Basics on airfoils and lift generation


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Basics on airfoils and lift generation

  1. 1. Basics on airfoils and lift generation Highspeed airfoil that will Airfoil for medium speeds that will produce more lift,Cl generate low Cl and low Cd. but also more drag Rather sharp stall at low ”Nice ” stall angle of attack Angle of attackChordline Smooth stall characteristics depends largely on : • Large nose radius Low speed airfoil, • High camber will produce much lift at low angles • Medium to high profile thickness of attack, much drag and smooth stall at medium angle of attack Camberline Angle of attack ”Paraglider airfoil” The angle between the A lot of volume gives much lift and chordline and flightpath smooth stalling at high angle of attack Also much drag as airspeed increases.
  2. 2. Upwash in fornt of the wing The air directly ahead of the wing Transition point will be slightly compressed and will The boundary layer Separation point cause a uward motion in front of becomes turbulent Where the airflow can no longer the wing follow the airfoil. Note : Not nesceseary the same as stall Stagnation pointDownwash behind the wing Where the air ”chooses” to go over or below the wing.The amount of downwashcorresponds to the mass This point wanders forward with increasing speed andsuspended wice versa Turbulent Laminar Boundary layer just a few cm deep
  3. 3. There will always be some degree of separation behind a wing during normal flight.Dunamic pressure 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. Static pressure Total pressure = Static + Dynamic ( Bernoullis law ) High dynamic, low static pressure 0 velosity ! Kritisk zone 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 airfoil Low dynamic, high static High pressure air from below moves up the pressure back of the airfoil, flowing against the direction of flight, Where the total velosity reaches 0 , Low pressure !! separation follows. Angle of attack Negativ flow ! Stall !! High Pressure !!
  4. 4. A stall will always happen because of a too high angle of attackHigh angle of attack = LOW airspeed and vice versa – 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
  5. 5. 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 beforeimpact. •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. 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 or d lateral control. Tip ch Center chord 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
  6. 6. My ”to do” list after a big collapse The glider flies again !Remember :It is always worse to do too much after a collapse. Keep 20 % on the left side; if the wing does not open:Relax, let the glider fly, preserve airspeed. • Apply right brake in long controlled strokesDo not pull the glider into a spin as you try hard to • Do not pump short, quick strokesreopen … • Do not overdo this, SPIN DANGER ! 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 change DO NOT try hard to keep heading – danger of spin !! • Apply about 20 % brake, both sides 60 % collapse …, rapid right turn induction Full speed ahead …
  7. 7. Nasty situations where the glider might stall unexpected .. Rapid sink• 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
  8. 8. Nasty situations where the glider might stall unexpected .. Entering rapid rising air Despite constant braking, the glider stalls …• 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 ofrelativ wind….• The glider stalls this wing and spins into the terrain … • Increase airspeed a bit when close to the terrain
  9. 9. Nasty situations where the glider might stall unexpected .. Wind gradient / windshear means that the wind speed or direction changes rapidly Wind gradient with altitude or distance. Severity will increase with the roughness of the terrain below. Wind speed Airspeed• 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 braketo 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 gradient • Allow the glider some forward pitching….
  10. 10. • Maintain your airspeed … ……… and avoid this …