|
A wind tunnel experiment was conducted to
measure the aeroelastic stability of a soft in-plane
hingeless rotor with swept-tip blades. The data from
this experiment will be used to assess current helicopter
analytical tools and to guide future improvements
to these codes. Aeroelastic and aeromechanical
stability are important elements in the
design of helicopters. Poor design, in this regard, can
lead to the loss of prototype aircraft and loads
problems, and can limit cyclic annoyances. For this
reason it is important for helicopter designers to have
analytical tools that accurately predict such
phenomena.
The development of accurate analytical methods
requires careful comparison of calculations with
experimental measurements. These methods, particularly
at an early stage in their development, benefit
from test data obtained with simplified rotor models
whose properties are accurately characterized. Two
such tests have been completed at Ames Research
Center. The first test, completed in 1995, was conducted
using a rotor with straight blades. The second
test was completed in 1999 using a rotor with swept-tip
blades. The swept-tip geometry introduces
additional bending/torsion coupling and provides
data that amplify the importance of the air loading at
the tip of the blade. Both types of rotor blades and
the swept-tip rotor installed in the wind tunnel are
shown in figure 1. Stability of the lag mode is measured
by exciting this mode and measuring the decay
with the Moving-Block analysis after the excitation is
terminated. The excitation is achieved by oscillating
the blades root pitch at the regressive lag frequency
by using hydraulic actuators in the nonrotating
system. Both rotors were tested over a range of
forward flight conditions on an essentially rigid test
stand in one of the Ames 7- by 10-Foot Wind
Tunnels.
In addition to the wind tunnel test, calculations
were made with a comprehensive rotorcraft analytical
tool, CAMRAD II, to demonstrate current analytical
capability. The second figure shows damping
measurements of the regressing lag mode for the
swept-tip rotor and CAMRAD II calculations. In the
figure the damping coefficient, or exponent, is plotted
versus the advance ratio (a measure of airspeed,
m = V/WR) for five collective pitch angles. The
agreement between analysis and experiment is good
for the lowest collective pitch angles; however, the
analysis overpredicts damping as the collective pitch
angle is increased. Further, the calculations do not
capture the up-down-up character of the measurements
with increasing advance ratio. An analytical
study has been initiated to look at the sensitivity of
damping to various physical model parameters and
analytical model sophistication.
Point of Contact: T. Maier
(650) 604-3643
tmaier@mail.arc.nasa.gov
Back To Top
Previous Paper
Return to Global Civil Aviation
Next Paper |
|
Fig. 1. Isolated rotor aeroelastic stability model with swept-tip rotor in the Army/NASA 7- by 10- Foot Wind
Tunnel; straight and swept-tip rotor blades.
|
|
Fig. 2. A comparison of theory and experiment for the
swept-tip rotor regressing lag mode stability in
forward flight for various collective pitch angles.
|
|
