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Astro Hog 25


Martyn K
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A brief update.

While waiting for the servos, I have got on with the fuselage. The upper decking and soft block around the fin have been added. I managed to do all the decking in 4 pieces. The sequence was start at the rear quarter, measure half way round the last and the end former to the central point and add about 10%. Allow for the fact that the cut is not 90deg.

Cyanoed the sheet to the upper fuselage side, then spray water onto the outside of the sheet - allow it to soak for a few minutes then roll and cyano the sheet to the formers.Repeat for the other side. Where they overlap, run a scalpel down the centre line and you get quite a reasonable fit. Repeat for the front 2 quarters then cut out the cockpit. Simples...

Sit back and admire your work then put your thumb through the decking... AAARRGGHH teeth.. push the decking back from the inside- lots of cyano and then apply filler...

Never mind - it wasn't actually as bad as it looks - more of a minor crack.. the filler looks a lot worse because it is so dark and it has also been fuel proofed which darkened it further.

The front cowling has been fabricated from 3 pieces of soft 1/2" sheet with 1/32" ply facing front and rear. Quite pleased with it, it looks a lot cleaner than the full size with the upright engine. The inner faces have been fuel proofed, painted black then fuel proofed again. It is held in place with a long allen head servo screw top and bottom into the engine mount bearers. There will be one locating dowel on the LHS protruding from the cowl into the Firewall bulkhead.

Finally a shot looking down the fuselage.. getting ready for covering soon.

I have used goldenrod snakes for the control runs. ( I am *really* sorry about that, but needs must and time is short - it really should be more traditional). A servo tray has also been built and glued in waiting for the servos. The rudder and steerable tailwheel have been added as well.

I am a bit concerned that I am running out of time with this build - I have got another long week away in early June and I need to get it finished and trimmed for the Fly-in.

My biggest worry is that it is looking very nose heavy. With the engine fitted, the CG is still in front of the LE - I have already fitted the Rx battery on a tray behind the TE. I may beef up the tailplane a bit more. If I need to put weight at the back, it may as well be useful weight.

More to come..

Martyn

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Hi Steve

I thought about that. If I hadn't already inserted the goldenrod tubes that may have been a good option. The backend is all built up now and I dont want to hack it all aprt again..

When I finalise the plan, I'll shorten the front end by about 15mm. I dont think the solarfilm will be enough to counter for that engine mass..

Martyn

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No problem Martyn....I've deleted them.....the forum seems to be running slow at the moment & every time you press "Add Posting" it posts another copy.....so please only hit Add Posting once....even if it doesn't seem to be doing anything it actually is working....ahem..embarrassed

The SC is very similar to the OS motors...identical in terms of exhaust & mounting dimensions but like most engines these days uses ball races for the crankshaft. The OS LA range are all "plain bearing" & are lighter & generally a bit less powerful. Many moons ago nearly all engines were plain bearing....nowadays I can only think of the OS LA range that has 'em......

I'm thinking that the CoG issue is ruling out the (even heavier) 0.30cu in four stroke....(sigh) sad

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Hi Steve,

Thanks for cleaning up the excess posts and the explanation on engines..

I think I mentioned earlier that the fuselage front would have to be lengthened to encapsulate the engine and mount. (the length of the engine is > 2/3 the size of a 61 which I used in the full size). When I spotted this, what I should have done was to move F1 (mount/firewall) back to keep the prop in the same place then split F2, the lower part being at the LE of the wing and the upper part back by about 10mm to allow space for the tank.

I'll redraw the plans like this shortly, this should put the CG about right and keep the general shape correct. Not a huge change, but far too much surgery do that now.

I still think a FS30 would be perfect - just needs the gremlins sorting..

Martyn

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  • 3 weeks later...

Things have moved on, so this is the first of 2 posts today. Things have been very slow. I have had engine problems on the aeroplane that I plan to use for my 'A' test. The bench has been cleared while I got that fixed and I have also managed to get quite a bit of flying done in the lst 3 or 4 weeks.

As you could see, construction was getting to the end, so a couple more shots:

More or less assembled..

From this angle, you can see that the cowl isn't quite right, so I planed and filed it down quite a bit more to get the blending better..

The metal geared '9g' servos now fitted in the wing for the ailerons. A much better idea. The gearing on this was much higher than the plastic geared servoe - I was getting +/- 2" of throw - I am not into 3D so I moved the linkage to the lowest hole in the servo arm.

This was allfollowed by a careful rub down and painted the front end with balsaloc to make sure I got good adhesion.

which get me ready for covering and final detail stuff.

Martyn

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Which brings me onto covering and finishing. Once again, I have opted for Solarfilm. I thought about making a potential nylon bag of bits, but I really haven't got the time. Solarfilm hides a multitude of sins (like using dark filler) and that was a major influence on the final decision.

So on with the film.. I am not sure there is any benefit of photos showing the film going on, so we need to flash forward almost 2 weeks. This really has been a situation of adding a bit every time I had a few spare minutes...

