CFM56-7B Ducted Fan Question

[byeplane]

Hi all,

I'm trying to get m head completely around turbo-fans and am using the CFM56 as a case study. I am having trouble understanding what it is that enables the fan of the engine to operate at higher altitude than a turbo-prop?

Above a certain altitude the air becomes to thin for a turbo-PROP to operate. I understand that the divergence of the air inlet of a turbo-FAN must help to raise the static pressure of air before it passes through the fan but I'm sure this does not the only factor...
Does the number of fan blades have anything to do with it, by sucking in more air?

[barit1]

The Tu-95 with four Kuznetsov NK-12 turboprops cruised as high at 45K. I don't think turboprops are intrinsically limited in altitude, but are usually optimized for a lower region.

And from Wikipedia:

A production DHC-5D Buffalo was used for breaking time-to-height records for the weight category 12,000–16,000 kg (26,430–35,242 lb) on 16 February 1976, reaching 3,000 m (9,836 ft) in 2 min 12.75 sec, 6,000 m (19,672 ft) in 4 min 27.5 sec and 9,000 m (29,508 ft) in 8 min 3.5 sec.

My recollection is that the CT64 engines would have taken it still higher, but required some "tweaking" of fuel delivery - this was an unexplored region for the engine. Besides, since the Buffalo was unpressurized, crew safety was a consideration.

[tdracer]

The primary advantage of a ducted fan vs. a propeller is speed, not altitude (although that extra speed leads to better performance at altitude). There are a number of inter-related aspects of that.

To a first approximation, a propeller is a constant power device. Since thrust is power divided by velocity, the available thrust of a prop falls off rapidly with forward speed.
Again, to a first approximation, a 'pure jet' is a constant thrust device, meaning the faster it goes, the more power it produces and thrust doesn't fall off.
A 'fan jet' is basically a compromise to get the best of both worlds. On a fan jet, thrust falls off with forward speed, but much less dramatically than a prop.

A big advantage for the ducted fan is the inlet - with an inlet, the airflow characteristics entering the engine are largely independent of the free stream velocity. It depends on the particular engine and installation, but at high power the airflow at the fan face is ~0.4 Mach, regardless if the engine is static or cruising at 0.8 Mach. When a propeller is going Mach 0.8, the blades are supersonic and the blades become very inefficient - but the fanjet blades are still seeing Mach 0.4 and are still near optimum performance. Further, much of the work of a propeller is lost as 'swirl' (unless it uses counter-rotation props, which can be quite efficient but are really, really noisy), where as the fan has exit guide vanes to eliminate most of the swirl.

[byeplane]

Perfect, thanks a lot!