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SF33 1/6 scale

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Peter Balcombe
Posts: 1399
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: SF33 1/6 scale

Post by Peter Balcombe »

Steve,
With regard to the wing ribs & false LE, I would suggest that you assume the CNC rib profiles are correct and fit the false LE so that it can get sanded to suit the rib profiles along the wing.
I would have done this, fitting an oversize false LE & sanding back to suit the ribs along the span, then fitting the top skins to sit on all rib edges & false LE.
(If the false LE was CNC cut & a little undersize then don’t worry as long as the skins fit well to the ribs & there is a minimal gap between false LE & skins. The LE will bond to the front edge of the skins & false LE if you use an aliphatic (or similar) glue which fills small gaps & dries hard enough to sand well to a smooth surface.)
With the top skin in place, largely locking in the wing D box profile, I would have turned over the wing, removed any rib building tabs & sanded back the false LE to the rib profile before adding the lower skin.

I find that the false LE & false TE profiles given by CNC design programs tend to err on the large side, so generally need sanding back to suit the rib profile.
Once both skins are on, they are sanded back flush with the false LE before fitting & profiling the LE.
Hope this helps.
Peter
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Steve Naidamast
Posts: 22
Joined: 25 Jun 2021, 17:51
Location: New York, USA

Re: SF33 1/6 scale

Post by Steve Naidamast »

Hi Peter.

Thank you very much for your reply and its informative notes.

I completely understand what you are saying by providing for an oversized Leading-Edge.

So my question then becomes, with what my email to Cliff described, would you simply add to the height of the existing Leading-Edge composite that I have already created (which turns out to not be high enough to accommodate the overlay sheeting comfortably, which is what my own solution was, or would you simply develop a new Leading-Edge composite altogether by simply purchasing new wood with greater measurements for the height?

Steve
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Peter Balcombe
Posts: 1399
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: SF33 1/6 scale

Post by Peter Balcombe »

Steve,
If you haven’t glued anything yet, then I suggest increasing the width of your wood to make a better job. (You can always butt glue another narrow strip to your existing false LE & LE strips to build them up before glueing to the ribs).

Cliff has reminded me that the SF33 design doesn’t have building tabs, so the wing is built up on the lower spar, flat on the building board with tip TE washout taper wedge inserted as shown on the plan to get the necessary tip twist. Note that although it isn’t shown in the rib section sketches, the plan view shows what looks like a 1/16” thick false LE with thicker LE strip (1/4”) on the front. (It wont matter much if you end up with 5/16” total instead of 1/4”, but a plan measurement would identify whether it’s 1/4” total, so 1/16” false LE & 3/16” LE).

Anyway,
I suggest that you initially fit just the 1/16” false LE to the front of the ribs (widening it if neccessary & positioning it vertically to make contact with the whole of the front of each rib). Note that a 0 to 5mm washout taper wedge should be fitted at the tip end TE as shown on the plan, which will cause the tip end LE to rotate downwards.
The LE strip is fitted only after both skins are in place so that it bonds to the front of both skins and can be profiled to the external wing thickness.

With the false LE sanded as necessary to get flush with the ribs, the top skin can be fitted, ensuring that it sits down onto the false LE all the way along. (It’s a good idea to use Aliphatic glue as this dries hard, but sands well, unlike Cyano which is harder & more difficult to sand back to balsa without taking off unwanted balsa! White wood glue is ok, but leave a “stringy” joint which is difficult to sand cleanly.
(I tend to use Aliphatic as my general glue for all wood builds, plus epoxy for high strength areas only. This makes joint sanding much easier.

With the top skin applied & carefully sanded back to the front of the false LE, turn the wing over, prepare the underside, fit wing retention blocks, cables, spoiler, servo hatch frames etc. before fitting lower skin.
Then sand off front of lower skin back to false LE to give a clean face with edges of skins top & bottom to fit the LE strip. The LE strip must be wide enough to cover front of top & bottom skins. Fix this with Aliphatic glue & then sand down flush with skins & to LE profile.

I hope this helps, but don’t hesitate to query if unclear.
Another SF33 was built by Stephen in Hungary after my build, so Stephen may add his thoughts if he sees this thread.
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Peter Balcombe
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Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: SF33 1/6 scale

Post by Peter Balcombe »

Steve,
I you haven't done so already, check out this build thread around Nov 2018 as this has photos of my wing build, showing a just a false LE fitted to front of ribs.
viewtopic.php?p=20644#p20644
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Steve Naidamast
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Joined: 25 Jun 2021, 17:51
Location: New York, USA

Re: SF33 1/6 scale

Post by Steve Naidamast »

Hi Peter...

