Sukhoi SU-26

[Martyn K]

I am assembling an ARTF Su-26 for 40cc Petrol. Its the Advanced Scale Models Kit and its OKish.. I acquired the unstarted kit from a club member who was giving up for the princely sum of £50 a couple of years ago. Kit quality isn't too bad although some of the joints in the fuse need a bit of work.

Anyway, my problem. One of the first tasks is to fit the tailplane into a slot in the rear fus. Its a nice fit so I carefully located, centred and triangulated it and marked the covering for removal. So far so good. Covering removed. Now how do I glue it in? On the inner frame, there are two ply locating slots for the tailplane. The fus has a GF moulded fairing already fitted which isn't really stiff enough to support the tailplane. If I apply Glue first, it will simply get wiped away when I slide the tailplane in. There is no way of getting glue (epoxy) into the slot afterwards.

How do you do it?

[Bob Cotsford]

I have done it with epoxy in the past by covering everything not to be glued in masking tape, slathering a good coat of epoxy where I hope to get a joint then assembling it with fingers crossed. Smears end up on the masking tape which can be peeled off leaving a (hopefully) pristine surface. Not sure I'd trust this method with 40cc up front though!

Is there absolutely no way of dribbling epoxy in via the fuselage or fin slot? If all else fails what about drilling a few discrete holes to pump in epoxy from a syringe?

[Martyn K]

Thanks Bob

I have followed the instructions which is basically what you have described and the glue is currently hardening. The fus isn't very long, so I may be able to load up a length of tube with epoxy and add a fillet on the inside edge of the frame. Something to investigate this evening I think. Its a tricky joint to get at..

The model instructions say engine size 26-35cc Petrol but I have a CRRCPro 40 idle at the moment. Probably need a 20x12 prop or similar. Wingspan is only 67".. <gulp>

Martyn

[Bob Cotsford]

My Edge 540 was about that size - with a 120 Surpass! Good luck on that one Martyn. Bicycle clips ready?

[Martyn K]

Just investigating the cost of 30cc Petrol engines.. for that very reason..

Recommended FS Glow range is 150 to 210 or 120 to 160 2 stroke glow..

[Martyn K]

I managed to get some glue (through the Elevator Servo holes) onto the lower surface joint between the fuselage and the slot in the inner wooden sides. A bit alarmed to see the glue come out the other side which suggested that the joint wasn't as good as I would have hoped for..

Hopefully, it will be OK now..

A bit more progress on this.

This shows the fairing on the very thin GF skin to tailplane join. Although this has been attached with epoxy and it all helps, I wouldn't trust this as a proper structural join

Elevators and rudder hinges use small robart pin hinges. I would have preferred something a little larger but they do seem fairly strong. Nice close gap on the elevators that will get sealed as well.

The pins were glued in with epoxy, to protect the hinge I added a drop of thin model railway engine oil onto the hinge pin that went exactly where I wanted it from a squeezy syringe type applicator. A couple of pounds from my local model railway shop that I spotted when buying plasticard and brass tube.

I mentioned that some of the inner frame joints were not brilliant. This is the worst. That side frame is meant to be slotted into the former. I am not going to try and unpick it, I'll add a fillet of epoxy though

The view down the inside of the fus. You can see how thin that GF skin is.

While I waiting for the glue on the tailplane join to set last night, I fitted the tail wheel assembly. 3 screws hold it in place with a dab of epoxy to help alleviate some of the abuse it will get. Quality of the fittings isn't too bad at all.

The main UC is held in place with 3 bolts. Its very sturdy (and also heavy). I way replace it with a CF version if I can find one.

The engine mount box. I need to mount the motor on 20mm standoffs. Of course the holes and prefitted T nuts are in the wrong place.

The old T nuts were removed and the holes filled with dowel epoxied into place.

