October 27, 2017, ©. Leeham Co: After a detour over Airbus’ A330neo first flight last Friday, we now continue with aircraft drag. We divided drag in two classes last time: drag from size and drag from weight.
These drag effects were not discovered at the same time. To make it more informative, we will mix in how aircraft designers uncovered these drag types over time. It took them centuries to understand what held their flying devices back.
The early aviators, like Otto Lilienthal (Berlin 1848-1896), had learned to test aerodynamic bodies and wings with a rotating arm apparatus. They used these tests to understand what forces acted on their flying machines.
The whirling arm apparatus (Figure 2) was invented by Benjamin Robins (1707-1751). He was one of the first to investigate and understand the drag of a body traveling through air.
With the whirling arm, Lilienthal and other aviation pioneers could test bodies and airfoils repetitively and under controlled conditions. With the arm suspended on a balance, lift force could be measured by how high the arm balanced at a certain speed and counterweight, Figure 3.
By measuring the rotation time of test items and comparing it to tests with balls or flat plates, the first drag data on bodies could be understood. Lilienthal used the whirling arm to do the first accurate measurements on wing airfoils, Figure 4.
The drawing shows the airfoil angled for increasing angle of attack on the right hand side. The vectors at different angles of the wing to the still air shows the size of the force for lift and drag (solid curve with dots). The dotted curve below is the same diagram when plotted for a flat plate wing profile.
Lilienthal could therefore show the difference between using a cambered airfoil over a flat plate for lift and drag. At 20 degrees angle of attack, the lift is much higher than for the flat plate’s lift at 37 degrees, and the drag (the horizontal component of the vector) is less than half.
This was the first time someone had done such detailed measurements and plotted them in a way which is still used today, the so-called Lift/Drag polar diagram.
The left diagram is when the cambered airfoil is lying flat and the wind comes from different directions. At 20 degrees downward wind direction the force vector from the cambered wing leans forward. The lift/drag force will increase the wings speed, something all hang gliders use today.
Lilienthal’s measurements were published and his diagrams and tables were used by others working on manned flight. The Wright Brothers used his published data to design gliders for flight experiments between 1900-1901.They found that neither lift nor drift (the name for total drag used at the time) for the gliders fit with Lilienthal’s data. Lift was poorer and drag higher.
There were two major missing links. Lilienthal and others at the time had no notion of induced drag. They could measure the drag increased with different shapes of bodies but did not couple it to the width of the objects. The thinking was that lift and drag came from the profile and how the air flowed around it, not from the shape of the wing. One needed a suitable profile and sufficient wing area to get the lift.
Lilienthal’s successful test with cambered airfoils was the new revelation. No one had noticed that these had varying drag with wing width. Users of Lilienthal’s results built their cambered wing as they saw fit. The first Wright glider had a stubby wing. It had an aspect ratio 3.5 wing whereas Lilienthal’s gliders had a 6.5 ratio. The result was a glider with a bad lift-to-drag ratio.
The other missing link was; tests with the whirling arm were made with Reynolds numbers of 330,000. The Wright Brothers gliders were flying with a Reynolds number of 1,100,000. Looking at the friction drag diagram from the second Airbus A340 Blade test article (Figure 5), we can see there is a clear difference in drag coefficient (drag per surface area) for Reynolds number 330,000 and 1,100,000.
The Wrights Brothers decided to do their own aerodynamics tests to understand why their glider didn’t have the aerodynamic data they though it would. Being practical and inventive, they decided to build their own wind tunnel, to verify lift and drag data on different bodies and wing profiles. What they learned during these tests we will cover in the next Corner.