Turboprop, turbofan, propfan

[Seiran]

Hello everyone,

I'm doing some research at the moment of why faster aircraft use turbofans compared to slower aircraft using turboprops. I understand the theory that the fan in a turbofan accelerates a smaller mass of air much faster than the large mass of slow air in a turboprop.

What I don't understand is how the turbofan can accelerate the air faster?

I know propellers and hence fan blades in turbofan become inefficient when airflow over them becomes supersonic so this implies that prop and fan have tip speed limits.

How is a propfan faster than a turboprop when their aerofoils are both limited by tip speed?

How is one form of fan/prop able to accelerate air faster than the other assuming they are both limited by around mach 1 tip speed?

[Seiran]

Could it be that the turbofan is able to produce a faster airflow because of a convergent duct after the fan that speeds the airflow up? (I'm not sure if that is the case)

So basically a turbofan and a turboprop work exactly the same whereas the turbofan controls its operating environment more with the divergent inlet to slow air for processing and then after the fan a convergent duct speeds it back up to greater speeds?

[procede]

A fan uses compression to slow down the air and increase the static pressure. The fan increases pressure even more and aft of the fan the air expands again (static pressure decreases) and speed increases. This limits the Mach effect.

A propeller simply adds pressure and thus speed. An unducted propfan is somewhere in between, as not having a cowl means that the air will partially go around the fan when compressed.

[MrSnuggles]

I see you already got excellent answers, so I'll just add to this a bit.

All blades are less efficient when the tips approach supersonic. Be it turboprops or helicopters. These blades are unshielded and so are subject to several unknown forces - turbulence, wind shear f.ex. - that might not even be noticed by the flight crew. (There are of course also the centripetal forces from the blades itself, but that is already designed for.)

To eliminate this unknown environment, the turbofan has its fan blades shielded. The inlet design gouverns the air flow to it to a much more reliable speed and consistency. For thrust purposes you have the compressor compressing the air after the fan.

On the Concorde they designed a very special inlet just to slow the air down when cruising at supersonic speeds for this reason. I strongly encourage you to read the thread about the Concorde when you have time. It is located here in Tech Log and is truly awesome!

[tdracer]

Previously discussed here:

http://www.pprune.org/tech-log/55651...urbofan-q.html

[Seiran]

Thanks for replying everyone, I have read that thread tdracer and whilst a good thread, doesn't quite answer my question.

I will try and think of how to better word my question but I am trying to seek some information to do with disc area I believe, I will try and think of a better worded question.

[Seiran]

Ok for this question lets assume two gas turbines of the same power, one driving a propeller and one driving a fan (for this assume basically a normal turbofan but with duct removed). (The numbers I throw out here will be for theory purposes and probably will not reflect that in real life)

If we compare a King air to a similar sized Citation the propeller will be much larger in diameter compared to the fan and have less blades, whereas the smaller diameter fan will have more blades to absorb the engine power. Smaller fan rotates at a higher RPM giving lets assume a tip speed of 200kts and the big prop rotates slowly but due to big diameter lets assume tip speed of 200kts.

I have been told via sources online a fan basically accelerates a smaller mass of air quick and a prop accelerates a large mass slowly. Given that the prop and fan in this case have the same tip speed and same blade area to absorb the same power I just can't get my head around how this works?

I don't think I have worded my thoughts to well but hopefully someone out there can understand what I am trying to get at here, I think I need to take a fluid dynamics course after I finish my instructor course!

[pattern_is_full]

On the basic question

- in jets, the burning of the fuel directly produces hot expanding exhaust that shoots out the back and provides thrust. In addition to whatever mechanical acceleration of air is achieved by blades (fan, compressor).

In jets, the combustion exhaust contribution to total thrust is anywhere from 100% (pure turbojets) to 15% (very high bypass turbofans)

- in prop planes, virtually none of the combustion exhaust produces meaningful thrust (although there have been various experiments and a few somewhat functional attempts to get thrust out of the engine exhaust over the years - cf: augmentor tubes). At best, combustion thrust in propellor engines is about 5% of the total, and usually closer to zero.

Now, in addition to that, you have the differences (as already mentioned) of air inlets, and exhaust nozzles - jets have them, props (except when ducted) don't. And those inlets and nozzles add various effects - such as reducing the onset of shock waves in the air intake, eliminating "sideways" loss of air pressure and thrust, and tightly controlling, shaping and directing the air outflow - the "jet wash".

Consider a fire hose. Put a nozzle on the tip, and you get a much faster and more concentrated flow of water, compared to the rather "sloppy" slower flow you get from a simple hose mouth, even with the same volume of water exiting the hose. The first is a dynamically-controlled "jet" - the second acts more like a propellor - a loose "unfocused" fluid flow, with less speed and effective power.

[Seiran]

full pattern, thanks for your answer,

I realised how to word my second question a bit better.

