# What is the most efficient symmetrical airfoil shape?

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?

• Hello JJayes, welcome to aviation.stackexchange. How do you measure efficiency? Lowest weight? Lowest weight / lift ratio? Highest lift / drag ratio? Lowest drag at 0 lift? in other words, what are you trying to achieve? Jan 4, 2019 at 15:06
• Do you actually want to generate lift with this airfoil or do you just want to minimize drag? Jan 4, 2019 at 15:09
• Hi, I am looking for a tube to be shaped to pass through air in a upright position while travelling at both 25kph and another st 30kph. I do not require any lift. The tube needs to be 40mm in width. I need it to provide the least possible drag. The weight is not a issue. Jan 5, 2019 at 19:36

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 does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review Feb 4, 2019 at 5:41
• 9 to 1 ratio thickness to chord added. Wind tunnel test for specific application may be best. Aug 23, 2022 at 10:44

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/