Computer simulations of the MD11 models
by Steve Smith
First, I needed to get the geometry of the wind tunnel
test section and the geometry of the MD11 model into a form that the
computer simulation can use. I used a type of CAD program to create a
"surface panel" definition of the model and the wind tunnel. This took
me about 3 weeks. I actually needed to do this step six times, because
I wanted to simulate the model installation in the wind tunnel at three
different angles of attack, and there are two different model sizes.
Next, I ran a simulation of the flow through the
empty tunnel to ''calibrate'' it  to make sure the flow speed in the
test section was the same as the flow speed in free air. Results: I put
a flow sampling point in the test section model, and the velocity at this
point was 0.972 times the free stream velocity. Since lift and drag forces
are proportional to the square of the velocity, V2, I need to adjust the
forces inside the tunnel in my computer simulation by dividing by 0.946.
December 19, 1997
I sent off the three computer runs for the 4.7% scale model inside the
wind tunnel, at angles of attack of 0, 5, and 8 degrees.
December 22, 1997
I got two of the computer runs back today. The third one is still running.
I submitted the three runs for the 7.25% model.
December 23, 1997
I got two cases back today. Merry Christmas
December 29, 1997
The cray computer is down today for maintenance
January 2, 1998
I got the remaining two cases back. I submitted the last 3 cases, just
the basic MD11 model in free air at the same three angles of attack.
January 3, 1998
I got all the computer results back today.
Determining the corrections to apply to the data
I made an Excel spreadsheet with all my computer
results of lift and drag coefficient. The first correction I did was to
adjust all the lift and drag coefficients from the models in the tunnel
for the empty tunnel calibration, by dividing by 0.946. Next, for each
angle of attack, I subtracted the result from the freeair computation
from the result in the tunnel, so I now have the ''deltalift'' and ''deltadrag''
caused by the wind tunnel at each angle of attack.
January 6, 1998
Today I used a curvefit routine to find the quadratic parabola that fits
through the three data points for each model at the three angles of attack.
I ended up with four parabolas, one each for the ''deltalift'' and ''deltadrag''
for the two models. I arranged to have the actual wind tunnel results
for the two models sent over to me from the wind tunnel data group. There
are actually mountains of data  I just got the data for the cruise configuration
(flaps retracted and elevators set for cruise) with the standard winglets.
I put these data into the Excel spreadsheet.
January 7, 1998
Today I actually applied the quadratic curvefits of my tunnel corrections
to the actual wind tunnel data. At each model angle of attack that was
tested, I just evaluate the quadratic equation for the deltalift and
deltadrag and add these to the lift and drag coefficients for the model.
After doing this for both models, I am ready to compare the lift and drag
characteristics of the two models again. Remember they should now be the
same if my corrections properly compensate for the wind tunnel effects.
Well, the results still show about 8% difference
in drag at the same lift. It looks like my corrections have compensated
for some of the differences, but not all. The 8% difference is really
big, considering that the models were basically the same, just different
size. There must be some more complicated effect that my computer model
doesn't account for. I'm going to have to think about this for a while.
