Medium-Speed Wind Tunnel Testing for High-Lift System Performance
by Steve Smith
February 14, 2000
What is a "High-Lift System"?
A Jet transport has wings designed to carry the airplane to high altitudes and high speeds. The wings are swept back to reduce the effects of compressibility as the airplane speed gets close to the speed of sound. Typical cruising speeds are from 75% to 85% of the speed of sound, or Mach 0.75 to 0.85. Wings designed to work well for these cruising conditions are too small and poorly shaped for creating high lift during take off and landing.
In order to take off and land on runways that are of reasonable length, the lifting ability must be improved. One option would be to simply make the wings bigger, but this would hurt the cruising efficiency. So instead, complicated mechanisms are used to make the wing change its size and shape, by putting landing flaps down, for example. The arrangement of flaps, leading edge flaps (called slats) and the mechanisms that extend and retract them, is called the high-lift system.
A well-designed high-lift system increases the lifting ability enough to allow the airplane to fly slower so it can take off and land from shorter runways. But, also, the less complicated the system is, the better, because of cost, weight, maintenance, etc. So we want to make the system create as much added lift as needed for a certain runway length requirement but not be any more complicated than necessary.
Balanced Field Length
There is a relationship between minimum flying speed and runway length, since the engines must accelerate the airplane up to flying speed before the end of the runway on takeoff, and the brakes must be able to stop the airplane on landing without running off the end. Actually, the runway length that the airplane is certified to use is based on a complicated procedure that insures that the airplane is safe in an emergency where one engine stops running.
Imagine that an airplane starts its takeoff roll, and just about the time it is ready to take off, one engine stops! The pilot has two choices. First choice would be to continue down the runway and try to take off. Because one engine has stopped, the acceleration is less and it will take more runway to get up to full takeoff speed. Or, the second choice would be to abort the takeoff and slam on the brakes and try to stop before the end of the runway. So, there is a balanced field length that always allows the airplane to do one choice or the other no matter when the engine stops. Before taking off, the pilot determines a "decision speed." During the takeoff, the pilot is checking the speed so that if an engine stopped, he would know right away. If the plane is slower than the decision speed, then he would abort the take off. If the plane is faster than the decision speed, continue the takeoff. In both cases, he is guaranteed to have a long enough runway.
Since the certified runway length is an important part of the airplane performance, the airplane designers must be sure that the design can meet the specifications. If not, customers would not buy that airplane because it needed too long a runway. The airplane company must be sure that doesn't happen, so they invest the time to be sure they understand the high-lift system performance.
High-Lift System Wind Tunnel Testing
Wind tunnel models for high-lift system testing are probably the most complicated and expensive models that we test. There are many small pieces to represent all the flaps and slats and all the small support mechanisms. The positions of these small parts must be carefully controlled to simulate the position on the real airplane. Sometimes they are positioned with very small gaps between the parts. Each of these small parts usually has small pressure ports with internal tubing connected to electronic pressure gauges, so we can measure the air pressure at many places on the flaps, slats and main wing. These pressure measurements are used to try and understand how the flaps are working and what can be done to make them work better. Sometimes, we even test models with the landing gear extended.
The actual wind tunnel testing is often slow too because we test many small changes in the flap positions to see which positions make the most lift.
High-lift system performance is one of the most difficult flow problems to simulate on the computer because the maximum lift condition is on the verge of stall. Stall is the condition where the flow just can't follow the wing shape anymore because of the large pressure differences, and, instead, the flow separates from the surface in a complicated turbulent eddy. When the flow separates, most of the lift is lost. These conditions can be approximated with Computational Fluid Mechanics (CFD) simulations but not solved exactly. So, these flight conditions are still tested thoroughly in wind tunnels. Pressure measurements through the little pressure ports give us indications of whether the flow is still attached or if it has separated. We tested the McDonnell Douglas MD-11 airplane in the Ames 12-foot pressure wind tunnel, one of the best wind tunnels for high-lift system testing. We tested the actual MD-11 high-lift configuration and also tested some simplified configurations that worked almost as well but would be much cheaper and lighter to build on the airplane. So, in future airplane design projects, the designers will have these data to help them make decisions about whether they need the complicated system to get the little extra performance or if they can get by with the simpler system. There is more information on high-lift system testing in some of the other field journals by Dale Satran and Steve Smith.