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Old Oct 17, 2013, 06:35 AM
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Build Log
Ben Shereshaw's Cumulus - Complete a Project Build Off 2014

Ben Shereshaw's Cumulus - BIPLANE PLUS Build Off July-Dec 2013

For those not familiar with the Cumulus, I include a photograph of Allan K's beautifully finished model

With only about 6 weeks available for me to build in, Its time I extracted the digit and made a start.

I am going to build the Klarich short kit for the 96” span version.

Like others before me, the 96” span wing will be built in 2 halves for transport etc. I will use straight ¼” carbon rods as wing joiners and by lowering the wings, I can put the rod straight thru the fuselage and hold them together in place with an internal rubberband so nothing will show re holding the wing on.. A bonus on this is that I get the full span as effective wing area The topper ( franni’s term, I think ) will be built as full length removable deck from front bulkhead to cockpit to access RX and battery etc.

Photographs below show wing basic frames built fairly quickly – the hassle is in the detail.

Guided by Franni’s build notes ( thanks Franni ), I studied the wing plan more than I normally would and did some measuring of the detail on the plan, particularly to get size of packing needed for spars etc.

The side view of the plan shows the wing bottoms on the bottom sheeting at front and the rear of the trailing edge. Packing would then be needed for front and rear spars and also the front of the trailing edge. As the measurements were being taken from a thick line dyeline print and would need to tie up with the accuracy of the precut ribs, I figured it would be simpler and more accurate to fit the bottom rear spar after wing removed from plan. This meant I would only need to pack up front spar and front of TE – less room for error. This also had a bonus in that the outer ribs were easier to fit as they need to slide in side ways, which they did easily.

The front view sketch on the plan of the wing tip showed just enough of the flat wing to extend a line out and work out how much to pack up the block wing tip. It would be too easy at the tips to build in wash-in if incorrect packing of both the tip and the curved sections leading to it.

The plan does not give any indications of packing needed which would have been helpful

For the benefit of anyone else building a Cumulus, I used the following packing which hopefully will prove ok in flight.

Front Spar – 1/16”
Front of 1 x 5/16 Trailing edge – 1/16” , rear pinned flat to plan. Slotted 3/16” for ribs. ( ribs cut a bit shorter to suit but chord still same )
¾” tip block-- ¼” packing under whole block. The ¾” is too much and I made the second wing with ½” sheet which has to be plane’d/sanded down towards the tip before adding the sheeting which will strengthen the fixture of the tip to the wing- otherwise the butt joint is going to break off on the first tip over on landing.

After building the basic wing out to the tip and while still pinned to the plan, I glued on the front curved section keeping it level with the leading edge. Then did same with rear curved section but raised it 1/16” to build in a little tip washout. After reinforcing the joints to the block tips with 3/16 triangular fillets I then DRY fitted W2 and W3 by sliding into place. The bottom spar is then packed up with ¼” so that it fitted into rib slots leaving ribs suspended above the plan. It should butt up with the tip block after trimming to length. Bottom of spar and block should be at same level. After any trimming of W2 and W3 to length at TE, the lot were then cyano’ed together.
W3 is precut as a blank and trimmed to suit with the plan and the final spar, LE and TE as guidance and then cyano’ed in place.

The 2 top spars are then glued in place – suggest slight steaming of the at tip end to bend down to meet ribs and tip block ( latter butted to). At this stage, the tip block should have nothing glued on top or underneath as it will be plane’d to correct taper and then top/bottom sheet glued on top for final sanding. Finally, wing removed and the rear bottom spar glued in place bending up ot tip to butt glue flush with bottom of tip block – needs sanding a little to mate up with top spar.

I also trimmed the first 3 ribs by 1/16 on top for full sheeting of the first 2 bays . The sharp eyes will also note short spars in the first 2 bays – this is for the tube for the rear carbon rod. The space between the 2 rear spars is insufficient for the rod. Assuming I get the CG at about 33%, ( where did yours end up Franni ? ) this puts it midway between this new rod position and the front rod trapped between the front spars – hopefully leading to a good balance of forces. Boxing in the first 2 bays should give a strong root wing section to take all the load.

I think that just about covers all my build to date and the thoughts behind my interpretation of the plan.

Apologies if this seems a bit long for an introduction but it does cover the initial build section.

Photos show some of points and also the full span of the 2 halves together. Looks wider than my 108” span Mamselle but only because the Cumulus has a 13” chord compared to Mamselle’s 18” chord and that makes Cumulus look wider.

Sundancer - can you add me to the started list - thanks

John
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Old Oct 17, 2013, 07:07 AM
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That's a pretty substantial start John; I've added the link and will be watching developments with interest.
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Old Oct 17, 2013, 08:16 AM
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Nice project and a great start. The Cumulus has always been a favorite design that I should consider building. I'm on board.

