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  #1  
Old 04-13-2008, 04:09 PM
Absolute Absolute is offline
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Why no ducted-fan piston engines?

As I understand it, most of the thrust produced by a high-bypass turbofan comes from the ducted fan, not the jet thrust produced by the turbine itself. These are the most efficient engines in use today, apparently.

So why are there no piston-powered ducted fan engines? We have turboprops, which are jet engines driving propellors - why not piston engines driving ducted fans?

And actually, while we're at it - why are turboprop engines used at all? I can understand piston-props, because they're cheaper, but if you're going to install a jet engine, why not just make it a turbofan?

Last edited by Absolute; 04-13-2008 at 04:13 PM..
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  #2  
Old 04-13-2008, 04:18 PM
Myglaren Myglaren is offline
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Quote:
And actually, while we're at it - why are turboprop engines used at all? I can understand piston-props, because they're cheaper, but if you're going to install a jet engine, why not just make it a turbofan?

WAG: I'd put it down to efficiency and ease of maintenance combined with reliability.
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  #3  
Old 04-13-2008, 04:34 PM
matt matt is offline
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Quote:
Originally Posted by Absolute
As I understand it, most of the thrust produced by a high-bypass turbofan comes from the ducted fan, not the jet thrust produced by the turbine itself. These are the most efficient engines in use today, apparently.
And here is your problem. The bolded part is not universally true. It depends how fast you're going.

At low speeds, propellers are more efficient than the ducted fans on the front of a turbofan. So if you're carrying cargo or airmail, or you just care more about fuel than speed, propellers are the way to go. And a turboprop will beat piston-plus-prop for efficiency and power-to-weight, with the possible exceptions of some funky turbocompound and flying diesel engines.

At higher speeds, turbofans beat turboprops for efficiency, but a piston-driven ducted fan won't give you any benefits at all.

Last edited by matt; 04-13-2008 at 04:35 PM..
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  #4  
Old 04-13-2008, 06:28 PM
Xema Xema is offline
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Quote:
Originally Posted by Absolute
These are the most efficient engines in use today, apparently.
Are you sure? Wiki seems not to agree.

Quote:
Advantages
- By reducing propeller blade tip losses and directing its thrust towards the back only, the ducted fan is more efficient in producing thrust than a conventional propeller at low speeds (normally considered under 80 knots, 40 m/s or approximately 90 miles per hour for an aircraft).
- For the same static thrust, a ducted fan has a smaller diameter than a free propeller.
- Ducted fans are quieter than propellers: they shield the blade noise, and reduce the tip speed and intensity of the tip vortices both of which contribute to noise production.
- Ducted fans can allow for a limited amount of thrust vectoring, something normal propellers are not well suited for. This allows them to be used instead of tiltrotors in some applications.

Disadvantages
- At higher speeds (above about 80 knots), the presence of the duct may create more drag than the extra thrust it provides and therefore its comparative advantage to an open prop is canceled.
- Good efficiency needs very small gap between tip blade and duct; Ducts are heavy and expensive.
- In general, a large propeller is more efficient than a small propeller, and ducted fans usually have very small propellers, and so can be inefficient compared to a larger standard propeller arrangement.
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  #5  
Old 04-14-2008, 05:37 PM
mwbrooks mwbrooks is offline
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Quote:
Originally Posted by Absolute
So why are there no piston-powered ducted fan engines? We have turboprops, which are jet engines driving propellors - why not piston engines driving ducted fans?
I'm not sure about the full-scale issues involved. Model airplanes have been using piston engines to drive ducted fans for many years now. I gather these engines turn at higher speeds and encounter a lot more stress than they would in propped applications.

For whatever reason, piston-powered ducted fans seem to be falling out of fashion. Model-sized turbine jets are available now for those who can afford them, and the newer technology electric-powered ducted fans (EDFs) seem to be filling in the smaller, lower cost niche. Some modelers are converting their old piston-powered DF models to EDF.

It is probably significant that turbine engines and most electric motors are said to be much more comfortable turning at high speeds than piston engines are.
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  #6  
Old 04-14-2008, 10:27 PM
Kevbo Kevbo is offline
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Reliability and durability are huge concerns with aircraft engines. Turbines are much more reliable than recips.

As for the model airplanes: Glow powered ducted fans don't sound like real jet engines, so somewhat distroy the desired illusion of a "jet" model.
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Old 04-15-2008, 12:01 AM
mwbrooks mwbrooks is offline
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Originally Posted by Kevbo
As for the model airplanes: Glow powered ducted fans don't sound like real jet engines, so somewhat distroy the desired illusion of a "jet" model.
Yeah, but I guess it's not as bad as hanging a prop and a muffler off the nose of an F-4 model. I haven't heard what an EDF sounds like yet. Turbines are still beyond my budget, but they sure are fun to watch. (And listen to. And smell, now that they run on kerosene.)
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  #8  
Old 04-15-2008, 04:36 AM
Richard Pearse Richard Pearse is offline
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Quote:
Originally Posted by mwbrooks

It is probably significant that turbine engines and most electric motors are said to be much more comfortable turning at high speeds than piston engines are.
Sure. Piston engines have a huge hunk of metal undergong a rapid 180 degree change of direction 5000 times a minute (in a typical aero-engine.) The only real stress a turbine is under is that they get very hot and every time you start one it rotates for a short period of time with little or no lubrication (also true for piston engines.)

