MFA Robin DR400

[Martyn K]

Its a while since I did a build blog here.. Retired two years ago and moved house 12 months ago (still ongoing would you believe although I can see the end of the 80 storage/transit boxes I bought). Add in to this a brush with Cancer and for some reason writing blogs wasn't anywhere near the top of my priority tree. ?

 

One of the benefits of a house move is a new shed, my new one is 12x10 and has an island bench which is 3 feet wide x 2m long (sorry about the mixed units). Always wanted an island bench and have so far built a replacement to my Mystic 64 and also a new F5J glider (from a kit) plus the usual maintenance stuff that this hobby entails. My Fury is also progressing slowly.. Model storage is up in the loft... ?

 

This kit appeared on Facebook about 3 months ago at a reasonable price.

 

The plan is try and build a reasonable replica of this:

 

 

Hope that Glyn doesn't mind that I used his lovely photo.

 

Being an MFA kit it obviously quite old, in face the box had solarbo balsa enclosed. The kit is quite complete but with caveats - more of that later.

 

 

Its a big box.. its been cluttering up my shed since it arrived so I felt compelled to empty the box, sort the contents into something more manageable and throw the box away.. I have a feeling that I may regret that later.

 

The model has a nice polyester GRP fus and cowl and spats and was designed for a Schneurle ported 60 2 stroke. I *think* I can squeeze an OS FS70 in without having to make and horrible holes in the cowl, we shall see. If I cant then I may go el*ctr*c.

 

 

Its a reasonable size, 1/5 scale and about 1.8m span and the box says that its 'almost' scale, the almost exception being the use of elevators instead of an all moving tailplane. Well, I'll put a fix to that.. hopefully. Lots of ABS bits for the cockpit interior, I suspect most wont be used..

 

Kit contents are very complete. Surprised that for an an early kit, it has laser cut ply parts, however, no laser cut balsa parts..

 

The bits were decanted into two boxes, wood and hardware with sheet and strip standing in the corner.

 

 

and

 

 

So, with that intro, its time to make a start..

 

More to come... as I used to say.

[Martyn K]

Making a start... As I wanted to use a proper all flying tailplane I started here.

 

 

This is the tailplane plan. The tailplane will be in 2 halves joined with a  pair of CF joiners. The V on the drawing is the correct angle to align with the fuselage and the horizontal line is the correct width of the tailplane.  This make the tailplane a little smaller, so I have made it a little wider to compensate for the loss of area. 20mm wider to be precise so wont be noticeable for Stand Off scale. The full size also has two large trim tabs that will be added later.

 

 

Perhaps a better view.

 

First irritation is that there are no tailplane ribs in the kit. Just an instruction to cut 11 ribs and a sheet of balsa - after cutting your own template I assume. As it happens, it wasn't a big deal. With no elevator, I don't need all that wood at the TE so I cut a 25mm TE from some 6mm balsa I had in stock and I made the ribs longer to suit. The second irritation is that the plan refers to part numbers (in circles) but there is no correlation with the instructions or the parts inventory..

 

With lots of packing, the tailplane was dry assembled.

 

 

... and then glued into place.

 

 

As you can see I didn't save a huge amount of wood but it all helps.

 

When happy, all glued into place except for the top spar which will get glued after the joiner tubes are installed.

 

The angled centre ribs are (at the moment) strips of 22mm by 5mm balsa slotted for the spar, LE and TE. 

 

Just noticed a couple of glue blobs that need removing.

 

When trimmed, the end ribs will be sanded to shape and then capped with 1/32 ply..

 

More to come but may be a few days..

[Martyn K]

I said it would be slow. .. 

Work in progress on the tailplane halves. 

 

The two halves separated and i used a length of snake outer, well roughened which was the only tube i had that would accept a pair of 4mm carbon tube joiners with a very snug fit. 

 

 

Basically with the tailplane pinned down over the plan, the joinets and tubes fitted and epoxied in place with appropriate packing to nearly centre it between the main spars and then allowed to set overnight. 

 

The webbing between the spars added thdn the lower sheeting, pva along the le seam and thick root spar any cyano to glue it to the ribs. The sheet needed thinning slightly alond the te joiner tube but not much. With that dry, the wing tip block and ply root facing ribs added. For info, i cut oversized rectangular ply blanks and take care to get the joiner holes in the right places. Glued to the root rib with pva, allowed to set hard and then razor planed and sanded to size. 

