Ducted Fan Pseudo-Jets

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sanman

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I would do a scale slow jet with larger high bypass fans, such as the A-10.
So you'd want a high-bypass turbofan powering an A-10 style light aircraft? That would have to be E-LSA, presumably.
If it was a piston-powered ducted fan version, then it could be LSA, and bypass wouldn't apply then.
 

sanman

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People DO care about efficiency. Fuel isn't cheap. Batteries aren't either. And both have weight and some risk.


People have been converting the Wankel since the 1970s, and it has not been widely successful. There are only two internal-combustion engine types: reciprocating, which includes all piston engines of any type and the Wankels, and the Brayton-cycle engines, which are the turbines.
Most homebuilders find that the quickest and cheapest way to get flying is to find an aircraft engine. These things have been developed and improved over many decades and most of their weaknesses have been addressed. They were designed to drive propellers. They were designed to be light and to run at high power settings. Using anything else means a lot more work, and often more money, especially if the builder doesn't have the skills and tooling and has to hire it done.


Go back a few posts and read Jedi's post on his experience with duct development at Boeing. Ducts on turbofans have gazillions of dollars sunk into their research. They wouldn't be using them if there wasn't a net gain in their application. For instance, open propellers at 650 MPH and high altitude just don't work well at all. It was tried in the 1980s, but was abandoned. Since then, turbofan fans have gotten larger and more efficient, with much higher bypass ratios, and I don't think we'll see any more unducted fans.

I do remember seeing that video awhile ago, along with some others. Propfan development was started because of the original oil price spike from OPEC, but later died due to falling fuel prices.


So it's not clear that ducted turbofan is inherently superior - except on lower noise and lower radar visibility, which benefits jet fighters.
 

sanman

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I would do a scale slow jet with larger high bypass fans, such as the A-10.
I was thinking about this some more, and the A-10's iconic look would really be nifty.

But I'm also now thinking that there'd be lawsuits after some nut puts their own autocannon in there and goes postal.
 

Dan Thomas

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I do remember seeing that video awhile ago, along with some others. Propfan development was started because of the original oil price spike from OPEC, but later died due to falling fuel prices.


So it's not clear that ducted turbofan is inherently superior - except on lower noise and lower radar visibility, which benefits jet fighters.
Go back and look at that video again. The airplane, in flight, is at a high AOA, indicating a very low cruise speed.

One must never forget that propeller tip speed must be kept subsonic, or drag mounts enormously and the noise becomes unbearable. Tip speed is figured by the simple triangulation formula: A² + B² = C², where A is circumferential tip speed, B is forward speed and C is the net tip speed. Those propfans are over 11 feet in diameter, similar to the props on WWII fighters, and those airplanes were limited to around 500 MPH max. Airliners cruise at well over 600. Open propellers just aren't going to work. They're fine on turboprop commuters like the Dash 8 series, which cruise at around 400 MPH.

Ducts on airliners are carefully designed to give positive returns. I already explained what they do and how they do it. Don't let wishful thinking blind you to the facts of physics.
 
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Malish

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Ducts on airliners are carefully designed to give positive returns. I already explained what they do and how they do it. Don't let wishful thinking blind you to the facts of physics.
I think this tread was about DUCTED FAN PSEUDO JETS - not about TURBO JETS! Like everything else ducted fan aircraft have their positive and negative things to offer and NEVER should be compered with turbo jet aircraft...
 

Urquiola

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UL-39 ALBI II


I recently got the book 'Roy Fedden', actual maker of Bristol sleeve valve engine line, from Rolls-Royce heritage, Fedden indicated 'Engineering is the British specialty', look at IA-36, also, that a new war was unavoidable, this is in line with the F Engels statement of UK wanting to be a manufacturer's land serving an agricultural Europe, this was around 1845, many people knew how to make steam machines, fabric, guns. If allowed, or the business gave enough profits, Airbus will attempt a monopoly in model airplanes, they suffer the insane 'french grandeur', but metal 3D printers changed aviation landscapes, a future could exist for 'handycraft flying machines'; about air transport and defense flight, Russia attempted unsuccessfully selling there, China aims at their own internal market, to serve far away sites, an airport is cheaper to build than a highway or a railroad. Be modest, this is the basis to avoid a crash. Blessings +
 

sanman

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Go back and look at that video again. The airplane, in flight, is at a high AOA, indicating a very low cruise speed.