Thank you again for your latest notes.

As to the Leading-Edge and its backing (the False-LE), I have already set that up and have the backing already glued to the Leading-Edge.

The Leading-Edge itself is 1/4 inch in width(thickness) and 1/2 inch in height. The False-LE is 1/16 inch with the same height as the Leading-Edge.

What I need to do is raise the height of the False-LE and the Leading-Edge so that when I apply the wing's overlay sheeting, it falls slightly below the composite Leading-Edge so that I can sand down the Leading-Edge so that it is flush with the overly sheeting.

I believe I have settled on adding a top-strip of about 1/8 inch as a heightner to the Leading-Edge so that the composite Leading-Edge will now be composed of three pieces instead of 2; the Leading-Edge, the False-LE(backing), and the top-strip that will add some height to the existing composite Leading-Edge

The attached JPG image shows my left wing layout as of now (wing ribs are not glued yet). You can just see the seam of my False-LE and Leading-Edge, which are currently glued together.

Steve
IMG_0683.JPG
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Peter Balcombe
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Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: SF33 1/6 scale

Post by Peter Balcombe »

Ok Steve, you should be able to get it to work - as long as you can butt the skins hard up against the rear face of the composite LE (ideally they will need a slight chamfer to account for the <90 degree angle with respect to the LE face.)
The other thing you need to do is make sure that the skin is down onto the front of the ribs & in a straight line between each one. (You could pin a length of strip Balsa placed along the very front of the skin into the composite LE, but you will need to stop glue sticking to this piece, say by putting a strip of cellotape along it to act as a release layer.)

The above is a reason why the false LE is added separately, before the main LE. ;)
Peter
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Steve Naidamast
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Joined: 25 Jun 2021, 17:51
Location: New York, USA

Re: SF33 1/6 scale

Post by Steve Naidamast »

Peter...

Yes, I believe I can get everything to work the way I have my layout set up currently.

And working over some wax paper on top of each rib should not be an issue.

Your suggestion of placing a 1/4-inch or 1/2-inch strip of 1/16-inch strip of Balsa as the front-most piece of the overly skin is a very good idea. This way I can manage more easily the straightness of the entire component across the length of all the ribs. I'll just have to sand a side very gently to create a slight angle so that the piece sits cleanly against the composite Leading-Edge.

The only thing I did not understand was your following statement... "The other thing you need to do is make sure that the skin is down onto the front of the ribs & in a straight line between each one."

My composite Leading-Edge is straight now. I would assume that any piece being fitted against it on top of the ribs would be straight as well.

I also have not sanded any of the ribs' tops; just the fronts, the rears, and the main spar notches so I could fit them cleanly within the plan outlines.

Steve
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Peter Balcombe
Posts: 1399
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: SF33 1/6 scale

Post by Peter Balcombe »

Stephen,
What I meant was that although your skin will butt up against your straight composite LE in the plan view, you don’t want the skins dipping or bulging in the vertical plane between ribs.
The length of strip Balsa will help keep the skin from bulging upwards as well as ensuring it is held down against the rib tops, but won’t stop any tendency to droop between them.
However, if there are no local pressure points forcing the skin away from a straight line, all should be well.
Peter
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Steve Naidamast
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Joined: 25 Jun 2021, 17:51
Location: New York, USA

Re: SF33 1/6 scale

Post by Steve Naidamast »

Peter...

OK, I understand what you mean now.

I'll make sure that the sheeting does not experience any dips or bulges. However 1/16-inch sheeting should not be susceptible to such issues.

At worst, I could cut a small forward notch in the ribs and lay down an upper mini-spar to ensure that no dips occur.

Thank you so much for all your help with this.

Steve
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Steve Naidamast
Posts: 22
Joined: 25 Jun 2021, 17:51
Location: New York, USA

Re: SF33 1/6 scale

Post by Steve Naidamast »

Hi Peter...

I have sanded down my composite Leading Edge and have begun the gluing of my wing-ribs. I tend to begin in the center of the wing, working to the wing-tip first before going towards the root.

I have a question about a statement you made regarding a strip of balsa right behind the Leading Edge. What did you mean by the following? ...
>>>
You could pin a length of strip Balsa placed along the very front of the skin into the composite LE, but you will need to stop glue sticking to this piece, say by putting a strip of cellotape along it to act as a release layer.
<<<

Thank you...

Steve
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