Thoughts on Aircraft Design
One of my COVID reads has been GA Airfoils by Harry Riblett. Some people love Riblett’s airfoils. They helped make the Sonerai, BD-5, and Bearhawk become better, safer planes. Some people hate them. There is some concern about their performance at low Reynolds numbers. Either way, there’s a lot to take away from his work.
I don’t have access to a wind tunnel, so I have to do my tests with XFOIL and XFLR5. I keep a Windows machine around just to use SolidWorks, which I also use for XFOIL. The learning curve is incredibly steep. Normally, I use a Mac, so I use XFLR5 more often. Of course, all I can do is click buttons, so the data provided is not 100% realistic.
The Thatcher CX4 I am building has a lot of interesting “features”. I don’t particularly love all of them, and I have a mental list of things I would revise if I build another. One interesting tidbit is that nobody in the builders’ community really knows what the airfoil actually is. I originally read that it utilized a Clark Y, which is not a particularly efficient airfoil. In fact, it’s a terrible, outdated choice from the 1920’s that shouldn’t be used on anything besides a model. Later, I found out that the CX4 uses a NACA 4412, which was famously used on the Aeronca Champ.
In order to not need a wing jig, Dave Thatcher drew a flat bottom on the 4412. It saves a lot of time to build the wing on a flat table without a jig. Unforunately, this decision degrades performance quite noticeably.
Now, using the 4412 is a 12%-thick airfoil with a maximum thickness at 30% of the chord. The equivalent Riblett airfoil is a GA30A-412. Again, using a Riblett airfoil does not magically solve all of your problems. However, it does yield some impressive performance gains.
Later, I considered the flat-bottom modification to the GA30A-412 and had some fascinating results. Instead of a noticeable degradation like on the NACA, performance change was almost negligible! Amazing! This is a fantastic little airfoil, and I wish someone in the Thatcher community would try using this instead. They’d likely see a bit of performance improvements in takeoff, climb, and cruise.
I’m not a fan of Riblett’s constant rambling, whining, and copyright violations in GA Airfoils. On the other hand, some of these airfoils are fantastic choices for GA to adopt.