Ducted Fan Pseudo-Jets

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Dan Thomas

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Reducing diameter reduces performance. Small turbines are inefficient and burn way more fuel per HP than a big turbine. Small propellers are inefficient, partly for the same reasons short wings have poor L/D ratios, and partly because the areas close to the hub aren't moving too fast and don't contribute much to thrust. Short blades lose a lot of their span to that.

The only reason someone wants a pseudo-jet is for the looks. Certainly ain't gonna get the jet performance. Form over function, fashion over practicality. That sort of thing loses its appeal real quick due to the loss of utility.
 

henryk

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posting the same thing so many times.
-I agree,BUT iff the "thing" seems VERY GOOD ?

BTW=it exist russian words=

"сила есть,ума не надо!"

-in auer case=iff we have big thrust, we can ignore the drag !

(in contrary,for economic "electric" fly we need LOW drag...)
 

Dan Thomas

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-I agree,BUT iff the "thing" seems VERY GOOD ?
What it "seems" like and what it is could be two very different things. If it's good, the idea will catch on and spread. If it isn't good, like most such things, it will disappear.
 

sanman

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Dear Sanman,
Archon is aimed at the low-cost, low-speed corner of the envelope that private pilots or LSA pilots can afford. Many of them also lack the skills to fly 200 mph airplanes.
Does low-cost automatically have to mean low-speed? Can the upper bound for low-speed not be pushed up a notch or two?
Archon has the boxy-shape which is not quite as rounded or streamlined as PJ-2 Dreamer. Presumably, that helps to lower the difficulties of fabrication and thus the cost. What's also appealing about Archon is its lower cost of operation. I'm not sure how much more expensive PJ-2 Dreamer is to operate compared to Archon, but it too is certainly cheaper to operate than a turbine-powered jet.

People who "feel the need for speed" can buy a stock L-39 for a lower price than developing an entirely new kitplane with a ducted fan.
L-39 as a turbine-powered jet trainer aircraft has a much higher cost of operation & maintenance than Archon or Dreamer. That cost of operation certainly does matter. Dreamer in particular seems to have been made to avoid such high costs. I'm not saying I know how to develop a new kitplane, but doing so could give rise to better choices for the consumer.
 
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sanman

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Reducing diameter reduces performance. Small turbines are inefficient and burn way more fuel per HP than a big turbine. Small propellers are inefficient, partly for the same reasons short wings have poor L/D ratios, and partly because the areas close to the hub aren't moving too fast and don't contribute much to thrust. Short blades lose a lot of their span to that.

The only reason someone wants a pseudo-jet is for the looks. Certainly ain't gonna get the jet performance. Form over function, fashion over practicality. That sort of thing loses its appeal real quick due to the loss of utility.
But there's also the cost factor. Turbine-powered aircraft are more expensive to maintain and operate.
 

Dan Thomas

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But there's also the cost factor. Turbine-powered aircraft are more expensive to maintain and operate.
That's a cost concern, not size and efficiency concerns that I was pointing out. Yes, turbines are expensive and are likely to remain so, even converted APUs. They run at very high RPM and so strength and balance are critical, and the turbine section has to be able to withstand high temperatures. They're not as simple as they look, either; they have complicated air-cooling systems. Only about 25% of the air is used in combustion; the rest is used for cooling.
 

Tiger Tim

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Does low-cost automatically have to mean low-speed?
Not necessarily, but the constraints you give yourself have costs elsewhere. In the case of the SF-1, the look was a big part of the whole package and there’s a lot of drag hiding in that design in order for it to look like an F-22 that you ordered from Wish.
 

sanman

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That's a cost concern, not size and efficiency concerns that I was pointing out. Yes, turbines are expensive and are likely to remain so, even converted APUs. They run at very high RPM and so strength and balance are critical, and the turbine section has to be able to withstand high temperatures. They're not as simple as they look, either; they have complicated air-cooling systems. Only about 25% of the air is used in combustion; the rest is used for cooling.
The main reason that people often even care about efficiency is due to its effect on cost. We often don't care about efficiency for its own sake, or for the sake of design elegance. Cost is often a overriding factor. Piston engines are cheaper to operate than turbines.
Is there some intermediate choice between reciprocating piston engines and turbines -- ie. intermediate on both cost and efficiency?
Is it the Wankel rotary engine? Or is there anything else available?


