Fanjet efficiency questions.

[AdamFrisch]

I have a series of fanjet questions that I haven't been able to get answers to. Here we go:

1. It's a known fact that the bigger you make a propeller/impeller/fan, the less horsepower it needs to move a set amount of air. This is why we see high-bypass fanjets getting bigger and bigger fans in the front. At some point, this fan is going to be too big to shroud in a housing and would have to be un-shrouded. Why, in the name of fuel efficiency hasn't this happened yet?

2. Any prop or fan will lose efficiency when the blade tip get close to transsonic speeds. In a big fanjet, the fan is normally driven by a second turbine stage at the end of the core turbojet engine. And since the blades are fixed pitch - how does the fan rpm stay below transsonic when it's direct driven by the turbine at the outlet? And if it doesn't (which I suspect), then it must lose a lot of efficiency. Which leads me to the my third question...

3....Why don't fanjets have variable pitch blades like the turboprop has?

4. Many blades in a fan or prop adds solidity and will require less power to shift air. The reasons turboprops rarely have more than 5-6 blades is that they have variable pitch hubs. A fanjet doesn't have VP, so they can have more blades. But there's a disparity here - if more blades are more efficient (as they keep telling us), then how come fixed pitch aircraft don't have more blades? Or reversed - why don't fanjets have fewer but variable pitch blades? You can't have your cake and eat it too - something has to be better than the other.

5. If a bigger fan increases fuel efficiency is taken to its logical conclusion, soon the fans will be so big as they will be hard to hang under wings without scraping the ground. Would it therefore be beneficial and fuel efficient to add more engines with higher bypass ratios so as to shift more air? Example: instead of having 2x engines rated at 10kN (in the core turbine, I'm not talking total thurst from the fan part) each and with a 100sq.ft fan area - would it be more beneficial to have 4x 5kN engines with 200 sq.ft fan in instead?

[hawk37]

Adam, you ask many excellent questions, and maybe have some of the answers embedded in your post. There have been may pprune posts along the lines of "jet engine efficiency" and while I cannot spar with those of immeasurably more intelligence than myself, I've great interest in this topic. May I humble request that you define some of the terms you use? Specifically;

para 1: "in the name of fuel efficiency"

para 2: "Any prop or fan will lose efficiency"

para 3: "lot of efficiency"

para 4: "more blades are more efficient"

para 5: "increases fuel efficiency"

I've come across "propulsive efficiency", "cycle efficiency", and "energy efficiency" to name a few. Google to view even more. Personally, I like to consider the term "fuel efficiency", which I believe is understood as TSFC, which some may use the generic term "efficiency". TSFC is thrust specific fuel consumption, which is, of course, in American units is, lbs of fuel burned, per lb for thrust produced, per hour. Sounds like a good discussion.

[AdamFrisch]

Thanks.

Yes, what I'm after is specific fuel consumption.

[Mark1234]

I'll take a stab, from a 'little knowledge' point of view:

1. First up, I'm not quite sure about your assertion, however, using the same amount of energy to accelerate a large mass of air to a lower speed produces more thrust than accelerating a small mass of air very to a much higher speed. So long as your aircraft isn't travelling at too significant a fraction of that gas stream velocity.

To the fan - if you unshroud the fan it becomes a propellor - propellors are wonderfully efficient at low airspeeds, and awful at high airspeeds (see Turboprops). They accelerate a very large mass, but can't get it going too quickly.

What the (divergent) inlet duct allows you to do is condition the airflow to the fan - slowing it. After the fan the duct converges, accelerating the airflow. This means the fan working in a lower airspeed stream. That will mean that the fan doesn't have to spin as fast for a given airspeed as it would if unducted/a propellor, yet you're getting the thrust stream up to a useful speed.

2. Partly see 1. I suspect that is, or will become a limiting factor on the size of fans. The blade angle at the back of the shaft isn't the same as the one at the front - it will be coarser. Note that some companies are working on geared fans - presumably because as the fan gets large, the need to keep RPM down is pushing the tail end RPM down too much to be efficient. I'm quite certain it's organised such that the fan doesn't go transonic, irrespecitve of what's going on with gas velocity.

