Ducted Fan Pseudo-Jets

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

pylon500

Well-Known Member
HBA Supporter
Joined
Dec 26, 2003
Messages
433
Location
Taree Airport Australia
I've always wanted to try something with a ducted fan, and some of the ideas that I've toyed with take note that many of these piston powered ducted fans don't seem to have a lot of blade area when compared to your typical large turbo fan airliner engine.
It seemed to me that the experimental versions were all just trying to put a propellor inside a duct instead of a fan.
Then from some ancient modelling stories I'd heard, it seemed the idea is to have large fan area (like your jetliners), but because there is so much blade drag, the air coming out the back has a lot of 'swirl', so you then need to have stator blades behind to 'unswirl' the air and redirect it rearwards for more thrust?
I was planning on using a Rotax 582 running direct drive to a 22" fan with something between 30~40% blade area of the duct, the debate was whether to use lots of narrow blades (reynold numbers problems maybe) or 4 or 5 broad low aspect shaped blades.
On a sideways note, although we in Australia took a step backwards and adopted LSA, amongst our assorted ultralight rules we have our Far103 equivalent, ANO 95:10, which doesn't have any stipulation on the number or type of engines we can use (think 4 engined Lazair), so the ducted fan idea went on the backburner while I contemplated something with 2 (or4) RC model turbines.
Should be able to do 4 really fast circuits. 🤔 😬:cool:
 

Hephaestus

Well-Known Member
Joined
Jun 25, 2014
Messages
2,363
Location
YMM
Just make sure you look up the saunders jethawk 2 - to see how not to do it :)

Wasn't someone here trying to revisit it and sort out the duct issues at some point?
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,592
Then from some ancient modelling stories I'd heard, it seemed the idea is to have large fan area (like your jetliners), but because there is so much blade drag, the air coming out the back has a lot of 'swirl', so you then need to have stator blades behind to 'unswirl' the air and redirect it rearwards for more thrust?
1637968544788.png

Flow straightening vanes. I drew in the red arrows pointing to them.

Note: that engine is not shown in its nacelle or inlet duct.

The turbine section also has them; in this picture they are the "rear support struts."

1637968807329.png
 

sanman

Well-Known Member
Joined
Feb 24, 2021
Messages
238
Ducted fan produce 30% more static trust than free prop(same diameter) with same HP engine and have better efficiency, but only to flying speed up to 100mph. If you need fly fester with ducted fan - you need to use more powerful engine to compensate for less efficiency.
But it's all compromise as we often do in aviation!

View attachment 118405
So that is Airbus E-fan 2.0, which is all-electric.
Is electric the future? How can it be, when it doesn't have the energy density of chemical fuel?
Can't hydrogen be the future instead? That's zero-emission, and it's lightweight.
Hydrogen turbojet or turbofan would burn quite hot, though.

Thrust vectoring with ducted fan can be done, but it wouldn't be much efficient as with turbojet engine - because much less exhaust air velocity of ducted fan system, than with turbojet.
Even if exhaust velocity is less, can we not compensate with bigger ducted fans?

For same reason afterburner can't be used on ducted fan propulsion system - the fan would simply borned up!
I thought that as long as fan is producing enough thrust, then afterburner flame will not reach it.
There are many videos on internet of people making afterburners with electric ducted fans, and they are not burning up:


I'm not sure how much thrust increase afterburner must produce to be worth it.
 

pylon500

Well-Known Member
HBA Supporter
Joined
Dec 26, 2003
Messages
433
Location
Taree Airport Australia
An afterburner is a way of expanding a flammable liquid to it's gaseous form, then expanding it further by adding air and burning the lot to form a rearward expansion.
It's basically a air/liquid rocket that uses the forward turbine stage as the front end of the rocket nozzle, and uses about the same amount of fuel.
(There are no turbine blades anywhere in an afterburner...)
As for the electric/afterburner, it's nothing new, Caproni toyed with this idea in 1940; Caproni Campini N.1 - Wikipedia

Did some searching and found this;
 
Last edited:

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
7,144
Location
US
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.
Can't hydrogen be the future instead? That's zero-emission, and it's lightweight.
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.
 
Last edited:

sanman

Well-Known Member
Joined
Feb 24, 2021
Messages
238
But can ducted fans reduce noise compared to a jet, or even a prop? Sometimes efficiency is not the only thing to be concerned about.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
7,144
Location
US
But can ducted fans reduce noise compared to a jet, or even a prop? Sometimes efficiency is not the only thing to be concerned about.
The quietest powered aircraft ever made use propellers.
If you want a quiet, practical powered airplane, use a propeller and turn it at a relatively slow tip speed. You'll get good efficiency, good power to weight, and maybe good performance per dollar spent.
 
