Extra 260

This is something I've had in the workshop for a very long time.
 
One time when our alcoholic friend was house sitting for us he managed to flood the west end of the house, while I was busy with the Wots Wot I noticed that the box for the Extra was water damaged. Hence the need to unpack and check.

I found everything so nicely packaged there was no damage except to the assembly instructions! Just dropped inside the box.
Naturally since the packaging was now open there didn't seem to be any point in putting it all away again.
The story is the same with all these ARFs, cut the excess covering away and glue, I only use 24hr Araldite, I don't like these 5 and 30min Epoxies, they never seem to set properly.
 
With 2 Elevator servos and a Pull/Pull rudder I became worried about the amount of weight in the tail
 

Needless to say, my Great Planes CoG tester was not tall enough so I had to make new test supports.
 
And discovered that she does indeed seem very tail heavy with a 150 four stroke 
 
This is not the engine I was intending to use but I couldn't find a good way to mount the ASP twin far enough forward.

This temporary lash up shows how far forward I need to go for balance
 
The recommended distance is 150mm from mounting box to prop driver.
 

Depending on air temperature, these 'fast' epoxies do set in the given time - but if you read the small print (who does?) it usually states it can take up to 24 hours to fully cure. 
Of course that depends on how well you mixed adhesive and catalyst, and how well you got the amounts correct! 

Re the c of g - - try the servos up front instead of down the back. Think of levers and fulcrums.

Eric, I used the provided mounting holes in the fuselage and the associated hardware. I did wonder about CoG but decided to go with what the designer provided. I suspect the rear end weight is to mass balance against the lump of motor at the nose.I decided to try and use the ASP160 but became very worried about the spindley 5mm mounting bolts.  While scratching around in the workshop I stumbled on the perfect solution, 
I realised that the rectangular hollow section used a a temporary support gave me the best of both worlds, rigidity, and light weight
 

Agreed. I don't like the way you have used blocks to extend the motor forward, but NOT used longer bolts that will go through the firewall as well, clamping the whole thing together.
As you have it in the photo, the mounting bolts are only just in the wood? extender blocks - being very close to the edge. Thus most of that block is doing nothing, structurally, and will fail by splitting at any vibration or torque stresses.
What is that metal plate for? If it is just adding weight, do those bolts through it just hold it to the box, or do they take any tensile load with the engine running? In which case, as Q pointed out, you are applying an outward/upward bendng load to the plate because the bolts are assymetric.

As an aside - ARTF builders are known for being very mean with the adhesives used. I'd suggest you make very sure that the firewall (painted black) is VERY positively bonded to the rest of the fuselage while you have good access to it - on both sides!

The more I look at that extender box - the more I think the model was intended for a BIG electric motor. I don't see any structure that can carry the oscillating torque stresses of an i/c through into the fuselage proper. That single central pre-drilled hole in the front of the box also says electric - it is for clearance of the back bearing, or maybe the stub shaft.

jon.in.swansea wrote:

Tony - have you checked that you can get this wee beastie out of the door to the boat room?

 

Err, damn!

All fair comments, having been brought up on a diet of Fox 19s in Combat wings I'm not at all happy with this box arrangement. This is an old Seagull model and the picture of the relevant page will put one question to rest.
 
The original intended engine mount screws were 4mm, to my mind undersized, and I've gone up to 5mm
 
that firewall is 10mm ply and firmly gusseted.
Note that I've used 5mm threaded rod screwed through the back nuts and locked with nuts. I had to remove one of the original back nuts to get my new one in.

Eric said "What is that metal plate for? If it is just adding weight, do those bolts through it just hold it to the box, or do they take any tensile load with the engine running? In which case, as Q pointed out, you are applying an outward/upward bending load to the plate because the bolts are asymmetric."
 
The metal plate is 3mm aluminium, I've used the original 4mm engine mounting back nuts to secure it to the firewall, I've threaded the holes 5mm and will put 5mm lock nuts behind it 
The standoff blocks are square aluminium extrusion.
 
They're machine cut so the cut ends are exactly parallel.
The pre-drilled holes in the engine back plate are 5.3mm which is how I decided on 5mm threaded rod (which I also happened to have in stock) I have to assume that ASP knew what they were doing when they drilled the holes for mounting.

I do like Tony's idea of gluing a bearer strip behind the plate, it's happening next.
I do intend to fuel proof everything before closing up, thanks Eric.
Any comments will be well received, I've got a very thick skin!