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        Poll ducting question: With or w/o intake lip inside fuse? Picture inside.

#1 marell01 Dec 08, 2008 09:06 AM

ducting question: With or w/o intake lip inside fuse? Picture inside.
1 Attachment(s)
Im designing the ducting for my new JAS 39. Fan to be used is either the minifan-size or the HW609 unit, All will fit inside.
The duct is quite long, my question is if I should make the duct all the way to the fan-unit or leave a gap and use the fan intake-lip. see picture below.

Sincerely Marcus

#2 Bob Reynolds Dec 08, 2008 10:44 AM

Idealy the duct to the fan should be FSA and then inlarge to the fan dia. A stinger in the duct leading up to and covering the spinner is neccessary to maintain the FSA. :D

The open ductwork creates a low pressure area right in front of the fan which is a no-no. :(

#3 AIR SALLY Dec 08, 2008 11:03 AM

I'll second that Bob

#4 marell01 Dec 08, 2008 11:15 AM

Ok, Ive seen your work Airsally, on the Draken. Nice ductings.... What did you do the plug from?

#5 marell01 Dec 08, 2008 11:47 AM

Maybe combine them???
1 Attachment(s)
How about combining the two above... Use of intakelip and let the intaketube go to thats OD? see picture...

#6 Ed Waldrep Dec 08, 2008 11:55 AM

Defiinitely not that way marell, that lets the area change which lets the air slow down and then speed up again which uses up energy which means more drag, bad idea, plus you would get some weird turbulence probably.

The inlet lip goes up at the inlet at the front of the plane or at the start of the inlets, then a smooth tunnel all the way to the face of the fan.

#7 WeMoTec Dec 08, 2008 12:02 PM

Is "hot" warmer than "cold", please anwswer my poll. :D

a) is the way to go.


#8 marell01 Dec 08, 2008 12:57 PM

OK, thanks... Then I dont have to redo my design :-)
Sincerely Marcus

#9 clausxpf Dec 08, 2008 06:04 PM

i think you should take a look at thise ..
it is down at post 13
i think it will get you into how it is done and for what reason.

Claus at area 52
velocity-rc team
ps... jeg taler dansk og svensk ...

#10 AIR SALLY Dec 08, 2008 08:32 PM

i carved my plugs from blue foam glassed bondoed and finished with epoxy primer ...very smooth

#11 AIR SALLY Dec 08, 2008 08:36 PM

guys should corners of the ducts be radiused or can corners work(90 degrees)

#12 TGoodwin Dec 08, 2008 08:57 PM


Originally Posted by AIR SALLY
guys should corners of the ducts be radiused or can corners work(90 degrees)

Rounded. :)

#13 TGoodwin Dec 08, 2008 09:02 PM


Originally Posted by WeMoTec
Is "hot" warmer than "cold", please anwswer my poll. :D

a) is the way to go.


I was thinking about answering the poll but this has been covered and debated about 1000 times in the past years.

The answer is "A" UNLESS you have a really badly designed duct and then the answer would be "B".



#14 pulsery2k1 Dec 08, 2008 11:49 PM

over in the foamey section I'm building a star max f-5
below are the results of fan testing out of fuse and in: note
the loss in fuse , how can this be corrected?

QUOTE:I ran the test to day these are the results

using the 77MM 7 blade unit with just the fan housing in
test stand using a 4s lipo at full charge

39.2 amps
513 watts
trust 1357 grams trust
then with a 6 " exust tube narrowed to 62 mm battery fully charged
40.3 amps
528 watts
1412 grams trust

then mounted in f-5 fuse ducting sanded and coated,ducting smoothed
for change in size of fan:with a fully charged battery
46.4 amps
547 watts
1118 grams trust
it seems power when up and trust when down , and quite a bite of
power loss , r vincent had mentioed that he had modifed his ducting
to over come this and would show us his setup.
I have the feeling that its not the airframe thats causing this to
not fly as fast as we would like but Issues with the ducting
inlets seem large enough and the ducting is smooth , possable
that the split exaust is problem?

#15 clausxpf Dec 09, 2008 04:43 AM

is the intake lips (on the plane) roundet?
Is the inlet-duct area ruled? 90-110% FSA
is it with very soft direction changes?
does the duct have a smooth surface? because dimples does not help in a duct..The only reason it works on a golf ball is it helps fill the vacum after the ball, and that way reduce the drag of the ball.
when looking at a duct .. have in mind that the air inside is moving fast, real fast
140 mph ( 200km/hour) or even more with the better fans. It hits like a sleadge hammer.
the split exhaust will give a little drag but still if area ruled it would be in the 5-8% area.
The drop in static thrust when mounted in the model is normal but it is only for the first couple of meters that it means something at flying speed things change to the better.

Claus at area 52
Velocity-RC team

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