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Old Nov 26, 2013, 03:30 PM
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Help!
747 foamie with 4 EDFs

Hi, I'm wanting to build a 747 foamie with 4 EDFs and make it as small and light as possible. I already have four 28mm ducted fan units powered by E-flite BL180m brushless motors. I also have a Spektrum AR400 receiver and Hitec HS-55 servos.

I know I'll need four separate ESCs but wondering what amperage each? Do I wire all 4 ESCs into one battery plug and then also wire all 4 receiver wires into one plug that goes into the receiver?

Also not sure if I should use the AR400 (it's pretty small and light) or would an Ultra Micro receiver be better (they seem to have built in ESC so not sure that would work)?

I'm planning on using only elevators & rudder for control surfaces so would my Hitec HS-55s work well or should I go with ultra micro?

All these parts have different ratings, plugs, etc., so it's confusing knowing what system to use in order to get the lightest setup but also something that is configured well and will have enough juice.

Thanks for any help!
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Old Nov 27, 2013, 11:41 AM
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Since your building an 747 i think speed isnt what you looking for 2s 800mah lipo would be the smallest you would want to use. If i was you i would go 4ch and use 2gr servos for the airlons and the hitec servos you have would be fine. Im guessing your doing a 30in WS i think it wouldnt be to hard to build it under 200grams witch would be a good slow flyer.

and yes you will need 4 esc's and wire them all together to one battery and one plug for the RX.

If this is your first scratch build i recomend you try somthing else thats more simple, what your wanting to do isnt hard but its not for a begginer. But i doupt you would listen to that i know i wouldnt lol So good look with your build and take lots of pics for us to see!
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Old Nov 27, 2013, 12:10 PM
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Thanks for the advice. I'm pretty new to rc planes but have modified my supercub to brushless, telemetry module, lights, etc. I also built a tiny 747 (14" wingspan) using electronics from a Champ (ultra micro) and a heli tail motor and prop in the nose. Surprisingly it flew beautifully, after moving the 1s 150mah lipo around to get the right CG.

What do you think about using the AR400. Would it be too heavy?

You're right, I should probably attempt something simpler but I'm a fool for trying to do something ambitious. I think once I get the electronics figured out, I'll be good. I'll probably start with flat foam parts to test the overall operation and then maybe spend more time on a full 3D body.
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Old Nov 28, 2013, 04:17 PM
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your servos, motors and batts will be most of the weight. the ar400 won't give you as much trouble.
i am not sure what servos and rx would be appropriate for a project like this but to me the servos (and in fact rx) seem more appropriate for larger craft. while 4 micro edf motors may generate a lot of thrust, they are still micro edf motors. i put one of those in a 2 foot wingspan glider and it flew fine but it had micro servos and used the umx mig 15 brick.
again, i don't have experience building what you are planning but the rx+servo choice given the motors seemed a bit mismatched (in my limited perspective).
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Old Nov 28, 2013, 10:58 PM
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First off the best way to learn is to just do it! Whats the worst thing that can happen!

I've got 2 of those itty bitty EDF's that are going into some kind of airliner - just don't know which one yet!

For a micro 747, here's what I think - start simpler - just cuz the real deal has 4 engines doesn't mean you need to use 4! Use 2 real and 2 dummy motors - much simpler to figure out.

HS55's are pretty big for this but do-able. My rule of thumb is the gear should weigh equal to or less than the airframe. So you have about 4 oz of gear including battery so a 4 oz foamy airframe is about a 2' wingspan max. That equals a pretty highish wingloading - which is fine for outdoors but not so good for indoors - where are you going to fly this?

I've found with these micro EDF's there is a sweet spot between size and weight and drag. If you up the the wing area to get the wingloading down, the drag quickly becomes too much for these tiny motors to push. If you make the wings too small, the wingloading becomes so high they have to fly too fast and are bloody hard to see/control. 4 EDF's seems like too much extra weight for the benefit AND this might take a version or 2 to get right.

So get some foam and bang one out - whatever you think is about right. Don't waste time or energy making it purty - just get one that is flyable first - THEN make a second with improvements - THERE WILL be improvements!!! Once that works, tackle the holly grail of 4 engines.

