June 23, 2023, ©. Leeham News: In our series about technologies that influence the efficiency of a new generation of airliners, we have covered the dominant drag of an airliner, air friction drag, and the second largest drag, induced drag, and what can be done about them. Now we look at Pressure drag and Interference drag.
We will finish with Transonic drag next week, which requires a full Corner to explain well.
Intuitively people associate the airflow drag of an object, like a car, with its frontal area. It represents the hole an object does in the air. Car’s drag is therefore referenced to the size of their frontal area.
The idea that drag is mainly associated with the hole an object does in the air is a misunderstanding of what causes drag. The drag of the made hole is called pressure drag and is surprisingly small for objects passing through the air as long as we have subsonic flow. This also applies to airliners, Figure 1.
When an object moves through the air, it forces the air molecules to the side at the front and to reclose at the rear of the object. The air molecule’s inertia creates movement forces, which create a higher air pressure at the front of the object and a lower pressure at the back of the object.
The pressure difference between the front and back of the object is counted as pressure drag. The pressure differences are surprisingly small. It comes from the air molecules’ capability to happily move back and forth in subsonic flow if a pressure force asks them to move.
As air is flowing around an object like an aircraft fuselage, it will force the air that runs around the fuselage sides to travel faster than the aircraft’s speed. It’s debatable to what drag type the increased friction drag of the speed delta shall be counted. Often it’s counted with Pressure drag as both pressure drag and the increased air friction drag are related to the object’s cross-sectional area.
As can be seen in Figure 1, pressure drag is a minor drag, even for objects with a large cross-sectional area. Given that it’s a rather small drag, the aircraft designer is conscious about the effect but is only changing volumes of the aircraft around to reduce it if it has no effect on the wetted area of the aircraft, as it’s a bigger evil.
We will also see next week that the volume and shape of an aircraft that flies at around Mach 0.7 to 0.8, as our jet airliners do, has a larger impact on the transonic drag, a more aggressive drag than pressure drag. Therefore, the shape and volume of different parts of a jet airliner are more shaped by transonic drag than pressure drag.
Interference drag is created when flight surfaces come close to each other at sharper angles at object intersections, Figure 2. A typical area with interference drag is where the engine pylons are mated to the wing or where a horizontal stabilizer meets the fuselage or the vertical stabilizer.
The air increases its speed as it circulates around the object (creating pressure drag), and when two air streams meet each other at sharp angles at an intersection, the air molecules from the two flows run into each other, sometimes to the level that one stream’s attachment to its object surface gets lost (boundary layer separation). The interference of streams and any separation of the boundary layer causes an additional drag called Interference drag.
We can see that the Truss Braced Wing has many such intersections, and elevated Interference drag is listed as one of the challenges of a Truss Braced Wing.
The interference problem is especially problematic if we have air stream speeds that are close to the speed of sound or even supersonic. Then the interference of the streams can lead to wider boundary layer separation as we will have a transonic shock created by the interference.
The areas where we have interference drag shall be kept small on an aircraft by avoiding sharp intersections. Where such intersections are unavoidable, make their angles less sharp, which is the purpose of fairings. One such fairing is the wing fairing, where the wing and fuselage intersect. Another is the typical bulbous fairing you see on T-tails where the stabilizers intersect.
The optimization of body shape and, if necessary, fairings at intersections to minimize Interference drag is an important part of aircraft design. It’s typically done with detailed CFD (Computational Fluid Dynamics) studies at the intersection areas.
We will discuss the problems of transonic flow on an airliner and its drag in the next Corner.