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Martyn's Mini Jazz

One for Satchmo

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Martyn K22/03/2014 22:31:06
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Here we go. I opted for the Mini Jazz after my Flea Fli nomination didn't get shortlisted (shame). So this the 2nd Mass Build model for me - although the first didn't actually qualify.

I bought the plan, canopy and cowl but baulked at the short kit - the postage cost was the killer.

Making a start. I am following the plan almost exactly - just a few tweaks here and there. Power will be via a SC15 that I adopted about 3 years ago. I have never actually heard that engine running yet. Ho hum...

This is a noddy guide - experienced builders can probably skip most of it..

Starting with the wing. Much like the Flea Fli, this is a parallel chord wing - the big difference is the size of the ailerons..

Start by cutting a 1/16" ply wing rib template as the master. This makes cure thal all the slots and the length of the ribs are consistent.

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There are many ways of doing this. At one time I used to swear by pin pricking, now I use Carbon paper. Hand draw the curves but use a straight edge for things like the LE, TE and spar slots where there are straight lines.

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It should look like this when you remove the plan. I use a Stanley knife with a new blade for cutting the ply, its a bit safer than using a scalpel.

Be very careful to cut the slots accurately, it is better to start under-size and increase the width/depth until you get a good snug (not tight) fit with your selected spar material.

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Good rib material should ideally be quarter grain balsa of medium light density. you should (just) be able to indent it by squeezing it between finger and thumb.

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It is more economical with this model to use 3" width balsa for the wing ribs - you can just get 2 out in one width, you cant get 3 with 4" sheet.

Basically, place the rib down on the sheet, press down with 2 fingers so it doesn't slip and then using a scalpel with a new blade cut round the edge of the template, Take care when cutting the spar slots - cut don't crush and when you rotate the rib and template make sure they stay together. Use the LE as a datum if one modes. Cut 12 ribs from 1/16" balsa sheet.

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The wing needs to be assembled with packing as you need to allow for the TE sheeting and the D box sheeting. As the model uses 1/4" x 1/16" cap strips, I have sliced 2 lengths using this little balsa stripper. I have also sliced the 1/16" false LE. and TE sheeting. Make sure you are cutting from a straight edge or you could end up with bowed parts.

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Here is the stripper that I use. It uses a vernier screw to set the width. I find it easy to use and is accurate if care is taken when guiding it past the sheet wood.

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One area where I differ. I like to use full length hinges made from the Kevlar cloth. This is sandwiched between an upper an lower TE, 1/4" wide and sliced for 1/8" balsa sheet. I have covered how I use this in the Flea Fli build blog.

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A close up of the material.

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More to come

BW

Martyn

Martyn K22/03/2014 22:31:32
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5084 forum posts
3678 photos

Start the build by pinning down the TE, ensuring that it is straight. Use one of the packing strips just in front of the TE for the ribs to rest on. The lower part of the rib is straight so it is quite easy to assemble this wing. Check for a fit on the spar, apply glue to the TE join, the spar join (I use Evo-Stik Resin W), then push the rib back into the TE, then pin the spar down with the second 1/16" packing piece under the spar.At this stage, the spar is only pinned down adjacent to the first rib. Check the rib is vertical using a square, then move to the second rib. Same process - check for a fit, apply glue place the rib back into the spar, check its vertical and then pull back into the TE then pin the spar adjacent to the rib.

Repeat for the other 10 ribs and allow the glue to dry. (one point, I cut two small slots in the centre rubs to allow me to get the servo leads from the servos to the receiver. Its easier to do this before you glue the ribs in.

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It should look something like this... Let the glue dry..

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Add the false LE. Note the wing is still pinned down. This is fairly straightforward. The false LE was stripped from 1/16" balsa using the template LE width to set the stripper. Apply glue to the front of each rib then attach the LE. Use a pin pushed through into the rib to hold it in place until the glue dries.

dscn0050.jpg When dry, unpin the wing from the board and then add the upper spar.

One tip, remove any excess glue before it dries. Glue runs just add weight, no strength and look a mess.

More to come

BW

Martyn

Bob Cotsford23/03/2014 09:50:41
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8583 forum posts
477 photos

"unpin the wing from the board and then add the upper spar." - what's the logic there Martyn? For myself, I would add all the structure possible while it's pinned to the board and held straight but I'm always open to new ways to get the job done.

Martyn K23/03/2014 14:51:07
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5084 forum posts
3678 photos

Hi Bob

Very simple really. You can't get the pins out from the lower spar once the upper spar has been added. (If you are careful, you can, but I usually end up damaging something).

Martyn

Bob Cotsford23/03/2014 16:00:35
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8583 forum posts
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simple as thatlaugh

Kevin Fairgrieve23/03/2014 17:46:35
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1670 forum posts
2979 photos

I put my pins in at an angle.

You can then pull them from behind.

Even after adding the top sheet.

wing (19).jpg

wing (16).jpg

Kev

Tony Bennett23/03/2014 17:57:27
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5082 forum posts
129 photos

fast and nice work sir.

