Hi Joe:
The spoilers on an airplane wing will only increase drag if they're
deployed
beyond some critical threshold. Certainly if a spoiler goes to 30 degrees
as a speedbrake or 60 degrees as a ground spoiler, there will be a lot of
drag. But a big reason for using spoilers (rather than more aggressive
aileron deflections) for roll control is because a few degrees of spoiler
will reduce lift on that wing and induce a bank, without the additional
drag
you'd get from additional aileron deflection.
We place pieces of tape on the upper surfaces of wings to trip the boundary
layers, and even those pieces of tape can raise the drag measurably. Tape!
A spoiler lifted even a minute amount in attached flow will cause the flow
downstream to detach, period. And we don't spend thousands of hours and
millions of dollars eliminating separated flow from aircraft for our health.
Every tiny little bit of separated flow causes huge amounts of drag. When
the B-47 was in use, workers had to wear special booties whenever they
walked on the upper surface of the wing. Scratches could lead to separated
regions which increased drag so much the airplane's mission could be put in
jeopardy. When the so-called "laminar flow" series (the NACA 23000 series)
of airfoils was introduced, engineers were disapointed with the drag
performance of the real wings built with those airfoil sections, compared to
those tested in the wind tunnel. They later found that dirt and dead bugs
(!) were causing the flow over the real wing to separate earlier than on the
pristine wings of the wind tunnel models, increasing the drag and
eliminating the benefits of the new design. Any other aerodynamicist will
agree with me, separation dominates.
Right, spoilers are used for roll control. And if you fly an airplane with
spoilers you'll quickly see that the airplane doesn't require much rudder
input to coordinate the turn. In fact, many airplanes with spoilers require
*no* rudder input. That's because the spoiler is increasing drag on the
downward-rolling (lower lift) wing. If it reduced drag, you would get the
same adverse yaw that you get in an airplane with ailerons. Lack of adverse
yaw is one of the side benefits of spoilers. But you don't have to take if
from me. Discussing the control system for the Boeing 707, Anderson writes,
"Spoilers are essentially flat plates that deflect upward into the flow over
the top surface of the wing, 'spoiling' that flow and, hence, decreasing
lift and increasing drag." For other quick references, see also Etkin's and
McCormick's books.
To look at it another way, if
an airplane suffers a disconnection of a spoiler actuator, the spoiler
won't
just lie flat on the wing, it will "float" up a few degrees. That
position
is where drag is minimized.
That will only happen if the volume below the spoiler is vented to the lower
surface or to the inside of the wing or if there is recirculating flow over
the spoiler. Otherwise, the spoiler *will* lay flat.
For a well-documented automotive example of spoilers reducing drag, we
don't
need to look any further than my own '87-91 Porsche 928, which boasts a
drag
coefficient of 0.34 and a pretty obnoxious looking spoiler. Contrast that
to the original 928 of 1978, which was aerodynamically identical aside
from
the lack of a spoiler, and demonstrated an unimpressive 0.41 drag
coefficient. (Intermediate model years had a small lip spoiler, not
unlike
that on a Milano Verde, which reduced the drag coefficient to < .4,
although
I don't remember the exact value.)
I can't say much about that without a picture and some analysis. Did you
make those measurements yourself are do they come from another source?
Rich Wagner
Montrose, CO, USA
'82 GTV6
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