Design & Build Sport Twin

[cymaz]

For a glow twin you need to concentrate on a few points.

• Always, always be ready for an engine to go out or sick.
• Get onto a good, methodical start up routine.
• A few useful ( and some tongue in cheek too) tips HERE

[Jon H]

Posted by cymaz on 18/01/2019 16:31:47:

For a glow twin you need to concentrate on a few points.

• Always, always be ready for an engine to go out or sick.
• Get onto a good, methodical start up routine.
• A few useful ( and some tongue in cheek too) tips HERE

This isnt true for glow...its true for all aircraft no matter how many engines and what they are fuelled by. if everyone did this i suspect accident rates would drop considerably!

[cymaz]

I put glow down because that’s all the multi engines run on. Would love to have a petrol twin one day.

[Former Member]

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[Jon H]

Posted by Percy Verance on 18/01/2019 18:59:38:

How about a Citroen 2cv?

i think the 40fp's will have more power

[Former Member]

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[jeff2wings]

A twin ? every flight an adventure ! just a thought, have a look for Ugly Twin on outerzone may give you some useful ideas on size for your project ?

[Piers Bowlan]

Posted by Peter Miller on 18/01/2019 08:48:29:

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?

Yes Peter, your Grumpy Skylark (RCMW March 2015) was inspired by the Grumman Sky Rocket. I sent off for a copy of the plan from Traplet with the intension of electrifying it. It had a wide chord, so plenty of wing area despite it's 49in span (18.76Oz/Squ Ft. wing loading, good for a twin). The model was adequately powered, by all accounts, by a couple of SC.12s although Peter thought a couple of .15s would be better.

I feel that the design is very scaleable but the plan (MW3740 from Sarik) doesn't seem to be available anymore.

Following with interest Nigel.

[Peter Miller]

I am sorry to say I no longer have my original drawing.

I know where it is but getting it out of the owner might take a few years.

[Nigel R]

Looking after my boys right now so must be brief.

Thanks for replies one and all.

On the landing speed and power front.

2 to tango design - very close to my spec so far - is 650sq in and 6.5lb. The 35 max motors used on that probably compare closely to a 40 fp.

The ugly twin is about 760sq in and 8lbs. 40 max motors have more go than an fp, at least the 40 max that I own has.

Wing loading for 6 or 6.25lb at 630 sq in is comparable.

I figure if I build to my normal method then I will end up with an airframe that is workable. My only real chance for large weight saving is in the engines and going down to a ball race 25 quite literally saves nothing on the fp. I have measured against a couple of other motors.

Jon a longer reply soon but I can't see that the prop disc argument pans out. Power required for a given prop is a calculable amount and it doesn't match when compared as you have. At a high level you have a given power on the airframe being converted to thrust with an efficiency dictated by the prop. Or props.

The worst I can do there is over motor the airframe.

More on both points soon!

Thanks again for all the interest.

[Peter Miller]

Over the years I have done quite a few twins ranging from two Cox 020s up to 19s (That one was a control liner) Also a B-17 with four G-Mark .12 flat two cylinder engines. (also C/L)

I have always found that the best way to go is to draw up what I liked with a bit of influence from other models and a lot of influence from my own previous twins and other designs.

Your first may not be exactly what you hoped for but I can promise that the second one will be a lot closer.

[Piers Bowlan]

Posted by Peter Miller on 18/01/2019 20:59:58:

I am sorry to say I no longer have my original drawing.

I know where it is but getting it out of the owner might take a few years.

Don't worry Peter I got hold of the Grumpy Skylark plan and build article too (RIP, RCMW!). What I liked about the design was the generous wing area, slim (clean) fuselage and the fact that it had a low, polyhedral wing. If you would like a copy Peter just PM me, as I know you sometimes like to revisit your old designs to develop a new, improved and maybe, now electric version!

Some years ago a friend of mine built a cartoon scale 76in span Mosquito from a plan. I am pretty sure that the model was powered by a couple of OS 25FP engines, which I thought might be a bit marginal. How wrong I was, as it was so clean and had plenty of wing area, in fact the problem was slowing it down to land as it didn't have flaps to create some drag. What I learnt was that a clean, lightweight twin doesn't need a lot of power to fly well and a low wing loading is sometimes more important than brute power. You can always throttle back (but keep a pair of bicycle clips handy just in case!)

2 to tango looks nice Nigel. The 35s used may have been equivalent to a modern 25 as it is quite an old design. I liked someones suggestion of removing the outboard wing panels for transport - outboard of the engine nacelles (just as long as they don't come off in flight .

[Jon H]

I look forward to your longer explanation Nigel but my method does work. I understand the HP numbers dont add up, but we are getting into power increasing with the square of the load and all that stuff as well and the differing torque delivery from both engines. Its also a nightmare trying to convert the pitch speeds and differing operating rpm. Direct comparison is very difficult.

If you take my twin with 2x 70's turning 13x6's at 10k. This works out to 1.23hp on each engine and 8.39 on thrust. The 155 runs a 16x8 prop at 8700 and that is 2.4hp with 14lbs thrust.

If you double the 70's to 16.8 thrust and 2.46hp everything looks good, but if you work out the power of the equivalent prop on a single, a 16.25x6 running at 10k you end up with a hp figure of 3, which is more than the 155 can provide. The two singles in this case offer more power (thrust) than a single of larger capacity.

The above figures are all calculated and not measured as measuring it all is not something i really fancy doing!