Not necessarily, but the constraints you give yourself have costs elsewhere. In the case of the SF-1, the look was a big part of the whole package and there’s a lot of drag hiding in that design in order for it to look like an F-22 that you ordered from Wish.
I'd think that the pseudo-ducts on SF-1 would exert that duct drag which Malish said increases significantly beyond 100mph.
The pseudo-ducts on SF-1 don't produce any propulsive thrust like the ducts on PJ-2 Dreamer which have fans inside them.
And yet SF-1's pseudo-ducts have bigger cross-sectional area than the ducts on PJ-2.

But why is it that nobody cares about duct drag for turbofans? Is it just because turbines are producing so much extra power that it doesn't matter?

-in auer case=iff we have big thrust, we can ignore the drag !

(in contrary,for economic "electric" fly we need LOW drag...)
So turbofan ducts will have similar drag problems as ducted fans, but it's just that we don't care because turbofans have plenty of power to spare?

But I again wonder about ionizing airflow, because ionized airflow is naturally more laminar/low-drag than regular air (diatomic gas), and furthermore the air flowing inside a duct should be easier to ionize compared to air flowing around an open aerobody. Also, if you add structures like inlet guide vanes, then it provides even more opportunity for contact & ionization of airflow.


 
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Dan Thomas

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The main reason that people often even care about efficiency is due to its effect on cost. We often don't care about efficiency for its own sake, or for the sake of design elegance. Cost is often a overriding factor. Piston engines are cheaper to operate than turbines.
People DO care about efficiency. Fuel isn't cheap. Batteries aren't either. And both have weight and some risk.
Is there some intermediate choice between reciprocating piston engines and turbines -- ie. intermediate on both cost and efficiency?
Is it the Wankel rotary engine? Or is there anything else available?
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.
But why is it that nobody cares about duct drag for turbofans? Is it just because turbines are producing so much extra power that it doesn't matter?
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.

 

henryk

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Is there some intermediate choice between reciprocating piston engines and turbines -- ie. intermediate on both cost and efficiency?
=HCCI (homogeneus charge combustion ignition ?)

=GUN engine (Detonation ignition)...

-thermal eficiency allmost 95 % !!!
 
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henryk

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be easier to ionize compared to air flowing around an open aerobody.
-how much energy for air ionisation ?

BTW= the aerodynamic drag can be elimineted (anihilation) by WEAVY flow...

Cd =0
eventually can be NEGATIVE (Cd<0) =became Thrust !
 

Tiger Tim

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But why is it that nobody cares about duct drag for turbofans? Is it just because turbines are producing so much extra power that it doesn't matter?
It’s all part of a bigger picture and a series of trade offs. You can bet the pros look down a hundred different avenues before choosing how the ducts are going to look for their turbofans. Where efficiency matters most (airliners) you can see they have enough duct in front of the fan for effective inlet anti-icing and enough behind the duct for thrust reversers and that’s about it. A fan installed in a jet fighter often has some fancy inlet ducting in front to reduce radar returns or put the inlet in the sort of pressure and flow the designers need, and enough behind the engine for afterburners and perhaps some form of thrust vectoring. Those needs overrode efficiency when they looked at trade-offs.

A lot of people will be thinking of RC scale (edited)ducted fans and their long ducts but I’d like to point out those aren’t practical airplanes. They get by with enormous power to weight ratios, cheater intakes wherever they can be hidden, and typical flight times measured in single digit minutes. Compromises are made.
 
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sanman

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=HCCI (homogeneus charge combustion ignition ?)

=GUN engine (Detonation ignition)...

-thermal eficiency allmost 95 % !!!
A derivative technology of HCCI is SCCI (Spark-Controlled Compressed Ignition), which Mazda now markets as 'SkyActiv' engines for its newest cars.
I wonder why they have 'Sky' in the name? Is Spark-Controlled Compression Ignition meant to ultimately be used on aircraft engines?

I'd read that PDE (Pulse Detonation Engine) suffers from low Thrust-to-Weight, because it has to be built heavier to withstand high peak loads.



-how much energy for air ionisation ?
Various papers mention that power requirements are low. Presumably it's proportional to air flow. But I don't see any papers on ducts specifically.
Maybe this will give a rough idea:

The power budget associated with these actuators is 10-100 watts per meter length of the actuator.
Since PJ-2 Dreamer has engine that puts out upto 230kw of power, then using few kilowatts for plasma drag reduction could be quite fine.

Furthermore, since it takes some small timespan for turbulent vortices to form, the plasma actuators can be operated intermittently in pulsed mode, for even more power savings:



Also, plasma can also be used to reduce noise levels:

 
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