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Old Apr 17, 2014, 12:45 PM
Oh no, not again!
jhspring's Avatar
United States, LA, Carencro
Joined Dec 2005
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Question
Fan Placement

At the moment, this is a hypothetical. I am an intermediate level builder and flier, but have just started EDFs. I have a Dynam Me 262 that I am getting really attached to. I would like to scratch build another jet.

Most of the 50's & 60's era jets had quite long inlet ducting. In following several build threads of things like Avro Vulcans, the fan placement seems rather arbitrary. Some put them at the front of the inlet for c/g purposes, some have them right at the back for a short exhaust duct. In the main, models seem to place the fans somewhere close to the c/g.

If we ignore c/g issues, is there an optimum place to site the fan? My thinking is that since the exhaust air is accelerated, that duct should be kept short to minimize energy losses. On the other hand, I notice that even with podded EDFs there is some length of exhaust ducting (to stabilize the efflux vortex?) I guess I am wondering if there is a formula, or ratio, or even a solid rationale for where to place the fan in a build like the Vulcan?

Appreciate any input.

Regards,
Jeff
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Old Apr 17, 2014, 01:13 PM
turbonut's Avatar
upland CA
Joined Dec 2004
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I find that its easy for the fan to push air out the long tail pipe..Hard to suck into a long intake..Bigger losses..but still getting the CG is key. the other problem is you can snake a intake around things in the fuse but that doesnt work with exhaust
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Old Apr 17, 2014, 02:24 PM
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...

my best bet is right in the middle of the inlet and outlet...
why?
less trouble with wiring from motor - esc, from esc - battery ....

.
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Old Apr 17, 2014, 05:02 PM
deltas are cool
AIR SALLY's Avatar
Tehachapi ,CA.
Joined Apr 2006
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place it for C/G and to get the most mass at the center of the plane . you dont want your fan way in the back and you batt way in the front . best to have most of it near each other as long as you caqn maintain C/G.
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Old Apr 17, 2014, 05:28 PM
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Quote:
Originally Posted by AIR SALLY View Post
place it for C/G and to get the most mass at the center of the plane . you dont want your fan way in the back and you batt way in the front . best to have most of it near each other as long as you caqn maintain C/G.
for some jets like the sapac jas 39 gripen, there is no
choice but placing the fan at the far back. If the fan
is close to CG then there will be a big chunk of mass place in
the back to get the right CG ... just does not make sense in this case :-)


.
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Old Apr 17, 2014, 09:14 PM
Da' Cajun
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Lake Charles, Louisiana
Joined Jun 2009
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I don't know why it works out this way, but every jet I've built has had the fan at the TE of the wing and the batts up on the LE.
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Old Apr 18, 2014, 08:00 AM
Oh no, not again!
jhspring's Avatar
United States, LA, Carencro
Joined Dec 2005
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Gentlemen, thank you for all the input. There is no substitute for real world experience and I appreciate your willingness to share. I hadn't considered moment arms, that putting the fans at the back would mean putting the lipo way forward, dampening pitch considerably.

I am surprised that there is enough negative pressure in the intake duct to collapse it. I would have thought that with good airflow there would be only a small pressure differential. If you have a lot of negative pressure so that the fan is working hard to suck (like a vacuum cleaner), that has to reduce the power output, right? I guess that's part of the reason for cheater holes.

Are there problems associated with making the intake diameter larger than the fan diameter to reduce intake air velocity and therefore the pressure differential?

Thanks,
Jeff
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Old Apr 18, 2014, 08:20 AM
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Your post #7 is exactly right. You get a lot of people put carbon rings round the intake ducting to stop it collapsing. They don't have a clue. They don't realize that if the duct has a tendency to collapse the fan is being starved of air, whether they strengthen it or not. So power is wasted. Just use a bigger intake. Make the intake area the same as the whole fan area if you can. In theory that can slow the plane slightly but in practice I have not noticed. Some say the intake area is ok if it equals the total area less the hub area. They are the people who have to put the carbon rings on. You will get people who disagree with this but it is my practical experience. It doesn't have to be 100% of the overall area, just as big as you can, but not greater than 100%. Don't make the outlet area too small either. 75mm for a 90mm fan is about as small as you should go. A streamlined fairing behind the flat back of the motor is a total waste of effort.

As other have said, place the fan and batteries so you can get the CG right and the wires from the battery to the ESC don't have to be extended. Within reason you can extend the wires from the ESC to the motor as much as you want, but overall the shorter any wires the better.

Don't assume all ARTF manufacturers know what they are doing. On my 53 inch span 90mm Black Horse ViperJet (the Graupner one is identical) if I put the battery where they said it was grossly nose heavy. I had to move the battery EIGHT INCHES back from the stated position to get the CG right. This meant they got in the way of the supplied intake ducts so I had to make new ones. At the same time I enlarged the intakes. I am right as the static thrust on the bench with no intake or exit ducting is absolutely identical to the static thrust in the plane with my intakes and a 17 inch long tailpipe with a 75mm diameter exit. Losses are zero. 53 inch might look big for a 90mm fan. Yes, you have to use a 'top' motor. But the bigger ones, provided they are light, fly with a lower angle of attack so the induced drag is low. So they are as fast as a smaller one.

A 'top' motor? As the diameter is restricted by the motor housing use as long a motor as possible. EG for 90mm (but it applies to all other sizes) HET 650-68 rather than 650-58 or Hacker B50-XL rather than B50-L or B50-S. High voltage low Kv is marginally more efficient than low voltage high Kv and need be no heavier as you use a larger number of smaller cells so the power capacity is the same. Putting a heat sink on the exposed part of the motor is just disguising the heat generated by drawing the heat away with the airflow and power is still being wasted. But put one on anyway.