To go back to your example, the two 40's give about .58hp each and 3.5lbs thrust . Double that gives 1.16hp and 7lbs.

My Dads Harrow bomber was 11lbs and 66 inches. It flew very well on 2x 25's which gave (if my memory is right) .72 combined hp and 6.6lbs thrust using 10x4's at 11k.

With that, if you went to the 70 inches and 10lbs i have suggested before you will be fine. You could probably go a bit smaller, perhaps 65? but down to 60 is really going to push it and when it comes to weight, twins are heavy, they just are, and anything 9-10lbs will be easy for the 2 40's.

[Jon H]

oh and to backup the comments made by KC on page 1 the testing done here suggests that the 40fp is at its best around 9500rpm so more load may get you more power (thrust).

again the waters are muddied by engine power (BHP) and actual useful thrust created by the prop at that rpm. Pitch speed also makes a difference to the performance in the air....

**LINK**

[Former Member]

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[kc]

The OS .40 FP seems to have come in an earlier type with iron piston in steel liner and a later type that was advertised as ABC ( Aluminium/ Brass/ Chrome ) but I don't know how they can be identified as they all looked the same on the outside ( to me anyway)

My point now is to say that for a twin you need to have 2 near identical engines, yet 2 of these which look the same might be different inside and perform quite differently or need different size props.

[Peter Miller]

Posted by Piers Bowlan on 19/01/2019 14:24:17:

Posted by Peter Miller on 18/01/2019 20:59:58:

I am sorry to say I no longer have my original drawing.

I know where it is but getting it out of the owner might take a few years.

Don't worry Peter I got hold of the Grumpy Skylark plan and build article too (RIP, RCMW!). What I liked about the design was the generous wing area, slim (clean) fuselage and the fact that it had a low, polyhedral wing. If you would like a copy Peter just PM me, as I know you sometimes like to revisit your old designs to develop a new, improved and maybe, now electric version!

Thanks for the opffer Piers but I don't think I will be building another twin now.I still have Grumpy Tigercub and don't fly that very often now.

[Karl-Georg Krafft]

Here is my version of an Ugly Twin. Two OS 20 FP and a span of 56 inches.A fine flier.

If you don`t want an ugly plane but a real beauty - google Mark Rittinger`s Super Sportwin.

Karl-Georg

[Peter Miller]

I did another twin, again for 2 .15s.it could be scalexd up.American Nightmare. I think it was a free plan in which case I will have it.

[Torsten Spitzner]

I also hankered after a smallish twin and finally chose 'The Big Apple'. Being my first twin, I kept postponing the maiden. I need not have worried so much, she is an absolute joy to fly. Rock solid and on rails. Easy to fly and easy to land. Powered by two .36 Evolutions spinning 10X6 APC props. Not too difficult to build either. Electric retracts and individual aileron and throttle servos. Fits easily into my Polo hatchback with room to spare.

[Nigel R]

Wow, too many posts to answer in one go, and some nice twins being posted, thank you all.

Charles

I am planning on engines upright for exactly that reason, plus, upright makes life a little easier when starting.

kc

Later ABC type have a different external shape casing. I have both. The ABC has what looks like a larger bump on the non-exhaust side. I don't know if that translates to a slightly different internal porting.

Jon,

I pulled some numbers from http://www.godolloairport.hu/calc/strc_eng/index.htm

10x6 at 11k is 422W - 0.575hp - 56oz static
12x6 at 11k is 876W - 1.192hp - 116oz static

Note, the FP, mine both tacho at 11k almost on the dot with a 10x6.
12x6 at 11k is pretty much exactly what my OS61SF tachos at. So, one way of looking at this, is this is like planning on a .60 in a 6lb model.

I've been looking to compare power at prop, rather than anything else. The point I noted, and remember from using other calculators as well, is that double the power, into a single larger prop, does roughly double the thrust.

Prop power / weight ratio on 876W and 6lb is 140W/lb. High, but, my experience is, that level is about good to pull through vertical eights with any kind of attempt at constant speed. At a wet takeoff weight, 7lb, is 125W/lb.

At a simimilar power comparison I'm very happy with the 800W electric model I'm flying, of 4.75lb. I would guess that has around 680W at the prop. It's the same ballpark - it has 145W/lb ("at prop" figure).

The prop disc area thing, well, bits of the prop move faster than other bits, bits of the prop are more efficient than other bits, but you know this, I simply don't see it is an apples to apples comparison.

Dave

Food for thought. Numbers I had calculated;

2 To Tango, 650sq, 6.5lb, WCL 10.8
Ugly Twin, 750sq, 8lb, WCL 10.8

I don't get 9.8 for the Ugly Twin figures?

Lazy Ace is a nice bipe, I agree there are some good structural choices in there, very little excess, although I suspect a slightly different flight performance being targetted.

My own build methods, you've seen already in the RM Trainer and Aerobat builds, this will be similar, although with some key material size choices being larger of course.

More soon!

[Former Member]

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[Nigel R]

[Jon H]

Posted by Nigel R on 19/01/2019 22:40:33:

I pulled some numbers from http://www.godolloairport.hu/calc/strc_eng/index.htm

10x6 at 11k is 422W - 0.575hp - 56oz static
12x6 at 11k is 876W - 1.192hp - 116oz static

Note, the FP, mine both tacho at 11k almost on the dot with a 10x6.
12x6 at 11k is pretty much exactly what my OS61SF tachos at. So, one way of looking at this, is this is like planning on a .60 in a 6lb model.

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

[Former Member]

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