Avon Sabre Spirit 78

[Harry Twist]

Very tidy Steve! I like the servo mounting frames.

[Phil Cooke]

Lovely work Steve, and some lovely hardware too! Very nice!

[Steve Houghton]

Thanks, Harry and Phil.

The KSTs were a very reasonable price from Phoenix models, comparable price to HS-65MGs, and I bought the HS-125MGs for a bargain price during the Barnstormers' closing down sale.

I'm trying to find anything else to do before returning to the complicated tail end bearings and fairings - I'll probably finish skinning the wings and mounting the fuselage servos next.

[Peter Garsden]

Very novel solution Steve. Wish I had thought of that. You won't be disappointed with the KST servos. I have used them extensively on many mouldies with much more torque requirement than you are using, in fact I think they are rated at about 5kg or thereabouts. Loads more than you will need. The only servos I have ruined with flaps not being cancelled before landings are Savox 255's. and Savox gave me my money back.

Good luck with the Hitec analogue servos. Still good, but not as good as the KSTs.

I have used Savox 255's for the flaps, so I am breaking my own experience. To be honest I don't think we will need flaps other than for thermal/reflex use maybe. Mine don't go down very far on crow anyway.

[Steve Houghton]

Pete,

Thanks for your useful experience comments about the servos. I know the KSTs are OTT on power, but they weigh only a few grams more than micro servos - 24g each.

I've just found that the HS-125MG arm mounting screw doesn't fit on one of the servos, and I can't work out what the thread size is. The screw from the other servo fits either, so the thread in the output gear isn't damaged. Trying other screws I found that M2 is too small and M2.5 is too big, so what can it be?

I may need to buy a replacement horn & screw set. It looks like HS-125MG has a unique set.

[Peter Garsden]

How annoying Steve. Have looked in my spares box and have 2 complete sets of arms mounting screws and Head bolt for the Hitec 125mg servo. If you pm me your address I can post it to you. Or just the horn bolt if that is all you need. We have to look after each other in these strangled times.

[Peter Garsden]

I probably have bits for the KST also as I have used that one many times.

[Steve Houghton]

Hi Pete,

That's a very kind offer, for which many thanks. However, I've just placed an order for some spares.

Looking online I've discovered that the screws for HS-125MG are M2.2, and standard Hitec servos are M2.6. No chance of sourcing these from my 'regular' size hardware trinkets box then!

So far, all the KSTs fit together nicely. I'm having a go at programming my JR DSX-9 TX today. It'll be the first time I've programmed any mixes for flaps/crow brakes etc., but the manual has a good section about setting up a four servo glider wing.

Regards, Steve

[Steve Houghton]

A Screw Loose

I'm pleased that the replacement screws for the HS-125MG have now arrived and one has been successfully fitted to replace the stubborn one.

Hatch-It

The aileron servo hatch covers are supported on 1/32" ply panels with 1/8" ply lugs for the screws:

A small border all around with three strategically placed screw mounts, which just about allow the servo to be inserted. This was always going to be tight, with round hatches limited in diameter by the rib spacing:

Like the proverbial square peg into the round hole, the servo nestles neatly into place:

Batten down the hatches

Hatch covers will be neatened up later, and will have pushrod holes cut appropriately. I thought about Martin and Gordon's hidden pushrod idea, but don't think I'll have clearance for the servo arm to make it work:

Graffiti

I hope no-one looks closely at the reverse side of the hatch covers, but I laminated an annular ring of 1/32" ply onto the 1/32" disk, making 6-ply in all, which is slightly more resilient than a single layer of 3-ply 1/16". The centre of the ring was in-filled with 1/32" balsa, then someone came along and scribbled all over them and left epoxy finger prints - sorry!

Roll your own

I used the old method of rolled gummed paper round a balsa strip former to make oval tubes to guide the servo wires complete with plug, leaving the tube former in place whilst glueing, to prevent tube collapse:

The tubes are just long enough to let the plug reach the inner end:

Hinge blocks added, and the two wing panels are ready for sheeting and joining:

[Steve Houghton]

Root blocks

I nearly forgot - I added a few extra blocks around the wing dowel area. Some in-fill between R1 and R2 to prevent R2 from distorting or collapsing sideways when drilled for the dowel, and small gussets outboard of R3 to support the LE a little more:

[Steve Houghton]

Pushing On

The wings are now sheeted and joined. Foolishly I sheeted the left panel before joining them together. I feared a poor join coming up, but thanks to the fabulous G&M wing jig I was able to pin and weight the panels into the jig absolutely flush together whilst the epoxy set, without being able to clamp the two R1s together:

Straighten up the Ply Right!

