Why propellers used for cargos

[ravachol]

I think my question is obvious Most of the cargo planes, almost every, use turbo props or turbofans; c 130, c 160, cn 235, a 400 m..
I am wondering that what makes props choosable for cargo planes ?

 

[brewnog]

A few reasons I know of, but Fred is your man.


Firstly, the greater thermal efficiency of a turbofan engine allows the kind of fuel efficiency which makes commercial aviation possible.

Secondly, the lower air velocity means that turbofans are generally much quieter than turbojets of equal thrust, which adds to passenger comfort and allows night time flying from city airports.

A converse argument might also be that military aircraft enjoy the lower radar signature of a low bypass turbojet, a characteristic which is not required for commercial aviation.

 

[russ_watters]

The OP question doesn't make a lot of sense to me - turboprops and turbofans as opposed to what? And a turbofan isn't generally considered a propeller - on a propeller plane (whether turboprop or piston), the propeller generates all of the thrust, whereas on a turbofan, the fan is only part of the thrust.

Did you mean turboprops instead of turbofans? If so, I'd generally say that most modern cargo aircraft are turbofan, not turboprop. If that is what you are asking, speed and altitude requirements are a critical difference between the two. The C-130, for example, is still around today largely for its spectacular low-speed performance. It was STOL before the term was coined. Essentially, if you decide you don't need to fly faster than, say, 350kts, then you can go with straight, fat wings, for high efficiency at low speed.

And propellers "grab" more air at low speed than jets, which is part of the reason for the invention of the turbofan. I never got to a propulsion class, but iirc, the fact that propellers are variable pitch means their power output is relatively constant with airspeed whereas jets are less efficient at low airspeed.

Besides cargo, the C-130 and the P-3 Orion (Locheed Constellation) are used a lot as Patrol aircraft because for that kind of aircraft, gallons of fuel per hour is a much more important measure of efficiency than gallons per mile.

 

[brewnog]

I jumped straight in and assumed the OP was asking why turboprops and turbofans were used as opposed to straight turbojets.

Ravachol, could you please clarify?

 

[Hawknc]

To just add a quick bit, turboprops are at their most efficient at around Ma = 0.5 or so, from memory, which makes them great for efficient travel at fairly low speeds. As Russ said, they can also provide a lot of power at takeoff, shortening the runway length required (the Caribou is a fantastic example of this, and a personal favourite aircraft of mine). Turbofans are a very different beast, with peak efficiencies in the high subsonic to transonic range, which is why you see nothing else on modern airliners. Larger cargo transports like the C-5 Galaxy use them for their range and speed, since they rarely land on very short grass strips like the Hercs sometimes have to. Turbojets are at their most efficient in the supersonic range, so they're once again not a wise choice for military cargo planes which often need to land in short spaces, i.e. at low speeds.

Basically, my understanding is that turboprops give the pilot the best control and fuel efficiency at relatively low speeds.

 

[ravachol]

i was exactly asking the same think that Brewnog thought; why turboprops and turbofans were used as opposed to turbojets for military cargo airplanes and thanks to all for answering.

 

[ajSpades]

The short answer is that turboprops (using a turbine engine to turn the propellor) and turbofans (turbojets with a large forward fan section that pushes unheated air around the turbine for additional thrust) are efficient.

At lower speeds (Mach <= 0.5) and for use on smaller aircraft (C-130) turboprops give the needed thrust at a good fuel efficiency. The same is applicable to large transports (C-5, C-141, 747, A300). Transports are all about carrying lots of stuff long distances, so efficiency is what you want.

Turbojets and low-bypass turbofans are more powerful for their sizes, but are much less efficient. These engines are used on smaller fighter and attack aircraft. Distance isn't so much of an issue, and takes a backseat to thrust and speed.