Biggles' Elder Brother - Moderator23/03/2014 19:05:19
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15748 forum posts
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Great blog Martyn - really clear and well explained. I'll follow this with interest.

BEB

Martyn K23/03/2014 20:51:09
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5084 forum posts
3678 photos

Thanks all - and I get the BEB seal of approval.. A high order indeed.

No car this weekend - so no flying so more building than usual. I am actually wauting for the resin on the fus of the Magic to harden so while I have time lets crack on.

This morning, I added the upper and lower TE strips. These were sliced from 1/16" sheet. BTW - there is actually a slight error in the plan here so I have worked around it. If you check the section showing the full cross section at the root, the top and bottom sheeting is 1/32". Additionally, the section on the template is therefore 1/16" too narrow for the false TE. So I have got round this by butting the lower sheet to the false TE. This brings the top of the false TE level with the top of the spar and then overlaid the top sheeting over the false TE. It fits well and makes sure that we have the correct thickness when we attach the ailerons.

dscn0051.jpg The lower TE sheet added. A little excess glue needs wiping away

Before you can fit the upper sheet, you need to chamfer the false TE section so that you get a nice linear progression down the length of the rib to the aileron junction.

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Sorry its not very sharp. New camera and it isn't as good at this sort of thing as my ancient Olympus which has now sadly passed away.

However, you can seethe taper ( the wing is upside down in this shot.). I have also added the upper TE sheet as well.

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Now that we have some support at the TE, we can start on the false LE. This also needs a chamfer sanding down. I find the best way is to use a long sanding block and sand lengthways at the correct angle until you hear the ribs catching.

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That sort of angle. You really need to avoid sanding the ribs though.

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A bit hairy, but this shot shows what you should be aiming for..

When happy, its time to attach the LE sheeting.

I find that the easiest way to get the correct width is to lay the wing on the sheet, lining the rear of the spar up with the edge like this

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Then roll the wing around the sheet until the LE touched. Mark in 3 places (tips and centre) then cut it about 2mm wider. Repeat for both halves

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More to Come.

Martyn

Martyn K23/03/2014 20:59:44
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5084 forum posts
3678 photos

Continuing..

The next bit is dead easy.

I use PVA glue along the spar and across each wing rib.

Peg the sheet into place like this. (BTW, these pegs are amazing - that are £1 from ASDA (bargain section) - probably totally useless as clothes pegs but they have a large flat surface for clamping - so no dents. Great colours as well).

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repeat for the other side. you should have the upper and lower sheet sticking out horizontally with glue on the ribs waiting for something to attach to them.

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Next. Spray the outside edge of each sheet with a little water and wipe off the excess. It needs to be slightly damp not wet. The wood will swell and help you form the curve.

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Using one hand, hold the panel vertical and squeeze top and bottom together over the false LE and then run cyano into the join. Hold for a few seconds (the dampness will help the Cyano go off), then quickly move down the panel at about 150mm at a time keeping running cyano into the seam. A wing like this will take less than a minute to seal both upper and lower sheet. Place the wing somewhere warm until the wood and glue are both dry.

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Seeemples...

So while the wing is drying - start on the tailplane and fin assemblies

 

 

Edited By Martyn K on 23/03/2014 21:23:02

Andrew Price 223/03/2014 21:19:48
821 forum posts

Watching with interest.

Martyn K23/03/2014 21:20:00
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5084 forum posts
3678 photos

The fin and tailplane is laminated using 2 pieces of 3/32" sheet balsa. The tailplane measurements were simply transposed from the plan, the fin outline was copied using carbon.

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Again, I am using Kevlar for hinge material so the same method described earlier.

 

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The bond is PVA for the wood areas and Cyano for the Kevlar area.

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and weighted down so that it doesn't warp while drying... Very important, PVA has a water based composition and will cause wood to warp when joining sheet like this

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Ditto for the fin. Note that I have extended the length of the fin slightly. It will sit in a slot in the top decking.

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Starting work on those huge elevators. The end pieces have been cut from 3/16" balsa sheet using the Carbon copy method. The elevator hinge is laminated from 2 pieces of 3/32" balsa with the Kevlar between. the pins sticking through the hinge are used to set the gap to allow the elevator deflect without binding.You will need to chamfer the hinges to allow the elevator to move.

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Assemble the elevator halves. I find that it is best to use a razor saw (most of the time), you can place your material over the plan or the parts already fitted and mark with the blade. Its quite easy to cut without crushing and (with a little practice) get good right angles so everything fits without gaps.

Don't forget the triangular gussets. They only weight a fraction of a gram each but extend the contact area for the glue providing a much stronger joint. I have made mine slightly larger and inserted additional gussets

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Exactly the same approach is used for the rudder.

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This shot shows how I have extended the length of the fin slightly

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When dry - unpin them from the plan and sand them smooth.. again, a large sanding block makes life much easier..

 

Back to the wing

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2 hours later and the LE sheet is dry. Plane and sand down the excess overlap of the sheeting - top and bottom. Try and keep the false LE nice and square.

