Share This Paper. Background Citations. Methods Citations. Citation Type. Has PDF. Publication Type. More Filters. In aircraft design a critical part of the design is the engine selection. This is typically making a selection from exiting engines. The canard offers lower trim drag, but in this case may require a higher wetted area. The only true way to determine whether a canard is a good idea for this or any aircraft is to design several aircraft, one with and one without a canard. UD Estimation For initial sizing, a wing aspect ratio of about 1 1 was selected With the area of the wing and canard both included, this is equivalent to a.
Comparing the sketch of Fig. This yields a wetted aspect ratio of 1. For a wetted aspect ratio of 1. This value, obtaine from an initial sketch and the selected aspect ratio, can now be used fordinitial sizing. For cruise, a value of 0. Takeoff-Weigh t Sizing From Table 3. For a subsonic aircraft the best SFC values are obtained with high-bypass turbof have typical values of about 0.
The extensive ASW avionicequatio s would not be included in that equation, so it is treated as a separate payload weight. At t IS altitude the speed of sound see Appendix A. The calculations in box 3. A "range trade" can be calculated to determine the Illcrease III deSIgn takeoff gross weight if the requ ired range is increase d.
In a similar fashion, a "payload trade" can be made. The given payload requirement is 1 0, lb of avionics equipment. Box 3. The results are plotted in Fig. The statistical empty-weight equation used here for sizing was based upon existing military cargo and bomber aircraft, which are all of aluminum construction.
For example, instead of the required 1 n. These calculations are shown in Box 3. This is a 9. This result sounds erroneous, but is actually typical of the "leverage" effect of the sizing equation. Unfortunately, this works both ways. If the empty weight creeps up during the detail-design process, it will require a more-than-proportional increase in takeoff gross weight to maintain the capability to perform the sizing mission.
Thus it is crucial that realistic estimates of empty weight be used during early conceptual design, and that the weight be strictly controlled during later stages of design.
There are many trade studies which could be conducted other than range, payload, and material. Methods for trade studies are discussed in detail in Chapter 1 9. The remainder of the book presents better methods for design, analysis, sizing, and trade studies, building on the concepts just given. In this chapter a conceptual sketch was made, but no guidance was provided as to how to make the sketch or why different features may be good or bad. Then more-sophisticated methods of analysis, sizing, and trade studies will be provided.
These are discussed in the next three chapters. This chapter covers selecting the airfoil and the wing and tail geometry. The P-5 1 was regarded as the finest fighter of World War II in part because of its radical laminar-flow airfoil. Recently, the low-speed airfoils developed by Peter Lissaman contributed to the success of the man-powered Gossamer Condor. Airfoil Geometry Figure 4. The front of the airfoil is defined by a leading-edge radius which is tangent to the upper and lower surfaces.
An airfoil designed to operate in supersonic flow will have a sharp or nearly-sharp leading edge to prevent a drag-producing bow shock. As discussed later, wing sweep may be used instead of a sharp leading edge to reduce the supersonic drag.
The chord of the airfoil is the straight line from the leading edge to the trailing edge. It is very difficult to build a perfectly sharp trailing edge, so most airfoils have a blunt trailing edge with some small finite thickness.
Total airfoil camber is defined as the maximum distance of the mean camber line from the chord line, expressed as a percent of the chord. In earlier days, most airfoils had flat bottoms, and it was common to refer to the upper surface shape as the "camber.
These terms are technically obsolete but are still in common usage. The thickness distribution of the airfoil is the distance from the upper surface to the lower surface, measured perpendicular to the mean camber line, and is a function of the distance from the leading edge.
The "airfoil thickness ratio" tic refers to the maximum thickness of the airfoil divided by its chord. For many aerodynamic calculations, it has been traditional to separate the airfoil into its thickness distribution and a zero-thickness camber line. The former provides the major influence on the profile drag, while the latter provides the major influence upon the lift and the drag due to lift.
In a similar fashion, an airfoil which is to have its camber changed is broken into its camber line and thickness distribution. The camber line is scaled to produce the desired maximum camber; then the original thickness distribution is added to obtain the new In this fashion , the airfoil can be reshaped to change either the. Airfoil Lift and Drag An airfoil generates lift by changing the velocity of the air passing ov:r and under itself.
