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Design & Build Sport Twin

Sport Aerobat for 2 x 40s

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Nigel R18/01/2019 08:20:56
3997 forum posts
688 photos
Hello all
This year's main project is an own design sport twin.
I've never built or flown a twin before. What could possibly go wrong!?
They do seem to offer an interesting sort of airplane for a fairly small increase in building time over a single engine. And they're quite rare. I've never even seen one at the local patch (at least, a proper one, made of wood with actual engines not the park fly foam electrickery stuff). I've wanted an IC twin for a while now (probably since finding RC planes at age 10) and with the previous project now finally leaving the ground under its own steam, it's time to start getting busy with this one.
I'm not planning on a blow by blow log for this build - probably more of a monthly roundup approach, although I will write a fair bit now about design choices, any later on any interesting bits as and when I think of them and / or stumble into the odd mantrap.
Peter Miller18/01/2019 08:24:09
11222 forum posts
1321 photos
10 articles

I shall follow this threxad with interest

Nigel R18/01/2019 08:24:11
3997 forum posts
688 photos
Engines and stuff
Motive power for this was decided on quite early - some time ago I received (another) box of glow motors from my dad (after his complete switch to the dark side). This particular box hid a pair of one of OS' finest moments - two 2nd gen OS 40 FPs, ye olde stereotypical learner motor. Easy to start, tune, unfussy about tank height, brilliant throttle response, reliable in the air, very light for their capacity, and although they're far from the most powerful 40 you'll ever find, in terms of power to weight, they seem very difficult to beat. Particularly when you start propping for sensible revs of around 10k or 11k. One of them had a silencer with the OS mute kit (the baffle and extension tube) - Ebay turned up a matching second mute kit. Also kicking around in the donated box was a pair of mounts that just about fitted the FPs, and a single 6oz tank. One extra tank, and they're good to go.
To keep myself amused, I popped both of the motors on a single plank of old 3/4" ply, hooked up a pair of bellcranks and whiled away an hour or two practicing starting and running the pair together, on the workbench, with the throttles linked together. It'll never fly, came the inevitable observation, and they were right, it didn't, but the two FPs nearly pulled the workmate over, unless something heavy (the IC flight box was handy) kept it in place.
In terms of power, the FPs are a bit like a "proper" ball raced schneurle 30; 6oz tanks are about right, presuming sensible revs and low nitro, so with that in mind I believe I should be aiming for a 6lb - 7lb airframe when finished, on the basis that (a) I don't want to underpower it, and (b) for a 3lb - 3.5lb model a single 30 would provide "good vertical" (if not prop hanging levels), double that gets 6lb - 7lb.
I looked at a couple of kits and plans, but none were quite the right size or quite the right shape. Dual Ace, nice, but a bit big and a bit scale-ish, Pegasus Twin Hornet, definite contender, just a bit small, more a 2x20 sort of size, and, er, other twin IC kits? They seem a bit thin on the ground. BillKits do a 36" funfly conversion, but that wasn't what I was after. Ripmax Harmony, ARTF, second hand only. There are plans out there of course, and a couple were close - RCM Super Sportster Twin, that I liked a lot, but, it was a kit bash, and I would not build the thing exactly as is, so that would be a near complete redesign anyway, Big Apple or Pica Duellist, all round a bit big and with retracts and taper wings a bit more complex than desired, Pete M's recent GTC, a bit small, didn't quite fancy doing another mid wing, Pilot Twin Ace, tiny bit small, for 25s, high aspect ratio wing with large span and really designed more for kitting than scratch building... none seemed to be exactly what I was after as they were.
So, a scratch build it is then.
(Although - it very nearly wasn't - the "2 To Tango" design, a plan build of almost exactly what I'm thinking of, was posted on outerzone within the last few days, but I've now done all the hard work of figuring my own numbers out for the airframe)
Nigel R18/01/2019 08:24:30
3997 forum posts
688 photos

Good to have you along Pete!

