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"SmokeDog" Stu Moment
Stu Moment is an air show performer who also participates in many aviation education activities for kids and adults.
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“Simple Aerodynamics"
Part 2
copyright 1984, 2004, Sublogic Corporation 

SmokeDog's Note: Much of this article is taken from a 1929 textbook. Some of the most clear and simple explanations were written in the early stages of the development of a field of study.

Most of this text can be absorbed by people ages 12 and up. If you are not ready to comprehend some of the math, don't worry. The verbal descriptions of properties of air will give you insights into the problems of traveling though the atmosphere.

The last article ended with illustrations of the term coefficient. Lets repeat these concepts........

Example. If the area of a flat plate is 6 square feet, the co-efficient K = 0.003, and the velocity is 60 miles per hour, what is the drag of the plate in pounds?
Solution: D = KAVsqared = 0.003 x 6 x (60 X 60) = 64.80 pounds drag.
For a square flat plate, the co-efficient K can he taken as 0.003.

SmokeDog's Note: In discussions of aerodynamics, we will often use the term "coefficient". A coefficient is a number often found through experimentation or observation, which allows you to compute results in convenient units (pounds, hours, dollars, etc.)

For example, you observe the results of cook times for a fully cooked turkey, given a variety of different trial weights of turkeys.

  Weight of Turkey Cook Time
Turkey #1 8 pounds 4 hours
Turkey #2 10 pounds 5 hours
Turkey #3 12 pounds 6 hours

You invent a coefficient, "cT" or "coefficient of turkey done time". For the Turkeys shown in the table, the coefficient cT=0.5.

Pounds of Turkey x cT = hours until cooked.

Different brands of turkeys may have different cT values.
Different aircraft parts may have different drag coefficients.

Different cars have different drag coefficients.

.....Before we move on lets relate the coefficient of drag to modern automobiles. The drag coefficient for autos is expressed in different units that the traditional aircraft drag coefficient.

The traditional aircraft drag coefficients usually give a number to multiply square feet of frontal area (the area of a structure or part as seen from the front) in order to give drag in pounds of force.

The modern automotive coefficient of drag, "Cd", appears to be in units of square meters of frontal area with results relating to horsepower required to push the car through the air given the drag produced. (Actually the force units are given in watts, but we know that 746 watts = 1 horsepower.)

The Cd of automobiles range from .25 to .5. The sleek, little Honda Insight has a Cd of .25. The low coefficient of drag, multiplied by a small frontal area, (multiplied again by velocity squared), give it a gas mileage of over 60 miles per gallon (MPG) at 60 MPH.

A low Cd really helps gas mileage. The smallish Honda CRV sport utility vehicle has a Cd of .5 and gives you 25 miles per gallon (MPG) on the highway. The big Chrysler Town and Country family van has a Cd of just .35 and gets 26 MPG, better than the smaller Honda CRV.

As your read in the last article, a blunt, square-ish front end and rear end contribute to a poor coefficient of drag. The rear end contributes more than the front end.

Let's continue with the 1929 text on aeronautics. Remember that this author uses the terms:

K for coefficient of drag
D for pound of drag
A for square feet of frontal area
V for velocity in Miles Per Hour

Streamline Forms. When a body is of such form that it does not cause turbulence when moved through the air, the drag is entirely due to skin friction. Such a body is known as a “streamline form” and approximations are used for the exposed structural parts of aeroplanes in order to reduce the resistance. Streamline bodies are fish-like or torpedo-shaped, as shown by Fig. 2, and it will be noted that the air stream hangs closely to the outline through nearly its entire length. The drag is therefore entirely due to the friction of the air on the sides of the body since there is no turbulence or “discontinuity.” In practical bodies it is impossible to prevent the small turbulence (I), but in well-designed forms its effect is almost negligible.

Fig. 2. Air Flow Around Streamline Body.

In poor attempts at streamline form, the flow discontinues its adherence to the body at a point near the tail. The poorer the streamline, and the higher the resistance, the sooner the stream starts to break away from the body and cause a turbulent region. The resistance now be comes partly turbulent and partly frictional, with the resistance increasing rapidly as the percentage of the turbulent region is increased.

Figs. 3-4. Imperfect Streamline Bodies.

The fact that the resistance is clue to two factors, makes the resistance of an approximate streamline body very difficult to calculate, as the frictional drag and the turbulent drag do not increase at the same rate for different speeds. The drag due to turbulence varies as V while the frictional resistance only varies at the rate of V° hence the drag due to turbulence increases much faster with the velocity than the frictional component. If we could foretell the percentage of friction, it would be fairly easy to calculate the total effect, but this percentage is exactly what we do not know. The only sure method is to take the results of a full size test.

Fig. 2 gives the approximate section through a streamline strut such as used in the interplane bracing of a biplane. The length is (L) and the width is (d), the latter being measured at the widest point. The relation of the length to the width is known as the “fineness ratio” and in interplane struts this may vary from 2.5 to 4.5, that is, the length of the section ranges from 2.5 to 4.5 times the width. The ideal streamline form has a ratio of from 5. to 5.75. Such large ratios are difficult to obtain with economy on practical struts as the small width would result in a weak strut unless the weight were unduly increased. Interplane struts reach a maxi mum fineness ratio at about 3.5 to 4.5. Fig. 3 shows the result of a small fineness ratio, the short, stubby body causing the stream to break away near the front and form a large turbulent region in the rear.

Results published by the National Physical Laboratory show streamline sections giving 0.07 of the resistance of a flat plate of the same area, with fineness ratio=6.5. In Fig. 4 the effects of flow about a circular rod is shown, a case where the fineness ratio is 1. The stream follows the body through less than one-half of its circumference, and the turbulent region is very large; almost as great as with the flat plate. A circular rod is far from being even an approach to a perfect form.

In all the cases shown, Figs. 1-2-3-4, it will be noticed that the air is affected for a considerable distance in front of the plane, as it rises to pass over the obstruction be fore it actually reaches it. The front compression may be perceptible for 6 diameters of the object. From the examination of several good low-resistance streamline forms it seems that the best results are obtained with the blunt nose forward and the thin end aft. The best position for the point of greatest thickness lies from 0.25 to 0.33 per cent of the length from the front end. From the thickest part it tapers out gradually to nothing at the rear end. That portion to the rear of the maximum width is the most important from the standpoint of resistance, for any irregularity in this region causes the stream to break away into a turbulent space. From experiments it has been found that as much as one-half of the entering nose can be cut away without materially increasing the resistance. The cut-off nose may be left flat, and still the loss is only in the neighborhood of 5 per cent.

(continued next week)

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“Simple Aerodynamics"
Part 2
copyright 1984, 2004, Sublogic Corporation