Figure 4. Note that the upper surface of the wing contributes about two-thirds of the total lift. In Fig. It can be seen that the effect of the aIrf?
As a side effect of the gen ion of lift th. For a cambe! Odd as it sounds, an airfoil in two-dimensional 2-D flow does not experience any drag due to the creation of lift. The pressure forces produced in the generation of lift are at right angles to the oncoming air. It is only in three-dimensional 3 -D flow that drag due to lift is produced. By definition, the lift force is perpendicular to the flight direction while the drag force is parallel to the flight direction.
The pitching moment is usually negative when measured about the aerodynamic center, implying a nose-down moment. Note that 2-D airfoil characteristics are denoted by lowercase subscripts Le. The center of pressure is usually behind the aerodynamic center. The location of the center of pressure varies with angle of attack for most airfoils. Cm where Lift, drag, and pitching-moment characteristics for a typical airfoil are shown in Fig.
The Reynolds number influences whether the flow will be laminar or turbulent, and whether flow separation will oc. A typical aircraft wing operates at a Reynolds number of about ten mIllIon. For example, dirt, rain, or insect debris on the lea?
This also can change the lIft and pItchmg-moment characteristics. Airfoil Families A variety of airfoils is shown in Fig. The early airfoils were? In these, the first digit defined the percent camber, t? While rarely used for wing desIg? The NACA five-digit airfoils were developed to allow shifting the position of maximum camber forward for greater maximum lift.
Six-series airfoils such as the 64A series are still widely used as a startmg point for high-speed-wing design. Airfoil Design In the past, the designer would select an airfoil or airfoils from such a catalog. This selection would consider factors such as the airfoil drag during cruise, stall and pitching-moment characteristics, the thickness available for structure and fuel and the ease of manufacture.
Modern airfoil design is based upon inverse computational solutions for desired pressure or velocity distributions on the airfoil. Toward the rear of the airfoil, various pressure recovery schemes are employed to prevent separation near the trailing edge.
These airfoil optimization techniques result in airfoils with substantial pressure differentials lift over a much greater percent of chord than a classical airfoil. This permits a reduced wing area and wetted area for a required amount of lift. Such airfoil design methods go well beyond the scope of this book.
Another consideration in modern airfoil design is the desire to maintain laminar flow over the greatest possible part of the airfoil. Laminar flow can be maintained by providing a negative pressure gradient, i.
This tends to "suck" the flow rearward, promoting laminar flow. As an airfoil generates lift the velocity of the air passing over its upper surface is increased. If the airplane is flying at just under the speed of sound, the faster air traveling over the upper surface will reach supersonic speeds causing a shock to exist on the upper surface, as shown in Fig.
The speed at which supersonic flow first appears on the airfoil is called the "Critical Mach Number" The upper-surface shock creates a large increase in drag, along with a reduction in lift and a change in the pitching moment.
The drag increase Merit. For example, the lower longerons in Fig. Had the longerons been placed lower, they would have required a kink to pass over the box. On the other hand, the purpose of the longeron is to prevent fuselage bending. This implies that the lightest longeron structure occurs when the upper and lower longerons are as far apart vertically as possible.
Only a trade study can ultimately determine which approach is lighter for any particular aircraft. In some designs similar to Fig.
A kink over the wing box is avoided by passing the longeron under or through the wing box. Weight is minimized when the stringers are all straight and uninterrupted. Another major structural element used to carry fuselage bending loads is the "keelson. A keelson is frequently used to carry the fuselage bending loads through the portion of the lower fuselage which is cut up by the wheel wells.
As the wing provides the lift force, load-path distances can be reduced by locating the heavy weight items as near to the wing as possible. Similarly, weight can be reduced by locating structural cutouts away from the wing. Required structural cutouts include the cockpit area and a variety of doors passenger, weapons bay, landing gear, engine access, etc.
An especially poor arrangement seen on some older fighter aircraft has the main landing gear retracting into the wing-box area, which requires a large cutout where the loads are the greatest. When possible, structural cutouts should be avoided altogether.