Soft landings,

Joe
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Old Oct 17, 2013, 11:26 AM
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Those wing tips look tricky,best of luck.KBO
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Old Oct 17, 2013, 03:05 PM
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Great to see John, looks good.

Nice to see another Cumulus build John, it is an outstanding flier with great glide performance. I do remember the tips as a bit tricky to sheet as there is some compound curving. I think I hot wired a bed out of foam to build my wings on top of. The bed had the same profile as the lower surface of the wing. It helped a lot with the positioning of lower sheeting and spars. Mine has a little more sheeting than the plan, particularly the Trailing edge to allow for the ailerons.
The tip areas get washout too using the method. Probably helps the handling a bit.
I think for the tips if you work progressively with CA and holding things in place as it goes off you can get the shapes. I usuall CA a strip on the inside at joints so that the skin butting to that edge has a step to sit on to align it.

Keep up the good work.

My Cumulus now has many years of regular contest work and is a regular winner in vintage duration contests her New Zealand including 2 Nationals. Testiment to a great design.

Allan
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Old Oct 18, 2013, 12:48 PM
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John, that's a fast start; puts me to shame. Yes, it's a beautiful model! The extra X bracing in the wing looks complicated. Are you going to do yours in blue too?
John
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Old Oct 18, 2013, 01:56 PM
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At the moment the jury is out on power and covering but if I go electric - I will cover with doped tissue over mylar and will try for the blue by dyeing the white modelspan tissue I have. Allan's semi translucent blue has really impressed me and the decision will then revolve roundcolour of contrastingtrim.

If I go 4 st - the sharp eyed will have spotted the magnum 52 box amonst the other stuff filed on my desk - then i am not sure re tissue vs doped polyester dress lining with fuel proofing.

An early check showed that the magnum 52 etc is 8 ozs heavier than electric setup and that before adding in weight for the slightly heavier construction plus fuel proofing
The x bracing is top one way and bottom of wing the other so not expecting any problems with it. I am not sure its necessary though as a wing warping measure.

decision decisions

john
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Old Oct 21, 2013, 09:46 PM
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With a closed D Box leading edge John the wing is more warp resistant than most. It is a very robust wing as drawn.
The 52 will still give it a pretty hot climb. If you are just going to potter around you could go smaller on the engine size. The original would have flown on 1/3 of a horse power. say 250 watts. Not much.

Allan
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Old Oct 22, 2013, 01:32 PM
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Quote:
Originally Posted by AllanK1 View Post
With a closed D Box leading edge John the wing is more warp resistant than most. It is a very robust wing as drawn.
The 52 will still give it a pretty hot climb. If you are just going to potter around you could go smaller on the engine size. The original would have flown on 1/3 of a horse power. say 250 watts. Not much.

Allan
Thanks Allan,

Given the weather, i prefer to potter and let model fly itself while I appreciate the beauty of it so the 400W emax motor on 3S should be ok

The 52 4 stroke is one of 2 still new in box that I bought for a twin job that never got built. So its time it earned its keep and there is nothing like a vintage pottering around on idle with plenty in reserve should it be needed - usually flying in wind when I shouldnt but here in UK, unless you fly in wind you very rarely fly - we have rain and thunder at moment

progress slow to date - hope to finish the wing joining rod/tube tonight and then get onto shaping tip and sheeting D section and 2 root bays which will finish the wings off apart from covering.

john
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Old Oct 23, 2013, 02:18 PM
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Basic construction of the wings nearing completion with “just” some sanding, webbing and sheeting to do – easier said than done.

Photos show –

Construction of the balsa “tube” for the carbon rod joiners to go into. 6mm rod used trapped between soft to medium 6mm sheet built between the 6mm sq short spars. The blocking is grooved to take the rod and ensure maximum contact between rod and balsa – getting it just right spreads the load evenly along the blocks and spar. Too slack and the load will shift to a smaller area with possible failure. Too tight and you will never get it out. After getting it just right, I drip some cyano along the groove to firm up the balsa groove. When happy that rod moves in and out with some friction, 1/8 balsa vertical webs glued both sides. The photo demonstrates the grip between the rod and the blocks in only one rib bay as the wing is only being supported by the rod and it is not rotating to heavyside down.

Next photo shows the 2 wing halves with the rods. Getting the geometry right for 2 rod and tube assembly – parallel in 3 dimensions – is essential

If the main spar had been close to CG, I would have only needed one carbon rod with a small 1/8” rod at the rear to ensure both wings at same angle of attack and in retrospect, it would have been better/easier to add the small additional spar at the CG point with the spars going out about 4 or 5 bays.

