The Theory and Practice of Model Aeroplaning Part 8

[Ill.u.s.tration: FIG. 27.--O T = 1/3 O P.]

-- 19. =The Blades, two in number=, and hollow faced--the maximum concavity being one-third the distance from the entering to the trailing edge; the ratio of A T to O P (the width) being 0048 or 1 : 21, these latter considerations being founded on the a.n.a.logy between a propeller and the aerofoil surface. (If the thickness be varied from the entering to the trailing edge the greatest thickness should be towards the former.) The convex surface of the propeller must be taken into account, in fact, it is no less important than the concave, and the entire surface must be given a true "stream line" form.

[Ill.u.s.tration: FIG. 28.]

[Ill.u.s.tration: FIG. 29.]

If the entering and trailing edge be not both straight, but one be curved as in Fig. 28, then the straight edge must be made the _trailing_ edge. And if both be curved as in Fig. 29, then the _concave_ edge must be the trailing edge.

-- 19. =Propeller Design.=--To design a propeller, proceed as follows.

Suppose the diameter 14 in. and the pitch three times the diameter, i.e. 52 in. (See Fig. 30.)

Take one-quarter scale, say. Draw a centre line A B of convenient length, set of half the pitch 52 in. -- scale = 5 in. = C - D.

Draw lines through C and D at right angles to C D.

With a radius equal to half the diameter (i.e. in this case 1 in.) of the propeller, describe a semicircle E B F and complete the parallelogram F H G E. Divide the semicircle into a number of equal parts; twelve is a convenient number to take, then each division subtends an angle of 15 at the centre D.

Divide one of the sides E G into the same number of equal parts (twelve) as shown. Through these points draw lines parallel to F E or H G.

And through the twelve points of division on the semicircle draw lines parallel to F H or E G as shown. The line drawn through the successive intersections of these lines is the path of the tip of the blade through half a revolution, viz. the line H S O T E.

S O T X gives the angle at the tip of the blades = 44.

Let the shape of the blade be rectangular with rounded corners, and let the breadth at the tip be twice that at the boss.

Then the area (neglecting the rounded off corners) is 10 sq. in.

[Ill.u.s.tration: FIG. 30.--PROPELLER DESIGN.

One quarter scale. Diameter 14 in. Pitch 52 in. Angle at tip 44.]

The area being that of a rectangle 7 in. 1 in. = 7 sq. in. plus area of two triangles, base in., height 7 in. Now area of triangle = half base height. Therefore area of both triangles = in. 7 in. = 3 sq. in. Now the area of the disc swept out by the propeller is

{pi}/4 (diam.) ({pi} = 22/7)

[Ill.u.s.tration: FIG. 31.--PROPELLER DESIGN.

Scale one-eighth for A B and B C; but sections of blade are full-sized.]

And if _d_ A _r_ = the "disc area ratio" we have

(_d_ A _r_) {pi}/4 (14) = area of blade = 10,

whence _d_ A _r_ = 007 about.

[Ill.u.s.tration: FIG. 32.]

[Ill.u.s.tration: FIG. 33.]

In Fig. 31 set off A B equal to the pitch of the propeller (42 in.), one-eighth scale. Set off B C at right angles to A B and equal to

{pi} diameter = 22/7 14 = 44 in. to scale 5 in.

Divide B C into a convenient number of equal parts in the figure; five only are taken, D, E, F, G, H; join A D, A E, A F, A G, A H and produce them; mark off distances P O, S R, Y T ... equal to the width of the blade at these points (H P = H O; G S = G R ...) and sketch in the sections of blade as desired. In the figure the greatest concavity of the blade is supposed to be one-third the distances P O, S R ...

from PS.... The concavity is somewhat exaggerated. The angles A H B, A G B, A F B ... represent the pitch angle at the points H, G, F ... of the blade.

Similarly any other design may be dealt with; in a propeller of 14 in.

diameter the diameter of the "boss" should not be more than 10/16 in.

-- 20. =Experiments with Propellers.=--The propeller design shown in Figs. 32 and 33, due to Mr. G. de Havilland,[35] is one very suitable for experimental purposes. A single tube pa.s.sing through a T-shaped boss forms the arms. On the back of the metal blade are riveted four metallic clips; these clips being tightened round the arm by countersunk screws in the face of the blade.

The tube and clips, etc., are all contained with the back covering of the blade, as shown in Fig. 35, if desired, the blade then practically resembling a wooden propeller. The construction, it will be noticed, allows of the blade being set at any angle, constant or otherwise; also the pitch can be constant or variable as desired, and any "shape"

of propeller can be fitted.

The advantage of being able to _twist_ the blade (within limits) on the axis is one not to be underestimated in experimental work.

[Ill.u.s.tration: FIG. 34.--THE AUTHOR'S PROPELLER TESTING APPARATUS.]

With a view to ascertain some practical and reliable data with respect to the _dynamic_, or actual thrust given when moving through free air at the velocity of actual travel, the author experimented with the apparatus ill.u.s.trated in Figs. 34 and 35, which is so simple and obvious as to require scarcely any explanation.

The wires were of steel, length not quite 150 ft., fitted with wire strainers for equalising tension, and absolutely free from "kinks."

As shown most plainly in Fig. 35, there were two parallel wires sufficiently far apart for the action of one propeller not to affect the other. Calling these two wires A and B, and two propellers _x_ and _y_, then _x_ is first tried on A and _y_ on B. Results carefully noted.

[Ill.u.s.tration: FIG. 35.--PROPELLER TESTING.

Showing distance separating the two wires.]

Then _x_ is tried on B and _y_ on A, and the results again carefully noted. If the results confirm one another, the power used in both cases being the same, well and good; if not, adjustments, etc., are made in the apparatus until satisfactory results are obtained. This was done when the propellers "raced" one against the other. At other times one wire only was made use of, and the time and distance traversed was noted in each case. Propellers were driven through smoke, and with silk threads tied to a light framework slightly larger than their disc area circ.u.mference. Results of great interest were arrived at. These results have been a.s.sumed in much that has been said in the foregoing paragraphs.

[Ill.u.s.tration: FIG. 36.--ONE GROUP OF PROPELLERS TESTED BY THE AUTHOR.]

Briefly put, these results showed:--

1. The inefficiency of a propeller of the fan blower or of the static thrust type.

2. The advantage of using propellers having hollow-faced blades and large diameter.

3. That diameter was more useful than blade area, i.e. given a certain quant.i.ty (weight) of wood, make a long thin blade and not a shorter one of more blade area--blade area, i.e., as proportionate to its corresponding disc area.

4. That the propeller surface should be of true stream-line form.

5. That it should act on a cylinder and not tubes of air.

6. That a correctly designed and proportioned propeller was just as efficacious in a small size of 9 in. to 28 in. as a full-sized propeller on a full-sized machine.

[Ill.u.s.tration: FIG. 37.--AN EFFICIENT PROPELLER, BUT RATHER HEAVY.

Ball bearings, old and new. Note difference in sizes and weights.

Propeller, 14 in. diam.; weight 36 grammes.]