Ch and Cv formula

[HELModels]

I'm using the following formula for Ch and Cv:

Ch = (A_hori/A_wing) * (tail_arm/avg_wing_chord)
Cv = (A_vert/A_wing) * (tail_arm/avg_wing_span )

This is recommended by Dr. Drela in another thread and probably been posted many times. What I am wondering is about average wing span. Is the average wing span the span between where the 2 average chords occur?


I assumed yes when I calculated Ch and Cv for my plane. I ended up with 0.7 for my Ch and 0.047 for Cv. I can say I have always felt the control authority and stability very good.

As some might recall, I am using an Aerobird Challenger wing and interestingly the wing area I calculated by dividing the wing up into rectangles and triangles is 167.5 and some article in a magazine quoted the wing area at 187 sq. inches. I seriously doubt my method could be off by 20 sq. inches.

The question is what is average wing span?

[HELModels]

If I divide the wing area, which is absolute, by the average chord I get 41.875 inches. If I measure wingtip to wingtip I get 42 inches. If I run a ruler to the point on the wing where the average chord is 4 inches, I get 30 inches between average chord locations. If I divide the wing area by the root chord I get 35.3 inches. The distance from tip to tip couldnt possibly be the average span when the LE is tapered, but I cant rationalize what would be average span.

Who has the sledge hammer?

[Ollie]

If depends on monoplane, biplane, triplane, etc. If you have a monoplane wing, then just plain one span. If you have two wings (biplane) then average the two spans for the Cv formula.

[HELModels]

Thanks Ollie, that makes sense. It also explains why my numbers were on the high end of the Cv range. A 42" wing vs. 30" wing changes the Cv to .034.

Acceptable ranges:

Cv = 0.02 - 0.035
Ch = 0.35 - 0.50

[Griphon]

Average wing chord used in those tail volume coefficient equations, is the just the mean aerodynamic chord.

[HELModels]

Right you are. The part that was confusing was Average Span. Average Span is the total span for a monoplane. Ollie was on the ball.