John Charnley: wind tunnels in aeronautical research

John Charnley, filmed at former RAE Farnborough, describes the use of wind tunnels in aeronautical research.

A wind tunnel has been the conventional, traditional work tool of the aerodynamics professional from early days. It’s used for providing information on the flow around all parts of an aircraft, very much concerned with the flow around the wings, because it’s the wings that produce the lift for making the aircraft fly. Now we’ve got a little example here of the sort of thing that can be done. Here’s a wing, and this would go into a wind tunnel, and measurements would be made of the lift and the drag as this sits there in the airflow. At low speeds there’s no problem, and you can look at the flow around the wing, you can put little tapes, little things like this, which demonstrate the flow around the wing. And they demonstrate when the flow is breaking down from a smooth, laminar flow to an interrupted turbulent flow. And when the flow goes turbulent then the drag of this surface goes up, the lift is destroyed and that’s an important parameter that you need to know in the design of a wing. So wind tunnels have been used for that sort of thing, professionally, from the word go. Then you do tests on the complete aircraft itself. You can measure the lift and the drag, you can measure the moments that are happening on the aeroplane, you can measure the forces required to move the controls, and all of this information goes to the design team to design new aircraft and examine why it is that existing aircraft aren’t performing as well as they might. That’s fine at subsonic speeds. Equally, there’s another regime associated with the flow at supersonic speeds. But at speeds between the two then the wind tunnels have difficulty. Because when the flow breaks down on the wing, you’ll understand that the flow over this top surface is speeded up compared to what it was at the front. And it can, as the aircraft – as the flow gets near the speed of sound, the flow goes past the speed of sound, it breaks down, and that causes a shockwave on the wing here somewhere. And in a wind tunnel that shockwave hits the sides of the tunnel and reverberates back again, and no longer can you believe the results that you get from the wind tunnel at transonic speeds. And that sort of design information needs then to be provided by doing the same sort of tests in flight. And so you do the same sort of thing, you put tufts of this sort on the wing of an aeroplane and you can take some photographs, if you're very clever, and show when the flow over the rear wing breaks down as you approach the speed of sound. And so this whole area of transonic aerodynamics needs to be explored in flight rather than in the tunnel. And one of the important things is to get a comparison between measurements you make in flight and measurements you make on a model of that same wing or aeroplane in the tunnel, compare the two, and try to understand how you can interpret the wind tunnel tests on some new design. And that’s what we were doing a lot of work along those lines, in flight, near the speed of sound.

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