Letters: September 2015

Which Aircraft Paint?

We are looking at having some military aircraft painted by a company out here in California. They have all the Mil Specs for the color we selected, however, I started receiving phone calls from the various paint manufacturers wanting us to select their brand of paint.

As a military aviator, I never thought about who supplied the paint on the aircraft I fly, as long as it looked good and wasn’t peeling and fading. Can Aviation Consumer provide me with some counsel on the pros and cons inherent in the different paint manufacturers, including PPG, Sherwin-Williams and Dupont? Do you guys have a preference?

Richard C. Bebb
CW4, U.S. Army

We ran this difficult question by Craig Barnett at Scheme Designers, our go-to for all things having to do with aircraft paint work. Barnett said the answer is easy if you select an experienced shop that specializes in painting aircraft and uses aviation-specific brands of paint (as opposed to vehicle paints or other industrial paints.) Aviation paints are more flexible for sustaining airframe vibration, better designed to cope with large temperature changes without cracking and are designed to be resistant to aviation fluids, including dyes, fuel and hydraulic oil.

As for selecting a paint manufacturer, once you pick the quality shop, stick with its preferred brand of paint. Every brand has different preferred techniques for spraying, as we’ll as desired environmental controls (temperature and humidity, to name a couple) for the best results. Drying times and the look of the finish varies.

Some paints provide an intentional textured finish, which may even appear like subtle orange peel, while others provide deep reflective finishes.

Barnett said most of the major paint manufacturers to include Sherwin Williams, PPG, Akzo Nobel and Axalta (Imron) make an excellent product, based on his many years of experience.

As was evident from the paint shop survey in our June 2014 issue, the final product, its longevity and finish quality is not only a function of the paint that’s used, but is strongly influenced by the quality of the work during the paint job. Bad preparation, incompatible materials—such as cheap primers not designed to work with the paint brand being used—and poor climate control may result in failed paint jobs, no matter what brand of paint is used.

Propeller Lessons

Nice composite propeller article by Rick Durden in the August 2015 issue of Aviation Consumer. Where I get puzzled, however, is when the manufacturers and writers don’t discuss the reasons why the three-blade props didn’t go any faster than their two-blade counterparts.

To those of us who went through aircraft design class in aerospace engineering, it’s a no-brainer. The propeller with three blades has more wetted area—hence, more drag—than its two-blade counterpart. More drag means that more energy is wasted pulling that extra blade though the air, thereby reducing blade efficiency (denoted as Greek letter eta). Eta, by the way, is the ratio of power delivered to the airstream divided by the power produced by the engine, typically between 82 to 85 percent. Constant-speed propellers keep eta at this efficiency level at pretty much all cruise speeds.

Going to the scimitar shape did increase eta a bit, however, because the third blade still has more wetted area, which most likely bumped eta back down to where it was before the third blade was added. On the other hand, if the scimitar shape were to find its way into a two-blade design, there would be even more power available. Maybe eta would get as high as 90 percent. Oh, now we’re getting somewhere, but that’s not going to happen, is it? There is your engineering lesson of the month.

David Rosing
via email

That’s good stuff, David. When we looked at the pre-scimitar props, airplanes with engines of less than 230 HP lost some cruise speed going from two- to three-blade props. Above that, the speed was a wash—no gain, no loss.

low-cost backup efis

I’m planning a glass cockpit retrofit and would like to install an electronic attitude indicator with airspeed and altitude tapes, plus battery backup. The L3 Trilogy, Aspen backup and Mid Continent SAM are out of my price range.

I recall reading in Aviation Consumer and AVweb.com there was a company developing an all-in-one EFIS for under $4000. All the unit needed was a electrical connection and a pitot static connections to it. Which unit is that?

Greg Wroclawski
via email

The unit is the $3595 Sandia Aerospace SAI-340, and it’s still snagged in the certification process, Greg. We check in with Sandia monthly hoping to get our paws on one for a review, but it’s just not ready yet. When it is, we’ll publish a full report.