Props Vs Jets

[Big Kev]

Just wondering what the major differences are operationally between say using a 40-50 set turbo-prop aircraft to say a 40-50 seat jet aircraft?

I'm interested to know why an airline would choose say a Dash-8 prop aircraft over say an Bombardier CRJ-200??

I suppose it would be something to do with range, pax numbers, baggage capacity, destination strip sizes etc, but if anyone could add to that then that would be great.

Thanks for your replies.....


BK

[EngineOut]

An answer from a novice here...

A prop is more efficient than a jet, generally speaking, it is moving a large mass of air at slow(er) speeds, compared to a jet propelling the air very fast which creates a lot of losses in efficiency. On a per hour cost, you will probably find a 50 seat DCH8 is significantly cheaper than a CRJ.

Anyone with knowledge care to post a 1 hour block fuel burn for each of these aircraft as a starting point?

However, over a sector of about 3-400nm and above, the speed advantage of a jet will make up for those losses and allow for greater utilisation. On short sectors the difference between a jet and a turbo-prop will only be a few minutes.

[Sheep Guts]

Engine out,
Sorry would have to disagree. The jet will win in the end due to speed. Also theres the so called Modern day travellers phobia of "Jets Only". This may not be prevelant in Oz much, but sure is in the USA, hence the rate of knots at which they are being retired and replaced with Jets.

DHC-8 will be cheaper until its gets Older then well...........

I have no idea on say the Range or CRJ to Dash 8-300/400.?
Prolly is better I suppose allthough the Bombardier site has range for Q400 but nothing for CRJ?

Also propellors lose efficencieny at altitude way before Jets. Plus also Props are very noisey :} . Just ask my Doctor, Im off to buy another NR Headset soon the LightSpeed G3 heard they are good.
Also there are fewer anf fewer Props on the market in that seating capacity, Dash, ATR and SAAB a re it I belive ooh and the Dornier but I think thats even a Jet now right.

Sheep

[BraceBrace]

The old days... turboprops vs turbojet.

Turboprops are constant power engines, from the moment you go full power, you get full power. However, once the speed builds up, it looses efficiency because of almost sonic airflow of the air passing the prop. You need to get the prop RPM down. Excellent for short runways. Not so excellent for high speeds.

Turbojet engines (single axis) are constant thrust engines, where power depends on the speed you are travelling at. So they initially do not have the same performance as a turboprop. Longer runways needed, usually higher speeds as well because of no propwash. But excellent for high speeds. They keep generating thrust when coming closer to the speed of sound.

Both are based on the same "core" which is very efficient when working at high rpm, high pressures, high temperatures. With a prop, no problem at low speeds. Use a gearbox to get the rpm down. But the more power you need, remember that the gearbox has to "transport" all this power.

This gearbox, even with an efficiency of 0,01%, will generate huge amounts of heat. So hot it would immediatly contaminate every drop of oil in the gearbox and lead to immediate failure of the unit. So... not very good for propelling a 777 (even if it would have a max speed of 250kts).

It's hard to "upgrade" the turboprop above a certain power level.

However, turbojet engines went high-bypass. Efficiency in the lower thrust ranges increased because of very sofisticated engineering (turbine blade technology etc...) that, considering the possibility of higher speeds, the lower noise level, increase of performance at low speeds... a small turbofan became a better option compared to a normal turboprop. Turbofans can (after some evolutions in engineering) now be "downgraded" in the section of turboprops.

Development of the gearbox has not halted however, P&W are working on a "geared turbofan", which is in the end the ultimate mariage of turbojet and turboprop. However, you won't see anything like it on a 777 (yet?). A318 is the target for this engine I believe.

Most airlines sticking with the turboprop do this because of: really small airstrip, history, passenger amount. There is no jet version of the Beech 1900 (yet). The Dash 8, well, who is buying it? I wouldn't say the aircraft is still very popular... on the contrary...

Just look at the world of the private aircraft, the "real" private jet has arrived...