FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1955
1955 - 0013.PDF
7 January 1955 13 f i-z UJ u u.u. 8 REGION OF SERIOUS FLOW INSTABILITY FRONT STALL Fig. 4. Velocity distribution in a vortex. 7 O 60 SO S *-o ANGLE OF INCIDENCE ,«x Fig. 6. Front stall and rear stall. Fig. 5. Starting vortex which induces circulation on wings. <S, i V. newspapers, money or film stars, and a term like "vorticity" per-haps suggests the horrors of mathematical symbols. Hence it is useful to understand what is meant by the statement that "circu-lation is an irrotational vortex, the core of which is formed by the aerofoil," and that "irrotational" implies zero vorticity, bothcharacteristic properties of potential flow. Anyone enjoying the use of a bath-tub has noticed the odd flow motion set up in thewater as soon as the plug is withdrawn. This is a vortex forming over a "sink" (which is a learned expression for the drain hole).Similar flow motions exist in the air, as we see from the play of leaves within miniature tornadoes around sheltered corners. Ahandy hydrodynamic experimental ground (preferred by Civil Servants and the like), namely, a stirred cup of tea, discloses thatthe circulatory motion has a velocity distribution resembling that indicated in Fig. 4. Such vortices are irrotational because thefluid mass moves as a whole, no element of it turning about its own axis. In a turbulent flow, however, vorticity is present: fluidparticles whirl about in a disordered manner. Viscosity is at the root of such disorderly behaviour, and flow within a boundarylayer may behave like this. But our circulation is irrotational, and therefore not turbulent. Circulation alone is incapable of producing lift: a rotatingcylinder induces lift only when moving through the air or when exposed to an air stream.We must also refer to the boundary layer, which is a physical fact and not fiction for the exercise of advanced mathematics. Airhas little viscosity, and all that there is of it plays its part within the boundary layer which enshrouds any body that is immersedin an air stream. In respect of high lift, a boundary layer is a Jekyll and Hyde. Without it, circulation cannot be induced,because the vortex starting it is formed from boundary-layer material which is shed as a "vortex sheet" from the wing (Fig. 5).But separation of the boundary layer from the dorsal surface of an aerofoil is the reason why the theoretically predicted lift ofpotential flow is not attained, and why we experience stalling (Fig. 6). Actually, potential-flow theory promises very highlifts (Ci_nuix values of between 6.3 and 12.6 respectively), and no stall for incidences of up to 90 deg (Fig. 7). Without a boundarylayer, we would obtain that. As commonly known today, a boundary layer may flow in twodiffering states. If it flows in a laminar state it gives minimum profile drag, but it is also likely to reduce the lift and to snap intoseparation (the abrupt stall). If it flows in a predominantly turbu- lent state, it adheres much better to the aerofoil surface, evenagainst an adverse pressure-gradient; higher maximum lift and a gradual stall are the consequences (Fig. 8). At the "transition Fig. 8. Modification of hydrodynamically effective aerofoil section, caused by partial boundary-layer separation at incidences near the complete stall: beginning of the lift decrease. _ 3 O 2O ) O Fig. 7. Aerofoil Hit, in the theory of potential flow and in practice. — 7 GOTTINGEN S5S (J. ACKERET] JOUKOWSKY AEROFOIL SECTION Fig. 9. Influence of a sharp trailing edge upon the circulation. A = Boundary-layer separa- tion ana icrmation of circula- tion-reducing vortices. B = Satisfactory flow at natur- ally induced circulation, with vortex formed as a result of abrupt velocity gradient in the boundary layer. C = Circulation weaker than the naturally induced strength (stagnation point forward on dorsal surface). D = Circulation in excess of natural strength (stagnation point forward on ventral surface). point," the state of flow within the boundary layer is supposedto change (in reality, this takes place along a distance). Merry games are played between it and the separation point, withprofound consequences on the low-speed flying characteristics of the aircraft. The lift coefficient is determined by the strength of the circula-tion, i.e., by the mean velocity of the revolving air mass. This, in turn, depends upon camber and incidence. For any givenincidence and camber, the circulation limit is imposed by the trailing edge (Fig. 9). In practice, it is, alas, even less. Thereason for such influence by the trailing edge is that it is the point at which the flow separates from the wing. The trailingedge is (or should be) a stagnation point at which, relative to the wing, the air is at rest. Another stagnation point is at the leadingedge. If the trailing edge were not a stagnation point, the circulationwould cause air to flow around the trailing edge. Theoretically, in an inviscid fluid, this cannot happen, since any flow arounda sharp corner implies infinitely great flow velocities, physically an impossibility. In practice, however, the viscosity of the air andthe presence of a thick boundary-layer at the trailing edge modify this to some extent. In most cases, separation actually takes placesomewhere injfront of the trailing edge, on the dorsal surface. This reduces the lift against the value of the theoretical one. Someflow around the trailing edge is thus possible, and there is no need to make this region needle-sharp. Cutting off the trailing edge and rounding the rest renders thecirculation theoretically indeterminate (Fig. 10). In practice, no more than an increased profile drag is experienced. If we now usesubterfuges to speed the circulation to more than the theoretical
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
