# Tag Info

42

It doesn't quite work that way. When an engine wears out it's rated thrust doesn't decline; its "ITT Margin" (or some similar phrase - basically its thermodynamic margin at takeoff) declines. When setting takeoff thrust on a turbofan there is a target N1, or target torque on a turboprop or turboshaft, or Engine Pressure Ratio on a pure jet, and the engine ...

37

Yes, and it has been demonstrated 30 years ago on the Tupolev 155. This is/was a hydrogen-powered version of the Russian Tu-154B tri-jet. Only one has been built and has since been retired after demonstrating the use of liquid hydrogen in 5 experimental flights. In total, the Tu-155 performed about 100 flights with several fuels, among them hydrogen and ...

34

"To firewall" is a phrase meaning to go to full power. Most aircraft throttle controls provide full power when moved to their furthest forward position - the direction towards the firewall separating the nose mounted engine from the cockpit in aircraft in the past. The phrase is still used, just as we "dial" a telephone even though the telephone dial is no ...

33

I analyzed a Pratt & Whitney F100 turbofan last semester in my aerothermodynamics course, so allow me to answer this question. The short answer: the un-compressed air provides the majority of an engine's total thrust since the compressed air powers the engine. Correction: I forgot to mention that the fans also compress entering air. That is, all air ...

32

Your misunderstanding lies in your thought that lift is smaller than thrust, while in fact, lift is much larger than thrust. The lift is provided by the wings. Their purpose is exactly to create a lift force (upwards force) while requiring relatively little thrust (forwards force). How well they do this is expressed by their lift-to-drag ratio (L/D ratio). ...

26

Your question already contains the answer. As you say kinetic energy is proportional to velocity squared, so it is easier to accelerate air from 0 to 100 m/s than from 100 to 200 m/s. The same is true for the air flowing through the propeller disk. Even if we replace the propeller by a black box, or better a black disk, which simply adds a bit of pressure ...

20

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 ...

19

For one, don't only look at the engine, but at the whole propulsion system. This includes tanks, piping, controls, pumps and the actual engine. Now the rocket looks much less favorable, especially if you size the tanks for equal running times. The rocket does not need any of the parts which are ahead of the combustion chamber of a jet and also does not need ...

16

Seems as though everyone has missed the simple, obvious answer: the rate at which the engine burns fuel. To take a concrete example, the Saturn V's first stage carried 205,400 gal/770,000l of kerosene fuel, which it burned in a bit less than 3 minutes: https://www.space.com/18422-apollo-saturn-v-moon-rocket-nasa-infographic.html By contrast, a Boeing 747 ...

14

It's just an expression. It means to push the throttle as far forward as it will go (all the way to the firewall, if you can), or full power.

14

Hydrogen works just fine on rockets. However "just fine" on rockets doesn't mean it is practical on an aircraft. The only way you can utilize $\mathrm{H}_2$ is storing it cryogenically. This is because $\mathrm{H}_2$ goes supercritical at $-240\,{}^\circ\mathrm{C}$, and no matter how hard you squeeze it beyond this temperature it would refuse to liquify and ...

14

Using water as the carrier and splitting it on-board to hydrogen and oxygen is a nonstarter. Electrolysis takes vast amounts of electric power, so instead of just water you need to carry water plus (big, heavy) batteries. If you use batteries, you're better off just using electric motors to drive the turbines, that would be lighter than an electrolysis setup....

13

There are at least two misconceptions in your question: The way I understand flex temperature is: It's a user interface for pilots to request a derating of engine power to reduce wear on the engine. This is not quite correct. A derating would imply that the engine thrust needs to remain at the derated level after an engine failure. A FLEX or assumed ...

12

The first diagram you link to shows three lines but does not indicate what they represent. I guess the bold line is thrust over speed. Then this diagram is correct for a turbojet. Thrust $T$ is the difference between the engine's exit impulse minus the entry impulse: $$T = (\dot{m}_{air} + \dot{m}_{fuel})\cdot v_{exit} - \dot{m}_{air}\cdot v_{entry}$$ The ...

12

On a non-FADEC engine (older airplanes - not this one) where the thrust levers are connected to the engine fuel control units with a cable circuit or a teleflex cable, you have to manually adjust each lever to wherever it has to go to achieve a given N1 (fan speed), and this can result in small variations in position of each lever when all engines are at the ...

