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ATO #105 - April 19, 2000

PART 1: Upcoming Chats
PART 2: Project News
PART 3: Medium-Speed Wind Tunnel Testing for High-Lift System Performance


QuestChats require pre-registration. Unless otherwise
noted, registration

Tuesday, April 25, 2000 10 - 11 AM Pacific
Regimes of Flight Aerospace Team Online Chat with Steve Smith

As an aerospace research engineer, Steve Smith spends his time doing
experimental research in wind tunnels and applying computer flow
simulations to evaluate new ideas for airplanes.

Tuesday, May 2, 2000 10 - 11 AM Pacific
Chat with Carolyn Mercer

Carolyn Mercer has and continues to find ways to use lasers and light to
measure aircraft performance during testing. Read her bio at

Tuesday, May 9, 2000 9 - 10 AM Pacific
Chat with Mary Reveley

Mary Reveley works with a propulsion systems analysis systems group to
determine how aircraft and engine designs will perform.
measure aircraft performance during testing. Read her bio at


"Regimes of Flight"

Regimes of Flight Art Contest, Grades 4-8
Begins April 17, 2000

Pick a hypersonic, supersonic, high, medium, or low speed plane.
Then reproduce it in the style of Marc Chagall, Peter Max or Edvard
Munch. Have fun and learn at the same time.

For more information go to

[Editor's Note: Steve Smith, is an Aerospace Research Engineer, who does computational and wind tunnel testing. Read his bio at http://quest.nasa.gov/aero/team/smith.html ]

Medium-Speed Wind Tunnel Testing for High-Lift System Performance

by Steve Smith

February 15, 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

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.

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