Scale Soaring UK

They say that confession is bad for the reputation but very good for the soul, if that is so my soul is about to get a major boost.

Back in 2018 I built a model of the MU28 primarily for slope aerobatics and secondly because there were very few models of this aircraft around. As it turned out this model was uncomfortable to fly, to the point of being a “complete dog”. It was very heavy, the Bex1809 airfoil was a bad choice, it would snap into a spin at the least provocation and it was nowhere near as aerobatic as I had hoped. It was so horrible to fly that I didn’t use it at all last year and it sat in a corner of my workshop unflown and unloved.
Recently I unearthed it and was at the point of stripping out all the radio and consigning the airframe to the skip when I had a change of mind: why not do a rebuild? I have nothing to build at the moment and the workbench is clear so why not change all the factors that make it a pig to fly?

So what are the problem areas and how do I change them?
Too much nose weight - remedy: build a new lighter tailplane and rudder.
Wings too heavy - remedy: build new set of wings paying more attention to weight.
Replace the Bex1809 wing section – remedy: use a tried and tested section like RG15.

I will start with the tail end as every gram I lose at the tail is 3 grams of lead coming out of the front and a 4 gram saving overall.
The existing tailplane halves were cut from blue foam, had spruce spars and the elevators were driven by two individual servos. Together they weigh 294gm (10oz).
After much thought I am going to replace them with built-up tailplane halves and using an AMT system to get rid of the individual servos. Ok, I know it is non scale but I would rather have an aeroplane that flies better.
The existing rudder had a built-up structure and weighed in at 140gm. I am convinced that, with care, I can build a new rudder lighter than that.
Brian.

Looks interesting Brian - If you are doing new wings with an eye (should that be ‘aye’?) for aerobatics, what about a Ritz or Quabeck section? Or have you previously used RG15 in that context? Just assumed something more symmetrical for aeros, although RG15 is still a fast and high-performing section

Simon

Hi Simon!
As you probably well know, back in the day, we used to build all our own F3b/F3f models rather than buying them from the continent. RG15 was my ‘go-to’ section as it was reasonably quick, could carry a fair bit of ballast and turned quickly without bugging out in the turn. The fact that it was designed for flaps meant that it also had a decent thermal performance for duration tasks. The slopes where I usually fly are not large and don’t generate enormous amounts of lift. (I am getting too old to climb The Bishop hill) Because of this I have to be content with pulling tighter manoeuvres with a bit less speed.
I felt that RG15 would be a lot more accepting of these conditions. I am still considering Ritz 1.5-30-10 and also perhaps HQ 1.5-10. I have looked at some of the MH and SD sections but I am not sure how well they would handle inverted flight.
As a little aside to this conversation: A long time ago I was sitting in a pub chatting to Ralph G and his friends at a Viking Race and asked him what profile he was using on his latest racer. He took a long sup of beer and then said Quabeck 1-10, then he winked.
Brian.

Great post Brian (apart from the blatant name dropping!). Sound reasoning, and if you are more about speed and maintaining energy in mostly upright manoeuvres then RG15 makes sense. I must admit I loved the MH32 section - How many sections (other than RG15) were successful at international level in F3B,F3J,and F3F in the same model? And there were some great SD sections as well (I think it was the 7037 that worked very well when ballasted, which it sounds like this model comes with ‘built in’). Best of luck with the winter work - it is a nice looking model, and it will be very interesting to see what comes out of the chrysalis after winter.

See you next year

Simon

It’s been a wet and mucky day so I have spent most of this morning and part of the afternoon preparing a kit of pieces for the tailplane halves. The ribs and spars have been cut and will ‘eggbox’ together and the lower skins have bee cut and joined slightly oversize. The tailplane crank has been cut from 2.5mm alloy and the brass tubes and bearings are ready to be glued into place with Araldite.
I am now away back to the workshop to start gluing components together.
Brian.

