Directory Suggestion

In your Homebuilt Aircraft Directory [December 2015], there are kits and plans that are no longer available. Why do you include them?

—Yvon Nowlan

There are many discontinued airplanes being built and flying. Readers are as likely to be shopping for them as they are for a new start. Including these planes allows potential buyers and builders to learn about these airplanes and projects, and compare them to those that are currently available. To help avoid confusion, we color-code the discontinued designs in red. —Ed.

Vortex Generators

I just read Mr. Ullman’s “Using Vortex Generators to Enhance Pusher Aircraft Cooling” [December 2015]. I always understood that a necessary predicate to proper NACA duct efficiency was a specific radius of the top edge of the intake opening (aka as the intake bar), and the intake’s overall placement in a position where the cross-sectional area was increasing. From the pictures included in the article, it does not appear that either of these conditions were met. The vortex generators would appear to be a draggy means to mimic the benefit that comes with proper location.

Kudos to Mr. Ullman’s efforts, but not mentioning these basic points may give rise to their repetition by others.

—Marc Bourget

David Ullman responds: Thank you for the comments. Before I tried my experiments, I put as generous a radius as possible on the top edge, as I read the original NACA reports before I began. The space is limited by the kit. Do realize that the kit fixes the position of the scoops and the space available for the radius. Also, many canards have NACA scoops in the same location. They arrived there after much trial and error of cooling scoops on the armpits and belly (all before my time). My effort was to improve what was there, not redesign the cooling system. I too don’t think putting the NACA scoops on the top of the fuselage in a low pressure area is a good idea, but that is where they are.

As far as draggy, I could not measure any change in speed. Since most of the VG is buried in the thick boundary layer, I don’t suspect they add much drag.


As I read “Completions” in the January 2016 issue, I was somewhat creeped out to see Bob Alexander’s RV-7 featured. Bob and his plane crashed in January 2015. Obviously you didn’t know this or you would not have published his plane. My thought is this: Maybe make a quick phone call to check on the status of the pilot and plane before going to print to avoid distress with family and friends? Hopefully they didn’t see the magazine.

—Mark Donahue

You are right—this is disquieting, and we sincerely apologize for any discomfort this has caused his friends and family.

Because we don’t run “Completions” every month, we tend to stockpile the submissions and use them in the order they are received (since we like to be fair to everyone). This is a rare occurrence where that clearly bit us. I do remember seeing the news of Bob’s crash when it occurred, but simply did not put the two pieces together when we published.—Ed.

Write to [email protected].


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