STOL-X(perimental)

[Martyn K]

Cheers Chris

[kc]

Most threads use more pages for this amount, so I assume Martyn does something different, perhaps reducing the file size for photos?

It's all excellent stuff and should help beginners with other models too.   Lots of tips that are not in books,  such as using the scalpel for marking etc

Edited By kc on 14/01/2015 19:18:51

[Martyn K]

Ok. Not much done on Wednesday evening, my File Server and SQL died and I have the unenviable task of trying to recover 220GB of files to the new box.

So Wednesday, I unpinned the fin and rudder and sanded them down - and forgot the photos

Back in the shed last night for another hour or so and started work on the tailplane. This is very straightforward, the only tricky bit is cutting out the curved end pieces

So.. pin prick the outline and make one pair. PR shows the last rib (actually 1/4 x 1/8 strip balsa) passing through the 1/4 sheet tips. I am not sure why he has one this..

I decided to cut them as one piece and worry about it later. So cut one pair and use these as a template for the second pair. Cut slightly oversize and sand back to the line.

You will end up with two matching pairs.

 

The plan shows that the leading edge join in the centre is a butt joint.

I prefer to use a scarf joint as this is stronger although a little bit more difficult to cut.

The easiest way (I think) is to cut 2 lengths of overly long LE pieces, overlap on top of the plan and then mark and cut. If you leave plenty of length in the strip you can make several attempts to get it right. It's worthwhile doing this, this joint is important, you don't want a tailplane to fold in mid air (I speak from experience).

I have already sanded the fin and rudder down, you will need to slot the base of the fin to drop into a slot in the tailplane - which will be cut exactly to size.

 

 

Assemble the tailplane - this time working from the elevator join forwards - symmetrically and sandwich the fin between the two centre ribs - but don't glue the fin in place yet.

Check that it is vertical - adjust accordingly.

Sorry - not sure why its on its side..

Then add the remainder of the ribs.

You can see where I have simply butted the last rib to the tip pieces rather than separate them. I cant see what difference this will make structurally. The peg clamp makes sure that the LE splice joint doesn't come apart while the glue is drying

I'll trim the excess wood with a Razor saw when the tailplane is lifted off the plan.

More to come - Sand and fit the hinges then starting on the fuselage next

Martyn

Edited By Martyn K on 16/01/2015 11:23:43

[Tony Bennett]

nice work sir.

coming along nicely.

[Martyn K]

Thanks Tony...

A little bit of progress over the weekend. Saturday was spent working on the 2nd man cave - my motorcycle workshop and yesterday I was at the Manchester Caravan Show. I do have a sad life outside model aeroplanes

The tailplane has been unpinned and gently sanded but on close inspection, I was unhappy with the strength so added this spar from 1/4 x 1/8 tapered to 1/8 x 1/8. Still needs a bit more reinforcement around the LE as well

So.. making a start on the fuselage. I thought that I had better check that my engine (OS26FS) will fit..

It looks like it will be OK but the cowl may need modifying slightly. I am going to use a commercial mount rather than bearers.

I have never plumbed a 4 stroke before. It has this pipe from the crankcase - the breather I assume, but I am unsure what to do with the pipe. Does it go to the exhaust or is the exhaust nipple used to pressurise the tank as per 2 stroke practice.. Answers on a postcard etc..

So now I happy that the engine will fit start on the fuselage.

The longerons are specified as medium hard 3/16 square. This means that you should only just be able to mark them with your fingernail when you push it into the wood. Select at least 6 (or more) fairly hard lengths of wood. The next thing to do is match them. To do this hold all 6 at one end and spread them slightly - fan like. Whip them up and down gently. The softer pieces will deflect more than the harder ones. You really need 4 that deflect the least but all deflecting a similar amount. Pair them off as upper and lower longerons for each side. You may have to discard some but these can be used as spacers.

When happy, cling film the plan and pin the longerons down. Don't make the mistake that I did though...

The very front spacer that sits behind the firewall stops at the middle longeron.. It's not clear until you check the plan view of the fuselage.

It should have looked like this..

Anyway, carefully fit the spacers. These should be a snug fit that does not distort the longerons when inserted.

After a while - it will look like this. Leave it to dry thoroughly. Note that I am using PVA for all my wood joints.

