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MU 28

B Sharp

MU 28

Post by B Sharp »

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. :oops: :oops: :oops:
Attachments
The flying PIG!
The flying PIG!
simon_t
Posts: 89
Joined: 17 Mar 2015, 21:45
Location: Herne Bay, Kent

Re: MU 28

Post by simon_t »

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
B Sharp

Re: MU 28

Post by B Sharp »

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. :)
simon_t
Posts: 89
Joined: 17 Mar 2015, 21:45
Location: Herne Bay, Kent

Re: MU 28

Post by simon_t »

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
B Sharp

Re: MU 28

Post by B Sharp »

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. :)
Attachments
Lots of tailplane bits.
Lots of tailplane bits.
The crank components.
The crank components.
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chris williams
Posts: 1549
Joined: 10 Mar 2015, 10:50
Location: Blandford Dorset

Re: MU 28

Post by chris williams »

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...!
Attachments
IMG_1974.JPG
B Sharp

Re: MU 28

Post by B Sharp »

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. :o
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. :)
Attachments
MU update 03.jpg
B Sharp

Re: MU 28

Post by B Sharp »

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. :)
Attachments
Marking out using the crank.
Marking out using the crank.
The actuation slot drilled and filed.
The actuation slot drilled and filed.
The new rudder.
The new rudder.
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Peter Balcombe
Posts: 1399
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: MU 28

Post by Peter Balcombe »

Good job Brian :) :)
Hope you are keeping on top of the workshop cleaning :D
Jolly Roger
Posts: 573
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: MU 28

Post by Jolly Roger »

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.
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