0
$\begingroup$

As far as I’m aware, high-bypass engines have far higher fuel efficiency than low-bypass engines, and as a result the effective specific impulse of the engine is increased. What I’m trying to figure out, and what no previous question I’ve found asks, is where’s the tradeoff?

There’s no free lunch, and a massive specific impulse boost for nothing doesn’t seem possible. What are you giving up with high-bypass engines?

$\endgroup$
1
  • $\begingroup$ Your first link is a question about the history of bypass engines, and your second is about the progression of bypass engines over history and into the future, though some of the background provided is certainly helpful. I’m not sure I see the connection to my question about engineering tradeoffs $\endgroup$ Aug 8, 2021 at 16:22

3 Answers 3

4
$\begingroup$

The bypass lowers the exhaust velocity of the engine. Which improves the efficiency, but limits the flight speed.

In jets (both turbojets and turbofans) the exhaust velocity does not grow with the inlet velocity. That means that with increasing flight speed the delta-V decreases, which increases propulsive efficiency, but the engine obviously can't produce any thrust when delta-V reaches zero, so this places an absolute limit on how fast the aircraft can fly.

Because efficiency is best close to the maximum speed, higher bypass is better for flying slower (airliners fly M0.8 to M0.85), while flying faster requires lower bypass (fighters tend to have only 0.5:1 to 1:1).

The other thing is that the core needs to be able to produce the power to drive the fan. So the bypass ratio raises as the designers learn to build stronger and more efficient turbines and compressors that allow higher compression ratio.

$\endgroup$
1
$\begingroup$

In addition to the good answer given by @JanHudec I can add a few things. To have the same thrust with a high bypass, the engine has to be a larger diameter, which has a bunch of downsides. This tends toward more weight, longer fan-blades, more gyroscopic forces etc. It can often lead to longer landing gear except on the 737, where instead the bottom of the nacelle was flattened. The engine-out drag on a high bypass would also tend to be higher.

$\endgroup$
0
$\begingroup$

In a conventional turbojet engine, all the air used to produce thrust by accelerating it flows through the core of the engine, where the combustor cans are. All of that air gets compressed and shot through the whole engine at high speed- but only a small part of it takes part in the combustion process. High speed means lots of frictional losses.

Bypass air (which has been accelerated only by the first-stage fan) does not flow through the (high speed) core and so dissipates less energy as friction, as pointed out by Jan Hudec. Here is a way to think about this in simpler terms:

Imagine a 100 horsepower O-200 engine as used in a Cessna 150. Its output drives a propeller, through which a significant mass flow of air is accelerated upon traversing the propeller disc, generating thrust. The power used to drive this "fan" is supplied by a relatively tiny mass flow of air through the engine which gets sucked in through the induction system, mixed with fuel, compressed, ignited, expanded, and exhausted. The pumping losses associated with this mass flow are small relative to the thrust developed by the fan- a very efficient arrangement.

So you can think of a piston engine driving a propeller as an ultra-ultra-ultra-high bypass ratio engine!

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .