Turbofan Duct

[pichanusakorn]

Hi all

I'm wondering: what is the main purpose of the turbofan duct? If it has certain effieciency factor, can't we put the same duct around the propellers for other smaller airplane, eg. Piper?

And why does the cross section of the duct has to be an airfoil shape?

I ask this because I saw a motor engine for model aircraft, it has a propeller and plastic duct (also an airfoil shape) around it.

Thanks alot

Bonpon2002

[Zeke]

They are aerodynamic devices use to present uniform subsonic flow to the turbine.

At higher speeds a shock wave formes between the pointy hub in the middle and the outer case to slow the airflow in a uniform way to the turbine.

The cross section also changes normally expanding the cross section from the start of the intake to slow and cool the flow down before it goes into the turbine.

The main limit on how much power/thrust an turbine can make is the amount of heat it can add the the airstream in the engine without melting the metal its made from, the cooler the intake air the greater maximum thatrust/power that can be produced because you add more heat before reaching your temperature limit.

Many airframe desigers also put vortex generators on the outside of them to aid with high angle aerodynamics downstream of the pod.

Z

PS The bit on the front of the turbine is normally called an intake, or as on a 727 intake duct.

[pichanusakorn]

Thanks alot for the answer. But I'm not sure if you understood it correctly... anyway thanks. Here, I would like to ask again:

IF I put a cone shape (with two open ends), with big openning after a propeller which direct the flow and speed up the flow to the smaller openning. Will that increase any thrust at all?

Personally, I don't think it will change because there is no change in mumentum right?

I don't know?

thanks

Bonpon

[Willit Run]

Way back when, in the early days of aviation, some of the radial engines had an airfoil shaped cowling ring in front of the engine, but that was there to aid in cooling the cyliders. The mear fact of haveing a ring or cowling around a propeller will not aid in thrust in any meaningful way. On high bypass turbo fan engines, the majority of the thrust is from the Fan discharge and not the core engine. The fan discharge air is often used to cool CSD oil or other types of cooling. Haveing the fan discharge seperate from the core exaust, which is extremely hot,is a way to seperate the air and have some flexibility with the engine. Also haveing the fan discharge air seperate, is a way to " save" the core from FOD since most debris is thrown outward away from the core engine. The amount of damage to the N1 compressor section can be quite substantial, and still have the core engine intact.. .I'm sure there is more to this that I can't recall right now, but I hope this helps.

[cwatters]

> IF I put a cone shape (with two open ends),. .> with big openning after a propeller which . .> direct the flow and speed up the flow to the . .> smaller openning. Will that increase any thrust. .> at all?

The answer is yes or no depending on the shape. .of the duct you are adding and the airspeed that. .the aircraft is operating at (and a load of other. .factors).

Here is one way to look at it...

Motors have a limited maximum power output. If the power being produced by the motor is the same with or without the duct then... The only thing you are changing by adding a duct is the EFFICIENCY with which that power is being applied to moving the craft through the air.

Adding a straight duct typically reduces the vortex drag caused by the tip of the blades but adds drag due to the friction of the air flowing through it.

A tapered duct will increase the velocity of the air coming out but will add drag so the volume of air per second will be reduced.

It's also interesting to think about the air flowing over the outside of the duct when the aircraft is moving. That 'sees' a taper going the other way so it's slowed down (= more drag).. . . .For more info raise the subject in the Jets section of the discussion forum at:

<a href="http://www.ezone.com" target="_blank">www.ezone.com</a>

..which is a forum for modellers building electric powered ducted fan aircraft.

Colin

[cwatters]

Sorry that URL should be...

<a href="http://www.ezonemag.com/" target="_blank">http://www.ezonemag.com/</a>

Colin

[John Farley]

Sorry don’t know how to put up a picture, but if you know what an Optica looks like or can find a pic of it that was an aeroplane that used your thoughts. The Lycoming 6 cyl engine drove a multiblade fan that had a duct wrapped round it. This increased the efficiency of the propulsion system when considered in isolation.

Snag was the duct produced a large amount of drag (as unwrapped it had approaching the same area as a wing and what is more the V inside the duct was of course considerably faster than the aircraft speed, so increasing the skin friction drag even further).

After some years of development the designers reckoned they could have got more performance from the overall aircraft with a conventional layout. There were other disadvantages of the ducted fan from both piloting and engineering points of view but these were not related to your notion

(If you can’t find a pic, imagine a 747 engine pod without its engine. Then add a couple of wings each side and support a tail from twin booms. Stick a nose pod for the crew ahead of the engine and duct and there you go)

[Zeke]

IMHO No,

Think of your question using a different fluid apart from air, water for example.