A bit of real world wing section experience, Brian... I once built a 3rd scale LO 100 with an RG 15 wing. It crashed on its maiden flight and suffice it to say, I wasn't too unhappy, as it seemed to fly horribly. I've only once strayed from the 3.5 camber on the HQ section, and built a 25/12 wing on a 3rd scale Club Libelle. To all intents and purposes, this model was unflyable. My 3rd scale Habicht wasn't too bad on a Ritz section, it was aerobatic enough and had no real vices. My current 1/4 scale Habicht, running on HQ35/14-12, the section I normally use, will do all the simpler manouevres, including inverted flight, and is super-safe at low speed.
But then, you'll know all about the latter having thrown your old K11 about a fair bit...!

Chris, you are not wrong about the HQ sections as I have used them extensively over the last many years and love their docile handling and high lift thermal capabilities. However I find that my HQ_3.5-12/14/16 machines do not handle outside manoeuvres very well and inverted flight tends to require oodles of down elevator.
For my sins, I came from the F3b and F3f communities where along with thermal capability the aircraft was required to fly a long, very flat course round a set of pylons for as many laps as possible. It was then required to be loaded up with as much church roof as possible and then fly the same course over 4 lengths as quickly as possible. This usually meant arriving at the turnpoint with a full head of steam in knife edge then pulling full up elevator. I found that RG15 performed all these tasks admirably. Possibly because we were using fully flapped wings and rearward CG positions the section did very well for many years until all todays glass machines became available and it is now totally outclassed.

Today I finished off the bottom shells of the tailplane halves and prepped them for the addition of the joiner tubes. The tubes were fitted to the tailplane halves with the completed tailplane crank in between. The crank keeps the tubes correctly located and sitting parallel spaced. After much checking the tubes were glued in place and further supported with balsa packing. The upper skins were joined and finally glued in place with aliphatic glue and clamped along the trailing edge. They will be left overnight to set before trimming tomorrow.
Brian.

It’s another wet day so the majority has been spent in the workshop.
The tailplanes are looking good with only the 1/32 ply root capping to glue in place and trim.
I have started work on the fuselage and currently I am tearing stuff out rather than building stuff in.
Last night I was sitting thinking about the whole process of building a flying machine. I remembered a gentleman called Ian Dunn who got me into model building as well as full size gliding and was my mentor for most of my youth. Ian had been an aircraft engineer during WW2 and a model shop owner thereafter. When I started flying control line, my models were always overweight. Ian’s advice to me was that an aircraft should not be built to withstand crashes but should be only strong enough to withstand the flying loads that were expected. His models were always light and had terrific performance (usually with relatively low power). Yes, when he crashed (which wasn’t often) his models broke but when my heavy models crashed they always broke!
When I was building the MU I think that I had forgotten this advice as everything had been beefed up in the expectation of high winds, demanding manoeuvres and hard landings on a rough slope. During the rebuild I will try to make amends!
I was working on the fin getting it ready to take the AMT mechanism. I used the crank to mark out the slot for the actuating rod. Yes, I know, it looks like it has a rather large throw but I would rather have it larger than I need now than have to file it out later.
I have also rebuilt the rudder, this time only making it as strong as it needs to be to control the plane. Once it is covered I will be able to tell you how much weight I saved.
Brian.

Good job Brian
Hope you are keeping on top of the workshop cleaning

I'm enjoying your rebuild Brian.

If you wanted to save weight, you could replace the 2.5mm aluminium sheet bellcrank with 3 or 4mm carbon sheet - stronger - lighter - what's not to love? Except maybe the cost!....Easycomposites will take £20 or so in return for some lovely pre-cured c/f sheet in a range of thicknesses. You could glue 2 pieces of 2mm back to back to get 4mm thickness? And you'll be able to get heaps of other control horns etc from the rest of the sheet. https://www.easycomposites.co.uk/high-s ... ibre-sheet

Just an idea.

You tell me this after I have already made the crank Rog? I had the alloy sitting there waiting to be used and it cost me nothing and I doubt that it weighs any more than a carbon version.
By the way, I am sitting this evening reading a copy of Airfoils at Low Speeds written by Selig, Donovan and Fraser. I am checking out the polars and lift diagrams of various sections to make sure I have made the correct choice.
Brian.