3. Most likely simple engineering - it's all spinning too fast, too loaded and the cost outweighs the benefit. Also remember it's damn hot in there... you can't really be using hydraulics to move things around..
However, in between the spinning blades, are stationary guide vanes which redirect the gas flow to each disc. These are variable in some engines.

4. The most efficient propellor is one very large blade - it minimises the interference between the blades. However, to absorb a given amount of horsepower, it will be impractically long (bang on the ground), and will only be able to spin slowly before the tip goes transonic. To absorb more power within a given diameter, add blades. Now, I don't know about the fan, but I'd assume the same applies. Also remember the front end of a turbofan is part compressor - I'm not sure how well a low solidity fan would perform the compress part.

5. More smaller engines (with high bypass) vs less larger. There's a certain amount of loss and friction inherent in any engine. I'm guessing, but I rather suspect if you halve the size of the engine, you're very unlikely to halve those losses - so 2 small engines have proportionally more losses than 1 big one. Add in manufacturing cost, complexity, maintenance etc..

But really, the important bit is 1. The fan is not simply a shrouded propellor - the duct is rather important to the operation of the whole thing. For an extreme example, Concorde: While in supersonic flight, the front end of those engines were still seeing subsonic airflow, thanks to a bunch of ducts, ramps and vents.

[barit1]

1. It's a known fact that the bigger you make a propeller/impeller/fan, the less horsepower it needs to move a set amount of air...

Well, it's not well known to me, and I've worked on performance of large fans for several decades.

You seem to have overlooked the role of the fan duct and nozzle. For a given fan diameter and RPM, you might choose a small-area nozzle, meaning the exit velocity will be quite high (but still subsonic) - with two implications: Lotsa noise, and lotsa horsepower (fan shaft torque) required to pump up that much air pressure.

Or, you might choose a larger exit area, less air pressure, less exit velocity, less noise, less hp required.

It's all a matter of what best matches the aircraft's mission.

[AdamFrisch]

Thanks Mark. I didn't know the duct slowed down the air to subsonic - makes sense.

I would however argue that the fan jet at the front is nothing more than an low pressure air shifter similar to a prop. It's not part of the turbine. In fact, the blades are often made out of composites as they never get hot and it's sole purpose is to give thrust by shifting big masses of air around a turbojet core.

As for the validity of the bigger/slower vs smaller/faster of airflows, I've been told that bigger/slower is more efficient for some reason. Don't really know why. But in helicopters (which I have some slight experience with) the bigger the rotor the less power it needs, that's a fact. It also gets quieter the bigger it gets, which makes sense. But a slower rotor also stalls quicker, so it's a balance (not really a concern in a fan jet design).

Most new helicopters have pretty small rotors with high rotor disc loading. This makes for fast and nimble helicopters, but they're very power hungry.

Interestingly, the abandoned propfan concept where the blades were unshrouded did reduce fuel consumption by 35% (compared to 90's turbofans, that is), but it did produce noise problems. Apparently in these fuel crisis times, engine manufacturers are taking a renewed look at the propfan concept.

Thanks for answering.

[chornedsnorkack]

But a slower rotor also stalls quicker, so it's a balance (not really a concern in a fan jet design).

A slower tip speed propeller loses its propulsive power faster.

It is possible to design a turboprop which is pretty fast. Tu-95 has swept wing, counterrotating turboprops - and it is very fast compared to ATR, or even Piaggio Avanti. But Tu-95 is extremely loud as well. How does Tu-95 sound compared to the jet B-52? And how does fuel efficiency compare?

Tu-114 is the fastest civil propeller plane, too.

And at the other end, it is possible to design a turboprop with a very large diametre, slow tip speed propeller. Sure, ground clearance is an issue when you have a low wing, like Tu-114 with its tall landing gear, but you can go to high wing, gullwing, engine pylons above wing...

Interestingly, the abandoned propfan concept where the blades were unshrouded did reduce fuel consumption by 35% (compared to 90's turbofans, that is), but it did produce noise problems.

How does Piaggio Avanti compare to jets of similar size?

[lomapaseo]

AdamFrisch

I would however argue that the fan jet at the front is nothing more than an low pressure air shifter similar to a prop. It's not part of the turbine. In fact, the blades are often made out of composites as they never get hot and it's sole purpose is to give thrust by shifting big masses of air around a turbojet core.