BTW: Make a MIG 15. It looks nice, is simple, and is perfect for EDF. Enlarge the tailplane a bit. 48-52 inches span with a good 90mm fan. The Wemotec MidiFan Evo is fine and low cost. Be wary of Chinese fans with 'top' motors. And their exaggerate thrust claims if they come as a unit with the motor. Don't mess around with strong formers to hold the wings on. Just hold them on with a carbon rod (solid is better than hollow) straight across the duct. As long as it is some way ahead of the fan it is fine. Divide the battery into two and put one in each wing. That way it doesn't obstruct anything and shares the weight better. Put them crossways behind the main spar and the CG should come out right. In front if is it is not (make the ribs and LE strong). A quick release hatch on the underside of each wing to get at them. You may be able to get at the wing securing bolts with these hatches too. 1.5 degrees incidence measured from the fuselage center line on the wing and tail so it doesn't fly nose up all the time. A little washout. Don't leave off the wing fences. If you fit retracts ignore the Migs complicated undercarriage with its funny angles, just do it straight and position the mechanism so the extended wheels are in the correct place. The position of the extended wheels on the real one tells you where the CG should be as it does for all scale tricycle undercarriage aircraft.
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Old Apr 18, 2014, 08:48 AM
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At the moment, lets ignore hypothetical. What era or better yet, what plane are you looking at to build? Wing plan form and intake locaton will decide a lot of what your asking.

I think once you let it be known what your building, more specific answers will follow. As always, lots of variables to deal with.
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Old Apr 18, 2014, 10:53 AM
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upland CA
Joined Dec 2004
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Jeff,
Plus one to Coolers post
As Cooler posted start with the plane design and go from there..best design for a fan is a pod like on airliners. But from there just make it work with your aircraft. if you can keep the mass near center. for better flying quality a larger intake is better to feed the fan but Will slow the plane down on the top end.
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Old Apr 18, 2014, 12:32 PM
Oh no, not again!
jhspring's Avatar
United States, LA, Carencro
Joined Dec 2005
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Again, thanks for all the info. I am reluctant to claim anything b/c this is nothing more than 3-views at the moment and I am 3 projects behind in the build queue.

What has my fancy is a CF-105 Arrow. The HK one is apparently a dog, very heavy and lots of trouble with the intake ducting collapsing, which prompted my question. I am thinking 1/12 - 1/13 scale, so 45-50" span, 70-80" length, twin fans. I am currently in the middle of my first depron build and it seems like an excellent medium for a relatively lightweight airframe.

The problem with the Arrow, or one of the problems, is that most of the internal volume is given over to intake and exhaust ducting, so I figured to make the ducting integral to the airframe, build it from the inside out. That's why I was wondering about where to place the fans, and the the intake/exhaust ratios.

I have searched all the threads I can find here on rcg, and most folks have opted for pusher over EDF for all the above reasons.

Regards,
Jeff
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Old Apr 18, 2014, 12:36 PM
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New Jersey
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A lot of planes have their fan placement dictated by battery configuration. If you have a bifurcated intake and the batts go in between then the fan must be moved back farther for example.
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Old Apr 18, 2014, 01:00 PM
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Another nice twin is the NA Vigilante. There is a Chris Golds plan of one of about that length (but larger span) available from Traplet (UK). I think they have US representation. Its all balsa however. His plans are superb.
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Old Apr 18, 2014, 01:51 PM
Do it Right, the first time!
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United States, MN, Hermantown
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Quote:
Again, thanks for all the info. I am reluctant to claim anything b/c this is nothing more than 3-views at the moment and I am 3 projects behind in the build queue.

What has my fancy is a CF-105 Arrow. The HK one is apparently a dog, very heavy and lots of trouble with the intake ducting collapsing, which prompted my question. I am thinking 1/12 - 1/13 scale, so 45-50" span, 70-80" length, twin fans.

Jeff:
Certainly you may change your mind, but it gives us a better idea of which direction your looking at going. Lots of decissions to be made, for sure. Twin 70's or 90's for instance. 6s, 8s or bigger? Retracts?
I enjoy the figuring out part of a build. What you going to use, where it's going, where's the CG at etc ? Tons of fun!!!

This is my current build using ModelPlanefoam, not Depron, but similar results. Fuse was then glassed.
http://www.rcgroups.com/forums/showp...&postcount=156

When you start this build, come down to the Scratchbuilt EDFs Forum, you'll be most welcome.
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Old Apr 18, 2014, 02:54 PM
Oh no, not again!
jhspring's Avatar
United States, LA, Carencro
Joined Dec 2005
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Gorgeous build, Cooler. I'll go read the whole thread.

Some of the obvious data I have worked out. C/g (25% MAC) is about at the sawtooth in the wing. Semiscale retracts. If I do the ducting right, there should be room for lipos in line along the centerline. I thought 6S should suffice. Separate elevators and ailerons as per original. Probably cheat on the wing section, I don't need a supersonic aerofoil and a little extra thickness helps with both strength and room for landing gear etc. Mostly depron, glassed, with balsa, CF and ply reinforcing as needed.

C/g is clearly critical as demonstrated here:
Avro Arrow Model Test Flight (2 min 33 sec)


Quite a striking looking aircraft, and as a Canadian ex-pat, with an uncle who worked at Avro at the time, I have always had an interest in it.

Regards,
Jeff
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