Boeing Trying to buy older 747s

 Boeing is acquiring previous versions of the 747 from airlines ordering its new, tough-to-sell 747-8. Of the 19 older 747s that have changed hands so far this year, Boeing has snapped up seven, according to data compiled by Ascend Online Fleets. That makes it the biggest buyer of the used jets in 2013.

 Boeing helps nurture demand for the newer 747-8—among a class of fuel-thirsty four-engine aircraft that airlines frown upon these days. New sales are pivotal to keeping 747 assembly lines humming as Boeing slows output 13 percent to 1.75 planes a month and stashes some unsold 747-8s in desert storage.

 the Ascend data show that this year’s sellers of 747-400s to the world’s largest planemaker are all buyers of the 747-8 family, which includes both passenger and all-freight versions. The buyers are Korean Air Lines  and Cathay Pacific Airways , as well as Cathay’s Dragonair unit and its cargo joint venture with Air China .

It entered commercial service in 2011, two years late, after Boeing diverted engineers to the delayed 787 Dreamliner. Production of the 747-400 ended in 2009. New features on the 747-8 include improved engines and an elongated version of the fuselage hump that gives the plane its distinctive profile.

Boeing’s buybacks help 747-8 customers avoid recording losses on older planes they would otherwise struggle to sell amid a global glut of used jumbos

 The 747-8 is a victim of Boeing’s engineering success:

The bottom line: Boeing is taking old 747s in trade in order to encourage sales of its newer $350 million 747-8 jumbo jet.

Ground effect aircraft

The total drag of an airplane is divided into two components, parasite drag arid induced drag. Induced drag is the result of the wing's work in sustaining the airplane. The wing lifts the airplane simply by accelerating a mass of air downward. It is perfectly true that reduced pressure on top o f an airfoil is essential to lift, but still that is but one o f the thing s that contribute to the overall effect of rushing an air mass downward. The amount of downwash is directly related to the work o f the wing in pushing the mass of air down and therefore to the amount of induced drag produced. At high angles of attack, induced drag is high. As this corresponds to lower airspeeds in actual flight, it can be said that induced drag predominates at low speed.


When a wing is flown very near the ground, there is a substantial reduction in the induced drag. Downwash is significantly reduced; the air flowing from the trailing edge of the wing is forced to parallel the ground. The wing tip vortices that also contribute to Induced drag are substantially reduced; the ground interferes with the formation of a large vortex.


Many pilots think that ground effect is caused by air being compressed between the wing and the ground. This is not so. Ground effect is caused by the reduction of induced drag when an airplane is flown at slow speed very near the surface.

Ground effect exerts an influence only when the airplane is flown at an altitude no greater than its wing span, which for most light airplanes is fairly low. A typical light airplane has a wing span of perhaps 35 feet and will experience the effect of ground effect only when it is flown at or below 35 feet above the surface (ground or water).


A low wing airplane is generally more affected by ground effect than a high wing airplane because the wing is closer to the ground. High wing airplanes are, however, also influenced by this phenomenon.


Pilots get into trouble because o f ground effect when they precipitate take-o ff before the airplane has reached flying speed. Take the scenario of a pilot trying a take-o ff fro m a poor field. He uses full power and holds the airplane in a nose high position. Ground effect reduces induced drag and the airplane is able to reach a speed where it can stagger off. As altitude is gained, induced drag increases as the effect of the ground effect diminishes. Twenty o r thirty feet up, ground effect vanishes, the wing encounters the full effect of induced drag and the struggling airplane which got off the ground o n the ragged edge of a stall becomes fully stalled and drops to earth.


Ground effect is also influential in landing. As the airplane flies down fro m free air into ground effect, the reduction of induced drag as it nears the runway comes into, effect to make the airplane float past the point of intended touchdown. In the common case of an airplane coming in with excessive speed, the usable portion o f the runway may slip by with the airplane refusing to settle down to land. A go around will probably be necessary. On short fields, approach as slowly as is consistent with safety.

An airplane also tends to, be more longitudinally stable in ground effect. It is slightly nose heavy. The downwash from the wing normally passes over the tail at an angle that produces a download on the tail. Ground effect deflects the path of the downwash and causes it to pass over the tailplane at a decreased angle. The tailplane produces mo re lift than usual and the nose of the airplane tends to drop. To counteract this tendency, more up elevator is required near the ground. During take-off as the airplane climbs out of ground effect, the download on the tailplane increases and the nose tends to pitch up.

