The Wright brothers approached aerodynamics in a thorough, practical, experimental
way. From their writings, it is evident that they were very concerned about
accurately determining the lift and drag of their aircraft.
But they were more practical engineers than theoreticians and many of
factors which affect lift and drag are better understood today
than they were by the Wright brothers. For your more complete understanding,
we present here a page which describes the modern theories of
viscosity and compressibility
effects on any moving object. The Wright aircraft were certainly subjected to
viscous effects, since this causes boundary layer separation and wing stall, which
the brothers experienced in flight, and has a large effect on overall aircraft drag.
Fortunately, the Wright aircraft flew at very low
velocities and were not subject to compressibility effects.
As an object moves through the air, the air molecules near the
object are disturbed and move around the object. Aerodynamic
forces are generated between the gas and the object. The
magnitude of these forces depend on the shape of the object, the
speed of the object, the mass
of the air going by the object and on two other important properties
of the air; the viscosity, or stickiness, of the air and the
compressibility, or springiness, of the air. To properly model
these effects, aerodynamicists use similarity parameters,
which are ratios of these effects to other forces present in the
problem. If two experiments have the same values for the similarity
parameters, then the relative importance of the forces are being
correctly modeled. Representative values for the properties of air
are given on another
page, but the actual value of the parameter depends
on the state of the gas and on the altitude.
Aerodynamic forces depend in a complex way on the viscosity of the
air. As an object moves through the air, the air molecules stick to
the surface. This creates a layer of air near the surface (called a
boundary layer)
which, in effect, changes the shape of the
object. The flow turning reacts to the boundary layer just as it
would to the physical surface of the object. To make things more
confusing, the boundary layer may lift off or "separate" from the
body and create an effective shape much different from the physical
shape. And to make it even more confusing, the flow conditions in and
near the boundary layer are often unsteady (changing in time).
The boundary layer is very important in determining the drag
of an object. To determine and predict these conditions,
aerodynamicists rely on wind tunnel
testing and very sophisticated computer analysis.
The important similarity parameter for viscosity is the
Reynolds number. The Reynolds number expresses the ratio of
inertial (resistant to change or motion) forces to viscous
(heavy and gluey) forces and is given by the equation Re =
velocity x density x characteristic length/viscosity coefficient. If
the Reynolds number of the experiment and flight are close, then we
properly model the effects of the viscous forces relative to the
inertial forces. If they are very different, we do not correctly
model the physics of the real problem and will predict an incorrect
lift.
Aerodynamic forces also depend in a complex way on the
compressibility of the air. As an object moves through the air, the
air molecules move around the object. If the object passes at a low
speed (typically less than 200 mph) the density of the fluid will
remain constant. But for high speeds, some of the energy of the
object goes into compressing the fluid and changing the density,
which will alter the amount of resulting force on the object. This
effect becomes more important as speed increases. Near and beyond the
speed of sound (about 330 m/s or 700 mph),
shock waves are produced that affect both
the lift and drag of an object. Again, aerodynamicists rely on wind
tunnel testing and sophisticated computer analysis to predict these
conditions.
The important similarity parameter for compressibility is the
Mach number, the ratio of the velocity to the
speed of sound. So it is completely incorrect to measure a lift
coefficient at some low speed (say 200 mph) and apply that lift
coefficient at twice the speed of sound (approximately 1400 mph, Mach
= 2.0). The compressibility of the air will alter the important
physics between these two cases.
The effects of compressibility and viscosity on lift are contained
in the lift coefficient and the effects on
drag are contained in the drag coefficient.
Navigation..
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page
- http://www.nasa.gov
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