Having completed the remaining wing sheeting, I trimmed the wing seat in the fuselage to see how well the leading edge fitted against F4 before fixing the 5mm ply facing. It was a poor fit, and rather than sanding away at the wing I glued a shim of 1/16" balsa behind the ply. I could then dry fit this and sand away at the balsa shim to level it up against F4. When satisfied, I finally glued the ply into place. The shim is just visible here, needing about 1/16" on one side and virtually nothing on the other. This is the bottom of the wing:

Fattening Up

I bought 3/8" thick TE stock but found it to be slightly undersize for my wing. I laminated a layer of 1/32" balsa to the underside of the TE, where it wouldn't show. I then sanded the top of the TE to fit, rather than risk thinning the lamination to nothing and leaving a thin glued edge that would probably lift in places:

What, no torque rods?

The torque rods will need to be flush with the lower surface of the wing to cope with the bottom hinged flaps. I thought it best to fit the root TE first, and then cut slots underneath for the full length torque rod tubes afterwards. That way I was more likely to get the TE correctly aligned with the wing profile, having the full depth of TE to glue on. Here's the top view:

The slots have not yet been cut, until I finish the wing seating area.

(is it June already?!!)

 

Edited By Steve Houghton on 04/06/2020 23:27:34

[Steve Houghton]

Ah, The Torque Rods!

Nothing very exciting tonight. Just trying to convince myself that I am progressing. The flap torque rod slots were gouged out on the bottom of the wing using the traditional sharpened brass tube, plus a round file:

The ends of the root TE were reinforced with 1/32" ply for ding-resistance, and to support the torque rods:

The torque rods had full length plastic tube bearings which were buried just below the surface, allowing a finishing filler of 1/32" balsa to be inserted to cover the slots, after the application of a smear of vaseline to the sensitive rotating areas:

Chiselled ends to the rods, to minimise the risk of bursting out of the flaps:

Ball links on the rods, and kwiklinks to be used on the servo end, although not yet trimmed to length and fitted. I noted Phil's comment about possible trim changes from the horns rotating, but mine are far too stiff - would need brute force to twist them round.

[Steve Houghton]

Make Way

F6 has now been trimmed back to make way for the flap torque rods. I added a cross-brace after the hatchet work, to compensate for the removed rigidity:

Are you Seated Comfortably?

I followed other builders' suggestion of lining the wing seat. In my case I cut the fuselage sides back by 3/32" above the wing using Steve McLaren's idea of pinning strips of balsa to the wing to act as the trimming guide - not shown here. I in-filled the missing corners of the F14 keels to make them up to full height, to support the seat sheet:

A little tirimming of the wing bolt plate to clear the torque rods, then I chamfered the bottom of F7 and added a 1/32" ply facing plate to protect the balsa fuselage planks:

Put on a Happy Face

Despite lagging behind most builders, I thought I'd put on a happy face with a cunningly shaped 1/16" ply load spreader plate under the wing bolts. I made mine the full width shown on the plan, aware that other builders have narrowed theirs down to fit within the under-wing fairing, still to be added:

I left a small gap at the back to allow for the addition of a final 1/32" ply facing when the fairing is complete:

The top side is left with a small shelf to be filled:

A single piece of 3/32" balsa filled the ledge nicely and didn't interfere with the wing seating:

Standoffs

I borrowed Dirk's idea of circular stand-offs on top of the wing bolt plate to prevent wing crushing:

I angled the stand-offs to suit the concave shape of the top of the wing:

Congratulations to those of you who have already finished (and flown!). I'm relieved we have been given a little more time, so I'm still hoping to see you up the Orme with a finished Sabre in due course!

[McG 6969]

Very neat and tidy build, Steve.

... and no worries, I'm way behind you with my Dog build progress...

Cheers & keep safe

Chris

[Steve Houghton]

Chris,

Thanks for looking in. I would say we're roughly neck and neck, although you've done more on the tail section of the fuselage.

Trying to keep safe - just ordered 200 face masks from Sarik, ready for the new rules for shopping only with a mask - bang goes the budget for my next balsa order!

[Steve McLaren]

Lovely neat building Steve. It's good to see so many different ways of doing things. Now that's a better way to do the wing seat. I should have done that! Have you created a little fillet fairing at the same time?

[Steve Houghton]

Hi Steve,

Thanks for you inspiration on this one, for using a template strip on the wing.

Yes, I followed other builders' fairing advice (Phil and Dirk, I think, but may have been others too), and left a lip over the rear 3 inches of the fairing to allow the building of a fillet. I've struggled to get my head around what shape each part of the wing fairing and fillets should be. It seems to take a long time for little progress, so will be glad to finish this bit.

