May 19, 2023, ©. Leeham News: Last week, we looked at the different drag types (Figure 1) that reduce an airframe’s efficiency.
We now look at the major drag types in more detail and what can be done to reduce them. We start with the dominant drag, skin friction drag.
During the cruise of an airliner, the skin friction drag represents the dominant drag type. Dependent on cruise altitude and speed, it can be more than or just below 50% of total drag, Figure 1.
Air friction drag occurs when passing air molecules scrub against the aircraft’s surface. It’s why its size is dependent on the total skin surface of the aircraft, the so-called wetted area. The skin friction drag is proportional to the wetted area and the skin friction drag coefficient, Cf.
One way to reduce the dominant drag for an airliner is to reduce the wetted area. The general method to lower wetted area is to make an aircraft that is efficiently packaged. You should also design the aircraft with a high aspect ratio wing.
Many believe a high aspect ratio is the way to lower induced drag. It’s a misunderstanding fueled by the fact the induced drag coefficient includes wing area (to normalize the coefficient) but the formula for induced drag does not. Wingspan reduces induced drag. The aspect ratio’s wing area has no influence on induced drag. It does influence skin friction drag, however.
Therefore, a high-performance wing has a large span (to reduce induced drag) and a low wing area (to reduce friction drag). A wing with a high aspect ratio (defined as span^2/wing area) thus is an efficient wing as it has a wide span that lowers induced drag and a low wing area that lowers skin friction drag.
A figure of merit for an airliner is the total wetted area per passenger seat. We will look more into wetted area reduction architectures and numbers next week.
The other way to reduce skin friction drag is to lower the skin friction drag coefficient, Cf. Figure 2 shows measurements of the skin friction coefficient in wind tunnels using flat plates.
Wind tunnel measurements are made with sub-scale physical models. It’s why the speed component is represented by the Reynolds number, Rl. The Reynolds number describes the ratio between the flow inertial forces and the viscous (air resistance to deformation) forces. If the Reynolds number in the wind tunnel is the same as the Reynolds number of the part (wing, fuselage…) in real flight, you can expect the measured data to be transferable to real flight.
The Reynolds number for airliner parts (wings, stabilizers,…) ranges between 10^6 and 10^8. As you see, the Cf for laminar flow at 10^6 is 0.0015, whereas the turbulent flow has a Cf of 0.0045, a three times higher drag level. Laminar flow is present as the air hits a surface, but as the boundary layer rubs against an uneven surface, it slows down and changes to a turbulent boundary layer.
Aero designers have tried for over 100 years to delay the transition to a turbulent boundary layer as much as possible on an aerodynamic surface. It requires a surface with a special profile to avoid slowing the boundary layer and a very smooth surface not to trip a turbulent flow.
In practice, it’s been hard to keep a part of the aircraft having laminar flow beyond a typical 10% of the surface length from where the air hits the wing/fuselage/stabilator/nacelle….
Boeing has designed the nacelles on the 787 to have extended laminar flow on the forward part (Figure 3). The nacelle lips have a special curvature and smooth surface.
Boeing also incorporated a passive boundary layer suction system on the horizontal stabilizer leading edges to extend the laminar flow area there. It was subsequently discarded, presumably as in-service contamination destroyed the laminar flow after a short in-service period, and the system added complexity and maintenance without tangible gains.
The latter is typical of why laminar flow in practice, despite hundreds of projects and tests, has not found wide-scale use. It requires high-precision manufacturing to create smooth surfaces but also constant cleaning and maintenance in operational use.
Reducing the wetted area of an airliner is the primary method to reduce skin friction drag. We will look at a special method in the next Corner.
The second method is to increase the amount of laminar flow on the aircraft. This has had limited success despite over 100 years of research and ideas and numerous tests. When the designs have reached operational service, very little of the promise of laminar flow has manifested itself.
Inertial forces may be vicious, but in this case they appear meant to be compared to a viscous fluid.
AI does create jobs. It created the need for a proofreader.
Can hardly wait for that AI copilot.
The “laminar flow wing” of the WWII fighter P51 is very obviously different from other wings of the era, when viewed end-on. I never knew what they were talking about until I looked at one. Apparently it delayed turbulence onset and increased the range of the aircraft.
The brilliant engineers of North American Aviation 👍
Besides the typo, “vicious” when you meant to say “viscous”, this sentence is a bit unclear in my opinion. The Reynolds number *is* the ratio between the the flow inertial forces and the viscous shear forces. The linear dependence on velocity is not separate from the force ratio.
Dynamic Pressure (inertial): rho*V^2
Viscous Shear Stress: mu*V/l
Force Ratio: (rho*V^2)/(mu*V/l) = rho*V*l/mu = Re
It was muddled; I corrected it. BTW, I changed in the previous article, friction drag is part of parasitic drag, as is form and interference drag. I separated it as friction is so dominating, but should have said it’s the dominating part of Parasitic drag. I have division between Parasitic, Induced and Wave drag in my book.
Most new long range aircrafts have carbon wings for increased stiffness and strength allowing wide span and low drag profile with fine surface finish. They are on narrowbodies like A220 and Russian MC-21. Still not yet on A320neo nor 737Max. We assume they will be robotic built on Airbus A320neo+ wing of tomorrow. Sadly they just wait for Boeing instead of give us the benefit now on a A320neo++
Airlines want 50-60% discounts off the planes of yesterday. You can keep your wing of tomorrow
However what really is holding up the next step is the automated airframe assembly. But it seems it cant be done without a clean sheet design. And the much vaunted ‘digital model’ engineering- mostly existing tech/design taken a step further is starting to fall flat on its face
we are all in awe of your technical expertise 🙂
compare the A350XWB gestation to the 787 more or less uncontrolled carambolageing path to a product.
No human involvement fully automatic assembly is further away than was thought in the 80ties. ( VW Halle 54 )
What you’ll see is massively robot assisted manufacture.
( mandatory prerequisite: You Must Not Hate Your Workforce )
Outstanding comment! 👍
Where did I say ‘no human involvement fully automatic’
You just made it up to make a point. Not sure what was the point about the A350 vs B787 either as neither were single digital model engineering.
I was thinking of the car body assembly by robots as an exemplar for a fuselage but its unknown but maybe for these 19 seaters in development.