JULY 1998 ~ CENTRAL VALLEY AVIATION NEWS
A Publication of the Fresno Flight Standards District Office~(209) 487-5306

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Flight Operations during the summer months, especially here in the central San Jacquin Valley, requires the diligent pilot to review performance values and basic aerodynamics of his/her particular aircraft.

BERNOULLI'S PRINCIPLE OF PRESSURE.
We recall this `ol Swiss mathematician's theory, where the pressure of a moving fluid (gas or liquid) varies with its speed of motion. Specifically, an increase in the speed of movement or flow would cause a decrease in the fluid's pressure. This is exactly what happens to air passing over the curved top of an airplane wing which provides us with what we call "lift".

How do you know that something is 2" long? You use a `ruler' or a standard of measurement! The same concept may be applied to our atmosphere to which ICAO has established the definition that the Standard Atmosphere @ sea level is 29.92" HG at 15·C.(59·F); additionally, the standard lapse rates (decrease) for pressure are approximately 1" Hg per 1,000 feet increase in altitude and 2· C. (3.50 F.) per 1,000 feet increase (up to the tropopause).

Since all airplane performance is compared and evaluated in the environment of the standard atmosphere, all of the airplane's performance instrumentation is calibrated for the standard atmosphere. Thus, certain corrections must apply to the instrumentation, as well as the airplane performance, if the actual operating conditions do not, fit the standard atmosphere. In order to account properly for the nonstandard atmosphere, certain related terms must be defined.

The more appropriate term for correlating aerodynamic performance in the nonstandard atmosphere is density altitude - the altitude in the standard atmosphere corresponding to a particular value of air density. Density altitude is pressure altitude corrected for nonstandard temperature. Under standard atmospheric conditions, air (at each level in the atmosphere) has a specific density, and under standard conditions, pressure altitude and density altitude identify the same level. Density altitude, then, is the vertical distance above sea level in the standard

atmosphere at which a given density is to be found. Since density varies directly with pressure, and inversely with temperature, a given pressure altitude may exist for a wide range of temperature by allowing the density to vary. However, a known density occurs for any one temperature and pressure altitude. The density of the air, of course, has a pronounced effect on airplane and engine performance. Regardless of the actual altitude at which the airplane is operating, its performance will be as though it were operating at an altitude equal to the existing density altitude

For example, when set at 29.92" the altimeter may indicate a pressure altitude of 5,000 feet. According to the airplane flight handbook, the ground run on takeoff may require a distance of 790 feet under standard temperature conditions. However, if the temperature is 20· C. above standard, the expansion of air raises the density level. Using temperature correction data from tables or graphs, or by deriving the density altitude with a computer (E6B or electronic), it may be found that the density level is above 7,000 feet, and the aircraft's takeoff ground run may be closer to 1,000 feet.

Density altitude can be computed by applying the pressure altitude and outside air temperature at specific flight level's (altitude) to a navigation computer (Circular slide and/or electronic). Density altitude may also be determined by referring to a density altitude chart.

Although not official, one may utilize a easy rule of thumb to quickly calculate, in their head, the current density altitude.

1. Find your pressure altitude by setting the altimeter to 29.92". Round off the altitude to the nearest 500': Example 4800' = 5000'

2. Find your outside air temperature (in C·),by reading your gauge, say it's 25·C.

3. Since the temperature lapse rate is 2·C/ 1,000', multiply 5 x 2; which equals 10·

4. Subtract this answer from 15· i.e. (15-10=5). This is the expected standard temperature for your pressure altitude of 5,000 ft.

5. Now, since your current temperature is 20· above standard (25-5), multiply 20 by 100 and add to your altitude. [20 x 100=2,000 & 2,000 + 5,000= 7,000.] Therefore, your estimated density altitude is approximately 7,000 ft.

A quick review of your aircraft's performance speeds is also necessary during the summer months. Since the air mass is thinner due to density altitude, you need to adhere more closely to them to obtain the necessary performance for your airport departure, enroute climb, enroute cruise, and destination airport landing perimeters.
True Airspeed (TAS)-the speed of the airplane in relation to the air mass in which it is flying.
Indicated Airspeed (IAS)-the speed of the plane as observed by the airspeed indicator. It is the airspeed without correction for indicator, position (or installation), or compressibility errors.
Calibrated Airspeed (CAS)-Means indicated airspeed of an aircraft corrected for positions and instrument error. CAS is equal to True Airspeed in standard atmosphere at sea level.

Vso-the calibrated power-off stalling speed or the, minimum steady flight speed at which the airplane, is controllable in the landing configuration.

Vs¹-the calibrated power-off stalling speed or the minimum steady flight speed at which the airplane is controllable in a specified configuration.

Vy-the calibrated airspeed at which the airplane will obtain the maximum increase in altitude per unit of time. This best rate-of-climb speed decreases slightly with altitude.

Vx - the calibrated airspeed at which the airplane will obtain the highest altitude in a given horizontal distance. This best angle-of-climb speed normally increases slightly with altitude.

Vfe- the highest calibrated airspeed permissible with the wing flaps in a prescribed extended position.

Va- Design maneuvering speed -the calibrated airspeed at which the aircraft will stall prior to potential structural damage while operating in turbulent air. This speed decreases with the decrease in gross weight of the aircraft.

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