Quantcast

Experimental Ducted Fan Aircraft Designs

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

vhhjr

Well-Known Member
HBA Supporter
Joined
Jun 29, 2003
Messages
94
As long as we're in the "ask the experts" mode, I have one. Why do electric ducted fans from 25 mm to 250 mm all have much higher blade solidarity (total blade area to swept area) than the fans used by the Dreamer, Airbus and others making the larger size units? Granted, the power density for EDFs is usually twice what the larger fans are, but the blade area ratios are much greater than than. EDF example below:

upload_2020-2-25_18-51-47.png
 

Blackhawk

Well-Known Member
Joined
Apr 18, 2008
Messages
229
Location
AUSTRALIA
Let's say 100 hp at 5800 rpm for the engine, something like a Suzuki G13. Direct-drive would be easiest but the fan RPM could be whatever works since you could also use a "backwards" redrive to increase fan RPM. Let's say the fan optimized for cruise at 100 mph at 75% power.
If you're going to increase the rpm through a belt drive or gearbox; remember you are reduceing the Torque of the final drive and you may need 120hp to get the same result as the direct drive 100hp engine. (but with higher rpm)
 
Last edited:

henryk

Well-Known Member
Joined
Mar 8, 2010
Messages
5,414
Location
krakow,poland
If you're going to increase the rpm through a belt drive or gearbox; remember you are reduceing the Torque of the final drive and you may need 120hp to get the same result as the direct drive 100hp engine. (but with higher rpm)
-this scenario will be thru,iff the eficiancy of gear is 80 %...

=in practice it is much better, but NOT 100 % !
 

Malish

Well-Known Member
Joined
Oct 11, 2013
Messages
547
Location
Russia. City of Volgograd
the power density for EDFs is usually twice what the larger fans are, but the blade area ratios are much greater than than.
"The Davis D-340 Cold Jet was a single staged high-pressure ratio axial flow compressor driven by
an internal combustion engine. Its maximum rpm was just over 18,000. The maximum tail pipe
velocity was just over 500 mph. The rotor blades were stainless steel and made to a tolerance
of .005 inches, with 350 hp" - This is why D-340 Cold Jet didn't worked, it's wrong approach for Ducted fan design in light aircraft. This setup require a lot of power to produce sufficient thrust and only can be used in Turbo-fan jet engines where thousands hp available to propel multi blade "compressor" style fan. For light aircraft "light" loaded fan should be used with number of blades is less as possible to have dissent performance and economy - because each "extra" blade(as airfoil) produce drag(parasite and induced) which required more power to overcome it.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,664
Location
Memphis, TN
Model plane ducted fans are trying to fit into a minimum space. You are trying to use that double power in something that should have triple power for the size. I remember seeing my first df model. Hangar Hobbies F9F8 with Violet fan with a screaming K&B .45. It was a world class model in looks. Engine turning 28,000 it barely got off the ground, and although it was doing about 90 mph, it could barely stay in the air. It weighed about 11 lbs. There was not enough room for more diameter. Later .60s and then .90 engines same diameter but more blades were designed to fill the same space to absorb the horsepower. The first model jets wiped out the glow DFs overnight. Only after someone stuck a big brushless motor in an old Violet fan and could turn it 35,000 rpm did they look back at DFs. Now with electric spinning rpm is easy. Battery duration and size is the problem. Model DFs have low mass and high strength. That’s something you can’t scale up to an advantage. Disadvantage is low Reynolds Numbers so you got to make those losses up somehow. More blades spinning faster. Still want a Joe Satia Me262 kit.
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
13,770
Location
Port Townsend WA
Don't copy a too small diameter turbojet and expect performance. Copy a large diameter turbofan, like the A-10.
 
Last edited:

poormansairforce

Well-Known Member
Joined
Mar 28, 2017
Messages
1,050
Location
Just an Ohioan
You're right Henryk, we will get cruise speed at 300 km/h(187 mph) as we promised. But most people interested in economy and good TO performance.
If someone looking for speed, they should buy our kit and install jet engines:wonder:
See, now I know your a fraud because that is way to much logical thinking!:D
 

vhhjr

Well-Known Member
HBA Supporter
Joined
Jun 29, 2003
Messages
94
Based on this discussion I think I will pull out half the blades and get some data with six. It appears I'm torque limited with all 12 anyhow at the extreme AOA they are set at.

I calculated the Kf for the 11" and the 17" fans and they are 17 and 14 respectively. Lots of room for improvement.

Vince Homer
 

Malish

Well-Known Member
Joined
Oct 11, 2013
Messages
547
Location
Russia. City of Volgograd
Based on this discussion I think I will pull out half the blades and get some data with six. It appears I'm torque limited with all 12 anyhow at the extreme AOA they are set at.
Initially we had our fan blades set at 32 deg., but engine couldn't develop full TO power(5000 rpm), it's was turning only 4750 - 4800 rpm at the throttle full forward. Then we set blades to 29 deg. - engine max. rpm's got up to 5000 and fan's thrust increased to almost 50 kg(110 lbs).
For new blades recommendations are - install them at 25 deg.
 
Top