Design & Build Sport Twin

[Ron Gray]

For what it’s worth, my GP Revolver - 60” WS, 6.5lb AUW, powered by a 52 and was clocked at 110 mph level flight. Somewhat overpowered (by design), nothing sedate about it, spoilerons necessary for landings, absolute hooligan.

[Nigel R]

Posted by Jon - Laser Engines on 20/01/2019 09:00:07:

a 60 in a 6lb model is a touch excessive wouldnt you say? I think my seagull cap was more than that and was a monster on an ASP 52

Er. Super hots, 54" span, piped 60, 6lb ish. Just saying.

I am a closet hooligan, I admit.

All that said I am thinking a few more inches span would not go amiss here. Another rib bay would take me up to mid 60s span and about 700sq in. Would certainly take the pressure off the weight target.

[Piers Bowlan]

Just a random thought Nigel... Have you considered a twin delta? Lots of wing area and you have now admitted to being a hooligan

[Nigel R]

Future project maybe Piers! That Dekan Delta type could work well for that, one motor at the LE tip?

 

Some replies I missed over the weeked...

 

Torzen - the Big Apple looks great, I have looked at the plan a number of times. Did you fit the airbrake/flap combination as per design?

 

Karl - Love it! Especially the 'dirty' decor.

 

Ron - the Revolver is a slippery looking beast. GP website says 560sq in, with a 7lb max dry weight, that's quite a wing loading for a sport model! Although still much lower category than EDFs.

 

-------

 

Thank you again all for input. I have played with some numbers again; scaling what I have by a few percent:

 

Span 61", chord 11-1/2", area 700sq in.

 

Fuselage will get a bit longer to match, by a couple of inch, so looking to make overall length of 49".

 

Still aiming to use FPs. If I have a high power to weight, I'm ok with that.

 

If the AUW is 6-1/4lb, my WCL (dry) ends up at 9.3. If the AUW is 7lb, and there is a full load of fuel 3/4lb, WCL is 11.5.

 

That should fall well within confines of easy sport handling I think. Going to 700sq in might also negate the need to fit flaps, in itself saving a few oz and cutting a good few hours of build time.

[Jon H]

Should be an interesting project, i look forward to seeing how it works out.

I flew my twin pulse yesterday and all were surprised by its performance. There were several 'oh really?' comments when i mentioned the engines were 70's and not 100's as most suspected. I was wrong about my props though, 12x7's at 10k. I remember now i ditched the 13x6's as i wanted more speed!

I also had my first engine out in a long time. The model has not flown in a long while and the right engine decided it wanted to be a shade richer in the cold conditions. It wasnt a problem, the normal single engine recovery of full power on the remaining engine and keep speed up saved the day as it always does.

[Nigel R]

Shh! Don't tell anyone your Laser cut, your reputation will be in tatters!

------

Revised fuselage numbers are thus;

An inch or two of "spinner"

9" of actual nose (which might be about 4" of cowl and 5" of tank bay in a single)

11-1/2" of wing saddle.

24" of rear fuselage

4-1/4" of rudder (ish).

 

Wing;

61" span, 11-1/2" chord, 700sq in area.

 

Tailplane;

22% of wing area, 155sq in.

Span 22-1/2", root chord, 7-1/2", tip chord, 6-1/4".

Of which elevator, 2" wide strip.

 

Rudder;

about 11" high, tapering from 4-1/4" down to 3-1/4", area 41 sq in

fin, not sure of exact dimensions, to be similar area to rudder

 

 

[Nigel R]

Wing construction;

 

 

I mainly use the same basic construction on all models if I can - built up structure, sheeted all over with 1/16. Rock hard balsa for spars - 1/4" square (or similar), easily cut from a single sheet, medium 1/4" strips for LE & TE. Mix of 1/8" and 3/32" balsa ribs with one or two liteply ribs - liteply at the root to carry the wing retaining dowel, and in this build, the ribs that adjoin the nacelles. These ribs also hold on to the main U/C blocks, so will be carrying a reasonable load. Normally I'd add some ply doublers around U/C mounts, but with the nacelles as well, it just seems easier to mop that all up with a rib of liteply. Rib spacing will be around 3-3/8" (intended to make 9 bays give a 30" panel size, giving an inch to fill up with the tip blocks). Spacing might be tweaked a bit to get nacelles in the right place for prop clearance.

 

The obvious difference with this one, vs a normal single engine design, is that the spars will need to be beefier than usual, to take the extra load of the two nacelles. On a single engine design of this size I would think about doubling up the front spars to about 30% span anyway. On this twin, the wing sheeting will probably be removed where the nacelles slot into the wing, so the spar will take more load around that bit of the wing. Maybe an extra rear spar might not go amiss.

