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Jan 23, 2020, 06:16 PM
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Build Log

25mm EDF Jet Build - DIY


Hey y'all,

I'm new to the RCGroups community, and the hobby in general, but I've always been impressed by other people posting about their great builds and fun ideas they try out. I'm especially inspired by FliteTest and everything they and their great community has put out to help people get into the hobby.

I've been flying a HobbyZone Super Cub (the old version) that I was given by some relatives... after the tail had been smashed. I fixed up the tail control surfaces with some foam board and it's been working well. I've finally got the inspiration to build a plane of my own... an indoor micro 25mm EDF jet!

So far, I've found and scaled down some community-generated plans for an Aero L-39 Albatros model (thanks Grifflyer!) and created a scratch-built EDF power system with a total diameter of 25mm. I'm using an old mini-quad brushed inrunner motor and clipped propeller, some 3d-printed parts, and a bit of thin (PVC?) pipe. I'm not sure what thrust it provides, or how to figure that out, but it has a nice kick to it when I hook it up to a 300mah 7.4v battery. Here's some pictures...

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I'm hoping to have the foam, or whatever material I'll end up using for the plane, to be about 2mm thick, and I might need some balsa-wood spars for structural integrity. I'd appreciate any help, guidance, or support y'all can provide, and I'll be sure to keep this thread updated with build pictures. I guess subscribe or whatever to keep updated.

Signing off for now,
kuhlrunner

(P.S. - I know this is a pretty ambitious project for a novice hobbyist, but come what may, the journey shall be the destination.)
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Jan 24, 2020, 07:41 AM
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Take a look at Nitro’s thread concerning his micro Hawk...You will learn a TON about micro jets from his build!!!!...Not to mention his other threads on balancing, etc....Perhaps he can chime in here and provide links to his threads........

You will probably want to go with carbon fiber rods for the spar(s)—if any are needed at all...Models as small as this can sometimes get away with no spars as the monocoque structure at times is strong enough. Lightness is the key here!....

Is your fan housing a store-bought piece of PVC or did you print your own?...You may just want to start with some guts from an AS3X MiG-15 with the fan unit from that...That brushed motor won’t provide anywhere near the power you’ll need for a micro EDF jet...I do have an AS3X RX brick out of a V1 MiG that I’m going to put up for sale...It has a built-in brushless ESC and ports to plug in your AS3X servos. With this brick and you purchasing the newer higher kv EFlite fan unit you can concentrate on building your model instead of engineering your own radio/power system. It’s just that as a beginner this will give you a head start where at least the radio/power system won’t disappoint you!!!

Kevin
Last edited by Kevin Greene; Jan 24, 2020 at 07:47 AM.
Jan 24, 2020, 12:47 PM
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Thought I have is the fan weight. Gotta be near weightless at the small sizes, especially for non BL power. I've been using 0.015 sheet plastic and a small ring of 1/32" ply glued around the fan housing. The fan housing is kept as short as possible for weight reduction, and I use a light weight vellum paper for the exhaust that is lighter than paper. You can almost see through the paper. The parts below are in a 28.5" P80 40mm EDF weighing about 8oz with battery.
Jan 24, 2020, 05:52 PM
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kuhlrunner
With a home created EDF it really pays to measure the thrust before you go any further with the air frame.
You may have to create some for of test stand to measure the thrust. The classic solution is to mount the EDF on a stand blowing vertically upward but far enough up so it can freely such in air. You then put the stand on ordinary kitchen type scales and zero the reading. Fire it up and you get a direct thrust figure.
This will then give you an idea of the size of the problem you are facing.

For scale reasons I had to use a very long duct on a 30 mm EDF. I built a small 3 leg stand, actually out of foam, for the complete EDF and duct.
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The thrust measured was not a lot but still sufficient to convince me it was worth proceeding.
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A twin 30 mm EDF and ready to fly it is still below that 250 g 'no registration' limit.

I hope this helps.
Jan 25, 2020, 01:10 AM
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Thread OP
Thanks for all the input!

Kevin -
I agree, Nitrocharged has some of the best tips I've seen on the forum. I can't seem to find the one you mentioned so a link would be helpful. I also agree that the plane's structure should be strong enough to support itself at such a small scale. I've already sorted out a RX and equipment setup that should work at micro scale. I might buy a manufactured edf to act as a fail safe in case this home built one doesn't succeed, which is looking somewhat likely...

AntiArf -
I've been paying careful attention to the weight of this thing, it's 9g itself so I'll have to redesign for weight reduction. I might make the entire shell out of card stock and see how that works out. As of now, though, I have some motor problems to work out, so I'm waiting for my setup to arrive so I can hook it up to a transmitter and test it.

Quorneng -
Thanks for the tip. I rigged up a little stand for it and hooked it directly to the battery, which in hindsight was a terrible idea and a rookie mistake. The reading got up to 2g thrust before it overheated, or had some sort of problem, and slowly decelerated. (If you know what happened please let me know) I'm going to have my esc and RX soon so I'll be able to test it legitimately. I feel like a doofus, but you know what they say... Live and learn.

So from this point, I'll be working on eliminating excess weight and figuring out motor things. If anyone has build tips or any equipment they swear by with micro edfs please add to the thread. I'm also looking for a better prop, because mine doesn't provide enough thrust as it is, so tips on that would be great. Thanks again for the help.

kuhlrunner
Jan 25, 2020, 03:30 PM
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kuhlrunner
I presume form the two wires leading from the motor it is a brushed type. This means that at the power levels required to make any sense in an EDF application the brushes will not last long. I suspect that is what has happened to your motor.
This is one of the reasons why a brushless motor is about twice as powerful for the same weight. They are limited by the maximum temperature the wire and the magnets can tolerate rather than the mechanical switching capability of the brushes over the commutator.
Jan 26, 2020, 12:21 AM
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Thread OP
Quorneng -
That does sound like the problem I'm experiencing. I've updated my hardware list with a 0703 brushless outrunner, a compatible esc, and a fun little servo and motor tester I saw in one of Nitro's videos. I'll redesign my duct and exhaust for lightness, as well as to fit a cropped Blade Inductrix prop.

I am getting into researching materials for my airframe, and I've heard that not only is Depron a light and thin building material that is exactly what I need, it is no longer produced and is hard to come by on sites like Amazon. I found some on HobbyKing for pretty cheap, but no way am I paying $75 for shipping from Hong Kong. Is there any material like Depron that is a little cheaper and more available? Quorneng did suggest to wait until I have a thrust reading, and I probably won't buy any material before that, but I like to think ahead.

kuhlrunner
Feb 05, 2020, 12:36 AM
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Thread OP
Hey y'all,
I thought I'd post an update just to establish this thread as definitely not abandoned.
I've designed and printed a 1g housing for the motor, but haven't had a chance to order hardware due to a busy schedule. I've been doubting my current choice of ESC, it's 7g even though it's really tiny, and I realized one of y'all might have a better idea of this than me. Any input is appreciated.
Also, I'm still looking for a good, stiff 2mm foam board that is cheap, lightweight, and readily available. What do y'all use for your tiny builds?

Signing off for now, and thanks for the support,
kuhlrunner
Today, 12:34 AM
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