 

With that bit done, I need to start on the horn mechanism. 

 

 

This is my revised plan. The horn is raked forward to give a little differential, more up than down. I dont think the full size was stressed for outside loops..  Note that there are no elevators with my version 

 

So, that will be this week's job. It will be tricky drilling the pivot hole to make sure the tailplane is horizontal.. No room for error.. 

 

More to come.. 

[Nick Cripps]

The "more" should include raking the horn backwards if you want more up than down...

[Eric Robson]

The drawing is correct Nick as the whole tailplane will be moving but the same effect could be achieved by advancing the servo horn forward on the splines.

[Nick Cripps]

Sorry Eric, the drawing is incorrect. When the horn is on the underside of the control surface, more up than down is achieved by raking the horn back from the pivot point. If you think about it, that is the same as raking the servo arm forward.

[Gary Binnie]

I would have thought any differential or travel limiting could be programmed in the transmitter? I would use a commercial 90° bell crank myself.

 

Just a bit concerned about using carbon tube for the tail joiners, carbon is either sound or broken (snaps suddenly). I would use piano wire here as used in many glider all moving tail arrangements and live with the weight penalty which might be offset by the engine anyway.

 

The Robin DR400 is an interesting design, I bought one on behalf of a gliding club and flew it back from Hamburg. Watching with interest.

 

 

 

 

 

[Martyn K]

Regarding the horn rake, i will need to have a think about that. I may just make it 90 degrees to be safe. ? 

 

The carbon tubes are very strong but i intend to fill the 2mm diameter hole with a length of piano wire. This stops the carbon tubes from shattering - something I have experienced in the past with an RC glider. 

 

Thanks for the tips anyway.. 

[Gary Binnie]

Should be sound as a pound using wire inners.

 

I reinforce 2 mm pushrods with carbon tube to stop them flexing, if they are really long (like an elevator pushrod) I just epoxy the threaded ends in.

 

 

 

[Martyn K]

I do the same on my classic aerobatic models to stop pushrods from flexing.. Very reassuring for very little additional weight. 

 

A bit of progress on the elevator. 

 

 

The joiner reinforced with 2mm diameter piano wire.! 

 

 

Glass Fibre Horn built (90 degrees) and locked with a sandwich of a 4mm internal diameter aluminium tube all held at 90 degrees with short lengths of dowelling. 

 

 

The tricky bit was marking and drilling the holes. Fortunately the fus has a flat base and the sides are upright so carefully marked and drilled wth a 2mm drill the main pivot. Put a length of wire through and checked it was horizontal, it wasn't so adjusted the hole with a needle file, opened out to 3mm, try again and tweaked again, then opened out to 4mm. It seems ok. 

With the main pivot in place, marked the rear joiner position and carefully cut the slots to allow the tailplane to go up and down. It still needs a bit more down but there is about 5 degrees up at the moment. It looks about right. 

 

 

 

With those bits done,  i glued an m4 washer to each side of the fus to act as a bearing and a spacer to stop the tailplane from binding. 

 

 

It now looks like this. A bit of a faff but i think it was worthwhile. 

 

More to come

 

[Martyn K]

Started work on the front end. The moulding needed copious quantities of fibre glass removing. I have taken out more than the instruction specified. The model had a dummy floor and a remaining partition behind the pilots which has now gone. That in addition to opening up the cowl air intakes was quite a time consuming exercise and i was glad when it was over.  I'll need to add a bit of stiffness to compensate for the gf that has been removed. More to come on that when I have thought about it.. 

 

 

And

 

 

I do like that cowl shape. 

There are two engine bulkheads. A 3mm ply former that sits inside and a 6mm ply engine mount bulkhead mounted on rubber standoffs. A very nice touch, shame that its not seem more often. The inner former has ply fuel tank mounts to support the supplied circular clunk fuel tank that is fitted laterally. A bonus is that all the mounting holes are clearly marked on the fus so were easy to drill. So, a couple of coats of fuel proofer and allowed to dry and the joy of trying to mount the engine. 

 

Rear former fitted no problem but using m4 nyloc nuts to support the mounts rather than the plain nuts supplied. 

 

The front former temporary fitted while I had a think about lining up the engine accurately. 

 

I rather hoped that it would fit angled slightly downwards but first I had to mark the former for the engine mount. No template for this, so this was my approach. 

 

Model on the bench and aligned so the former was vertical. 

