40 % Pucara twin Turbo Prop

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Ken McCormick
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Joined: November 27th, 2009, 1:53 pm
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40 % Pucara twin Turbo Prop

Postby Ken McCormick » April 8th, 2014, 9:17 am

I would like to say I was inspired and just wanted to share this story and updates with you ! … but the truth is it was Andy Johnson who will be the pilot that reminded me that it was part of the LMA ethos to share these sorts of projects and he is correct so better late than never.
About 8 years ago I was quite taken by some pictures I read in one of those monthly aircraft magazines which gave details and photographs of the Pucara and remembered the bad press it received during the Falklands war.
I since learned its poor record was based partly due to the sabotage and bombings on the ground of most of the fleet to ensure that air combat was avoided against our own forces. Having spoken to aircraft engineers who evaluated the captured stock it seems it was a pretty awesome threat and good flying machine but fortunately neutralised at the time.
So you either love it or hate it.
gayden 2013 1a.jpg
gayden 2013 1a.jpg (263.96 KiB) Viewed 9454 times



A tricky wee bit was the flap construction.

Flap detail.jpg
lots of head scratching but Jan came up this solution
Flap detail.jpg (328.83 KiB) Viewed 9446 times


At some stage the whole structure starts to get a feeling of strength and after a patient number of hours the result starts to show.

Start of sheeting.jpg
This is a very satisfying stage when you have spent all day with pins and pegs
Start of sheeting.jpg (361.35 KiB) Viewed 9443 times

John Greenfield
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Re: 40 % Pucara twin Turbo Prop

Postby John Greenfield » April 9th, 2014, 6:32 am

Ken

In the last picture is that bottom stringer really butt jointed against each former !!!?
Structurally it would be much better if the formers were notched and the stringer was full length.

John

Phil Clark
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Re: 40 % Pucara twin Turbo Prop

Postby Phil Clark » April 9th, 2014, 11:16 pm

It would also be nice to see some (at least) 45 degree scarf joints in the balsa sheeting.

Phil

Ken McCormick
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Re: 40 % Pucara twin Turbo Prop

Postby Ken McCormick » April 10th, 2014, 7:55 am

Hi John .
The stringers above were planned prior to CNC cutting but the bottom one was not and I took on board your comments at the show and fitted these with a small plate only to increase the glue area of the sheet. After long discussions as to whether it added or not too the strength then it was decided not but in constructing the sheeting it was an aid to keeping the shape and then using glass cloth as the final cover.

Thanks for the comment as it always raises discussion and improves learning and me being a relative newbie to the big model world its good to know there are people who are interested enough to comment and therefore always improving.

Ken

Ken McCormick
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Re: 40 % Pucara twin Turbo Prop

Postby Ken McCormick » April 10th, 2014, 8:06 am

Hi Phil.
Thanks for the comment re Scarf (scarph) joints.
on the ply sheet and balsa sheet we have chosen a method related to finger jointing. Its not clear on the photograph but we have machine cut the ends to the finger shapes on each of the sheet which should give us the strength along the length as effectily there is double the glue area.
I am comfortable making joints with this method on the thin sheet simply because making a tapered Scarf on sheet for me is not practical. in testing the finger joint the sheet breaks before the joint.
So I don't know if there is any difference.

Anyways thanks very much for your comments as I said to John its great having such expertise looking over my shoulder.

Ken

Stuart Solomon
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Re: 40 % Pucara twin Turbo Prop

Postby Stuart Solomon » April 10th, 2014, 9:52 pm

Finger joint is just as good if not better than a scarf joint. My industry has been using it for many years as it is mechanically stronger, after saying that, the joints shown would need to have a slope of 20 degrees at most ;)

Ken McCormick
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Re: 40 % Pucara twin Turbo Prop

Postby Ken McCormick » October 27th, 2014, 6:39 pm

Sorry I haven't been updating here but things have been so hectic.
Well the Pucara went to Gayden with I thought a pleasing progress from last year and the only negative comments were that it is was felt to be too big. Jan and I have been using a new construction method with use of a special PVC very light foam sheet bl did with balsa and or lite ply and the result has been stunning. At the show I challeged a number of visitors and fellow members to break the test pieces, the money was safe in my pocket. Bottom line is our inspector has now passed this stage of construction and eliminates all worries of twisting the fuz in the event of a bad landing. The idea came from a crowd called easy composites and gives us the opportunity to gain lots of strength without a huge penalty on weight.
The show was great for me not because of all the nice things said about my wee Pucara but the exchane of really useful ideas and assistance to help me complete this project. More photos soon

Simon Wright
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Re: 40 % Pucara twin Turbo Prop

Postby Simon Wright » October 28th, 2014, 7:43 pm

Ken, really nice to chat with you about your impressive Pucara project. It certainly caught my eye as I came up the escalator. Look forward to seeing more construction photos in the coming months.
LMA2345 now that should be easy to remember!

Mike altham
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Re: 40 % Pucara twin Turbo Prop

Postby Mike altham » November 4th, 2014, 12:37 pm

Ken,

Looks like an interesting build.

Just wondering can you tell me more about the PVC foam and balsa skin????

Im looking for a light weight but strong method.

Mike.

Noel Rumers
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Re: 40 % Pucara twin Turbo Prop

Postby Noel Rumers » November 8th, 2014, 3:29 pm

Would this be Devinicell stuff.
Rohacell is more or less the same.
Noël


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