RCU Forums - View Single Post - undercambered vs flat bottomed wing
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Old 03-20-2002, 04:53 PM
  #12  
Ollie
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Default Sopwith Pup

I went to the Balsa USA web site and found that the 1/3 scale Pup is a Sopwith Pup not a Buhl Pup. The description of the model strongly implies that the airfoil is scale or scale-like.

I checked some pictures of the full scale 1916 Sopwith Pup and estimated that the airfoil is thin at about 5 or 6% thickness and the undercamber is not extreme with a mean camber of about 4% or so.

The high speed of the model will be mainly limited by the parasitic drag of the struts and wires and the interference drag of the biplane configuration rather than by the profile drag differences of the airfoils under consideration. The drag of an open cockpit, cowled rotary engine and landing gear will further dilute the effect of airfoil drag differences.

It will take twice as much thrust to increase the top speed by as much as 40%.

The low speed end of the speed range will be determined by the low wing loading of less than 16 ounces per square foot and the maximum coefficient of lift of the airfoil. The stock airfoil will undoubtedly have a lower stall speed than a flat bottomed replacement. The relatively low wing loading and good maximum lift coefficient will allow the model to turn very tightly.

BTW, the top speed of the full scale Sopwith Pup was 111 MPH so the top speed of a 1/3 scale model should be 37 MPH.