Ducted Fan Pseudo-Jets

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opcod

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People need to also remember, if you park a Ferrarri in the street.. the speed limit, we don't care and all do see a nice <Ferrari> even if it<s a fake one. At least they fly with it.
 

nicknack

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I had opportunity to see first official flight of Albi. My impressions?
Airplane looks very cool :cool:
Takeoff has been long and noisy = ducted fan has much lower efficiency at low speeds.
Noise has been higher (in compare with prop airplanes) even during flight.

When you compare performance of airplanes with prop and Rotax 915i (JMB VL3, TL Strem, Shark etc.) you can see that is possible to achieve better performance.

Albi and Dreamer are using 200hp+ engines and performance is still lower than prop airplanes. Seems that ducted fan small prop has significantelly lower efficiency at speeds where most of LSA airplanes are operating. In my opinion both this projects would work better with small turbines - for example from PBS
I know - than is not possible LSA registration. However Albi (even Dreamer) would be beautifull experimental jet. Maybe could be altrnative for "Beast one project" ;)
The UL39 ALBI is indeed going to be powered by a turboshaft version of the PBS-TJ100. In some ways it is closer to a high-bypass turbofan. The best engine for it would have been the DGEN 380 or 390 with about 600 to 900lbf thrust.

It's the version called UL-39 ALBI II M DUCTED FAN TRAINER

Honestly, I think after that much effort, it's simply better to bite the bullet and make a small jet with a jet/turbofan that is more reliable than piston engines with shafts and extended fans, have decent (500+nm) range when flown in a single seat configuration, at least able to go over 200kts, landing stalls of ~ 55kts or lower, keep the wingloading max 20 lb/sqft. The plane may require 3000ft+ runways.

Will not sell many but I suspect there will be a market of around 5 a year and may increase if there's a well developed transition program like the subsonex jet(A dual seater will be good for training). For instance the subsonex jet started shipping in late 2015 and now there's 18 flying and a few more being built. Not cheap but if airframe is around 100k to 150k and engine around 60k to 100k, there could be a solid niche market with reliable sales.


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The other interesting designs are the Subsonex 2 place jet (JSX-2T) and the Beast One.
Nick
 

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nicknack

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The early high bypass turbofan called the Astafan is nearly a ducted fan with a jet core. The ducted fan had a variable pitch design for optimising thrust at different speeds ie idle takeoff vs cruising.


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crusty old aviator

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Martin Hollmann wrote a book about ducted fans & props. He shows how the drag on a duct becomes so high above 100 mph that it’s no longer tenable above 130 mph. I asked Moeller about this and he walked away in silence (I guess he was an early proponent of “alternate facts”). Given this limitation, your YT follower should consider alternate propulsion configurations if he desires to travel faster than 100 kts...
 

Riggerrob

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Having a bunch of flames shoot out the back of a cylinder doesn't indicate meaningful thrust is being produced.
At each point in the fluid flow aft of the ducted fan (or, aft of the compressor in a jet engine), the pressure must decrease (else the flow would reverse direction). For any ducted fan that can provide meaningful, practical thrust in manned aircraft using a piston engine, the relatively low pressure aft of the fan puts a significant cap on any potential for producing thrust from an afterburner. Now, factor in the fuel burn needed to achieve any "afterburner" (?) thrust and we can pretty much put the idea to bed.

Another idea that is frequently mentioned but which is impractical is use of hydrogen as a fuel for aviation internal combustion engines. Check out the problems/weight of storing it, the practical implications of making it, and the challenges of moving it. Hydrocarbon fuel can be "greener" and much more practical.
Storing hydrogen is a major problem. Since hydrogen atoms are so tiny, they can leak ... permeate ... thorugh most conventional pressure cylinders. This requires specialized storage facilities. Even so, some hydrogen will leak out after a few days ... forcing you to top off hydrogen tanks just before every flight.
Hint: Hans von Ohain only burned hydrogen in his first jet engines, but quickly switched to petroleum. Old Hans might have known a thing or two about jet engines ... back during the 1930s ....
 

Vigilant1

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Storing hydrogen is a major problem. Since hydrogen atoms are so tiny, they can leak ... permeate ... thorugh most conventional pressure cylinders. This requires specialized storage facilities. Even so, some hydrogen will leak out after a few days ...
And, then there's hydrogen embrittlement. Hydrogen doesn't weaken all materials, but a lot of common, useful ones. It's just not a very practical fuel, but it has some ardent boosters who keep dreaming the dream.
 

sanman

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Storing hydrogen is a major problem. Since hydrogen atoms are so tiny, they can leak ... permeate ... thorugh most conventional pressure cylinders. This requires specialized storage facilities. Even so, some hydrogen will leak out after a few days ... forcing you to top off hydrogen tanks just before every flight.
Hint: Hans von Ohain only burned hydrogen in his first jet engines, but quickly switched to petroleum. Old Hans might have known a thing or two about jet engines ... back during the 1930s ....
Alright, point taken - hydrogen is difficult to store compactly. Meanwhile, methane, propane, butane are significantly easier to store, while having comparatively lower carbon content relative to other hydrocarbons - that means less CO2, and also less engine coking problems. They also offer comparatively higher specific impulse, though not as high as hydrogen. You wouldn't necessarily have to store hydrogen for long periods if you could generate it with a reformer.

