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C of G Seagull PT -19

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Winco Steve19/07/2019 14:32:08
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198 forum posts
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Hi all,

Just finished the above kit ( Seagull PT -19 Fairchild ) and about to check it all over and set the C of G. However, the instructions state ‘ the centre of gravity is located 6.5 to 7.5 cm back from the leading edge of the wing measure at the wing tip’. Not sure why the inclusion of ‘wing tip’? Back from leading edge, sure, but wing tip? Any directions would be most appreciated as Sunday looms and club day for maiden flight!

cheers all,

Winco Steve

Edited By Winco Steve on 19/07/2019 14:33:16

Martin Harris19/07/2019 14:57:41
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8870 forum posts
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That's not uncommon with Seagull. Run a piece of string from tip to tip and (easier if the fuselage top section is removable like my Yak?) to establish the datum and mark the C of G on the place where you'll balance it (normally upside down on a low winger) and dig out your C of G rig (or Mk 1 fingertips) to check.

A piece of masking tape should protect the finish if you are using the wings as the measuring point.  I suspect that you might be able to connect the wings together off the model with the joining tube to use the string "trick" more easily.

If in any doubt, remember the old adage - a model with a too forward C of G flies badly but with a too rearward one flies once!  It's often worth a Google for owners' experiences with recommended C of G positions which have been known to be specified incorrectly!

Edited By Martin Harris on 19/07/2019 15:18:57

Winco Steve19/07/2019 15:47:52
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198 forum posts
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Hi Martin,

Thanks for reply. The wing sections are permanently joined by way of support rod (epoxy) . So, a line across the wing top surface from wing tip to wing tip. Line 6.5 to 7.5 cm in from leading edge? Have I got that right? Fix back on plane, upside down on C of G machine? Presumably, the balance points can be anywhere, equidistant, along that line? Using a Great Planes C of G machine as fingers fat and wobbly! Never do I miss this part out, too many early days mishaps.

Cheers,

Winco Steve

Denis Watkins19/07/2019 16:32:32
3912 forum posts
61 photos

These are very lightly built Steve

Very stable in flight

Suitable as a 1st low wing trainer

And lovely colours

I put Hobbyking Oleo Legs on mine to look the business

Martin Harris19/07/2019 16:33:09
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8870 forum posts
221 photos

I haven't any experience of your model but with my Seagull Yak, I used string to establish the wingtip position on the fuselage and measured the specified C of G position back from there.

It worked fine for me on that model - which incidentally needed tail weight to balance the rather larger than specified engine.

Bottom line, failing any response from someone with direct experience of the model is to err on the side of caution. One simple precaution might be to input the planform data to an online calculator (example) and see how closely it aligns with the method I've advocated.

Edit: I see Denis has had one - any advice on the measurement method and behaviour at the specified setting?

Edited By Martin Harris on 19/07/2019 16:37:24

Winco Steve19/07/2019 17:31:10
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198 forum posts
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Hi both,

Martin, thanks for that link it looks very interesting and will assist me I’m sure. I’ve given up for today as it’s 34

degrees and I’m starting to melt! Club day Sunday so hopefully will have it sorted. I will get back on this thread with results. Yes , Denis, a very light model but it does look pretty! I got it to add to my collection for something to sling in the van and get a bit of fun flying again. My other models seem to take an age to set up! I have another Seagull, the At Texan. That looks super too.

Best wishes,

Winco Steve.

Denis Watkins19/07/2019 18:05:04
3912 forum posts
61 photos

Just seen Martin's C of G query

this model has to be said, is very forgiving

And like most Seagull models, if you install all the heavy bits in their cut outs, then the C of G is not far away

With this one, the C of G was set 7cm from the tip LE, using 2 lads with a finger under each wing tip

But to add, the PT19 is very like a small Astro Hog, and flys as such, and is very stable

Edited By Denis Watkins on 19/07/2019 18:06:03

Winco Steve20/07/2019 11:55:06
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198 forum posts
1 photos

Hi again,

Hopefully Martin or Denis is about? Or anyone else who has a PT19 Seagull. Tried recommendations but way out. One way very nose heavy the other very tail heavy. Can someone give me the exact way Seagull are instructing you to locate the C of G? Leading edge at root is normal but where/ why /how at wing tip? I might just be getting thick in my old age or possibly suffering from sun stroke but non of this adds up. Using the C of G machine the balance, at wing root end , just out from fuselage, is about 90 cm??

yours, confused,

Winco Steve

Denis Watkins20/07/2019 13:48:06
3912 forum posts
61 photos

9cm at the Leading edge, Yes Steve

Goes through the Leading pilots head

I measured

The wing, as you know is tapered, so as advised, ran a string from one tip to the other

At the field, without being inverted, one lad at each wing top lifts the model with a finger under the 6 -7 Mark

That did for me and flys beautifully

Winco Steve20/07/2019 15:00:28
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198 forum posts
1 photos

Hi Denis,

Thanks for your reply. I will have another go to see if I can sort it.

Regards, Winco Steve

Martin Harris20/07/2019 22:56:00
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8870 forum posts
221 photos

Sorry - out flying this afternoon but Denis had the best info...

Winco Steve21/07/2019 10:22:51
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198 forum posts
1 photos

Hi all,

Last update! Measured 7 cm in from leading edge at wing tip just before it curves. Piece of wooden batten to join dots each end. That gave C of G about 9cm just outside of wing root/ fuselage. Placed machine points here and nose dipped a little too much. Added some weight ( 20 gm I think, small weights used for balancing cycle wheels ) to tail and just dipping nose down. That’ll do for me.

Will give it a try Wednesday!

TTFN Winco Steve

Martin Harris21/07/2019 11:12:30
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8870 forum posts
221 photos

Winco Steve26/07/2019 06:53:25
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198 forum posts
1 photos

Hi all,

Just an update. Whilst conversing on this forum I also sent an email to Seagull . They responded quite promptly and confirmed the measuring technique/ position that was suggested and employed. See my previous reply. They also sent a diagram of how to measure C of G which they stated that they will include in future construction instructions. Unfortunately, I am unable to download this as it is a pdf. I tried, several times, with photos etc but no joy.

Anyway, test flight didn’t go well as model kept going nose down during taxi. Problem, weeds and stones on grass runway. Solution, I hope, much larger wheels? Retry Sunday!

Regards to all, Winco Steve.

Denis Watkins26/07/2019 07:54:18
3912 forum posts
61 photos

Progress Steve, At least some progress

This site works on jpegs

So I screenshot the pdf to my gallery as a jpeg, then post,

I overdo freeing off wheels on axles, for the reason you state

Most of mine are tube brass bearings on the piano wire

Or more recently using Dubro Low Bounce wheels are a tight fit on standard size diameter piano wire

So, said wire is taken down with wet and dry paper and polished, until the Dubro wheels run free

Laborious, by there are only two wheels

Edited By Denis Watkins on 26/07/2019 07:55:30

Winco Steve26/07/2019 08:05:59
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198 forum posts
1 photos

Good morning Denis,

I did try the photo trick to convert from pdf to jpeg but this failed also. I attempted this first on IPad then on my Mac computer. No joy, just would not upload. Gave all the right signals it would work, but when upload button pressed, failed to appear in album. Even started all over with deleting all, open new album and try again. Nothing. Better spending the time on my models! Ha Ha!

Guys with front wheel tricycle undercarriage don’t suffer quite the same as the ‘tail draggers’. They seem to bounce along easier and, of course, have the nose support. Replaced the PT 19 wheels with a set from an old Wots Wot. They should be up to it.

All the best, Winco Steve

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