Chapter 122: Boundary Layer Partition

Chief Engineer Yu had departed, leaving Yang Hui in the care of Professor Zhu from Beihang University. With a group of more than ten researchers from the base and Professor Zhu’s dozen or so graduate students—who served as assistants—they began the practical aerodynamic testing.

There were numerous aerodynamic tests for aircraft: power simulation, interference and separation of multiple bodies, aeroelasticity, dynamic derivatives, unsteady tests at high angles of attack, spin tests, free-flight wind tunnel modeling, hinge moment testing, and many more. Altogether, these major test categories exceeded a dozen.

Aircraft are a subset of flying vehicles, but not all flying vehicles are aircraft. Some wind tunnel tests are unique to certain vehicles, such as parachute tests or Magnus effect experiments, which are unnecessary for airplanes.

Among all the wind tunnel tests for the new craft, the most crucial was the jet-powered intake testing—a major operation, since the design was to switch from a nose intake to lateral intakes, a rather complex modification.

The new aircraft’s intake redesign had to ensure the engine’s operational requirements. This directly affected engine efficiency, whether the engine could function normally, and its thrust output.

In fighter jets, the intake’s role is even more critical. Fighters must decelerate and compress incoming high-speed airflow; this begins in the intake. Large subsonic passenger jets, on the other hand, scarcely have proper intakes—merely engine nacelles.

Modern fighter jet intakes are classified by their operation—adjustable or fixed—or by their configuration, such as three-dimensional axisymmetric or two-dimensional rectangular.

The J-7 fighter adopted a supersonic, externally-compressed triple adjustable intake with a central cone, designed for high-altitude supersonic flight. However, the switch to lateral intakes in the new aircraft raised new issues: the intake must match the engine’s operating conditions for optimal performance.

The J-7 was a high-performance, Mach 2 fighter. When flying above Mach 1.5, its low-speed intake became unsuitable for the engine, necessitating an adjustable intake design to alter the intake’s cross-sectional area and vent excess airflow.

Lateral intakes inherently added weight. If coupled with cumbersome adjustment mechanisms, one could imagine how unwieldy the new intake would become.

Thus, through Yang Hui’s coordination, the new intake design sacrificed high-speed performance to prioritize low-speed capability, followed by structural robustness for higher speeds. This made the design process much simpler.

Without pursuit of high-speed performance, the aircraft’s Mach number naturally decreased. Intake efficiency during high-speed flight became irrelevant, and the complex adjustable cross-section mechanisms could virtually be abandoned.

Given the new small bypass turbojet’s increased airflow demands, only extreme conditions needed to be considered; vent valves would suffice.

Considering the new aircraft’s flight requirements, a simple pitot intake became the optimal choice. Though it was the least efficient, it was adequate so long as it met the needs.

These initial intake design proposals were documented for all involved personnel to review. Professor Zhu approved the adoption of this simple intake for the new aircraft, so preparations began to test the intake’s shape.

Indeed, it was a matter of testing, not designing—the most reliable approach was actual wind tunnel trials. Professor Zhu set this intake testing project as the first priority.

“According to the initial design, the nose intake is changed to lateral intakes, so we must account for the effect of the boundary layer. The thickness of this boundary layer is a major item to be tested.”

How to explain the boundary layer? Simply put: air flow is viscous. In the new lateral intake design, airflow first passes over the nose surface before entering the intake. During this process, friction occurs between the airflow and the nose surface, creating viscosity. The layer of airflow closest to the nose slows down compared to the rest—this slow layer is called the boundary layer.

Aircraft engines cannot allow boundary layer air into the intake, as it creates a small zone of sluggish airflow, incompatible with the surrounding high-speed air—a phenomenon known as airflow distortion. Such distorted airflow can trigger compressor stall in the engine, possibly shutting it down.

Thus, the boundary layer must be isolated from the engine. There are various methods to achieve this.

Early nose intakes and standalone engine nacelles did not require boundary layer considerations, as the incoming air had not passed over any other surfaces and thus remained free of boundary layers.

For military aircraft with lateral intakes, the only way to isolate the boundary layer is by using boundary layer splitter plates, but this raises another issue: different aircraft shapes generate boundary layers of varying strength and thickness.

To determine the proper distance between the splitter plate and the fuselage, the thickness of the boundary layer must be tested—requiring genuine wind tunnel experiments.

Of course, the splitter plate itself also creates a boundary layer, but its effect is minor; setting vent holes on the plate’s inner side can siphon away this boundary layer.

To facilitate these tests, preliminary theoretical calculations and analyses were necessary, followed by wind tunnel testing to measure steady-state and dynamic performance.

Theoretical analysis required time, as did model construction—different tests demanded different models.

Model construction followed specific ratios:

First: the model’s length should not exceed the height of the test section;

Second: the model’s length should not exceed 0.6 times the test section’s width. The wings could be truncated, so long as the section above the intake was preserved.

Third: …………

The creation of test models was itself a field of study; without careful research and adherence to standards, correct data could not be obtained.

Theoretical analysis was Professor Zhu’s specialty—not that he did it personally, but that his students were his best labor force. Having Professor Zhu personally undertake tedious, less technical theoretical analysis would be a waste.

“Liu Jun, take your team and conduct a theoretical analysis of this boundary layer first. We’ll proceed to actual testing afterwards. This project is a rare opportunity—make sure you learn well.”

Such is the duty of a teacher: not only must one consider the standard project issues, but also the students’ education. A responsible educator faces great challenges, especially in higher education.

Liu Jun, whom Professor Zhu addressed, stepped forward with several classmates. At first glance, he seemed a reliable student, but his lack of glasses was surprising; one might question how he managed his academic journey.

Touching his closely cropped hair and wearing his dependable expression, he smiled and said, “Theoretical analysis is our forte; we guarantee quality and timely completion.”

With that, Liu Jun led his team away to begin the analysis. Professor Zhu nodded and quietly said to Yang Hui, “Liu Jun is the most accomplished among my current students—entrusting him with this theoretical analysis is absolutely safe.”

Yang Hui had no worries about this; with sufficient knowledge, theoretical analysis came easily. If a group of graduate students struggled with it, that would be truly odd.

Yang Hui considered whether multiple aerodynamic projects could begin simultaneously, rather than one after another, as the current approach felt inefficient.