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Scratch Build Twin

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G-JIMG24/05/2020 17:00:14
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151 forum posts
114 photos

Haven't posted for a while - that will teach me to continually say, "I'll do that when I get the time dear"!

This is the first twin I've ever built, so I was worried about getting everything perfectly aligned, particularly the engines. To aid me I built a simple jig on my workbench. The two balsa blocks are held in place with double sided tape and are angled for the correct wing dihedral. The two liteply panels allow me to check that the x, y and z axis of the motors are identical.

wing jig.jpg

Using the jig, the wing halves went together pretty easily - huge sigh of relief!

Having joined the wing I was then able to sort out the wiring and the programming of the Transmitter (I use a Futaba T7C). I even managed to programme the Flaps onto a 3 position switch so I can have No, Half and Full Flaps.

The wing has a 13.2 deg Dihedral so getting the main undercarriage at the correct angle was another concern. Having partially built the fuselage I was able to temporarily assemble the model and check the angles.

trial fit uc angle.jpg

The retracts are all at the correct angle and at the correct height - another sigh of relief!

Note, the Red Nose has nothing to do with Comic Relief (although some might think my efforts are a joke!). I learnt a long time ago that I am incapable of sanding a dome shape, it always ends up lopsided and/or with unsightly flats. So, a few years ago, at the Nationals, I spied a box full of large spinners behind a vendor's counter. Most had bits missing or were too large for most customers. Long story short, I bought the entire box for £1 and now use them as aircraft noses, simply selecting the spinner with the correct profile and cutting off the portion I need.

Now it's starting to look a bit more like an aircraft.

trial fit 1.jpg

trial fit top view.jpg

Jim.

Rob Fairweather 124/05/2020 17:21:20
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121 forum posts
11 photos

Superb looking aircraft

Timo Starkloff27/05/2020 20:49:28
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397 forum posts
885 photos

Very detailed and accurate

cymaz27/05/2020 21:12:42
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9253 forum posts
1195 photos

There’s a hint of Constellation about her. Only a hint mind..face 1

Martian27/05/2020 21:33:54
2527 forum posts
1163 photos

That is looking really good 😎

Jon Laughton27/05/2020 21:44:18
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1217 forum posts
72 photos

Very very Classy - I like it a lot! yes

Denis Watkins27/05/2020 21:55:22
4443 forum posts
112 photos

Exactly what I was just thinking Jon,

That model is just so smart

G-JIMG31/05/2020 21:26:23
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151 forum posts
114 photos

Thanks for all the kind comments, much appreciated.

Having joined the wing halves, and verified all the relevant angles were correct, I have spent the last couple of days carefully modifying the underside of the wing to accept the battery box. It would have been easier to plan this in from the start, but as I said in an earlier post, I hadn't really got my head around battery placement when I started the build.

Anyway, I made a jig from scrap balsa block that maintained the correct dihedral and carefully cut away sections before replacing them with liteply. Once I had a plywood base, I was able to attach the battery box.

The small gap between the wing and the side of the box is for the 'Pod' cover. It slots in very neatly.

I have painted one of the ESC/UBEC connectors Green to remind me which one to connect first. The Green one powers the RX. The other black/red wire is for the lower anti-col located on the 'pod'.

batt box.jpg

The batteries slot in easily and will eventually be restrained by Velcro straps. I will install the straps after the final paint.

batt front (2).jpg

The 'ledge' at the front sits flush on the lower fuselage and will eventually house two wing securing bolts, the other two will be at the rear of the wing.

batt front (1).jpg

Jim.

Edited By G-JIMG on 31/05/2020 21:27:54

Edited By G-JIMG on 31/05/2020 21:28:19

Timo Starkloff08/06/2020 21:07:03
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397 forum posts
885 photos

Maybe this website is interesting for you?

www.kingairmagazine.com/

There are also many stories about the history of American general aviation aircraft:

www.kingairmagazine.com/category/historical-feature/

Timo

Edited By Timo Starkloff on 08/06/2020 21:07:36

Robin Fowler09/06/2020 09:04:30
33 forum posts
34 photos

Hi Tim,

I am new to this forum so had to take a quick look back through your thread to try and see how you got to this point (of virtually fully enclosing batteries so that cooling is minimal) and found that some links no longer seemed to work. I was therefore thwarted to a large degree, although I did pick up that you thought (rightly in my opinion) that you should have planned this earlier.

I wonder if you would be prepared to indulge me by setting down your expected AUW for the model and the details of the power train so that I can reassure myself that your beautiful creation is likely to fly? Sorry to sound like a prophet of doom - not the ideal introduction I know! - but I would hate to see the product of your labours end with a puff of smoke in the air and a sickening crunch! Even more so if I had done nothing to prevent it!

With best wishes,

Robin

G-JIMG09/06/2020 09:51:36
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151 forum posts
114 photos

Thanks for the Links Timo.

Robin,

Thanks for your concern.

The estimated AUW was 15lb, but it appears the final build will be nearer to 14lb.

As this was my first excursion into electric flight I passed the details (wing area, AUW, etc.) to George at 4-Max for his advice. He came up with a suggested Batt/ESC/Motor combination but, to make sure, I sent him one of the 4-bladed props and he conducted a series of bench tests for me. Test results show the power train is fine.

Jim.

Robin Fowler09/06/2020 16:04:17
33 forum posts
34 photos

Thanks Jim,

It provides some measure of reassurance to know that George has been involved early on. However his bench tests will have been conducted with all gear out on the bench so cooling will not have been a problem for him at that stage. He will also not (I assume) have run the system for a length of time anywhere near what you will be expecting for a flight, so while he said the power train is fine the question of whether it will be fine for the duration of a flight is unanswered.

