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Miles Magister from Dave Platt plan


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As requested elsewhere I am going to do a build blog of my current build, a Miles Magister from the Dave Platt plan.

 

I picked up the plan from Aerofred and had it printed out at my local printers at 120% to take the wingspan to 80" approx.

 

It will be electric powered and is targeted as a replacement trainer for my Ivan Pettigrew DHC-1 Chipmunk which is now showing its age (16 years old) having been flown in all weathers.

 

Target weight is around 4.5lbs, as I will be using construction techniques learned with my Ivan models to keep the weight down and strength up.

 

I will not be doing a "bit by bit" blog but will be doing a "bits of interest" log. The full build will be hosted on my own website and found here.

 

I already have an issue with the plan in that the aerofoil section used is incorrect and is thicker than necessary.

This may be because it was designed and built in 1967 when radios were somewhat heavier and motors less powerful. That is probably also why the plane weighted 10lbs.

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1st "bit of interest"

 

I am using an old technique used in the period for the tail feathers, in this case the tailplane.

 

A thin core of 1/16" balsa to provide the shape required. The tailplane and elevators have an 8" chord which means I can join 2 pieces of 1/16" x 4" of balsa to provide the core.

 

PICT0257-640.jpg.bd5fe26af4d2db5da2ad9385d13d2717.jpg

 

The split rear tailplane spar has been glued to the core and the elevator section of the core removed to have the elevator front split spar fitted.

 

Next the leading edge, which will be made up of 4 laminations of 1/16" strip stuck to the core. The strips will be wetted with a water mister on both sides of the strip only where the corners are. The strip will be held in place using a mixture of pins, clothes pegs and weigh in the form of expired 6v lead acid gel batteries.

 

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1st strip in position.

 

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2nd strip

 

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3rd strip fitted and the whole assembly left weighted to dry flat. Final strip will be added tomorrow night.

 

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  • 4 weeks later...

I was expecting that the laminations would extend down either side of a central core, but they seem to be top only.  Does this mean you have to put another lot of laminations on the other side or is the core not actually central?

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Laminations are on both sides of the 1/16" core KC, which will be sanded to aerofoil section together with the rib strips once the elevators are completed. Received my piano wire yesterday so the joiner / control horn is one of the jobs for this weekend since the weather is un-flyable.

 

I hope to progress the front of the centre section too over the weekend. Pictures to follow of course.

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I looked for this Dave Platt plan ( the basis for this design  ) and found it seems to be a Radio Control Modeler (USA ) plan rather than Radio Modeller.  The american spelling of ' Modeler' was a good clue.   The very clear and detailed plan was another clue.   It's on Outerzone as well as Aerofred.

 

Dave Platt used a slotted mainspar ( 'eggcrate' construction ) which was fashionable decades ago , so I will be keen to see what Andy specifies for his model........

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Working on the "picture paints a 1000 words" theory...

 

PICT0265web.jpg.31c9a6ec2f463c32abbbf61bec6b74f8.jpg

 

Upper surface is the front of the tailplane showing the core and the 2 sets of laminations.

 

Lower shows the front of the wing centre section without the front rib sections - tomorrows job.

This wing is going to be split centrally as I found the 6'+ wings of the DH Chipmunk were great for sweeping stuff off shelves and getting damaged.

The upper and lower 1/8" x 1/4" spruce spars can be seen together with what will be the central aluminium wing joiner pushed into place.

The rear 1/16" webs can be seen above which will be sanded down after the front rib sections and webs have been fitted.

This will mean the wing joiner will be sandwiched in a box of spruce and balsa - light and strong, same as used on the Twin Otter.

 

Similar will be done to join the central sections to the outer sections, although double webs will only be fitted up to the rib outside of the end of the aluminium joiner.

Edited by Andy Gates
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Very interesting build!    I see you have not used the slotted main spar but gone for the more popular top and bottom spruce spar with, presumably,  webbing fitted individually between each rib.   However this does not seem the same as your Chipmunk which seems to have the front part of each rib detached from the rear part and fitted either side of a solid spar.   Unusual design on the Chipmunk ( if I understood the photos correctly ) and I thought you might have used that for the Magister.

 

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Thank you KC.

 

Centre section has 1/16" vertical grain webbing on both sides of the 2 spruce spars as does the 1st 3 ribs on each outer panel.

 

This together with the split ribs as per the Chipmunk build, provides a nice strong channel for the wing joiners to be fitted into.

 

PICT0271-640.jpg.ed887d07d552d9b3e56b6b137843d4f5.jpg

 

The centre panel to outer panel joiners will be glued in permanently once lightened, the centre panel joiner will only be glued into one side so I can have a 2 piece wing.

This also allows me to reduce the spar thickness reducing weight the overall weight.

The Chipmunk has 2 x 1/4" square spars stuck to the upper and lower edges of the 1/8" balsa spar, the other machines listed below have 1/4" x 1/8" spars. The Chipmunk only has 1 spruce spar that goes all the way to the tip, the other stops short.