Nigel R18/01/2019 08:27:33
3997 forum posts
688 photos
Sizes and shapes
I'd better sketch out the airframe.
I know a few things.
- I want something around 6lb - 7lb to match the available powerplants.
- I want a reasonably agile sport aerobat, not a lightweight trainer nor a heavily loaded warbird - so a "middle ground" WCL (cubic wing loading) of around 10 to 12 would be appropriate. A more salient point, much heavier starts getting a bit hairy to bring in on our short grass strip, and would need flaps or brakes or other added complifications.
- I want a span of 60" - or a bit under - because, much of anything beyond 60" starts being a bit of hassle to transport. Preferably it will be a few inches under.
- Conventional layout, symmetric section, not too heavy on the building requirements.
Fairly short design brief.
More to come later...
Peter Miller18/01/2019 08:48:29
11222 forum posts
1321 photos
10 articles

Grumman Sky Rocket???

Wide chord wings will give plenty of area for shorter span. Engines right out in front of the fuselage for easy access for starting, a later version had a trike U/C.

I do have a book on the aircraft with 3 views. I don't suggest full scale but a "stand off based on" model?

Denis Watkins18/01/2019 09:09:19
4551 forum posts
124 photos

Brilliant idea Nigel, size and power within the realms of the average club flyer.

My twopenerth

7lb build margin puts no pressure on you and easily achievable but

5ft span 7lb is a high landing speed,

and more comfortable sports 6ft span are 6 - 7lb

For flying, and more importantly landing, and nice size transport model at 5ft

I would try 5.5lb - 6lb as max

I know you are building flaps in, but I have a 5ft 7lb Delta with flaps that comes in like a steam train

Have not measured, but the model must arrive at the ground in excess of 15mph and there is no margin for error

As the end of the strip looms closer

Former Member18/01/2019 09:21:47
1322 forum posts

[This posting has been removed]

Nigel R18/01/2019 09:37:13
3997 forum posts
688 photos

I do like the Sky Rocket Pete, bags of character, but for this build I'm thinking more a trike gear, light civil, sort of feel.

Denis, appreciate the thoughts, I believe 6lb would be quite the result here, 6-1/4lb more likely. I have done my sums - let's hope correctly - and added up the lumps of kit and airframe components, and my best guess is 6-1/4lb. Nearer 7lb would be a little disappointing, much more likely if I had regular beefier ballrace type 40s - those FPs are light motors.

Flaps would certainly steepen and slow the approach a little. They are definitely in the mix as an idea. I do want to keep things simple though, so if I can get away without them I will do so. My Chilli Wind is like your delta I think, comes in with little room for error, as it is unwilling to slow up (well, you can fly around with up held in but I am not a big fan of coming in on 'the back of the curve', our site has to much turbulence from hedges etc to make that workable). But that is regularly flown and happily so. But, flaps on this, never say never.

Note - the '2 To Tango' I mentioned, mid 90's design, 62" span and about 650 sq in, turned out with foam wings and two old crossflow - but ballrace - 35 Max engines (physically very similar size/weight to FPs I believe), was listed at 6-1/2lb. I think I can improve a few oz with built up.

Nigel R18/01/2019 09:39:11
3997 forum posts
688 photos

Thank you Dave, and good to have you along (Denis also!)

As you say I can adjust chord and weight to aim my loading at the right point. I think flaps represent a third avenue, aerodynamic aids, to help landing performance.

Order of importance - WCL came first, powerplants dictate weight, car dictates span!

Edited By Nigel R on 18/01/2019 09:50:52

Jon - Laser Engines18/01/2019 09:53:55
5565 forum posts
271 photos

Nigel you might want to rethink your model size.

I would say that 70 inch and 10lbs will easily fly on your two 40fp's. I flew a 66 inch (if i remember rightly) pilot dual ace on a pair of 25fp's and had no trouble, the pica dualist is a similar size and was seriously fast on os 32s. My RMX Harmony was extremely fast with 2 ball raced 40's and even more rapid with 2 52's. So rapid i nearly fluttered the tail off it!

I currently fly my sport twin of 76'' and 14lbs on two Laser 70's. Performance is more or less unlimited from a sport point of view and it will fly on one engine. The DB models twin tub is 83'' span and designed for 30-40 2 strokes. It is a slow flying model though and not intended for aerobatics

If you consider that an acrowot will fly quite nicely on a 40fp two of them will clearly pull a bigger model. While i know the acrowot will not exactly be a rocketship on a 40fp it will fly and be aerobatic. I flew one like this for some time in the dim and distant past.

