undercambered vs flat bottomed wing
 

Does anyone know how a under cambered wing compares to a flat bottom wing in flight?? Specifically a slow flying bibe.

Airfoil Comparison
 

The most meaningful way to characterize an airfoil is by its mean camber line and maximum thickness, not by flat bottomed or undercambered. Draw the chord line from the nose of the leading edge to the trailing edge. Then draw a line that is half way between the upper and lower surfaces of the airfoil. The maximum distance of the mean camber line from the chord line, expressed as a percent of the chord length is a meaningful number.

The next most meaningful number is the maximum thickness expressed as a percent of the chord.

These two numbers are encoded in the NACA four digit designations, precisely because they are the most meaningful way of briefly describing an airfoil.

undercambered vs flat bottomed wing
 

An undercambered airfoil provides a higher coefficient of lift for a given angle of attack than a flat-bottom airfoil in most cases. The penalty, however, is in penetration. Since an undercambered airfoil achieves a higher CL at the cost of a higher CD, the flat-bottom airfoil will have an advantage in penetrating the wind.

The undercambered foil is what is often called a "one speed airfoil', and a flat-bottom or semi-symmetrical foil is more adept at cutting through the air with less drag. Look at the polars for each airfoil type and you will see the relationship between camber and thickness, as Ollie outlined. You will see that for less camber you get better penetration, but you will also get a lesser lift coefficient. Everything is a trade-off in airfoil selection, and undercambered airfoils are the preferred choice for slow airplanes that need copious amounts of lift without regard to the associated drag. In other words, SLOW. Like doing all your flying with the flaps down.

undercambered vs flat bottomed wing
 

Yesterday we were flying in 15 MPH winds. I was flying with a wing of 8% thickness and 3% mean camber (undercamber). Some of my flying buddies were flying planes with flat bottomed airfoils of 12% thickness and about 4% camber. All the planes had about the same wing loading of 8 ounces per square foot. My plane was able to penetrate noticable better than the others.

I maintain that you can't generalize on the basis of flat bottomed versus undercambered.

undercambered vs flat bottomed wing
 

Ollie,

Point well taken. I was generalizing like crazy with my last post, for the sake of simplicity. Truth is, the terms "undercamber" and "flat bottom" are not very clear and well-defined.

When I think of an undercambered airfoil, I think of something like the NACA 6409 or one of the old Benedek airfoils of free-flight fame. But if you examine a foil like the Wortmann FX60-100 (everyone sent scrambling now for their copy of SOARTECH 8) or the Eppler E387, there is a noticeable cusp to the underside near the trailing edge. Undercambered? By definition, yes, but by commonly-understood behavior, decidedly no.

Someone might argue that the Clark Y is a good example of a flat-bottom airfoil, but it has a convex underside, as does the SD3021. To build a wing flat on the table using one of these foils, you gotta use tab feet or a jig. The full-size Piper Cub used the Clark Y, most models of the Cub use a modified Clark Y, with the bottom surface perfectly flat from trailing edge to leading edge radius-- a true 'flat bottom'.

That said, the characteristic shape of a given airfoil does have an effect on Cl and Cd, in addition to the camber and thickness values. The polars for two airfoils of identical camber and thickness, but with different contours, will reflect this difference, primarily at high alpha, where the boundary layer is most stressed.

The bottom line is that trying to generalize about flat-bottom airfoils or undercambered airfoils is like trying to generalize about Republicans and Democrats. Works if done loosely, but not if specifics are addressed.

undercambered vs flat bottomed wing
 

I have a Fly Baby 72 that has undercambered wings. This thing absolutely refuses to land. I mean it floats and floats. The usual nervousness in landing is not there. I mean, I have to force this plane to land. I sometimes circle the landing strip 3-4 times to lose enough airspeed to drop down. I can take it up and thermal for about 2-3 hours if I want to. If that is what you're looking for, definitely go undercambered.

VP

Airfoil Qualities
 

Don,
The truth, or as close to it as we can get, is in Soartech #8, Summary of Low-Speed Airfoil Data, X-Foil, etc. as you well point out. The truth is in the numbers! Trying to deal with the truth through qualitative descriptions is bound to distort the truth. The frustration is in trying to explain the truth to anyone who can't or won't read the numbers or who doesn't understand them after they read the numbers. How far should we go in distorting the truth to popularize the information? As the King of Siam said,"It is a puzzlement!"

undercambered vs flat bottomed wing
 

First, thanks for all the response........Ollie, If I were to build an airplane with a under cambered wing, could I convert it to flat bottom by just filling in the area between the leading and trailing edges??????? Seems for a wider flight envelope this mod would probably be desired.

Undercamber Conversion
 

Skip,
Yes, you can convert an under cambered airfoil to a flat bottomed airfoil and it will probably extend the speed range to a higher range. However, in thickening the airfoil you probably also will increase the thrust required to drive the airfoil at higher speeds. The drag force increases as the square of the airspeed, other things being equal. If you would measure the airfoil mean camber, thickness and nose radius I would stop hedging my answer.

Please, oh, please, give me the numbers instead of the qualitative description. Get out your ruler and measure the blankety, blank thing. If that's too much trouble, send me an accurate drawing of the airfoil and I will measure it for you. If you haven't got a specific airfoil yet, it is too early to be asking questions about it.

undercambered vs flat bottomed wing
 

Ollie, I currently fly a 1/4 scale pup....I have got the itch to build Balsa USA 1/3 scale pup......the 1/3 scale utilizes a under camber, and as you can tell I have no experience with this airfoil. Also I don't have numbers for this wing.....Just wondering if I should consider modifying the wing for up close and slow maneuvering.

undercambered vs flat bottomed wing
 

Skip,
Since you put it that way, I don't know.

Sopwith Pup
 

I went to the Balsa USA web site and found that the 1/3 scale Pup is a Sopwith Pup not a Buhl Pup. The description of the model strongly implies that the airfoil is scale or scale-like.

I checked some pictures of the full scale 1916 Sopwith Pup and estimated that the airfoil is thin at about 5 or 6% thickness and the undercamber is not extreme with a mean camber of about 4% or so.

The high speed of the model will be mainly limited by the parasitic drag of the struts and wires and the interference drag of the biplane configuration rather than by the profile drag differences of the airfoils under consideration. The drag of an open cockpit, cowled rotary engine and landing gear will further dilute the effect of airfoil drag differences.

It will take twice as much thrust to increase the top speed by as much as 40%.

The low speed end of the speed range will be determined by the low wing loading of less than 16 ounces per square foot and the maximum coefficient of lift of the airfoil. The stock airfoil will undoubtedly have a lower stall speed than a flat bottomed replacement. The relatively low wing loading and good maximum lift coefficient will allow the model to turn very tightly.

BTW, the top speed of the full scale Sopwith Pup was 111 MPH so the top speed of a 1/3 scale model should be 37 MPH.

Airfoil for Pup
 

The AG17 airfoil is thin and undercambered like the airfoil of the full scale Sopwith Pup. The AG17 has a very low drag coefficient which is minimum (Cd<0.007) at a lift coefficient of 0.12. This airfoil would tend to minimize the power required to push the Pup to the top speed you desire. The coordinates and polars can be found at:
http://www.charlesriverrc.org/articl..._allegro2m.htm