The Theory and Practice of Model Aeroplaning Part 10

In Figs. 44 and 45 are given two very efficient forms of cambered surfaces for models.

[Ill.u.s.tration: FIG. 44.--AN EFFICIENT FORM OF CAMBER.

B D Maximum Alt.i.tude. A C Chord.

Ratio of B D: A C :: 1:17. A D 1/3 of A C.]

[Ill.u.s.tration: FIG. 45.--ANOTHER EFFICIENT FORM.

Ratio of B D to A C 1 to 17. AD rather more than 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.

[Ill.u.s.tration: 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 att.i.tude similar to that adopted in the Wright machine, as in Fig. 47.

-- 7. Apart from the att.i.tude 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."

[Ill.u.s.tration: FIG. 47.--ATt.i.tUDE OF WRIGHT MACHINE.]

The nature of the aerocurve, its area, the angle of inclination of its chord to the line of flight, its alt.i.tude, etc., are not the only important matters one must consider in the case of the aerofoil, we must also consider--

-- 9. Its =Aspect Ratio=, i.e. the ratio of the span (length) of the aerofoil to the chord--usually expressed by span/chord. In the Farman machine this ratio is 54; Bleriot, 43; Short, 6 to 75; Roe triplane, 75; a Clark flyer, 96.

Now the higher the aspect ratio the greater should be the efficiency.

Air escaping by the sides represents loss, and the length of the sides should be kept short. A broader aerofoil means a steeper angle of inclination, less stability, unnecessary waste of power, and is totally unsuited for a model--to say nothing of a full-sized machine.

In models this aspect ratio may with advantage be given a higher value than in full-sized machines, where it is well known a practical safe constructional limit is reached long before theory suggests the limit. The difficulty consists in constructing models having a very high aspect ratio, and yet possessing sufficient strength and lightness for successful flight. It is in such a case as this where the skill and ingenuity of the designer and builder come in.

It is this very question of aspect ratio which has given us the monoplane, the biplane, and the triplane. A biplane has a higher aspect ratio than a monoplane, and a triplane (see above) a higher ratio still.

It will be noticed the Clark model given has a considerably higher aspect ratio, viz. 96. And even this can be exceeded.

_An aspect ratio of_ 10:1 _or even_ 12:1 _should be used if possible._[37]

-- 10. =Constant or Varying Camber.=--Some model makers vary the camber of their aerofoils, making them almost flat in some parts, with considerable camber in others; the tendency in some cases being to flatten the central portions of the aerofoil, and with increasing camber towards the tips. In others the opposite is done. The writer has made a number of experiments on this subject, but cannot say he has arrived at any very decisive results, save that the camber should in all cases be (as stated before) very slight, and so far as his experiments do show anything, they incline towards the further flattening of the camber in the end portions of the aerofoil. It must not be forgotten that a flat-surfaced aerofoil, constructed as it is of more or less elastic materials, a.s.sumes a natural camber, more or less, when driven horizontally through the air. Reference has been made to a reversal of the--

-- 11. =Centre of Pressure on Arched Surfaces.=--Wilbur Wright in his explanation of this reversal says: "This phenomenon is due to the fact that at small angles the wind strikes the forward part of the aerofoil surface on the upper side instead of the lower, and thus this part altogether ceases to lift, instead of being the most effective part of all." The whole question hangs on the value of the critical angle at which this reversal takes place; some experiments made by Mr. M.B.

Sellers in 1906 (published in "Flight," May 14, 1910) place this angle between 16 and 20. This angle is much above that used in model aeroplanes, as well as in actual full-sized machines. But the equilibrium of the model might be upset, not by a change of att.i.tude on its part, but on that of the wind, or both combined. By giving (as already advised) the aerofoil a high aspect ratio we limit the travel of the centre of pressure, for a high aspect ratio means, as we have seen, a short chord; and this is an additional reason for making the aspect ratio as high as practically possible. The question is, is the critical angle really as high as Mr. Seller's experiments would show.

Further experiments are much needed.

FOOTNOTES:

[37] Nevertheless some models with a very low aspect ratio make good flyers, owing to their extreme lightness.

CHAPTER VII.

MATERIALS FOR AEROPLANE CONSTRUCTION.

-- 1. The choice of materials for model aeroplane construction is more or less limited, if the best results are to be obtained. The lightness absolutely essential to success necessitates--in addition to skilful building and best disposition of the materials--materials of no undue weight relative to their strength, of great elasticity, and especially of great resilience (capacity to absorb shock without injury).

-- 2. =Bamboo.=--Bamboo has per pound weight a greater resilience than any other suitable substance (silk and rubber are obviously useless as parts of the _framework_ of an aeroplane). On full-sized machines the difficulty of making sufficiently strong connections and a liability to split, in the larger sizes, are sufficient reasons for its not being made more use of; but it makes an almost ideal material for model construction. The best part to use (split out from the centrepiece) is the strip of tough wood immediately below the hard glazed surface. For struts, spars, and ribs it can be used in this manner, and for the long strut supporting the rubber motor an entire tube piece should be used of the requisite strength required; for an ordinary rubber motor (one yard long), 30 to 50 strands, this should be a piece 3/8 in. in diameter, and weight about 5/8 oz. per ft.