New holes marked and drilled and old T nuts reused. I managed to put a slight axial twist in the motor when I marked and drilled it but that wont be noticeable. No downthrust and a few degrees side thrust as the firewall is skewed by 5mm over its width. The prop should be still centre in the cowl which will need to be butchered to accommodate the HT lead, plug cap and venturi for the carb.

More to come

Martyn

[Martyn K]

Quick question. I have spotted that the wooden firewall and engine mount frame don't appear to be fuel proofed. What fuel proofer is better for petrol powered airframes?

Thanks

Martyn

[Colin Leighfield]

My Top Gun Edge 540 was 69” span and I thought that had all the power I could ever use with an O.S. FX91. This one is going to be interesting. Have I heard that petrol engine powered planes don’t need fuel proofing?

[Gordon Whitehead 1]

Hi Martyn

Cellulose is fuelproof against petrol and paraffin, though I can't say what modern synthetic oils do to cellulose once they've passed through the engine combustion chamber. I'd use epoxy resin (either finishing or laminating variety) in the engine bay anyway as it'll help to protect the glued joints against seepage. Epoxy did the job a treat in the engine bays of my petrol Jungmeister and Ultimate bipes.

Don't forget to tell us when you do the maiden flight.

Cheers

Gordon

[Martyn K]

Thanks for the advice both. I have given the engine and tank bay area a coat of z poxy. Looks better for it.

Colin. The instructions say 25 to 35cc petrol so 40cc shouldn't be massively overpowered although getting the cowl to fit is proving to be a challenge. Photos to follow soon

Here is a dry first assembly to whet your appetite though. That's a 20 inch diameter prop which is slightly too big. I have ordered a 18x10 Falcon cf prop which should be quieter 

Edited By Martyn K on 13/07/2019 12:10:59

[Colin Leighfield]

Martyn, the FX91 has probably got as much go as a 25/30cc petrol anyway, so I don’t doubt you will make a go of it. I can see the attraction of the plane, it looks that little bit out of the rut. Very interesting, I can see why you want to do it.

Actually I am itching to try petrol, I have got the new version of the RCGF 10cc motor in the shed, a bit less ambitious than your 40cc weapon, but I’m itching to find a suitable plane for it. It is rear induction so need to take that into account when choosing the plane for it.

[Martyn K]

A bit more progress.

Getting the cowl to fit was a challenge, The cowl needed 3 holes cutting, Carb venturi, exhaust and spark plug cap. That too k most of Saturday and basically involved taking all the protruding bits off, fitting the carb so the prop clearance was correct then carefully marking and drilling small holes and making adjustments as they were opened out. In the main it went well but the plug cap - because it was angled - did not go quite as well

Carb Venturi

Exhaust. For some reason, I cant find the piccie for the plug cap but I'll so that later when I get the HT lead routing sorted

The exhaust will have a secondary silencer fitted.

The exhaust collector (I wont call it a silencer as that would be an offence on the ears) needed to be adjusted to fit in the curved section at the front of the cowl so was attacked with a hack saw then a piece of aluminium cut and ali brazed into place.

First cut

 

and after..

 

Finally last night, I fitted the aileron servos and horns. One change from the instructions, the push rods and servo arm are perpendicular/ parallel to the hinge line, which I think is correct.

Absolutely no slop whatsoever, I used aluminium servo arms and M3 ball links. The rods came with the kit and are very good quality.

Finally, another model shot..

A bit more to come.

 

Martyn

Edited By Martyn K on 15/07/2019 13:45:48

[Martyn K]

Forgot to mention, I have ordered a Falcon 18x10 Carbon prop from the prop guy. Apparently these are much quieter than wooden or plastic props as they don't flex. However, at £50 a pop I cant afford to break them..

Edited By Martyn K on 15/07/2019 13:47:44

[Former Member]

[This posting has been removed]

[Martyn K]

Thanks Steve

I thought that as well

I read some test reports on RCU (I think). The consensus was an 18x10 was a good match. A 20" prop is just a bit too big, just clipping the grass with the fus at landing angle.

