Chapter 10

 

Chapter 10

Design of Airscrews: How they work, pitch and diameter, setting out blanks.

IN writing this book the main underlying purpose is to present the subject of scale model aircraft in as simple a way as possible consistent with scientific investigation, and therefore we have carefully avoided any mathematical formula.

However, when we discuss the design of airscrews we are dealing with a subject which could involve us in a maze of wonderful figures. And even suppose we designed our propeller accordingly, the chances are that the finished article might not be the very best choice for our job.

So what we propose to do is to give the reader a simple method of laying out his own propeller in accordance with the best accepted principles, and then, by a system of trial and error, eventually evolving the best propeller for the particular model under consideration.

If the builder keeps to the same scale in building his models he will, after a time, have an assortment of various propellers of approximately the same size, and by interchanging them it may be possible to find the best one for any particular aircraft.

It must always be borne in mind that however well the model has been designed and built, and however much rubber has been stowed in the fuselage (and the rubber is the motive force), if we cannot convert this energy efficiently our flights must perchance be of a poor order.

It is the propeller alone that converts our power within the model to powered flight. Therefore, however nice a particular propeller appears on a model, if it does not deliver the goods it is no use to us! - Although we generally term a propeller a propeller, it is a very loose phrase, because a propeller in reality should be placed behind the main-plane and act as a " pusher ".or " propeller." The propeller in front, as is the more general position, is really termed a " tractor," or one that pulls.

However, to suit our purpose we often use the word " airscrew," which embraces both types, although even this word is a relic of earlier days, when people imagined that its blades " screwed " their way through the air similarly to a screw worming its way through . a nut.

Actually, however, we must imagine each blade to be an airfoil or wing,^and in revolving through the air it is developing lift or thrust, giving forward motion and drag in the form of torque.. '. And just as in our wing we have incidence or angle of attack, so we must have this angle of attack in our airscrew. This angle, too, must be kept within reasonable limits, otherwise our propeller will stall.

A beautiful model built by Mr. E. Dyer.A beautiful model built by Mr. E. Dyer.

The "thrust" or pulling power of the airscrew is used to overcome the drag (or wind resistance) of the aeroplane, and the thrust depends on the diameter and speed of rotation. ' It also varies with the forward speed of the machine. With the model flying at a certain speed, then, there will be a certain speed at which the airscrew must revolve, and irom this speed of rotation and the forward speed and drag of the machine we should be able to determine the best airscrew. The design of an airscrew on scientific lines, however, is rather out of the question, since there are so many forces on the model we are unable to measure. By far the best plan is to try two or three different pitches, since the diameter is fixed by the scale, and a strand or two, more or less, of rubber on the motor will vary the speed of rotation.

Fig.18Fig.18

The " pitch " of an airscrew is the distance it travels, forward (theoretically) in one complete revolution. See Fig.18. However, there is a certain amount of slip, usually at least 25 per cent, so we seldom have an airscrew that is more than 75 per cent efficient. The pitch business can be likened to a gearbox on a car. A high pitch is like the high or top gear; it travels a long way in one revolution, but requires a lot of power, whereas, a low pitch, like low gear, can do with less power per revolution, as it does not travel so far, but it will turn over faster. If the pitch is too large the airscrew will stall and not generate sufficient thrust to pull the machine through the air. On the other hand, the pitch must be high enough to suit the flying speed of the aeroplane. There is quite a wide margin between too large and too small a pitch, which is another reason for trying different pitches to find the best.

We now want some guide as to what pitch "to try, and since we have a number of different diameters, this is best done by a pitch to diameter ratio. One writer is of the opinion that this should be from 1 to 1-3. That is, if we have a pitch/diameter ratio of 1'3 we should have a pitch of 13 in. for a diameter of 10 in., or a pitch of 7'8 in. for a diameter of 6 in. The other writer considers that pitch/diameter ratio should be lower, and might usefully vary with the ratio of diameter to wing-span. He thinks that if the diameter/span ratio is, say, one-seventh, as with the B.A. Swallow, the pitch/ diameter ratio should be one-half to two-thirds, and where the diameter/span ratio is, say, one-quarter, the pitch/diameter ratio could be about 1/1. Here, then, is another reason for experiment.