When dry, carefully unpin and then apply a layer of cling film over the completed half then build the other fuselage half directly over the first half. This way you should get 2 identical halves which should line up nicely when we fit the cross spacers.

While this is waiting to dry, I fitted the hinges for the rudder and elevators. A task I absolutely hate doing.

I bought one of these SLEC gadgets to assists with the alignment.

Its a nifty little parallelogram with a slot that helps centre the blade. These special 'V' blades are supplied with the kit but I cant seem to manage them very well.

I simply clamp (between the fingers) the parallelogram over the hinge boundary and use my scalpel to cut the slots. It sounds simple, but this is something I always struggle to get accurate.

However, with a bit of perseverance..

I can get them to fit and they appear to be centred correctly..

and done..

Last task last night was to start cutting the formers. It looks like there is an error on the drawing ( as mentioned in one of the other STOL threads), the formers drawn on the plan are incorrect.

Here is the ply firewall former ready for cutting though. Marked out using carbon paper and the outline cut using the bandsaw and the centre hole drilled with a 10mm drill and opened out using a fret saw then a file.

Mor

[Bob Cotsford]

ref your nipples, the breather vents to atmosphere, exhaust pressurises the tank as per 2 stroke practice.

[Martyn K]

Thanks Bob, I suspected that may be the case but I wasn't too sure if crankcase breather was recycled in some way

M

[Bob Cotsford]

Most newer and larger 4 strokes feed the breather back to the inlet tract either through internal channels or via an external pipe to the carb manifold but I've got various brand engines that just pump gunge over the underside of the model just like your OS. Those include ASP, RCV, Laser and Saito.

One trick is to collect the gunge in a small (1 or 2 oz) tank but then you need room to store the collector and access to empty it. The Stol looks to fall short on both counts so just duct it out the underside where the gunge won't show as much

No kevlar sheet hinges this time Martyn?

[Martyn K]

Cheers Bob

I have plenty of Kevlar sheet, but not on this model. As this one is meant to be a tutorial type blog, I have tried to use readily available tools and materials. With hindsight, possibly a mistake

Martyn

[Martyn K]

A bit more done last night.

First a photo of the two halves on top of each other with the cling film liner.

One thing you do need to watch out for. The intermediate layer of film is quite loose - I managed to trap it between one of the joints. As it is not meant to stick to anything, it didn't add much to the structural stability.

If you follow this, make sure that you pull the film tight after laying it out over the first side.

The front Former - F1 - is about 8mm too tall and will need to be trimmed top and bottom to fit. This former butts up against the rear of the two cross pieces at the front of the cabin.

Whereas the rear of the two formers, F2 - at the rear of the cabin is simply sandwiched between the two fuselage sides. You will need to make sure that these are vertical or the other side wont line up correctly when you attach it.

This shows F1 clamped and with a square and battery holding the Former vertical. The second side has been added before the glue dries and the rear is clamped together to make sure that the two halves aren't skewed

Don't glue the rear yet - it needs formers inserting..

However, you do need to support the fuselage strategically to prevent a banana forming. I used a craft knife but anything will do. As the longeron material should be quite hard and there is very little weight, it shouldn't distort very much.

 

While all this was drying, I started work fitting the engine mount. I had already cut out the Firewall bulkhead (3mm birch ply) and given it a couple of coats of fuel proofer.

The plan doesn't show any thrust lines other than those down the datum. This is a bit unusual for a high winger which usually have a little right and downthrust is required and on this occasion, I am really hoping that the draughtsman hasn't had a bad day.

Downthrust is quite easy to add at a later date, however, adding right thrust on a centrally mounted engine will skew the prop arc off the centre line and it looks odd.

So working from the plan, a cruciform drawn onto the former marking the lateral and vertical centres. The SLEC mount has some nice cutaways under the mount which are used for alignment which you can see in the photo below. I used a 3mm drill spun between my fingers to mark the former then drilled that 4mm as well.

 

The SLEA mount needed re-drilling - 4mm - the unused holes are just a little too close to the centre fuel pipe hole

The bolts are simply run through from the rear. I have also placed a spot of epoxy around each bolt head.

Finally last night, I added the 3/16" cross sheeting between F1 and F2 - grain running traverse and doublers are also added at the top of the fuselage. Note the weights to clamp it all together while the glue dries

I wont get much don't today as it is club night tonight so the next update will probably be Thursday.

