Question Consider a KFm3-TGK airfoil. It has a thinkness and camber of 4%. Also consider an angle of attack of -1 degrees. More information on this airfoil can be found at http://airfoildb.com/foils/549 . If I were to place one of these airfoils above another one, similar to a biplane arrangement; based on the actual thickness dimensions of the airfoil, how far would I need to distance them so as not to interfere with each other's flows? Is there a simple way to calculate this?
Answer The effect of one airfoil above another does not have a perfect answer. For engineering purposes the effect of an airfoil is considered in relation to the wing span and not the chord of a wing. One wing span is generally understood as the limit of this effect http://en.wikipedia.org/wiki/Ground_effect_%28aerodynamics%29 There is no formaula that relates to camber or chord.
As far as the airfoil is concerned there is very little difference between an object being the ground or another wing. There is some difference but not enough that engineers give it extra consideration in designing a plane. The Beech Stagerwing is often used as an example of an attempt to work around the interactions of two airfoils, but in reality the top wing was placed towards the rear (staggered) to improve visibility, not to effect the airflow.
So the wings need to be as far as possible apart for best efficiency. In other words, a biplane is a less efficient lifting surface than a mono plane. The rule of thumb used by engineers is the wings should not be closer than the chord of the widest wing.
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