 

 

Fit the cowl and measure to the centre of the prop driver hole. Transfer that back to the former and draw a horizontal line. The former is nicely centred so that was easy to get the vertical centre. 

 

Fortunately, i am using a circular nylon mount so i can use the notches to centre the mount. Draw around the mount. I can now simply rotate the engine and mount knowing the prop driver is still central. 

 

Mount held in place with a single woodscrew and test fitted the cowl, which didn't fit. Tried several angles but the os70fs was binding. 

 

As a last resort, I tried the engine inverted and amazingly it fitted almost perfectly with the cylinder head sat in the lower air intake. 

 

That almost just stops the cowl from sitting perfectly.

 

 

I can force it in place but that is just a recipe for a cracked cowl so I will need to remove some of the cowling from underneath the air intake. Remember the engine is rubber mounted so I will need to leave space for wobbles as well.. 

 

The black line shows  roughly how much i will need to remove. 

 

So , the question is, fellow forumites, do I get the dremel out or do i take a bow to the gods of electrickery? 

 

More to come.. 

[Martyn K]

I bit the bullet last night and cut the cowl. The good news is that I didn't need to take quite so much material out. 

 

 

From underneath it's not hugely noticeable 

 

 

And from the side its almost invisible. 

 

So with that out of the way, the firewall was drilled and captive T nuts fitted and the engine mounted. 

 

The engine is very slightly too low so I need to trim/sand a modicum off the rear lower of the cowl to tilt it down slightly. 

 

Also need to check the fuel tank level as it was originally intended for a side mounted engine. It may need to be lowered somehow. Not very straightforward on this model. 

[Martyn K]

On holiday for the past 4 weeks so nothing done for well over a month.  Hiwever,  I did get a few hours of solitary confinement in the workshop today. 

 

As i have removed the 'cockpit floor' from the moulding,  i have replaced this with 4 of 6mm sq spruce longerons terminating in a taper about 100mm behind the cockpit area.  A 2mm lite ply floor then glued to tje lower longerons just above the wing root

 

Rudder and elevator standard mg bb servos added supported in an additional 3mm lite ply servo tray and the all flying tailplane connected up. 

 

The throttle linkage also fitted along with the fuel tank.  The throttle servo is a 9gm bb mg servo secured on a foam pad. It should be ok but I'll keep an eye on it. 

 

 

The r/e servos will sit under the rear seats so should be invisible when complete. 

 

Next to fit the noseleg and shroud

This was a real faff,  mainly because there was no drill mark at the top for the uc leg to fit through so an educated guess and careful adjustment.  At least its all out of sight. 

 

The method of securing the shroud to the leg is quite neat. Uses a custom arm/axle that could be fairly easily replicated. 

Two screws hold it to the shroud and a flat on the leg locks the axle securely. 

 

With the cowl pushed on,  its starting to look quite Robin like

 

The fuel tank sits laterally across the fus,  held in a ply cradle with elastic bands.  I'll do a test engine run and if ok try and find something more permanent to hold it. 

 

Next job is to shape and fit and connect the rudder then start work on the interior.  The steerable nose leg will slso need connecting up.  I also need to find a home for the rx,  battery and power switch. 

 

Really enjoying this and pleased with the progress.  🙂

 

More to come.. 

 

 

 

[Martyn K]

A bit of progress. Not been too well over the past few weeks but i started painting the fus. 

 

 

I used klass kote from fighteraces sprayed on using an automotive touch up air gun. It really is very nice to use. Lettering was traced from a side view, it is very slightly slanted. 

 

 

Needs a little more work as i managed to mark the white slightly when i removed the lettering mask. 

 

Made a start on the wing. The wing tips and ailerons are supposed to be built on a jig. This is to build in some washout due to the extreme taper on the tips.  Unfortunately, the jig pieces were missing so i had to guessimate and make my own. One of the problem is that the chunky wing trailing edge and aileron leading edge need a fair amount of twist which loads up the structure. 

 

The left tip went ok. As soon as the sheeting was on, i removed the tip and planed off the bevels to allow aileron movement which removed some of the stress

 

 

Lots of clamps to hold that twist... 

 

The right tip and aileron was a little more problematic and the aileron bowed. A bit of dampness and splitting the join at the TE and then clamping flat for 48 hours seemed to sort that. 

 

And

 

 

I have just made a start on the centre section panels.  The instructions are rather lacking and the plan isn't much help especially regarding servo mounting for the ailerons and flaps but I  am sure I'll get it sorted. 