Slight delay to the build at the moment, due to house decoration taking priority. My eldest daughter is assembling her first Ikea flat-pack tonight, so I'll try to encourage her to progress to model building if I can!

[Phil Cooke]

Steve - no probs with your build lag, between you and me we will be putting the Sabre Mass Build back into 2021 - I will be going to press shortly on this - a tough decision but with the current second waves brewing across Europe and such uncertainty still about travel its just not right to run the event half cocked and not enable a number of over seas flyers the chance to join us. There are a lot of builds still at risk of not being completed in time, myself included if im honest, so you are not alone!

Nice progress on the wing seat and fillets, yes they are quite tricky mine took far too long too!

Hope the Ikea work goes well!

[Steve Houghton]

Phil, Thanks for the rumour - I look forward to the public announcement!

Who knows, with even more time to build, and more builders able to participate and finish, this could be the 'massiest' mass build ever. I hope you bring your wide angle lense for the group photo!

Ikea desk now completed and looking good - ready for her 'not going' to University in the Autumn, and having to study her degree online from home.

[Steve Houghton]

Now, where were we?

After a short delay, work is continuing. I struggled with the shape of the underwing fairing and had to remove my first attempt, leaving some of the wing sheeting a little scratched. However, I ended up with this, complete with rust marks from the pins (which don't appear to be rusty!):

The Blender

To blend the wing trailing edges into the fuselage I inset a couple of triangular(ish) pieces of 1/16" ply, sitting flush with the fuselage planking. From the few underside photos I've seen of the Sabre, the outer edges of fairing pieces meet the trailing edge at roughly 90 degrees:

The ply pieces are level with the bottom of the wing, and roughly continue the underside profile of the airfoil:

The plan is to build up the fairing on top of these ply platforms, and to bridge the step up to the 3/32" balsa wing seat, which I left oversize for cutting down later:

Hopefully, progress will be a little more rapid over the next few weeks.

[McG 6969]

Hi Steve,

Glad I'm not the only one...

Struggling at the wing seat fairings and the TE fairing as well at the moment.

Quite a lot of sculpting/sanding for not to much satisfactory result...

But then, we'll get there, young man.

Cheers & keep careful

Chris

[Phil Cooke]

Yep, the little wing fillets are tough features on this one for sure, they've had a few of us scratching our heads and pondering... keep up the good work Steve - great to see you updating the blog!

Stay safe and well,

Phil

[Steve Houghton]

Still Here!

No, I've not given up - just short of time.

The wing fillets have now been in-filled with balsa and sanded down - not much to show after all the work:

 

The step at the front of the fillet is lined with thin ply for protection:

 

Pitch and Yaw

The HS225MG elevator and rudder servos have been mounted as far forward as I could, using 1/8" ply plates mounted on top of F4 and F5, and a gap between them for my sliding RX mounting plate on top of the battery box. Balsa pushrod for the elevator, but rudder connection still undecided, although I'm still hoping to use a closed loop. Using the outer hole on the elevator crank and the second hole on the servo, the elevator throw is too much, so I may need to move to the inner hole. I prefer mechanical reduction to using throw reduction on the radio:

 

 

I needed a 15mm deep balsa filler on F5 to raise the rear ends of the plates to be parallel to the fuselage centre line:

 

The front ends sit on the remnants of the F4 cross piece that was left after I cut it away in the middle for the RX slide-in plate. No need for any additional supports underneath the plate, as the rear mount holds it level:

 

Fairing well

I have finally tackled the awkward tailplane fillets, placing the outside edges 60mm apart at the front and 42mm apart at the rear:

 

The top edges ended up lower than the F15 saddle, so I added a layer of 1/8" balsa to the top of each one, then sanded them down to be flush with F15. One side needed a shaving off F15 as well, to level the sides up:

 

I'm quite please with the fit, which needed a tiny sliver of balsa to fill a couple of gaps between the fillets and the fuselage:

 

Tailplane Tubes

Since my tailplanes are going to be removable, I fitted tubes into slots in the lower laminations of the tailplanes. Brass at the front and aluminium at the rear, as that's what I  had in stock. They sweep forward slightly to accommodate the taper in the fuselage fairings, becoming parallel when the planes are spread slightly at the front:

 

The top laminations were then slapped on with aliphatic and left for two days to endure the glue in the middle had dried:

 

I'm considering a mod to the front tailplane pivot bushing, which I'll expand on later. Stay tuned for the next exciting episode.

 

 

[Phil Cooke]

Great to see you back at the bench with an update to your build blog Steve!  ?

[McG 6969]

... glad to have you back at the Sabre indeed, Steve.  ?

Looking forward to the next episode.

 

Cheers & stay safe

Chris