Martyn's Mini Jazz

[Martyn K]

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.

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.

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.

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.

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.

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.

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.

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.

A close up of the material.

More to come

BW

Martyn

[Martyn K]

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.

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

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.

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 Cotsford]

"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 K]

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 Cotsford]

simple as that

[Kevin Fairgrieve]

I put my pins in at an angle.

You can then pull them from behind.

Even after adding the top sheet.

Kev

[Tony Bennett]

fast and nice work sir.

[Biggles' Elder Brother - Moderator]

Great blog Martyn - really clear and well explained. I'll follow this with interest.

BEB

[Martyn K]

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.

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.

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.

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.

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

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

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

More to Come.

Martyn

[Martyn K]

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).

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.

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.

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.

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 2]

Watching with interest.

[Martyn K]

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.

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

 

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

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

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.

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.

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

Exactly the same approach is used for the rudder.

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

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

 

Back to the wing

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.

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.

More to come..

Martyn

 

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

[Martyn K]

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.

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.

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.

 

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.

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.

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.

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

A shot of one of the joints.

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

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 Fairgrieve]

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

[Masher]

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 K]

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 Fairgrieve]

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 K]

Thanks Kev

Helps keep it nice and simple

[john stones 1 - Moderator]

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

[Martyn K]

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

[Martyn K]

Day 4 - Tuesday. Not much done tonight. We had builders round so the only activity was to finish off the ailerons.

Hard points have been added and a little bit of filler here and there. The hinge width has been adjusted (by carefully cutting away material) so that I can get about +/- 30 degrees.

These are inlaid into the wood by carefully marking the extremities then cutting a number if parallel slots with the scalpel about 1/32" deep. Carefully remove the excess wood by slicing it out a little bit at a time.

Day 5 - Wednesday

Tonights first job was to fit the servo rails followed by the centre sheeting. The rails are cut from 3mm Birch ply.

These are cut about 8mm wide and placed across the ribs where the first rail is fitted. Make sure it is parallel to the main spar, Mark the 2 ribs where the rail intersects (4 points) using a scalpel then carefully cut the slots in site. Take your time and check regularly that you haven't exceeded the the thickness of the rail. It should sit flush with the top of the rib.When happy, glue it into place.

Place your servo into position and then place the second rail so that it butts up to the servo like shown. Don't get it too tight or you wont be able to get the servo in or out. When happy and the rail is parallel, mark the ribs like before and fit the second rail.

Before you glue the second rail in, check that you can actually get the servo in or out, There should be about a 2mm gap.

When happy, glue the second rail in and repeat for the other aileron servos making sure the rails are the same distance from the main spar.

We can now add the centre sheeting. This is best done in about 5 separate pieces. I used one piece from the spar back to the front servo rails, one piece from the TE forward to the rear servo rails, 1 piece of cap strip in the gap at the outer edge of the servos. Measure the remaining width and subtract the width of both servos and then fit the centre piece of sheeting.

Finished.

Turn the wing over and add the upper centre sheeting.

This is cut from two pieces. However, before you get too far ahead, you need to add the Y lead for the servos and a hole to let the wire out. I use a piece of sharpened tubing to cut the circular hole (actually the tailboom for a 450 sized heli).

Feed the wires through then glue the sheet down followed by the smaller piece to close the gap. Glue and allow to dry using pins to hold the sheet down.

Allow the glue to dry then sand away any ridges.

We can now make a start on the fus.

More to come

Martyn

[Martyn K]

Day 5 - Wednesday continued

The outline of the formers are transferred to a suitable piece of 3mm lite ply using Carbon paper as described earlier.

These need to be carefully cut out. I use a Stanley knife but the trick is to cut the 'holes' out (if they are present) before you cut the external outline. Work from corner to the centre, taking care that the blade does not go into the former material which will weaken it.. Also mark any datum lines etc. but be careful you don't inadvertently cut these lines.

Cutting the inner before the outer. You can see that there isn't much material around the edge.

Use a straight edge to guide you, it makes life much easier.

After about an hour you will have a set that looks like this.

I don't have huge pieces of carbon paper so I settled on the pin prick method for the fuselage sides.

