Expert: Mark Janus - 3/1/2006

Question
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Followup To
Question -
what is a tradeoff in replacing a single airfoil with multitude of airfoils of the same performance ? Exp: by downscaling airfoil dimension 2 times I should be able to afford 4 identical airfoils that would all weight half of the original. Would those 4 not perform as well as single original ? I can guess the answer is NO since all comercial planes are monoplanes, but what are the primary reasons ?

Answer -
Sorry for the major delay in getting back to you...  when I first tried to reply the AllExperts system was messed up...  after they fixed it, your question slipped my mind...  the AllExperts system just reminded me...

Inside the wing of commercial airplanes the manufacturers utilize the significant volume of the large wings to carry fuel and various other mechanical systems...  The wing also has significant structural members in order to carry the great weight of the aircraft.

The volume/surface area ratio increases with the size of the object (think of a circle...  the area of a circle increases by radius squared whereas the circumference increases by radius alone)...  The drag of an aircraft is highly influenced by the surface area...  so a larger volume/surface area ratio is desired for efficiency...

Hope this helps and again sorry for the major delay...
Mark

Thx Mark for your reply. Now I understand why flying wing design is contemplated for comercial planes. However, what if I don't need that volume ? Let say I have an object of a given shape (sail boat, glider, flying car) and I need to slap an airfoil on it. I would preffer to do multiples since a) their combine weight is less, b) are easier to fold, c) can use lighter design. But can multifoils achieve same (better ?) prformance as a single foil of the same surface ?  

Answer
Basically the answer to your question is... possibly....  hows that for a straddle-the-fence answer

Anytime you have multiple bodies in close proximity there will be mutual interference....may be constructive may be detrimental....

Yesteryear sailing ships had multiple sails for constructive interference and other reasons

A commercial airline uses multiple airfoils for landing and a little bit for take off....  basically they are increasing wing area though, along with beneficial interference....

Also, if you are suggesting reducing the chord while leaving the span alone (increased aspect ratio), that actually makes for a more aerodynamically efficient wing....although from a structures perspective a long thin wing is not a rigid as a long wide (and or thick) wing.....

As you can see there are many trade-offs and it really depends on your objective....

Basically an airfoil of a given shape when analyzed non-dimensionally will have a particular lift and drag coefficient (non-dimensional lift CL or drag CD) which is independent of airfoils size.  These are non-dimensionalized by (divided by) the planform area and freestream dynamic pressure (1/2 density velocity^2).  so to get the dimensional lift back one simply takes the CL and CD and multiplies by the planform area times the freestream dynamic pressure.

Hope I've helped in some way...