• Version
• 133 Downloads
• 2.07 MB File Size
• 1 File Count
• June 15, 2016 Create Date
• June 15, 2016 Last Updated

Base Pressure Measurements on the XB-70 Airplane at Mach Numbers from 0.4 to 3.0

Full—scale flight base pressure coefficients obtained from the XB—70 prOpulsion
package are compared with predicted values based on a combination of cold jet flow
wind—tunnel models and data from a two—engine side-by-side jet, full-s cale aircraft.
At cruise Mach numbers the base pressures on the full—scale aircraft were higher than
predicted, resulting in a favorable increment of about 2 percent in terms of lift-drag
ratio.

At low supersonic speeds near a Mach number of l. 2, the negative base pressure
coefficients were about three times larger than predicted, which would result in a sig—
nificant lift—drag-ratio decrement.

The investigation showed that the net calculated effect of underestimating the base
drag at low supersonic climbout speeds, even though overestimating the base drag at
cruise, can seriously reduce the range potential of the aircraft, depending on several
operational factors that can influence transonic excess thrust. The trend of this range
decrement (with respect to the transonic excess thrust) emphasizes the need for a
base drag prediction based on models with a higher degree of similitude throughout the
transonic and supersonic speed range.

It is well known that base drag can be a relatively large part of the total drag of
multieng‘ine aircraft at low supersonic Speeds. For some large supersonic aircraft
the base component of drag can become a serious performance and Operational problem.
This problem occurs when the flight trajectories for maximum transonic excess thrust
become incompatible with operational restrictions that may be common to supersonic
transport aircraft. Unfortunately, the incompatibility is most adverse at the Mach
numbers at which base drag coefficients are the highest. Therefore, base drag
becomes of interest operationally in addition to its effects on efficiency.

Many investigators have studied afterbody and base aerodynamics to determine
which fluid dynamic parameters are most important, what is the most realistic analyt-
ical flow model, and how base drag can be predicted and reduced. Theoretical and
semiempirical methods have been developed to predict base pressures for design pur—
poses (refs. 1 to 10), and the adequacy of these methods continues to be assessed pri—
marily by comparison with wind—tunnel results.