Last edited by Richard Pearse; 04-15-2008 at 04:37 AM..
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  #9  
Old 04-15-2008, 09:41 AM
Capt B. Phart Capt B. Phart is offline
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Originally Posted by 1920s Style "Death Ray"
The only real stress a turbine is under is that they get very hot and every time you start one it rotates for a short period of time with little or no lubrication (also true for piston engines.)
Jet engines are under lots of centrifugal stress (or centripetal, if you want to nitpick) and that, combined with the heat, is generally what limits their output/efficiency.
The hotter the engine can burn the fuel and the faster it can spin the better, but that stresses the output turbine blades particularly.
Early jets were limited mainly by the metals available at the time, as the blades would stretch or crack. High temperature alloys have come a long way since then
(and some turbine inlet stages now use hollow blades to operate with gas temperatures higher than the melting point of the alloy they’re made from)


A ducted fan plane:-
The Stipa Caproni
Cute eh?
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  #10  
Old 12-24-2008, 04:36 PM
Absolute Absolute is offline
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I want to resurrect this thread - I got busy when I first started it and never followed up.

I understand there are reliability issues...but what about cost? There are plenty of people flying piston-powered Cessnas because they can't afford a jet, but they'd like to be able to fly faster, more efficiently.

A piston-powered ducted fan wouldn't be as reliable as a jet engine, but wouldn't it be cheaper than a jet, and more efficient (at high speeds) than a prop?

Is the issue simply that you need a lot of power to get to the speeds where a ducted fan is more efficient than a prop, and piston engines simply can't deliver that much power economically?
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  #11  
Old 12-24-2008, 07:27 PM
LSLGuy LSLGuy is offline
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Cessna-sized aircraft are an economic dead end.

Most of the technology there is 1950s or earlier, and the production runs are so small and the liability so huge that there is no real effort being made to invent anything new for that part of aviation.

If it was a booming market, you might find people expending effort to determine whether a ducted-fan driven by an ICE would work better than a traditional 2 or 3 blade freestream propeller driven by the same ICE.

But for now, it doesn't really matter what the aero-theorists might say would be more or less efficient. It is economically impossible just as much as a lead zeppelin is physically impossible.
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  #12  
Old 12-24-2008, 11:07 PM
Sam Stone Sam Stone is offline
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I wouldn't be too quick to write off 'Cessna Sized Aircraft'. There is actually significant innovation going on now, for the first time in decades. This is mainly due to deregulation - the creation of a "Light Sport Aircraft" category, which has lower entry requirements, but is also the result of composite aircraft technology.

For example, the Cessna Skycatcher is a thoroughly modern version of the Cessna 152. It's got a glass cockpit, control sticks instead of yokes, and significantly better performance than the Cessna 152 on the same horsepower. It's also six inches wider inside, which is important to those who have ever flown in a 150/152.

The price is still out of control - $111,000 for a two seat airplane with an old technology 100 Hp motor, but that's a function of the problems you listed - too small a market, which prevents economies of scale.

Then there's the Cessna 350, which is an advanced composite four-seater with stupendous performance, an advanced glass cockpit with weather, 3-D track display, full autopilot, etc. Unfortunately, it's half a million bucks. But I'll bet Cessna is selling a lot of them.
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  #13  
Old 12-25-2008, 12:19 AM
ElvisL1ves ElvisL1ves is offline
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Quote:
Originally Posted by Absolute View Post
Is the issue simply that you need a lot of power to get to the speeds where a ducted fan is more efficient than a prop, and piston engines simply can't deliver that much power economically?
It isn't just power, it's weight and size, too. You can get a given amount of power in a much smaller and lighter package with a turbine engine.

A duct adds weight and friction drag, too, that may offset any efficiency improvements, especially at lower speeds. Plus, it adds serious airframe packaging headaches, especially on a single. It's been done, though, just not often.


Cessna, btw, has suspended production of the 350 and 400 at the former Columbia facility. The new high-end piston market is currently pretty soft, and the VLJ's now coming into service are taking a chunk out of it.
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  #14  
Old 12-25-2008, 02:52 AM
Sam Stone Sam Stone is offline
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That's too bad. I got a good look at them in September at the Reno Air Races at the Cessna booth, and they looked fantastic. But I can imagine half a million dollars for a 4-seat piston plane is a pretty hard sell.
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  #15  
Old 12-26-2008, 03:39 AM
Hail Ants Hail Ants is offline
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Quote:
It is economically impossible just as much as a lead zeppelin is physically impossible.
Not true!
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  #16  
Old 12-26-2008, 08:05 AM
Bosda Di'Chi of Tricor Bosda Di'Chi of Tricor is offline
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Quote:
Originally Posted by Capt B. Phart View Post


A ducted fan plane:-
The Stipa Caproni
Cute eh?
That's insane! It ain't an airplane, it's a barrel with wings & a prop!
__________________
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"He is an abomination of science that curdles the milk of all honest men!"~~One Dr Chouteh, possibly commenting on Bosda Di'Chi.Or not.
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