Ducted Fan Pseudo-Jets

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

sanman

Well-Known Member
Joined
Feb 24, 2021
Messages
110
The Miller JM-2 started out looking like this:
<snip>
It performed better with an open prop than with the ducted fan.
With a ducted fan, we can have thrust vectoring, can we not?
I don't think thrust vectoring can be done as easily with an open propeller




Imagine doing 'Cobra' maneuver in an Archon, thanks to Ducted Fan Thrust Vectoring! 😜
(and then if you have twin ducted fans, you can do independent vectoring on each, for greater roll control)
 
Last edited:

John.Roo

Well-Known Member
Joined
Oct 8, 2013
Messages
1,082
Location
Letohrad / Czech Republic
I want to ask about Ducted Fan technology, given the recent marketplace entry of a couple of new aircraft, the PJ-2 Dreamer and the UL-39 Albi, both featuring ducted fan propulsion powered by piston engines.




Firstly, what is the scope for development of this technology? What are its limitations?

Nextly, is there any other parlance for this type of aircraft? Pseudo-jets? Quasi-jets? Jet-like? (Anything to help my Google-searching)

So there is Electric Ducted Fan technology, which has developed under the r/c model and drone market.
But with PJ-2 and UL-39 we now see the advent of piston-powered Ducted Fan propulsion -- the key being that reciprocating piston engines are allowable under LSA regulations, whereas turbines are not.

One problem seems to be in power-to-weight, given that you need a powerful piston engine and gear transmission to bring the fans up to useful rpm.

What are the ways to solve these problems, while still falling under LSA restrictions?
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" ;)
 

Malish

Well-Known Member
Joined
Oct 11, 2013
Messages
620
Location
Russia. City of Volgograd
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.
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!
Story 007 (Large).jpgStory 005 (Large).jpg
E-Fan 2.0 web.jpg
 

Malish

Well-Known Member
Joined
Oct 11, 2013
Messages
620
Location
Russia. City of Volgograd
With a ducted fan, we can have thrust vectoring, can we not?
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.
For same reason afterburner can't be used on ducted fan propulsion system - the fan would simply borned up!
 

John.Roo

Well-Known Member
Joined
Oct 8, 2013
Messages
1,082
Location
Letohrad / Czech Republic
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 118403View attachment 118404
View attachment 118405
Thanks - interesting info!
Problem is that Albi has much smaller "fan prop". So for typical use is easier to use larger "free" prop (diameter 1 700 - 1 750 mm). Other technical problem what needs to be designed perfectly are air intakes / outtakes able to affect propulsion efficiency a lot.
Anyway - I like this projects 👍 Once - when batteries capacity will increase significantelly - will be ducted fan nice solution ;) Airbus e-Fan shown one of possible ways. Pitty that there are not so many technical informations and performance data available and project has been cancelled.
 

Malish

Well-Known Member
Joined
Oct 11, 2013
Messages
620
Location
Russia. City of Volgograd
Problem is that Albi has much smaller "fan prop". So for typical use is easier to use larger "free" prop (diameter 1 700 - 1 750 mm). Other technical problem what needs to be designed perfectly are air intakes / outtakes able to affect propulsion efficiency a lot.
In case of Albi, their fan was looking more like turbine - 13 blades and I thing it's was wrong designed for 150hp engine:
ul39-004.JPG

And design of air duct is very impotent to:
Ducted fan design 024 (Large).jpgDucted fan design 025 (Large).jpg

On Albi air duct was copied from real L-39 aircraft and it's was wrong for ducted fan operation.
 

vhhjr

Well-Known Member
HBA Supporter
Joined
Jun 29, 2003
Messages
189
No one would design and build a ducted fan powered aircraft because they were looking for efficiency or simplicity. You get a jet without the FAA hassles of a real flying blowtorch, the high engine, fuel costs and maintenence. Worry about the lower performance per horsepower would certainly be offset by the "wow" factor of having your own jet like aircraft. If you want efficiency and simplicity there's plenty of scratch built and kit planes with a propeller on one end or the other available that meet that criteria.

On the plus side there are some off-the-shelf electric ducted fans currently available in thrust ranges that just might work with the addition of a hybrid system. With batteries only you would have trouble getting out of sight of the airport in any design that wasn't gliderlike.

My feeble efforts have produced 11 inch and 17 inch diameter ducted fans that need refinement to get to the usable stage. The highest thrust I have gotten from the 11 inch version is about 45 lbs. on 26 HP.

1637937103973.png1637937138428.png


Vince Homer
 

Attachments

rv6ejguy

Well-Known Member
HBA Supporter
Joined
Jun 26, 2012
Messages
4,707
Location
Calgary, Alberta, Canada
Thanks - interesting info!
Problem is that Albi has much smaller "fan prop". So for typical use is easier to use larger "free" prop (diameter 1 700 - 1 750 mm). Other technical problem what needs to be designed perfectly are air intakes / outtakes able to affect propulsion efficiency a lot.
Anyway - I like this projects 👍 Once - when batteries capacity will increase significantelly - will be ducted fan nice solution ;) Airbus e-Fan shown one of possible ways. Pitty that there are not so many technical informations and performance data available and project has been cancelled.
The new Albi is using a larger fan, different engine, different duct. I think they learned a lot on the first one. This is pioneering stuff without a lot of good technical data to build on. You have to design and test yourself, learn as you go.