FIgure 8. Large conce? Figure 8. The box carrythrough simply continues the wing box t. The fuselage itself is not sUbjected to any of the bendmg moment of the wing, which minimizes fuselage weight. However, the box carrythrough occupies a substantial amount of fuselage volume, and tends to add cross-sectional area at the worst possible place for wave drag, as discussed above. The wing panels are attached to fittings on the side of these fuselage bulkheads.
While this approach is uSilally heavier from a structural viewpoint, the resulting drag reduction at high speeds has led to the use of this approach for most modern fighters. Like the ring-frame approach, the wing panels are attached to the side of the fuselage to carry the lift forces. However, the bending moment is carried through the fuselage by one or several beams that connect the two wing panels.
This approach has less of a fuselage volume increase than does the box-carrythrough approach. Many light aircraft and slower transport aircraft use an external strut to carry the bending moments. While this approach is probably the lightest of all, it obviously has a substantial drag penalty at higher speeds. Aircraft wings usually have the front spar at about of the chord back from the leading edge.
Additional spars may be located between the front and rear spars forming a "multispar" structure. Multispar structure is typical for large or high-speed aircraft. If the wing skin over the spars is an integral part of the wing structure, a "wing box" is formed which in most cases provides the minimum weight. The heat shield may be constructed of titanium, steel, or a heat-proof matting.
On the other hand, an "integral" fuel tank in which the existing structure is simply sealed and filled with fuel will require no clearance other than the thickness of the skin. There is no easy formula for the estimation of structural clearance. The designer must use judgement acquired through experience. The best way to gain this judgement other than actual design experience is by looking at existing designs. Aircraft with the landing gear in the wing will usually have the gear located aft of the wing box, with a single trailing-edge spar behind the gear to carry the flap loads, as shown in Fig.
Ribs carry the loads from the control surfaces, store stations, and landing gear to the spars and skins. A multispar wing box will have comparatively few ribs, located only where major loads occur. Another form of wing structure, the "multirib" or "stringer panel" box, has only two spars, plus a large number of spanwise stringers attached to the wing skins.
Numerous ribs are used to maintain the shape of the box under bending. These are further discussed in Chapter 1 5. First-order structural sizing will be discussed in Chapter For initial layout purposes the designer must guess at the amount of clearance required for structure around the internal components.
A large airliner will typically require about 4 in. The structure of a conventional fighter fuselage will typically require about 2 in. For a small general aviation aircraft, 1 in. The type of internal component will affect the required clearance.
During World War I, the only "sensor" in use was the human eyeball. Camouflage paint in mottled patterns was used on both sides to reduce the chance of detection.
Radar acronym for Radio Detection And Ranging , the primary sensor used against aircraft today, consists of a transmitter antenna that broadcasts a directed beam of electromagnetic radio waves and a receiver antenna which picks up the faint radio waves that bounce off objects "illuminated" by the radio beam.
Detectability to radar has been a concern since radar was first used in World War II. Chaff, also called "window, " consists of bits of metal foil or metallized fibers dropped by an aircraft to create many radar echos that hide its actual echo return.
Chaff is still useful against less-sophisticated radars. Chaff obscures the actual location of the aircraft, but does not allow the aircraft to pass unnoticed. RCS is usually measured in square meters or in decibel square meters, with "zero dBsm" equal to ten to the zero power, or one square meter. Actually, the RCS of an aircraft is not a single number.
The RCS is different for each "look-angle" i. The following comments relate to typical threat radars seen by military aircraft. There are many electromagnetic phenomena that contribute to the RCS of an aircraft. Imagine shining a flashlight at a shiny aircraft in a dark hanger. Any spots where the beam is reflected directly back at you will have an enormous ReS contribution. Typically this "specular return" occurs on the flat sides of the aircraft fuselage and along an upright vertical tail when the radar is abeam the aircraft.
Note that this ReS reduction approach assumes that the designer knows where the threat radar will be located relative to the aircraft. Also, this assumes a monostatic radar. Another area of the aircraft which can present a perpendicular bounce for the radar is the round leading edge of the wing and tail surfaces. If the aircraft is primarily designed for low detectability by a nose-on threat radar, the wings and tails can be highly swept to reduce their contribution to ReS.