Two identical hp engines with appropriate gearing for the following: assume a larger diameter two blade propeller rotating slowly giving tip speed of 200 knots and a certain blade area, now assume a smaller diameter 4 blade propeller with the same blade area as the two blade prop. The smaller prop has a higher RPM to achieve 200 knots tip speed. Each prop blade has the same aspect ratio.

1.Is the only real difference in thrust produced due to the blade interference?

2.Is the only reason we have different blade configurations due to different design configurations, eg ground clearance?

3. Would the two props in above scenario give the same amount of thrust?

I will no doubt have another set of questions after this.

Thanks everybody, my mind just has an insatiable need to understand everything to do with flying/aircraft etc

[tdracer]

Seiran


This might make things a bit clearer as to the design compromises involved:

To a first approximation, propellers are a constant 'power' device (it doesn't much matter the source of the power - piston, turbo, etc.). Thrust = Power/Velocity, so the faster you go, the less thrust a prop produces. That means great takeoff performance, but lousy high speed performance.

Again, to a first approximation, turbojets are a constant 'thrust' device - so using the same equation, the faster a jet goes, the more power it produces. Lousy low speed/takeoff performance, great high speed performance.

There is a third relationship - fuel efficiency. Jets accelerate a small amount of air a large amount, while props accelerate a large amount of air a small amount. Maximum efficiency comes when you accelerate an infinite amount of air an infinitesimally small amount - meaning props are more efficient.

Turbofans combine the best of both worlds - they achieve much of the good low speed performance and fuel efficiency of props, while losing relatively little thrust with increasing forward speed so power output increases with speed.

Highly simplified, but that's the basic concept.

[Seiran]

So tdracer,

Basically the ducting in the turbofan allows it to accelerate the air faster (ignore the jet thrust from turbine exhaust)?

In my above scenario with the two blade and the four blade, would both of those props move the same amount of mass at the same air speed? ie produce the same thrust?

[tdracer]

There is a small efficiency loss as you add blades to prop (basically losses due to interference with the wake from the leading blade) but we're not talking big numbers here. The main difference when adding blades is you can do the same work (i.e. move the same amount of air) with a smaller radius prop.


The big magic with a turbofan is the way the inlet slows the airflow before it hits the fan - again see the discussion in the other thread I reference.

[Seiran]

Thanks again tdracer,

With regards to the two and four blade prop question, the larger two blade prop would have a larger disc area, would this have an effect on the mass and speed of the propeller outflow in comparison to the smaller disc area of the four blade prop?

[barit1]

As mentioned in earlier threads, the exit swirl (i.e. vortex around the engine certerline) is a significant factor. A simple prop operating in the free airstream, while doing its job of accelerating the air, imparts a rotation too.

Said vortex represents an investment of energy, but does no useful work in creating thrust. It's an energy loss.

But there are ways of recovering this energy. One is to use counterrotating (umm - contrarotating...) props, one behind the other. The aft prop will redirect the vortex into more or less straight flow.

The turbofan uses a row of fixed stator vanes behind the fan rotor to do the same job. Since the flowpath between adjacent vane airfoils is divergent, the airflow is slowed as well as redirected, so now its static pressure within the fan duct is increased. Finally, this higher-pressure air is forced through a nozzle to accelerate it to a speed where it can propel an aircraft efficiently at high subsonic cruise speed.

[MrSnuggles]

assume a larger diameter two blade propeller rotating slowly giving tip speed of 200 knots and a certain blade area, now assume a smaller diameter 4 blade propeller with the same blade area as the two blade prop. The smaller prop has a higher RPM to achieve 200 knots tip speed.

Imagine all else equal, the 2-blade and the 4-blade would have the same angular velocity (RPM) to produce the same tip speed velocity. The velocity of the tips is not dependent on the number of blades, rather on the angular velocity developed from the rotational axis - the centre of rotation.

If you lengthen the blades on one (does not matter which) and do not increase centre speed, the angular velocity of the tips increases and you would have to speed up the centre of the shorter one to achieve the same speed tip velocity.

Example (all numbers are for illustration purposes only):

Two all else equal fans rotating. Diametre 100 cm. One is a 2-blade and one is a 4-blade. To achieve same tip velocity you let both rotate at 100 RPM.

Now you change fan blade length on one. Different diametres, one is 100 cm, next one is 200 cm. All else equal. If you keep both rotating at 100 RPM then the velocity at diametre 100 cm is the same. Velocity at tips for the 200 cm diametre fan is larger.

The larger tip velocity for the longer blades is because the magic of angular velocity that messes up ordinary thinking. While they have the same speed at 100cm, the longer blade tip has to travel a longer way to "keep up with" 100 RPM and thus it has a greater speed.

This thought experiment is of course performed in vacuum-like circumstances. No consideration to drag or other real world performance issues at all.