Martyn

[Nigel R]

Manual says 780 square inches of wing, and 10lbs weight... 40cc should be "spirited" !

It's looking good, anyway

[Martyn K]

Have to admit that I am more worried about the CG and AUW rather than the power. I can always use the throttle.

My Mama Mia has a 975 sq in wing and Webra 90 on a pipe and AUW of 10lbs. That is definitely under powered, mainly because of a very bulky draggy fus. This is quite draggy as well although less wing area.

Its the less wing area that really bothers me...

Martyn

[Nigel R]

It'll be relevant at landing

But yes, that is a big fuselage on the 26 and will soak up a fair bit of power being dragged through the air.

[Martyn K]

I have a feeling that fast and smooth will be the necessary landing style. Those uc legs are very beefy, probably for a very good reason..

[Gordon Whitehead 1]

I'm waiting with bated breath to see what additional silencing you're going to fit Martyn, to keep our farmer's wife happy.

Ref landings, for more years than I care to remember I've always drooped strip ailerons as flaperons to help slow the beasties for landings. Some elevator coupling might be needed when the flaperons go down to counteract unwanted changes in attitude. My latest beasty using the setup is my Acrostar, and it was especially valuable this afternoon in the zero wind conditions. Just don't droop the ailerons more than their up-throw, and if necessary arrange to couple a modicum of rudder with aileron to help turning when the flap switch is operated. Coupled rudder isn't always needed, and is found by experiment - which needs a few successful landings of course

Gordon

[Callsign Tarnish]

I've flown many 26's, smaller, same size and bigger and I love them. The radial cowl does increase drag but what's nice is that on the down lines it doesn't build speed as quickly so you can fly all manoeuvres at a reasonably constant speed. Don't expect it to be as fast as an Edge / Extra etc. The same characteristic means you need to keep some power on for the final portion of the approach otherwise the rate of speed decline can accelerate and catch you out.

I still have one of these in a box so interested to hear how you get on.

[Martyn K]

Thanks both. I think that is reassuring Tarnish.

Regarding silencing Gordon, I have a secondary silencer that will fit onto a length of tube cut from an old Tuned pipe header which has a nice 90 degree bend. If that isn't quiet enough then I'll get a 'proper' header and canister as used by the IMAC guys. They manage to get the noise down to very acceptable levels.

Prop noise may be more of an issue - hence the CF prop.

I'll try out the drooping ailerons. How much deflection do you use?

Martyn

Edited By Martyn K on 16/07/2019 09:28:27

[Rich too]

Following with interest!

[Gordon Whitehead 1]

Hi Martyn

My Acrostar flaperons' first stage flap deflection is 5 degrees, and full flaperon is 8 degrees. I haven't needed to program in any elevator or rudder mix to correct attitude or help turning.

Aileron deflection on the flight mode I use for all normal flying and landing is 10 degrees each way with no expo, so on full flap each aileron retains a bit of up-throw at full stick deflection.

I always set up new model control surfaces with 10 degree deflection each way, irrespective of kit recommendations if it's a kit, and progress from there. The Acrostar's ailerons and elevator are just right for me with these deflections, but it needs as much rudder as possible which works out at 30 degrees.

The model flick-rolls easily with these deflections on rudder-elevator alone with no aileron input needed. ie no need for extreme elevator throw to flick-roll.

On occasion I have forgotten to raise the flaperons for the following flight (brain-fade after a fast battery change) on both this model and its predecessor, the Sebart Katana, and wondered why the axial rolls were a bit slower than normal until the penny dropped! No unwanted tip-stalls have ever occurred.

I think the main point to remember on landing is to keep the nose down a bit to maintain airspeed against the lift-induced drag which could slow the model to stall speed, rounding out for landing only when the runway threshold has been passed. This is my SOP for all flapped landings anyway.

Gordon

[Martyn K]

Thanks Gordon

That's really helpful. I will follow your guidance (as usual )

Martyn