As for the validity of the bigger/slower vs smaller/faster of airflows, I've been told that bigger/slower is more efficient for some reason. Don't really know why. But in helicopters (which I have some slight experience with) the bigger the rotor the less power it needs, that's a fact. It also gets quieter the bigger it gets, which makes sense. But a slower rotor also stalls quicker, so it's a balance (not really a concern in a fan jet design).

Most new helicopters have pretty small rotors with high rotor disc loading. This makes for fast and nimble helicopters, but they're very power hungry.

Interestingly, the abandoned propfan concept where the blades were unshrouded did reduce fuel consumption by 35% (compared to 90's turbofans, that is), but it did produce noise problems. Apparently in these fuel crisis times, engine manufacturers are taking a renewed look at the propfan concept.

Your questions are fine for a technical forum, however your presumptive statements of fact have a tendancy to turn off a responder since to respond implies either an acceptance or an argument first.

The design engineer can achieve simple soultions both by gearing between the fan and the turbine as well as by use of variable pitch blades. Both have been done in experimental type gas turbines. However there are lots more considerations or balances between competing requirements that need to be addressed in order to make a viable product. Not the least of which is comparable safety (certified standards).

As for the products currently in the marketplace they have also had to meet a cost of ownership standard which includes maintenance consideration (how long and how much cost)

As for bigger engines one of the pinch points is still the landing gear and its length to keep the engine above the ground.. Yup you can put the engine above the wing but that has its drawbacks as well.

[chornedsnorkack]

If a bigger fan increases fuel efficiency is taken to its logical conclusion, soon the fans will be so big as they will be hard to hang under wings without scraping the ground. Would it therefore be beneficial and fuel efficient to add more engines with higher bypass ratios so as to shift more air? Example: instead of having 2x engines rated at 10kN (in the core turbine, I'm not talking total thurst from the fan part) each and with a 100sq.ft fan area - would it be more beneficial to have 4x 5kN engines with 200 sq.ft fan in instead?

This is a reason why Airbus built A340-500/600 rather than A330-500/600.

[barit1]

3....Why don't fanjets have variable pitch blades like the turboprop has?

Fair question. VP blades would give greater efficiency over a wide thrust range, and the performance guys would love it.

But VP for a fan turning 3000-4000 rpm is a different matter than for a prop turning ~1000 rpm. It's another system wth failure modes to be considered. Airlines are in an approach-avoidance mode re variable fans; They like the fuel efficiency, but fear the maintenance requirements and impact on dispatch reliability.

BTW, newer turbofans DO have fewer, and bigger, fan blades than early engines. Count 'em.

[PETTIFOGGER]

1. It's a known fact that the bigger you make a propeller/impeller/fan, the less horsepower it needs to move a set amount of air. This is why we see high-bypass fanjets getting bigger and bigger fans in the front. At some point, this fan is going to be too big to shroud in a housing and would have to be un-shrouded. Why, in the name of fuel efficiency hasn't this happened yet?

Answers seriatim: It is the relative speed of the air that is important for thrust. Higher bypass ratios are getting bigger because they use or ‘recover’ more energy produced by the gas generator, or core. A fan without a housing is a propeller, and different laws of fluid mechanics apply. Simply put, the air (flow field), when it reaches the ducted fan, slows down or hesitates at the face of the fan. This is part of the reason that the fan remains more efficient as speed increases. The lateral swirl, which is lost energy (slip) from a propeller, is contained by the ducting. It has happened. Apart from being noisy there was a return to the inefficiency of propellers at the air speed required.

3....Why don't fanjets have variable pitch blades like the turboprop has?

Because it is unnecessary, as the engines’ 'sweet-spot' is designed to coincide with the normal cruise speed required for a range of commercial aircraft.

4. Many blades in a fan or prop adds solidity and will require less power to shift air. The reasons turboprops rarely have more than 5-6 blades is that they have variable pitch hubs. A fanjet doesn't have VP, so they can have more blades. But there's a disparity here - if more blades are more efficient (as they keep telling us), then how come fixed pitch aircraft don't have more blades? Or reversed - why don't fanjets have fewer but variable pitch blades? You can't have your cake and eat it too - something has to be better than the other.

