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Scratch Build Twin

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Martian30/03/2020 13:53:25
2462 forum posts
1157 photos

Come on Jim keep posting i have only just found this thread and what a pleasure to follow

john stones 130/03/2020 14:12:34
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11445 forum posts
1516 photos

Yep I've been viewing, you do need responses though not just views, feedback fuels your mojo. wink

Toni Reynaud30/03/2020 15:10:57
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423 forum posts
58 photos

Also watching with interest. Impressed with the CNC gadget.

G-JIMG30/03/2020 16:54:10
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144 forum posts
105 photos

Sorry, didn't realise you were out there!

G-JIMG30/03/2020 17:05:38
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144 forum posts
105 photos

Made progress with the wing today.

For aileron control I am using a Futaba 3150 Servo and a homemade RDS (Rotary Drive System). The drive is via a 1/8" diameter steel rod passing through a short brass sleeve at the TE. The Aileron pocket was made using carbon fibre strips separated by 1/8" plywood. The pocket is securely glued into the aileron so it cannot flex, and the rod is a very snug fit in the pocket to ensure there is no flutter.

6 aileron control.jpg

40 aileron pocket.jpg

41 aileron pocket installed.jpg

Jim.

G-JIMG30/03/2020 17:16:35
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144 forum posts
105 photos

As I said at the start, this is my first excursion into an all electric model so I've produced a wiring diagram for it. Hopefully it will prevent me making wiring errors as the build progresses!

shadow model wiring.jpg

fly boy330/03/2020 17:24:17
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3670 forum posts
22 photos

Yes please carry on this interesting build. I too am an ic fanatic, but interested in your power conversion when available. Cheers

Simon Chaddock30/03/2020 19:29:31
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5683 forum posts
3021 photos

Jim

That wiring diagram looks sound enough to me. The only query is the UBEC providing power to the lighting when it appear the power is also connected via the red wire from channel 7.

Is there a reason for using a battery for each motor?

There is an argument that a reducing power on one motor in a twin can be more serious than both failing.

I have a twin set up like this for structural reasons. I specifically keep the flight time shorter than necessary to avoid the risk of taking a battery down too low and not being able to spot it in time.

In hind sight I would have added a battery cross connection.

Martian30/03/2020 20:59:33
2462 forum posts
1157 photos

Simon it looks to me as though that ubec powers the rx and the lighting is teed

G-JIMG30/03/2020 21:20:54
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144 forum posts
105 photos

Martian, you are correct, that UBEC powers the Rx and the lighting.

Simon, the Motor/ESC/Batt configuration was arrived at by George of 4-Max. I sent him the approximate weight and wing area of the model, along with one of the 4 blade props, and he tested various combinations to get the right set up.

The hardware consists of:

Motors: PO-3547-800
ESCs: PP-TESC45AU
Batts: PPL-40C4S-3700

Jim.

Simon Chaddock30/03/2020 22:14:21
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5683 forum posts
3021 photos

Jim

Ooops! Should have spotted that UBEC is powering the rx, servos as well as the lights.

I was not querying the power itself but only if for any reason one battery runs down before the other you will have asymmetric thrust. Not a problem as long as you spot it is happening.

G-JIMG31/03/2020 14:21:25
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144 forum posts
105 photos

It's occurred to me that not everyone will know what a heavily modified King Air 350 looks like, so here's a few pictures of a professionally made model.

ac side.jpg

ac top.jpg

ac front.jpg

Jim

G-JIMG31/03/2020 14:32:35
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144 forum posts
105 photos

Being a scratch build, one potential problem is the manufacture of the fibre glass parts such as the Engine Cowlings.

Fortunately I have access to a 3D Printer so I was able to print out the relevant part in two halves.

nacelle side.jpg

nacelle front.jpg

The print included the exhaust stub but I will have to remove that in order to make the cowling mould.

Jim.

G-JIMG31/03/2020 14:36:28
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144 forum posts
105 photos

Printed the other half of the cowling using a different plastic but the end result was still pretty rough (it's only a hobby level printer).

The finish was certainly not good enough to make a mould with so I spent a lot of time today filling and sanding.

30 engine cowling 3d print.jpg

Jim.

Chris Walby31/03/2020 14:48:20
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1194 forum posts
297 photos

Hi,

Nice long nacelles so should not have too much of an issue getting the C of G although I would construct the nacelles/fuselage with the option of moving the batteries around when the time comes to check + they are close to the fuselage which has its benefits.

A couple of electric and IC Mosquitos have everything as far forward and some lead to achieve C of G where as my Dual Ace needed nothing. I have another twin tricycled which was sitting with 700g nose wheel weight and I am currently engineering as much back to the tail as possible. My point is that the model may not reflect the full size with respect to weight distribution.

Regarding your RDS, are you providing access panels to adjust the aileron RDS grub screws, only asking as sometimes its easier (if you have the option) of putting the ailerons on different channels and thus you can adjust their centre points separately?

Are the flaps fuselage to nacelle or extend to the outer side of the nacelle - just interested wink

Martian31/03/2020 14:56:40
2462 forum posts
1157 photos

I am certain that you will do it justice Jim

G-JIMG31/03/2020 15:06:29
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144 forum posts
105 photos

Hi Chris,

The main undercarriage rotates forward, so there's not enough room in the nacelle for a battery. Fortunately the aircraft has an under-fuselage pod that is large enough to house both batteries. At some point I will have to figure out how to make it easily removeable whilst still secure in flight.

Regarding the RDS, there will be underwing access panels for both servos. Unfortunately I only have 7 channels available, so putting the ailerons on different channels is not an option.

There are two flaps each side. One extends from the fuselage to just outboard of the nacelle and the other extends from there until the aileron. The real aircraft has Fowler Flaps but that's a step too far for me at this stage. Designing and building them into a new wing is a project for another day.

Jim.

David Hall 931/03/2020 16:53:22
237 forum posts
15 photos

I noticed that your UBECs are wired in parallel  (even more if the ESCs are supplying power too). This is something that I plan to use when I next need to supply more power than one BEC can provide. Not all BECs will do this happily, I understand. Have you tried this on the bench?

 

 

Edited By David Hall 9 on 31/03/2020 16:55:40

Chris Walby31/03/2020 17:05:34
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1194 forum posts
297 photos

David,

Where does it show the BEC's in parallel?

shadow model wiring.jpg

The upper BEC powers the U/C only, notice the red wire cut to the RX.

This is standard practice when using U/C as if it jams then the BEC will burn the U/C motor out or its inbuilt safety will function but it will not brown the RX out.

I don't think its ever good practice to parallel BEC's, if the ESC BEC is not large enough then put a separate UBEC/BEC in that can handle the expected load. I had an interesting chat with George at 4Max regarding my Laser 180 YAK and his advice was keep it simple and have one 20A UBEC (It does not have U/C) but in the event of a servo failure the UBEC voltage will not drop below the RX min voltage as it clears the fault.

 

 

Edited By Chris Walby on 31/03/2020 17:07:42

Edited By Chris Walby on 31/03/2020 17:08:16

Edited By Chris Walby on 31/03/2020 17:13:24

David Hall 931/03/2020 17:48:06
237 forum posts
15 photos

I see your disconnect now... in the red lead.    

 

Edited By David Hall 9 on 31/03/2020 17:49:18

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