A few questions about Jet engine.

[LikeABoss]

I can't seem the understand something, and this may(probably is) a silly question to most people.

I get that most of the air goes around the engine (through the fan) and not within the core(bypass ratio), but don't understand why that is, nor what benefit that gives?

Can somebody please explain?

[redsnail]

I goo goo'd Benefits of a high bypass ratio engine.

This is what wiki (http://en.wikipedia.org/wiki/Bypass_ratio) has to say. Amongst other things, much better fuel efficiencies. As ever, read wiki with a grain of salt.

[ReverseFlight]

When the B747 entered service, I was pleasantly surprised by how quiet it was compared to the then popular B707s, VC10s, DC10s and TriStars. Many people were asking, since the B747 was twice the size of the average commercial jet at that time, surely it must be twice as loud ?

Read about the benefits of reducing shear in high bypass engines.

[Fliegenmong]

Not a heavy driver, but I think Reverse Flight has it, the 'shear' of 'core' engine exhaust / thrust against the relatively slow surrounding air causes the ear splitting scream, whilst a High ByPass ratio produces a gradual effect from the relatively slow to the High By Pass air, to the core exhaust / thrust air, lessening the 'sheer'

Is that roughly it?, I may not have articulated that as well as possible, I reckon I grasp it though.....

(Edited to change 'ration' to 'ration'....no high high Bypass rations at the Deli :p)

[Lightning Mate]

The Froude formula will give you the answer with regards to propulsive efficiency.

[4015]

A high bypass ratio engine provides thrust in two main ways.

The core, which acts the same as a turbojet. This chucks out high velocity, hot air in order to generate thrust. This high flow velocity causes a lot of noise for a small increase in thrust, as was evident in the late 60's as jets got bigger and more powerful.

The FAN part of the turbofan works more like a propeller. This chucks out lower velocity air (relatively cold but still heated due to the work done on it, probably not important to OP), but lots of it to generate thrust. Low flow velocity, less increase in noise for an increase in thrust.

Thrust is, as you may have gathered, a force, and is therefore approximately described as a simple one: Force = Mass x Acceleration

Since the acceleration is across the engine (between the front and the back), and we are only interested in what's going on at the back of the engine, we can say that the thrust is approximated by: Force = Mass x Velocity

From this, we can see that there are two ways to increase the thrust; increase the velocity of the flow or increase the mass of air being chucked out the back. The turbofan uses the fan to increase the mass flow rate, increasing the trust, and giving all the lovely benefits listed in this and the previous posts. Hope this helps!