[Seiran]

Does the disc area of the propeller have anything to do with the mass and velocity of the propeller outflow?

Assuming 100hp is transmitted through a 2m diameter prop and the same 100hp on another set up is transmitted through a 1m diameter prop.

For the same power to be transmitted we would have to have the same blade area in each set up wouldn't we? This same blade area would mean that in the 1m diameter prop we have a higher disc solidity/disc loading.

Does the 1m prop have a higher outwash velocity than the 2m prop?

[megan]

At best, combustion thrust in propellor engines is about 5% of the total, and usually closer to zero

During WWII Rolls Royce put much research into maximising the jet effect from the exhaust of the Merlin, achieving a figure of 10%. Likewise the Wright Cyclone in the T-28 produced 10% from the stacks.

[WeekendFlyer]

Originally Posted by Seiran

What I don't understand is how the turbofan can accelerate the air faster?

Let's start at the inlet. If you look closely at any turbofan engine mounted in a nacelle (such as on airliners and business jets) you will see that the inlet area is smaller than the fan area. The inlet duct is divergent, so the air slows down before it meets the fan. A careful combination of fan diameter, blade shape, fan RPM and duct divergence ensures the air passing the tips of the fan blades remains subsonic. The fan and following stator stage then increases the pressure of the air but leaves the velocity largely unchanged. This excess pressure is then converted to velocity in the bypass duct, which is convergent. I suggest you might wish to read up on Bernoulli's theorem, divergent and convergent ducts, and Momentum Theory, which should be covered in most fluid dynamics text books. Wikipedia has some good articles on these topics also.

On your questions regarding turboprops and prop fans, a turboprop has to accept air at the airspeed the aircraft is flying; it does not have the benefit of a divergent inlet duct to slow the air down. It thus has a lower outlet velocity also, but for a given diameter has a higher mass flow and thus better propulsive efficiency at lower speeds. Turboprops also lose energy to causing the air mass to rotate (whirl) as it leaves the propeller disc, something turbofans do not suffer from because stator blades behind the fan convert the whirl into additional rearward velocity.

Prop fans are somewhere in between turbo props and turbo fans. They are normally 2-stage contra-rotating to minimise whirl losses, have complex blade shapes to reduce tip losses and generally have many blades with a low aspect ratio, permitting high disc loading and high RPM while keeping the tips subsonic. This provides most of the benefits of a turbofan in terms of having a higher exit velocity than s propeller, but without a big heavy and draggy duct that turbofans have to have. The downside is the noise!

Just one last point: the simplified thrust equation. Thrust = air mass flow x velocity difference across the rotor disc (Vout - Vin), but propulsive efficiency is all about outlet velocity. For a given thrust you can either have a small mass flow and a massive velocity difference, or you can have a huge mass flow and a small velocity difference. The former case is for a pure turbojet, the latter for a turboprop. The former is great for high flight speed but has low propulsive efficiency; the latter has lower flight speed but good propulsive efficiency. The key point here is that the outlet velocity sets the maximum level flight speed of the aircraft, because to create thrust the outlet velocity must exceed the aircraft flight speed. However for propulsive efficiency the velocity difference should be small and the mass flow large, hence the trend for larger diameter turbofan engines over the last few decades.

Regards, WF

[riff_raff]

Here is an example of a large turboprop aircraft designed for efficient high-speed (>380 kts) cruise. The audio should give you some idea of why this particular approach is not more widely used.

While this aircraft proved that you can actually operate a propeller at supersonic conditions, there was a good reason it was nicknamed the "Thunderscreech".

While the theoretical comparisons between props/un-ducted fans/turbofans is interesting, in practice most commercial aircraft will use turbofans for the foreseeable future. One only needs to look at the recent turbofan development work being done at P&W, GE and R-R. It is very impressive and is progressing at a far more rapid pace than propellers and propfans.

Consider R-R's UltraFan program, which they plan to put in service within a decade. It will have variable pitch fan blades, a reduction gearbox driving the very large diameter fan at optimum speed, a BR >15:1, and an overall PR >70:1.

GE is currently testing subsystems for their adaptive cycle engine, which will significantly improve efficiency of turbofan engines used on very high-speed aircraft.

[barit1]

At best, combustion thrust in propellor engines is about 5% of the total, and usually closer to zero

Stepping back a couple decades, the R-2800 18-cylinder aircooled piston engine presented a cooling challenge, and the cooling drag was substantial. But it was a reliable and well-respected machine and successful in the DC-6, Martinliners, and Convair twins.

Convair took a different approach to cooling it, and ducted the engine exhaust into augmenter (eductor, if you will) tubes in the aft nacelle, where the cooling air was drawn out at the wing trailing edge.

I doubt this created any thrust in itself, but since it improved cooling with less external drag, it had the same overall effect as a small jet.