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MU 28

B Sharp

Re: MU 28

Post by B Sharp »

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. :)
Attachments
Work continues - slowly.
Work continues - slowly.
B Sharp

Re: MU 28

Post by B Sharp »

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! :o
I am going to be a bit busy next week so it may take a little time before the next instalment.
Brian. :)
Attachments
Fitting out - servo, brake wiring and hinge mountings
Fitting out - servo, brake wiring and hinge mountings
Getting the wing mounting set.
Getting the wing mounting set.
B Sharp

Re: MU 28

Post by B Sharp »

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. :)
Attachments
Wings complete ready for covering.
Wings complete ready for covering.
The fuselage with re-profiled wing roots.
The fuselage with re-profiled wing roots.
B Sharp

Re: MU 28

Post by B Sharp »

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. :)
Attachments
Setting up the tailplane to the wings and Fuz.
Setting up the tailplane to the wings and Fuz.
The operational all moving tail.
The operational all moving tail.
New rudder temporarily hinged to the fin.
New rudder temporarily hinged to the fin.
B Sharp

Re: MU 28

Post by B Sharp »

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. :)
Attachments
The finished and installed rudder.
The finished and installed rudder.
Mike F
Posts: 114
Joined: 18 Mar 2015, 14:08
Location: Gloucestershire

Re: MU 28

Post by Mike F »

Hi Brian

Covering looks good. Any comment on the Easycoat product?

Cheers
Mike
B Sharp

Re: MU 28

Post by B Sharp »

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. :(
B Sharp

Re: MU 28

Post by B Sharp »

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. :D :D :D
Attachments
Two covered wing panels.
Two covered wing panels.
B Sharp

Re: MU 28

Post by B Sharp »

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. :)
Attachments
Aileron being attached and held in place with clamps.
Aileron being attached and held in place with clamps.
Detail of hinge line, hinges and clamp.
Detail of hinge line, hinges and clamp.
Setting up the servos, pushrods and horns.
Setting up the servos, pushrods and horns.
Control throws more that adequate.
Control throws more that adequate.
B Sharp

Re: MU 28

Post by B Sharp »

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. :) :) :) :) :)
Attachments
Careful surgery on the nose!
Careful surgery on the nose!
The lead mice have been feasting on my nose weight.
The lead mice have been feasting on my nose weight.
The removable nose weight no longer required.
The removable nose weight no longer required.
The MU balanced up at 30% MAC.
The MU balanced up at 30% MAC.
The complete model almost ready to fly.
The complete model almost ready to fly.
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