George will also (again I assume) have expected that the usual practice of ensuring adequate cooling airflow over the components will be followed (or did he warn you of this knowing that you were new to electric flight?) and you are apparently not providing any appreciable cooling airflow at all.

The batteries WILL heat up during flight, and so will the ESCs. The motors MAY also heat up - depending on how much spare capacity they have. If you stick with the arrangement you have I advise a good long run on the ground (not in the workshop) before flying to test battery temperature.

The King Air at 1:10 has I believe just under 3 sq.ft. of wing area ( how much less depending on version) so an AUW of 14 pounds will result in a wing loading of more than 75 oz./sq.ft - twice as much as any model I have ever flown since the bad old days of brushed motors and Nicads. That puts your model well outside my experience but my expectation is that to generate enough lift for flight those wings are going to have to be pulled through the air very fast, which calls the motors' capability into question. The pitch of the props may also need increasing (I don't know what they are - or the kv of the motors. If high kv less pitch will be needed that for low kv.)

Anyway I have said my piece. My maths has been known to let me down (not my strongest suit at school - which was a long time ago), but I have over 30 scale designs published for electric power in my CV so I have a pretty good feel for what to expect by now.

Shutting up!

Best wishes,

Robin

Edited By Robin Fowler on 09/06/2020 16:11:56

Edited By Robin Fowler on 09/06/2020 16:12:21

G-JIMG09/06/2020 17:24:56
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151 forum posts
114 photos

Hi Robin,

If you look at the head on picture further up this page you'll notice there is an Air Intake at the bottom of each nacelle. The ESCs (and UBECs) sit directly in line with those intakes, so should have more than enough cooling.

Jim.

Chris Walby09/06/2020 17:30:24
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1237 forum posts
303 photos

Just out of interest Jim what is the estimated wing loading? Having asked that I have some models that have high wing loading and fly very well if you give them the respect they are due.

G-JIMG09/06/2020 17:46:34
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151 forum posts
114 photos

The build is slowly progressing. I have installed the wing mounts and mounting nuts in the fuselage and everything lines up perfectly. I am now covering the fuselage but haven't taken any pictures. I figured you all know what balsa sheeting/planking looks like laugh.

The real aircraft has lights mounted on the Nose Wheel.

nose lights.jpg

The lighting kit I'm installing doesn't have enough outputs to install real lights, so I have made some dummy lights for effect.

I habitually keep anything that might be of use for modelling and realised the tips of plug protectors were the perfect size for the lamp housings.

pin covers.jpg

I cut the tips off the power pin protectors, sprayed the inside with chrome paint, the outside with white paint and glued some acetate to the front face. I made the support from some Litho Plate and glued the lamps to it. The assembly will eventually wrap around the Nose Leg and be clamped in place by a M2 bolt/nut.

landing lights.jpg

Not perfect but the overall effect should be OK from a distance.

Jim.

Robin Fowler09/06/2020 18:02:20
33 forum posts
34 photos

Absolutely Jim. The ESC's will be just fine so long as there is also enough outlet area - about 1.5 x the intake - otherwise the intake air has nowhere to go. (I usually place that inboard of the rear of the nacelles where it is less visible as a departure from scale. It's the batteries that are worrying me... What airflow will take away their excess heat in the fuselage?

I agree with Chris that heavy models can fly, but they can be prone to tip stalling and they waste a lot of power just overcoming the drag of the airframe with AOA and speed higher in order to achieve the required lift.

Robin

Robin Fowler09/06/2020 18:14:14
33 forum posts
34 photos

I love to see a bit of lateral thinking at work! Great idea for the lights.

Of course there is nothing to stop you having working lights. You can get high intensity LEDs that run of anything 4V to 12V for cars and they don't flash so can be independent of the lighting system. They just need to be switched on when the gear is down. A judiciously placed microswitch is all you need for that. I use landing lights as the equivalent to 'three greens'.

Robin

G-JIMG09/06/2020 19:21:19
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151 forum posts
114 photos

Robin,

ESC cooling -

32b cooling air.jpg

The intake air passes over the ESC before exiting through the hole in the underside of the nacelle (that area has since been cut away). Additionally, further back in the nacelle, there is an underslung oil cooler. I will use this to disguise another 1"x3/4" opening.

As for the battery, the cradle will have additional slots cut into it (also helps with weight). The cover replicates the under-fuselage pod on the real aircraft. This has panels installed at the front and sides which, on the model, will simply be rectangular holes. The cover is slightly raised at the rear, leaving a large slot for the air to escape.

Thanks again for the advice, nice to have input from someone who has been there/done that.

Jim.

Robin Fowler09/06/2020 21:57:18
33 forum posts
34 photos

Thanks for the explanation Jim. That eases my mind.

Hopefully you can build the tail end light enough so as not to need to move the batteries forward to achieve correct CofG. - Though even if you couldn't there is always the possibility of sitting the batteries on Velcro pads on the cockpit floor and ducting cooling air forward from the front of the cupola leaving the entire rear open as an outlet. This would allow the sides of the cupola to be solid, which would look more scale - plus you wouldn't be able to look through the cupola from front or rear.

Thank you for the information.

Best wishes,

Robin

G-JIMG18/06/2020 17:55:39
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151 forum posts
114 photos

The real Cockpit has a split windscreen to the front and two small windows to each side.

cockpit.jpg

Consequently the cockpit internals are not as visible as they would be on an aircraft with a bubble canopy. Even so, I have expended a little time and effort to give the impression of a King Air cockpit, although I haven't gone to the lengths of making 3D instruments.

The pilot is a 1/10th scale combat figure with the majority of the internal plastic removed. The only changes I had to make were to overpaint the US flag on his shoulder and manufacture a headset for him.

cockpit from left.jpg

cockpit from right.jpg

Jim.

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