Fig. 45.—Another Efficient Form.
Ratio of B D to A C 1 to 17. AD rather more than ¼ of A C.
The next question, after having decided the question of aerocurve, or curvature of the planes, is at what angle to set the cambered surface to the line of flight. This brings us to the question of the—
§ 6. Dipping Front Edge.—The leading or front edge is not tangential to the line of flight, but to a relative upward wind. It is what is known as the "cyclic up-current," which exists in the neighbourhood of the entering edge. Now, as we have stated before, it is of paramount importance that the aerofoil should receive the air with as little shock as possible, and since this up-current does really exist to do this, it must travel through the air with a dipping front edge. The "relative wind" (the only one with which we are concerned) is thereby met tangentially, and as it moves onward through the air the cambered surface (or aerocurve) gradually transforms this upward trend into a downward wake, and since by Newton's law, "Action and reaction are equal and opposite," we have an equal and opposite upward reaction.
We now know that the top (or convex side) of the cambered surface is practically almost as important as the underneath or concave side in bringing this result about.
The exact amount of "dipping edge," and the exact angle at which the chord of the aerocurve, or cambered surface, should be set to the line of flight—whether at a positive angle, at no angle, or at a negative angle—is one best determined by experiment on the model in question.
Fig. 46.
But if at any angle, that angle either way should be a very small one. If you wish to be very scientific you can give the underside of the front edge a negative angle of 5° to 7° for about one-eighth of the total length of the section, after that a positive angle, gradually increasing until you finally finish up at the trailing edge with one of 4°. Also, the form of cambered surface should be a paraboloid—not arc or arc of circles. The writer does not recommend such an angle, but prefers an attitude similar to that adopted in the Wright machine, as in Fig. 47.
§ 7. Apart from the attitude of the aerocurve: the greatest depth of the camber should be at one-third of the length of the section from the front edge, and the total depth measured from the top surface to the chord at this point should not be more than one-seventeenth of the length of section.
§ 8. It is the greatest mistake in model aeroplanes to make the camber otherwise than very slight (in the case of surfaced aerofoils the resistance is much increased), and aerofoils with anything but a very slight arch are liable to be very unstable, for the aerocurve has always a decided tendency to "follow its own curve."