This works for me - your mileage may vary. (don't over think it!!!)
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Old Nov 28, 2013, 11:13 PM
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Just read my ramblings and it needs a little more info.

Weight thoughts...
Gear weight is fixed. Pick the best gear you got and weigh it all. Every gram counts so spending a couple bucks can help. HS55's are 8 grams X2 so that's 16 grams of servos. Use 3 gram servos (or linear actuators) and you can save 7 grams - that's the weight of most micro RX!
Batts - aim for 80% of the C rating. I trust we just want it to fly and aren't concerned about how long. So 2 of these EDF's draw about what? 8 amps a side? IF we need 16 amps a 30C 3S 800MaH pack is fine - weight 70 grams or so.

FOAM - really isn't all that heavy. I've found doubling the overall size doesn't always mean double the weight - which seems counter intuitive. The reason is in the structural bits - motor mounts and CF spars etc - those weights are pretty much fixed no matter what size (within reason). Adding an extra 6" of wingspan won't increase the weight as much as it will improve the wing loading - make sense? But again, with mirco EDF's the drag from size increases can change the flight envelope significantly. I've done some scratch builds where a 1" change in wingspan went from requiring full power to fly to cruising at 1/4 throttle - you need to experiment with this - haven't found any formulas yet.

Does that help?
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Old Nov 30, 2013, 08:56 AM
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Thanks cmdl and Hepdog for the advice. I think going with only two motors and two fake ones is a great idea. I haven't actually fired up one of those EDFs yet, so probably don't appreciate how powerful they are relative to their size.

After a lot of research, here's what I'm going with initially:

RX: Spektrum A6310 (http://www.spektrumrc.com/Products/D...odID=SPMAR6310). It's really tiny but still has 6 channels and the ports for the plugs. I didn't want an RX with built in ESC because I'm going to be running two motors. Plus I want a nice 2 cell system that I can easily take out and build into another model by using plugs rather than a lot of soldering.

Servos: Spektrum A2010 (http://www.spektrumrc.com/Products/D...odID=SPMSA2010). They're only 2.5g each. Decided not to use linear servos as these ones are more protected and versatile for many planes. They also plug in nicely to the A6310.

Battery: (2) Thunder Power RC 2-cell 250mAh 25C (http://www.horizonhobby.com/products...-THP2502SPP25J). My 28mm EDFs are used in the UMX Habu and that comes with a 200mAh battery, so figures two 250 mAh batteries would be a good way to go. I was told it's better to go with two batteries, so I got a Prophet Sport LiPo charger (http://www.horizonhobby.com/products...arger-DYNC2005) because it charges through the balance lead. So I'm going to make a Y-harness and plug both batteries in then set it to the lowest 0.5A charge setting to charge and balance both at the same time. That should provide faster charging (rather than doing one at a time) and also help balance them to be the same for flight.

ESC: (2) E-Flite 10A ESCs (http://www.horizonhobby.com/products...r-10a-EFLA7300). I think I could have used 5A ESCs but was told a higher rating wouldn't affect performance, just add another 1g of weight each. So decided to go with something I could re-use for another plane with a standard motor/prop later.

Motors: (2) E-Flite BL180m (http://www.horizonhobby.com/products...-EFLM30180MDFA). I bought two extra EDF units (http://www.horizonhobby.com/products...unit-EFLDF180M) that'll just spin in the wind to give that 4-engine look but not have any power.

I'm trying to put together a 2-cell system I can re-use for many different projects, so I've sacrificed a few grams of weight for more "plug and play" ability and hoping I will be able to size the airframe to carry all that gear and that the motors will be able to cope!

Any thoughts before I start would be appreciated. Hepdog - I agree with you - I'm just going to cut this out of 6mm depron with flat surfaces at first to test things out. If that's a success, then I'll build a nicer, more realistic 3D airframe.
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Old Nov 30, 2013, 04:51 PM
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All looks good and I really like that you are thinking about other uses - I've ripped and reused the gear out of more planes than I can remember.

