👉 State for an foil the lift/drag ratio is whether less than on greater than one. Give reason.
In case of an foil, the lift/drag ratio is always greater than one. The reason behind that is as follows –
For an foil oriented with relatively small angles of attack, the loneof ii” Separation is limited to a small region close to the trailing edge. Thus turbulem Wake and consequently the pressure drag is quite small. The total dragt I’ mainly due to friction drag. The streamlines in upper part of airfoil are crowded 14 which indicates velocity is greater than free stream velocity and thus pressure it is less than the free stream pressure. While on underside the stream linesan farther apart, which indicates low velocity and consequenty high pressure. Q This gives rise to a high pressure force in upward direction or high lift. Thu; ” lift/drag ratio is always greater than one for an airfoil.
in actual practice also more lift is better as it serves the purpose of l supporting airfoils, while drag is an evil. Usual ratio of lift/drag for an airfoil lies in between 6-20.
What is magnus eject ? Explain clearly.
The flow pattern around a circular cylinder subjected to circulation ii is shown in fig. 5.6 (c). As we can see the flow pattern is assymetrical about i the horizontal axis, velocity is maximum at 9 = 90° and minimum at 9 =270’. Thus, the effect of circulation is the enforcement of velocity in uppetlhli portion and diminution of velocity in lower half portion. This happens dueto the fact that with clockwise circulation, the flow velocity and velocity duetv circulation are in same direction around the upper half portion and consequentll get added to give a higher velocity. While on the lower half portion, two velocities being in opposite direction, get subtracted and combined velociil becomes lower.
Now according to Bernoulli’s principle pressure on the high velocity side, will be lower than the pressure on the low velocity side. This pressure difference on two sides will exert a lift force on the cylender in upward firectio. This generation of lift by spinning cylinder in a fluid stream is called Magnus effect.