And, then there's hydrogen embrittlement. Hydrogen doesn't weaken all materials, but a lot of common, useful ones. It's just not a very practical fuel, but it has some ardent boosters who keep dreaming the dream.
I'd read that there's been recent advances in using hydrogenated graphene in composite overwrapped pressure vessels to resist permeability and susceptibility to the hydrogen embrittlement.
 

sanman

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=
The early high bypass turbofan called the Astafan is nearly a ducted fan with a jet core. The ducted fan had a variable pitch design for optimising thrust at different speeds ie idle takeoff vs cruising.
Can variable blade pitch be used on piston-powered Ducted Fans to increase their efficiency across a broader flow regime?
Or what about a variable nozzle at the exhaust end of the duct? Can that help?

Also, if it's possible to have a Ducted Fan with a turbojet core, then is it possible to have a turboprop with a turbojet core? (I guess that would make it a prop-jet?)
 
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Urquiola

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May I inquire what document those pages are from and if you might share? Thanks!
The document is 'Ducted fans', by Hovey. I purchased a copy from Zenith Aviation books, no longer operating. Booklet seems out of print, you can look for it at sites as Abebooks, Amazon, eBay, libraries, or try finding something similar. Blessings +
 

henryk

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Malish

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The UL39 ALBI is indeed going to be powered by a turboshaft version of the PBS-TJ100. In some ways it is closer to a high-bypass turbofan. The best engine for it would have been the DGEN 380 or 390 with about 600 to 900lbf thrust.

It's the version called UL-39 ALBI II M DUCTED FAN TRAINER
If consider cost of power plant and maintenance - this aircraft will have no use for average GA pilot/owner :(
And for use as a trainer aircraft no one will need it too 🤔
 

Malish

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Can variable blade pitch be used on piston-powered Ducted Fans to increase their efficiency across a broader flow regime?
Or what about a variable nozzle at the exhaust end of the duct? Can that help?
All this will help, but at what complexity and cost? This will make it "million dollar" aircraft that nobody can afford for privet use...
 

Malish

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Can variable blade pitch be used on piston-powered Ducted Fans to increase their efficiency across a broader flow regime?
Or what about a variable nozzle at the exhaust end of the duct? Can that help?
All this will help, but at what complexity and cost? This will make it "million dollar" aircraft that nobody can afford for privet use...
 

vhhjr

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Ducted fans don't make very good supercharges because they don't develop much pressure. My 11" fan deveops around .5 psi and the best RC DFs develop about 1 psi. I have a small Roots type supercharger and it deveops 4 - 6 psi. Air flow alone will not increase an IC engine output. It take pressure to cram more air/fuel mix into the cylinders. Also the volume of air used by the engine is very low compared to the DF output volume. Using RC DFs as an example, the flow velocity out can be 250 - 300 feet per second. That's about 30 ft3 per second. The RC DF would be drawing at least 4000 watts or about 5 hp if an IC engine was used. At a fuel burn rate of .5 lb/hp/hr and an air fuel ratio of 14:1 that would be 500 ft3/hr or .14 ft3/sec, or about .5% of the DF air flow. So, the volume of air drawn off for the engine would be almost undetectable. The increase in fuel burn due to the 1 psi boost would increase the HP output to 5.3 HP. A 6% increase in engine output is useful and essentially free, but not a game changer. It would also require the engine to have a pressure carburetor or be fuel injected.

Vince Homer
 

vhhjr

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All this will help, but at what complexity and cost? This will make it "million dollar" aircraft that nobody can afford for privet use...
Variable inlet vanes would be very useful. They would act much like having a variable pitch prop on a conventional aircraft.

Vince Homer
 

sanman

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Variable inlet vanes would be very useful. They would act much like having a variable pitch prop on a conventional aircraft.

Vince Homer

So these things don't actually spin like turbine blades, they instead just swivel to regulate the inlet flow.



So the idea would be to run your engine at constant rpm to optimize engine efficiency, and then use the variable inlet guide vanes to change the thrust?
So that means you could go for an engine with a narrower power band, but which might be lighter in weight. That would be a turbine.
But in the world of piston engines, narrower power band tends to be associated with 2-strokes, with diesel cycle and also with lower number of cylinders, if I recall correctly.

Which engines might that fit the profile?

 
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cluttonfred

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Well then how in the world could you do a jet-like piston-engine design without a ducted fan? Oh, right…. ;-p

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Martin Hollmann wrote a book about ducted fans & props. He shows how the drag on a duct becomes so high above 100 mph that it’s no longer tenable above 130 mph. I asked Moeller about this and he walked away in silence (I guess he was an early proponent of “alternate facts”). Given this limitation, your YT follower should consider alternate propulsion configurations if he desires to travel faster than 100 kts...
 
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