I strongly recommend you go for a bigger model. 70 inches with engines will fit in anything but the smallest car in one piece. If you have a little car a two piece wing is an option.

If you didnt mind a mod on an ARTF find a hangar 9 pulse 60 and modify it to take two engines. That is what i did with my twin as i modded my pulse 125. Its an easy mod and the pulse is a good candidate for twin conversion due to its long rear fuselage and powerful rudder. It also has straight wings and will not stall unless you are really daft with it. You can also add retracts if you so choose to clean it up in flight.

Former Member18/01/2019 10:18:46
1322 forum posts

[This posting has been removed]

kc18/01/2019 10:45:10
6589 forum posts
173 photos

You might get some inspiration from the Dennis Tapsfield Doublet plan RC 1193 62 inch twin which looks a bit Beech 18 ish in the plans handbook photo.

Edited By kc on 18/01/2019 10:47:04

Edited By kc on 18/01/2019 11:17:48

Nigel R18/01/2019 10:47:42
3997 forum posts
688 photos

Jon, many thanks! One thing I notice about many twin designs, is that they are based around high aspect ratio wings (Pilot Twin Ace is 7:1, the Duellist is near 6:1) although there seems little rhyme or reason behind that. And if I was going for a long skinny wing I would completely concur, but, my thinking is to have a more compact wing, rather than to lower the area, to keep the chord reasonably large.

On the power/weight thing, my prior experience with fast aerobats, is that 5lb of (even quite slick and smooth) model is way too much for a 40FP to pull by itself - 4lb, getting there, 3.5lb, cooking on gas. As it has been a while since I have flown much IC, my personal reference point with this kind of power is an Irvine 36 on a Chilli Breeze, which came in at 3-3/4lb, that was "about right", and the Irvine has more go than the 40FP, and was regularly flown on a small strip. Hence I'm talking 7lb as upper limit. Will 40FPs be overcooking it for 6.25lb? Not by much, I believe.

I will certainly be using a rectangular planform, simple to build plus good manners (as you suggest).

Bear with me a bit...

From my figures for, weight and WCL, I can figure wing area, add the third constraint (maximum span) and the wing emerges.

If I pick (yes, a bit "from up my sleeve" 630 sq in of wing area - with a 6lb the WCL figure comes out at 10.5 and at 7lb AUW the WCL it is 12.2. I think that range should work OK for me. In fact I'm certain that range will be good, I've just finished my last model, on which WCL is 10.6 and is a delight to bring in on our strip.

All that now remains is then to figure the exact proportions of wing.

For some aspect ratios between 5:1 and 6:1 I could get 630 sq ins with...

AR 5:1, span 56", chord 11-1/4"
AR 5-1/4:1, span 57", chord 11"
AR 5-1/2:1, span 58", chord 10-3/4"
AR 5-3/4:1, span 60", chord 10-1/2"
AR 6:1, span 61-1/2", chord 10-1/4"
...excuse a bit of rounding. In truth I think any of these planforms will fly much about the same. I suspect I'll go with the 57" x 11" arrangement, comes in just under my 60" max span. Gives a few inch leeway to add some span if I change my mind a bit.
I thought (briefly) about building a tapered wing, but for simplicity sake, I will leave that for the next twin.
Nigel R18/01/2019 10:50:29
3997 forum posts
688 photos

kc, thanks, I have indeed seen the Doublet, I found some useful structural ideas in there, although the fuselage is much larger and has far more detail work that I want to do!

kc18/01/2019 10:58:21
6589 forum posts
173 photos

The various Dornier twins offer prospect of a simple box fuselage or a sleek cowled look. A very sport scale or cartoon scale Dornier might be possible.


kc18/01/2019 11:14:32
6589 forum posts
173 photos

The thing about the OS 40FP - at least the original version- was it would not rev but it would turn an 11x 7 at the same revs as a 10x 6. I found this by accident and later found the engine tests in magazines confirmed this too. Might not be the same on later ABC versions but just in case it is it might be worth ensuring there is clearance for the larger props. Of course with a twin it's not just a matter of a taller u/c but making the props clear the fuselage!

