42
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Accepted
Do multi-engine jets need all engines with equal age to reduce asymmetry in thrust and fuel consumption arising out of deterioration?
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 ...
- 126k
39
votes
Accepted
Why do pilots slowly give the plane more thrust during takeoff?
The engines may accelerate slightly differently, resulting in an asymmetric thrust. Spooling up slowly at first (normally to about 60% N1) prevents this. After that, you can accelerate up to full TO/...
- 2,751
37
votes
Accepted
Can hydrogen gas be an alternative fuel source for a gas turbine engine?
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 ...
- 230k
35
votes
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What does it mean to "firewall" an aircraft engine?
"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 ...
- 844
32
votes
How can lift be less than thrust that is less than weight?
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 ...
- 12.9k
29
votes
Why do pilots slowly give the plane more thrust during takeoff?
A pure engine-side viewpoint:
Engines don't really like their power level being changed quickly. Doing so, one increases the thermal and mechanical stress over the engine parts, as well as the ...
- 1,182
26
votes
Accepted
Why is thrust inverse to speed in piston engines?
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 ...
- 230k
20
votes
Accepted
How does the bypass air provide thrust?
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 = \...
- 54.3k
19
votes
Accepted
How do rocket engines produce more thrust than aircraft jet engines?
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 ...
- 230k
17
votes
How do rocket engines produce more thrust than aircraft jet engines?
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 ...
- 2,577
17
votes
Why do pilots slowly give the plane more thrust during takeoff?
High-powered single prop planes may react violently if the throttle is opened abruptly. Torque reaction and P-factor should be smoothly dealt with...
- 11k
17
votes
Why do turboprops not have a thrust value in the specifications?
Because the turbofan's thrust value is derived from the integrated fan unit that is part of the engine. It's all an integrated product of the engine manufacturer.
A turboprop's thrust making ...
- 126k
17
votes
Accepted
How can a turbofan's fan generate thrust in cruising flight if air is being slowed down?
While in cruise, air is actually pushed into the propeller or fan, there is no "sucking in" taking place which would imply a lower pressure at the fan than the surrounding air.
Source: TU ...
- 61.3k
16
votes
Accepted
How do airplanes with thrust lines above CG still fly without a down pitch moment?
What you generally do with a high thrust line is place the horizontal stabilizer surface directly in the propeller wash to help offset some of the trim change caused by thrust changes.
When you add ...
- 126k
16
votes
What determines maximum speed of a propeller aircraft - Thrust or Power?
People, let's not confuse the newbie with endless comments, but write a proper answer!
Why power?
Power is reasonably constant over speed, so it is more useful than thrust when characterizing a ...
- 230k
15
votes
Can hydrogen gas be an alternative fuel source for a gas turbine engine?
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 ...
- 1,906
14
votes
What does it mean to "firewall" an aircraft engine?
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.
- 2,468
14
votes
Can hydrogen gas be an alternative fuel source for a gas turbine engine?
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 (...
- 10.2k
14
votes
Accepted
Flex temperature isn't actually "fooling" the FADEC, right?
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.
...
- 54.3k
12
votes
Accepted
How (and why) does engine thrust change with airspeed?
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 ...
- 230k
12
votes
Accepted
Are thrust levers synchronized by default when pushed/pulled?
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 ...
- 126k
12
votes
Accepted
What is the general mathematical relationship between increasing an airplane's weight and the needed thrust to fly?
While the takeoff and stall speeds increase, the cruising speed doesn't need to change. Assuming that the cruising speed is kept constant, the thrust increase can be derived as follows:
At cruise, the ...
- 4,332
10
votes
Accepted
Is 1 propeller always more efficient than 2?
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 ...
- 230k
10
votes
How does the bypass air provide thrust?
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 ...
- 61.3k
10
votes
Accepted
Why do turboprops not have a thrust value in the specifications?
It's a bit of a historical reason. In the olden days of only piston engines powering aeroplanes, the unit of measurement for the capability of the engine was the Brake Horse Power BHP. The engine ...
- 61.3k
10
votes
Accepted
Why is the Cirrus Vision SF50's turbine angle so acute?
There are two questions here: why so and how is it handled.
For the first one, I'll refer to the Jpe61's answer. Basically, it's just a reasonable compromise for a compact single-engine passenger jet.
...
- 9,013
10
votes
How does a fan in a turbofan engine suck air in?
Starting with the marine prop example you used in the comment on John's answer. It creates high pressure behind the prop and low pressure in front of the prop, so the BOAT moves forward. The water is ...
- 28.4k
9
votes
Accepted
Does a turboshaft's exhaust provide any thrust?
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 ...
9
votes
Accepted
Are there any airliners capable of vertical climb with engine thrust only by pointing the nose up?
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 ...
- 101k
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