Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 60

Using Table 2.1 and several of the equations from earlier in this chapter, the density altitude can also be determined using ratios. An aircraft at an indicated altitude of 1800 ft has an altimeter setting of 29.70″ Hg (sea level) and an outside air temperature of 75 °F. Calculate the density altitude:

 Pressure altitude (P.A.) = 2005 ft is found using Figure 2.3.

 Referencing Table 2.1 for P.A. 2000 ft, δ = 0.9298

 Using Eq. 2.2, θ = 1.031

 Using Eq. 2.5, solve for the density ratio

 

 Using Table 2.1, it can be interpolated that with a density ratio of 0.902, the resultant density altitude is 3500 ft. This makes sense as the sea level pressure is lower than standard and the temperature is above standard for that altitude, which results in lower air density (higher density altitude).As discussed, density altitude influences aircraft performance; the higher the density altitude, the lower the aircraft performance. So, in the previous problem, even though the aircraft is at an indicated altitude of 1800 ft, the density altitude for performance calculations is 3500 ft. Low air density equals a higher density altitude; high air density equals a lower density altitude. Therefore, aircraft performance charts are provided for various density altitudes.