_Bamboo may be bent_ by either the "dry" heat from a spirit lamp or stove, or it may be steamed, the latter for preference, as there is no danger of "scorching" the fibres on the inside of the bend. When bent (as in the case of other woods) it should be bound on to a "former" having a somewhat greater curvature than the curve required, because when cool and dry it will be sure to "go back" slightly. It must be left on the former till quite dry. When bending the "tube"

entire, and not split portions thereof, it should be immersed in very hot, or even boiling, water for some time before steaming. The really successful bending of the tube _en bloc_ requires considerable patience and care.

Bamboo is inclined to split at the ends, and some care is required in making "joints." The ribs can be attached to the spars by las.h.i.+ng them to thin T strips of light metal, such as aluminium. Thin thread, or silk, is preferable to very thin wire for las.h.i.+ng purpose, as the latter "gives" too much, and cuts into the fibres of the wood as well.

-- 3. =Ash=, =Spruce=, =Whitewood= are woods that are also much used by model makers. Many prefer the last named owing to its uniform freedom from knots and ease with which it can be worked. It is stated 15 per cent. additional strength can be imparted by using hot size and allowing it to soak into the wood at an increase only of 37 per cent.

of weight. It is less than half the weight of bamboo, but has a transverse rupture of only 7,900 lb. per sq. in. compared to 22,500 in the case of bamboo tubing (thickness one-eighth diameter) and a resilience per lb. weight of slightly more than one half. Some model makers advocate the use of =poplar= owing to its extreme lightness (about the same as whitewood), but its strength is less in the ratio of about 4:3; its resilience is very slightly more. It must be remembered that wood of the same kind can differ much as to its strength, etc., owing to what part of the tree it may have been cut from, the manner in which it may have been seasoned, etc. For model aeroplanes all wood used should have been at least a year in seasoning, and should be so treated when in the structure that it cannot absorb moisture.

If we take the resilience of ash as 1, then (according to Haswell) relative resilience of beech is 086, and spruce 064.

The strongest of woods has a weight when well seasoned of about 40 lb.

per cub. ft. and a tenacity of about 10,000 lb. per sq. in.

[Ill.u.s.tration: FIG. 47A.--"AEROPLANE ALMA."

A very effective French Toy Monoplane.]

-- 4. =Steel.=--Ash has a transverse rupture of 14,300 lb. per sq. in., steel tubing (thickness = 1/30 its diameter) 100,000 lb. per sq. in.

Ash weighs per cub. ft. 47 lb., steel 490. Steel being more than ten times as heavy as ash--but a transverse rupture stress seven times as high.

Bamboo in tube form, thickness one-third of diameter, has a transverse rupture of 22,500 lb. per sq. in., and a weight of 55 lb.

per cub. ft.

Steel then is nine times as heavy as bamboo--and has a transverse rupture stress 44 times as great. In comparing these three substances it must be carefully borne in mind that lightness and strength are not the only things that have to be provided for in model aeroplane building; there is the question of _resistance_--we must offer as small a cross-section to moving through the air as possible.

Now while ash or bamboo and certain other timbers may carry a higher load per unit of weight than steel, they will present about three to three and a half times the cross-section, and this produces a serious obstacle, while otherwise meeting certain requirements that are most desirable. Steel tubing of sufficiently small bore is not, so far as the writer knows, yet on the market in England. In France very thin steel tubes are made of round, oval, hexagon, etc., shape, and of accurate thickness throughout, the price being about 18s. a lb.

Although suitable steel tubing is not yet procurable under ordinary circ.u.mstances, umbrella steel is.

-- 5. =Umbrella Section Steel= is a section 5/32 in. by 1/8 in. deep, 6 ft. long weighing 21 oz., and a section 3/32 in. across the base by 1/8 in. deep, 6 ft. long weighing 195 oz.

It is often stated that umbrella ribs are too heavy--but this entirely depends on the length you make use of, in lengths of 25 in. for small aerofoils made from such lengths it is so; but in lengths of 48 in.

(two such lengths joined together) the writer has used it with great success; often making use of it now in his larger models; the particular size used by him weighs 13 grammes, to a length of 25 in. He has never had one of these aerofoils break or become kinked--thin piano wire is used to stay them and also for spars when employed--the front and ends of the aerofoil are of umbrella steel, the trailing edge of steel wire, comparatively thin, kept taut by steel wire stays.

-- 6. =Steel Wire.=--Tensile strength about 300,000 lb. per sq. in. For the aerofoil framework of small models and for all purposes of staying, or where a very strong and light tension is required, this substance is invaluable. Also for framework of light fabric covered propellers as well as for skids and shock absorber--also for hooks to hold the rubber motor strands, etc. No model is complete without it in some form or another.