Area Rule of an Aircraft

The area rule is an important concept related to the drag on an aircraft or other body in transonic and super sonic flight. The area rule came into being in the early 1950s when production fighter designs began pushing ever closer to the sound barrier. Designers had found that the drag on these aircraft increased substantially when the planes traveled near Mach 1, a phenomenon known as the transonic drag rise illustrated below . This increase in drag is due to the formation of shock waves over portions of the vehicle, which typically begins around Mach 0.8, and this drag increase reaches a maximum near Mach 1. Because of its source, this type of drag is referred to as wave drag.

Increase in wave drag at transonic Mach numbers

Since the physics of supersonic flight were still largely a mystery to manufacturers, designers had no idea how to address this problem except to pr ovide their aircraft withmore powerful engines. Even though jet engine technology was rapidly advancing inthose days, the first generation of jet- powered fighters was hampered by relatively low-thrust engines which limited them to subsonic flight. The US Air Force hoped to overcome this deficiency with its first dedicated supersonic fighter, the F-102 Delta Dagger.

Since the transonic drag rise was still not fully understood, the F-102's designers chose an engine they believed would provide enough thrust to reach a maximum speed of about Mach 1.2. However, initial flight tests of the YF-102 prototype indicated that theaircraft couldn't even reach Mach 1. The Convair engineer s were baffled by this lack of per formance until a NACA researcher named Dr. Richar d Whitcomb developed the arearule. Whitcomb experimented with several different axisymmetric bodies and wing-bodycombinations in a transonic wind tunnel. What he found was that the drag created on these shapes was directly related to the change in cross-sectional area of the vehicle from the nose to the tail. The shape itself was not as critical in the creation of drag, but the rate of change in that shape had the most significant effect. For the mathematically inclined, we can say that wave drag is related to the second-derivative (or curvature) ofthe volume distribution of the vehicle.

Whitcomb area rule test models: (a) cylindrical fuselage, (b) fuselage with wings, (c) bulged
fuselage, (d) waisted fuselage with wings

To illustrate the point, four of Whitcomb's experimental models are drawn above, representing a simple cylindrical fuselage, the same fuselage with wings attached, a bulged fuselage, and a "pinched" fuselage with wings. What Whitcomb discovered wasthat the addition of wings to the basic cylinder produced twice as much drag as thecylinder alone. He also found that drag rose by the same amount if a simple bulge were added to the cylinder , the bulge being of equivalent volume as the wings. However, if he reduced the cross -sectional area of the fuselage over the region were the wings were attached, shown as body "D," the total drag was about the same as that of the cylinderalone. The conclusion of this r esear ch was that shaping the vehicle to create a smooth cross- sectional area distribution from the nose to the tail could drastically reduce the drag on an aircraft. The area rule tells us that the volume of the body should be reduced in the
presence of a wing, tail surface, or other projection so that there are no discontinuities in
the cross-sectional area distribution of the vehicle shape.



Effect of the area rule on overall vehicle shape

Whitcomb's findings are related to a more theoretical concept called the Sears- Haack body. This shape yields the lowest possible wave drag for a given length and volume. The variation in cross-sectional ar ea for a Sears-Haack body, illustrated in the following figur e, tells us that wave drag is minimized when the curvature of the volume distribution is minimized. The closer the volume distribution of an aircraft or other high-speed vehicle comes to the ideal Sears-Haack body, the lower its wave drag will be.


Volume distribution of a Sears-Haack body

Whitcomb's research was a major breakthrough in supersonic aerodynamics and had
an immediate effect on the design of the afor ementioned F-102 fighter. Convair engineers quickly redesigned the aircraft's fuselage, taking the area rule concept into account, to create the "waisted" or "coke-bottle" fuselage. This modification, plus a new engine, allowed the aircraft to easily exceed Mach 1 and achieve a maximum speedover Mach 1.5.


Effect of the area rule

Today's supersonic fighters are fitted with much more power ful engines than were available in the 1950s, so the area rule is not as essential to their design as it used to be. However, it has found greater application to subsonic aircraft, particularly commercial airliners since they cruise at the lower end of the transonic regime. A good example is the Boeing 747 known for its distinctive "hump." This hump, which houses the cockpit and upper passenger deck, increases the cross-sectional area of the forward fuselage and has the effect of evening the volume distribution over the length of the aircraft. As a result, the 747 is able to cruise efficiently at a slightly higher speed than most other airliners since the increase in transonic wave drag is delayed.

Adverse yaw and Aircraft Turns

To understand what ADVERSE YAW is, we need to first explain the axes of motion for an
airplane. An aircraft in flight can rotate around three different axes, as illustrated below.




