# Tag Info

45

Now that's an interesting phenomenon! I do not believe that this is a case of an illusion of any sort, or the engine dilating. What the video probably shows, is the engine slightly turning towards the window as takeoff thrust is being applied. Engine mounts are not 100% rigid, this can easily be observed on pretty much any passenger flight if you can see ...

39

It's simple. Cheap (development costs amortized decades before) and reliable. They made money for airlines. Or, you know, they wouldn't have bought them. There's more to operating costs than fuel burn, and in any case, fuel prices in the mid 90s were cheaper than at any time since the 20s in constant dollars. Fuel was so cheap that Air Canada was ...

25

Turbojets and turbofans are very similar indeed: both are turbine engines; both create thrust from jet exhaust; and both have a rotating implement in front that can be called a fan. Although in the case of the turbojet, it isn't called a fan but the compressor first stage. $\$ Junkers Jumo 109-004 So what is the difference? There are five types of ...

23

There are basically 3 limits that the engine faces, temperature (maximum turbine entry temperature or maximum compressor exit temperature), pressure (maximum compressor exit pressure) and stress (maximum stress in the blades as a result of spool speed). Varying the OAT for a specific engine design will hit one of these limits. When the OAT increases, the ...

22

In a turbojet, all the air goes through the engine proper, through the combustion chamber and all the stages of compressor and post-combustion turbine blades. In a turbofan, some of the air is just pushed by a fan around the rest of the engine. This is the "bypass". As Harper points out, it's not fundamentally different from a turboprop or extracting ...

19

The main gear sits between the wings and retracts inwards, such that the wheels will rest in the lower fuselage. Lengthening the main gear would either require to shift the gear attachment points outward or to retract one leg a bit forward and the other a bit backwards, such that both wheels will sit behind each other instead of side-by-side, as they do now -...

19

The bypass air is accelerated by the fan at the front of the turbofan engine. This changes its velocity and therefore its momentum, which is the definition of a force (in this case: thrust): $$F = \frac{\text{d}}{\text{d}t} p = m \frac{\text{d}}{\text{d}t} v = m \cdot a$$ This thrust contributes to the total thrust of the engine. How much will depend on ...

15

Completely different design philosophies They are both turbine engines, and that is where the similarity ends. In a turbojet, the compressor-burner-turbine package is optimized to make thrust. A turbofan engine is a type of turboshaft engine. These use a compressor-burner-turbine core, but use a secondary set of turbine blades to convert its thrust ...

14

The Airbus A380 comes with two engine choices: Rolls-Royce Trent 900 Engine Alliance GP7000 The thrust reversers are not part of the core engines. The reversers are developed by the company which produces the engine nacelle, in this case Safran Nacelles. They developed nacelles for both engine variants for the A380, including the first electrically ...

12

There are a couple of sources of loss throughout the process, as indicated in the figure from an old paper format uni book. I've had to translate the labelling, open to suggestions there. The percentages are valid for a high bypass turbofan manufactured in the late 80s. The total energy input starts with the fuel flow: chemical energy per second. Combustion ...

12

What you see is called a flat rated engine. It means the maximum thrust from the engine is constant below the flat rated temperature (usually 30°C). Above that temperature, thrust will decrease due to the EGT (exhaust gas temperature) limit. In order to achieve a constant thrust at lower temperatures, the N1 needs to be decreased accordingly. (CFM56-5A ...

10

The rotational inertia of the fan will continue to push air through the engine after the fuel supply has been reduced. Reduce the fuel supply too quickly and the result is like holding a match in a gale: the wind blows the flame out. To prevent this, modern engines use digital controls (FADEC) to ensure that the engine is kept within its operating ...

9

First, the airframe and systems were well-engineered and had few flaws and good aerodynamic qualities. Second, the cost to re-engine it with higher bypass ratio engines would have been high, and as such needed to be included in the cost-benefit analysis and balanced against the costs of operating the plane as-is with its original engines. The expected ...

9

A bypass fan provides thrust in the same way a propeller provides thrust: by increasing the energy content of the gas mass passing through the disk. The added energy is most effectively converted into thrust by allowing it to expand until internal pressure is equal to ambient pressure, so all added energy is converted into kinetic energy. This expansion ...

8

Airlines normally maintain what are called Engine Build Units (EBUs) in their spare engine pool. The engine will be pre-assembled up to the point where variations in position on-wing have to be accounted for. Components beyond that point are only installed at the time the engine is installed on-wing. Designers try to minimize engine "handing", mirror ...