[Nigel R]

 

Nacelles

 

Like most of my thinking, the thinking here is, as simple as possible. Balsa sides, maybe some thicker 3/16" sheet to avoid the need for doublers, or, a standard fuselage type construction, 1/8" + 1/32" ply. Some triangle stock in the corners and soft top/bottom sheet to allow for some shaping.

 

Plan is to make a tank & servo hatch on the underside of the nacelle. The engines will be upright (ease of access / operation), so the underside is nicely away from the oily stuff. Forward of the tank, standard block balsa cowling can partly hide the motors.

 

As for size, the tank bay needs to be just large enough to fit in the tank and micro servo, behind the tank bay the nacelle can fair down to nothing over a distance of 6" or so. In front of the tank it is all block cowling, and that size will be dictated by wherever the prop driver may sit. The gap between the ribs is near 3", so width of nacelle will be about 3", height likely to be around the same.

Edited By Nigel R on 21/01/2019 10:43:21

[Nigel R]

Tail

 

Strip balsa framework, sheeted with 1/16. Most likely, I will do solid control surfaces. My usual efforts, basically.

 

Gear

 

It's a standard sort of sport airframe. So nothing big or fancy needed here.

 

Ailerons and elevator (and flaps if they happen) just need a standard size standard sort of servo. I have some JR NES517 I can use. They have good resolution and centreing. Not super strong, but that isn't needed.

 

Rudder, I have a higher torque Hitec HS475, which I picked up in a rare moment of extravagence (well, steady on, it was second hand from ebay). The rudder on a 60 size airframe is starting to need a bit of grunt to move; especially thinking about maneuvres like full power knife edge. My (er, well, the torque calculator website I used) calculations show the 517 would be about on its limit; whereas the 475 is about 50% stronger and thus not overtaxed.

 

Throttles, Hitec micros, HS81 or HS85 or similar, one per side, snuck in behind the tank, or next to it, or thereabouts, and thus make a short job with no awkward linkage geometry to contend with (in theory).

 

Interestingly (or not, depending on your predispositions) the stall torques and current for the old standard analogues (and even the high power Hitec) are quite low (3/4A ish) compared to the newer digital equivalents (about 1-1/2A). So the old standard of a 4 cell AA pack is quite sufficient for a handful of analogues. And normal sort of wiring sizes are big enough. And the slide switches (albeit a quality branded effort, not a cheapo nasty one) are all good enough. Nothing big or fancy needed.

 

------

 

I have laid out the wing in devwing, to get some nice accurate rib templates. I just draw a few construction lines on the board, spar and rib positions, and build up from there, I don't really use a plan per se.

 

I've drawn the fuselage on paper, but it is all straight lines when cutting wood, so it will be easy to transfer. I think at this stage, nothing remains but a few hours at the workbench. Except buying a nice stack of timber that is.

 

Maiden flight pegged for September!

[Jon H]

Posted by Nigel R on 21/01/2019 10:36:25:

Shh! Don't tell anyone your Laser cut, your reputation will be in tatters!

yeaa. It was my own fault, i knew it was a little lean on the ground and it faltered on takeoff. I should have taken the hint but didnt so it quit when i did a roll. Such is life eh?

Anyway your design ideas sound good to me.

For the wing/nacelles i recommend you build a structure that takes the nacelle loads and transmits them to the fuselage. You can then 'decorate' this structure with the wings afterwards. When i modified my pulse 125 i made the nacelles from a lamination of hard 1/4 balsa and a 1/16 ply doubler on the inside. These had a slot in them and they were simply slotted over the main spar. To make sure they were all the same i tack glued 4 sheets of 1/4 ply together and cut them all out on the band saw in one go. The ply was then glued on and cut to fit the spar slot. I have some photos somewhere i think.

Given that the wing of the pulse was never intended to take anything more than the flying loads its doing just fine as it now accepts the engine loads and undercarriage loads as i moved the gear from the fuselage to the wing when i installed the retracts. I didnt add anything to the wing structure and its been fine.

[kc]

Nigel I hope that you will consider keeping enough info so that you would be able to produce a plan for publication in due course. We need new and original plans in our favourite mag!

I have just noticed that Big Apple is a plan on Outerzone-- in case anyone else didnt realise it's here

[Nigel R]

kc, I'll keep that in mind, I will definitely have enough on the wing, it is in CAD (Devwing) already - I can try to get the fuselage drawn in CAD at some later point - I know all the measurements - although I am not familiar with any of the common CAD tools yet. Let's see how she turns out first!

jon - you make a good point with the wing/nacelle marriage. I think I'm thinking along similar lines by planning on using a liteply rib to tie the spar/sheeting structure to the nacelle sides?

