Scale Soaring UK

Hey John,

Its been a very busy time flying and refurbishing models, a Fair K8, and a Chris Williams 1/4 scale T21. I am doing as much flying as possible before winter sets in.

I am trying to sell my original T21, a 'Mike Trew plan' 1/5 scale (3.5 meter wing span) no luck so far. No problem - I will just keep it and fly it. I guess that not too many people are into vintage sailplanes. If you know of anyone that may be interested please let me know, I am certain we can negotiate a good deal.

Getting back to the Tern, the wings are now well on the way, see attached pictures. Still to do are the rubber bands supports and the rear wing joiner box. The wing wiring, aileron servos and electric air-brakes (250 cm) will complete the installation. For the ailerons I have Hitec wing servos (HS125MG), at 6 volts they give a torque rating of 3.5 kg. From past experience they will be more than enough for the job.

Still to complete is the the bit of sheeting at the bottom of the wing root, this was left open to give access for the band supports, rear joiner and wiring. Finally I will cover with 'Antique Oratex'. The wing D box was 'sheeted' with 1/16" balsa, the ply was great, but the estimated cost was a bit high, and I had loads of balsa in stock. I might see if I can get a paint or stain to mimic the ply after covering.

Please note that using balsa means that the wing is now 1.2 mm too thick, and needs adjusting to ensure a neat join at the root. The wing tip blocks have a 1/64 ply sheet in the middle, to give a datum line for sanding and additional 'robustness'.

I will now move on to cutting the ailerons from the section and facing them off. I intend to use the covering to make a continuous hinge. thus eliminating any air flow through the gap. they will be hinged from the top and articulated from the bottom of the wing.

I have a question re-the ailerons, do you think they need any diagonal bracing to increase their rigidity, or in your experience are they rigid enough ?

Well, that's it for now. I am getting nearer to completing the model and will keep you posted as and when I finally finish the building.

Once again thank you for your help and advice.

All the best

Barry A

Hi all, or any one following this log

I have now started doing the final rear wing joiner box fitting. One side done, as per picture it is a good idea to leave the rear joiner box 'free' till this point then you are able to check the wing incidence v's the tail-plane, around 1.5-2.5 degrees positive AOA for the wing should be about right. This should give a stable sit in the air during flight.

Although you cannot see it from this picture there is also about 2 degrees of dihedral in the wing. This is achieved by lifting the tip 1" during building. Also as per John's plan do remember to build in the 3 degrees of wash out on each wing to ensure good manners in flight. Having checked with John he confirmed that this was important, and should not be ignored.

Spot the 'deliberate error'. I mis-measured the rear fuselage fairing length and have had to add an small infill piece. I hope this will not be too obvious once sanded and stained. It can happen to us all and I take comfort that I am not alone in seeing one measurement on the ruler and noting down another on my plan. Think thrice, measure twice and cut once. Well at least I have not got two right wings !!!!!

All the best

Barry

Lovely build Barry. Really enjoying it.

There's something a bit Jules Verne about the Tern... like a fictitious design in some futuristic comic book of the late 19th century. You could have an aerial adventure, or possibly plunge deep into the ocean inside its hull-like fuselage.

In reality, it was built at a time when there wasn't yet a set notion what a glider should look like, so designers were still trying stuff out. These days sailplane design is more gradual evolution than revolution, a little tweak here or there to squeeze small gains. The only revolution I think I can see (no pun intended) is the nose-mounted electric sustainer, which I think will become the norm. And great for us modellers...

Anyway, thanks again Barry.

Rog

Hi All again,

I know what you mean Roger it is a very unusual model , it is of its time and reflected the fact that gliding had not really got going. In fact the wings could almost be for a power model, not really the high aspect ratios we consider the norm today.

In the 'head on' picture below you can see the slight dihedral on the wings. Technically speaking the real Tern had a completely flat top wing with dihedral only on the bottom of the wing as determined by the rib size. However it is a good idea to add a couple of degrees 'actual dihedral', please note that you will also need to adjust the root rib to ensure a good fit on the fuselage.

Well despite falling over in the modelling room (I have only drunk tea), I have now managed to get both wings on and ensured that they are the same in terms of AOA. The rear wing joiner boxes need to cure and harden over night so that they are totally solid before we can move on to the next phase. this will be the installation of wiring, servos and air-brakes and then on to preparing for covering.

