Martyn's Mini Jazz

[Tony K]

Posted by Martyn K on 26/03/2014 22:46:02:

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.

For formers like this, I prefer to drill a hole in each corner and cut to the holes. That leaves a radius in the corner which, I think, improves the structure. I also find there is less risk of damaging the frame when cutting.

Of course, the size of the radius depends on what has to be fitted through the former.

Good informative blog for the less experienced builder.

[Martyn K]

That's a very good point Tony. However, for F2, the hole you can see above is for a square edge tank shaped. I would have to make a bigger hole - either wider or taller to get arced corners - there isn't enough material to keep the tank in the same position.I think this is why the designer doubled up this former

Martyn

[Martyn K]

Today's feeble efforts.

Before breakfast I cut and glued in the top rear decking.

This is cut slightly oversize from 3/8 sheet. The plan shows 1/4" but this was better quality and lighter than the 1/4" that I had available. Note the slot for the fin which is not shown on the plan.

By teatime, the glue had dried and 10 minutes with a plane and coarse sanding block and it was ready for a polish with fine sand paper.

The next thing I want to get done is get the wing fitted correctly. I am using captive star spike nuts. These are quite long so I have laminated a piece of 3mm lite ply to some 1/4" balsa and clamped it in position. This really needs to be secure so let it dry thoroughly. After the photo was taken I added an additional bar clamp across the fus so making absolutely sure the PVA glue was properly forced into the grain.

You can see that the balsa has been left very slightly proud. When dry, this will be sanded down to get a good wing seat contact. This makes sure that the wing does not distort when the wing mounting bolts are tightened up.

Put the fus aside to dry. These are the times to do the odd-jobs on models like this, The first one is to box in the cut-out in the wing to allow the fuel tank to fit. Before you do this, make sure that the wing locating dowel is glued into place.

The last job tonight was to fit the upper decking . This needs a small arc former - cut from 6mm balsa (the plan suggests 3mm balsa). The one piece deck is cut from 3mm balsa. The wood that I was using was too stiff so it was given a good soaking in water for about 20 minutes. This softened the wood nicely. These decks are tricky to fit. You need to chamfer one edge then roll the fus around the sheet to get the correct width. Cut it a couple of mm wider.

Plane back the other edge - wet balsa doesn't plane or sand well and tends to strip rather than shave - this means that you are likely to take slightly too much off - as I have done here. When it gets sanded back I'll add a little filler to close the gap..

The next jobs will be the wing mounting, then tailplane and fin and then fit the radio gear in the fuselage

That's about it for tonight, more to come.

Martyn

[Martyn K]

Time now to check everything lines up properly. Centre the wing and mark it in several places so you can quickly recheck alignment.

Note that the wing mounting bolt holes are also marked.

First, using triangulation with the wing mounted centrally, check the distance from the same point on each wing tip back to the centre of your dead straight fuselage.

When the wing is straight and triangulated equally, clamp it into place and drill the wing mounting bolt holes..

I also changed my mind about the wing mounting plate. Close examination this morning and I felt it wasn't up to the job so I removed it and replaced it with a plate made of 1/2" x 3/8" beech.

This was cut and drilled using the original as a template and then with the wing and bolts in place, it was glued back into position.

No photos for this stage, but we know the tailplane will be horizontal when we fit it (see above/previous posts), so we simply centre the tailplane and then measure from each tip to a central point on F1. When happy, glue and pin into place so it doesn't move.

Finally repeat for the fin. I was very lucky (or accurate), mine simply slotted in and it was dead vertical. Check though by measuring from the top of the fin to each tailplane tip. Again, the measurements should be equal.

Let it dry. You can then take a photo....

or two...

Note that I still have the clamp squeezing the rear end together while the glue dries.

OK - while the glue is really drying time for the next odd-job. The undercarriage.

Most sensible people will but the customer Carbon Fibre undercarriage from Carbon Copy. I am not sensible, mine is bent from piano wire.. The main legs are 12swg and the strain arm is from 16swg.

Its basically the same size as the carbon copy version - it just doesn't look as nice. The legs are bent up - I used my 'Wirebender' for the main legs and a pair of pliers for the 16swg strain arm.

They are bound up with thin copper wire (after being thoroughly cleaned) and soldered up. I am not doing a how to solder exercise here - there are many tutorials from far better solderers on this forum..

The wheels are held in place with a few turns of copper wire wrapped around the end of the leg and then soldered up.

Finally tonight, a 1/8" ply u/c plate has been glued into place ready to receive the saddle clamps that will hold the undercarriage into place.

BTW, you can see that when I sanded the top decking so that it faired into the fuselage the gap disappeared - no filler required

More to come..

Martyn

[Levanter]

Hi Martyn. I have really enjoyed your post and there is always an advantage in being a laggard as I can crib other's ideas. It's like sitting at the back of the class!!

