Expert: Paul Soderman - 12/12/2008

Question
QUESTION: i would like to know how much distance is needed between a front wing and a rear wing if the front wing is at 6degrees and the rear wing is at 0 degrees.i'm trying to get the distance so the two don't act as one wing. thank you marty

ANSWER: Marty
The front wing will have a lift force and trailing wake that will induce a downwash on the trailing wing.  Even though the rear wing is at 0 degrees angle of attack, the induced downwash will cause the rear wing to have a down load (or lessen it's upload if it is cambered).  That in turn will have a small effect on the front wing.  The downwash can be approximated as:

E = Cl/(pi * AR)
where E is the downwash angle in radians near a wing
Cl is the lift coefficient of the wing
AR is the aspect ratio of the wing

Far behind the front wing (say one span distance), the value of E will double.  At infinity, the rear wing will not feel anything from the front wing.  The effect on lift can be estimated if I knew the wing geometry, speed, and fluid density (air or water).  

To really answer your question I need to know what you mean by "act as one wing".  Downwash angle is usually a small number, but the wings induce flows that interfere with one another and never really act as one wing.  Maybe you can clarify your problem better.
Paul




---------- FOLLOW-UP ----------

QUESTION: i'm dealing with ground effects due to the closeness of the wings to the water. what would be the best shape of wing to use. also i ment by act as one wing is, does the front wing recognise the hole between the two or does it not. thank you marty

Answer
Marty
Optimum wing shape depends on many factors such as mission (speed, load,  drag, elevator size, etc.).  A wing with an elliptical loading is nice because it will have minimum induced drag.  If lift curve slope and angle of attack are independent of span, this leads to an elliptically varying chord distribution.  But that chord distribution is hard to build.  Most aircraft have simpler wings and the drag is not far from ideal.

As to your question about the hole, the answer is yes.  But I am unsure of your actual geometry and mission.  Good luck.
Paul