Note that this and many other approaches to reducing the ReS will produce a penalty in aerodynamic efficiency. It is also important to avoid any "corner reflectors," i. Another contributor to airframe ReS occurs due to the electromagnetic currents that build up on the skin when illuminated by a radar. This effect is much lower in intensity than the specular return, but is still sufficient for detection. The effect is strongest when the discontinuity is straight and perpendicular to the radar beam.
Carried to the extreme, this leads to diamond- or sawtooth-shaped edges on every door, access plate, and other discontinuity on the aircraft, as seen on the B-2 and F-1 This has advantages in ease of construction and signature analysis, but offers a large number of sharp edges to create diffraction returns, and so is no longer in favor Ref.
Current stealth design begins by configuring the aircraft such that all "big" returns, such as from perpendicular bounces, are "aimed" in just a few directions. For example, if the leading edges of the wings and tails are all straight and set at the same angle, there would be a huge radar return from that angle direction, but little return from other directions.
This would presumably offer a small probability that the aircraft and threat radar would be mutually oriented in exactly the angle of high return, and the aircraft would be undetectable from all other angles. It is also common practice to "aim" the wing leading-edge return in the same direction as the edge diffraction return from the trailing edge. This is done either by using identical sweep angles at the leading and trailing edges thus, a wing with no taper, as on the B-2 , or by aligning the left wing leading edge at the same sweep angle as the right wing trailing edge and vice versa.
This creates a diamond wing as on the F and an early Mikoyan research aircraft. Once all wing and tail returns are "aimed" in the same direction, the returns from doors, access panels, and other discontinuities can be "aimed" in the same direction by alignment of their edges. This is clearly seen on the B-2 where virtually every feature on the aircraft, including weapons, bay dodrs, gear doors, inlets, nozzles, and access panels, is constructed using only lines which are parallel to a wing leading edge.
This design approach leads to an aircraft planform composed entirely of straight, highly swept lines, much like the first-generation stealth designs. However, the desire to eliminate the edge diffractions caused by the facets of first-generation stealth now produces designs in which cross-sectional shapes are smooth, not sharp-edged.
The steep angles on the fuselage sides as shown in Fig. Such shaping can be seen on the B-2, F, and F RCS can also be reduced simply by eliminating parts of the aircraft. A horizontal tail that isn ' t there cannot contribute to the radar return!
Similarly, RCS can be reduced if the nacelles can be eliminated through the use of buried engines, or better yet, by eliminating the entire fuselage through the use of the flying-wing concept. This approach is used in the Northrop B In addition to reshaping the aircraft, detectability can be reduced through the use of skin materials that absorb radar energy. This will reduce not eliminate! However, one can probably assume that such use will reduce or eliminate any weight savings otherwise assumed for the use of composite materials.
For most existing aircraft, the airframe is not the largest contributor to RCS, especially nose-on. A conventional radome, covering the aircraft ' s own radar, is transparent to radar for obvious reasons. Therefore, it is also transparent to the threat radar, allowing the threat radar ' s beam to bounce off the forward bulkhead and electronic equipment within the radome.
Even worse, the aircraft ' s own radar antenna, when illuminated by a threat radar, can produce a radar magnification effect much like a cat's eye. Radar energy gets into these cavities, bounces off the engine parts, and sprays back out the cavity towards the threat radar.
Also, these cavities represent additional surface discontinuities. The best solution for reducing these RCS contributions is to hide them from the expected threat locations. For example, inlets can be hidden from ground-based radars by locating them on top of the aircraft Fig. Exhausts can be hidden through the use of two-dimensional nozzles.
Cockpits provide a radar return for a similar reason. One solution for this is to thinly coat the canopy with some conductive metal such as gold, causing the canopy to reflect the radar energy away. Missiles and bombs have fins that form natural corner reflectors. The carriage and release mechanisms have numerous corner reflectors, cavities, and surface discontinuities.
Gun ports present yet another kind of cavity. The only real solution for these problems is to put all the weapons inside, behind closed doors. The many techniques for tricking radar and ECM go beyond the scope of this book. However, designers should be aware that there is a tradeoff between the aircraft's RCS level and the required amount of ECM.