Cut a length of 1/4" soft balsa and glue into place. I used PVA in the centre and a little Cyano along the seams. This hardens the joint quite nicely. It is also rigid enough to start carving and sanding to shape straight away.

dscn0077.jpg

That is what it should look like with the LE sanded to shape. Take your time with this and keep checking the profile. Small wings don't work especially well, keeping the LE shape accurate and consistent helps a lot.

Yes - that is Solarbo balsa - I think I have had that piece of wood for over 30 years...

Finally for tonight (and before I forget) I have added a liteply doubler and balsa infill at the TE in the centre to carry the wing bolt load.

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More to come..

Martyn

 

Edited By Martyn K on 23/03/2014 21:28:55

Martyn K24/03/2014 22:13:03
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Day 3:

Back on the wing today. Nothing done before breakfast but I did manage to get out for a while at lunchtime.

The wing-tips were cut and anti-warp strips added. Those little strips at the rear of the tips help stop the tip from bowing when covering. They are shown on the plan, it is very tempting to leave them off but it is better not to.

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No need to wait for the glue to dry. The tips were checked for a good fit then carefully aligned between the LE and the false TE. All done by the Mk1 eyeball. Left to dry for the afternoon.

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the triangle reinforcing pieces have been added top and bottom. The plan says 1/8" sheet, I used some light 3/16" sheet which gives a better blend into the wing surface. These are added upper and lower on each wing-tip. Ideally, the grain should be down the longest length but as this is totally enclosed, the grain running lengthwise will suffice.

The next job is to add the webs.These do 2 things, they prevent the wing spars buckling under load and they also add considerable torsional rigidity. If you dont believe just how much rigidity they add, try twisting the wing before and after fitting.

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The grain on shear webs runs vertically. This is because balsa is strongest in compression along the grain, not across it. On a wing this thick, I suggest that you pin the wing back down again before you insert the webs. This will ensure that the wing stays flat while they are being added. If you build a warp in at this stage, you probably wont get it out again,

 

Carry on until all are done. One thing that I have changed with the webs is that I have placed the web behind the spar rather than in front of it. The reason why the torsional rigidity increases is because you have created a tube comprising of the LE, the upper and lower sheeting and finally the webs. Moving the web as far back as possible increases the size of the tube which helps with the torsional stiffness of the wing.

When you insert the webs, you need to allow for the cap strips which are fitted next. As these sit on the ribs and butt up to the LE sheeting, the web must be aligned with the top of the rib, not the top (or bottom) of the spar/sheet junction.

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The strips are cut 1/4" wide from 1/16" sheet balsa and glued in place using Cyano. Hold each strip in place for a few seconds until the glue sets.

I haven't added the centre section sheeting yet. That will be done when I am ready to install the servos.

Probably the biggest change I have decided to make it to use built up ailerons. I am not convinced that this will save a great deal of weight, but it is more in character with the rest of the model. The plan has been annotated with the revised built up view using 1/2" x 3/16" and 1/4" x 3/16" strip and 3/16" sheet gussets.

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The front edge of the aileron is laminated with the Kevlar to form the hinge and the same methodology of construction is used as the elevator and rudder.

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The packing is necessary because the wing-tips lift the false TE slightly off the board.

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A shot of one of the joints.

AT the moment the wing is still pinned down waiting for the glue to dry.

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Quite pleased with the progress so far.

I may start the fuselage tomorrow evening.

More to come.

Martyn

Edited By Martyn K on 24/03/2014 22:18:34

Kevin Fairgrieve25/03/2014 17:11:08
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1670 forum posts
2979 photos

I went down the built up aileron route. To be honest the weight saving is very small.

If I were to build another (and I probably will) I will go down the solid route.

The build is looking top notch.

Kev

Masher25/03/2014 17:21:11
1106 forum posts
79 photos

Nice details Martyn.

I would also go solid next one - for the tail too. I have actually ended up adding a little extra weight to the tail - but that could be avoided by better planning in the cockpit area so the battery could be moved a bit

Martyn K25/03/2014 18:22:03
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Hindsight is wonderful and i wish i had gone down the solid route as well. The built up ailerons look great but if any newbie builders are following this i suggest you stick with the plan.

Did either of you taper your ailerons or did you leave them flat? (Apart from rounded edges)

Martyn

Edited By Martyn K on 25/03/2014 18:23:02

Kevin Fairgrieve25/03/2014 19:08:51
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1670 forum posts
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Left mine flat.

Also left the trailing edge flat.

I seem to recall reading somewhere that this helped to prevent flutter.

I may of course be completely wrong.

Kev

Martyn K25/03/2014 20:03:47
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Thanks Kev

Helps keep it nice and simple
john stones 125/03/2014 20:29:06
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11528 forum posts
1517 photos

are you going to add a hardpoint to fix aileron horns to Martyn

I can see the two gussets is this where linkage will be

not picking fault bud i'm just nosy wink

Martyn K25/03/2014 22:40:45
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5084 forum posts
3678 photos
I have only had an hour in the shed tonight. Mainly sanding the ailerons. I have also inlaid some 1/32" ply where the aileron horns will be fitted. So - a very pertinent question.

Got my car back today so managed to get to my lms for the wheels tank and engine mount. So ready to start on the fus now.

BW
Martyn

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