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Why is the fan in a turbofan different than a propeller in a small airplane? Why do propellers typically have 2 or 3 blades, but the fan has a lot more? And why is the fan shrouded but the propeller open?

I read that a ducted fan is more efficient at very slow speeds (it generates more static thrust than an open prop), but at high speeds the drag becomes too great and you're better off with an open propeller. Which is why they use a ducted fan in hovercrafts, for instance. But if that's true, shouldn't the fan in a turbofan be open (unshrouded), considering the high speeds (higher than a propeller)?

Also, how does the fan get away with supersonic tip speeds and the prop don't? Would the fan be more efficient if tip speeds were lower?

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The simple answer that covers the majority of engines is that a fan has a shroud. The possible exception are unducted fans or Open Rotor Engines, which are a hybrid between a turboprop and a fan engine.

A more scientific answer is based on the difference in disc loading: How much power per disc area can be pumped into the fan for propulsion. For propellers, this is distinctively less than for fans. Some examples:

  • Cessna 172 with O-320 engine (150 hp) and McCauley 1C160 prop (1.9 m diameter): 39.45 kW/m².
  • P-51D with Packard-Merlin 1650 (1590 hp) and Hamilton-Standard prop (3.4 m diameter): 130.59 kW/m².
  • Lockheed C-130H Hercules with Allison T-56 engine (4590 hp) and Aeroproducts propeller (4.1 m diameter): 259.25 kW/m².

With jets, this comparison needs power when thrust is listed. Therefore, we make the heroic assumption that the speed at the fan is Mach 0.4 in static conditions and thrust is split according to bypass ratio (which is completely imprecise but will do for the purpose here):

  • BAe 146 with Lycoming ALF 502 (31 kN static thrust, 5.7:1 bypass ratio and 1.02 m fan diameter): 4254.35 kW/m².
  • Boeing 747-200 with Pratt & Whitney JT9D (213 kN static thrust, 5:1 bypass ratio and 2.34 m fan diameter): 5388.74 kW/m².
  • Boeing 777 with GE90 (388.8 kN thrust at take-off, 8.4:1 bypass ratio and 3.124 m fan diameter): 6077.23 kW/m².

I guess by now you see where this is heading: Turbofans are simply a different class when thrust per frontal area is concerned. And for this to be possible, three conditions must be met:

  1. A well-designed intake which produces uniform flow over the engine face. The shroud is only the consequence of providing the fan with an intake.
  2. A high solidity factor of the fan/propeller (the ratio of the total blade area of the propeller to the disk swept out when the prop turns)
  3. High dynamic pressure at the engine face, so the engine power can be absorbed by the fan/propeller.

Note that those conditions depend on each other: Without the solidity, much less power could be absorbed. Without the intake, the supersonic flow at the fan tips would produce horrible losses and noise.

Now for the crazy ones. The misfits. The rebels. The troublemakers. The round pegs in the square holes:

Here it is hard to make a clear call whether these are fans or propellers. Their proper name "propfans" hints at that already.

A last word on terminology

Efficiency is a measure how much effort is needed for a certain result. An efficient prop needs less power per unit of thrust produced. It has few blades, turns slowly and has a big diameter. A subsonic fan would be more efficient, but would create much less thrust for a given size.

If you want to express the capacity for thrust creation at a specific diameter, use disk loading, not efficiency.

