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Design & Build Sport Twin

Sport Aerobat for 2 x 40s

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Charles Smitheman18/01/2019 13:15:12
216 forum posts
18 photos

Oh yes I fitted flaps, and they are great. However the model slows down much better than a single I suppose due to having two prop discs.

stu knowles18/01/2019 13:43:16
532 forum posts
44 photos

I have flown quite few twins and I too think that for your engines 60" is a bit small. This is based on a DB Expo 80 with 2 x OS 25FP. It has just enough but no power to spare especially with the floats fitted. With wheels it will fly quite happily on just one engine. I look forward to seeing how you develop your own design whatever parameters you choose.

Jon - Laser Engines18/01/2019 13:58:38
4441 forum posts
164 photos

I hear what you are saying Nigel but the model is just too small

If you forget about the power of a 40fp and how it compares to another engine or model and just look at the numbers it is easier to understand. When it comes to twins, two 40's offer more power than a single 80 as they have a larger combined propeller area than the 80 could swing at the same rpm.

So your 10x6's have an area of 78.5 and you have 2 of them giving 157.

Some maths later means that you end up with equivalent performance to a single engine model turning a 14x6 at 11000rpm.

a 14x6 apc at 11k gives in the order of 14lbs of calculated thrust.

This is far in excess of what you need for a model of the light weight/small dimensions you are discussing and it would easily be enough for a very exciting 70 inch model of 10lbs or more.

I notice this with my two Pulse 125's. One is powered by a single laser 155 and weighs about 12-13lbs i would guess (not actually weighed it). The twin version of the same model has two 70's on it and is 14lbs. It is noticeably heavier than the single engine version. Anyway both fly very similarly despite the twin being heavier and having less capacity in the engines.

This is why there are often long debates with customers regarding the choice of engines for their new twin project. They always think they need more than they do.

I also just remembered my dad's very heavy (11lbs) 66 inch span Harrow bomber. a pair of 25's pulled it to far greater than scale speed without issue.

I understand that you have concerns about physical size for transport, and thats fine, but from a pure power point of view 70 inches and 10lbs will be a walk in the park for a normal sport wing.

 

Edited By Jon - Laser Engines on 18/01/2019 13:59:37

Richard Harris18/01/2019 14:11:14
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1991 forum posts
1823 photos

Nigel,

Sounds like a nice project, if all goes well why not draw up the plans ,pencil some words down and take plenty of clear photos of your build for a possible submission into the magazine?

Drop Graham a PM, I'm sure he will be enthusiastic and would guide you the right direction regarding the type of photos he requires.

Its not often you see twins published and we all like to see something different, especially if its a good looking design.

I will be watching thumbs up

Rich

Peter Miller18/01/2019 14:12:56
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9741 forum posts
1152 photos
10 articles

Looking at all those figiures. I am reminded that Grumpy Tigercub is 53" span and is fast and fully aerobatic on a couple of 15s and doesn't slow down much on one engine

Little Miss Honky Tonk is 59" span with 650 sq.inches of area and is fuly aerobatic on half throttle on one SC 52 four stroke.

Either this project will need to be bigger or it will go like a rocket and land like a space shuttle.

cymaz18/01/2019 16:31:47
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8308 forum posts
1148 photos

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 - Laser Engines18/01/2019 16:40:03
4441 forum posts
164 photos
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!

cymaz18/01/2019 16:51:12
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8308 forum posts
1148 photos

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

Percy Verance18/01/2019 18:59:38
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7566 forum posts
145 photos

How about a Citroen 2cv? smiley

Jon - Laser Engines18/01/2019 19:21:36
4441 forum posts
164 photos
Posted by Percy Verance on 18/01/2019 18:59:38:

How about a Citroen 2cv? smiley

i think the 40fp's will have more power

Percy Verance18/01/2019 19:49:36
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7566 forum posts
145 photos

And more credibility.........

However, the 2cv reputedly handles ploughed fields better........

Edited By Percy Verance on 18/01/2019 19:51:33

jeff2wings18/01/2019 20:09:24
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755 forum posts
1763 photos

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 Bowlan18/01/2019 20:53:20
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1707 forum posts
41 photos
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. yes

Peter Miller18/01/2019 20:59:58
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9741 forum posts
1152 photos
10 articles

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 R19/01/2019 11:54:13
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2483 forum posts
406 photos
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 Miller19/01/2019 13:42:03
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9741 forum posts
1152 photos
10 articles

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 Bowlan19/01/2019 14:24:17
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1707 forum posts
41 photos

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 crook.

Jon - Laser Engines19/01/2019 15:03:59
4441 forum posts
164 photos

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 - Laser Engines19/01/2019 15:10:02
4441 forum posts
164 photos

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**

David Mellor19/01/2019 16:25:04
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1287 forum posts
620 photos

Nigel,

I can't help but notice something quite fascinating in the discussions between yourself and Jon. Which is that the Ugly Twin is precisely mid-way between your wingspan/weight target and Jon's recommended wingspan/weight target.

 

Before I go any further, the Ugly Twin is (at WCL=9.9OPCF) far closer to your declared cubic wing loading target (of 10 to 12 OPCF) than that of the 2-to tango design, and it seems to me, therefore, that the Ugly Twin design is perhaps going to be the more useful of the two models to use as a reference from time to time.

 

Now... those weights. Your preferred AUW target is around 6 pounds, Jon suggests 10 and the Ugly Twin is precisely 8 pounds.

 

Then the spans. Your preferred span is around 60", Jon suggests 70" and the Ugly Twin is precisely 66".

 

Next the WCL. Your preferred WCL range is 10 to 12, Jon's suggestion can easily meet that target and the "Ugly Twin" is also bang on your target (the 2-to tango is way higher than your target).

 

Jon has helpfully given a power curve in his link which is consistent with an output of around 475 Watts per engine at around 9,500 to 10,000 rpm. Using an online calculator (**LINK**  ) gives much the same answer matched to a nominal 11 x 6 APC prop at 10,000 rpm (i.e. 464 Watts per engine and 66 ounces of static thrust per engine).  I use static bench testing and APC props a great deal and have always found this particular online calculator to give a pretty good match between measured power (in Watts), rpm and measured static thrust (in ounces) on the bench.

 

So, if you take 950 Watts as your indicated peak power for the two engines working together, and, say you are looking for a peak power to weight ratio of around 140 Watts per pound, your indicated AUW would have to be no more than 6.7 pounds.

 

Alternatively, if you stick to your 6 pound AUW target, your indicated power-to-weight ratio would be 158 Watts per pound which is actually pretty high, and you might struggle to keep the weight to that tight target to cope with the higher imposed stresses (if you wanted to fly it aggressively).

 

So... it looks like there are some useful reference points to guide you in your quest!

 

Just one more morcel of food for thought.  Designers - like artists - tend to have a characteristic "signature" in that their models bear a degree of self-similarity to each other.  I think you've spotted this with Boddo's designs. This "signature" often shows up in the cubic wing loading of the designer's models.  Of all the successful, proven designs that I have studied, the designer with the lightest touch in my view is the late, great Chuck Cunningham.  If you search Outerzone and study his designs you'll see what I mean - especially his larger Lazy Ace bipe and Miss Texas.  His choices of timber sections and construction methods are of the lightest, stiffest and strongest around, in my humble opinion, and may inspire you! 

 

Dave

 

 

Edited By David Mellor on 19/01/2019 16:52:53

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