Just for fun I weighed the wings and they came out at 650 gm for the left wing and 648 gm for the right. Still servos, wire, covering and air brakes to add to this yet.

All the best

Barry

Hi Barry,
The answer to your question about aileron bracing is `yes', and my apologies if it is not shown on the plan.
I have included a phot of my Tern in flight to illustrate this.
Sorry about delay in replying I had been away for a few days, very impressed with the model.
Kind regards

John

Thanks John,

Yes I do see what you mean about the bracing. I was planning to do this even though it was not on the plan. As yet I have not cut the ailerons from the wing, but they seem to need a little bit more support.I shall do this as per your model, its has been a very satisfying model to do so far. The wing joiner boxes (front and rear) are now fitted, and glued. I will wait a couple of days for the '24 hour' epoxy to cure and fully harden and move on to aileron servos and air brake installation, and finally covering. I will keep you posted on the progress. Had a good days flying my ASW 24 at Bopeep (NE) today, very light wind and lift but great fun.

Thank you for your advice, it is much appreciated.

All the best

Barry A

Yes indeed it is nice to talk building as well as flying. I went again on Wednesday and 'maiden-ed' my Chris Williams 1/4 scale T21. We got there early and got in 4 flights before the wind picked up to gusts of over 20 mph. Flying was not the issue it was on the ground where the model was trying to ground loop in the gusts. But all in all a very successful first set of flights. Went home tired but happy !

The advice you took from CW re-adverse aileron drag (more up than down) is so vital to these slower vintage style models, it makes those turns a lot safer.

All the best

Barry

Covered wing of the Tern, with 'Faux' scale decals added

Dear Tern watchers,

The wings are now completed and covered with vintage Oratex, only the ailerons to cover and hinge. See the pictures above and below. I have added some 'Faux' scale decals from 'Andy Schafer Graphics', he does some really great stuff and the service is very good. The real aircraft did not have have these on it, but I think they set the model off very well. This will not be for the purists I know, but I like them. There was however lots to do before covering, and in a rough order this is what was done:-

1) Aileron servo and electric air-brakes fitted, with capping strips and sheeting pieces. All plugs and extension wires were secured using good quality string with a bit of 'cyano' on the knot.
2) As per the plan I intend to use cup hooks and rubber bands to secure the wing, these need fitting and reinforcing from the inside. That is why I left the bottom root sheeting till last of all.
3) In order to get the wing bands through the fuselage we need to use a long rod to catch the band and pull it though. At the 'hooking' points I fitted phenolic tubes (cardboard impregnated with epoxy) in the fuselage; through which the rod can be directed, otherwise you might just grab the wiring loom and cause a problem. These tubes also have another benefit as they will serve as compression struts to absorb any forward/back ward motion of the wing on less than perfect landings.
4) The ailerons were stiffened up using diagonal bracing (1/4" x 1/16" hard balsa) as per the picture from John of his Tern, this was needed as they felt a little flimsy. Please note this is not shown of the plan.
5) Before the lower inboard sheeting was completed do please get the wings onto the Fuselage and connect everything to make 100% sure that the electrics work.
6) Having done all the this, there was just the final sanding of the wing. Once completed I used a soft brush to ensure that the surface is dust free before covering.

Well once I cover and hinge the ailerons I can set the ATV up on the Transmitter, I plan to use about 1.5" up verses 0.5 inches down. so a a fair amount of differential. I usually use about -15% exponential to soften the ailerons, although in real terms this is not a lot. Then all that remains will be trying to paint the leading edge to mimic a ply finish, final balancing and maiden flight. I will post some pictures when it is totally completed.
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All the besthttps://scalesoaring.co.uk/phpBB3/download/file.php?mode=view&id=17354
download/file.php?mode=view&id=17353
Barry

Hi Barry, you don't know me but I know John Slater and his very nice Tern. May I suggest that an aileron differential you have of 1.5 in up to 0.5 in down is rather excessive for good control. A ratio of 3 to 2 or 2 to 1 would give you better control plus a "bootful " of rudder to assist on these early vintage types. Anyway great build and I am sure will enjoy flying the Tern. Regards John.