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What is the most efficient symmetrical airfoil shape to make for a covering for a round upright bar that measures 40mm outside diameter?
The speed of travel will be 25Kph and I also need a shape to travel at 30Kph. I assume these will be different to some extent?
Truncation (chopping off the rear portion of the airfoil) does not reduce drag, although reducing up to 20% of the chord does not significantly increase it. As Peter says, "trade off between more separation and more wetted surface area. Hence "boat tail" bullets.
The 3:1 ratio of chord to width is apparently a bike racing specification. Nothing new here, the full airfoil produces the lowest drag. Hence the cone they put on the back of the Space Shuttle while it was piggybacked on its 747.
What they did to meet the 3:1 specification was to truncate 66% of a 9 to 1 airfoil. This produces the lowest drag within the specification.
The Kamm tail shape was designed for automobiles, providing a weight savings (as racers constantly accelerate and deccelerate) and to make the vehicle a little shorter, which can help changing lanes in a crowd. Two different applications. But a fully symmetrical airfoil, such as the struts of a Cessna 172 will do the job.
For an aircraft strut, if strength allows, make it as long and thin as possible until drag values begin to rise. 9 to 1 rings a bell from boat hull design, and would be a good place to start.
This site discusses tube width and length ratios for least drag
https://bikerumor.com/2011/04/14/scott-f01l-aka-f01-finally-released-to-the-public/
https://www.bikerumor.com/wp-content/uploads/2011/04/Airflow-tubing-visual.png
Tyler just had a chance to speak directly with Scott regarding the final profile, and reported back that they found that a 9:1 ratio profile with a truncated tail was indeed the fastest, lightest, and most stiff. According to Scott, Scott Aero Science engineers began with various tubing shapes first in CFdesign programs to identify shapes before producing prototypes that would then be validated in the wind tunnel. Over 100 hours of wind tunnel validation time was invested solely in the F01 project using Drag2Zero facilities in Mercedes-Benz Grand Prix Wind Tunnel. Starting with raw tubes, shapes were identified and built into complete test mules, and then analyzed against current competitor designs. Over 60 tube shapes were tested in order to identify the optimum “cut ratio”. The engineers selected tube shapes that are wider than normal NACA aero tubes. They were designed with a “leading edge” and a ratio of height to width that complies with UCI regulations, while still maintaining the lowest air disturbance vis-àvis both individual tubes and the overall frame structure. The ratios of the aggregate tubes are between 6:1 and 12:1 resulting in a 9:1 average ratio, well within the UCI limits and actually going the opposite direction in relation to other manufacturers in this regard. The result is a virtual tube shape, a tube that is not NACA shaped but a truncated cross section, which acts in the same way. Each and every tube in the frameset is analyzed for cut position along the “chord”, or length of the tube profile. Most tubes are cut retaining between 25-35% of the overall chord length. This science of tube shape is a contrast to the Kamm tail designs that emerged 80 years ago in the auto industry in that we use the leading edge of the tube rather than simply cutting the trailing edge like some competitors practice.
Here's another site discussing the same shape. https://www.trekbikes.com/us/en_US/inside_trek/kammtail_virtual_foil/
