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klm744
Topic Author
Posts: 109
Joined: Sun Jun 10, 2001 12:53 pm

Jet Engines And The Noise They Make

Sun Sep 30, 2001 12:20 am

What part or engine process makes the engine loud? For example is it the mechanical sounds i.e. the actual motor spinning the fans etc., or is it the actual ignition of the air/ fuel mixture, or is it the rush of the large amount of air out of the back of the engine. I realize it is probably a combination of all of those reasons, but which is responsible for the majority of the sound you hear as an airplane flys over your house.

Also, why has there been a trend in making engines with larger front fans. Such as the 777,747 as opposed to engines that look like little tubes such as the 737-200, DC-8 etc. Does this have something to do with them being high bypass? And for that matter what is high bypass?

I have noticed that along with the change of engine shape, the sound has changed. For example the 732 has a deep sound as well as a roar. The newer jets have less of a roar and only that deep bass sound. Why?
 
TWAalltheway
Posts: 142
Joined: Mon Jul 30, 2001 11:17 pm

RE: Jet Engines And The Noise They Make

Sun Sep 30, 2001 12:30 am

The reason why the engines have larger front fans is because that it what makes the engine quieter. The old "tubes" such as PWJT8D's have a low bypass ratio, which means most of the air that goes into the engine is used for thrust, and very little goes around the engine. On newer engines, such as CFM-56, they have a high bypass ratio, where most of the air goes around the engine, and little is used for thrust. (CFM's have approx. a 6:1 bypass ratio, JT8D's have approx. 1.8:1 bypass ratio.)

Based on my knowledge, what you hear from your house is the huge amounts of 500 degree air rushing out of the back of the engine, creating lots of noise. What you hear from inside the cabin is a combination of the turbines and the air. Hope this helps.

TWA all the way!



 
klm744
Topic Author
Posts: 109
Joined: Sun Jun 10, 2001 12:53 pm

RE: Jet Engines And The Noise They Make

Sun Sep 30, 2001 1:14 am

So if I understand correctly, the less air going into the engine the quieter it will be. In that case, how do the newer engines produce more thrust? It seems that with less air to work with they would have to really compress it to make up for the relatively lower amount of air going in, and it would be louder. Also, does the air that is sucked into the large front turbine that is bypassed contribute to the thrust or is it relatively weak compared to the bulk of the thrust.
 
palebird
Posts: 75
Joined: Sun Jan 28, 2001 1:41 pm

RE: Jet Engines And The Noise They Make

Sun Sep 30, 2001 4:43 am

There is a bit of a problem here. Bypass ratio is the ratio of air that the engine is moving that is not going through the core of the engine and being used in the combustion process. Original jet engines used 0 bypass as all the air going into the intake was used for combustion and the thrust was the energy coming from the tailpipe. DC8 (JT3 and 737(JT8) engines used a ratio around 50-50. Half the air going into the intake was used for combustion and the other half was used as fan thrust air. These engines employed , for example,a two stage fan on the JT3. 50% of these first two stages of compression were used as thrust with no combustion. The other 50% of intake air went inside the engine for combustion to drive the engine and supply the other 50% of tailpipe thrust and, in addition, turn the two stage fan. In actual fact the fan is a propellor inside a duct. Hence the terminology "ducted fan" and "unducted fan". All of the new generation turbine engines are ducted fans. Now with the advent of complex computer design systems they are able to design very complex blade designs which move large amounts of air very efficiently. So you have very large bypass ratios whereby 90% of your thrust is actually the air being moved by the fan and the thrust coming out the tailpipe is negligible. So, in effect, we have come full circle and are dealing with complex propellors of a sort. As for noise the intake and movement of air is the biggest contributor to noise. Popular belief is that the combustion process is the main contributor but that is "generally" false on commercial jets.When hush kits were developed for the JT3 and JT8's this is where almost all of the mods were made. To the intake.
 
aeroguy
Posts: 66
Joined: Wed Aug 15, 2001 1:33 am

RE: Jet Engines And The Noise They Make

Sun Sep 30, 2001 5:06 am

I think you can classify gas turbine aircraft engine noise into two categories, internal and external.
Internal noise is what comes from all the turbomachinery inside the engine. It's usually associated with the high pitched whine that you hear. I think the biggest source for internal noise on a high bypass ratio engine is from the fan.
The external noise is caused by the interaction of the high speed exhaust jet and the ambient air. This is highly dependent on the velocity of the exhaust jet. In fact, sound intensity is proporational to the 8th power of velocity. There have been and still are some interesting efforts to get better mixing of the exhaust jet and ambient air so as to cut down the intensity of the "shearing" interaction.
Usually, the solution to cut down noise has been to reduce the exhaust velocity by raising the bypass ratio. For example take two engines with the same thrust but one is low bypass ratio, the other high bypass ratio. For the same amount of momentum change imparted to the air, the low bypass engine moves less air but at a higher velocity than the high bypass engine. It's that higher velocity exhaust jet that gives a low bypass ratio engine it's loud roar.

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