Lift in Ground Effect continued....
#41
Gets Weekends Off
Joined APC: Jun 2009
Posts: 317
Ryan I was just being a smart #$%. It's just that most pilots have only flown straight and level and can think only in this dimension. I just hate this representation of an aircraft with the four forces with lift going upward. That was just my point. Just like you said, lift has no relation to the ground yet we make it appear this way in every flight training books.
Up for another trivia question? From straight and level flight, what is the fastest way to accelerate? (be precise in the answer)
Up for another trivia question? From straight and level flight, what is the fastest way to accelerate? (be precise in the answer)
Next question, precise really? Point the nose down 90 degrees and if you want to die put in full power. When the wings rip off you will get a real good reduction in drag.
#43
Gets Weekends Off
Joined APC: Jun 2009
Posts: 317
Reduce to a zero lift AOA, I figured that was inherent with the push the nose over and align the direction straight down. I see what you mean by be specific now.
#45
Ryan I was just being a smart #$%. It's just that most pilots have only flown straight and level and can think only in this dimension. I just hate this representation of an aircraft with the four forces with lift going upward. That was just my point. Just like you said, lift has no relation to the ground yet we make it appear this way in every flight training books.
Up for another trivia question? From straight and level flight, what is the fastest way to accelerate? (be precise in the answer)
Up for another trivia question? From straight and level flight, what is the fastest way to accelerate? (be precise in the answer)
#47
#48
Gets Weekends Off
Joined APC: Apr 2007
Position: B744 FO
Posts: 375
A report I remember from a SSA Convention presentation:
[PDF] AIR FORCE- INSTITUTE ýOF- TECHNOLOGY
Glider Ground Effect Investigation
Thesis
Nathan H. Jones
Captain, USAF
They used a Blanik & a Grob glider in flight tests to investigate Ground Effect (at KEDW)
[PDF] AIR FORCE- INSTITUTE ýOF- TECHNOLOGY
Glider Ground Effect Investigation
Thesis
Nathan H. Jones
Captain, USAF
They used a Blanik & a Grob glider in flight tests to investigate Ground Effect (at KEDW)
Last edited by 727gm; 10-19-2009 at 10:00 AM. Reason: Add. info
#49
Line Holder
Joined APC: Aug 2009
Posts: 62
RE: Thin Airfoil Theory
Note: I'm omitting equations and explaining things in terms that are hopefully understandable, because, well, no one likes equations.
There's some good reading on this topic here:
Thin Airfoil Theory
Basically, thin airfoil theory is a way of approximating the performance of a given airfoil to get the lift and moment coefficients for both symmetric and cambered airfoil.
Assumptions:
1. Thickness is neglected, but the derivation is still considered relevant for airfoils with a thickness-to-chord ratio of 10% or less.
2. Inviscid (approximately valid for Re >> 1)
3. Irrotational (fluid lines do not curl; they are parallel to one another at any given position at any moment in time)
4. Incompressible (that rules out anything moving faster than about M=0.3)
Basically it is only applicable to 2-D airfoils, not 3-D finite wings. This in turn neglects induced drag from wingtips, as well as a handful of other tip and root-interference related factors.
Basic steps to the derivation:
1. Place a vortex sheet along the chord line of an airfoil.
2. Calculate the variation of the vortex strength per unit length along the vortex sheet, such that the camber line becomes a streamline of the flow and the Kutta condition is satisfied at the trailing edge (vorticity strength = 0 at the trailing edge).
3. Obtain the velocity distribution due to vortex strength along the length of the airfoil.
4. Find pressure as a function of velocity.
5. Calculate the lift coefficient and the moment coefficient for the airfoil.
Conclusion
Thin airfoil theory provides a simple, practical, reliable method of calculating airfoil properties, within the limitations of its assumptions. It is extremely useful for designing and analyzing airfoils. All you need is the mean camber line for the airfoil, and you can calculate all of the important properties of that airfoil. The major weakness of thin airfoil theory is its inability to calculate drag; that must come from later analysis.
Note: I'm omitting equations and explaining things in terms that are hopefully understandable, because, well, no one likes equations.
There's some good reading on this topic here:
Thin Airfoil Theory
Basically, thin airfoil theory is a way of approximating the performance of a given airfoil to get the lift and moment coefficients for both symmetric and cambered airfoil.
Assumptions:
1. Thickness is neglected, but the derivation is still considered relevant for airfoils with a thickness-to-chord ratio of 10% or less.
2. Inviscid (approximately valid for Re >> 1)
3. Irrotational (fluid lines do not curl; they are parallel to one another at any given position at any moment in time)
4. Incompressible (that rules out anything moving faster than about M=0.3)
Basically it is only applicable to 2-D airfoils, not 3-D finite wings. This in turn neglects induced drag from wingtips, as well as a handful of other tip and root-interference related factors.
Basic steps to the derivation:
1. Place a vortex sheet along the chord line of an airfoil.
2. Calculate the variation of the vortex strength per unit length along the vortex sheet, such that the camber line becomes a streamline of the flow and the Kutta condition is satisfied at the trailing edge (vorticity strength = 0 at the trailing edge).
3. Obtain the velocity distribution due to vortex strength along the length of the airfoil.
4. Find pressure as a function of velocity.
5. Calculate the lift coefficient and the moment coefficient for the airfoil.
Conclusion
Thin airfoil theory provides a simple, practical, reliable method of calculating airfoil properties, within the limitations of its assumptions. It is extremely useful for designing and analyzing airfoils. All you need is the mean camber line for the airfoil, and you can calculate all of the important properties of that airfoil. The major weakness of thin airfoil theory is its inability to calculate drag; that must come from later analysis.
#50
Thanks Runge.
If someone wants a mathematical breakdown including how the Laplace Equation is satisfied by the flow on a thin airfoil, feel free to speak up. Not many pilots care at that level although it makes for interesting reading if you happen to have your old Calc III book in the house.
If someone wants a mathematical breakdown including how the Laplace Equation is satisfied by the flow on a thin airfoil, feel free to speak up. Not many pilots care at that level although it makes for interesting reading if you happen to have your old Calc III book in the house.
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