Got to use up that chequered OrcaFilm - cost me an arm and a leg...

I put a lady driver in - got to be able to blame someone if I crash it... (wonder if that will get past the mods )

The bits outstanding are:

Linkages - rudder and elevator

CoG - Currently the CoG is about 25mm too far forward..

Update the plans to reflect the minor changes

the fin/rudder look too small for the model. Time will tell.

Weigh it. - it feels very light..

I'll stick some more detail shots up later

Martyn

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End of build, here is the detail.

The all up weight came in at just a 1.350Kg (just under 3lbs in old money) dry but apart from that, ready to fly.I am not sure if that is heavy or not, it doesn't feel heavy and it is a 400 sq in wing. Now to need to calculate the loading... never to sure what the units are nowadays.

The CoG came out about 25mm too far forward, easily fixed with a 50mm M8 coach bolt at the rear end. Possibly still about 5mm too far forward, but this will keep it tame for its first flights.

Some of the detail snaps..

The radio installation. I prefer Big Bandits approach for rubber mounting the servo tray. I have actually screwed each servo through a 2mm strip of foam to give them some isolation against vibration. Rx is a FrSky 2.4GHz unit with a hack module in the Futaba Tx. Twin aerials are routed out through 1mm holes in the fuselage side. I use exactly the same approach on all my powered models for the on/off switch, basically a steel push/pull wire with a pull on.

Just behind the wing mounting bolt plate, you can see the Rx battery.It *should* be possible to get it through the slot that you can see, I just hope that I never have to actually try..

Elevator and rudder linkages are very straightforward..

Another rear shot showing the tail wheel. A simple commercial item linked directly to the rudder.

The front end, although that looks light a tear in the film, it just an odd reflection from the anglepoise light that I use..

The hidden aileron linkage. Thanksto Sparks for the idea. You can also see the 1/32 balsa infill between the aileron ribs. Quite tricky to get thosein place with the aileron in place and the linkage attached. I was quite pleased with the result.

The coach bolt weight at the rear.I drilled a 8mm hole then screwed it in flush, looked quite neat, unscrewed it a bit to drop some cyano in there and I couldn't get the thing back in again as far as I would like. It isn't going to fall out!

Finally, the main undercarriage mounts, simple 10G saddle clampsheld on with long hex servo screws pilot drilled in at an angle of 30degs. The undercarriage legs and wheels came off a Gangster 52 that I crashed about 20 years ago!

That's about it. If the wind drops sufficiently then I will maiden it tomorrow morning. Fingers crossed..

Best wishes,

 

Martyn

 

 

Edited By Martyn K on 26/05/2012 22:27:10

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Well, it got 2 flights today (very breezy) and like all maidens, it was eventful. The 1st flight was almost a disaster after a Tx failure which I managed to recover about 10m off the ground.

CoG is still too far. Forward, it was a bit sluggish on elevator but apart from that it flew beautifully
2nd flight ended abruptly when the engine suddenly died and I bent an U/C leg on arrival but apart from that, I am delighted with it. Hopefully get some airborne shots later.
Martyn
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Posted by Steve Hargreaves - Moderator on 09/05/2012 12:47:39:

No problem Martyn....I've deleted them.....the forum seems to be running slow at the moment & every time you press "Add Posting" it posts another copy.....so please only hit Add Posting once....even if it doesn't seem to be doing anything it actually is working....ahem..embarrassed

The SC is very similar to the OS motors...identical in terms of exhaust & mounting dimensions but like most engines these days uses ball races for the crankshaft. The OS LA range are all "plain bearing" & are lighter & generally a bit less powerful. Many moons ago nearly all engines were plain bearing....nowadays I can only think of the OS LA range that has 'em......

I'm thinking that the CoG issue is ruling out the (even heavier) 0.30cu in four stroke....(sigh) sad

Steve, SC 25 two stroke is 72 grammes heavier than the sc 30 four stroke, which is approximately 20 grammes lighter than the OS 25 LA, it's the silencers wot does it!!!! Incidentally, the os 30 4 stroke is the same weight as the os 25 LA.

Edited By John Gibbs on 28/05/2012 08:23:26

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Hi Steve

Thanks, there were a couple of other minor problems.

I will be updating the plans, probably next week as I am on holiday for a week. One of my clubmates has said that he wants to build one as well so I will redo the drawing for him as well.

It flies easily with an SC25, the first take off was on just over half throttle and it fairly cleanly took off without any problems. I think it would be OK with .20 but it may struggle with some of the vertical manoeuvres.

I am going to add another 50mm coach bolt as ballast at the rear end, hopefully, that will sort out the CoG. The expo on the elevator was -60 (far too much) that has been reduced to -30 for the next flight.

The tailpane is flexing slightly - it needs more torsional stiffening. I'll extend the spars full length

My pilot ejected on the first flight - never found. That will teach me to make adverse comments about women drivers..

looking forward to the next flight - hopefully calmer and less turbulent conditions.

Martyn

Edited By Martyn K on 28/05/2012 15:32:55

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