7

Short answer This space is part of the inter-turbine duct (ITD), a duct which is required to maintain a satisfactory flow continuity between HPT and LPT rotating at different speeds (in the ratio of 1:3) and having different diameters. The duct is made of the gap and the larger (in the axis direction) LPT stage 1 vanes. Note this duct is a diffuser as the ...

7

It doesn't bypass everything, just the combustion chamber. Notably, the air still goes through a fan. High-bypass turbofan engines make most of their thrust from the ducted fan. Very much like a turboprop except the blades are smaller and enclosed by the cowling. @Harper's answer on What is the difference between turbojet and turbofan engines? explains ...

6

This was only a problem on the 1st generation of turbojets, where the fuel controller was effectively a needle valve controlled by the thrust lever and the "fuel control" was the pilot's brain. These engines also had the reverse problem; you couldn't "slam accelerate" them without overtemping and/or flaming out. Once hydromechanical fuel controllers came ...

6

Because lengthening the main gear struts constitutes an almost complete structural re-design of the main wing structure, with dramatic consequences on cost, lead time, re-certification, weight increase etc. The main landing gear of most swept back winged aeroplanes retract behind the rear spars, in the kinked section close to the fuselage. Picture above is ...

5

The MD80 was not obsolete at the time of its first flight and the decade thereafter. The market situation for 150 pax aeroplanes around 1980: Boeing B727-200: first flight 1967. Three JT8D-7/9/11 engines. MD-80: first flight 1979. Two JT8D-200 engines. The first generation of JT8D had a bypass ratio of 0.7, the JT8D-200 engines had an increased bypass ...

5

The difference between a ducted propeller and a turbofan is mainly determined by the difference between a propeller and a fan. A propeller has relatively few blades, which are relatively long and slender. A fan has many blades, with a relatively large chord. Like a household fan. A parameter to catch blade count and chord relative to blade length, is the ...

5

Two worlds There are two elements constraining propeller use: The thrust we can get from a free propeller is lower than from a ducted fan at the same airspeed. Propeller efficiency quickly decreases with Mach number. Propeller performance vs speed. Source For these reasons fans are the natural choice for large and fast aircraft, and propellers for slower ...

5

I suspect it's because the business case just wasn't there. Every retrofit project I've been involved in boiled down to the money. Retrofits aren't cheap and ultimately the mods have to recoup those costs through improved revenue or decreased operating expenses. Looking at your references, there was some structural changes to the wing and landing gear ...

5

There are four reasons why a propeller aircraft will exhibit a left-turning tendency: torque, spiraling slipstream, P-factor, and gyroscopic precession. Torque is due to Newton's third law of motion. As the engine spins the propeller, the propeller spins the airplane in the opposite reaction. Since most propellers spin clockwise as viewed from the cockpit, ...

4

Turboprops and turbojets - or, more broadly, jets - produce thrust in somewhat different ways. First of all, let's address the way thrust is produced. Per Newton's 2nd and 3rd laws, force equals acceleration times mass, and an action (accelerating the air) produces an opposite reaction. After canceling out the variables (the math is easy to find), thrust is ...

4

There's also benefit to parts commonality between aircraft, where if all of the DC-9 variants that an airline was flying were using JT8D engines, then that's one less airframe difference to be concerned about when it comes to maintenance and parts supplies.

4

The answer is unequivocally no. Not only is it not possible, but what would be the point of designing them to do that? The only thing that expands in the turbo jet engine are the turbine blades themselves, but the amount they expand is infinitesimally small.

3

After reading all the answers I felt that none of them really explained the answer in a way understandable to a layperson, so I will attempt to do so. First of all, both types of engines will burn fuel to generate energy, which is ultimately used to accelerate a stream of air towards the rear of the aircraft to create thrust. They differ in the method by ...

3

A burning turbine consists of compressor stage, burning stage and gas turbine stage. Both compressor and turbine stages consist of sets of stator and rotor blades and the rotors are connected via shaft so part of the work the turbine generates can be used to compress the intake air. The output from the turbine stage is a high-velocity jet of hot air-fuel-...

3

Assuming that the net thrust of a turbojet is constant is not correct. It is assumed to be constant (for simplicity by the aircraft performance engineers and usually valid for low subsonic speeds), but in reality, the performance is not constant, and it also varies with altitude. This is best shown by a simple simulation of a turbojet engine. The following ...

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