You can to a certain extent and that is the thinking behind it: a fan jet is a compound engine.The thrust physics of propellers and ducted fans are different. As a general rule for propellers, fewer blades are more efficient. More blades are generally used when the optimum disc size cannot be accommodated on the wing, but there are other considerations, such as nvh, weight, balance and proportionality.

5. If a bigger fan increases fuel efficiency is taken to its logical conclusion, soon the fans will be so big as they will be hard to hang under wings without scraping the ground. Would it therefore be beneficial and fuel efficient to add more engines with higher bypass ratios so as to shift more air? Example: instead of having 2x engines rated at 10kN (in the core turbine, I'm not talking total thurst from the fan part) each and with a 100sq.ft fan area - would it be more beneficial to have 4x 5kN engines with 200 sq.ft fan in instead?

Again, it is a question of aerodynamics and proportionality. From the viewpoint of efficient thrust per lb of fuel, the fewer fanjet engines the better.
I hope that this helps. I am happy to be corrected by others. These are un-researched answers and should not therefore be used for an academic paper or talk. That would need citations e.g. from Newton, Reynolds, Navier-Stokes to name but a few. As well as giving the mathematics, this site also gives various animated displays: http://www.grc.nasa.gov/WWW/K-12/airplane/aturbf.html.

[AdamFrisch]

A fan without a housing is a propeller, and different laws of fluid mechanics apply. Simply put, the air (flow field), when it reaches the ducted fan, slows down or hesitates at the face of the fan. This is part of the reason that the fan remains more efficient as speed increases. The lateral swirl, which is lost energy (slip) from a propeller, is contained by the ducting. It has happened. Apart from being noisy there was a return to the inefficiency of propellers at the air speed required.

But let's for a minute assume that the core of the fanjet contributes very little to the forward motion and assume that the fanjet produces most of the thrust (which I think I've heard is the case).

Now, if that's a "sweet spot" and an efficient way of reaching high cruise speeds and thrust envelopes it begs the inevitable questions: why not use ducted fans on piston and turboprop engine aircraft then? Why is the propeller still around if the fanjet is such a fantastic thing of efficiency and so good at reaching higher speeds?

[barit1]

why not use ducted fans on piston and turboprop engine aircraft then? Why is the propeller still around if the fanjet is such a fantastic thing of efficiency and so good at reaching higher speeds?

Well, it's been done with mixed success. I watch the Fantrainer fly at Hanover '87, quite impressive, but it seems to have had its problems too. Maybe someone can fill in details.

[PETTIFOGGER]

But let's for a minute assume that the core of the fanjet contributes very little to the forward motion and assume that the fanjet produces most of the thrust (which I think I've heard is the case).

It is the gas generator that produces all the energy by continuous thermal reaction. Most of the thrust is produced by converting this energy to rotational force to drive the fan; some thrust is produced by the jet efflux from the gas generator. The difference between the two is the bypass ratio.

Now, if that's a "sweet spot" and an efficient way of reaching high cruise speeds and thrust envelopes it begs the inevitable questions: why not use ducted fans on piston and turboprop engine aircraft then? Why is the propeller still around if the fanjet is such a fantastic thing of efficiency and so good at reaching higher speeds?

Because they have different range, speed and payload requirements. The propeller is more efficient at lower a/c speeds. The advantages are debated in Keesje's recent thread proposing a new short range turboprop.The faster contra-rotating exceptions mentioned by chornedsnorkack are too noisy and do not meet the current sound pollution regulations.

[barit1]

It is the gas generator that produces all the energy by continuous thermal reaction. Most of the thrust is produced by converting this energy to rotational force to drive the fan; some thrust is produced by the jet efflux from the gas generator. The difference between the two is the bypass ratio.

Well, that's close. Actually the bypass ratio is defined as the mass flow rate (i.e. kg/sec) through the fan nozzle divided by the mass flow rate through the core nozzle. Since the two are different temperatures and velocities, the thrust ratio is somewhat different. (For a typical high-bypass fan engine, the fan nozzle accounts for 75-80% of total thrust)

[PETTIFOGGER]

barit1,
Yes, exactly, thank you. My apologies for being slapdash.
rgds

[AdamFrisch]

Well, that's close. Actually the bypass ratio is defined as the mass flow rate (i.e. kg/sec) through the fan nozzle divided by the mass flow rate through the core nozzle. Since the two are different temperatures and velocities, the thrust ratio is somewhat different. (For a typical high-bypass fan engine, the fan nozzle accounts for 75-80% of total thrust)

So my questions remains. If this fan that is in effect nothing more than a shrouded propeller (fixed pitch, at that), providing about 80% of the thrust and regularly manages to get these aircraft to cruise at almost sonic speeds - then why aren't all aircraft designed with this ingenious high speed device?

