OT : Why do they set the engines so far apart on Jet planes ?

Speculation: these high-bypass engines are so massive and produce so much thrust that their intakes may have to be far enough away from the fuselage and each other to ensure clean intake airflow and avoid (whatever the term is for cavitation when it happens in the air}...

It is much easier to fly a full scale with the engines set in or close to the fuselage also. But, a full scale must meet certain criteria to be certified. It must be controllable down to VMC (minimum controllable airspeed) with the critical engine out for one thing. This results in a need for a lot of rudder authority. From personal experience, it is easier to recognize engine failure and make the proper control inputs when you are actually in the cockpit, rather than observing it from the ground.

In a model there is not a need for baggage requirements, or many fuel tanks in the wings, etc.

Could be many factors.. One thing is that if they are heavy, stringing them out along the wing will enable the wing to be built more lightly.

You may be sure its a compromise...aircraft design ALWAYS is.

I think you may have nailed it DT. The term you're looking for is called "compressor stall" and when you've experienced it you'll never forget. it's not dangerous unless you remain in that regime a little too long. It's characterized by a series of very loud and violent explosive "BANG"s. Reduce the AoA and you're OK. IMHO, Since wings are designed so that a stall begins at the wing root and progresses toward the tip, designers moved the engines away from the wing roots so that they would be out of the initial stall area. Good thinking ;-)

Goedendag ;-)

The term cannot be 'cavitation' because cavitation by definition only occurs in a fluid. See

Vriendelijke groeten, Ron van Sommeren near Nijmegen, the Netherlands

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| > ... whatever the term is for cavitation when it happens in the air ... | The term cannot be 'cavitation' because cavitation by definition only occurs | in a fluid.

Air is a fluid.

| See

| You probably meant 'stalling'.

(No idea what he meant. Perhaps the jet designers just wanted them further away from the passengers to reduce noise and vibration, or didn't want the exhaust anywhere near the tail of the jet?)

I wrote --

| In article , | Ron van Sommeren wrote: | | | > ... whatever the term is for cavitation when it happens in the air ... | | The term cannot be 'cavitation' because cavitation by definition only occurs | | in a fluid. | | Air is a fluid. | | | See

Perhaps you meant to say `liquid' rather than `fluid'. The wikipedia page you mentioned does --

Cavitation is a general term used to describe the behavior of voids or bubbles in a liquid.

though it later seems to also use the terms `fluid' and `liquid' interchangably. Since cavitation involves liquids becoming gasses (or possibly dissolved gasses out of a liquid), I don't really see how it could happen in a gas. So using the term `fluid' probably isn't really accurate, unless I'm missing something.

Doug,

Both 'fluid' and 'liquid' are translated the same in Dutch 'vloeistof' (try

Vriendelijke groeten ;-) Ron

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I can imagine the violence a compressor stall could wreak - could it cause an engine overspeed condition and start throwing buckets?

/daytripper

Nope,. It is simply a backfire..the engine spits flame out of the front and may stop.

I love Babelfish. One of the most amusing things you can do is to translate a passage into Japanese or Korean and then translate it back to English. For instance, this is what happens when you translate this post to Korean and back:

"I love the Babelfish. There is a possibility you doing and one thing of most joyful days transit under translate, it faces each other it translate the that time English back with the Japanese Korean language and. When for example and when you translate this point in the Korean language and after, this happens and me is the thing,"

Here it is, translated to Japanese and back:

"I love Babelfish. The road it translates one of the funniest things where you can do in Japanese or Korean, next it is to translate to English. For example, as for this when this post is translated in Japanese and the back section, is something which happens"

In English, anything that flows is a fluid, whether air, water, petrol, beer, or volcanic lava. However, if it is in gaseous form, as is air at standard temperature and pressure, it is not liquid, which is a state between a solid and a gas. Does that help? Three states: Solid, Liquid, Gas. I'm sure your physics textbooks say that in Dutch just as they do here! :-D

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As a fellow old fart - you forgot the fourth state that the young'uns are taught these days - plasma. Yeah, I know...

Ed Cregger

But the how of it is what we want. The turbine engine has three stages: Compressor, combustion sesction, and turbine. The compressor compresses air and feeds it to the combustors, where fuel is injected, burned, and the resulting expansion is turned into velocity which drives the turbines. The turbines spin the shaft that drives the compressor, and 60 to 80% of the energy thus created is fed back to the compressor to keep it going. The remainder is used as thrust or to drive another turbine section to drive a fan or propeller. The maximum pressure in the engine is in the diffuser at the aft end of the compressor. The diffuser is a divergent duct that reduces the velocity of the airflow and thus increases the pressure (Bernoulli's Theorem) for combustion. In the combustors the pressure actually drops, for if it didn't it would force air back out the compressor and the whole thing would quit. It's more velocity we want, not pressure. Compressor stall has several possible causes, such as interference with incoming airflow, or more likely the pilot advanced the throttles too quickly and created a rapid pressure rise in the combustors that the compressor couldn't keep up with (the throttle controls fuel flow only, and the whole rotating mass needs time to speed up). The flame finds its way out the wrong end of the machine and makes ugly noises.

Dan

"The Natural Philosopher" wrote

I was on a commercial flight one time and experienced a compressor stall(s).

We were landing, an just as we went to reverse thrust, I heard (and felt) a loud BANG, then about a half second later, another BANG.

As we were leaving the plane, the first officer was standing at the cockpit, bidding everyone a farewell. I asked him what the bangs were. He said, "compressor stall. Kinda' gets your attention, doesn't it?

I think the captain made him stand there and answer all the questions, since the F.O. was the one who caused them.

Doug, This is hard to accept, but accept it you must. Air IS a fluid. NASA has entire super computer farms for running fluid dynamic computations on various shaped objects at various speeds through the atmosphere.

Yes, cavitation is just a low pressure area in a flow. In liquid that pressure can drop to the point that gas bubbles form and then collapse and then doing all kids of nasty things.

Jim

On Feb 22, 1:06 pm, "Six_O'Clock_High" > could happen in a gas. So using the term `fluid' probably isn't

Yup, air is a fluid but not a liquid. When we talk of cavitation we're generally thinking liquids. In air, the failure to smoothly follow the low-pressure side of a compressor blade or propeller blade or wing or any other airfoil is called stall. Liquids are not compressible. Air is, and so its density can vary. A liquid can be gasified if we remove enough pressure from it, while air just gets less dense when we reduce its pressure. When I owned a boat, I could see a steady stream of tiny bubbles behind the prop while cruising. It wasn't exhaust; this was a straight-shaft inboard with transom exhausts. The bubbles were vaporized water coming off the front of the prop blades and would disappear as the pressure recovered and turned them back to liquid. Air doesn't behave that way over the low-pressure surface; it gets turbulent at high AOA and its pressure will actually increase, not decrease, and thrust or lift is lost.

Dan

And of course its all so very old anyway. They used to call it Earth, Water, Air and Fire.