The carbon version would be over 40% lighter (size for size) but in real terms you're right this wouldn't amount to much. Maybe drill some lightening holes in your aluminium crank

This morning I finished off the tailplane halves shaping the leading edge and tips and trimming the 1/32 ply root facings. I then spent the rest of the morning covering them with white ‘Oracover’.
I then weighed the original tailplane assembly which came in at 296gm (10.3oz). The new tailplane assembly including joiners and crank were weighed and came in at 162gm (5.6oz) – a saving of 134gm (4.7oz). This saving will mean that I should be able to remove 375gm (13.2oz) from the nose of the model and the whole model will be 509gm (almost 1lb 2oz) lighter overall.
Although I can’t be totally sure of these figures until the model is balanced when complete it is a promising start to the project.
Brian.

Well done Brian, it’s coming along nicely.

That's an excellent result, Brian.

Thanks gents. I have to say that I am finding this whole process very therapeutic.
Brian

After my last post I was at the point of fitting the all moving tail (AMT) mechanism into the fin.
I have not been in the workshop for the last couple of days as October time is harvest time and I have been stocking up my freezers and larder for the winter. However my time was not wasted as while I was lifting potatoes I was also planning in my mind the next few steps in the rebuild. I suddenly realised that if I set up the tailplane in the fuselage now I would have no ability to adjust the tailplane parallel to the wing later on and even the slightest difference tends to show. As the joiner tubes, wiring and retainers are mounted solidly in the fuselage any fine adjustments have to be made at the tail.
So as soon as I get a wet day I will start on the wing construction.
Brian.

A few days of heavy rain has allowed me to leave my other duties and get back into the workshop.
I have decided to rebuild the wings using a fully built up wooden structure as I feel that I can produce much lighter wing panels. The original wings were cut from blue foam and had great big spruce spars let into the surface before being covered with veneer attached with ‘Copydex’. There was also judicious use of glass cloth round about the root spar/joiner area. Together they weigh 2.835kg (6lb 4oz) which seems very heavy.
I have drawn new wing plans on paper using the exact same planform as before. I have also been prepared a kit of parts comprising wing ribs, fully tapered main spars, false leading and trailing edges and joiner tubes ready to start construction.
This afternoon as the rain continued to pour down I made a start on the right hand wing.
Brian.

Brian,
Do you not put sub ribs in the LE D box or do you just not bother drawing them?

Robbie, I don't generally add intermediates in the D box. On this wing the ribs will be at the same distancing all the way to the tip. So the rib spacing you see at the root continues out to the tip. In addition I use 3/32 soft/light balsa sheeting to make the wing more handleable.
Brian.

The last few wet days has seen work slowly progressing. The ribs were attached to the bottom spar. The false leading and trailing edges were added and trimmed. The joiner tube was threaded though W1, 2 and 3 then blocked in between the spars. The upper spar was then glued in place before adding the webbing. 1/16 ply webs were used back and front of the first five bays and hard 1/16 balsa webs to the back of the spars all the way out to the tip.
Brian.

Nice, Brian.

Yesterday I attached the front underside sheeting forward of the spar. After trimming I added the ¼ plywood hard points to the root rib for the incidence peg and the front and rear retention pins. I then pinned the wing to the building board to hold it straight and then added the front upper sheeting forward of the spar to complete the D-box.
This afternoon, after trimming up the D-box sheeting, I have added the rest of the upper surface sheeting back to the false trailing edge including the airbrake opening.
The wing is now upside down on the bench waiting for me to fit in the servo mounts, the block reinforcement at all the aileron and flap hinge points and the servo wiring before I sheet the underside.
Brian.

The wing build has progressed quite a bit in the last few days. I have installed the servo mounts and wiring in the wing shell. I have also added all the balsa hard points for the ‘hinge-point’ hinges to the false trailing edge. The remaining underside sheeting was added and servo access hatches formed followed by the balsa leading edge. I then started on the flap, incorporating a plywood trailing edge as well as balsa hard-points for hinges and the horn. The aileron has been carved from stiff but light balsa and seems to fit well. I am now just waiting for the glue on the top sheeting of the flap to set before final sanding.
I always have difficulty in setting up and drilling the holes in the wing and control surfaces for the ‘hinge-point’ hinges. As I have ten on each wing it was a job I was not relishing. I have ended up making a drilling jig which will allow me to drill at right angles to the TE and at the correct angle into the surfaces. As you will see from the image below it comprises a wooden frame with two brass tubes to guide the drills. The smaller tube takes a 3mm drill for the hinge shank and the larger takes a 5.5mm drill for the clearance hole at the surface for the hinge knuckle itself. I have tried it out on some scrap wood and it seems to work just fine.
Brian.