Aircraft axes of motion


First, the air craft nose can rotate up and down about the y-axis, a motion known as
pitch. Pitch control is typically accomplished using an elevator on the horizontal tail.
Second, the wingtips can rotate up and down about the x-axis, a motion known as roll.
Roll control is usually provided using ailerons located at each wingtip.
Finally, the nose can rotate left and right about the z-axis, a motion known as yaw. Yaw control is most often accomplished using a rudder located on the vertical tail.



Aircraft control surfaces

However, the effect of one control surface is not always limited to just pitch, roll, or yaw alone. When the deflection of one control surface affects more than one of these orientations, we say that the orientations are coupled. The most important of these coupled interactions is adverse yaw. To better understand the concept, let's study a picture of what happens when the pilot deflects the ailerons to roll the aircraft.




Effects caused by aileron deflection


As you can see, the aircraft rolls because one aileron is deflected downward while the other is deflected upward. Lift increases on the wing with the downward-deflected
aileron because the deflection effectively increases the camber of that portion of the wing. Conversely, lift decreases on the wing with the upward-deflected aileron since the camber is decreased. The result of this difference in lift is that the wing with more lift rolls upward to create the desired rolling motion.

Unfortunately, drag is also affected by this aileron deflection. More specifically, two types of drag, called induced drag and profile drag, are increased when ailerons are deployed. Induced drag is a form of drag that is induced by any surface that generates lift. The more lift a surface produces the more induced drag it will cause (for a given wingspan and wing area). Thus, the wing on which the aileron is deflected downward to generate more lift also experiences more induced drag than the other wing. Profile drag includes all other forms of drag generated by the wing, primarily skin friction and pressure drag. This profile drag increases on both wings when the ailerons are deflected, but the increase is equal when the ailerons are deflected by the same amount. However, the induced drag on each side is not equal, and a larger total drag force exists on the wing with the down aileron. This difference in drag creates a yawing motion in the opposite direction of the roll. Since the yaw motion partially counteracts the desired roll motion, we c all this effect adverse yaw.

We can correct for this effect in several ways, the most important methods being:

1. FRISE AILERON

The concept behind this particular kind of aileron is to minimize
the profile drag on the wing with the down aileron while increasing the profile
drag on the wing with the up aileron. This difference in profile drag counteracts
the effect of induced drag thereby creating a yawing motion that at least partially
cancels the adverse yaw effect.





Frise ailerons

Frise ailerons accomplish this differential profile drag by maintaining a smooth
contour between the upper surfaces of the wing and aileron, causing very little
drag, while the bottom surface of the aileron juts downward to create a large
increase in profile drag. Although this approach is simple and does provide some
relief, the performance of Frise ailerons is very dependent on operating
conditions. For this reason, such ailerons are often only partially effective at
overcoming adverse yaw.

2.DIFFERENTIAL AILERONS

Another approach to solving adverse yaw is to deflect the ailerons by differing amounts. The deflection of the down aileron is typically much less than the up aileron so that the additional profile drag is very small compared to that on the up aileron.



Differential ailerons

As in the case of Frise aileron, this differential profile drag produces a yawing
motion that at least partially offsets the adverse yaw, but the effect is limited.

3. SPOILERS

Spoilers are long narrow flat plates typically fitted along the upper surface of both wings. In normal flight, spoilers lie flat and generate no effect on the aerodynamic performance of the wing. However, the spoilers can be raised upward into the air flow to generate large turbulence that reduces the lift and
increases the drag on a wing.




Spoiler

When used in coordination with ailerons, a spoiler can be used to reduce the lift
and increase the profile drag on the wing with the up aileron. As a result, the
wing with the down aileron experiences a large increase in lift and a small
increase in drag while the wing with the up aileron experiences a large decrease
in lift and a large increase in drag. These effects combine to create the desire ed
roll motion and a complimenting yaw motion that is called provers yaw.

4.Cross-coupled controls

One of the most effective solutions to adverse yaw is
to couple the ailerons and rudder so that both sur faces deflect simultaneously.
As the ailerons create a yaw motion in one direction, the rudder automatically
deflects to create a yaw motion in the opposite direction. The two effects
counteract each other eliminating the undesired yaw. This form of cross-coupling
was often built into the cable-and-pulley control systems of older air craft. The
problem was recognized even as early as the Wright brothers who incorporated
such controls into the Wright Flyer. In addition, most major aircraft today utilize
some sort of computerized fly-by-wire control system, and it is rather trivial to
program cross-coupled control measures into the automated systems.