Only thing that might be problematic is the 2 windmilling EDF's. Put them on for static but take em off in flight - A LOT of drag penalty having 2 holes plugged like that.

Can't wait to see what you bang out. Might even join in the fun!
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Old Nov 30, 2013, 10:55 PM
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@bigredair - consider http://www.hobbyking.com/hobbyking/s...dProduct=18827 these rx work well (i used them on my GP sabre and never had a problem). much cheaper and bind without problems to spektrum tx.
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Old Dec 01, 2013, 01:02 PM
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Bigredair,

Don't know which building technique you'll use, but I'd go with 2mm or 3mm depron. That's if going box (Jetset44 style), or frame and skinning. Interested in some multi EDF builds myself. Subscribed!

Ksqm

Ksqm
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Old Dec 09, 2013, 03:39 AM
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Hi guys,

I finally have a model ready to test fly! Built it out of 6mm depron flat parts as I didn't want to invest too much time until I've tested the gear/size/flight characteristics. Thanks Hepdog for the advice about leaving the two fake engine fans off. I'm going to do that.

Here are the photos (before it gets smashed up!) and also a breakdown of the plane's weight. It's about 28" wingspan, 110 square inches of wing area, and just under 100g of gear. So hoping I'll have enough lift surface and thrust to make this a decent slow-ish flyer. The wings are totally flat, so probably eventually giving them a profile will help with lift. No ailerons for this one. Sorry, don't know why the pictures are getting rotated!
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Old Dec 09, 2013, 11:39 AM
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That was quick! Looks pretty good - well built -= should fly! Where are you putting the CofG?
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Old Dec 09, 2013, 11:48 AM
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COG is about 1cm forward of the CF rod. At least that's where the plane balances when holding it with my fingertips. I have no idea how to properly locate that on a swept wing plane. I put extra velcro to allow the batteries to be moved foward or back to adjust the COG. Any ideas for locating that?

Here's a quick video of the engines spooling up to 100% power. Produces some pretty nice thrust:
RC Foam 747 Engine Spool-up (0 min 36 sec)
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Old Dec 09, 2013, 01:12 PM
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When I build one offs with no plans I usually build a quick n dirty "chuckie" glider to find the CoG - a plane exact in all dimensions but without the expensive and heavy bits - will usually survive many throws trying to find the sweet spot. I think your CofG is about right though....maybe just a hair forward of the motor power lines. Better to be a touch nose heavy than tail heavy.

Are you test flying indoors or out? Concrete floors suck for test flights. Also I'm sure you know this but with the motors under the wing at full power she will want to climb or pitch up - which is a good thing if your ready for it. The only other thing I see is how you added that angled step up on the rear fuse - that might cause some up pitch with increased speed - only one way to find out for sure though LOL.

Vid was cool - those little motors sure do scream! Try to get video of the maiden!
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Old Dec 09, 2013, 01:30 PM
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I built this off some 747 plans I got from this forum. I built a tiny 14" one with a heli tail motor/prop in the nose and used the ultra micro gear from a HobbyZone Champ, and it flew beautifully, once I got the CoG right.

Not sure how I'd counteract the nose-up rotational force of the engines at full power - I guess that just comes with this kind of plane? The angled step wasn't really planned. When I started cutting out the parts, I realized that because of the 747 profile, I couldn't run a flat horizontal fuselage section all the way back to the tail. So the only way seemed to be stepping it up. Eventually once I get a good feel for how this flies, I want to build a full body version (like this one:
Small Boeing 747 pusher prop - PLEASE READ VIDEO INFO (1 min 13 sec)
- uses 4 pusher props, not EDFs, though).

I don't have an indoor place to test, so it'll have to be over grass at the park once the weather is better and no wind. Trying to find a local sports facility to fly at. It's hard enough testing flying a lightweight foam plane, so it's extra hard fighting any breeze.

How do you find CoG just hand throwing your "chuckie" that has no equipment? Isn't the CoG a factor of equipment loading? Or is there a kind of "natural" CoG based on the planes design? Then do you adjust the plane's design until you get a balanced natural CoG and then load the equipment and keep the CoG in the same place?

Will definitely try to get a video of the maiden flight!
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