Archive for the ‘ airfoil ’ Category

High altitude flight Re, new airfoil KS415/14.3

The Reynolds number at very high altitude is very low. Here is an article about airfoil study for 60000 ft altitude flight. My previous airfoils are not very suitable in a small aircraft at 60000 ft, they require longer chord to be efficient. I made series of new airfoils for short chord and high altitude and ended up with the KS415/14.3.

altitude = 20000 m
velocity = 80 m/s
wing chord = 0.8 m (80 cm)
Re = 396331.94
M = 0.2711

Therefore it is beneficial that the airfoil used in this kind of aircraft is such that provides maximum L/D at low Re, here around 400000.

Here are some simulations:

Then some airfoils that I created:


More simulation at low Re, two conditions: 80 m/s at 600000 ft and 111 m/s (400 km/h) at 60000 ft:

Added case 154 m/2 (300 kts) at 60000 ft:

Of these, the KS415 exhibits the lowest drag. Here is the geometry of the KS415:

Here is a smoothed version of KS415/14.3:

And simulation for a Reynolds number range:

KS400 airfoil


KS400 wing at altitude 20 km, speed = 155 kts

Here is the dat-file. Download it here: KS400.dat

Works from Re 500 000 up.

More simulations to follow later.

Why Diamond uses Wortmann FX63-137?

I have been thinking over and over again why Diamond has chosen the Wortmann high lift airfoil FX63-137 on its aircraft. However, I am suspecting what might be the reason (not confirmed though since anybody on Diamond booth e.g. in Oshkosh is usually never able to answer to my questions). Here is my theory about it:
– The FX63-137 has high L/D at fairly high alpha and thus Cl (as the airfoil is such that the Cl rises rapidly as a function of alpha). This is maybe not the best configuration for cruise where a low drag bucket at low Cl is desirable. On an airfoil which has best L/D at low Cl, the climb has more D component (because high lift devices cause drag) and while getting more L with high lift devices. It might be close to the optimal climb optimisation on the chosen aspect ratio on those planes and compromise is drawn to cruise and it is not seen as a bad thing because competition is not faster but usually slower, it does not take so much to win e.g. a C172 in efficiency and speed after all. So it might be that with a lower drag cruise airfoil e.g. DA42NG with the very heavy diesel engines might have somewhat poorer climb rate on single engine situation or it might not climb alltogether if the airfoil was not optimised to provide low drag on high Cl.
– Comparison between the DA40 and Cirrus SR20 kind of potentially shows this: the Diamond shows significantly better climb rates with a quite similar AR and quite similar wing loading (SR20 takes some toll on that, but not that much in comparison if a light loaded SR20 and heavy loaded DA40 is compared), despite of the fact that the SR20 has more sophisticated flaps and the SR20 has 20 hp more engine power available.
– This can be also evidenced on best climb rate speed: with similar wing loading, the best climb rate speed is much higher on the SR20 than it is on the DA40, which partly indicates that the sweet point in the L/D occurs at lower alpha on SR20 than on DA40. SR20 also requires quite accurate angle of attack and thus speed to climb optimally whereas the DA40 is not that critical which would also indicate that the low drag bucket of the FX63-137 is broader than on the (according to UIUC data site) Roncz airfoil on the SR20.

So this is just my home-brewn theory style thinking, is based on collected information and my experience with flying the Diamond DA40, DA42 and Cirrus SR20 and SR22. I might be wrong as always, but here is some food of thought if you have been thinking why there is this airfoil with high L/D at high Cl and the airfoil also has fairly high pitching moment which some find undesirable because of for example trim drag.


You was able to see the polar of KS118 on the previous blog post. Here is the airfoil in question, it is very similar but slightly different from KS125.


Here is a wing simulation with this airfoil:

Comparison between NACA and KS118, 2D-simulation

KS118 Cl-alpha polar, including stall region at 1M, comparison with NACA23012 and 23015 included:

New airfoil KS125


Dat file in QFLR5 format:

Here are the polars:

KSNLF51 airfoil – high L/D, high lift, low to medium Re

I created this airfoil one day. I was looking for getting high L/D at low Re. This is pretty nice. I tried simulations as low as Re 100000-300000 (not included in here, you can try by yourself with QFLR5).

Airfoil KSNLF51:

KSNLF51 L/D graph:

Data file:

Re 1 million, Mach 0.3

Re 3 millions, Mach 0.3

Re 5 millions, Mach 0.3

License: Creative Commons Share-a-like

Comparison between some airfoils:

Some analysis for NLF215F

Above picture as full size 2560×1600

Some NLF215F polars in text format