Last edited:

Hephaestus

Well-Known Member
Joined
Jun 25, 2014
Messages
2,363
Location
YMM
NASA says yes ducted fans should be quieter (think goodyear blimp).

Practical application - I've never seen one quieter than a propeller aircraft.

The RFB fantrainer and Edgely optica are some of the few that weren't screamers - the optica was super quiet by reports if memory serves.
 

opcod

Well-Known Member
Joined
Aug 28, 2010
Messages
101
Location
Canada
Duct are not the worst to make noise.. indeed a pulso jet is the king. But an efficient ductfan need to turn high rev and that is uber unpleasant. But all in all, with a 13in you can take off with a silent. But again, it's the mission, going up to 2k is plenty, doing a 1hr flight.. quite a joke.. having to wear headset.. no way.
 

Arthur Brown

Well-Known Member
Joined
May 1, 2016
Messages
139
Location
London

Is a motor cycle engine (they don't say which) driving an internal fan to give the appearance of a jet aeroplane. Other videos used to exist -may still but...

Personally I'd be looking at using a large vehicle cooling fan driven at high speed. Injection moulded ready made fans ready balanced and with some future stock availability seem like a good plan rather than any attempt to fabricate something

Ok it's a BMW 1000cc Aircraft Data OK-UUH-01, 2016 Skyleader UL-39 Albi C/N Not found OK-UUH-01
 
Last edited:

rv6ejguy

Well-Known Member
HBA Supporter
Joined
Jun 26, 2012
Messages
4,752
Location
Calgary, Alberta, Canada

Is a motor cycle engine (they don't say which) driving an internal fan to give the appearance of a jet aeroplane. Other videos used to exist -may still but...

Personally I'd be looking at using a large vehicle cooling fan driven at high speed. Injection moulded ready made fans ready balanced and with some future stock availability seem like a good plan rather than any attempt to fabricate something
1000cc BMW engine.
 

Cardmarc

Well-Known Member
Joined
Jun 16, 2021
Messages
71
Well, The Switchblade 'flying trike/car' is using a ducted fan on a as yet unproven engine. But they seem to have engine issues, haven't flow yet. The way they set the intakes up for that fan make me think it won't produce the thrust necessary to fly well, if at all. But they maintain otherwise.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,592
The quietest powered aircraft ever made use propellers.
If you want a quiet, practical powered airplane, use a propeller and turn it at a relatively slow tip speed. You'll get good efficiency, good power to weight, and maybe good performance per dollar spent.
Yup. But the guys that want a "jet" without the million-dollar price tag are determined that they will do it. Of course, they might be happy with it, but I bet that the novelty will soon wear off for most as their airplane will just be another relatively slow homebuilt that needs an 8000-foot runway.

Kind of like the WAR warbird replicas. An obviously fake P-51, maybe, that sure doesn't perform or sound like a real P-51. Just a big RC model. I'd rather take that money and time a build something that has some utility. How many WAR replicas are still flying now?

But that's just my taste. Other people see things differently. I understand that. Some are even willing to jump out of airplanes that aren't on fire.
 

cndg

New Member
Joined
Nov 25, 2021
Messages
1
Ducts almost always make things worse. e.g.

There is a HUGE amount of ******** out there about ducts - basically - if it's on paper, it's false. If you see an actual experiment, *maybe* it's real.
 

sanman

Well-Known Member
Joined
Feb 24, 2021
Messages
238
Ducted Fan which is mechanically driven by piston engine can also be similar to Supercharger, if its airflow is fed to engine, right?

Can Ducted Fan be combined with Supercharger somehow?
(Okay, technically Supercharger is mechanically driven compressor and not fan, but still, can they not be combined into a single powertrain?)
Supercharger has an energy cost, but liberates more energy than it uses up.

Also, would it be useful to ionize the air inside the duct, to increase laminar airflow and reduce efficiency losses?

In an actual turbojet, about 1% of the exhaust is in a plasma state, due to the heat of combustion.

In a ducted fan (cold jet), if we could ionize the airflow perhaps by charging the fan-blade surface and duct walls, then some of that diatomic air would become ionized plasma, which could have smoother, less turbulent flow characteristics. So that would cost energy, but would it provide more advantage?
 
Last edited:
Top