Cascading Wings for Ultralights: Do they really increase lift by 333%?!?!?
The classic NACA 650-18 airfoil has a Clmax of 1.5 according to airfoiltools.com ( a modified 650-18 is used on the Zenith 701).
Add non-slatted flaps, and the Clmax jumps to about 2.2.
What if one arranges the wing in a cascading fashion as shown below? Javafoil shows the Clmax jumping to 5, that’s an increase of 333% over the plain wing!!!
Not by just adding the same planform, but dividing the original chord by 4, so the a/r ratio is 4 times the original....
So that the quadplane is 400% more efficient than the original monowing, less a quadplane inefficiency of about 50%, so that the quadplane is still 200% more efficient than the monowing(maybe).
This probably only works ( if at all ) for very slow speeds (Re 300-800k) where the form drag and induced drag is very low.
I understand Javafoil uses Xfoil, which I’m told is a robust airfoil performance prediction program and is accurate up to an AOA of +/-20 deg.
Alternately, I’m told the VLM method (XLFR) is good up to only a Clmax of 1.5, +/- 20 deg.
So…. the million dollar question is… is a Clmax of 5 realistic, or is this beyond the scope of Javafoil/xfoil, even though the AOA is only about 8 deg….?
Ultralight Design Data:
See charts and tables below.
Flaps: 20% chord, 50 deg deflection
Aspect Ratio: 7
Atmospheric conditions: Standard
Wing configuration: see chart below
Therefore biplane ( not multiplane) lift efficiency as per the table below should be: .26/.34=76% efficiency vs mono wing (Is this correct?) ( ignore the big green arrow, it's from the source I found the table)
Reynolds #: 880k
is there a glitch in the software as per the comment below:
and the Clmax is about: climax 5 / 4 wings = 1.25
given the Clmax for a monowing is 2.2, and a biplane at 50% gap is 22% less efficient, a quadplane with 4 wings can easily be 40-50% less efficient, giving a clmax of about 1.3, which is close to the clmax of 1.25 above.
So what is the likely Clmax, 5 or 1.25?