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  • $\begingroup$ So the shroud is basically an aerodynamic complement to the fan intake? But also, I was under the impression that the fan acts a little like a compressor, that raises the pressure of the air and then the air is ejected at a higher speed than the intake speed. I say this because of the nacelle shape. The exit usually has a small diameter, which to me looks like a convergent nozzle. I know the jet core uses this principle, but does it apply, to a smaller extent, to the cold flow of the fan as well, or is the shroud merely to control the intake flow? thanks $\endgroup$ – Southbob May 1 '16 at 16:59
  • $\begingroup$ @Anonymous; You are right, the flow contracts while being accelerated. See this answer for a propeller, where the same happens, just without a wall between inner and outer flow. The shroud just follows the streamline contour. $\endgroup$ – Peter Kämpf May 1 '16 at 18:15
  • $\begingroup$ OK, @PeterKämpf, can you please be consistent in your usage of fractional notation? For props and kW/m², you use the American decimal to differentiate the meters from fractions thereof 1C160 prop (1.9 m diameter) and 4254.35 kW/m², yet you use the European comma for jets 3,124 m fan diameter. It took a moment to wrap my head around just how huge that fan was, then to realize you switched methods on me. :) $\endgroup$ – FreeMan May 2 '16 at 12:40
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    $\begingroup$ @FreeMan: Sorry, that one slipped through. I did the list by copy and paste and did not look at the decimal point. Thank you for catching it! $\endgroup$ – Peter Kämpf May 2 '16 at 12:42
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    $\begingroup$ You and me both, @reirab, you and me both... :) $\endgroup$ – FreeMan May 2 '16 at 17:03
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In simple terms, the working principle is different:

A propeller is a rotating wing. This means that its main working principle is the Bernoulli principle. A difference in pressures (i.e. low pressure in the front and high pressure in the back) causes the plane to move forward. Is the same as in the helicopter rotors. See https://en.wikipedia.org/wiki/Lift_(force) and https://en.wikipedia.org/wiki/Airfoil

The turbofan is mainly a reactor. This means that its main working principle is Newton's third law. The high velocity air that is forced trough the nozzle pushes the plane forward. The same as in a garden hose. See https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion

To answer one of your additional questions, the one that is less efficient at high velocities is the propeller. This is because, in the propeller, at high velocities the air is flowing from the front to the back causing the pressure to rise at the front thus stalling the propeller.

In the other hand in the turbofan, the additional pressure at the front actually helps the air to be forced trough the nozzle increasing performance.

At extremely high velocities, however, the drag from the fan becomes very high and the speed is limited again (but at a higher velocity than in the propeller.) To achieve even more velocity, the rotating blades are removed and the air is forced trough the compressor because of the very high pressure at the front (See ramjets.) The working principle of ramjet is also Newton's third law.

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  • $\begingroup$ @SMSvonderTann, A turbo<b>fan</b> is a ducted fan and is clearly the kind of ducted fan the OP is about. $\endgroup$ – Jan Hudec May 3 '16 at 5:10
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    $\begingroup$ No, the principle is not different. Propeller and fan (turbocompressor) are both rotating wings. A pressure difference is the only way to make air apply force on anything, so they both use that too. And they also both use air as reaction mass, because one simply can't work around the laws of motion. $\endgroup$ – Jan Hudec May 3 '16 at 5:21
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    $\begingroup$ You're right indeed Jan. All laws of physics are present in both devices. The difference is that, by design, the main principle is different. For example, in the propeller, the air trough the blades comes mainly from the sides (not from the front.) For this reason, in propellers, Venturi effect is more important than Newtonw's third law. In the other hand, in the turbofan, the air behind the blades is compressed and accelerated before leaving the nozzle, making Newton's third law to having a greater impact than Venturi effect. $\endgroup$ – Krauss May 3 '16 at 10:31
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    $\begingroup$ First of all, the Venturi effect is specific to enclosed structures, like a pipe. I think you are referring to the Bernoulli principal. I second JanHudec. Both propellers and fan blades use the exact same method of reducing pressure to accelerate air. The Newtonian reaction force is the result of accelerating that air. There's no order of importance. The more Bernoulli you have, the more Newton you have. $\endgroup$ – TomMcW May 3 '16 at 18:10
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    $\begingroup$ @Anonymous: What is different is the intake. It slows down the air in case of the fan, so it never "sees" the actual flight speed. $\endgroup$ – Peter Kämpf May 4 '16 at 16:54
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First, let's start with the simplest machine, the screw, which converts rotational motion to linear motion.

A fan is a screw whose primary purpose is to move the fluid (air for a plane or water for a boat) which surrounds it.

A propeller is a screw whose primary purpose is to move (propel) the object that it is attached to, such as a boat or a prop plane. And indeed, mariners use the term screw instead of prop.

