickness

Cl

Angle of attack

Stagnation pointWhere the air ”chooses” to go over or below the wing.

This point wanders forward with increasing speed and wice versa

Upwash in fornt of the wing The air directly ahead of the wing will be slightly compressed and will cause a uward motion in front of the wing

Transition pointThe boundary layer becomes turbulentSeparation point

Where the airflow can no longer follow the airfoil.Note : Not nesceseary the same as stall

Downwash behind the wingThe amount of downwash corresponds to the mass suspended

Laminar

Turbulent

Boundary layer just a few cm deep

There will always be some degree of separation behind a wing during normal flight.If we increase angle of attack, this point moves forward.A stall Follows when this point has moved sufficient far forward, and typhical at around 30% of chordline.

Kritisk zone

High dynamic, low static pressure

Low dynamic, high static pressure

High static pressure moves up and around te trailing edge to equalize the low pressure there.The airflow can not follow the airfoil due to the velosity decrease past the highest point of the airfoilHigh pressure air from below moves up the back of the airfoil, flowing against the direction of flight, Where the total velosity reaches 0 , separation follows.

Dunamic pressure

Static pressure

Total pressure = Static + Dynamic( Bernoullis law )

High Pressure !!

Low pressure !!

Negativ flow !

Stall !!

Angle of attack

0 velosity !

A stall will always happen because of a too high angle of A stall will always happen because of a too high angle of attackattackHigh angle of attack = LOW airspeed and vice versa – ALWAYS !ALWAYS !

• On a paraglider, you control the angle of attck with your brakes.• In unaccellerated flight, the glider will stall at the same brake pull position( Exception: if you have trimmers on your risers )• Find, and know this point !!• Spend a lot of time flying near this point, straight and turning.• Practice flying really slow then gently stalling and then back to slow flight.

• This is much harder than it sounds, and more difficult than the common fullstall manouver

• All wings react different near and at the stalling point, but some similarities exist.

• Less brake pressure, and often pulsating brake force• Strange yawing motions• The sensation of sliding/falling backwards

Tip chord Center chord

Paraglider – Wash IN• To increase resistance to collapses, the tips fly with a bit higher angle of attack than the center.• This leads to the tips stalling before the center.• The tip section is the most effective generator of lateral control.When this portion of the wing stalls, directional control is lost …• The wing enters a spin.

Airplane/ Hangglider – Wash OUT• To prevent spins, the tips fly at a slightly lower angle of attack.( This can be done by geometric or aerodynamic washout )• The center will stall first, and directional control remains also after a stall

Stall VS Spin • By reason of the way a paraglider is rigged; a stall can quickly lead to a spin …• Almost without exception, when a pilot uncontrolled hits the ground, there has been a rotation before impact.

•The glider stalls, and a rotation develops into a spin …• After a collapse, the glider enters a spiral or enters a spin due to overcorrection by the pilot.

Full speed ahead …

Keep 20 % brake, lef and right ..• Wait a bit to see what the canopy does now, many times it will reopen by itself• Axcept a certain heading changeDO NOT try hard to keep heading – danger of spin !!

Remember :It is always worse to do too much after a collapse.Relax, let the glider fly, preserve airspeed.

Do not pull the glider into a spin as you try hard to reopen …

Keep 20 % on the left side; if the wing does not open:• Apply right brake in long controlled strokes

• Do not pump short, quick strokes• Do not overdo this, SPIN DANGER !

My ”to do” list after a big My ”to do” list after a big collapsecollapse

The glider flies again !

60 % collapse …, rapid right turn induction

• Apply about 20 % brake, both sides

Nasty situations where the glider might stall unexpected ..

• You are flying slowly …• Suddenly you enter heavy sinking air …• Flightpath becomes steeper, angle of attack becomes greater ….• The glider might stall …..

• Release brakes a bit if entering heavy, sudden sink

Rapid sinkRapid sink

Nasty situations where the glider might stall unexpected ..

Entering rapid rising Entering rapid rising airair

• You are flying slow, near the terrain …• The inner wing is suddenly lifted ….• This wing will increase it`s angle of attack due to the vertical component of relativ wind….• The glider stalls this wing and spins into the terrain …

• Increase airspeed a bit when close to the terrain

Despite constant braking, the glider stalls …

Nasty situations where the glider might stall unexpected ..

Wind speed

Airspeed

Wind gradientWind gradient Wind gradient / windshear means that the wind speed or direction changes rapidly with altitude or distance.Severity will increase with the roughness of the terrain below.

• You are flying above the windshear and are maintaining slow, but safe airspeed• You enter the windshear, and suddenly your airspeed falls quickly …• Lift is reduced, and the flightpath becomes steeper…• Angle of attack increases further …• A stable glider will tend to pitch forward, and a common mistake is to apply brake to compensate for this …. ( NOT GOOD !! )• The already high angle of attack increases even more …• The glider stalls ….

• Release the brakes a bit when entering a wind Release the brakes a bit when entering a wind gradientgradient• Allow the glider some forward pitching….Allow the glider some forward pitching….

• Maintain your airspeed …

… and avoid this …

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