The only gain in my view would be an increase in the losses ;(

The design of the intake and duct would induce losses.

Z

[Keith.Williams.]

John,

What do you mean you can't do pictures????

That was the best description I've seen in this forum for a long time. I definitely visualised an Optica.

I don't know if you've seen it, but there is a post from a guy asking about the second part of yor Flyer item on stalling. He had received no replies last time I looked.

[Michael Murphy]

While we are talking ducts, has anyone considered the loss of efficiency in the 727 and L1011 S-Duct? First of all, the air (at high volume and velocity) is bouncing off walls, creating drag. Second, would the shape of the duct not generate lift and thus an up-load on the tail, which would require an even greater down load (and more drag)? I have often wondered if the DC-10/MD-11 layout with a straight duct was more fuel efficient. The DC always seemed to cruise more level than the L10, at least from the perspective in the cabin. I don't have the fuel specifics on the two widebodies, but note that the DC10 survived economically much longer than did the Lockheed. That is not to say that the L10 did not enjoy technical advantages not shared by the MD products.

TR <img src="confused.gif" border="0">

[M.Mouse]

Good drawings of the Edgley Optica here:

<a href="http://web.dreamsoft.com/JACKBALE/plan0064.htm" target="_blank">Optica drawing</a>

and a picture here:

<a href="http://www.cofe.ru/avia/E/photos_avia_E-2.htm" target="_blank">Optica photograph</a>

[John Farley]

Keith and M.Mouse

Thanks

[ft]

T. Runaway,

"First of all, the air (at high volume and velocity) is bouncing off walls, creating drag."

Air does not "bounce of walls". It's a fluid, remember? "The air slides along the walls" would be a more appropriate description. You'll lose a bit of energy to friction but not very much.

Consider how much longer pipes are used every day without much resistance at all. How much harder is it blowing through a pice of rubber house if you bend it into a U (without deforming it) than if you keep it straight? <img src="wink.gif" border="0">

Take a look at cut-through diagrams of jet engines. Many have the air going back and forth. There are engines with the air flowing backwards through the combustion chamber, turning at each end. You've seen those small turboprops with the exhausts right behind the propellers, right? Those take in air, turn it around, send it forward through the engine and turn it around again before letting it out. All with very little loss of efficiency.

"Second, would the shape of the duct not generate lift and thus an up-load on the tail, which would require an even greater down load (and more drag)?"

That's a negative. In the end, no air is accelerated downwards since it is all being let out through the engine in the same direction it came in from.

"I have often wondered if the DC-10/MD-11 layout with a straight duct was more fuel efficient."

You probably lose a bit of pressure in the inlet. On the other hand, the DC-10 probably has a significantly heavier tail structure to carry around the world which also adds to fuel costs. What's to prefer? I don't know and I don't think there's much of a difference. Won't lose sleep over that one anyway - even though it could be fun to read a more detailed analysis. <img src="smile.gif" border="0">

"The DC always seemed to cruise more level than the L10, at least from the perspective in the cabin."

Ramp rumours has it that the DC10 should have a different angle of incidence... it's a fact that I spent lots of morning hours shoveling cargo in and out of the bulk cargo bay in the tail to get the W&B right. :/

Cheers,. . /ft

[ 24 January 2002: Message edited by: ft ]</p>

[Michael Murphy]

Thanks for the info, FT, and the challenge to do some more research. Being not unfamiliar with Mr. Bernoulli's Theorem, I apologize for the earlier literary exuberance on my part. Your post sent me scurrying to the internet where my initial and admitted lay research (not being a fluids engineer) suggests this may be a tad more complex than presented. The more research, the more questions. Too much to compress into a short post, but those with equally bent interests may wish to look at:

<a href="http://www.hq.usace.army.mil/cemp/e/Et/efficien.pdf" target="_blank">http://www.hq.usace.army.mil/cemp/e/Et/efficien.pdf</a>

and some of the other academic/engineering sites using duct/friction/airflow as key words.

What intrigues me is the amount of air that goes into the engine - a cylinder bounded by the diameter of the inlet and equal in length to the forward movement of the aircraft. If there are inefficiencies at relatively low CFM, as indicated in the above URL, are they multiplied at very high volumes? Density at altitude is yet another factor. I'll grant you this is edge of the boundary-layer stuff, but there may be something to be learned here, even if it is not mainstream.

BTW, I am unconvinced about the no-download argument, having conducted practical tests and having pondered Newton's Third Law.

