TA

I've always had a few curious questions about engines that I wonder if anyone here can answer?

1.) It is my understanding that the main engines are started using hydraulic pressure from the APU -- is that correct? How does one start a jet engine without an APU? Is there an electric starter?

2.) What kind of engine is the APU?

3.) I know that there exist single engine turbo-prop aircraft -- are there any single engine jet aircraft in use?

4.) Are small multiengine turboprop aircraft much cheaper to produce and operate than small multiengine jet aircraft? Is this why choosing between for example a DH8 and a Canadair jet is a big difference?

Thanks to anyone who can enlighten me!

UAPremierExec

TA

Cool -- thank you for the reply:

3.) I know that there exist single engine turbo-prop aircraft -- are there any single engine jet aircraft in use?

non commercial single engine jets do exist, like the F-16 fighter


what I meant here is, "are there any *passenger* aircraft with just one jet engine?" It's hard for me to recall any!


4.) Are small multiengine turboprop aircraft much cheaper to produce and operate than small multiengine jet aircraft? Is this why choosing between for example a DH8 and a Canadair jet is a big difference?

...turboprops are cheaper than jets because the main force of thrust is from the spinning blades, not the thousands of blades in the engines... I personally prefer props to jets, but everyone thinks I'm crazy anyway...


How much cheaper are they to buy? And do they run more efficiently?

I also don't mind turboprops. It's strange that people associate them with accidents, turbulence, or a bad flight. I think they're orders of magnitude safer than piston engines, right? And almost as safe as large commercial jets. I guess seeing a propeller bothers some people?



[This message has been edited by TA (edited 05-27-2002).]

gladtohelpifican

I hate to be the one to correct you on this but I feel I must. A jet engine uses AIR to start the engines. It goes like this.....APU supplies "bleed air" to the starter, the starter spins up to about 10-15,000 RPM, then a starter gear engages the main turbine....the rest of UAPremierExec's explination is correct...pressure, fuel etc.....He is also correct in his statement about not needing an APU to start the engines, but you do need an electrical source, and an air source. Once one engine is started you can cross bleed start the other engines using the air off of the running engine. An APU is convenient way to supply an Aircraft with lights,and heat/air while it is parked at the gate. It is a lot cheaper to plug it into a ground power source and use pre conditioned air to heat or cool it....That is why you always see the big (Yellow)hoses connected to the Aircraft while it is at the gate. Next, no single engine jets that I am aware of in the commercial field. Turbo props are cheaper to build and operate, tend to be a little more reliable and require less work to repair. Piston props are a thing of the past in commercial aviation strictly due to cost and speed. I hope this helps.

eastwest

Not that I can recall.

cblaisd

That makes two of us. Give me an UA EMB over a CRJ any day. In fact, I'd much rather do LAX-OAK/SFO/SJC on an EMB than a 737.

JS

I also prefer turbo-props.

Efrem

Technically, there are single-engine jet-powered commercial passenger aircraft, but AFAIK they're all sightseeing helicopters.

LarryJ

The regualtions put a lot of restrictions on single-engine passenger airplanes in commercial service which is why you don't see them very often.

jsmeeker

Good answers so far. But as usual, the answer is "it depends".

There are jet engines that can be started via electric motors. You won't find them on large commercial airliners, but you will see these a lot on small, business jets. Those *are* jet engines, just like on an airliner. Just smaller.

I can't think of any single *jet* engine *passeneger* airplane. Somone already mentioned the F-16, but I know that is not what you were thinking of.

And yes, turboprop is *much* more reliable than a piston engine. That's not to say piston engines are unrealible, but the turboprop is much more reliable. Less stuff going on in the turboprop. Its continously spins in one direction. Ignition happens *once*. All "phases" occur simultaneously (suck, squeeze, bang, blow). Piston engines have lots of parts mvoing up and down and in circles (pistons and crankshaft). Valves open an close. Ignition fires over and over and over. Etc. Lots of stuff going on there.

TA

Thanks for that info!

Is there a minimum size for a jet engine? I'm always amazed at how large (and how small) jet engines can be. For example, from a 777 GE90 to the small ones on a citation jet.

And the early ones that were essentially no bypass ratio. (you can still see them on really old 737s in service)

jsmeeker

There are some very small jet engines. APUS really are jet engines, as mentioned aabove. But if you mean engines that actually power the plane for flight, there *are* smaller ones that the one you see on the smallest citation. Note the engines on the Citation X are rather large for the size of jet. just look at how it dominates the look of the plane). Anyway, check out the Williams engine. The Williams EJ22, to be used on the Eclipse, is *very* small. Weighs a mere 85 pounds. Thrust to weight ratio of 9.5!!!

Check this out.

http://www.eclipseaviation.com/500jet/ej22engine.htm

Efrem

If you really want small, it's quite practical to build a jet-powered model plane. There are books, such as Gas Turbine Engines for Model Aircraft by Kurt Schreckling and Keith Thomas or Model Jet Engines by Thomas Kamps and (again) Keith Thomas, that tell you how.

If you aren't all that interested but want to see a few, you can visit http://www.gasturbine.pwp.blueyonder.co.uk/page2.htm or check out the links on http://mdavis19.tripod.com/turbine/links.html .

What this comes down to is that, to someone designing an airplane to carry people, there is no relevant lower limit to the size of a jet engine. It becomes a business question of an engine company seeing a market for small ones and wanting to address it.

UAL Traveler

Correction: turbojet and turbofan engines do not operate in a manner where a burning gas creates a vacuum (you might be referring to the thermodynamic Lenior cycle upon which the old German V1 'Buzz Bombs' were based during WWII.

The jet engine is based on the thermodynamic 'Brayton' cycle. Air is compressed in the compressor, and then energy is added by burning a fuel/air mixture in the combustion chamber which leads to a further increase in pressure. This is the critical phenomenon that permits the jet engine to operate. Gas at high temperature and pressure enters the turbine, where some energy is extracted (to power the upstream compressor in the engine. Since the gas in the combustion chamber was pre-pressurized, and then heated to higher pressures and temperatures, enough energy remains downstream of the turbine to[list=1][*] provide thrust in a turbojet/fan engine, or[*]to power a second turbine (e.g. turboprop).[/list=a]

UAPremierExec

i was trying to keep it simple :P

Air + Fire = spinning motion

[This message has been edited by UAPremierExec (edited 05-30-2002).]