I saw in some of the contributions that everyone is giving up on built up ailerons. This is where I have saved the most weight so far. 10 whole grams! I did think about the warren truss arrangement but without the straight ribs. This did not work very well with my idea to put a wide rib inline with the horn as the spacing and angles would have been odd. In the end I decided to just have straight ribs to line up with the wing ribs on the basis that as the aileron is inset and protected by the wing-tip I could get away without the diagonals. Time will tell. I am going to taper mine as that will also get rid of more weight but leave about 2mm on the trailing edge. I am banking on the model not being fast enough to have flutter problems and I bet your Kevlar hinges are really stable anyway.

I really liked your idea to put the shear webs on the aft side of the spars to increase the D section. Is there any reason (apart from possibly more difficulties in gluing the sheeting) why the webs cannot be fitted before the sheeting as I would then run the sheeting over the web as well.

I am certainly going to copy your wire undercarriage. Much more springy than carbon and I am going to need that!

Great blog and very clearly described - Thanks

[Masher]

Looking good Martyn!

Can't wait to see what scheme you choose and see how you do the fitting out

[Martyn K]

Hi Masher

Thanks. My colour schemes are always simple... easier to repair. My biggest worry is that CG looks like its going to be too far forward so am trying to shift heavy bits rearwards at the moment. Servos in the tail and the battery behind F3.

So back to planning at the moment although there has been a little progress over the weekend. I'll update later

Martyn

[Masher]

Martyn

Although I have ended up with a bit of weight added to tail, my initial flights were with some added to the nose! (senior moment I think!!!!)

Point is, I don't think CG is critical on this model so don't worry too much until after you've flown it. As long as it's not tail heavy you should be Ok

Martin

[Martyn K]

Thanks Martin, that is reassuring. At the moment, the model is balancing on the LE... Empty tank, no prop, but not covered either

Martyn

[Martyn K]

A brief update - I have spent the weekend performing end-game activities - ie flying - despite the petty miserable weather.

But some progress.

Its now sitting on wheels...

And the view from the underside. Its worth reiterating that I have uprated this undercarriage mounting plate to 3mm birch ply. The 16swg strain brace provides a nice springy (front/aft) undercarriage for those (regular in my case) heavier than expected arrivals. Small woodscrews holding it all together.

The remainder of my free time was spent getting the cowl to fit.

The first job was to trim the rear back so that the 'flange' has been removed and the curve has also gone where the flange joins the main cowl. Take care - a large sanding block is your friend. If you take too much away, you will find it will be too small for the firewall. Make sure that the base is also even - ie the cowl isn't tilting in any direction.

Next job - you need to know exactly how long the cowl needs to be.

Measure the length like this, then subtract 1mm to allow for a small gap between the prop driver and the edge of the cowl.

You need to transpose this measurement onto the cowl. The easiest way (and I forgot to photograph this - you also need 3 hands to do the photo..) is to use 2 squares. A large one standing vertically with a smaller square places against this so you have a guaranteed parallel reference to the building board. If its not clear, I'll go back and redo this. Rest a pencil on top of the second square and adjust the gap between the two squares so the pencil point is the same height as the measurement you have just made. Bring the cowl up to the pencil while resting on its base and draw a line around the cowl by moving the cowl against the pencil point, not the other way around.

Check its correct then check again then cut it to length. Save the bit you have just cut off in case you measured it wrong 3 times on the trot..

Using the same method, mark the positions of holes for the needle valve, the cylinder head and the exhaust. Cut them out using a sharp knife and lots of patience..

After a couple of hours you should end up with something like this..

The needle valve orifice is slightly offset as this was the only way that I could get the cowl to fit. The cowl sits on top of the front undercarriage clip - this is intentional to help let some cooling air through - however, I think it will need a bigger hole.

I did a quick balance at this point. At the moment, the CG is on the LE of the wing. The SC15 that I am using is somewhat heavier that the OS15 used in the original so I am planning to fit the Rudder and Elevator servos in the tail.

The plan shows that the elevator linkage is a 'Y' push-rod arrangement. Clearly I will not be able to use this so I have added a 16swg saddle linking the two elevator halves.

Bend and cut the saddle to shape. and drill a hole in the fus just behind the tailplane. I then milled out (using a 1/16" drill in a dremel) a couple of slots to take the wire. Fill the slots with epoxy and insert the sadle. Add a small strip of polythene on top of the glue and add a large clamp - let it set..

The photo doesn't do this justice it actually looks better in reality but you will end up with a mirror glass finish over the join with the wood thoroughly impregnated with epoxy. Ample strong enough for a model this size.

Make sure that both elevator halves are at the same angle before the glue sets though.

More to come..

Martyn

Edited By Martyn K on 07/04/2014 18:53:31

[Martyn K]

Progress

Servo Fitting. I mentioned in my last post that I was fitting the servos in the rear of the fus for the rudder and elevator to help move the CG back a bit...

A quick check to make sure that I have enough depth to cut the holes for the servos and slots were cut. An additional bonus is that the servo leads were just long enough to reach back to where the receiver will be fitted.

To avoid a weak spot, the servos are offset in both the longitudinal and vertical planes.

When happy that the servos fit and don't bind against each other, 3mm lite ply strips were added front and rear for the securing screws to bite into.

Leave to dry..

