Semi-empirical prediction of tone noise due to counter-rotating open rotors
Counter-rotating open rotors (CROR) were extensively studied to power medium-size aircraft in the 80s aft er the first increases in fuel costs. Indeed, their efficiency i s greater than that of turbofans and of single prop fans. They have again become a topical subject due to the new incre ase of fuel costs and to the risk of oil shortage. They however raise a serious acoustic issue because noise is gen erated by the two rotors and by their interactions, and due to the lack of any shielding nacelle. An original and fast semi-empirical method is proposed to predict radia ted sound levels. The objective is to rapidly assess if certification rules can be fulfilled, and to estimate the possib le impact of a future fleet on noise contours around airports. This work is thus focused on takeoff and approach conditions (advancing Mach number lower than 0.3). It is shown that direc tivity of a tone is mainly determined by a Bessel fu nction, and a parabolic pattern is suggested for overall sound pr essure levels. The shape of third-octave spectra is based on the large number of interaction tones which are present in each frequency band. Finally, some information is given on overall sound levels.
High speed turboprop aeroacoustic study (counterrotation). Volume 2: Computer programs
The isolated counterrotating high speed turboprop noise prediction program developed and funded by GE Aircraft Engines was compared with model data taken in the GE Aircraft Engines Cell 41 anechoic facility, the Boeing Transonic Wind Tunnel, and in the NASA-Lewis 8 x 6 and 9 x 15 wind tunnels. The predictions show good agreement with measured data under both low and high speed simulated flight conditions. The installation effect model developed for single rotation, high speed turboprops was extended to include counter rotation. The additional effect of mounting a pylon upstream of the forward rotor was included in the flow field modeling. A nontraditional mechanism concerning the acoustic radiation from a propeller at angle of attack was investigated. Predictions made using this approach show results that are in much closer agreement with measurement over a range of operating conditions than those obtained via traditional fluctuating force methods. The isolated rotors and installation effects models were combined into a single prediction program. The results were compared with data taken during the flight test of the B727/UDF (trademark) engine demonstrator aircraft.
Early Cretaceous Feathers from Victoria
Unusually well preserved fossil feathers from Lower Cretaceous (VaIanginian-Aptian) claystones at Koonwarra, Victoria, are the oldest (c. 110-125 million years) bird remains yet found in the southern hemisphere, They are among the oldest bird remains known in the world, being antedated only by the toothed Archaeopteryx from the late Jurassic of Bavaria, and being more or less the same age as Gallornis from the Lower Cretaceous of France. The Victorian fossil feathers are evidence for essentially worldwide distribution of birds soon after the beginning of the Cretaceous, if not before. The feathers could conceivably have come from toothed birds.