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Fokker Red Baron (Dr. I) Fokker Red Baron (Dr. I) von Richthofen's Fokker Dr.I serial 425/17 Sopwith Triplane

Photos' source are here: Fokker Dr. I (Red Baron), and here: a Sopwith triplane. They are probably not the actual airplane but only (claimed) authentic models.

It makes sense to me that the Sopwith had ailerons on all three wings. But the Dr. I only had them on the top wing.

My question is, what is the aerodynamic explanation of this design? Why not have ailerons on the middle and on the bottom wing? Which design is better?

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The top wing is always the most efficient, as the lower ones suffer reduced lift due to interference from the air flowing beneath the one above. Ailerons will therefore be most effective on the top wing. If they do the job OK then there is no need for any more. Reasons for adding more below include:

  • The upper set are likely to be (or found to be) inadequate.
  • Making both wings identical to simplify production.

The outermost part of the wing is also the most effective position. So if a short aileron is inadequate then the designer faces a choice of two less efficient additions, to widen it inboard or to place another one underneath. Another issue is which brings more drag or weight penalties, a small aileron which must be sharply deflected or a larger aileron which needs more subtle movement.

Such decisions were often based on the designer's personal experience and preference rather than firm knowledge. They would stick with what they knew rather than take arbitrary risks. Historically the Sopwith came first, following their earlier successful biplanes such as the 1 1/2 Strutter and Pup in having identical upper and lower wings with short ailerons. Fokker responded in his own equally successful house style, with a single aileron on the top wing only, having a long span and outboard balance tab.

Which design is better depends on how you define "better" - less weight, less drag during turns, less cost, optimal distribution of aerodynamic forces through the structure? The basic technical tradeoff is between a single large and strong upper aileron vs. several smaller and lightly-built lower ones. I doubt if anybody has ever crunched realistic numbers.

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