Multiplex Acromaster

This is a real photo taken by Darren against an evening sky 
 

This is multiplex's design for a 3D aerobatic model, it succeeds and a lot more, I first came across this model when Stan asked me trim his out, we set up Stan's on the conservative side and softened it with 40% of exponential,  it needed no trim adjustment and flew so well doing axial rolls with no other input than the ailerons, I knew then I had to have one.  It is made of Epalar foam which seems to have the lightness of expanded polystyrene with the the strength of durability of EPP

The Kit comes in an impressive box the sort that attracts children, how ever old they are, it is also very well packed and would be very unlikely to be damaged by the clumsiest of carriers. I bought mine at the Blackbushe  airshow 

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the box, I think it says 'for masters and those who want to become one'

the parts, nearly a hundred

The motor I wanted to fit was a Hacker A30 14L, but the man from West London Models insisted it needed theA30 10 XL  to perform properly. The motor felt very heavy for a light model, it works out well because it sets the C of G perfectly. I bought a 14 x 7 prop and this also looks on the large size, attempts at running with a watt meter have stopped at 250 watts due to cowardice.   At the moment I think this motor is too big and I am considering fitting an A30 12L

Battery
I have used a few 2200 ma 3 cell cheap lipos from Hong Kong at about 10 a time, these work fine and just get warm. I have used 1500ma 20C as well, cells up to 4000 ma could be used, there is plenty of room and it will carry the weight. very little full throttle is needed so the lipos have an easy time.
You will need 4 batteries to keep it flying, one in use and 3 on charge at 1 C, as you will not want to stop flying 

Servos
I broke two micro servos and it now has 4 Hitech 81MGs

Speed Control
This is old 60amp Tower Pro, the throttle curve needed a bit of exponential to soften it at small openings, it has a nasty habit of cutting the power to near zero when the battery is exhausted it would be better if it gave some sort of warning like oscillating 

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 The Hacker fitted to the adjustable mount

The A30 XL has a lot of wire tightly wound

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The battery compartment, big

The Wing comes in 2 parts and needs gluing on all touching surfaces, two carbon tubes join the two wings

Building 
The kit is very easy to put together with very clear instructions and numbered diagrams. It is glued together with medium super glue, not foam safe. The only tools needed are a small knife and a screw driver.
The only thing I changed was the elevator snake, fitting a carbon push rod instead. Very little skill is needed except for fitting the the hinges, take care to get them is the centre and wipe the glue off the hinge

Radio set up
I used my old Futaba FF8, all travels to 100%, 40% elevator to ailerons (snap flap) and 40% of exponential to soften the controls. I adjusted all linkages to the correct length so no sub trim was needed, this will allow the full movement of the servos in both directions. I have never been a fan of dual rates but  this model needs them, one for 3D and one for patterns

Flying
For the first flight Darren followed me to the strip with my camera, this was embarrassing as the prop flew off while it was 4 feet high and pointing up 45 degrees, Darren supplied a spanner and a second attempt with half throttle, it leapt forward and vertically up.
The only surprise is that no down is required for inverted, at first it seems strange. Rolls are lightening fast, inverted is just the same with a symmetrical wing, prop hanging is average but I can not hold a 45% high alpha without wobbling the wings
The Acromaster does all the eights with ease, the fast axial rolls make the Cubans look good and with a symmetrical wing the outside loops are just the same as the inside ones. I was a little disappointed with the spin, it is not as flat as I would like, the inverted spin is better much flatter.
The model flies just like a park flier as low speed and and will land slowly with the tail dragging. It will do 3D but not as well as the ultra light modern balsa/carbon models or can used to practice pattern flying, not to forget the best part, just for pleasure and fun.
I found the model  hard to fly at speed with a reward centre of gravity , this can easily adjusted by moving the battery forward 

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First flight without the decoration, take off  in less than 3 metres and will climb vertically

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knife edge is easy with very little correction needed

A bit wobbly at high alpha

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a poor landing liable to break the prop

Prop hanging, fairly easy  

Two faults have been reported with Acromaster, the motor mounting breaks out and the undercarriage is weak, both of these faults may be due to poor landings or oscillations building up at certain motor speeds.

Further reading, there are several detailed expert reviews of the model on the web type in  acrmaster review to a search engine for more.

Specification:
Wingspan 950 mm
Overall fuselage length 500 mm
All-up weight min. 920 g
Wing area 36.6 dm2
Wing loading (FAI) min. 25 g/dm2

Phil

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