[kc]

Personally I would always prefer a hand drawn plan to a CAD one. Much nicer to see the person's own style and hand written comments just like plans from years ago. But having a plan available is what matters most whether CAD or drawn by pencil and drawing board.

[Peter Miller]

When I am drawing a plan on my big AO drawing board that very often a minor change to a curve or angle can transform a design from something that just doesn't look quite right into a really nice shape.

You can't see such small changes on a computer screen

[Nigel R]

It's definitely easier to move around the plan when its physical and in front of you to get things "right". Maybe that's something you pick up in the CAD environment?

I was thinking of laser cutting when I made that post, but I suppose that can be done from hand drawn plans?

[kc]

SLEC advertise " plan tracing, laser & router cutting service"

Drawing boards forever!

[Levanter]

Posted by kc on 21/01/2019 12:59:05:

SLEC advertise " plan tracing, laser & router cutting service"

Drawing boards forever!

I would love to get hold of a decent one. Not sure where I would put it but I would find somewhere. So many thousands must have been scrapped and I wish that I had grabbed one.

Levanter

[kc]

I made a drawing board from 3mm ply on a light frame 5ft by 2ft. The ply takes drawing pins well. I worked out an unusual way to hinge it so that it goes up onto the wall with the drawing facing out ( for reference) while coming down to sit a comfortable angle ( 30 to 40 degrees?) to rest on the worktop for drawing.

You dont really need the complicated parallel arm Tee square arangement found on professional boards. Most planes have only 1 horizontal line but many true vertical line, therefore a Teesquare along the TOP edge works well for our purposes. Therefore the Teesquare holds itself nicely on the top edge when the board is at about a 30 to 40 degree angle for comfortable drawing.

A useful refinement would be to recess a few of the powerful hatch type button magnets into the top of the board so a few mating magnets or a steel rule will hold the paper in place without pins. The magnets or steel just needs to allow the Teesquare to hang from the top without obstruction to moving left or right.

Edited By kc on 21/01/2019 13:44:00

[Peter Miller]

I used to use a large sheet of 1/4 ply with a Tee Square.

Then, when I worked at the school one of the teachers was given an AO size board with parallel motion straight edge. This sat in a class room and was used as a spare table. Then the kids started vandalising it.

I offered to buy it from the teacher and he agreed. Two ounces of tobacco and it was mine. I had it off the stand and got the lot in the back of my Reliant Rialto Estate and home before anyone could say anything!!!

[Nigel R]

If it's not nailed down!

Well, the die is cast, SLEC have my order, I now await the Ecuador Gold before proceeding!

[Jonathan M]

Just popped over for a gander - very interesting project!

Jon

PS re drawing-boards, I have two at work: an old-school T-square with a softwood board (roughly A1) which is my favourite, and a modern A0 parallel-motion type which is maybe a bit more precise, but doesn't give me that warm fuzzy feeling.

[Nigel R]

Thanks Jon

Hope to have a useful update soon as I now have a big stack of timber, and I've been drawing up (no fancy boards here - just the kitchen table) wing / tail layout, and figuring out a few details of fuselage construction (it was worth drawing out in full). Aiming to make a start a cut some wood the next few days, although it'll mainly be kitting up to start with.

[Nigel R]

Small update, more to report that this project is alive and well, than anything else!

I have to apologise for the lack of juicy workshop photos, but I'm not quite up to having any structures together yet. I have done lots of kitting up tasks for the wing - cutting strip goods for spars and LE / TE, cutting the ribs, getting the sheeting prepared, cutting the UC blocks, getting some doublers ready, etc. All the bits that construction articles condense into that age old phrase, "first, cut a kit of parts..."

With the wing, I've gone for a double spar arrangement. The front spar is hard balsa, 1/4". I'm going to double it up to nearly mid span. This will have webs of 1/8" (inboard) and 3/32" (outboard) - these were cut from the left overs from the ribs. The rear spar is a single 1/4" all the way out. Webbing on that is to be 1/16" (again, largely dictated by contents of scrap box). That was a lot of webs to cut out!

The ribs at the nacelle location are cut from some hard quarter grain that came from the scrapbox (originally some fuselage sides in a kit, which I replaced with lighter stock at the time). The root ribs are liteply, mainly to provide a secure place to bury the wing dowel (the middle part will largely be cut out for the aileron servo). All the other ribs are 1/8 (inboard) or 3/32 (outboard). I probably could have used 3/32 all over. However a few choices around the centre section are "a grade up" due to the two lumps of metal hanging off the wing.

Next update should see some structure appear.

[Nigel R]

[Nigel R]

ribs in progress, note hi tech spar slotting widgets