On the subject of airflow reversal in turbine engines, especially of the free sort, what of the PWC PT-6, with its 1080 degrees of directional change from free air to exhaust? I have always wondered if the relatively inferior fuel specifics of that engine (despite its many other sterling qualities) was yet another example of the many twists and turns through which gas had to thread itself, as it wended its way around the Gas Generator and Power Turbine. My own facetious theory of the PT6 is that half the power produced by that little engine is because the air is so frigging frustrated, all it wants to do is escape!

Cheers

TR. . <img src="confused.gif" border="0"> <img src="smile.gif" border="0">

[ 25 January 2002: Message edited by: Thermal Runaway ]

[ 25 January 2002: Message edited by: Thermal Runaway ]</p>

[John Farley]

TR

[quote]What intrigues me is the amount of air that goes into the engine - a cylinder bounded by the diameter of the inlet and equal in length to the forward movement of the aircraft.<hr></blockquote>

Does not have to be like that. With high peformance military jets slamming the throttle shut at high IAS causes a lot of spillage and associated buffet and intake banging type noises.

No big deal though. Only just a nitpick.

Mind you the spillage could destabise some older designs. On one of my early flights in the FD2 I slammed the throttle shut at about 1.6 and the nose got a big wiggle on left right as the air went in one trouser leg and out the other as it were with the bifurcated intake. Big sideways. When I got back and complained the Rolls guy told me "Everybody knows you must not close the throttle at high speed in the Fred, you were lucky the fin did not come off." I pointed out that everybody did not know .........or I would not have done it.

[ 26 January 2002: Message edited by: John Farley ]</p>

[Keith.Williams.]

Bonpon,

Although previous posts have shed a good deal of light on this subject, I'm not entirely sure that they have answered your original question "what is the fan duct on a turbo-fan engine for?". I will attempt to do so and as usual, will go for simplicity rather than completeness.

The fan serves two functions. Firstly, it is the low pressure compressor. The air passing through the fan is compressed, then some of it is passed into the engine core, where it is compressed again and used for combustion, cooling and thrust. The second and arguably more important function of the fan is to provide additional thrust.

The relative proportions of total thrust produced by the hot gas flow and the fan, are generally equal to the ratios of airflow through each. A modern high by-pass engine for example, might have a by-pass ratio of 10:1 This means that for every eleven pounds of air going through the fan, 1 pound goes through the engine core, and the other 10 pounds by-passes the core. But this 10 pounds produces about 90% of the total thrust. So in one sense we can consider the fan to be simply a propeller. To understand the function of the duct we must consider the problems affecting propellers.

Propellers are simply a means of converting the rotating power of the engine into thrust. Their blades are aerofoils, which, by rotating at high speed, generate a total reaction. The forward-facing component of this is thrust and the majority of the remainder is the propeller torque. This is wasted in opposing rotation.

Like wings, propeller blades produce tip vortices which in turn produce induced drag. The magnitude of these tip vortices increases, as the total reaction of the blades increases. So if we produce a lot of thrust, we can expect strong tip vortices and lots of (energy wasting) induced drag. This problem can be greatly reduced by placing a duct around the tips, to reduce the tip vortices. Because tip losses increase with thrust, ducts are most beneficial in highly loaded, high thrust systems. So our modern turbo-fan producing 80000 or 90000 pounds of thrust will gain a lot from a duct, whereas a small piston/prop creating perhaps 2000 or 3000 will not.

The second problem affecting a propeller is that its angle of attack is proportional to the relative magnitudes of RPM and TAS. So as airspeed increases, the angle of attack decreases. This reduces aerodynamic efficiency, thereby causing thrust to reduce rapidly as speed increases. This limits the maximum speed that can be attained by propeller aircraft. Variable pitch props reduce this effect by increasing blade angle as TAS increases. Although this maintains a more or less constant angle of attack, it also tilts the total reaction away from the direction of flight. This increases the proportion of the total reaction that is wasted in the form of propeller torque. So the variable pitch prop system is not entirely successful. But by controlling the TAS of the incoming airflow, a fan duct can provide much greater control of angle of attack without the need to vary blade angle.

Ducts designed for subsonic flight commonly form a slightly divergent passage ahead of the fan. This slows down the incoming air, and increases its static pressure. This in turn increases the airspeeds at which turbo-fans can be operated efficiently. Also by producing a slightly convergent passage behind the fan, the duct accelerates the air, to increase the thrust produced.

The third problem is that of tip speed. The airspeed over the tip of a propeller blade is made up of rotational speed and aircraft TAS. Rotational speed is proportional to blade length and RPM. So if we use long blades and high RPM to create lots of thrust, the tip speeds will soon become very high, as TAS increases. When tip speed approaches the speed of sound, the blades create shock waves and just like wings, generate a great deal of extra drag. In a turbo-fan this problem is addressed into two ways. Firstly, by using lots of short blades rather than a few long ones. And secondly by using the duct to reduce the speed of the incoming airflow as described above.