The photo showing the 2 wing tips end on illustrates the before and after razor planning and sanding with a flat permagrit tool. The plan called for 5/8 sheet tips, Klarich supplied ¾” precut tips and I stuck the first one on before I twigged it was far too thick, and substituted ½” sheet tip for the second wing. This is one I sanded first – always pick the easiest first.
I indicated above that unless the 1/16 sheeting extends over the block tip, it will break off on first tip over on landing and sure enough mine broke off when sanding – just could not support it enough to grind away extra material and I had added additional corner webs to strengthen the butt joint. Fortunately cyano was at hand for quick repairs and I ended up sanding holding the wing vertical by the tip to avoid straining the joint. I also used the permagrit tool to taper the outer 3 ribs inline with one another and down over the tip block. Now to tackle the hard ¾” block supplied but if it too breaks off, I will substitute the soft ½” sheet I used on other tip.

Finally 2 photos showing the full 96” span – I am 6 ft – emphasises the need for splitting the wing in at least 2 parts.

I will vertical web the spars for the full span and leave out the spar trusses as not many rib bays are left after webbing at root and tips. I will also leave out the diagonal drag trusses as I am not sure they are needed. By all accounts the model flies quite slow and between the stiffness of the frame and the doped skin, it’s hard to see that the drag will distort the wing particularly with the double set of ½” wide spars.

That’s all for now

John
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Old Oct 23, 2013, 04:21 PM
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I realize this is an electric version, but that is still a substantial model, and can easily experience the flight loads of a 4S cousin. Those CF rods are rather small-ish looking diameter-wise for a 96" wing. One presumes (safely, I think) you've built other models of similar size with the same joiner arrangement, yes? You've had no issues with stress cracking those rods?

Inquiring (Eaglet) minds want to know!
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Old Oct 25, 2013, 07:17 PM
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[Sound of crickets chirping...]

Ouch, I hope I didn't say something that was taken wrong! My curiosity was about the size of those CF rods, how they seemed a little small-ish to me, which is by no means an "expert's opinion". My building efforts tend to be ham-fisted and over-built as a rule, so seeing a much smaller structural member for a wing of this size than I was expecting comes as a surprise. If I could use a similarly-sized joiner method on my Eaglet, then all the better!

Peep?
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Old Oct 25, 2013, 09:24 PM
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I wondered the same thing about the size of the CF rods, with no disrespect intended. The Cumulus has a very large wing without additional support via struts. This would be a very good time for Fanny to check in with her expert thoughts. I for one have seen wings like the Cumulus design fold in flight, and it can happen in an instant; we do not want to see that happen to John's Cumulus, that's for sure.

Soft landings,

Joe
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Old Oct 26, 2013, 04:34 AM
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Quote:
Originally Posted by cpmcgraw View Post
I realize this is an electric version, but that is still a substantial model, and can easily experience the flight loads of a 4S cousin. Those CF rods are rather small-ish looking diameter-wise for a 96" wing. One presumes (safely, I think) you've built other models of similar size with the same joiner arrangement, yes? You've had no issues with stress cracking those rods?

Inquiring (Eaglet) minds want to know!
Sorry for late reply but have been busy and have also been carrying out a rather crude test to satisfy your concern and also convince myself that I am building strong enough wing joiners that dont weigh a ton

I have used carbon rods in other models - the latest being the 80", 5 lbs weight Debutante and that only has one 1/4" carbon rod at the main spar / Cg / lift centre of pressure point. The smaller 1/8 carbon rod at the rear is simply to ensure both wing halves have same angle of attack.

As I had never actually tried testing the rods to see what they could take, your question triggered me to do just that and I have attached a drawing of both the test and the Cumulus design sketch
IF - repeat - IF my math modelling of the forces on my Cumulus and the mechanics is correct, then the 2 rods as used in the Cumulus are capable of taking more than 6G as my test did not take the rod to break point.

Doing a bit of Googling found this comparison with steel

http://www.carbonfibertubeshop.com/t...roperties.html

which showed that the carbon epoxy rods are more than 10 stronger than high carbon steel ( I doubt that when we buy piano wire that we are buying high grade steel )

One point to note is that steel joiners will bend if overstressed and hopefully you will notice the increased dihedral of the model and make a hasty landing,
Carbon fibre rods will fail catastrophically with similar results to model if at height.

I do make other points in the sketch which I believe to be most important.

Note that I am not an aeronautical or structural engineer and that the above is simply my understanding of the forces involved

thanks for asking the question and hopefully someone with better understanding can confirm my interpretations

now back to the building board

john

reloaded the sketch with a clearer copy in the next post
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Old Oct 26, 2013, 05:02 AM
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trying again to upload a better scan of the carbon rod sketches in above post

john
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