TTS Module 08

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Module 8 Aerodynamics for

EASA Part-66 Licence Category 81 and 82

2nd Edition

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Preface Thank you for purchasing the Total Training Support Integrated Training System. We are sure you will need no other reference material to pass your EASA Part-66 exam in this Module. These notes have been written by instructors of EASA Part-66 courses, specifically for practitioners of varying experience within the aircraft maintenance industry, and especially those who are self-studying to pass the EASA Part-66 exams. They are specifically designed to meet the EASA Part-66 syllabus and to answer the questions being asked by the UK CAA in their examinations.

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The EASA Part-66 syllabus for each sub-section is printed at the beginning of each of the chapters in these course notes and is used as the "Learning Objectives" . We suggest that you take each chapter in-turn, read the text of the chapter a couple of times, if only to familiarise yourself with the location of the information contained within. Then, using your club66pro.com membership, attempt the questions within the respective sub-section, and continually refer back to these notes to read-up on the underpinning knowledge required to answer the respective question, and any similar question that you may encounter on your real Part-66 examination. Studying this way, with the help of the question practice and their explanations, you will be able to master the subject piece-by-piece, and become proficient in the subject matter, as well as proficient in answering the CAA style EASA part-66 multiple choice questions.

We regularly have a review of our training notes, and in order to improve the quality of the notes, and of the service we provide with our Integrated Training System, we would appreciate your feedback, whether positive or negative. So, if you discover within these course notes, any errors or typos, or any subject which is not particularly well, or adequately explained, please tell us, using the 'contact-us' feedback page of the club66pro.com website. We will be sure to review your feedback and incorporate any changes necessary. We look forward to hearing from you. Finally, we appreciate that self-study students are usually also self-financing. We work very hard to cut the cost of our Integrated Training System to the bare minimum that we can provide, and in making your training resources as cost efficient as we can, using, for example, mono printing, but providing the diagrams which would be better provided in colour, on the club66pro.com website. In order to do this, we request that you respect our copyright policy, and refrain from copying, scanning or reprinting these course notes in any way, even for sharing with friends and colleagues. Our survival as a service provider depends on it, and copyright abuse only devalues the service and products available to yourself and your colleagues in the future, and makes them more expensive too.

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Units The metric units of pressure are dynes per square centimetre, where the dyne is the force required to accelerate 1 gram by 1 centimetre per second. The Systeme international (SI) units of pressure are Newtons per square metre, where the Newton is the force required to accelerate 1 kilogram by 1 metre per second. The Newton is equal to 105 dynes. Although largely obsolete, the Imperial system of units is still encountered , and pressure is expressed in pounds per square inch. In meteorology the unit of pressure is the millibar (mb), which is equivalent to 1000 dynes per square centimetre . Before the introduction of the millibar, meteorological pressure was measured in terms of the length of a column of mercury in a barometer that the weight of the atmosphere could support.

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Length of Column of Mercury

Air Pressure

Proportional to Pressure

Figure 1.3- Principle of the Mercury Barometer

Module 8.1 Physics of the Atmosphere

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Variation of Pressure in the Atmosphere At sea level, pressure generally varies between 950 and 1050 mb. In tropical revolving storms and tornadoes, however, pressures may fall much lower. With increasing altitude the mass of overlying air decreases and so the pressure falls. Pressure values of the International Standard Atmosphere are given below:

Altitude (ft) 40,000 30,000 20 ,000 10,000 0

Pressure (mb) 187.6 300.9 465.6 696.8 1013.25

Pressure Pressure (psi) (in Hg) 2.72 4.36 6.75 10.11 14.7

29.92

Pressure (mm Hg)

760

From the table it should be noted that at about 18,000 ft . the pressure is half the sea level value. Also , it should now be apparent that the rate of pressure decrease with height is not constant. In the first 10,000 ft. the pressure falls at a rate of approximately 1 mb per 30 ft but between 30,000 ft and 40,000 ft the pressure decrease is closer to 1 mb per 88 ft .

Pressure Altitude The altitude at which a given pressure occurs in the International Standard Atmosphere is called the pressure altitude. If, for example, the pressure at the top of Mount Everest were determined as 300.9 mb, then the pressure altitude would be 30,000 ft. Assuming the same mean sea level conditions, and two columns of air of the same height, but differing temperatures, then the cold air would have a greater mass than the warm air due to the density difference. The pressure of the atmosphere , however, is caused by the mass of overlying molecules on a unit area. The pressure above the column of warm air is therefore higher than that above cold air. Because a higher pressure is found at a lower level , then the pressure altitude above warm air is lower than the pressure altitude above cold air. Alternatively it can be expressed that the true altitude of an aircraft is more than that indicated (assuming the correct mean sea level pressure has been set on the subscale) above warm air, and less than that indicated above cold air.

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Module 8.1 Physics of the Atmosphere Use and/of disclosure is governed by the statement on page 2 of this chapter .

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Pressure Corresponding To 700mb

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A Pressure Altitude Of 10,000ft

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1013mb

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1013mb

1013mb

Figure 1.4 - Pressure Altitude -The effect of temperature on pressure

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Module 8.1 Physics of the Atmosphere Use and/or disclosure IS governed by the statement on page 2 of th1s chapter.

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