In the case of turboprop and turbofan, the turboprop uses its prop as the primary means of motion, whereas in the turbofan, the fan simply increases the velocity of the air, but doesn't provide the primary means of motion for the aircraft.

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  • $\begingroup$ the gasses expelled by the turbine itself $\endgroup$ – rbp May 2 '16 at 7:56
  • $\begingroup$ now we're getting into etymology. when the word turbofan was devised, there were only low-bypass ratio fans and the exhaust gasses provided the primary means of propulsion. even as high-bypass fans were developed, the name stuck. $\endgroup$ – rbp May 2 '16 at 8:08
  • $\begingroup$ feel free to upvote, then :) $\endgroup$ – rbp May 2 '16 at 8:13
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    $\begingroup$ That's correct in the case of the low BPR engines, but not in today's civil engines for fuel efficiency purpose, as explained here: What is the bypass air in a turbofan engine actually for? $\endgroup$ – mins May 2 '16 at 8:19
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    $\begingroup$ Between this answer and your comments on Peter's answer it seems you're pointing out that prop vs. fan is a continuum, which is true. But other than the propfans Peter mentions there are distinct differences between props and fans used today. That what the question is about, not the difference in terminology $\endgroup$ – TomMcW May 2 '16 at 19:28
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I guess both the propeller and the fan act in the principle of conservation of momentum, call it 'action-reaction', it send backwards a mass of air that pushes the device forward, the speed of expelled air mass, and the airspeed of airplane have an influence on which type of propulsive method gives better results, a lot of air at a not very high speed, or a lesser mass of air expelled very fast. German engineers during WW II thought that propellers would be good up to a bit above Mach 1, but the contra-rotating propellers version of the F-84 Thunderstreak jet fighter proved so noisy that those close to the running engine or airstrip got sick. Here is a brief YouTube audio recording of the F-84H 'Thunderscreech':

You may like the attached chart, from the booklet: 'Ducted Fans for Ultralight Aircraft', by R. W. Hovey. It shows the efficiency of different propulsion systems according to AirSpeed of a flying machine. Typical Propulsion systems efficiency

There's a more detailed analysis of the subject in a 2009 Thesis by Leighton Montgomery Myers, Pennsylvania State University: 'Aerodynamic Experiments on a Ducted Fan in Hover and Edgewise Flight' http://www.engr.psu.edu/rcoe/theses/Myers_Leighton.pdf

Also: AIAA-98-3116 NASA / TM--1998-208411 General Aviation Light Aircraft Propulsion: From the 1940's to the Next Century Leo A. Burkardt Lewis Research Center, Cleveland, Ohio Prepared for the 34th Joint Propulsion Conference cosponsored by AIAA, ASME, SAE, and ASEE Cleveland, Ohio, July 12-15, 1998 National Aeronautics and Space Administration Lewis Research Center, July 1998. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980209647.pdf

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    $\begingroup$ It would be helpful to include some of the information from these references to make it more clear what the conclusion is. $\endgroup$ – fooot May 3 '16 at 20:59
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    $\begingroup$ Agree with @fooot, we try and stay away from being a link only (or reference only) site. It's better to pull the relevant information into your answer. $\endgroup$ – Jay Carr May 3 '16 at 22:14
  • $\begingroup$ The articles cited have many pages, adding just a few, or some charts, will make losing information. The first image is the basic info, more is available in the links. Thanks for your interesting feedback $\endgroup$ – Urquiola May 3 '16 at 22:43
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A fan is not necessarily a screw, it's any object used to move air towards something. The helix shape happens to be quite ideal but a propeller does not need to be a helix-shaped thing either, just an object used to propel something in a fluid or gas. (Propulsion through a solid is like time traveling: probably impossible. Unless we can imagine something like a tunneler spitting out whatever comes in the other end to move forward.)

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    $\begingroup$ Welcome on aviation.SE. You should make some editing effort, use caps at the beginning of sentences and simple dot at the end. You may also enhance your answer by adding references and links for further readings. $\endgroup$ – Manu H Sep 22 '16 at 12:04

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