FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1910
1910 - 0340.PDF
(TED APRIL 30, 1910. CORRESPONDENCE:. The name and address oj the writer (not necessarily for publication) MUST in all cases accompany letters intended fo> insertion, or containing queries. Correspondents asking questions relating to articles which they have read in FLIGHT, would much facilitate our work of reference by giving the number of the letter. NOTE.—Owing to the great mass of valuable and interesting corre spondence which we receive, immediate publication is impossible, but each letter will appear practically in sequence and at the earliest possible moment. CAMBERED PLANES. [482] I have read " Naval Constructor's " article on " Aerial Propellers," and also the letters in your correspondence columns ie " Propellers," and concur in every respect with your remarks at the end of Mr. F. C. Harrop's letter (352). The aeroplane and the propeller-blade are undoubtedly analogous cases, and if concavity is correct practice for one, it must be correct for the other. Likewise, if a dipping edge be advantageous to the aeroplane, a dipping edge should also be incorporated in the design of a propeller-blade. The angle which the tangent to the leading edge of an aero plane, or, to speak more correctly, an aerofoil, should make whh the line of direction of flight, should vary with the velocity of flight ; and similarly, the angle which the tangent to the leading edge of a propeller-blade makes with the helical line of direction of movement should vary from point to point along the leading edge, owing to the increased peripheral speed as you get further from the axis of revolution. This angle should be such that the propeller-blade or plane enters the air in a direction parallel to the lines of flow of the air. The designers of hydraulic centrifugal pumps and turbines are careful to arrange that the vanes or blades shall enter the liquid in this manner, and the designers of steam turbines are equally in agreement on this point. The economic result of such design is well proven. The purpose of an aerofoil or propeller-blade is to take hold of a mass of air and communicate motion to it in such direction that the reflex force generated simultaneously dominates and acts in opposi tion to the force of gravity in the case of the aerofoil ; or propels the mass of the aerodone in its direction of flight in the case of the propeller-blade. It 15 perfectly obvious then that as the air enters in lines ot flow parallel to the surface at the leading edge of the plane that the •surface of the plane must have such a concave curvature as will impart velocity to the mass of air coming within its sweep in such a manner that the reactive force balances gravity. The pressure on such a curvature will at any point act at right angles to the tangent at that point : consequently the upward or vertioal component in the case of an aerofoil of such pressure will get less, and the bickward or horizontal component get greater, as the angle which the tangent to the curvature makes with the line of direction of flight gets greater. It is a matter of calculation to find out at what such angle the upward lifting component counterbalances the weight of engine required to generate it and overcome at the same time the backward retarding component. Having settled this angle of trailing edge, all that remains is to connect it with the angle of leading edge with the most suitable and efficient surface of curvature. There are two important reasons deciding that the grading of this curve should be parabolic. Firstly, as is well known, the most advantageous and efficient manner in which to change the course of a fluid is to lead it in a path of gradually increasing curvature. Secondly, and this is most important, that ws may take advantage #f every inch of sustaining surface, the curvature should be such that as the air be given downward velocity the surface must follow it up, keeping tip an even pressure on it, in a similar manner to that in which the f>rce of gravity acts upon a falling body. With such a graded surface, the centre of pressure will remain relatively stationary for all variations of forward speed, other condi tions being equal, and we get rid of the dangerous tlugoid oscillations in the flight path. We have now to decide on the plan form, and are helped in this •decision by the knowledge that we must have as little side-edge as possible for the air to escape past; in other words, the greater the breadth in proportion to the depth for a given area of surface the better. The area of the surface will be dependent upon the weight it is designed to carry, and upon^ie speed at which it is designed To travel. The depth will be determined by the area and by the speed at which it is designed to travel. The breadth will of course be determined by the area and by the depth. Considerations of construction may modify the plan form, as for instance, designing to obtain greater strength for least weight, &c. It is curious that in every design of aerofoil I have examined, and they are many, the curvature has been formed gradually flatter towards the trailing edge ; this formation evidently entailing a falling off of the pressure reaction, with accompanying inefficiency. Apologising for the length of this letter, the importance of the subject being my justification, BELTON T. HAMILTON, M.I.A.E., A.M.I.Mech.E. Willesden Lane, N.W. [The above most interesting letter touches several fundamental points in aerofoil design, and also therefore—admitting the analogy between the aerofoil and the propeller—in the design of propeller- blades. Before it will be possible accurately to design either the deck of an aeroplane or the blade of a propeller with any degree of certainty as to its efficiency, reliable data will have to be collected on every point that Mr. Hamilton has raised. While agreeing in the main with the gist of our correspondent's remarks, we should like to suggest that his statement with respect to the camber is especially debatable where it relates to the main tenance of a uniform pressure reaction from entering edge to trailing edge. Approximately parabolic curves are often used in setting out the section of an aeroplane deck, but, as Mr. Hamilton remarks, the curvature flattens off towards the trailing edge. We judge that our correspondent takes exception to this design, but it seems to us that there may possibly be practical grounds for reducing the pressure intensity in the vicinity of discharge. We are not so much referring to any question of possible loss through disturbances of the class that characterise the air stream in the vicinity of the extremities of the plane, as to the probable influence that the trailing portion of the plane has on the stability of the whole member. From a purely theoretical point of view, a camber represented by the arc of a circle would make a satisfactory plane. The brothers Wright found that it was desirable in practice to shift the maximum versine close up to the leading edge, thereby converting the circular into a parabolic arc, and modern practice seems to approve of the extending of the trailing portion of the plane so that the trailing edge shall always be well below the level of the leading edge. Some reference is made to this matter in Lanchester's " Aerodynamics," p. 281, where the author speaks of curtailing the dipping front edge. In connection with the dipping front edge itself, it is important to bear in mind the effect of the cyclic up-current as giving an upward trend to the air before it meets the leading edge of the plane. This point, to which we have had occasion to refer in FLIGHT before (notably Vol. I, p. 296), is apt, if ignored, to confuse any discussion on the attitude of a plane in flight.—ED.] 15-20-H.P. ENGINE WANTED. [483] Could you or some reader of your valuable paper tell me where I could obtain a cheap flight engine (15-20 h.p.), with or without propeller, for sale or hire ? Blackrock, Co. Dublin. G. HUTTON. AERONAUTICS AND THE NAVY. [484] With regard to Mr. Griffith Brewer's article on " Aero nautics for the Navy," it may be interesting to some of your readers to point out that the author does not consider man-lifting kites as part of a warship's equipment. As a matter of fact, at this moment, one, at least, of our battleships is experimenting in this direction. The kite does not possess the grave disabilities which, in my belief, would make the use of captive balloon? at sea only possible on rare occasions. CHARLES C. TURNER. THE RIGHT SORT OF WORKMAN. [485] In your issue of March 19th, 1910 (No. 408), there appears a plea and suitability of pattern-makers for aeroplane work, and what is required of the mechanic who takes, in hand the work of aeroplane construction. No one will question that he must be a good judge of wood, strength, and quality of accuracy; but to state of all wood-working trades, for constructional purposes or manipulation, he is pre-eminent, is taking to himself that which is very wide of the mark, and bordering on 338
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
