Leading edge aeronautical research on the plains

And on the planes

You don’t think of leading edge aeronautical research being conducted in the General Aviation industry, especially in Wichita, Kansas. But Cessna did just that, in the early 1950s, and on its own planes.

And not only was it successful, but it was incorporated in some famous long run production airplanes–unfortunately, not Cessna airplanes. And what the other people’s production airplanes used was somewhat, but not exactly, the kind of things we researched at Cessna. As will be seen the whole story is full of such intrigue.

boundary layer control diagram
A 1956 Cessna report explained the concept of boundary layer control (BLC).

It began in Germany in the WWII era, during which one of Germany’s companies did some pioneering work with a type of boundary layer control (blc) effect. The company was named Arado, and they intended to apply the concept to improve the low speed performance on one of their military transport airplanes–to shorten takeoff and landing distances by getting more lift and reducing stall speeds. Thus the concept was popularly called the Arado blc system, which increased lift by delaying airfoil stall by a combination of 1) sucking in the stagnant boundary layer in the inboard wing flap area, and then by 2) blowing energized free stream air over symmetric downward deflected ailerons (on right and left wings acting in unison to increase lift) and maybe short “outboard” flaps, too.

The unique thing was they used a rocket-like jet pump located about midspan inside each wing to suck in the inboard air and induce added energy by entraining it with the jet’s exhaust, and direct that same (now higher speed) air over the outboard trailing edge devices. The end of the war terminated their development effort, without getting to a production application.

So how did that effort migrate to Wichita, Kansas? By Wichita University (now Wichita State University) contracting to translate “captured” German technical documents into English for the Federal government–and some of them were about the Arado blc system.

That information caught the eye of the Dean of its School of Engineering, who sometimes served as a consultant to Cessna, with which he coordinated to offer to do further research on the concept for the Office of Naval Research (ONR), which bought into the idea. And they convinced the Army Transportation Corps to help support the endeavors.

So both the University and Cessna created absolutely new research departments to contract for and carry out the projects. The University would conduct extensive and exploratory wind tunnel tests and Cessna would use the results to modify and test fly a couple of airplanes to provide real life confirmation. The wind tunnel tests included a fairly large model of one of Cessna’s airplane’s wing incorporating blc and, quite differently, a complete model of a multi-engine transport airplane designed with the idea of using the blc concept (as improved by the new research) in the tunnel tests.

The University bolstered their research by employing two German engineer-scientists, who had had experience with the Arado development, brought to the U.S. under the government’s “Paperclip” project–the same one that brought Werner Von Braun to this country to help with, or was it lead, our early space endeavors.

Cessna looked overseas, too, by hiring an escapee from Communist Russia–but he got out in 1917, came to the United States, earned a Master’s Degree in Aeronautical Engineering from Michigan University, and founded Wichita University’s Aero Engineering Department in the late 1920s.

Cessna’s later CEO, Dwayne Wallace, and several of his student colleagues and later company officers, were taught by that Russian gentleman, Alex Petroff. The Engineering School was closed down during the Depression, so Alex had gone on to work for other airplane companies, but his former students then hired him in the 50s to come and head up Cessna’s new Research Department.

Cessna’s effort on the ONR contract was more diverse than the University’s. The Cessna Research Department was the intermediary between the University group and the rest of the company and had both drawing board people and technical people on staff to do this, but relied on company board designers to provide the actual detailed modifications to the airplanes, and our own Flight Test, Aerodynamics and Preliminary Design Group to do the flight test program. The new Research Group was co-located with our FT, A & PD Group in the back of the Experimental Hangar.

Cessna 170
Cessna tested their “boundary layer control system” on the 170.

The airplanes utilized were, in sequence, a modified Model 170 and a modified Model L-19, it with a more “powerful” flap system (and a new squared off tail). Let it be known that these were modified to be research airplanes only, and despite what some people conjecture, with no serious thought that these mods might be incorporated into these or other Cessna production models.

One departure from our flight test group’s normal procedures was that one of my usual jobs, flight test observer, couldn’t be conducted because the airplanes were filled (in space and weight) with the components that powered, conducted and controlled the blc and with some unusual, for us, instrumentation carried. I was, however, able to do some flight test engineer duties, which included a little aerodynamics, on the project.

Putting the models used in perspective, and knowing that our development model numbers were in chronological order, the blc modified 170 was our Model 309 (initiated just ahead of the Model 310 twin) and the modified L-19 our Model 319 (begun just after our Model 318, later to become the USAF’s jet T-37). The 309 initially used the true Arado system, with a chemically powered jet pump in the wings. It was abandoned fairly early for a couple of reasons. The entrained blc air was so hot it expanded the aileron metal and caused interference during their deflection for roll control. And chemical leakage sometimes caused eye irritation and loss of vision for the pilot. But most conclusively, jet pump efficiency was found to be very low in all cases.

