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Seagull 80" DH Chipmunk

A bit of light relief

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Danny Fenton07/03/2019 23:56:43
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9082 forum posts
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Okay so it was muted that some people might be slightly interested in an artf build I am doing for flying only comps this year. I had hoped to use my Hawker Fury but with very little time remaining before the first event I thought it prudent to put something together quickly.

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Queue the Seagull models Chipmunk. At 80" it is not as large as my Black Horse Chippy which is 85" but bigger than my Dennis Bryant Control line model which is 70"

I started by fitting the motor and the motor mount. It is all very straightforward. Seagull suggest 8 or 9 cells LiPo which is crazy. I will go 5S with a 500kV motor. The 4258. this motor can give around 1200 watts, depending on how you prop it. 900 - 1kW will do me, I think a 15 x 10 should do it.

I will let the pictures tell the story. it is all very straightforward stuff.

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Okay I lied, the first picture shows that you actually need to pack the motor out further than the travel of the adjustable bulkhead will allow. 145mm to the prop back plate from the motor bulkhead on the airframe. I added 6.5mm of SLECs finest ply, which brought the motor to 142.5mm a quick check of the cowl and this will be fine

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The spinner backplate needed the rear edge chamfered to allow it to sit flat on the prop backplate, due to the radius on the prop adaptor.

I used this to check the cowl allignmet could be achieved, all is good. I will fit the cowl later.

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I then worked out where the speed controller was going, this is a HobbyKing 60/70A 1-8 cel, non BEC controller. I will be using a seperate BEC and a receiver pack. l will be trying one of Chris Botts tasty twin power feed circuit boards, with an FRSky receiver.

Next up was the servos. I was going to use my trusty HK 5010 but then in a moment of madness splurged out online at Webbies and got some HiTec HS425BB and fitted the two in the fus, rudder elevator. there are twin feeds, one to each elevator, but a coupler means only one elevator servo. A nice touch would have been to shuffle the servos a bit so two could be used for redundancy.

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Cheers

Danny

Danny Fenton08/03/2019 00:04:48
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9082 forum posts
3928 photos

I then moved on to look at the tailplane and elevator. trying to lift it out the box made me realise that Seagull had dropped a clanger here surely? The tailplane and elevator weighed a massive 180g!!!

So what to do.... day one of the build, motor mounted and two servos..... that was as far as I got before I modified the kit.....

I scribbled around the offending item and started making a plan on paper. Can't remember the last time I did this, I use cad normally. Anyway a tailplane was duly manufactured. The kit one weighed in at 116g my replacement 32g..... so a very worthwhile exercise. It still needs covering but I cannot see that adding 84g!!!

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sanded

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while making this tailplane, which by the way was by making a 1/16th core and then adding medium 1/4 around the edges on one side. with some 1/8 ribs. this was flipped over and mirrored on the other side, before some careful sanding revealed a tailplane buried inside. Who'd of guessed

Anyway that pic is actually a bit of a cheat, as the other skin isn't fitted yet, but it wont weigh much

Cheers

Danny

Denis Watkins08/03/2019 07:09:50
3736 forum posts
178 photos

You really show how it should be done Danny.

Solid front end work, precision bolted, and looking ship shape, the motor as solid as it can be

Using what is best of ARTF, then rejecting the tail, is a bold worthwhile move.

That covering looks to be Cub Yellow.

Following with interest.

McG 696908/03/2019 07:14:46
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2503 forum posts
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... subscribed, Danny. yes

Danny Fenton08/03/2019 07:52:34
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9082 forum posts
3928 photos

Thanks chaps

Yes i made sure i had some Cub yellow Oracover before I started

I will change the hinging in the elevator, and the flaps. Really dont like the fittings.

I really mustnt alter this too much it needs to be a quick build so I can get back onto my Hawker Fury.

Cheers

Danny

Chris Bott - Moderator08/03/2019 08:09:43
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Come on Danny, keep up..

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To be serious for a minute - mine is built straight from the kit. It needed lead up front so your weight saving at the rear should multiply up. We'll be able to do a direct comparison but I bet you just saved half a pound overall.

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Jon - Laser Engines08/03/2019 08:20:29
4723 forum posts
174 photos

Im surprised the tail was so heavy. Is there not some hardwood or something important in the middle of it that your lighter version is lacking? You wouldnt want to build the super light edition and then find there is some important structural reason the original is so heavy.

Interesting model, i have had enquires from customers about it so im interested to hear how it performs in the air...even with the vegan power plant

Danny Fenton08/03/2019 09:01:30
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9082 forum posts
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Yours does look lovely Chris

Jon, reports from a guy Stateside that has flown his reports its a lovely flier. However the C of G (as usual with Seagull it seems) is way out, and needs bringing forward a fair bit.