How have you arranged the ply TE Brian?
Is it 1/16” ply butt jointed to 1/16” balsa, then add & sand top skin to get a ply TE?

I’ve currently got a solid balsa TE on the Albion flap, but wondering if a built up one with ply strip would be an improvement or not.

Peter

Hi Brian

I have been using a variety of "jigs" to drill hinge holes for years and it really does make the job easier and more accurate. May I suggest a slight mod to your jig to make things even easier?

Draw a line on the top of the jig that is centered on each of the brass tubes to use as a reference. On the wing, or whatever part you are hinging, assemble the fixed and hinged part in the correct orientation and mark the locations of the hinges and then using a soft pencil and a set square extend the marks forward and rearwards just far enough so that they are visible when the jig is in position. It is now a simple job to align the mark on the jig with the mark on the model to ensure the holes for the hinges are in the correct position and the holes in both parts line up.

Stay safe.

AEB

Peter, absolutely spot-on. I don't feather the balsa sheeting before gluing - I simply pin the sheet in place and clamp the trailing edge below a long piece of spruce. When the glue is set I plane and sand the sheeting down to the ply trailing edge.

John - good call mate. I have drilled all the holes for the flap by peering down the tubes to find my location marks. I will now go out and make the adjustments before I do the aileron. By the way the jig works perfectly and I have the neatest set of hinges EVER.

The flap end the ailerons are now basically complete and have been taped temporarily to the wing. The wing tip was glued in place and carved to a pleasant shape (the full size machine had tip plates on the wings and these, on the model, were constantly getting ripped off when landing in heather and tussock grass). I have checked out the fit of the wing to the fuselage and I am pleasantly surprised that everything lines up perfectly.
As mentioned above I have drilled out the mounting holes for the flap using my home made jig and a dry check shows that it fits well and gets the amount of movement I was expecting.
Brian.

Brian,
The flap hinging looks very neat.
I will be very interested to see the result on the ailerons (top hinged?) as I may well give the installation method a go myself.

Do you use a mylar strip to cover the lower gap when flaps are not deployed?
I tend to use bottom hinged flaps & with a small bevel for up flap deflection, there is only a small top gap.
Swings & roundabouts no doubt

Do you think that your built-up flap arrangement is likely to be any stiffer than a solid balsa TE?
I like the idea of the ply TE to get a well defined straight edge & the overall assembly is probably a bit lighter than solid balsa - depending on balsa density. Harder grade balsa no doubt equates to stiffer, but heavier.

Peter, I often use bottom hinging when I need the flap to come down to the vertical. In this case I only need the flap to slow the model down and to provide additional lift on approach, the air brakes doing the rest. The main use of the flaps on this model is to work in conjunction (and in opposition ) to the elevator. I will cover the gap on the underside with white mylar.
The flap is built with medium grade straight grain balsa and are quite stiff and definitely lighter than a solid alternative. However the aileron is carved from hard, but light, straight grained balsa as it is rather long and quite thin towards the tip and I didn't want it twisting in flight.
After I have completed the covering and hinging I will attach a thin strip of white vinyl along the hinge line just to tidy it all up.
Brian.

The right hand wing is now basically complete barring the covering. I finalised the root rib area, installed the incidence peg, wired up the connector plug and drilled the retaining peg holes. The bare wing half, including flap and aileron weighs 1lb 3oz less than the original right hand wing. I am sure that the film covering is not going to weigh anything like that so I am quietly pleased so far.
I have now pinned down the spar for the left hand wing (remember left hand, left hand .....) and I will start the fettling of the wing ribs in the next couple of days.
Brian.

After a week or so of working on another project I have finally got back to the MU28 left wing panel. All the ribs are glued in place the wing joiner tube added and then the top spar. The interspar webbing has been added; 1/16 ply in the first six bays and hard 1/16 balsa (crossgrain) out to the tip. I have attached the front underside sheeting and dry fitted the electric airbrake aft of the spar. Weather permitting (if it rains) I will pin the wing down again and attach the top sheeting which should stabilise the wing substantially.

