# I analyzed 2 nearly identical flying wing in XFLR5, how come the one with the thinner tip chord has a higher cD?

I made two flying wings, one with half the wing tip chord as the root chord and one with the root and tip chord being equal.

I then did VLM2 analysis on both at 7 m/s, and used the same airfoil on both. surprisingly, the wing with the thinner wingtip chord had a higher coefficient of drag (which i believe leads to higher drag). How come?

Everything I know leads to less drag, less induced drag because a thinner chord, thinner airfoil due to proportions, and same parasitic drag because everything else is constant. What causes the one with a thinner wingtip to have a higher CD? Does this make sense, or is it just the inaccuracies of trying to calculate CL and CD?

XFLR5 project file

I designed another plane with the same 30 deg sweep, same airfoil and an aspect ratio of 10 (span .5m, root & tip chord .05 m, no twist). Performance keeps getting worse. Only AOA at 0 makes sense to me but that's out the window.

• Are you certain they've been non-dimensionalized properly? Have you tried comparing CL^2 vs CD between the two configs? – JZYL Dec 17 '19 at 0:02
• I can't find cl^2 so I compared Cl^3/2 vs cd (not sure if this is what you were going for), and it look similar, but again the thinner chord tapers off after 2-3 deg AoA. I ran the analysis through 3d panel and LLT analyses and got very similar results. – ohitstarik Dec 17 '19 at 0:11
• What do you mean you can't find it? You just have to plot CL, multiply by itself and plot against CD on Excel – JZYL Dec 17 '19 at 0:13
• Oh duh. completely forgot you could export data for a second. Yeah gotcha, just did it and updated the main post. Cl^2/cd ratio higher now on the main wing. – ohitstarik Dec 17 '19 at 0:32
• Could you share the XFLR5 project file? – AEhere supports Monica Dec 18 '19 at 15:11

In addition to Chris' answer, I have had a look at your project file and could not readily reproduce your problem. I suspect the issue was in a combination of poor discretization (I increased the resolution of the airfoil and wing somewhat) and incorrect methodology (fixed-speed instead of fixed-lift).

Here are the XFLR5 results for fixed-lift tests (using 0.4 kg) on the following designs:

• Baseline: your original wing ($$AR=5.0, \;S=0.05\;m^2, MAC=1.0$$)

• Tapered: your original tapered variant ($$AR=6.67, \;S=0.038\;m^2, MAC=0.78$$)

• Tapered - equalized AR: the tapered variant with the root chord increased to match the aspect ratio of the Baseline ($$AR=5.0, \;S=0.05\;m^2, MAC=1.08$$)

• Tapered - equalized area: the tapered variant with the span extended to match the area of the Baseline ($$AR=8.8, \;S=0.05\;m^2, MAC=0.78$$)

As expected, efficiency gains are proportional to AR increases. You can have a look at the updated project file here (make sure to click Show all Polars).

• This is it i believe, I used fixed speed instead of fixed lift. I still have a couple semesters until my aero classes. I'm guessing we use fixed lift comparing the two airframes because they have their own optimized flight window right? i.e., the higher AR aircraft would want to fly a little faster to perform better, etc. – ohitstarik Dec 24 '19 at 19:26

The way to think about this is that the CD of a wing is going to be strongly dependent on the aspect ratio $$AR$$, the taper ratio $$\lambda_T$$, the chord and twist distribution along the wing, as well as the airfoil distribution. Is the airfoil t/c constant across both wings? That will also have an effect.
• Does your method show the breakdown between viscous drag ($C_{D_{p}}$) and induced drag ($C_{D_{i}}$)? From the screenshot it looks like you're running a viscous analysis (coupled to a vortex lattice?). If you're only looking at $C_{D_{i}}$, then higher AR should give lower drag. If you're including $C_{D_{p}}$ there may be more going on. – Chris Dec 19 '19 at 15:44