# Tag Info

26

It's not military vs civilian, but subsonic vs supersonic-capable Note that subsonic military aircraft use the same engines as civilian aircraft, even if their names might be different. The KC-135 used initially the J-57 which was called JT-3C when used in the Boeing 707-120. Now they fly the CFM-56, which is used on the Boeing 737 and the A320. The C-5 ...

25

Yes in principle, but some modifications are advisable. What limits the maximum operating altitude of a jet engine (besides the thrust needed to climb up there) is the length of the combustion chamber and the absolute pressure of the air entering it. Since the atmospheric pressure drops with altitude and the compression ratio of the compressor stays ...

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 ...

19

On modern turbojets the bypass air provides (at least) two things Thrust The air does bypass the engine core, but it is accelerated by the N1 fan and provides thrust as it is expelled rearward out of the engine. Thrust from the bypass air can contribute more than half of the total thrust produced by the engine (upwards of 80% of the total thrust for some ...

11

Increasing the bypass actually decreases the thrust. A good example is the CFM56-5B variant (sortable table on Wikipedia). The reason is easy to grasp, the more air bypasses the core, the lower the core's power. The next question would be how did the TF39 achieve higher bpr and thrust? Through an extra half-fan and extra turbine stage. It will make sense by ...

11

fooot has pointed out the GE F101 (powers the B-1 Lancer) has a BPR (bypass ratio) of >2:1 and comes with an afterburner. Other examples are the Turbo-Union RB199 (1.1:1) and the Volvo RM8 (0.97:1). Generally, anything below 4:1 is a low BPR, and bypass air is needed for cooling. Pure turbojets resort to other means of cooling, such as the ram air inlet on ...

11

Because the priorities for military aircraft (engines) are different. While it is true that the high bypass turbofans have better fuel economy (in cruise) and are less noisy, the low bypass engines offer significant advantages when we take into account their intended use in combat aircraft, such as: The response of the low bypass turbofans to throttle ...

10

It is 9.6:1 not 9:6:1. 9.6 parts air are bypassed for each part that goes through the core.

10

If we define thrust T of a turbofan as $$T = \dot{m} \cdot (V_e - V_0)$$ with: $\dot{m}$ = total mass flow through both the gas generator and the fan $V_e$ = average exhaust speed $V_0$ = airspeed we can see that thrust is proportional to mass flow. At higher altitude the air density is lower, so mass flow through the engine is lower, but as long as ...

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 ...

9

Although of course the engines will be important in allowing an aircraft to reach its service ceiling, the choice of service ceiling has more to do with the aircraft than the engines in this case. Airliners are designed to to be economical to operate. Flying higher puts more stress on the fuselage to maintain the same cabin pressure. To guard against the ...

9

An airplane engine provides thrust by accelerating air (plus some combustion products) backwards. As air is accelerated backwards the plane is accelerated forward thanks to conservation of momentum. Momentum is proportional to velocity but kinetic energy is proportional to velocity squared. The result of this is that it is notionally more efficient to ...

9

Can you add reheat to a high bypass turbofan engine? Sure. The real question is what is the purpose for doing so? The bypass ratio of a turbofan is based largely on its operation regime. Turbojets or low bypass turbofans are excellent choices for operations above Mach 1 to around Mach 2-3. Larger bypass ratios are more suitable to high subsonic and ...

9

The need for a turbofan was realized as early as the mid-30s. The first jet aircraft were subsonic and the poor suitability of the propelling nozzle for these speeds due to high fuel consumption was understood, and bypass proposed, as early as 1936 (U.K. Patent 471,368). The underlying principle behind bypass is trading exhaust velocity for extra mass ...

8

Bypass ratio is the ratio of the overall flow to the core flow (which goes eventually through the combustion chamber). Overall flow is either the flow captured by the intake or going through the propeller disc. Even for piston aircraft it is possible to define a bypass ratio if you interpret the air which flows through the cylinders as the core flow. A ...

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

The bypass ratio of a turbofan engine is the ratio of the mass flow rate of the air that bypasses the engine core (i.e. it doesn't undergo combustion),$\dot{m_{f}}$ to the mass flow rate of the air that passes through the core $\dot{m_{c}}$ and undergoes combustion. The bypass ratio is usually given as the mass flow of fan by the mass flow of core i.e. \$\...

6

Early turbojets were so inefficient that adding a fan was considered but not implemented because that would had made the engines even more sluggish and narrowed the operating limits even more. If you compare the Jumo 004 with the EJ200, you will find that both are of similar size, have 8 compressor stages, but vary widely in their compression ratio and ...

5

The bypass air is actually what gives the jet engine most of its thrust. As the air enters the engine, some of it goes to the turbine core and runs the whole engine. But most of the air goes through and is sped up by the large fan giving it the thrust. Doing it this way increases the efficiency because the engine moves more air, although at a slightly lower ...

4

Bypass ratio (BPR) alone never tells the whole story. Take a low BPR engine and add to it a bigger fan, now you have higher BPR. Does that mean better efficiency? No. The reason is simple, by keeping everything the same, the same combustor/turbine do not have the right power/design to turn the now bigger fan at the optimum speed (slow fan). But if you use ...

3

The Cessna website mentions the engine type used (emphasis mine): The Citation Hemisphere will be powered by two next-generation Safran™ Silvercrest™ engines delivering over 12,000 pounds of thrust. The Silvercrest engines incorporate the latest advanced, field-proven technologies to offer unrivaled performance in its class in terms of direct operating ...

2

https://en.wikipedia.org/wiki/Pratt_%26_Whitney_F100 gives a maximum afterburning thrust for the F100-PW-220 of 105.7 kN, so you were pretty close there. Power is not listed on wikipedia, but you can get there. It lists "Specific fuel consumption: Military thrust: (0.73 lb/(lbf·h))", so all you need to know is the lower heating value of jet fuel, and you ...

2

So if thrust happens when there is an action-reaction pair with the air and fan blades, is there additional thrust provided when the mass of air pushed back by the fan blades exits the back of the engine? The thrust provided by the action of the fan blades against the air is the same thrust as the thrust provided by the air exiting the back of the engine. ...

2

Yes a lot of the thrust is from the discharge of the fan being crammed into the convergent nozzle and forced to accelerate. Similar to a regular jet engine where most of thrust is coming from the air coming out of the turbine being forced to accelerate by the convergent nozzle of the tail pipe. Best way to envision it is to look at a balloon that you blow ...

2

You can model the fan disc driving the bypass air flow as a well-shrouded propeller inside a short duct. It does indeed generate thrust which, as in the general case of a propeller, is transmitted to the engine case via a thrust bearing that supports the fan disc and thence to the airframe via the engine mounts.

2

Essentially, it is more efficient from a propulsion point of view. Let me explain this with a very simple example. Imagine that you are over a skateboard and you would like to propulse yourself using your hands to give yourself the boost. You have 2 options, either you use your hands over another person over an skateboard or you impulse yourself using a ...

2

In the late 60s/early 70s, P&W proposed an engine (the JTF17) for the projected USA Supersonic Transport (SST) which would have introduced afterburning in the fan-generated section of the airflow: a fan-burner. This doesn't quite match the sense of the questioner's description but you should always beware of assuming that the most trodden paths are the ...

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