Baffler

^ I have carefully read about turbo-fan jet engines on How Stuff Works dot com, and I just can't get my mind around it. The concepts of burning gas and thrust are all well and good, but I can't figure out how an engine that's basically open at both ends, with fans at both ends, can avoid having the thrust come out at both ends so that it pushes both ways and just stands still.

But you did answer what would have been my next question, which was why not use a turbine instead of a reciprocating engine. Cost would do it. I'm glad that commercial planes have turbos, though. I've been told they're much more reliable.

 

I understand that Boeing (many years ago) managed to get 737's certified for gravel runways with the use of gravel deflectors for the landing gear.

 

Well, you know what they say about different strokes for different folks....

 

I know this.

Okay, this is the part I don't get. Since the turbines extract energy (to drive the compressors) I don't see how they can act to "further pull" the gas toward the back. If they are extracting energy, they are retarding the backward movement.

And the crux of my problem is that if the turbines are extracting enough energy to run the compressors, it would seem to cancel the effect of the push, because the burning (expanding) gas is pushing outward against the compressors as much as against the turbine.

I know it works. I just cannot conceptualize how the compressors, driven by the turbine, can counteract the effect of the gas, which is pushing outward with equal pressure in all directions.

This happened once on an episode of Firefly.

And yet, commercial aircraft are statistically much safer than private planes.

 

I found this video it might help you Daniel to understand. [ame="http://www.youtube.com/watch?v=e-Iu4q6_9iw&feature=youtube_gdata_player"]Turbojet Turbofan Turboshaft Turbo - YouTube[/ame]

Aloha,

ED

Posted from my iPhone via the Tapatalk app.

 

Thanks for the explanation and the video.

I guess that makes sense. Sort of. (I cannot find a head-scratching emoticon...)

Thanks for finding and posting that. I can't read that fuzzy scan, though. I mean, I can read it, but it hurts my eyes to read more than a page.

Your explanation above of how the air gets into the combustion chamber is much more useful.

I've never been mechanically inclined. Another one that baffles me is the Stirling Cycle engine. I have two working model Stirling engines, and I've read the How Stuff Works article on it also, and it's totally beyond me. But they're cool. One is supposed to operate from the heat of your hand, but my hands are too cold, so I use the toaster. The other operates from a tiny alcohol lamp.

 

That makes sense, though I would not have thought of it. Of course the gas is moving in complicated ways. But for someone to figure all that out and realize you could use it to make a propellerless plane was pretty smart.

Awwww... Thank you. :redface:

 

The Wright Brothers were bicycle men. The bicycle is a fundamentally unstable vehicle, requiring constant attention and manipulation to maintain control, but in return for the fundamental instability, it gives a high degree of maneuverability.

Their flyer, with its "canard" design, was similarly unstable, but maneuverable. As bicycle men, this did not bother them, it probably seemed natural to them. But wider adoption of flying machines required a control mechanism that was more stable, allowing sustained flight without unduly tiring the pilot. Thus nobody afterward adopted their control design.

Being first is so important to us, that people will do many things that are utterly pointless if they can be the first to do them. In the case of the Wright Brothers, they demonstrated what some folks thought was impossible: manned heavier-than-air flight. It was a significant achievement, even if others were close on their tail and their design was ultimately rejected.

 

Unstable at rest, maybe, but the gyroscopic effect of rotating wheels make a significant difference once it's moving. It's even more apparent on motorcycles, with their heavier wheels and greater speeds. Some vehicles are more difficult to control than others, but they all require constant attention, even if it is from an autopilot.

From our perspective of a-hundred-and-something years later, the Wright Flyer almost looks like it's flying backwards. Still, I've seen canards on airplanes built long after 1903. Twisting the wings to enable turning was a great idea - modern 'flaps' are pretty much the same thing, in principle.

 

I have proven to my own satisfaction on numerous occasions how unstable bicycles are. Eventually, I became satisfied that the proposition was well-enough established that I quit performing the experiment.

The difference between a car and a bike can be seen in the length of time I can let go of the steering wheel before the car veers off from a straight flat road, vs the length of time it takes a bike to fall over and dump me onto the pavement after I let go of the handlebars. This is not changed by the fact that some riders can control a bike without hands. They are still maintaining a much greater degree of active participation than needed by a car.

Similarly a modern airplane vs the Wright Flyer.

 

Regarding turbines- their fuel control system is much more sophisticated and complex then ICE's too. With turbines running very close to meltdown temps in normal ops, it's very easy for rookie pilots to do major damage during "hot starts", and over-torquing, improper use of APU's during startup, fuel contamination, etc.
Startup procedures are very exacting and require monitoring several gauges simultaneously while also controlling the throttle, fuel cutoff switch- while also being prepared for an instant abort start procedure in the event of an impending hot start. You have 2 seconds to abort in the event of a hot start!!

Starting a $150k small turbine is not for the faint of heart-
Here's the startup procedure for a typical single engine turbine helicopter:

 

^ Cool!

"Never attempt ANY helicopter operation without proper individual instruction."

Well, DUH!!!

There are two hiking lodges I like where access to the lodge is by helicopter. (Once there, the hiking is out and back from the front door of the lodge.) I never realized how critical the starting procedure is.

OTOH, when I worked on the farm I drove a tractor that started on gasoline and then ran on diesel, and its starting procedure was a bit complicated as well:

Set compression lever to low, fuel switch to gasoline, spark to ON.

Press and hold starter button and wait for engine to catch. It would turn over VERY slowly until it did catch, and then horrid dirty smoke would spew out.

Wait 5 minutes for engine to warm up, during which time the smoke would more or less clear up.

SIMULTANEOUSLY turn fuel switch to diesel, move compression lever to high, and turn off the spark. Now the blackest, filthiest, stinkiest smoke imaginable would spew out like a pipe from hell.

Wait another 5 minutes for the engine to warm up even more, during which the smoke would clear to "normal" diesel stink-filth.

Drive.

The tractor had no power steering or cabin. The seat was just a shaped piece of metal on a sort of spring-metal blade. Not too bad, really. After a day of wrestling the wheel your arms felt like they were going to fall off.

(Sorry for the digression.)

 

Now think of the same procedure- but with twin turbines

I had the pleasure of flying for NY Helicopters several years ago.
In addition to the single turbine Jet Rangers, Long Rangers, A-109 and several AS350 A Stars- we had several twin turbine helicopters including a few AS355 Twin Star's and three Sikorsky S58's that had been converted to twin turbine. As a young (25yo) co-pilot I remember being extremely nervous starting the twin turbine models (always under the supervision of a seasoned captain).
All our aircraft had a red lamp keyed to the TOT gauge that would illuminate (and stay illuminated) if you over-temped or hot-started. It was infamously referred to as "the lava light". It was pretty much common knowledge that if you brought a ship home with the lava light on- you could just clean out your locker upon landing- and don't dare show up to collect your last paycheck!

 

I guess that's why when I fly Horizon, first they start one motor, and once it's running, then they start the other. Never both at the same time.

(Horizon flies twin-engine turbo prop planes on short routes like Spokane to Seattle.)

 

Well, here's an interesting development: a jet engine with no moving parts!