I suspect it's very simple - the sheer excess in horsepower is providing these numbers. The abandoned propfan experiments showed that it did not suffer in speed compared to the shrouded fan, so it's certainly not the case that a prop can't be just as fast or effective.

I have a sneaking suspicion that if you designed an aircraft with exactly the same amount of horsepower powering either a shrouded fanjet, or an unshrouded fan/prop, they would probably be exactly as fast and fly as high.

[barit1]

Once again, it depends on the mission.

If you need to take off from small airfields, the prop does the job because of its great mass flow and static thrust. (If you need to take off vertically, turn the prop axis 90 degrees and make the blades longer... )

If you need to cruise long distances at high speed, the fan does the job, with the tradeoff of requiring many acres of concrete.

[VinRouge]

I know of certain uni research into embedded PZT material in Carbon fibre blades that optimizes flow angles at the first (big stage).

As for your questions:

1) thrust is dependent on M(dot)*(V2-V1) IE the increase in speed if the air (for every action reaction etc). If you incresed the area any more, thus decreasing the increase in velocity, you would have a corresponding decrease in thrust at higher speeds. Also, think of all that extra drag!!! Big flat round disk = lots of drag!!!

2) Gearing in some respects, multiple stage turbines (up to 3???) and yes, they have transonic blade tips on some blade sections on the outer regions. (especially in the HP compressor). Asfor variable pitch, forget it!!!! Weight penalties together with lots of complex gubbins to go wrong. Turbofans are great as they are so uber reliable. I wouldnt want to see a 3 metre front end fan lose pitch control to fine pitch and spin off the hub as it would probably rip the wing off!!! (think of all that drag and torque effects)

A lot comes down to material limitations. Work out the centripetal forces acting on the root of a CF fan blade. Its Huge. Absolutely huge. I think its similar to having a couple of tonnes hanging on the end of the blade. Thats before you even get into the forces involved as a result of work done on the Air to produce thrust. Rule of thirds from what I remember. 1/3 of the stress at the root due to squeezing air, 2/3 due to centripetal forces. Or at least thats what I last calculated at uni whilst carrying out a CFD/FEA test on an experimental blade

[BelArgUSA]

Hola Adam -
xxx
A lot of data and information about design of engines so far presented here.
Sure there are ways to improve on design. But some are not.
Look at the progress of the jet engines for the last 60 years.
xxx
From the centrifugal flow compressors (not so efficient) to the axial flow...
Using afterburners (relight) designs...
From single spool turbojet engines into the bypass and fan engines...
Design of fans on compressor section, or aft-fan (CF700/CJ805-23) on turbine stages.
And finally into high bypass turbofans, all that to increase improved mass acceleration.
xxx
Using SFC comparisons (pardon my advanced "Alzheimer's"), I remember -
JT3C (J-57) which had some SFC of about .75 (early 707/DC8s)...
Fitting a forward fan (JT3D), it reduced the SFC down to .53 (later 707/DC8s).
There was the JT8D (727 and old 737s) with a SFC of .58 - Essentially a bypass design.
xxx
Then came the JT9D class engines (52,000 thrust) reducing SFC to .38...
If I recall - JT9D-7Q fan thrust produces 41,000 lbs of the thrust.
The core of the engine only contributes to 11,000 lbs of thrust.
xxx
Fan blade design, variable inlet guide vanes, variable stator blades are used as combination.
I recall the J-79 (CJ805) turbojet having 15 stages of variable stator blades.
And inlet designs to reduce inlet speeds below supersonic.
xxx
What is next... Tell me. I only know the past, not the future...?
I am retired, replaced a subscription of Aviation Week by one of Playboy Magazine.
I hope future designs will include sea water as fuel, do not attempt using my wine and beer.

Happy contrails, bottoms up.