38.The aerodynamic characteristics of an airplane. Lift and resistance of an airplane with the count of an interference. Polars of an airplane at different Mach numbers.
It may be seen from Sections 4.1 and 4.2 that vortices can be used to represent lifting flow. In the present case, the lifting flow generated by an infinitely thin cambered plate at incidence is represented by a string of line vortices, each of infinitesimal strength, along the camber line as shown in Fig. 4.12.
39.Principle of operation of a propeller. Geometrical and kinematical characteristics of propellers.
A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade. Propeller dynamics can be modelled by both Bernoulli's principle and Newton's third law. A marine propeller is sometimes colloquially known as a screw propeller or screw.
Each section of a propeller (or of a wing) has a certain angle of incidence and is moving through the air at its unique angle of attack - both are independent. On the other hand, a mechanical screw or a knife moves through a rigid material exactly in the direction which is given by its pitch or angle of incidence - a screw with different pitches along its radius would get stuck, a propeller does not.
Geometrical and kinematical characteristics of propellers.
the number of blades B,
the axial velocity v of the flow (flight speed or boat speed),
the diameter D of the propeller,
the selected distribution of airfoil lift and drag coefficients Cl and Cd along the radius,
the desired thrust T or the available shaft power P,
the density rho of the medium (air: ~1.22 kg/m³, water: ~1000 kg/m³).
40.The aerodynamic characteristics of propellers. The main operational regimes of propellers.
A propeller creates a thrust force out of the supplied power. The magnitude of this force is not constant for a given propeller, but depends on the velocity of the incoming air and the rotational velocity of the propeller itself. Thus tests of propellers usually cover a wide regime of operating conditions.
Propellers having the same shape, but scaled by a size factor behave similar. In order to make a comparison of propellers of different size easier, aerodynamicists try to get rid of the units. Then it is possible to use the results of a small scale wind tunnel model to predict the performance of a full scale propeller. Similar to airfoils and wings, the performance of propellers can be described by dimensionless (normalized) coefficients. While an airfoil can be characterized by relations between angle of attack, lift coefficient and drag coefficient, a propeller can be described in terms of advance ratio, thrust coefficient, and power coefficient. The efficiency, which corresponds to the L/D ratio of a wing, can be calculated from these three coefficients. These coefficients are helpful for the comparison of propellers of differing diameters, tested under different operating conditions.
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