The noise would be a big concern. From the videos, very loud with an unpleasant note.
 

Urquiola

Well-Known Member
Joined
Aug 23, 2013
Messages
252
Location
Madrid, Spain
Moller used a Wankel in a ducted fan configuration in his VTOL project. Never went anywhere except tethered hover.
Yeah! Moeller was not allowed free flight by FAA, just tethered flight, as they did in Romanian Antonov 'Izdelie', meaning 'experiment', with a Custer Channel Wing machine, engine in fuselage, propellers, on channel leading edge, Custer had propellers in trailing edge, Izdelie moved propellers by gears and shafts.
The Moeller machines tookoff with rope loose, not under tension.
The 'freedom-motors' Wankel are sound, but today, the design trend would be generators, Wankel, turbines,.. inside fuselage, electric engines, lighter, less prone to fail, as propulsion mean. Comments? Thanks. Blessings +
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,380
The 'freedom-motors' Wankel are sound, but today, the design trend would be generators, Wankel, turbines,.. inside fuselage, electric engines, lighter, less prone to fail, as propulsion mean. Comments? Thanks. Blessings +
Which would mean more weight and even less performance. It was dismal enough as it was.

Moller spent more than 50 years and $100 million on that idea. There's a lesson there: It's Not As Easy As It Looks. Wishful thinking has to be backed up by physics. Period.
 
Last edited:

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,380
With a ducted fan, we can have thrust vectoring, can we not?
I don't think thrust vectoring can be done as easily with an open propeller




Imagine doing 'Cobra' maneuver in an Archon, thanks to Ducted Fan Thrust Vectoring! 😜
(and then if you have twin ducted fans, you can do independent vectoring on each, for greater roll control)
Scaling up RC models doesn't work. Those things perform spectacularly because of their high power-to-weight ratio, doable because of those tiny engines that produce lots of power for a few hours before burning out. When you double the size of the airframe, you square its area and cube its weight. Twice as big, eight times as heavy. Four times as big, 64 times as heavy. Need big engines for that, which makes it even heavier. And since the weight increases so quickly, the airframe must also be much stronger than simple scale, which really adds the weight.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,380
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
 

jedi

Well-Known Member
Joined
Aug 8, 2009
Messages
2,805
Location
Sahuarita Arizona, Renton Washington, USA
Good explanation. Duct inlet design is a big deal with Boeing and others for good reasons. There are many ways to mess it up.

I worked with Sid Welling, the designer of the fish mouth inlet on the early high bypass 737s. It was a big project and very successful. With a body of revolution (round duct) you can do one calculation and it applies all around the circle. The fish mouth needs different calculations for each clock position and each calculation is affected by the calculations to the right and left. Do that for airspeed from zero to M = 0.9. Then include angle of attack from 0 to 20 degrees and a bit of yaw or crosswind on takeoff.

One percent plus or minus in efficiency is enough to sink a project. Without Sids work there would be no 737 Max issues. Boeing would have needed a new airframe model with longer gear legs. The fish mouth inlet was Boeing’s 737 solution to the 707s high bypass re-engine issues. The 757 is an example of overcompensating on gear length and fuselage stretch problems of the DC-8 B-707 lessons.

I am sure that is an over simplification but you can get the picture. All that was above my pay grade. All I know is Sid smiled a lot when we did the test flights and we never did have a CMF engine surge that I can recall. On the other hand, 727 surges are something you do remember.
 
Last edited:

emotodude

Active Member
Joined
Nov 5, 2020
Messages
36
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 118403View attachment 118404
View attachment 118405
May I inquire what document those pages are from and if you might share? Thanks!
 

Lendo

Well-Known Member
Joined
Feb 6, 2013
Messages
875
Location
Brisbane
Ducted Fans are great and they look great, their even safer, however the cost is efficiency, why? because to get them working well, is beyond most designers experience and knowledge. A number of experimental designs have tried and they did indeed look good, but sadly they all ended up a failure in performance as there's too many variables for a simple duct to accommodate. However don't let me stop anyone from trying.
George
 

rv6ejguy

Well-Known Member
HBA Supporter
Joined
Jun 26, 2012
Messages
4,707
Location
Calgary, Alberta, Canada
One of my YT followers commented on the Raptor NG video I did and said he was building a DF aircraft which would address the usual poor performance. Was aiming for around 200 knots. I know nothing of his qualifications or experience but he promised to share data as he progressed. It was already under construction at the time of the video a couple months ago. We shall see...
 
Top