Basic Turboprop vs turbofan Q
 

Am thinking myself in circles here and might be beating my head against a brick wall. So if anyone can set me straight I'd appreciate it.

I'm finding it hard to understand why turbofans are able to operate at vastly higher speeds and alts than a turboprop. The majority of an turbofans thrust comes from the fan component. And I understand that this is partly due to the fact that the fan is ducted, therefore the fan experiences airflow at a lower mach compared to free stream velocity. Now if this is the case then why don't turboprops simply slap a cowling around the prop component allowing the Ac to operate at higher speeds? I'm sure there is a ridiculously simple explanation to this but I'm blind to it at present.

A turboprop engine is driving a propeller. One can certainly shroud it (see the Stipa-Caproni as an example). This would reduce tip vortices and might do the efficiency some good, but increase total mass and would not turn it into a turbofan.

A turbofan engine core is driving an enlarged compressor stage consisting of an intake system (reducing the incoming air speed and turning it into pressure), a rotor (adding energy to the airflow), a stator (roughly converting speed to pressure) and a nozzle (converting pressure to speed).

The question is, if I may rephrase it, at what point does a shrouded propeller become a fan? With the attendant benefits: no need for blade pitch control and higher Mach and altitude for same efficiency.
I'm curious too.
(My apologies, if I misunderstood, byeplane.)

It may be solely a convenience of terms...

However, it could be that multiple fixed, overlapping blades constitute a "fan", where separated blades constitute a propeller.

As to the definition of fan vs. prop:

A fan is a part of the jet-engine compressor (among other things) - a propellor is not.

The turboshaft engine in a turboprop will perform absolutely normally with the prop removed (assuming one has a governor to prevent overspeed) - or with the prop held motionless, as with an ATR in "hotel" mode. A fanjet with the fan removed will be a degraded engine. And with the fan braked - won't run at all.

As to trying to make a prop work at faster speeds:

The usefulness of a propellor is that it moves more air, more slowly, and is more fuel efficient in a low-to-moderate speed scenario. And requires a much smaller, simpler (read, cheaper) powerplant - turbine or piston.

As airspeed increases, a prop gets less efficient - regardless of whether it is ducted or not. In fact, ducting is most effective at low airspeeds (and high prop rpms): Ducted fan - Wikipedia, the free encyclopedia (That link also lists other pros and cons of ducted fans/props).

Turbojets/turbofans get MORE efficient at higher speeds. So there is a cross-over point where it just makes more sense to simply use a turbofan, than keep trying to push the prop model faster.

If anything, the preferred approach, engineering-wise, to a "hybrid" engine is not to duct a prop, but to unduct a fan.

Propfan - Wikipedia, the free encyclopedia

Although neither has really been successful in attracting customers, over the slow-prop/fast-jet model. Noise being one primary consideration.

Pattern Is Full - with regard to your statement that an engine will not run with the fan braked,I have seen a JT9 in a 747 running at idle with the fan stopped - it was a recently installed engine and the fan tips had dug into the attrition liner.
I was not on the headset and thought the engine had not been started but then noticed the smoke from the breather was flowing forward into the outlet of the fan duct. On speaking to the engineer in the cockpit he remarked that his N1 indication had failed - the EGT was a little higher than normal. We had an extremely difficult job freeing the fan then we ran it for a while to wear the fan into the attrition liner.

The fundamental difference between a turboprop and a turbofan is the turbofan has an inlet, fan exit guide vanes, and an exit nozzle. Those three things all help optimize performance of the turbofan relative to the turboprop.

The big part is the inlet - as a turboprop goes faster, the air the prop sees keeps going faster. Pretty soon, the prop tips go supersonic and the efficiency goes south FAST. With an inlet, at a constant corrected fan speed, the air speed at the fan face is almost constant regardless of the freestream airspeed - the inlet almost works like a nozzle in reverse, slowing the airflow at cruise airspeed such that the air hitting the fan at max power, the airspeed at the fan face is ~ 0.4 Mach. Hence the fan can stay efficient at very high airspeeds without the fan going supersonic - unlike the turboprop.

Putting a shroud around a prop doesn't help - you need an inlet.

Thanks guys, a lot of helpful info which has helped to give me a clearer idea. I think I was over thinking it to a point.

cheers!

Apparently the tips of large turbofans do go supersonic. The Trent reaches Mach 1.4 at the tips, supposedly. This allegedly causes the 'buzzsaw' noise.

There's a thread on the Tech Log some, 'Large Turbofan Noise' it's called.

Apart from the shrouding and close blade spacing, I myself can't see how the fan differs fundamentally from a propeller and those don't like going faster than Mach 1 (the Harvard had prop tips that could exceed mach 1 which gave it that slapping sound sometimes).

The fan air can't be compressed very much can it? Perhaps the close spacing of the fan blades allows the inevitable shock waves to be harnessed to good use???

Confirmation about tip speeds:

"To optimize the cycle the
bypass flow has to be raised to a pressure of approximately 1.6 times the
inlet pressure. This is achieved in the fan by utilizing very high tip
speeds (1500 ft. per sec.) and airflow such that the bypass section of the
blades operate with a supersonic inlet air velocity of up to Mach 1.5 at the
tip" Quoted from "The Jet Engine" by Rolls Royce plc 5th edition 1996 page
26.

Yes, fans go supersonic at max power (takeoff and sometimes max climb, especially at lower altitudes). And yes, that causes the well known 'buzzsaw' noise. For takeoff and initial climb, max thrust trumps fuel efficiency.

However you'll notice the 'buzzsaw' noise goes away at cruise, when fuel efficiency is the primary concern. At cruise power settings the fan tips may go supersonic, but just barely - nowhere near 1.5 Mach. Creating shock waves is simply not efficient (unless you start talking about something like a Ramjet/Scramjet, which use an entirely different cycle relative to fanjets). If you look at the newer (relatively) high speed turboprops, you see highly swept props at the tip - for the same reason you see swept wings on jet aircraft. It delays the onset of supersonic flow...


BTW, 1.5-1.6 max pressure rise is pretty typical for the fan portion of fanjets at high power.

Interestingly, when you listen to a takeoff without much of a derate, the “buzz” from the Trent reduces noticeably approaching rated power as the air around the tips (now supersonic) forms a kind of sound attenuating/reflecting layer. When the power reduces at acceleration altitude, the buzzing returns.

pattern_is_full is essentially right, but overstates the cases somewhat.

I like to compare in terms of bypass ratio. A typical modern turbofan runs about BP = 4 to 8. A turboprop will be at least double that; Big low-speed prop, small core. This gives lower fuel consumption, better TO performance, and other well-known attributes.

Now put a REALLY BIG fan on it, slow it down and rotate it so the shaft is vertical. They call this a helicopter. Bypass ratio might be in the hundreds, great static thrust, lousy high-speed performance.

OK? :8

BTW, regarding "Hotel" mode: Any multi-spool engine can run with the low-speed spool locked, provided its lube and other systems will tolerate it. A helo with a rotor brake is another example.

I won't claim this is true for all large turbofans, but a bit of ice locking up the LPT probably won't damage anything as long as it's restricted to idle running until the fan frees up.

I've seen the fan-stage frozen from a little puddle of frozen water on an ALF502. Started right up and stabilized at the correct N2. The pilots were pretty stumped why they weren't getting N1 until they shut down and had a look.

Back to the original post - We're getting to the point where these large-bypass high-thrust engines are becoming more like ducted-fans than traditional 'turbo-fan' engines.

It'll be interesting to see if the unducted-fan technology comes back in the next decade or so.

Likely the most prolific kinda-operational example of an un-ducted fan/prop-fan engine is the Progress D-27 on the An-70.

I guess I'd say a fanjet reading zero N1 is not running in good health - but I appreciate (and stand corrected by) the examples of when it has happened.

as others have said, incidents of locked fan (and their associated aft stages) have occurred. Usually due to the blades windmilling backwards on the ground in a stiff breeze in combination with tight clearances. There is a complicated reason why it is more likely to be due to backwards motion.

This can easily go unnoticed during walk-around and then during the start sequence there is no N1 for a period of time. Somewhere after the engine lights off and begins to spool up the high rotor the stuck fan blades overcome this friction simply due to the torque from pressure drop across the driving turbine rotor.

I know that GE does a lot of outdoor testing; shutdown overnight, testers intentionally exposed them to freezing rain up the tailpipe. When restarted they have no N1 rotation for a minute or so until the ice melts and is exhausted.

I think they place a time limit on locked N1, shutdown temporarily then restart until N1 turns OK. This is probably because the LPC (on the fan shaft) pumps air for seal pressurization.

The turboprop-powered Tu-95 can do 510 kts (575 mph, Mach 0.87) at 40,000 ft. That's not exactly slow. In fact it's faster than the cruise speed of many turbofan-powered civilian airliners, and way faster than many business jets.

That also has 4 engines, with 8 props that go super sonic. Apparently they are incredibly loud, and I doubt they are very fuel efficient. I'm sure you can make a turboprop go faster than a jet, but it suffers some serious penalties as a consequence.

(Having said that, the B52 still uses turbojets for some reason, and I doubt thats a paragon of efficiency either)