Various alternatives were tried for this configuration, including a complete departure that used electrical power for axial fans in the wings, and some research success was achieved–but without the jet pump the latter was not a true Arado system. But it was the preamble for the only, and very successful, configuration for the Model 319 blc–hydraulically powered axial fans, with the power extracted from the airplane engine (so not a real Arado system either).

And success was complete and historical on the 319, with the major criteria being the lift coefficient achieved–this being the ultimate technical measure of lifting capability of a wing. When the pilot came back from the formal flight to determine the airplane’s maximum lift coefficient, I jumped in and calculated the figure based on the raw data (not adjusted for instrumentation calibrations) and was astounded that the number that came out was 5–about double the 2.5 plus figure that could be achieved with the best combination of fancy flaps, droop, slots and slats and such at that time.

That startling figure, 5, was later confirmed with the instrumentation corrections used. And I have never seen that measure of lift performance beaten or even equaled with other configurations since then.

Then why wasn’t the technology adopted for other Cessna models?

Take the Model L-19 (which was our development Model 305, by the way), from which the 319 was adapted. It had a takeoff and landing capability of 600 feet, over the “standard” 50 foot obstacle. The 319 could do it in 450 feet with blc operating. That’s a huge percentage improvement, but still only 150 feet in physical measurement.

Would that be worth the carried cost, weight, and operating and maintenance expenses for the given improvement in field length performance? Short usable field length is important but it is the most infrequent and shortest duration part of the flight operation or mission. Thus when the Army suggested blc be considered for the L-19, we in the engineering department said that it would “not likely” be attractive.

F-104 fighter
The supersonic F-104 fighter used a version of Cessna’s BLC design to shorten landing distances.

So where was blc used in practice? Primarily in our Cold War supersonic jet fighters, like the much produced F-104 and F4Hs (but also, and actually first, the T2V jet carrier trainer, which had shorter production and is not further discussed here). But they used a somewhat different blc approach, made possible by their unique configuration and mission. It was used only to reduce landing distance, as takeoff distances were already relatively short because of having all that thrust available for cruising at supersonic speeds. And the (high pressure) air came from bleeding it from the jet engine located conveniently close to the inboard flaps, which were the only trailing edge devices, in this case, “blown” by it.

That bleeding arrangement additionally allowed the engine to be operated on approach at a higher RPM for immediate use for go-arounds without producing such amounts of thrust that it would increase landing distance.

The aftermaths of our work were sobering. I was really disappointed when looking for acknowledgement of the contribution of our University-Cessna research for ONR to the configurations used on the 104 and F4H–for which I found none. And why was that the case? I think because we did our research on nominally 12% thick airfoils that were standard on our airplanes, and more consistent with the likely use of our research on transport airplanes.

One of the German engineer-scientists afterwards made a career for himself of trying to sell that blc transport concept to other potential airplane company customers – without success. The supersonic fighters had wing thicknesses in the 3 to 4% range, making the applicability of our results uncertain.

I was further disappointed that some observers criticized, as definitive, the stalling characteristics of particularly the 319 at maximum lift. And they were admittedly challenging. But what would you expect of a wing at a very high angle of attack with really exorbitant lift being created?

A slight yaw, or difference in wing twist, or even the different efficiencies of the blc pumps would cause one wing to stall slightly before the other, with heavy roll likely to put the airplane on its back before being corrected. But great field performance had already been safely demonstrated using landing, or takeoff, speeds normally considered very low but with a hefty margin above the stall velocity with blc operating. And to provide further safety proven “stall” warning devices, available even in the 50s, could have been utilized to assure that the necessary speed spread was maintained by other operators.

A historical note: As time went on and our research efforts were successful some differentiation from other blc applications, say for drag reduction, was wanted. So our approach was called “Circulation Control” in some quarters, thus by word selection emphasizing the lift increases realized.

So, what happened to the Research Departments at Wichita University and Cessna after completion of the ONR/Transportation Corps contracts, plus some limited additional wind tunnel work done in the same discipline for the Air Force? They were disbanded.

F-4 Phantom
Even the F-4 Phantom used elements of boundary layer control, especially useful for carrier operations

But other successful research was conducted, including a device of my creation (for my Masters Thesis from Wichita University) that resulted in further reductions in takeoff and landing distances over that achieved with just blc, were that to be desired.

In comparative assessments, though, you would have to consider that energy was added, by the pumps or fans in the wings, to the air employed in our blc system, a requirement that likely might be absent in other, more passive, high lift approaches. (The invention mentioned just above that I contributed did not require any additional energy extraction.)

There is hidden meaning in the title to this article, since while as implied it was advanced research for the time, the flight test work didn’t apply to the supersonic fighters with blc in part because of the difference in leading edge contributions to stalls, given the sharp noses of those thin wings. But the final shun came when the extended wind tunnel work included one model with blowing over a midspan leading edge flap, and that feature was added on the F4H–with still no credit shown for Wichita’s contribution.

Summing up, overall what we accomplished was so good. Why didn’t anybody notice?

Leave a Reply

Your email address will not be published. Required fields are marked *