The original tailplane seems to have a lot of ply in it Jon, and includes a ply spar, and granted it is stiff. I could swat cricket balls with it! I know my structure looks light, but it is rigid, especially once the skins are on. This is more or less how Brian Taylor makes tailplanes. The thought of flutter did briefly cross my mind but its a Chipmunk and not meant to belt around the sky at mach 1

Chris wasn't it decided that the C of G should be around 115mm back from the leading edge?

The paper instructions suggest 130mm and the online version 100mm so the US modeller felt safe at 115mm, and he reported it flew well at that.

Cheers

Danny

Jon - Laser Engines08/03/2019 09:21:12
4723 forum posts
174 photos

If they have carved the tail from a railway sleeper then thats fair enough.

On the c/g thing, i have not flown one of these but have flown the Seagull Hurricane. I have also flown many YT models and on those all the forums were full of 'the cg is wrong' posts. In my experience the cg on all of these models is accurate but the rates they recommend are miles off and make the model way too sensitive, usually in pitch, leading to cries of bad c/g. I see it in youtube videos all the time too and its a bit like everyone crying that an overheated engine was lean. It wasnt lean at all, you just didnt cool the poor thing so it died! I usually ignore instructions and eyeball rates these days but always favour small movements. In general too little is better than too much.

I dont know what the chord is but if 130mm is a 3rd of it then i would stick to that and bring the rates down.

Danny Fenton08/03/2019 12:16:44
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9082 forum posts
3928 photos

Okay the saga continues.

Firstly I have been caught out with the tailplane incidence before so wanted to make doubly sure this one was right. feedback is that the model flies okay so I am going to stick with the incidence specified by Seagull.

I set the fus on the bench and weighted it down so it wouldn't move. I didn't give a hoot as to the datum, I just wanted the fus to stay still.

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I fitted the original tailplane using a rubber band to hold it still, and took a reading on the incidence meter, -2.

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I then started sanding my tailplane support seats until the height was good, and the angle was the same -2 degrees.

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It came out well, though i should have used 1/8 balsa as the tailplane is a little thinner in profile than the original. It is probably slightly more accurate this way, although that was not the reason for the extra work.

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Final weight, less film, 44g

Cheers

Danny

Danny Fenton08/03/2019 12:18:55
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9082 forum posts
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Now do I add shrouds and rounded leading edges to the elevators

"The devil most definitely is in the detail"

Chris Bott - Moderator08/03/2019 12:22:59
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130mm is 33% of root chord but that doesn't take into account the slight forward sweep of the planform. So it might be OK but then again I wouldn't go blindly with it, personally.

I asked a number of folk who have other Chipmunk models in various sizes where their CGs were. Scaling from those, then 115mm seems more like it, especially for a maiden.

Interestingly, Seagull appear to have ammended the instructions. This is now available as a PDF download saying 100mm.

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Jon - Laser Engines08/03/2019 12:31:27
4723 forum posts
174 photos

Oh thats interesting. I tempted by one of these myself so i will give it a go with both given the opportunity. i just need to find the space thats all

Danny Fenton08/03/2019 18:24:23
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9082 forum posts
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The madness continues with elevator construction.

The ones supplied by Seagull aren't sheeted in balsa, they are sheeted in lite ply!

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Same technique as the tailplane, 1/16th core with bits of "what I have lying around" to make the thickness.

Cheers

Danny

Braddock, VC08/03/2019 19:20:53
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1621 forum posts
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If you remove it carefully you can re-use the covering material, may require a dab or two of balsa lock but that's ok too.

Danny Fenton08/03/2019 19:23:51
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9082 forum posts
3928 photos

I have a roll of Cub yellow Oracover to make good any bits. I am not into trying to re-use film covering

Cheers

Danny

Braddock, VC08/03/2019 19:39:34
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1621 forum posts
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Waste not want not or, in jockonese, every mickle maks a muckle.

Danny Fenton08/03/2019 19:54:44
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9082 forum posts
3928 photos

I have left the parts alone just in case they are needed.

Cheers

Danny

Braddock, VC08/03/2019 20:03:47
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1621 forum posts
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Better still!

Wingman08/03/2019 21:12:33
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1086 forum posts
404 photos

Oh dear Oh dear! What a kerfuffle. It's a Seagull ARTF - they are all good flyers straight out of the box in my experience and almost Hangar 9 quality and it's to be used for flying only comps you say - it's a chipmunk so sedate to moderate flying is the order of the day and weight won't matter as regards a pound or so - looks to me like OCD is creeping in heresmiley or is it a guilty conscience for not building a competition model from scratch??wink 

Edited By Wingman on 08/03/2019 21:14:52

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