Originally Posted by
chornedsnorkack
Just why is a winglet better than extending the wingtip in the same direction?
A wing with infinite length has ZERO spanwise flow as there is no pressure differential along its span. No vortices are then created and therefore no drag as a result of the vorticies. So you are right, a longer wingspan, or ASPECT RATIO, is good.
However, the longer the wingspan, the more bending (fatigue) it creates at the wing root where it connect so the fueselage (longer moment arm). With more bending comes the requirement of more strength and it's associated weight penalty with a beefed up airframe, which cancels out the original bennefit.
Also, a bigger wingspan has issues when getting into the passenger terminal area for parking.
So winglets do increase the SPAN of the wing but just in a vertical direction, the result is the same. Less spanwise flow, less wingroot bending and the weaker vorticies are actually moved up and away from the main airflow over the wing.
Most books/texts don't tend to actually explain HOW the vorticies increase drag - which is nice to know (INDUCED drag from lift).
Imagine a pen stuck straight out the top of a wing. This is called the TOTAL REACTION FORCE. As the angle of attack increases (creating more lift), the pen tilts back. The TR force is broken down to a vertical component, LIFT, and a horizontal component pointing backwards called INDUCED DRAG. It's this horizontal force which opposes thrust that winglets try to reduce.
How?............
Every wing has an associated UPWASH and DOWNWASH. With low pressure on top and high pressure underneath, air approching the leading edge of the wing wants to move up to the the lower pressure. This increases the "angle of attack" (AoA) of the air as it hits the wing (increases lift). As the air passes behind the trailing edge an opposite downwash is created. This reduces the EFFECTIVE angle of attack. So the pilot needs to select a higher pitch attitude to produce the same lift.....and unfortunately, out "pen" gets tilted further back and with it, an increase in the horizontal component of INDUCED DRAG.
Wingtip vorticies exacerbate the DOWNWASH, so reducing the size of the vorticies, reduces the downwash. This means the pilot doesn't have to increase the AoA as much for a given amout of lift. Also, having the winglet tips high above the main wing also aids in stopping the vortices impacting the airflow over the wing.
Hope this schpeeeeel helps you
