5

I am getting very confused with these terms. I'm trying to analyse a span loading distribution given as cl(y)c(y)/ccL. This distribution is almost elliptical but when I plot just cl(y) it changes and I have more cl near the tips due to the reduction in chord.

This made sense from my point how view since the AR is low, the sweep is high and the taper ratio is small. It is like the figures I remember for these cases. However, when I have searched in google I have seen this image: cl(y)*c(y)/c*cL distribution

This wing is pretty similar to mine, and the distribution is similar to my cl(y) distribution. However, the parameter here is cl(y)c(y)/ccL!!

I don't understand completely the difference. When we talk about elliptical distribution for minimum induced drag it is about cl(y) or cl(y)c(y)/ccL? Why is this parameter used and plot and not cl(y)?

JGG
  • 341
  • 3
  • 6

1 Answers1

7

Minimum induced drag is reached when the circulation distribution over span is elliptical. Circulation is indeed proportional to the product of the local lift coefficient and the local chord (c$_l \cdot$c). The diagram in your question normalizes this with the overall lift coefficient c$_L$ and the mean chord $\overline{\text{c}}$.

But this is only true if you disregard the structural weight of the wing. Using less load near the tips will reduce the wing root bending moment and, consequently, wing mass, so less lift and wing area are needed in order to carry the same non-lifting load. Even though your drag coeffcient might be higher, your overall drag will be lower.

The optimum will also depend on the size of the wing. Small wings should indeed use an elliptical circulation distribution because their structural weight is low, but in large airplanes a more triangular distribution will be better.

One last word: Please try to avoid the high loads near the wingtips if you want a nice, docile stall behavior.

Peter Kämpf
  • 231,832
  • 17
  • 588
  • 929
  • Thank you so much for your answer! However, I don't fully understand it. As far as I know, L(y)=rhovcirculation(y). Therefore, if you have an elliptical circulation distribution you should have an elliptical lift distribution. This would explain why people use these terms indistinctively.

    With this statement I can't see how lift distribution and circulation distribution could be different in shape. Also, I can't understand your statement "Circulation is indeed proportional to the product of the local lift coefficient and the local chord (cl⋅c)".

    Thank you again!

     – JGG Jan 29 '18 at 08:20
  • Just in case someone have the same question some day: of course cl(y)c(y) is proportional to the circulation. cl(y) is normalized with c(y) and therefore cl(y)*c(y) is l(y)/q. Lift and circulation distributions have of course the same shape. This must not be confused with cl(y) distribution. The graphs show the effect of some parameters but this cl(y)c(y) distributions can be changed with twist. If you have an elliptical distribution of lift in a wing with small taper ratio and calculate cl(y), it will have a similar shape to that cl(y)c(y) distribution but that is not the load distribution! – JGG Jan 30 '18 at 00:23
  • @JavierGarcia: Is there anything still unclear? Should I expand the answer? Let me know if you think it can be improved. – Peter Kämpf Jan 30 '18 at 09:06
  • No, everything is clear now! Thank you! – JGG Jan 30 '18 at 21:53