More to come

Martyn

 

 

 

Edited By Martyn K on 20/01/2015 09:57:35

[Martyn K]

I managed to get an hour or so in the shed yesterday.

We can now get the cross members added. I use a SLEC building jog, this has been another very worthwhile investment, it doesn't get used much but it makes life much easier when building a straight fuselage.

Assemble the jig and clamp the fuselage into place.

Use a square to ensure that the fuselage is central at the rear..

Also check that its not twisted. One the end piece has been glued, it should be possible to lift and the flip the fuselage and it should still fit between the clamps perfectly. It really is worthwhile checking this now before you put the rest of the crosspieces in.

Cut and add F3

The slots in the former sides are for doublers that will be added later

The next bit is slow and time consuming.

Basically, cut and fit - {you will need to sand a chamfer on each) the lower set of cross pieces. However, before you glue them in, cut an identical one for the top but don't fit it - yet. Work as far down the bottom as you can until you cant get your hands through the gap.

Then add the top pieces. They should all fit perfectly..

As soon as this is dry, flip the fuselage and add the remainder of the now top pieces, then flip and complete.

I need to start thinking about the radio installation, I was originally intending on rear mounted servos but as the engine is rather light and there is plenty of space in the cabin area I think that I will fit snakes.

More to come

Martyn

[Martian]

Excellent tutorial build log Martyn

[Martyn K]

Thanks Martian

One thing I forgot to explain was why I do the bottom pieces first. Basically, as you fit the cross piece, I rotate the fuselage so that it is touching the board where the cross piece will be fitted  so that when I push down on the cross piece, I know that it is flush with the outer edge of the longerons and also slightly rotated to allow for the curved taper towards the tail.. Helps to get a good fit

If you do them alternately, top/bottom, then the problem you get is lack of access, so do the bottom first then the top

 

 

Edited By Martyn K on 22/01/2015 13:41:51

[Toni Reynaud]

I was given one of these last year. I fitted a Jeti 45 3 700 kv motor running on 4 cells and 11x5 prop, and it flies at half throttle most of the time. I can't remember what the power was, but I think in the region of 350W. There are no wingtip slots, but it does have the flaps.

My particular example is horrible to fly - one would expect a vintage looking high wing moderate dihedral plane to be releaively forgiving and stable, but this one isn't (not under my control, anyway.). It needs to be positively flown all the time, and it has a really vicious tip-stall tendency which can catch one out now and again. The flaps are fun to play with, but I have yet to get a really successful and relaxing flight from it. I did use it to drop some poppy petals during our Remebrance Day event, which wasn't bad, but it isn't my most favourite plane.

Just thought you'd like to know!

Edited By Toni Reynaud on 25/01/2015 09:40:24

[McG 6969]

Dear Mr K,

Hello Martyn,

I'm a silent witness of your build as I desire to learn as much as possible regarding balsa construction.

So far each of your steps turned into a new lesson for me.

Allow me a question though: when you unpinned your horizontal stab, you decided to reinforce it with some triangular(-ish) spar on the top. Looking a bit closer at the concerned picture, it looks like the front angle of the leading edge of the stab is not on the center line of the fuselage (in relation to the small pencil line marked on the spar).

As I don't think it being some picture parallax issue, is there some technical reason or explanation to it?

Many thanks & keep on posting.

Chris

Brussels, Belgium

[Martyn K]

Toni,

Thanks for the update, that's the first colour photo of a STOL that I have seen. I wasn't aware that it had any nasty vices - I enjoy a challenging model to fly but thanks for the warning

Chris - its not really meant to be like that, its an artefact of a splice joint rather than a butt joint. Its displaced to the left slightly because the right side LE passes over the left LE. Any errors in wood thickness - it looks like the piece that I cut for the left side may have been slightly narrower and this error is magnified because of the angle of the joint.. Its not worth worrying about although I agree it would look neater if it was more accurate.

Not that much done since last week. Had rather a busy weekend and only really managed to get some quality wood time last night and Friday night

so..

One area where I really wish that I had done things differently. The plan shows that the front of the fuselage is deflected in - but no cut marks are shown

My wrong solutions was to half cut and crack the joint as there was no way that I was going to deflect those longerons by so much without an uncontrolled fracture. In hindsight, it would have been MUCH better to saw the front off completely and remake the joints up properly. But - we are where we are..