11

What they need to watch out for is applying the right stabilization process (thrust not to be applied in one go), even if both engines are brand new. Manufacturing tolerances and age affect the acceleration profile and/or idle thrust, and it may lead to veering off the runway. Any residual thrust difference thereafter would be minimal if perceptible (...

10

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 ...

9

Yes, the exhaust of any engine does provide some thrust (very little for piston engines). The amount of jet thrust for turboprops is in the range of 4%-15% of propeller-produced thrust. For a helicopter, it's only 2%-4% of rotor lift, but it's directed backwards. The helicopter in your picture would get 100-150 lbf of thrust from the engine exhaust, ...

9

No, there are few planes that can do this at all. In order to be able to climb out straight up you need a thrust-to-weight ratio greater than 1. In other words you need enough thrust to propel the full weight of the aircraft up without the wings generating lift. Wiki provides a brief list of aircraft and associated thrust to weight ratios, the Concorde ...

9

High performance sailboats can sail faster than wind speed in certain conditions. Therefore, your question is not as absurd as it might seem at first glance. However, the technique that enables sailboats to do so is not available to airplanes, at least not in flight. Sailboats use the speed difference between water and wind. Their keel keeps them from being ...

9

Indeed, exhaust gases can be expanded beyond Mach 1, and doing so will result in higher thrust from higher propulsion efficiency. The problem is that pressurised gas is expanded by constricting the cross-section area when lower than M1, and expanding it when higher than M1. So in order to achieve full expansion to ambient pressure $p_0$ at supersonic exhaust ...

8

The CF34-3and CF34-10E are quite different engines. The -10 produces more thrust for a number of reasons: It has a higher mass flow. The CF34-3 has a 44 inch fan, while the -10E has a 54 inch fan. So, the mass flow will be higher in the -10E, and thrust is related to mass flow. The bypass ratio is different, the -3 is 6.2 while the -10E is 5.4. This is ...

8

Ducts are only used for highly loaded propellers of a very small radius, namely the fan stage of turbofan engines. For lower disk loadings a shroud or duct would create more friction drag than it saves in efficiency. If the tip Mach number of the propeller is low enough (say below Mach 0.9), then the single propeller of maximum diameter will be the most ...

8

First off, your calculation for 9.8 W does not seem to be based on any actual physics. Check the units! I can't tell if you did $P=F\cdot m$ or $P=\frac{F}{m}$, but the result is either the nonsensical quantity of joule-kilogram per meter, or acceleration respectively. A correct calculation based on keeping something aloft by propelling air downwards would ...

8

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 ...

8

Yes, thrust force vector must have a force equal to (but opposite) the combined gravitational force and aerodynamic drag force (from velocity) to climb at constant speed. In horizontal flight, thrust equals drag to fly at constant speed. But some of the drag comes from the wing creating lift. Since the larger wing moves "a lot of air a little", ...

7

The rocket engine produces the same thrust regardless of the speed it moves. Differently, the thrust of the jet engine depends on velocity and declines as velocity increases, because of the ram drag. It is largely useless if the engine speed approaches the exhaust velocity. The exact formula for efficiency can be found here:  \eta_p = \frac{2}{1 + \frac{...

7

Hydrogen works just fine as turbine fuel, and does so in space launch turbopumps. Achieving full efficiency, power, and engine life on hydrogen will require tweaks to a pre-existing engine, of course. On the environmental side, H2 normally burns hotter than hydrocarbons, which produces more N2O, but combustion temperatures can be regulated, and have to be ...

7

Thrust is not decreased. The drag penalty means that more thrust is required to achieve a performance condition than without the drag penalty. In level flight, more thrust is required to go the same speed. When climbing, more thrust is required to achieve the same climb rate. Your maximum thrust available is what the engine can produce, so the extra drag ...

7

Short version of (by now) two other answers: a sailing vessel needs a sail in one medium and a keel in another: a foil in the water, conventionally; ropes dragging on the ground; electromagnets in the storm wall of Jupiter's Great Red Spot, in a science fiction story; reaction wheels in inertia, in a solar sail. Slightly longer: it needs two things (airfoil,...

Only top voted, non community-wiki answers of a minimum length are eligible