# Low Speed Airfoil Design Paper

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#### BBerson

##### Light Plane Philosopher
HBA Supporter
Low speed airfoil design paper:
See if this file loads below. What do you think about the claimed "Liebeck" 3.0 max CL?
I think "single element" could mean "no flaps"

edit: neat, I found I could edit the title with thread tools (upper left)

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#### Starflight

##### Well-Known Member
That's funny; I only noticed Cl* = approx. 2.2 (admit to speed reading some chapters). At a wing chord of 1 meter, how fast would you be flying to obtain that 2.2 Cl ?

#### poormansairforce

##### Well-Known Member
If I'm reading it correctly on pg. 17 it refers to a Cp of 3 which is not Cl. The graphs on 24 and 31 show a Cl of 1.8 or a little over. Pg. 27 clarifies it better.

It is confusing as written.

#### fly2kads

##### Well-Known Member
HBA Supporter
My understanding is that Dr. Liebeck designed his series of airfoils originally for high-altitude aircraft, with different design requirements than the typical homebuilt or GA aircraft. For example, the Liebeck airfoil used as an example in the attached paper has a design lift coefficient of 1.8. A typical homebuilt will only spend a few moments of each flight at that condition.

You're close on the definition of a single element airfoil. It's one that is a single, continuous body with no gaps for air to flow through. A foil with a deflected plain flap would still qualify. A foil with a slotted flap or Junkers flap would not.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
If I'm reading it correctly on pg. 17 it refers to a Cp of 3 which is not Cl. The graphs on 24 and 31 show a Cl of 1.8 or a little over. Pg. 27 clarifies it better.

It is confusing as written.
I don't see Cp on page 17 .
Seems to say design lift coefficient is 1.8 and Max lift coefficient would be greater at 3.
Quote: "
"Comparison of these form parameter variations for two very different
"looking" sections clarifies much of the difference in stall behavior between the sections. On the Liebeck section, as angle of attack is increased beyond the "design" value (design lift coefficient equal to 1.8), the recovery region form parameter level is shifted progressively upward until a value of approximately 3.0 is reached, at which point turbulent separation begins".

#### Victor Bravo

##### Well-Known Member
HBA Supporter
The Liebeck airfoil earned a very bad rap on the small ultralight glider "Minibat". I don't have the formal education to discuss why this was, or what exactly was wrong with it, but it is reasonably well known that these exotic looking airfoils were very problematical

#### poormansairforce

##### Well-Known Member
I don't see Cp on page 17 .
Seems to say design lift coefficient is 1.8 and Max lift coefficient would be greater at 3.
Quote: "
"Comparison of these form parameter variations for two very different
"looking" sections clarifies much of the difference in stall behavior between the sections. On the Liebeck section, as angle of attack is increased beyond the "design" value (design lift coefficient equal to 1.8), the recovery region form parameter level is shifted progressively upward until a value of approximately 3.0 is reached, at which point turbulent separation begins".
No, it doesn't say Cp but I suggested that it was referring to that since it was about pressure recovery. That's where pg 27 comes into play where it shows the upper surface Cp rising to above 3. And since we are talking about the upper surface it's a negative number. Also, it says right there on that graph that Cl is 1.8.

#### poormansairforce

##### Well-Known Member
The Liebeck airfoil earned a very bad rap on the small ultralight glider "Minibat". I don't have the formal education to discuss why this was, or what exactly was wrong with it, but it is reasonably well known that these exotic looking airfoils were very problematical

Horrible stall properties.

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#### BBerson

##### Light Plane Philosopher
HBA Supporter
Yeah, a bit higher CL is worthless if the stall is cliff like straight down, as shown.
I guess nothing useful in the paper.

#### tdfsks

##### Well-Known Member
The Liebeck airfoils are not practical. If you find and read his original papers you will quickly see why. First the optimisation method that be uses imposes a single design point so the airfoil is optimised for one flight condition. Second, and more importantly, the high lift is achieved by designing a section with the shortest possible pressure recovery region on the upper surface and this implies that the boundary layer is on the verge of separation over the entire length of the pressure recovery. A small disturance can thus cause the sudden separation of the flow as opposed to a more progressive separation on a more conservative design. This behaviour was quickly discovered when these sections were used on the wings of racing cars .... at critical points turbulence or cross flows would cause sudden separation, instant loss of dowforce and a spin into the wall. Yes the airfoils produce the high CLs claimed in a wind tunnel, but their performance is too unpredictable to be of practical use.
There are later papers in which attemps are made to design airfoils that are optimised for mutliple design points, which are generally more practical ....

#### pictsidhe

##### Well-Known Member
The Liebecks do indeed have spectacular lift. But they also have a spectacular stall. I got hald way through the paper, it is interesting to me.
The SWIFT foot launched gliders have a laminar rooftop airfoil that has a lot of lift. I failed to find anyone complaining about its stall. I have coordinates somewhere.

#### poormansairforce

##### Well-Known Member
The interesting thing for me was that the profile is kind of like one half of an ice cream cone airfoil used in aerobatics. The ability to command a stall of an entire wing panel is useful in that environment!

#### poormansairforce

##### Well-Known Member
The SWIFT foot launched gliders have a laminar rooftop airfoil that has a lot of lift. I failed to find anyone complaining about its stall. I have coordinates somewhere.
Could it be that the swept wing tames it down?

#### plncraze

##### Well-Known Member
HBA Supporter
My guess is the second cannot generate enough moment to get it to stall.

#### tdfsks

##### Well-Known Member
The SWIFT foot launched gliders have a laminar rooftop airfoil that has a lot of lift. I failed to find anyone complaining about its stall. I have coordinates somewhere.
There is a paper by Richard Howard on the design of that section. Basically it is a much more conservative design in terms of the pressure recovery and has more practical design constraints imposed. Its CLmax is more like 1.7 compared to approx 2.3 for the Liebeck L1003.

Also we should remember that the Liebeck section was the product of a research project to design a single element airfoil with the maximum possible lift. It was never intended.

#### Retiree

##### Well-Known Member
Thanks for posting this reference, it looks like it has a lot of interesting info.
Doug

#### Tiger Tim

##### Well-Known Member
All technical stuff aside, the page that gives the comparison of the physical shapes of each airfoil really surprised me with how normal MacCready's airfoil was. Maybe there's some voodoo going on when you look at it up close but at first glance it's not far removed from what I've always used on stick and tissue models.