Some aircraft are simply beautiful.
The sleek lines, graceful transitions, elegant proportions, or raw power captivate many of us at a very young age. We grow up yearning to take flight and idolize those brave aviators that made the impossible possible.
Some aircraft never fly yet steal center stage at any museum they adorn.
Often times we can agree when something is beautiful, but like classical vs impressionist vs post modernist artwork, what makes something beautiful becomes contentious.
Artists and designers throughout history are biased towards the beautiful. While the useful is valuable, the beautiful is venerated. No one is immune to this phenomenon, even shade-tree experimental aircraft component designers such as myself.
“There is no art of any kind without its own philosophy”.
- Frank Lloyd Wright, 1959
If you don't know who Frank Lloyd Wright was, he was an American architect and one of our best.
A structure might be beautiful but it must also be functional, and this complication is precisely why I love architecture. Form and function that is clear in philosophy and execution is beautiful because of its design, not because of its ornaments.
But what happens when the ugly is more useful than the beautiful?
What happens when the design philosophy leads to poor performance?
And what does any of this have to do with Cubs?
Pressure Cowls
Somewhere between the PA-11 and the PA-18, Mr. Piper decided the time had come to stop pushing air through the engine to cool the low-power Lycoming and Contiental engines of the time. Yes indeed, it was time to pull air through the engine.
The new 90HP engines bolted to the nose of the "Super Cub" required greater cooling capacity than the old 65HP J3 engines. Even as early as the 1920's, aircraft designers knew there was much efficiency and speed to be gained with pressure cowl designs. Today this design is applied to most every GA and experimental aircraft on the planet.
Experimental Cub pressure cowl in its natural backcountry habitat, circa 2019.
Pressure cowls are not terribly difficult to fabricate, however they do involve a great deal of imprecise precision in the internal baffling.
The basic design intent is to utilize cowl "cheeks" to create a low pressure area behind the engine. This causes the intake air to be pulled through the nose and out the bottom/back of the engine. The greater the pressure differential between the top and bottom of the engine, the better the results. In practice this design works great, right up until it doesn't.
Colorado Air is Thin
On a hot day at low altitudes on the front range of Colorado, the air is (on average) 25% less dense than at sea level. The rated horsepower of your air cooled engine drops through the floor, and in turn you ask your engine to work harder just to keep your airplane aloft. Higher workloads coupled to less cooling potential (fewer air molecules means less cooling potential) means a cooling design that works at sea level don't work worth a damn in the somewhat-high-country.
Now take your already marginal cooling system up to 11KMSL with a density altitude of 13K and your CHTs will be screaming for mercy. If you think 11K DAs sound high, I live at 9600MSL with summer DAs over 11K MSL on the ground.
Look at the following cowl closely:
There is nothing exceptional nor out of place here and this is a common cowl design used by many Cubs across the country.
On closer inspection however it should become obvious that my lower cowl was full of holes. Holes for air filter, holes for oil coolers, holes for exit ramps, holes for exhaust tubes,...lots and lots of holes. How on earth are we going to create a low pressure zone at the bottom of the motor if we keep opening big holes in the lower cowl?
This design might have worked at sea level, but slow your Cub down with big tires, big gear, gurney flaps, and jump up to 9K DA and things stop cooling. You have an airplane generating high heat but also flying so slowly the marginal pressure cowl can not keep up with the cooling demands.
So, what do we do?
In my case, I initially tried to do more of the same, just better. My hope was to maintain the beautiful, but also attempt to increase the low pressure vacuum at the rear of the engine. In practice this meant I needed to build an entirely new pressure cowl, but this time with less holes and bigger, chubbier cheeks.
The philosophy of this cowl design iteration was simple: larger cheeks (both on the sides and bottom), and only a single sealed hole for the air filter. I call this cowl iteration my "fat guppy phase" as the cowl strayed from the streamlined beautiful path however still attempted to remain true to the pressure cowl intentions.
This design did work better, however it did not work well enough for me to be satisfied. It was also my first iteration of ugly, but since it did lower my CHTs in standard Colorado summer operating temperatures I slowly became more comfortable with ugly-but-useful.
Back to the Drawing Board
I was still not ready to give up on the beautiful, so I modeled out a pressure cowl that was sure to solve all of my problems. Plenums and a pressure cowl!?