 

More to come. 

 

[DaveyP]

Looking good Martyn, one of my favourite aircraft. Very good build, I do like the all moving tailplane and the fact you're sticking to I.c.

[Martyn K]

A little bit more progress this week. 

Wing centre section is built in two halves and then joined using ply joiners at the le and spar. I also added a balsa brace at the TE. 

 

 

The TE floats in mid airso is blocked up and double checked with a straight edge. The steel threaded rod holds the ribs down against the packing while the glue dries. 

 

 

The wing tips go on before the centre panels are joined. Note the oversized TE has been added. 

Not sure i like assembling in this way. Much prefer rib tabs to set the height of the ribs. 

 

 

Wing halves joined. I didn't sheet this half so that i could see what was happening and try and get a decent join when the sheet does go on.  In the photo above, you can see the mounting plate for the aileron horn. Glued to the rib and also the bottom sheet. I have added a 1.5mm balsa doubler to add a bit of stiffness. 

 

 

Which led onto sheeting top and bottom and mistake number 1.

The lower sheeting should have gone out 1 extra bay. A workaround follows. 

 

 

Cap strips and shear webs and assembly mistake number 2.  Which i chose to ignore. Can you spot it? 

I have widened the servo bay to comfortably accommodate two aileron servos. Plenty of space under that fuselage. 

 

The workaround for mistake number 1 was serendipitous.  I didn't like the mounting arrangement for the flap horn (glued directly onto the sheeting) so i added a strip of 5mm balsa between the ribs and glued a small section of underside sheet to that. The horn is attached to a 1.5mm ply plate and glued to the upper edge of the balsa brace. The sheeting is only needed for the slot for the flap pushrod. 

 

 

A small strip of balsa covers the beech undercarriage block. 

 

Underneath it looks like this. 

 

While the glue is drying, i designed and started 3d printing a set of horizontal servo mounts. 

 

More to come.. 

 

[Piers Bowlan]

Looks like a nice quality kit Martyn, a shame MFA have long gone.

Enjoying your excellent build and the all-flying tail-plane modification. 
I wondered where you will mount it’s servo.

A nice flying aeroplane the DR400.

[Martyn K]

Thanks Piers 

 

The rudder and tailplane servos are mounted under the rear seats. Rudder will be closed loop.

 

The throttle servo sits between the front seats but is side mounted so only the arm may show. I still need to find a home for the reciever and battery and switch. The reciever may go in the wing. Not really thought it through yet.. 😂 

[Martyn K]

Still slow progress. I can just about manage an hour or so standing in the workshop before my back complains too much. 

 

This weeks efforts have been fitting ailerons flaps and linkages which has taken far longer than it shoukd have. 

 

The hinging went fairly well for me with reasonable tight joints along the trailing edge although the dihedral break flap/aileron junction still doesn't look quite right. 

 

 

Horns fitted as per plan, the flap uses these micro alloy indoor horns which have been reused. The ailerons use small plastic horns which I had in stock. I didn't like the ones supplied. The aileron horn looks decidely weird at 90 degrees to the te but the linkage comes straight out. Dictated by the position of the hard point. However it seems to work ok. 

 

 

flap horn

Aileron horn

 

Servos and servo brackets all went in ok. 

 

And

 

All linkages terminate with clip locks and clevises at the aileron servos. Soldered washers will secure the pushrods to horns after it has been covered. 

 

Next job is to remove the pushrods and horns, sort out the wing mounting then cover the ailerons and flaps and underside of the wing then permanently glue the hinges in.. 

 

More to come.. 

Edited October 27, 2022 by Martyn K

[Martyn K]

The wing mounting was fairly straightforward. Originally the kut had a laser cut ply sub former that carried two nylon wing bolt mounts. However, because i have modified the interior, i couldn't use this so had to settle for a single m5 steel bolt through the te of the wing. This was reinforced with a small ply plate. The Tee nut sits in a 6mm ply plate bonded to the gf fus at the end of the cut out area.  If that makes sense.. 

 

 

Two dowels locate at the le. These are glued into the ply brace. All pretty traditional stuff. 

 

A balsa wing fairing blends the gf fus into the lower wing root area. 

 

Looks better after it was sanded down. 

 

With that done, it was time for a dry assembly. 

 

 

The gap to the wing fairings leaves a bit to be desired. 