Before you start marking and cutting, make sure that the wood you have selected is the same density. By all means weigh them - they should be within a few grams of each other but it is better to check that they deflect under the same load. This ensures that when you come to pull the two sides together at the rear, the fuselage will be straight and will stay straight.

You need to make sure that the wood - under the plan is actually totally under the outline. It is very easy to get 80% of the way round than find that you are pin pricking fresh air.

Locate the wood be 'feeling' around the outline and when happy pin the plan to the wood underneath and then through to the building board so that NOTHING moves.

You will notice the little white triangles on the plan. This shows the outline of a single piece of wood. Note that there can be several sets of triangles on some plans outlining sheet sides, doublers or formers - it can get confusing so make sure you know what you are trying to cut or mark. This one is easy - just one set of triangles.

When happy, I use a map pin to plot the outline. For straight edges, you only need to mark the outlines once every 50mm or so, curves need marking every 3mm

This is the first side after marking there is no need to draw to join up the dots, they are very clear where you need to cut. When cutting, keep your scalpel vertical. This needs to be accurate as it is used as a template for the other side.

When happy, use a sharp pencil and use the first side as the template for the second and cut the second side out.

When both sides have been cut, sand them so that are the same size - this should be minimal - and then you can see if the wing fits where it should.

It does! Always a relief when something looks like its going to fit.

Finally tonight, a group shot of where we are up to after Day 5

 

There wont be much happening now until next week as I am going away to the seaside for a long weekend.

More to come next week..

Martyn

 

 

Edited By Martyn K on 26/03/2014 23:18:47

[Martyn K]

So.. back from my long weekend away, but very little time in the shed.

Next job was to add the formers and join the funeral sides.

F2 is laminated from 2 pieces of lite-ply. Note the doublers top and bottom - although I am not sure whether there should be any at the top. There is a slot for them in F1 but not F2.

A it out of sequence, but the sides are pinned down using map pins. pushed through the plan the side and into the board underneath. Nothing will move.

While waiting for F2 to set, the F1 Firewall bulkhead combination - again laminated from 2 pieces of lite-ply is drilled for the engine mount. Note that it has been offset by 2mm to the right - looking from the front so that when the right thrust has been added the prop driver will be on the centre line of the fuselage.

The rear side of F1 combination

I have decided to build in the right thrust (and down thrust) rather than using shims so the fuselage side is offset (slightly shorter - by about 2mm) on the right hand fuselage side.

The fuselage is parallel from F3 forward so it is quite easy to get this right.

With F3 fitted, it was time to bend the fuselage sides so that the curve actually fits. The 3/16" sheet isn't brilliant at bending but a light spray with water and then wiped off using a piece of kitchen towel and after a few minutes it becomes quite easy to bend around. Lots of bands to hold it down while the glue and water dries.. Note that at this stage, the rear of the fus hasn't been joined. If you look you can see a balsa brace holding the sides apart, without this, F3 started to crack.

A shot looking down the length of the fuselage just after gluing. The excess has been (mainly) wiped off.

When dry, glue F1 into place checking that the side and downthrust is correct.

With a model this size, I find it easier to get the wing mounted and then align everything against it. Check the wing is central and then mark the LE where the dowel needs to be fitted

Drill through with a 4mm drill, test fit then open out to a 6mm drill, including drilling into the web between the spars. Now is the time to sand the wing seat for a goof fit it its not perfect - mine wasn't quite right.

When happy, a cut out for the tank is needed. For the purpose of this demo, I used a felt tip - I wish I hadn't!. The cut out needs to be lined with 1/16" sheet balsa.

Finally for tonight, we can join the rear of the fuselage.Mark the centre of F1, F2 and F3 and using a long rule, line up the rule with the centre marks and pull in the rear of the fus making sure that the rear of the fus intersects with the straight edge, slot the tailplane in and check that it is level with the wing (using the eyeball).

Leave it to dry and that is where I am tonight..

More to come

Martyn

[john stones 1 - Moderator]

You'll be covering wing in blue then Martyn

good blog very well put over and informative

[Martyn K]

No comment.. I didn't even have enough sense to use a water based pen either John..

I think black may be a very good choice .

The good news is that the blue is on the inside of the fus...