Maybe I copy something like this:
...or this:
Fifty hours of 3D modeling later, I had something modeled in digital space that I might be able to produce in composites:
It is definitely slick and beautiful. If I were able to somehow create this in carbon fiber, it sure would be cool. Drunk with design eye candy I started creating full sized molds for the cylinder plenum, but reality soon set in. This project was to be a sizable undertaking requiring a set of skills I did not yet possess.
If only there was a precedent set by some creative old timer to solve the problem of cooling high horsepower engines with airplanes that fly very slowly.
Of course, a precedent was already set and shown to work way back in the 1960s; the banner tow guys figured it out long ago.
Somehow these crafty A&Ps found a way for O320s/O360s to pull huge banners on hot summer days for hours and hours at a time. By absolutely not caring what the airplane looked like, they apparently solved the exact problem I was designing for. The solution seemed simple enough: give the air-cooled engine more air by shoveling it in.
Of course, the same solution was also presented by my STOL hero Frank Knapp.
Frank Knapp and Lil Cub ditched tradition and stripped off anything and everything that didn't help him win. In doing so, he won often and won big.
In my obsession with the beautiful, I was blind to the simplest solution presenting itself this whole time.
Once overcoming this mental hurdle, I was able to 3D print a positive mold and hand layup a fiberglass J3-style cooling eyebrow in a single weekend.
At a later time I could make a proper mold, but my motivation was to iterate quickly and get a first revision built to prove or disprove the old idea. Could J3-style cooling eyebrows cool a 180HP Titan O340? If we are going to fail with this idea, lets fail fast.
At the very least, my design philosophy became greatly reduced:
- Push rather than pull air through the motor
- Strip off all extra complexity & weight
After a bunch of sanding, paint, and fabrication of many 3D printed brackets and wire stays, V1 was ready to test:
Yes. I can hear you now: this could be a lot of extra drag.
And yes, this is quite ugly.
Parasitic Drag
Even before my cowl modifications, my slick-cowled airplane hit a drag wall at ~95MPH. It happily cruised at roughly 75MPH and happily lands in roughly 250' distance (in Colorado elevations, not your pea-soup-atmosphere sea level elevations).
What does an no-cowl-high-drag-nose do to those numbers?
As it turns out, the numbers didn't change much.
I added a bunch of horsepower, removed a bunch of weight, but ended up with pretty much the same cruise speeds. Given the prop and the following graph, this outcome makes perfect sense.
Simply put, the airplane is so darn slow that at 75MPH I am at the low end of the parasitic drag curve even with large cooling eyebrows. Anything above 90MPH might as well be Chuck Yeager speeds, but anything below 80MPH is no big deal no matter how many silly drag-inducing contraptions I bolt to my cylinders or wings.
Who knew?
(We both do, now)
Weight & Complexity
If you have read any other articles on this site, by now you can see I am willing to trade complexity for simplicity and convenience for weight savings. I redesigned a stainless exhaust to save a few pounds and even removed my starter to save a few more pounds.
Just how much did the old nose bowl weigh?
If you zoom in you can see 9.88 pounds is displayed on the pull scale.
How much did all of this extra aluminum cowl weigh?
How much does all of that precision baffling weigh?
The upper cowl, cowl cheeks, and lower cowl weigh 12.4 pounds. Ignoring the baffling material, in total I removed 22 pounds directly off the nose of my airplane and replaced it with under 2 pounds of fiberglass cooling eyebrows.
What would you give up to save 20 pounds off your nose?
All of this is moot if the eyebrows don't work, so...
Do J3-style cooling eyebrows work on a Titan O340 in thin Colorado air?
In short, yes.
I currently have 24 hours of flight testing in temperatures up to 84 degrees Fahrenheit in DAs between 6 and 8K. Break-in CHTs were no higher than expected and are now dropping, as expected. I am able to push the engine as hard (if not harder) than expected, and CHTs can be immediately influenced with mixture or RPM settings.
I absolutely prefer the look of a sleek pressure cowl.
I absolutely believe a pressure cowl can be made to work our extreme Colorado conditions.
That said, the simplicity and weight savings of cooling eyebrows should not be overlooked. Isn't that why we love our dinosaur air-cooled Lycoming/Continental engines in the first place? Simple, lightweight, & reliable.
Are these eyebrows ugly? Yes.
Functional? Absolutely.
I am currently building a proper negative mold that will allow Ted Waltman to help me lay up slick, cool-looking, beautiful carbon fiber cooling eyebrows. Check back soon for updates.