 

 

So, out with the solartex to cover the wing.  Only 11 sq m left... 

 

 

Wing now covered and painted white, i still have the red trim to do. 

 

 

Now, if you are still there, can anyone suggest what has happened to the colour scheme under the wing root and fus looking underneath.  I thought it was red, but it doesn't look very red to me.. 

 

 

I dont think its black either and its not shadow.. Very confusing.. 

 

 

Edited November 12, 2022 by Martyn K

[Gary Binnie]

I think it's unwashed oil and dirt, they get filthy under there.

 

I used to fly a DR400-180 Regent as a glider tug, we operated without the wheel spats as they filled up with cut grass and eventually locked wheels.

 

[Martyn K]

Thanks Gary. I am going to write to the current registered owner but I suspect that you may be correct..

[Gary Binnie]

It's from normal operation, not leaks, though leaks do add to the grime. It's a difficult area to clean, you have to lie on your back and use detergent that then drips in your eyes!

 

This photo is of our group-owned Tiger Moth during an engine change that I did, the large tube running vertically on the firewall is the crankcase breather which is open to atmosphere, the DR400 and other Lycoming-powered aircraft have the same arrangement.

 

 

   

[Martyn K]

It really is about time that I updated this. I had a lousy winter building season, the fracture in my back 'moved' in November and bending over a bench was a very short lived affair. It didn't help that my SD card in my phone died and I have lost a number of photos (about 3000). <sigh>

 

However, the Robin has progressed slowly. The wings finished, covered and painted and markings applied (sorry lost photo).

 

Then started work on the interior.

 

Instrument panel was 3D printed than the dials were from the freebie that came with RCM&E a few years ago. (Hint - it would be nice if that could be done again, I cant print as fine as your printers).

 

The seats were a bit of a challenge. They were cut from 9mm foam (B&Q) but sorting out that distinctive pattern was a challenge.

 

The best I could do was copy a part of the pattern from least distorted seat base, stretch it in GIMP then join the stretched parts in a chain. Printed using a cheapo inkjet printer.

 

 

The velour is felt from Hobbycraft.

 

So, two singles and a bench seat, fabricated and fitted.

 

 

I am really quite pleased with this. The cockpit rear wall is 1.5mm balsa with vinyl film, still attached to its backing, glued in place using 3M spray adhesive. This will hopefully stop it from going wrinkly. The floor and walls are covered in thin foam sheet (£1 A4 sheet) also from Hobbycraft.

 

 

Another shot. The gap between the front and back seats is too large.. I used the windows as reference but of course the window positions aren't quite to scale.

 

The pilot was 3D printed from a commercial file. Its not the best print I have ever done, its actually a glider pilot but I dont care about that too much

 

 

He looks a bit moth eaten but only because I overdid keeping the wall thickness as thin as possible to try and save weight. The seat belt clamps are a bit big - so probably are the seat belts as well.

 

A couple of dry assembly shots while waiting to fit the canopy..

 

and

 

Getting it balanced was a bit of a worry. I needed to keep the weight under 5kg to qualify for BMFA light scale comps and it was already very close to that at this stage.

I fitted an onboard glow and fitted the C cell and the Rx battery in front of the crankcase. I still needed about 40g of weight to get the CG in the place marked on the plan.

A strip of lead was screwed above the crankcase but the model would still tip backward on its wheels. I couldn't do anything with the the undercarriage as the spats would prevent that from happening. I was also a bit nervous about the amount of travel needed for the all flying tailplane. I definitely didn't want a twitchy model. Another 40g of lead was added... 😞 

 

The canopy was completed with litho strips with obligatory pva rivet spots (got to keep Danny happy) and a final assemble photo.

 

 

and 

 

The wing walkways have been subsequently added.

 

The spinner needs to be red with a white spiral but that can wait.

Time for the grand weigh-in..

 

 

 

Suspended from a tree in the garden, it weighed this much..

 

 

Drat and double drat Mutley, foiled again.

 

So its now a candidate for BMFA Flying only..

 

Two failed attempts at a Maiden. The first, the engine started and ran beautifully but the nose leg linkage was flexing too much (offset noseleg) so back to get that fixed. I tried again on Sunday at the NW Area Scale Fly-in but a leaking fuel tank put paid to that effort. Now waiting for a replacement tank. One of the joys of using NIB 25 year old hardware.

 

 

 

 

Edited May 23, 2023 by Martyn K