By the way, has anyone any experience with ‘Easycoat’ white covering material, I can get it in a 10metre roll from Balsacabin?
I often use Oracover but it is getting very expensive. I have tried to get 10 metres of Hobbyking white film covering but it is out of stock with no promised date.
Brian.

I have managed to fit in a little more work in the last week on the MU in between working on other projects. The wing is now fully sheeted and the leading edge added and shaped. The next step is finishing the wiring loom.
I fitted the completed right hand wing onto the fuselage and then fitted the new left hand wing in order to finalise the incidence peg position. Using my electronic incidence gauge I was able to get the two wings to match with only the minimum of jiggling about. However when everything had set I was annoyed to find there was a 0.1 degree difference! Ah well, the best laid plans!
I am going to be a bit busy next week so it may take a little time before the next instalment.
Brian.

I have managed to get some time in the last two days to get some more work done on the MU. Since my last post I have completed the wing structures, made ailerons and flaps, drilled all the hinge holes and completed the wiring harness.
The wings were fitted to the fuselage and filler was added to the fuselage wing roots so that the wings now fair smoothly into the root mounts. I was surprised as to how little filler I had to apply. The fuselage was weighed before adding the filler and sanding to profile then weighed a second time. The fuselage had increased in weight by a total of 62gms right over the CG position.
I am now in a position to start the covering process. On the first build the foam/veneer wings were covered in car vinyl which is quite thick and quite heavy. This time I am using “Oracover Easycover” iron on film which is a first for me – I will let you know how I get on with it.
Brian.

I’ve made quite a bit of progress today including a couple of major steps. With both wings now structurally complete it was now possible to fit and align the tailplane. I made a suitable length elevator pushrod from 8mm carbon fibre tube with a fixed clevis at the tail end and an adjustable clevis at the servo end. I was able lay all of this up, attached to the elevator crank which was temporarily attached to the outside of the fin at one end and the servo horn position marked on the outside of the fuselage at the other end. Once the tailplane crank was attached inside the fin I was very pleased when the pushrod fitted exactly.
I was then able to assemble the whole airframe on the dining room table and eye up the fit of the tailplane against the wings. Initially I thought that the port tailplane was just a smidgen high in comparison to the wings, but on moving a bit to my right it appeared that the starboard tailplane was similarly a smidgen high, so it must be an optical illusion.
I was then able to tidy up the sides of the fin before replacing the lower section of rudder post which had been removed to gain access for the tailplane crank. I then drilled new holes in the rudder post for the rudder pin hinges and corresponding holes in the new rudder.
The final task was to use wet and dry sandpaper on the fin and on the fuselage at and above the re formed wing roots to prepare them for a new coat of white paint.
Brian.

There was nothing much on the TV last night so boredom forced me back out to the workshop. I covered the rudder in white film and glued in the control horn. The hinges were also epoxied in place and the rudder checked for adequate movement. I installed the closed loop system, solid at the rear and with adjustable clevises at the servo end.
I have set up the rudder throws but have also dialled-in about 50% exponential for a gentler feel around centre.
I have tidied up all the servo leads in the centre section of the fuselage grouping them with very small cable ties then plugged up and located the Rx in its final position. So now the fuselage is complete ready for the wings.
I spent a little time re-calculating the CG position in the traditional method which gave me a 30% CG location at 101mm back from the LE at the wing root. I then used my computer CG app to plot the CG at 15%, 12.5% and 10% static margin(SM). I was pleasantly surprised to find that it gave 102mm at 15% SM, right on the manually calculated 30% point. The 12.5 SM calculation gave a CG of 109mm and the 10% SM a CG of 116mm.
Brian.

Hi Brian

Covering looks good. Any comment on the Easycoat product?