So sanding a small chamfer on the uprights where they join the firewall, align and glue and clamp it into place

Checking that the centre line is maintained

Leave it to dry - in my case for 2 full days..

You can put the fuselage jig away now...

There is 1/16" ply Undercarriage plate that runs from the firewall backwards. As far as I can see, this carries the landing loads and also transfers the load back from the engine through the airframe. To improve the glue contact area, I added 2 large gussets from F1 forwards

and also a small length of triangle stock across the base of the firewall

The top braces were also added last night. A bit fiddly, you will need to chamfer the slot in the top of the firewall to get a good glue joint..

Back looking at the plan...

I have decided to add a small gusset at the upper junction of the engine bay former with the upright carrying F1 and also extend the lower infill from the Firewall to F1 so that we have a linear load path from the engine bay through to the infill that sits in the cabin area under the wing. I really don't like stress discontinuities. I will also add another gusset backwards into the cabin area from the new upper gusset I have marked above

You can see that the 1/16" ply undercarriage plate has been added as well. The rear edge of the plate has been sanded down to a near point to help it blend in with the lower fuselage - photo to follow..

It probably worth mentioning that all these little tweaks to the design are adding weight. Individually, not a great deal, but cumulatively they may be significant. The wood I tend to use for gussets and infill is fairly light, but if you are following along don't get too carried away. I like my models to be fairly rugged, I prefer to build than spend time carrying out repairs and would rather correct what I perceive may be design flaws rather than just keep my fingers crossed that every landing/arrival with be a 0.1G floater.

More to come...

Martyn

[kc]

Toni seems to imply the STOL is difficult to fly but Peter Russel said in his article ( it's online here) " As far as normal flying is concerned, if you can fly at all, then you can fly the STOL" and many other people built them too.

Maybe there is something wrong with Toni's ? Maybe a warped wing or tailplane, or overweight?

Edited By kc on 26/01/2015 11:45:13

[Martyn K]

I have to admit that I was rather surprised by that. My only thought was that Toni's model was slat less and I remember that PR spent a lot of time optimising the wing for use with slats at slow speed. It could be that they are important for the slow speed handling and are not just ornaments..

M

[kc]

It would appear that without slat is OK PR's article says " Slats again were not part of the original concept – it had been hoped that the new wing section wouldn’t need them. As with the Mk.1 however, the ultra-low airspeeds that were possible with the new plain flaps down still caused a certain amount of uncertainty in the yaw/roll area. So although the need is not desperate, slats of the type suggested on the drawing will be incorporated in due course. "

[trebor]

Would you recommend this build to a novice.

[Martyn K]

Hi trebor

That is an interesting question. It is not an overly complex build, there is just a lot more to it than the plan suggests. It is very similar in construction to models that were designed early post war, hence it is also quite a cheap build.

If you are competent with hand tools and confident that you can cut and sand fairly accurately then by all means have a go at it. Getting the correct wood selection is important. I would expect that it may be a slow build for you but with the assistance you will get here if you get stuck, I am sure you could make a reasonable job of it. I would suggest that its is possibly an easier build that a Jocasta but slightly more complex than the DB Tyro range (because of the open/lattice fuselage).

Good luck

Martyn

[trebor]

Did I see a link to a wood kit somewhere ?

[kc]

The Tyro would be more suited to a new flyer it's tough and simple, easier to build. Likely to withstand hard landings while a STOL will take longer to build and probably won't stand the continual heavy landings. STOL could be a 2nd or 3rd model but at that stage you might want an aerobatic design.

Tyro Major is a plan pack - plan and ribs for 17.50 from DB Sport & Scale, you supply your own wood

[Toni Reynaud]

I agree with the choice of the Tyro - my second plane ever was the Tyro Major back in about 1976. Easy to build and a very good beginners plane.

The problems I have with flying my Stol are probably of my own making, but it's nowhere near as nice as my Depron Wot For look-alike in the air.

[Martyn K]

Trebor - I would agree with kc and Toni that the Tyro would be a much better first build model. The fuselage construction is far more simple or less fiddly. However, if you do want to build a STOL then there will be people around who will help you.

M