 

This is the pattern I used on the Mini Catalina, Mosquito, Beaufighter and the Twin Otter, the Twin Otter being the only other machine with an aluminium joiner. All the others have 1/8" ply joiners around 1" deep, the Twin Otter has aluminium joiner due to the lack of gap between the spars of approximately 3/8".

 

I have some wiring to add to the wing panels soon, together with the false trailing edge and aileron spar.

 

I hope the above provides a suitable insight as to why I have modified this build in the way I have.

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Is there some great advantage in having the centre section glued to one outer panel?  My view is that wings with wheels attached are a pain and more likely to get damaged.   Having the centre section with wheels able to be left on the fuselage if required might be an advantage in storing and transport.

 

 

 

 

 

 

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A slight misconception there I believe.

 

The centre section is in 2 parts, so each outer panel has half of the centre panel attached to it. 

 

I have split the wing as I find the Chipmunk wing to be a bit unwieldy. I hope to be joining the centres to the outer panels later tonight so I will take some photos as I do it.

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For those who didn't notice it earlier Andy has put more photos of construction on his own website.  

 

The construction is very interesting because it does not follow usual size wood for this size of model.   The 1/8sq framing for the fuselage seems much smaller than I expected and looks to be very delicate at this stage.    Will this all be skinned with sheet Andy?

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Correct Trevor, the only way I could see of getting the weight down to where I wanted it.

 

Hi KC,

Part formers will be added to the frame structure for the complete upper section and the lower nose section prior to the whole lot being sheeted with 1/16" balsa. The Magister has fairly flat sides and lower tail areas so that is sheeting straight onto the framework. All fairly standard IP practice - and pretty much as per the Chipmunk you have seen.

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I guessed it would be sheeted in 1/16th but I was wondering if there was some reason why the sideframes weren't sheeted  before assembling in the jig?  The frames look so delicate without sheeting that it looks a very tricky job to assemble into a square fuselage.    I am sure I wouldn't have the patience!

Andy's Chipmunk is mostly fresh air inside - I couldn't see anything inside the fuselage!   No bulkheads visible yet it seems quite strong enough.   Flew very impressively on a 3S2900 where most models of that size ( 75 inch span)  would have used a large chunk of Lipo perhaps 5S or 6S.   It certainly made me think again about construction methods for electric models.   The traditional construction of glow models with thick sheet balsa sides with 1/32 ply right back beyond the wing seems inappropriate now for electric.

 

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Current weights, wing 400g, fuselage frame 33g.

 

I could have fitted sheets to the frame first but something would be fighting during the curve taking effect. Either the side frame members would be under compression (sheet being tougher than the sticks)  or the cross members would be under more compression than needed and may cause breakage.

Adding sheet after, allows all the wood to be as relaxed as possible.

 

I too was brought up on 1/8" sheet balsa sides with ply doublers and more 1/8" sheet as cross sheet under and over, with ply or thick balsa formers all on a 36" span plane. No suprise it flew fast.

 

The Ivan way does seem to have the best of both worlds, strong yet light and not overly intricate as per some of the newer laser cut machines. As you say, electric does not carry the same levels of vibration as IC so we can scale down significantly.

 

Because of the reduced weight, the planes can fly slower. This means in the case of an accident, because e=mc2 - not only have we reduced m(mass) but also c(speed) which is squared. So if I have dropped speed from 10mph to 15mph, c2 changes from 225 to 100 which is a reduction of over half the energy without including the weight reduction too.

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I knew there would be a good reason!   I suppose your way is somewhat like laminating - bend one layer and then attach the other layer.   I think it would be possible to counteract that by saying wood is pretty good under compression but fails in tension - so attaching the wide balsa might help prevent failing in tension and also prevent some joints springing open.  In the end both the 1/8 and the sheeting are bent just as much.   But whatever works works!

 

Are there any other published RC designs that are as lightly built as those by Ivan Pettigrew?  I have not noticed any.

 

I think Einstein said ( or was it one of my flying pals... )  that not only do light planes crash less but they crash lighter too .  

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I just noticed in the article by Dave Platt he said the wing incidence of 2 degrees on the full size had been removed.  This was tucked away under the 3 view and colour diagram, so I wonder if you noticed it and followed suit or not.

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I have picked up on the incidence issue as it can be seen on the plan.

I am still undecided especially as I have change the aerofoil section.

I may build at zero and then pack the TE if required, but not really made my mind up yet.

 

Added the first bit of fuselage sheeting held in place with weights, all 6.3kg of them. Does it need to be stronger?

 

PICT0280-640.jpg.4dec2b5264bc7af1e80f0cd6859bd8e8.jpg

Edited by Andy Gates
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3 hours ago, kc said:

 

Are there any other published RC designs that are as lightly built as those by Ivan Pettigrew?  I have not noticed any.

Robin Fowler has done a number of lightweight designs. He has a novel and very economical way of making wing ribs.

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