Cheers
Mike

Mike, the rudder was covered with some white Oracover that I had left over from another job.
I covered the underside of one wing with white Oracover Easycover. My first impression was that it was very thin and my second impression was that it was semi opaque. The film went on the structure quite well but when finished I could see all the balsa grain, the sheeting joints, etc, in fact I could see every single little mark on the balsa surface. I suppose if I was building a cheepo sport model it might have been all right. However as I was building a model of a full size glass fibre glider the finish not appropriate.
The Easycover was stripped off (quite easily) and I am presently waiting on a delivery of proper Oracover to finish the wings.
Brian.

Today, wonders of wonders, my 10 metre roll of white Oracover arrived in the mid day post. It was posted ‘first class’ on 1th Dec and I reckon that the post office must have given it a first class, round the world trip.
However, I got straight down to work this afternoon and the two wing panels are completely covered and trimmed. The flying surfaces will get the same treatment in the next few days. Perhaps not tomorrow as it is some sort of winter solstice religious festival where everyone eats and drinks too much and watches hours of TV.
Brian.

Christmas is over and we are back in solid lockdown here in the frozen north.
The hinging of the control surfaces is going quite well. I have the aileron and flap hinged on the right hand wing and they are looking quite neat. I used little wood and rubber band clamps to keep the surfaces exactly in line while the epoxy on the pin hinges set. I think that the hinge line is very acceptable.
This morning I have been setting up the push-rods for both surfaces. I am happy with the range of movement that I am getting from both aileron and also the flap. If you look at the attached photos the flap droop as shown will be more than adequate for landings.
Brian.

Today was W&B day for the MU. That is weight and balance.
I am really pleased with my efforts to slim down this model. The tailplane was reduced from 294gm to 148gm. The left wing from1216gm to 904gm and the right wing from 1180gm to 892gm. The fuselage was reduced from 2740gm to 2600 (hopefully 2458gm when final trimmed). The total completely assembled weight used to be 5.526kg (12lb 3oz) and is now 4.640kg (10lb 3oz) or hopefully 4.498kg (9lb 15oz) when fully trimmed out. This means that I have hopefully saved just over just over a kilo in weight in total.

Balancing proved to be a bit of a problem. I first set up my balancing rig to for a CG at 116mm from the leading edge which represented a static margin of 10%. When I placed the fully assembled model onto the rig it proved to be nose heavy. I had already removed all of the removable nose weight which had been needed to balance the original model (removable weights shown in an image below). The permanent weight remaining in the nose was airgun pellets which had been cast in with epoxy resin. The only solution was to get my electric drill with a 10mm drill bit into the fuselage to make some big holes in the in the moulded weight. I managed to avoid breaking through the fuselage shell but inevitably ended up removing more weight than desired.
From there however it was a simple matter to bolt back in a small square of lead to achieve the 116mm balance point. I followed by adjusting the rig to give me a 109mm (12.5 static margin) balance point by adding a further disk of lead. Finally I set the rig to 102mm (15% static margin) balance point with a final disk of lead.
As I carry out the first few trimming flights I will gradually remove these pieces of lead to achieve the final trim that I want.

Tomorrow I intend to reassemble the model and set up the control surface movements and settings. I will also take the opportunity to set the longitudinal dihedral to 2 to 2.5 degrees. I will do this by setting the wing to 0 degrees with my electronic level and then set the all moving tailplane to – 2/2.5 degrees. Hopefully that should give me a good chance of success on the maiden flight.

Well that’s it for this rebuild for now. It’s a bit too cold for me to stand on the side of a hill in a 15mph wind at present so the trimming flights will have to wait for a while. However I will post the results here when the rebuilt MU has a couple of fights under its wings.
Brian.

Well done Brian

Today I eventually got reasonable conditions to maiden the rebuilt MU28. When I got to the top of my westerly slope the wind was blowing 18 to 22mph and it was quite warm for this time of the year.

The MU was launched and rapidly gained height almost directly overhead followed by me inputting a shed-load of down trim. (It was my fault as I had been very conservative with the longitudinal dihedral) An additional couple of beeps of left trim had the MU flying around quite happily.
My initial response was that the model was flying way, way, way better than in it’s previous existence. A dive test showed that it was still a little nose heavy. A couple of rolls show the model is much more responsive, even when just using the ailerons without the flaps coupled in. Loops were big and open mainly due to a soft elevator and I will reduce the exponential and increase the movement. Stalls were a non-event with no sign of wing drop or spin. Just so much better than before!