Nigel R18/01/2019 12:17:42
3997 forum posts
688 photos

That's worth knowing. I have't tried an 11x6 or 11x7 on mine; but, I do know they both turn a 10x6 at just over 11k.

Nigel R18/01/2019 12:46:56
3997 forum posts
688 photos
Fuselage sketch
Ok, long one, this...
Setting the twin thing aside, if were making a regular single engine design, I would aim to make the fuselage about 75% of wing span - measuring prop driver back to rear of fin. This is one of those old rules of thumb for conventional single engine monoplanes - and it works well for regular shape wings (not so good for long thin ones or short stubby ones), so I have no reason to do much different for this. Minor change here, I think a little extra rudder authority would be good, I will stretch that a couple of inches at the tail end.
So if span is 57", I can get my total fuselage length, around 45". Perhaps add an inch or two more for good luck, because extra help for rudder authority. Say 47" total then.
As I also know my root wing chord, 11", and - another rule of thumb - I need about two wing chords of fuselage behind the wing, to find the elevator hinge line location. Add a bit to that, because I like to set my rudder hinge line a bit further back than the elevator hinge line - allows the elevator joiner to poke through the fuselage. Let's call that length 23" total.
Rudder will probably be about 4" wide, gut feel, no proper numbers for that just yet.
That leaves, in front of the wing, 47" - (11" + 23" + 4" = 9"... that would be from where the nosering location in a conventional single would be, to the front of wing.
So far:
An inch or two of "spinner" (well, what would be spinner, in a single).
9" of actual nose (which might be about 4" of cowl and 5" of tank bay in a single engine design)
11" of wing saddle.
23" of rear fuselage
4" of rudder (give or take).
I can draw that out on the wood as I go, or, I might even yet draw it on some paper first to see what it looks like first (would probably be a good idea).
Fuselage width, gut feel, standard sort of size for a 40-50, would be 3-1/2" around the wing saddle and pull the side in for a nice taper both front and rear.
I anticipate the lower fuselage will be easy enough to work out - I'm planning on a box, with a flat top, with a wing saddle cut out. In my mind, make the box about square at the wing TE, then stick a deck on top - fuselage sides will thus be about 3-1/2" deep at the rear of the wing - same as fuselage width. At the wing saddle front, depth is a bit more, to allow for a wing peg and a bit of fairing around the LE. The nose will then curve upwards nicely toward the front. The rear, from TE, straight taper up to the tailpost. Former sizes I can work out as and when - they're rectangles - no big deal, and I only need 3.
What of the top bit? I have a Pegasus Magician canopy doing nothing. Design brief item #4! It is 3-1/2" wide and of the right general sort of size for this. I can make a front deck (semi circular) and rear deck (half an ellipse) to suit it's profile. I'm sure it can be made to look pretty good; I'll cut some formers for the decks as I go.
Structurally, standard stuff here I expect, sides, 1/8" balsa, some 1/32" ply doublers over the wing saddle forward to whatever former carries the nose leg, thick(ish) wood up front to allow for some shaping with the sanding block, 3/32" balsa for rear top decks and underside crossgrain sheeting. No surprises there.
Lastly, that front 9", I think the front half of that will need to be detachable, or else I won't be able to get to the noseleg, and, the space in the nose area might be needed to mount an RX battery (or some lead - perish the thought), for balance reasons. It's good to have an option or two to move the battery around up your sleeve (so to speak).
Charles Smitheman18/01/2019 13:12:58
226 forum posts
18 photos

Hello Nigel, this is an Interesting project.

The engine choice sounds ideal to me.

My only twin so far is a refurbished Ripmax Harmony with two OS52fs.It is fabulous to fly.

One aspect with this one is the sidewinder engines. I have noticed a tremendous amount of vibration when the engines go out of sync. They are as smooth as silk on their own.

I wonder if placing the engines vertically is better so that their vibration "planes" are not coinciding.

Of course your two strokes will probably have less vibration anyway.

I am interested to hear anyones experience with this. Possibly the stucture on the Harmony wing is not solid enough.

Another challenging aspect of the Harmony is prop clearance, an aspect to keep in mind.

I find the wing with two engines very awkward to handle during transport and storage, perhaps having stub wings fixed to the fuselage with plug in outer sections would be easier to manage.

Have fun.


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