IR missiles can sometimes be tricked by throwing out a flare which burns to produce approximately the same IR frequencies as the aircraft. IR fundamentals are more thoroughly discussed in Ref. Many short-range air-to-air and ground-to-air missiles rely upon IR seekers. Also, sensors can detect the solar IR radiation that reflects off the skin and cockpit transparencies windows. This reduces both exhaust and hot-part temperatures. However, depending upon such an engine for IR reduction may result in selecting one that is less than optimal for aircraft sizing, which increases aircraft weight and cost.
This will also increase fuel consumption slightly. For example, the H-tails of the A-lO hide the nozzles from some angles. Unfortunately, the worst-case threat location is from the rear, and it is difficult to shield the nozzles from that direction!
Plume emissions are reduced by quickly mixing the exhaust with the outside air. Mixing can also be enhanced by the use of a wide, thin nozzle rather than a circular one. Another technique is to angle the exhaust upward or downward relative to the freestream.
This will have an obvious thrust penalty, however. Sun glint in the IR frequencies can be somewhat reduced by the use of special paints that have low IR reflectivity.
Cockpit transparencies which can't be painted! Emissions due to aerodynamic heat are best controlled by slowing the aircraft down. Also, fIghter aIrcraft usually have radar only in front, which leaves the eyeball as the primary detector for spotting threat aircraft which are abeam or above. In simulated combat, pilots of the small F-5 can frequently spot the much-larger F-1 5s well before the F-5s are seen.
However, aircraft size is determined by the mission requirements and cannot be arbitrarily reduced. Background contrast is reduced primarily with camouflage pamts, usmg colors and surface textures that cause the aircraft to reflect light at an intensity and color equal to that of the background.
Frequently aircraft will have a lighter paint on the bottom, because t. Different parts of the aircraft can contrast agamst each? To counter this, paint colors can be vaned to lIghten the dark areas such as where one part of the aircraft casts a shadow on another. Also, 'small lights can sometimes be used to fill in a shadow spot. Canopy glint is also a problem for visual detection. These can be reduced by techmques previously discussed for IR and glint suppression.
There are also psychological aspects to visual detectIon. If the aIrcraft does not look like an aircraft, the human mind may ignore it. In air-to-air combat seconds are precIOus. To this end, some aircraft have even had fake canopIes! Aircraft noise is largely caused by airflow shear layers, primarily due to the engine exhaust.
A turbofan falls somewhere in between. Blade shapmg and mternal duct shaping can somewhat reduce noise. Piston exhaust stacks are also a source of noise. Within the aircraft, noise is primarily caused by the engmes. Internal noise will be created if the exhaust from a piston engine impinges upon any part of the aircraft, especially the cabin. Wing-mounted propellers can have a tremendous effect on mternal nOise.
All propellers should have a minimum clearance to the fuselage of about 1 ft, and should preferably have a minimum clearance of about one-half of the propeller radius. However, the greater the propeller clearance, the larger the vertical tail must be to counter the engine-out yaw.
Jet engines mounted on the aft fuselage DC-9, B, etc. An aircraft can be "killed" in many ways. A single bullet through a non-redundant elevator actuator is as bad as a big missile up the tailpipe!
This refers to the product of the projected area square feet or meters of the aircraft components, times the probability that each component will, if struck, cause the aircraft to be lost. Vulnerable area is different for each threat direction. Typical components with a high aircraft kill probability near 1. The FMEA considers both the ways in which battle damage can affect individual aircraft components, and the ways in which damage to each component will affect the other components.
During initial configuration layout, the designer should strive to avoid certain features known to cause vulnerability problems. Description This best-selling textbook presents the entire process of aircraft conceptual design — from requirements definition to initial sizing, configuration layout, analysis, sizing, optimization, and trade studies.
One of the very best texts on conceptual design. Rakin Tajwar rated it it was amazing Jan 13, Book ratings by Goodreads. Orbital Mechanics Vladimir A. This website uses cookies to improve your experience while you navigate through the website. Out of these cookies, the cookies that are categorized as necessary are stored on your browser as they are as essential for the working of basic functionalities of the website.
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