After 10 minutes I landed to remove some nose weight and adjust trims and throws. The landing was another non event, however a massive gust of wind lifted the model before I could get to it, turned it upside down and slammed it back into the ground. I thought that I had managed to escape any damage but closer inspection showed that the servo mounting tray had broken loose as had the battery tray. Oh golly gosh I said!
It won’t take long to fix and I will be back up the hill as soon as possible.

Brian.

Well done on its first flight Brian and sorry about the mishap - these things happen. It sounds like its performance is transformed - ample reward for all your meticulous work.

I seem to think the original Mu28 had some kind of automatic flap system driven by a pendulum in the fuselage. I'll have to dig out the relevant Martin Simons book.

Best wishes,
Rog

Your memory serves you well Roger.
There is a diagram of the weighted swinging lever (in horizontal plane to react to ‘G’ forces) & damper (to prevent unwanted oscillations) connected to the pilot’s manual flap control.
The swinging lever could be locked to allow the pilot full manual control for take-off & landings plus tail slides (in order to allow the pilot to choose which way to recover).
The combination of symmetrical wing profile with automatic camber change made it easier to perform negative loops, Cuban eights and rolling circles.

It’s described on pages 54 & 55 of the Martin Simons Sailplanes 1965-2000 book.

MU28 update.
I have now managed a couple more flying sessions on my local slope with the MU and all the basic trims and flight modes have been established. I will do fine trimming of control throws and mixes over the next months to get it just the way I want.
The rebuild of the model has been totally justified as the MU is now a pleasure to fly and all the nasty tendencies have disappeared. The choice of RG15 with the CG set at 34% root chord (9% static margin) is perfect and has resulted in a fast flying and buoyant aircraft. There are no more flick rolls when up elevator is applied; to the point that consecutive square loops are a dawdle. Rolls are now a lot quicker and they stop where requested due no doubt to the much lighter wing panels (especially out at the tips). Inverted flight requires a small amount of down elevator but that is to be expected. Landings are very easy and predictable which is a good thing as landing on my postage stamp patch of grass in a sea of heather can be challenging at times. All manoeuvres that I am capable off should now be easily achieved with the MU. On its last flight I managed to gain enough height to try my favourite manoeuvre; a horizontal square 8 with half rolls in the down-lines, and was quite pleased with the result.

As a by-line to this rebuild, I have been watching Mike F’s build of the Phase 5 aerobatic slope soarer. It has been a while since I had one of these in my collection of models and I quite fancied something similar. It would have been way too simple to build my own Phase 5 so I sat down with my drawing pens and a roll of paper to design something similar, but different! I ended up drawing a “cartoon” scale MU28 with a span of 2.00 metres which makes it 1:6.5 scale. The side elevation is pretty much to scale and so is the plan view except that the fuselage has been put on a severe diet and is only wide enough to house the radio. I have used foam/veneer wings with a root section of MH32 and a tip using RAF27. The fuselage is mainly 1/32 ply sides with balsa top and bottom to achieve a more pleasing shape. The model is finished but is awaiting suitable weather for it’s maiden flight. I now have two MU28s.
Brian.

Hi Brian

Please to hear that you are very happy with the refurbished Mu28.

Your mini version has worked out well too, looking forward to reading about its first flights. What is its unballasted flying weight?

Regards
Mike

Here is an update on the little 1:6.5 MU28.
The final weight is 1.46kg or 3lb 3oz in old money. I reckon that this is quite light.
The first flights were yesterday afternoon in a 10mph wind and onto a less than spectacular slope. I managed five 10 minute flights with landings inbetween to make changes.
Although the wind was lighter than I would have wished the little MU performed very well. It is very buoyant and gained height easily. I initially had to rack in a whole pile of down on the all moving tail as I had too much longitudinal dihedral wound in. I was able to remove a chunk of lead from the nose to bring the CG back to the 10% static margin point.
After a bit of trimming and adjustment the final flight was really good with lots of aerobatics which it handled with ease. The stall/spin is very benign and it can be rotated very quickly in all axis.
I think this is going to be a fun machine, especially when I let it loose on a good slope in a decent wind.
Brian.