# Tag Info

41

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 ...

36

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 ...

31

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). ...

18

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 ...

15

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

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....

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

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 ...

10

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 (...

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 ...

7

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 ...

7

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 ...

7

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 ...

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

Yes, the exhaust does provide some thrust. The amount of thrust is in the range of 4%-15% of prop thrust for turboprops. 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, compared to 3,000-5,000 lbf of lift provided by the rotor. This thrust ...

6

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{...

6

Much like how a gas pedal controls the fuel input of a car's engine, so do the thrust levers on jet-liners. The coasting equivalent is idle thrust. The engine keeps running, but with minimal fuel flow. The minimum fuel flow is not fixed as it is a function of altitude, airspeed, and temperature. Nowadays it is typically managed by the FADEC, so the pilot (...

6

Your plan is to not store the hydrogen, but generate the hydrogen inflight. The problem is that generating hydrogen inflight is even more awkward than storing it. The component chemicals you combine to make hydrogen are much, much heavier than the hydrogen itself (hardly a surprise since hydrogen is by far the lightest atom)... and both heavier and more ...

5

First of all, the thickness-related drag increase is small, especially for airfoils below 15% of thickness. Thickness by itself mainly determines structural efficiency and wing volume and is chosen on those merits. A very common choice for GA planes is 15% - 16% at the wing root tapered to 10% - 12% at the wing tip. Next, much depends on the wing's bracing. ...

5

Please refer to the following website, it explains the plug programming.: http://nandang-smart.blogspot.com/2014/09/cfm56-7b-identification-plug.html You may read: WHEN ENGINES ARE MOVED BETWEEN AIRCRAFT AND MAY REQUIRE A DIFFERENT THRUST INSTALLATION THIS N1 MODIFIER IS USED IN CALCULATIONS TO UPRATE OR DERATE A GIVEN ENGINE AT AN APPROPRIATE LEVEL. ...

5

Does it mean the same percentage of takeoff thrust might provide different thrusts according to the operator? Not just per operator, it is different per engine installation in one fleet. That's why calculations are done according to each airframe's AFM. And as the engine ages, its thrust decreases* (and fuel consumption increases), and this is reflected ...

5

For any airplane to fly level at a constant altitude, the amount of lift generated by its wings must be equal to its weight. If lift exceeds weight, the plane starts to climb. If weight exceeds lift, the plane starts to descend. In a constant rate climb/descent, lift equals weight; a simplification that ignores the propulsive force vector. The engine & ...

5

Because a wing produces way more lift than drag. Thrust must equal drag, not lift. ...thrust-to-weight ratio as low as 0.3. That's a drag-to-lift ratio, actually. And that is why you find fixed wing aircraft everywhere, otherwise all aircraft would be VTOLs

5

No, the thrust levers are not automatically synchronized. You could set one engine to 100% and the other to full reverse thrust, just by moving the levers. Having said that, there are some airplanes that kind of do what you're talking about. For instance, most Airbus designs have detents in the throttle position which basically tell the computer to take ...

4

When originally developed, an engine design will have a "thermodynamic limit" that sets the maximum energy you can pull out of the basic core configuration. Because the initial development is super conservative, when first developed there will be ample thermodynamic margin in the design, allowing more energy to be passed through it with various detail ...

4

The answer is simpler than others mention here. It is simply that the rocket operates at much higher temperatures than gas turbines do. This translates to more thrust at the greater temperatures. Turbine blades in turbojets would melt at such high temperatures. Rockets cool the outer lining of the rocket combustion chamber. This is done by using liquid ...

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