One must never forget that propeller tip speed must be kept subsonic, or drag mounts enormously and the noise becomes unbearable. Tip speed is figured by the simple triangulation formula: A² + B² = C², where A is circumferential tip speed, B is forward speed and C is the net tip speed. Those propfans are over 11 feet in diameter, similar to the props on WWII fighters, and those airplanes were limited to around 500 MPH max. Airliners cruise at well over 600. Open propellers just aren't going to work. They're fine on turboprop commuters like the Dash 8 series, which cruise at around 400 MPH.

Ducts on airliners are carefully designed to give positive returns. I already explained what they do and how they do it. Don't let wishful thinking blind you to the facts of physics.
Hi, thanks for your comments. So what is it in a ducted turbofan that prevents the blades from crossing the sound barrier and producing sonic booms?

And is the distinction between a propfan and a regular prop mainly in the number of blades, or also in the shape of those blades?
 

Dan Thomas

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Hi, thanks for your comments. So what is it in a ducted turbofan that prevents the blades from crossing the sound barrier and producing sonic booms?

And is the distinction between a propfan and a regular prop mainly in the number of blades, or also in the shape of those blades?
Way back on the first page of this thread I posted this:

Jets use turbofans to improve thrust for fuel consumption, and to reduce the noise. They are big fans, and are in ducts so that the airflow into them can be slowed to keep the blades subsonic. Those ducts are divergent, making them diffusers, which slows the air and increases its pressure before it ever gets to the fan. Bernoulli at work. The blade tip clearances are really tiny with respect to the fan diameter, too. Then the fan discharge is often narrowed down to increase its velocity again, since K=MV². Increasing velocity has a bigger effect on thrust than increasing mass.

Most of the homebuilt ducted fans I saw appeared to be constant-diameter ducts. One needs to use all the available tools.



1637946550261.png



1637946569682.png



Pressure Recovery - an overview | ScienceDirect Topics

Now, pay attention to this:

1. As a gas's velocity increases, its temperature and pressure both decrease.
2. As a gas's velocity decreases, its pressure and temperature both increase.
3. A divergent duct widens as the gas passes through it. In this case, the gas is air, and the divergent duct is the turbofan's intake. That means that the air SLOWS DOWN in there, and its PRESSURE AND TEMEPERATURE INCREASE. This does the following:

The slower air means that there is more room for higher tip speeds before they reach supersonic speeds.

The higher pressure means that the fan is dealing with denser air, meaning more mass moved, meaning more thrust created.

The higher temperature means that the speed of sound in that air is higher, again making more room for higher tip speeds. Remember that the speed of sound in air is dependent entirely on the air temperature, not its pressure, as is widely believed. Air temperature decreases with altitude.

So it's not just a straight-walled tube. It's MUCH more complex than that even though it appears simple.

A supersonic jet fighter's intakes form shockwaves in them that slow the air so the engine can operate even though the airplane is moving far above the speed of sound. The airflow in any turbine slows as the compressors compress it, and by the time it reaches the combustion section its pressure is pretty high, and is moving very slowly. The combustion causes expansion that increases its velocity, not its pressure, since an increase in pressure would stall the compressor section and the engine would never run. The velocity drives turbines that drive the compressor, and also a fan or propeller or helicopter transmission in those applications. Around 75% of the energy goes to compress the incoming air; the rest is thrust or shaft horsepower.

1638471486558.png

And that's not all. Between every pair of rotor or stator blades throughout that engine we find divergent and convergent ducts, to decelerate or accelerate the air or combustion gases. The above graph only shows that velocity change in the turbine section, bit it's happening in the compressor section, too.

When I taught this stuff I found that most students did not find it intuitive. Some had a hard time believing it, but if it isn't true you'd better not get on an airliner, because it won't go anywhere.

The propfan used many blades to absorb the horsepower, and their blades were designed to operate near supersonic. If they had been supersonic everyone for miles around would have known it.
 

sanman

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Feb 24, 2021
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239
PJ-II "Dreamer" is single engine aircraft with twin ducted fans:
You've pointed out that over 100mph, the increases in drag lower the efficiency of the ducted fan propulsion.

Would you be willing to consider the idea of trying to ionize airflow inside the duct, in order to reduce drag?
Ionized airflow is more laminar than regular air (diatomic gas) and therefore less turbulent with lower drag characteristics.
Please consider that because duct is surrounding the airflow, it provides better opportunities to ionize that airflow.
There are multiple possible methods of ionization: contact ionization, corona discharge, helicon antenna




Marketing suggestion: If you can make modified version with ionization, you can call it PJ-2B Twin Ionized Exhaust Fighter (T.I.E. Fighter) 😎

Ducted fans already provide similar sound fx:

 
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