The lower sheet is 3/16" light balsa, cut well oversize but with the grain running across the fuselage.

Lots to sand away...

Fitting the throttle servo. the front doublers have 2 pieces of 1/8 x 1/4 hard balsa glued to them and a 3mm lite ply servo tray fitted across the fuselage. Check that you have sufficient clearance above the top of the servo with the horn fitted before you commit the glue. Its a very tight fit even with these mini MG servos.

As Levanter observed there is very little space to get the push rod from the engine alongside the tank. I have used 18swg piano wire carefully cut to length and then dog-legged to an offset servo arm. This worked better than expected.

There isn't enough space for adjusters on the linkage so all trims will need to be done from the transmitter. Make sure that you can get the throttle fully closed and fully open doesn't bind. Sort the linearity using curves.

And a dusty shot showing the situation at the engine. The throttle arm needs to be removed to get the linkage into place.

As a matter of interest (I think), I don't use Z pliers to make my Z links. I bend the end by 90 degrees. I then perform a second bend 1-2mm away at 90 degrees to that. That is easy to do holding the wire in a conventional pair of pliers. You end up with a linkage that (held vertically and working from bottom to top) goes up - then away from you for 1-2mm then turns left (or right) by 90 degrees. I simply then twist the wire so that left or right bend point upwards. You only get one attempt at the twist though but I haven't had one fail yet.

Finally, the fuselage base has been sanded flush with the fuselage sides and a small tail skid added to keep the rudder off the ground.

That just about finishes the construction. The next post will be with the model covered. I promise you, you will need you sunglasses on .

I don't do how-to's on covering as this is one area that I am useless at..

More wild Jazz to come.

Martyn

[Levanter]

Martyn - I am jealous. I wish I could make your progress. Come to a bit of a grinding halt with work getting in the way. It will be another week before I can get back on it. It doesn't have to be a how - to but it would be great to see what you do. I keep making mistake after mistake.

I really like you U/C. I bet it weighs no more than carbon or even less but much more springy and practical.

[Andrew Price 2]

What with waiting to see your colour scheme AND the long awaited maiden of Monz's Bucc, I'm a nervous wreck!!

Build just gets better & better Martyn.

Andy.

[Biggles' Elder Brother - Moderator]

Nice one Martyn. Excellent blog, a first class guide for the less experienced. No excuses now folks, Martyn;s shown you how! Its still not too late to sign up to this years Mass Build

BEB

[Levanter]

Martyn. Sorry I was talking about you doing the covering. You can't get us all worked up about the scheme and not show us your work in progress.

[Martyn K]

Time for those sun glasses..

The model is finished in day-glo yellow/green, midnight blue and white. A bit of silver and black check on the tips and rudder (it's a thing I have) for contrast. It's a bit bright.

Underneath

 

Probably worse

A couple of other shots. The cockpit has been sprayed on the inside with metallic blue acrylic car paint.

Also shows the RX switch position.

Linkages at the tail are simple wire links. Adjustment is carried out by sub trims.

The front end..

And a final assembled shot...

 

 

I am quite pleased with that. I still need to fuel proof the cowl and seams and a prop to go with that delightful pink spinner.

Then time for a maiden crash..

That won't be for a week or so though.

 

Martyn

Edited By Martyn K on 15/04/2014 22:18:27

[john stones 1 - Moderator]

Looks good that Martyn

Its boisterous

[Martyn K]

It looks far louder in daylight. Think police car colour schemes. It just needs a flashing blue light.

Martyn

[Ian Jones]

I wish I'd put a decent engine on the front of mine instead of the wimpy electric thing I chose because it's supposed to be easy - which it wasn't; I gave what's left of it to Joe Barnes.

t'uther Martyn at our field went for a maiden today and chickened out just because an aileron wasn't working and I took my camera especially. I hope you won't let me down, eh?

[Martyn K]

Thanks Ian.

Only one aileron working - wimp - two is just plain greedy, one is fine.

I am not sure what to do about surface throws though. At the moment it seems like a frightening amount (except rudder - you cant have too much rudder. I have a feeling it will be 50% expo and 40% rates.

Would appreciate some advice on this from those who have set one up before.

M

[Andrew Price 2]

I have now truly seen the light!! Hope it flies as good as it looks. Andy.

[Bob Cotsford]

who needs a flashing blue light with that spinner? It actually looks very smart if your sunglasses have a heavy tint

[Concorde Speedbird]

I don't know the throws, but if you use dual rates and set some conservative rates just to get the aeroplane in the air and then try full rates once a few mistakes high it should be fine. This is what I usually do.

Looks great, but that spinner is awful!

CS

[Masher]

Job's a good 'un Martyn, well done!

I'm still on very small throws for aileron with mine - I think the phrase is something like "she has plenty of aileron authority!".

Look forward to seeing how maiden goes - did C of G come out Ok with those backend servos?

Martin

[Martyn K]

OK, I give in - I'll change the spinner..

The CG is just slightly too far forward now - I'll do a final check when I get a prop and replacement spinner fitted. I think a bit of lead at the rear will be required though

Martyn