Lift-induced drag

About: Lift-induced drag is a research topic. Over the lifetime, 2861 publications have been published within this topic receiving 41094 citations.

Measurements of polymer induced drag reduction and polymer scission in Taylor flow using standard double-gap sample holders with axial symmetry

Abstract: The rheological properties of 1–50 ppm poly α-olefins in Varsol 80 were studied using laminar and Taylor flows. In Taylor flow, all the studied poly α-olefins displayed polymer induced drag reduction and polymer scission. The measurements were carried out using a commercial rheometer equipped with a standard double-gap sample holder with axial symmetry. We find that this particular instrument even at the maximum obtainable geometrically averaged shear rate of 15,000 s −1 yields, an accuracy and a reproducibility better than ±2.5%. This unique precision for measurements in the presence of Taylor vortices, the small amounts of sample and time needed to carry out the measurements, and the general availability of the rheometer, suggests that these previously unexplored properties of this instrument will be of significant value for future investigations of polymer induced drag reduction.

The profile drag of a hawk's wing, measured by wake sampling in a wind tunnel

Abstract: The distribution of dynamic pressure behind a Harris9 hawk9s wing was sampled using a wake rake consisting of 15 pitot tubes and one static tube. The hawk was holding on to a perch, but at an air speed and gliding angle at which it was capable of gliding. The perch was instrumented, so that the lift developed by the wing was known and the lift coefficient could be calculated. The mean of 92 estimates of profile drag coefficient was 0.0207, with standard deviation 0.0079. Lift coefficients ranged from 0.51 to 1.08. Reynolds numbers were nearly all in the range 143000–194000. The estimates of profile drag coefficient were reconcilable with previous estimates of the wing profile drag of the same bird, obtained by the subtractive method, and also with values predicted by the ‘Airfoil-ii’ program for designing aerofoils, based on a digitized wing profile from the ulnar region of the wing. The thickness of the wake suggested that the boundary layer was mostly or fully turbulent in most observations and separated in some, possibly as an active means of creating drag for control purposes. It appears that the bird could momentarily either increase or decrease the profile drag of specific parts of the wing, by active changes of shape, and it appeared to use the carpo-metacarpal region especially for such control movements. Further investigation in a low turbulence wind tunnel would help to resolve doubts about the possible influence of airstream turbulence on the behaviour of the boundary layer. Note: Present address: Department of Zoology, University of Bristol, Woodland Road, Bristol BS8 1UG, England.

Some Trim Drag Considerations for Maneuvering Aircraft

Abstract: The results of a preliminary analytical and experimental study of trim drag characteristics at maneuvering lift coefficients have been summarized. The study included aft-tail configurations at subsonic and supersonic speeds and canard configurations at subsonic speeds. It is shown that the tail load required to minimize trim drag is highly dependent on the wingbody drag-due-to-lift characteristics with examples presented for both the full and zero leading-edge suction cases. For the high drag case (corresponding to zero leading-edge suction), which tends to be typical at high maneuvering lift coefficients and high speeds, rather large uploads on the tail are required to reduce the trim drag problem. The analytical predictions of trim drag characteristics compare well with the experiment for the aft-tail configuration. At supersonic speeds, reductions in down tail load required to trim, obtained by increasing tail volume and thereby allowing a favorable rebalancing of the aircraft, result in relatively large reductions in trim drag for aft-tail configurations. At subsonic speeds, the experimental studies for the canard configuration exhibit considerably higher trim drag than the analytical prediction as a result of canard stall.

Induced drag - Historical perspective

Abstract: Induced drag is associated with the shedding of vorticity along the span of a finite wing, especially its tip region; for most subsonic aircraft configurations, induced drag constitutes about 50 percent of total aircraft drag throughout the flight envelope. NASA and the U.S. aircraft industry have aggressively studied induced-drag reduction methods. The state-of-the-art CTOL commercial aircraft wing is as a result of these efforts virtually optimal, with a total induced drag lying within a percent of the theoretical minimum. Many of the devices currently under study for induced drag reduction are added to wingtips, yielding benefits through their effects on the wake vortex as well as through forces generated in the flowfield.

On vortex strength and drag in bluff-body wakes

Abstract: In a recent paper (Griffin & Ramberg 1974) the authors studied the vortex-street wakes behind forced vibrating rigid cylinders. All experimental conditions were within the regime of wake capture or synchronization between the vibration and vortex frequencies. Both mean and fluctuating velocities in the wake together with the length of the vortex formation region were measured as functions of vibration amplitude and frequency at a Reynolds number of 144. The viscous vortex strength, age and spacing at this Reynolds number were then obtained by matching a model for the vortex street with the mean and r.m.s. velocity profiles obtained from hot-wire measurements. These results are employed here to determine the steady drag force on the vibrating cylinder by means of the von Karman drag formulation. The drag coefficients determined in this way are in agreement with the recently published direct force measurements of Tanida, Okajima & Watanabe (1973) and Griffin, Skop & Koopmann (1973) at Reynolds numbers of 80, 500-900 and 4000. From these results a direct relation is drawn between the increased drag on resonantly vibrating structures and changes in the vortex strength, spacing and formation in their wakes.