So in a turbo-fan the duct reduces tip losses, controls incoming airflow speeds, and reduces the rate at which thrust drops with increasing TAS. In a sense it could do all the same things for a propeller but the benefits would be much less. . .These must then be balanced against the additional weight and drag costs described above by John.

. .TF,

Although your suggestion that the rear inlet duct in a Tristar might produce lift, appears logical, we need to consider the entire duct. At the front end it curves from horizontal to downward pointing. This will indeed accelerate the air downwards, and hence produce an upward force on the aircraft. But the rear end of the duct curves from downward pointing to horizontal. So the vertical velocity given to the air by the forward end is reduced to zero by the rear end. The overall vertical acceleration and hence upward force is therefore zero.

It is true however that long, contorted ducts reduce efficiency. That is why most designers use the minimum duct length necessary to achieve their objectives. A high engine as in the DC10 will produce a nose-down pitching moment, which needs to be balanced by the nose-up moments from the lower, wing mounted engines and (probably) a bit of trimming. The Tristar designers' decision to go for a low engine was probably aimed at eliminating this nose-down pitching moment. Having made this decision they were then stuck with the need for a long curved duct. The use of reverse flow combustion chambers in engines is a similar balance between costs and benefits. Reverse flow reduces efficiency slightly, but this is usually outweighed by the benefit of having shorter and hence lighter engines.

[John Farley]

Keith. .Excellent words sir

[Keith.Williams.]

Thanks John,

I work on the basis that if I make enough posts, (and they're long enough), some will make sense!!!!

Another aspect of this subject occurred to me today (I was driving at the time). It is that the (few ) modern airships currently in existence tend to have ducts on their (multi-bladed) props. I suspect that this is because the extra parasite drag caused by the duts at very low speeds is outweighed by the benficial effects of improved prop eficiency. This is of course pure conjecture on my part.

Any airship designers out there?

[John Farley]

I should think you are right there. It must also keep the diam down, so less support structure needed to keep the prop away from the hull plus less risk to handlers on the ground and a nice package with which to use vectored thrust. I seem to remember seeing pics of self docking car ferries that could extend similar podded props from their hull.

[Black Baron]

Great thread, thanks.

I was just wondering,
1/ What is the rpm of a ducted fan? I only know N1 etc.
2/ How much thrust is made by the fan. ( As in an example of an engine and how much thrust it makes)
3/ Does this vary with TAS, IAS, and altitude/ temperature.

Thanks for your time.:ok:

[enicalyth]

re L-1011 duct S-bend. at least the three engines are co-planar. s-bend did have good ram efficiency by all accounts. on the ground the co-planar middle engine allows the designer to use it for reverse thrust if he wants. on a dc-10 it isn't an option... it will lift the nose.

[PT6ER]

enicalyth,

You will find the center engine on a DC10 / MD11 does in fact have a translating cowl/blocker door style thrust reverser.

Interesting idea though since I saw the results of a Citation X, aft cg tests and thrust reverser deployment...messy!

[barit1]

#2 DOES have a reverser, although it is inhibited until the nose gear squat switch is closed.

Related subject: One astute F/E wrote a paper that the DC-10 should cruise more efficiently with #2 at a reduced power setting, because trim drag would be reduced.

He would have been correct except for the fact that thrust falls off faster than fuel flow as the golever is retarded (i.e. SFC deteriorates).

[barit1]

The real reason for the fan duct is to get the fan pressure ratio up so it creates some real velocity going out the fan nozzle. An unducted fan (all the engine guys have experimented with open fans) is great at lower mach, but its propulsive efficiency drops off pretty fast as aircraft speed increases.

Black Baron asked about RPM - for the 747-size engine it's around 3500 RPM on the fan (called N1 in the commercial world, or NF or NL in military markets). Bigger engines turn slower, little ones turn faster.

[idg]

TriStar S duct.

I saw an explanation about the design of the S duct from Lockheed. When originally concieved the L10 was going to be the proverbial 'bus' and so quick on and off loading was required. To make use of the centre doors with airstairs or bridges would have brought the wing engines very close to equipment using those centre doors.

The solution was to move the engine farther out on the wing which incidentally also improved wing bending relief. The downside was the need for a bigger rudder to keep the Vmcg/Vmca speeds under control.

Lockheed considered a DC-10 type arrangement but settled on the S duct to give a full length rudder. As has been mentioned before this also minimised the pitching effect of a high mounted #2 engine.

At the time of the L10 line closure they were also considering using the space above the S duct as an extra fuel tank to give the balance effect now often achieved with a tailplane tank.

:ok: