The Theory and Practice of Model Aeroplaning Part 9

A propeller of the static-thrust type was, of course, "first off,"

sometimes 10 ft. or 12 ft. ahead, or even more; but the correctly designed propeller gradually gathered up speed and acceleration, just as the other fell off and lost it, and finally the "dynamic" finished along its corresponding wire far ahead of the "static," sometimes twice as far, sometimes six times. "Freak" propellers were simply not in it.

[Ill.u.s.tration: FIG. 38.--"VENNA" PROPELLER.

A 20 per cent. more efficient propeller than that shown in Fig. 41; 14 per cent. lighter; 6 per cent. better in dynamic thrust--14 in. diam.; weight 31 grammes.]

Metal propellers of constant angle, as well as wooden ones of uniform (constant) pitch, were tested; the former gave good results, but not so good as the latter.

The best angle of pitch (at the tip) was found to be from 20 to 30.

In all cases when the slip was as low as 25 per cent., or even somewhat less, nearly 20 per cent., a distinct "back current" of air was given out by the screw. This "slip stream," as it is caused, is absolutely necessary for efficiency.

-- 21. =Fabric-covered= screws did not give very efficient results; the only point in their use on model aeroplanes is their extreme lightness. Two such propellers of 6 in. diameter can be made to weigh less than 1/5 oz. the pair; but wooden propellers (built-up principle) have been made 5 in. diameter and 1/12 oz. in weight.

-- 22. Further experiments were made with twin screws mounted on model aeroplanes. In one case two propellers, both turning in the _same_ direction, were mounted (without any compensatory adjustment for torque) on a model, total weight 1 lb. Diameter of each propeller 14 in.; angle of blade at tip 25. The result was several good flights--the model (_see_ Fig. 49c) was slightly unsteady across the wind, that was all.

In another experiment two propellers of same diameter, pitch, etc., but of shape similar to those shown in Figs. 28 and 29, were tried as twin propellers on the same machine. The rubber motors were of equal weight and strength.

The model described circled to the right or left according to the position of the curved-shaped propeller, whether on the left or right hand, thereby showing its superiority in dynamic thrust. Various alterations were made, but always with the same result. These experiments have since been confirmed, and there seems no doubt that the double-curved shaped blade _is_ superior. (See Fig. 39.)

-- 23. =The Fleming-Williams Propeller.=--A chapter on propellers would scarcely be complete without a reference to the propeller used on a machine claiming a record of over a quarter of a mile. This form of propeller, shown in the group in Fig. 36 (top right hand), was found by the writer to be extremely deficient in dynamic thrust, giving the worst result of any shown there.

[Ill.u.s.tration: FIG. 39.--CURVED DOUBLE PROPELLER.

The most efficient type yet tested by the writer, when the blade is made hollow-faced. When given to the writer to test it was flat-faced on one side.]

[Ill.u.s.tration: FIG. 40.--THE FLEMING-WILLIAMS MODEL.]

It possesses large blade area, large pitch angle--more than 45 at the tip--and large diameter. These do not combine to propeller efficiency or to efficient dynamic thrust; but they do, of course, combine to give the propeller a very slow rotational velocity. Provided they give _sufficient_ thrust to cause the model to move through the air at a velocity capable of sustaining it, a long flight may result, not really owing to true efficiency on the part of the propellers,[36] but owing to the check placed on their revolutions per minute by their abnormal pitch angle, etc. The amount of rubber used is very great for a 10 oz. model, namely, 34 strands of 1/16 in. square rubber to each propeller, i.e. 68 strands in all.

[Ill.u.s.tration: FIG. 41.--THE SAME IN FLIGHT.

(_Reproduced by permission from "The Aero."_)]

On the score of efficiency, when it is desired to make a limited number of turns give the longest flight (which is the problem one always has to face when using a rubber motor) it is better to make use of an abnormal diameter, say, more than half the span, and using a tip pitch angle of 25, than to make use of an abnormal tip pitch 45 and more, and large blade area. In a large pitch angle so much energy is wasted, not in dynamic thrust, but in transverse upsetting torque. On no propeller out of dozens and dozens that I have tested have I ever found a tip-pitch of more than 35 give a good dynamic thrust; and for length of flight velocity due to dynamic thrust must be given due weight, as well as the duration of running down of the rubber motor.

-- 24. Of built up or carved out and twisted wooden propellers, the former give the better result; the latter have an advantage, however, in sometimes weighing less.

FOOTNOTES:

[24] _Note._--Since the above was written some really remarkable flights have been obtained with a 1 oz. model having two screws, one in front and the other behind. Equally good flights have also been obtained with the two propellers behind, one revolving in the immediate rear of the other. Flying, of course, with the wind, _weight_ is of paramount importance in these little models, and in both these cases the "single stick" can be made use of. _See also_ ch.

iv., -- 28.

[25] _See also_ ch. viii., -- 5.

[26] Save in case of some models with fabric-covered propellers. Some dirigibles are now being fitted with four-bladed wooden screws.

[27] Vide Appendix.

[28] Vide Equivalent Inclinations--Table of.

[29] One in 3 or 0333 is the _sine_ of the angle; similarly if the angle were 30 the sine would be 05 or , and the theoretical distance travelled one-half.

[30] _Flat-Faced Blades._--If the blade be not hollow-faced--and we consider the screw as an inclined plane and apply the d.u.c.h.emin formula to it--the velocity remaining the same, the angle of maximum thrust is 35. Experiments made with such screws confirm this.

[31] Cavitation is when the high speed of the screw causes it to carry round a certain amount of the medium with it, so that the blades strike no undisturbed, or "solid," air at all, with a proportionate decrease in thrust.

[32] In the Wright machine r.p.m. = 450; in Bleriot XI. r.p.m. = 1350.

[33] Such propellers, however, require a considerable amount of rubber.

[34] But _see also_ -- 22.

[35] "Flight," March 10, 1910. (Ill.u.s.tration reproduced by permission.)

[36] According to the author's views on the subject.

CHAPTER VI.

THE QUESTION OF SUSTENTATION THE CENTRE OF PRESSURE.

-- 1. Pa.s.sing on now to the study of an aeroplane actually in the air, there are two forces acting on it, the upward lift due to the air (i.e. to the movement of the aeroplane supposed to be continually advancing on to fresh, undisturbed _virgin_ air), and the force due to the weight acting vertically downwards. We can consider the resultant of all the upward sustaining forces as acting at a single point--that point is called the "Centre of Pressure."

Suppose A B a vertical section of a flat aerofoil, inclined at a small angle _a_ to the horizon C, the point of application of the resultant upward 'lift,' D the point through which the weight acts vertically downwards. Omitting for the moment the action of propulsion, if these two forces balance there will be equilibrium; but to do this they must pa.s.s through the same point, but as the angle of inclination varies, so does the centre of pressure, and some means must be employed whereby if C and D coincide at a certain angle the aeroplane will come back to the correct angle of balance if the latter be altered.

In a model the means must be automatic. Automatic stability depends for its action upon the movement of the centre of pressure when the angle of incidence varies. When the angle of incidence increases the centre of pressure moves backwards towards the rear of the aerofoil, and vice versa.

Let us take the case when steady flight is in progress and C and D are coincident, suppose the velocity of the wind suddenly to increase--increased lifting effect is at once the result, and the fore part of the machine rises, i.e. the angle of incidence increases and the centre of pressure moves back to some point in the rear of C D.

The weight is now clearly trying to pull the nose of the aeroplane down, and the "lift" tending to raise the tail. The result being an alteration of the angle of incidence, or angle of attack as it is called, until it resumes its original position of equilibrium. A drop in the wind causes exactly an opposite effect.

[Ill.u.s.tration: FIG. 42.]

-- 2. The danger lies in "oscillations" being set up in the line of flight due to changes in the position of the centre of pressure. Hence the device of an elevator or horizontal tail for the purpose of damping out such oscillations.

-- 3. But the aerofoil surface is not flat, owing to the increased "lift" given by arched surfaces, and a much more complicated set of phenomena then takes place, the centre of pressure moving forward until a certain critical angle of incidence is reached, and after this a reversal takes place, the centre of pressure then actually moving backwards.

The problem then consists in ascertaining the most efficient aerocurve to give the greatest "lift" with the least "drift," and, having found it, to investigate again experimentally the movements of the centre of pressure at varying angles, and especially to determine at what angle (about) this "reversal" takes place.

[Ill.u.s.tration: FIG. 43.]

-- 4. Natural automatic stability (the only one possible so far as models are concerned) necessitates permanent or a permanently recurring coincidence (to coin a phrase) of the centre of gravity and the centre of pressure: the former is, of course, totally unaffected by the vagaries of the latter, any shifting of which produces a couple tending to destroy equilibrium.

-- 5. As to the best form of camber (for full sized machine) possibly more is known on this point than on any other in the whole of aeronautics.