Blade area, too, plays a part, and is varied by the width, since the diameter will be fixed. This again has some connection with the pitch, since a wider blade will give more thrust and will stand a larger pitch.

This width should not be too great or it will look ugly. A larger diameter would be much better, though not to scale. Looks and efficiency would be a big improvement on scale diameter with very wide blade.

If we are building an advanced model with two airscrews, we could work out the pitch as above, and then multiply it by 1'4, i.e. the square-root of the number of airscrews.

If we consider Fig. 19, this will give us a good average arrangement suited to most purposes. Although it is drawn full-size for an airscrew 12in. diameter and 12 in. pitch, the figures we require can be substituted. Should we wish to make an airscrew that is to scale in width (we can fly a model successfully with one), the lengths a, b, c, d in Fig. 2 should be taken from the full-size airscrew.

These dimensions would be taken across the flat face of the blade, ignoring the effect of the twist.

Most probably, however, our propeller will be about 10 in. dia., with a pitch of between 7 in. and 13 in. Assuming we are building a model of 1 in. to the foot scale.

Decide on the size required, and, referring to Fig. 19a, draw the radius of the propeller OR, then draw OP, which is equal in length to the pitch divided by 6. Divide OR into four equal spaces, marked 1, 2 and 3, and mark another point 4 at ^ inch from the end R. Join these points to P. Next draw the centre line for Fig. 19b and mark off the lines 1, 2, 3 and 4 to correspond with Fig. 19a.

The lengths b and c are now put in, and should be equal to the diameter of the propeller divided by 9. Mark,off the lengths b and c, Fig. 19a, and draw a line shown dotted from the end of b.

This gives the lengths d and w, which should be marked off in Fig. 19b.

Draw a smooth curve round all the points, and from this curve measure the length of d. Put in the length of d in Fig. 19a.

Draw the centre line for Fig. 19c and the lines 1, 2, 3 and 4 as before, and mark off the lengths W, E, F, G and H,corresponding to the length of these lines in Fig. 19a. Draw in the curves as indicated, and we have a side view of one blade.

Draw a centre line and lines 1, 2, 3 and 4 in Fig. 19d, and the length of the lines j, k, 1 and m from

Fig.19Fig.19

Draw in a curve through these points and point Z as indicated:

The centre XY is determined by the size of the boss or spinner, and should be made accordingly.

It is advisable to make the airscrew out of a good close-grained hardwood, such as walnut,

as it will probably get quite a lot of knocking about, and the slight extra weight involved will be in the right place.

Where a three-bladed airscrew is considered, the total blade area will be approximately the same as a two-blader, so the width of each blade will be less.

On examining the different sections in Fig. 19e it will be noted that the angle of the blade increases towards the hub. As the airscrew is revolved it will be seen that the tip of the blade has farther tp go in one revolution than the root of the blade, although in a forward direction the distance is the same, hence we get the twist effect when looking end on to the airscrew. The most effective part of the propeller is between a quarter and one-third of the radius from the tip, so we are careful in making this part the widest and the truest to pitch. Actually the pitch at the root is not correct, but we make it as near as we can, or even fair it off so that it makes as little disturbance to the air as possible in the form .of a boss or spinner.

We can always take a little off the width of the blades if we find that, with the power available, the propeller'^ too slow, although, as we shall see in the chapter on " motors," another loop of rubber may cure this trouble. On the other hand, a propeller that whizzes round with not much speed to the model probably has not enough blade area, assuming the pitch and workmanship to be up to standard, unless if happens to be very small in diameter compared with the wing span. A little experience here is the best way of learning, but we must not expect to fly before we can run.

 
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