ORIGINAL: adam_one
I only wanted to know if the chart shown, is a rendition of someones actual testing .
I like good, qualified , simple charts -
Is it a provable piece of work?
Y/N
Thank you
Dick,
The chart was "borrowed" from an article included in the last issue (no. 10, Sept 04) of the Fly RC Magazine.
The article's title is just: "Optimizing airfoils for small RC Models" based on Jef Raskin's research (
[email protected]).
The chart is missing a curve labeled "thin airfoil". A properly selected thin airfoil, when compared to a plate of the same thickness, would have a higher maximum CL. I assume that phenomenon being represented in the chart is that, for low Reynold's numbers, a thick airfoil would require a pressure recovery that is not feasible, which would limit the maximum CL. An airfoil of the same thickness as the plate would probably not suffer from this problem.
Banktoturn,
It is stated in the article that the chart is to give a feel for how different kind of airfoils' lift changes dramatically at different Re's.
At very low Re <5000 a flat plate is best (usch), at higher Re a curved plate is best and somewhere between Re 50,000 and Re 100,000 conventional airfoils become superior.
As for how much thin is thin, one may assume that thickness between 1% to 3% of the chord may be considered a thin airfoil / plate.
The author has also come to the conclusion that an airfoil known as "44" was the best choice to his MiniStick (Bob Selman Designs with Re = 21,000).
After he has replaced the original 6% camber airfoil with his "44", which has 4% camber made of a curved sheet foam, the plane climbed faster, had better glide, penetrated the wind more effectively and maintained the altitude at a noticeable lower throttle setting (half throttle instead of 3/4 as with the original 6% camber).
The "44" airfoil has 4% camber made by two arc circles where the max thickness is at 40% from LE.
More details in the actual article...
adam_one,
I should correct my 'definitions' from the previous post. There are three variables, or wing characteristics, that are relevant in this discussion. They are maximum thickness, chordwise thickness distribution, and camber. A section with a small value of maximum thickness (a relative characteristic) is 'thin'. A section with a constant thickness along the chord is what we have been calling a 'plate'. A section with zero camber could be called 'flat', although this term seems natural only for a plate, since a traditional section with no camber does not look flat. The terms 'thin', 'plate', and 'flat' are exclusive, and should not be confused. I think that much of the confusion on this topic results from interchanging these terms.
I take issue with the claim that 'flat plates are best' for low Reynold's numbers. Thinner sections (smaller maximum thickness) tend to be better for low Reynold's numbers, but for any given Reynold's number, the optimum section (say, for maximum CL) is not a plate, even if it may be thin, nor is it flat. I don't have access to the text of the article from which you borrowed the chart, but it sounds as though the '44' section is neither flat nor a plate. I would contend that a plate is seldom if ever best, aerodynamically, although it may be good enough that its simplicity makes it the 'best' choice for some applications.
banktoturn