Wednesday, October 8, 2008

Wing droops on laminar flow section

If you have wondered why Cirrus has the discontinuity on the wings. This may answer to that to some extent. I have not found any factual information about the airfoil section used on the Cirrus other than that it is a natural laminar flow section. Cirrus VK-30 used the Jeff Viken NLF414F airfoil. I don't know if the SR20/SR22 uses the same airfoil or a different NLF section.

Anyway in this NASA tech paper it is explained how the stall resistance can be made better with the wing droop. The wing droop on the NASA test C210 actually indeed resembles the discontinuity on the Cirrus SR20/SR22 wing. Please have a look:
Wind tunnel results of the low-speed NLF(1)-0414F airfoil

Notable thing is that the Vmax-probe did not have this wing droop or any other means to prevent tip stall. And it crashed on landing possibly according to NTSB report and Bruce Carmichael's book, because of unfavorable stalling charasteristics at low Re of the airfoil caused a hard landing (which the pilot did not survive). NLF414F is not to be used without some means to prevent tip stall and to soften the otherwise very sharp stall at low Re.


Allen said...

I asked the tour guide at the Cirrus factory about this "dogtooth" in the Cirrus wing. This is a safety feature. The outer wing, beyond the dogtooth stalls before the inner wing stalls. During a partial stall the outer wing stalls, but the inner wing with the ailerons can control the plane.

John said...

I just finished reading the Nasa white paper on spin resistance and came away with the exact opposite conclusion, that is that the outer drooped section resists stalling.

"the increase in lift-curve slope beyond stall was caused by the fact that the outer wing panel continued to produce lift to extreme angles of attack".

Is this right? I am coming at this as a potential RC beginner and was researching removable droops on the outer leading edges of the wings of some trainers.

Thanks, John

Karoliina Salminen said...

John & Allen:

John, you are right. Allen, you are thinking backwards.

The outer wing stall should be always prevented, and the inner wing should always stall first, otherwise the aircraft enters spin when stall occurs. Cirrus does not do that, and evidently the outer wing stalls after the inner wing, and the ailerons in the _outer_wing_ continue to be effective through the stall, possibly because of the wing cuff in the outer wing.

Wing droop is one of the ways to prevent tip stall. There are also other ways to accomplish that, e.g. wing twist and using a different profile at tip and also using less taper, but according to the NASA paper, the droop looks like a solution which does not cause that much penalty. Excessive twist in wing reduces efficiency significantly as the outboard wing is not producing lift in cruise, but is only producing drag.