Every Pilot Should Know About Curves

Page 1 ... 24 JANUARY 1971 power would be required to fly straight and level ... Suppose you don't have the altitude to spare — you've got the airplane cocked ...
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Every Pilot Should Know About Curves (Reprinted from "The Gosport", Newsletter of EAA Chapter 193)

By Floyd L. Brown, Jr. (EAA 48070) President, Chapter 193, Inc. 8606 Hipps Road, E. Jacksonville, Florida

power would be required to fly straight and level at 65 mph as at 135 mph. The power-available curve has been added as a light dashed line to show what can happen to your excess horsepower (the difference between power available and power required) as the speed is varied. The rate of climb is directly proportional to the excess horsepower available. Notice that as you slow down below the speed of minimum power required you have less excess horsepower and, because of this, the airplane's rate of climb will decrease. To increase the rate of climb you would have to lower the nose to get a more-proper climb speed which is one of the reasons this area is sometimes called the "region of reversed command." So, the slower you fly in this area, the more power is required and the less excess horsepower exists. You can get out of the problem by one of two methods (or both): (1) Add full power to have excess horsepower to accelerate to a higher speed as you gradually lower the nose; (2) Drop the nose sharply to trade altitude for air speed. Or, you can do both. Suppose you don't have the altitude to spare — you've got the airplane cocked up right at the stall at an altitude of 20 ft. above a boulder-strewn field two miles short of the runway and are using full power just to stay airborne. Well, fellows, be prepared to buy the farm! Remember, something like this can happen from being too late on power, and a settling condition which increases angle of attack the same as using elevator to increase angle of attack.

VERY PILOT SHOULD know something about curves, E or specifically about the power curve, familiarity with which might keep you alive to learn more about other curves. This is the power-required curve, which is a plot of horsepower required to fly the airplane at various air speeds, and is usually drawn for a particular weight and altitude. The power required is proportional to the drag to

be overcome, so it would be better to look first at the drag problem. Drag basically is broken into two parts, parasite and induced drag, and the drag that your homebuilt experiences

in all speed ranges is a result of the sum of these two types of drag in various combinations. Parasite drag is the sum of drag caused by skin friction, form drag, and interference drag which is caused by interference of air flow between two or more components of the airplane. Parasite drag increases with the square of the velocity. Double your air speed and parasite drag goes up four times. Triple your air speed and parasite drag goes up nine times. Induced drag is caused by the fact that the wings are producing lift (wing-tip vortices are also a result of the wing's lift). Induced drag is proportional to the square of the angle of attack of your wing. The slower you fly, the greater the induced drag — because, to maintain altitude as you slow up, the angle of attack must be increased and the effect of the increase is squared. Induced drag is highest just before the stall. Note in Fig. 1 that the total drag of the airplane is always the sum of the parasite and induced drags. Also, in straight and level, unaccelerated flight, thrust must equal drag, so the drag curve is also a direct measure of thrust required by the airplane at various speeds. Let's look at the drag versus velocity curve in terms of horsepower. Fig. 2 shows the power-required versus veloci ty curve for the airplane just mentioned. This is the general shape of the power curves for all airplanes. The shaded area denotes the "back side of the power curve" and it extends back from the speed of minimum power required to the power-on stall speed. As you slow the airplane, more and more power is required to maintain a constant altitude. Points A and B of Fig. 2 show that the same amount of 24

JANUARY

1971

FIG. 1

100 REGION OF _ _ _ REVERSE COMMAND I