Wat Did You

... 12 (J's!

By AMTECH Services R. D. 8, Mansfield, Ohio

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N CONNECTION with our advertising in SPORT AVIATION, we often receive inquiries like this one: "Dear Sirs: "I am designing and building a two-place airplane and would like to know the thickness of the

spars to stand 12 G's. What would be your cost for this calculation?"

In turn, we reply: "Unfortunately, the situation is not as simple as you may think. The spar design is one of the last steps and there is no such thing as just one '12 G's' design requirement. Instead, the aircraft is designed for a number of certain flight conditions known as the 'flight envelope' which, in turn, determines the 'G's' or, more correctly stated, the (air) load factors. From these the air loads are determined and shear forces, bending and torsional moments are calculated; they, in turn, govern the dimensions of various members of the aircraft structure such as wing spars and fittings, struts, thickness of skin, fuselage longerons, tail, etc. "It is a long, tedious work. There are no short cuts. And the faster the aircraft, the more details must be considered . . . . " It appears to us that many designers of homebuilt aircraft are either unaware of or have only a nebulous idea about the aircraft flight envelope. Even some pilots are not quite sure what it is or what it does to an aircraft, although some may remember that the air speed should be reduced in turbulent air, that under "high

speed" conditions flutter may develop and that one does not pull out of a dive suddenly, to mention a few. Of course, we can only read about the others who cannot tell their story. So, what is the flight envelope? The flight envelope is a graphic presentation (diagram) of limit flight conditions within which the aircraft must be operated in order to prevent structural damage and failure. During past decades the flight envelope changed somewhat; with more knowledge about turbulent air, maneuvering forces, human capability, technology and other related factors the flight envelope of today is not as crude as it used to be. In addition, the designer has a choice of selecting the design category. In this country

known at present as Federal Aviation Regulations (FAR), currently in process of being reissued. The new FAR Volume III which encompasses the design requirements for Normal, Utilty, and Aerobatic Category Airplanes is now available from the United States Government Printing Office, Superintendent of Documents, in Washington, D.C. 20402. Price: $5.50 plus $1.50 for foreign mailing. The current Part 23 succeeded the Civil Aeronautics Manual 3 (May, 1962): Airplane Airworthiness; Normal, Utility, and Aerobatic Categories. CAM 3 is more voluminous and has references which may facilitate the under-

standing of various requirements, although there are some typographical errors. Also, some changes are evident in the new Part 23. Another good source of information is the FAA's booklet "Basic Glider Criteria" which contains sample

calculations of the flight envelope, examples of good design practices, and advice on structural testing of aircraft movements. It also covers the design of auxiliary powered sailplanes. "Basic Glider Criteria", reprinted 1969, FAA 5.8/2: G49/962, is available from the same Government Printing Office source for a price of $1.00. So, the question comes up: What does the flight envelope look like? Fig. 1 shows a typical flight envelope for an airplane. The horizontal axis of the diagram is the air speed V (mph.), the vertical axis is the load factor n. Thus, the flight envelope is also known as the V-n diagram. A simple definition of the load factor n is the ratio of an air load L (acting normal to the assumed horizontal axis of the aircraft) due to a maneuever or gust to the aircraft weight W. n = L_, W

and since weight W = M X G, the air load L = M(n X G); thus the expression for example, "6 G's" or "8 G's," etc.

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the FAA issues the design requirements. Other countries have similar regulations.

While the FAA design requirements are primarily directed toward certification of production aircraft, any designer of homebuilt airplanes would find a wealth of information in them. To questions like: How strong must a wing spar or a fuselage longeron be in order to fly safely through rough air?; how many "G's" should the aircraft be designed for?, etc., helpful suggestions can be found by studying the FAA design requirements, also

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V mpt,

FIG. 1 SPORT AVIATION

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12 G's? . . .

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The flight envelope actually consists of two separate, general flight conditions known as the maneuvering and gust criteria.

V .ph

FIG. 2

V »p k

FIG. 3

The maneuvering part (Fig. 2) is due to various maneuvers, not neccsarily of an aerobatic nature. The gust part (Fig. 3) is based on gust loading encountered by the aircraft. The maneuvering envelope is simply defined by the FAR, as shown later on. The gust loading (Fig. 3) requires some elaboration for better understanding.

When an aircraft is hit by an up gust, as shown in Fig. 4, the angle of attack cc will suddenly be increased by an amount A«. This increase in the angle of attack produces a corresponding increase in lift coefficient, as shown in Fig. 4, thus the lift will be increased, i.e., the wings will experience a larger load. The lift increase AL = AC L X S X p X v2 where "2 S = wing area (ft.2), /> — air density (lb. sec.2/ft.