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Author Topic: Sig Acromaster spearmint  (Read 2604 times)

Offline Paul Smith

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Sig Acromaster spearmint
« on: January 13, 2017, 07:49:32 AM »
I built this model.  It's not overweight and the LA15 runs OK.  But I've been unsatisfied by the square-turning performance.  Tail weight and additional flipper travel haven't helped.

I did the usual trick of extending the wing to the full extent of the LE & TE.  Maybe I have too much wing area for the tailplane.

So now I've begun spearmint.  I'm going to build a bigger tailplane and see what happens.  Results in April.

A similar fix did wonders for a Sig Twister with the same problem.
Paul Smith


Offline Chris Brainard

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Re: Sig Acromaster spearmint
« Reply #1 on: January 13, 2017, 09:13:33 AM »
Is it nose heavy? The last one I built balanced well with a TD .09. Plenty of power for the weight (18 oz.), but not very good squares, maybe due to our elevation (Denver, Co one mile above sea level). The next one will be scratch built with a goal of 12 oz. and a Big Mig .061. These are more the size of a 1/2A stunter.
Chris

Offline Paul Smith

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Re: Sig Acromaster spearmint
« Reply #2 on: January 13, 2017, 09:58:39 AM »
On this and a couple of other planes, I got the feeling that the tailplane was being pushed to its limits and the nose wasn't coming up enough.  I've added tail weight with no results other than jumpiness.

I'll make the tail enough bigger to see if it helps.  I can always cut it down if it's too big.

Downsizing planes to 15 size can end up with things like an undersize tail.
Paul Smith

Offline paw080

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Re: Sig Acromaster spearmint
« Reply #3 on: January 14, 2017, 02:56:28 PM »
On this and a couple of other planes, I got the feeling that the tailplane was being pushed to its limits and the nose wasn't coming up enough.  I've added tail weight with no results other than jumpiness.

I'll make the tail enough bigger to see if it helps.  I can always cut it down if it's too big.

Downsizing planes to 15 size can end up with things like an undersize tail.


Hi Paul, what does your Akromaster weigh?  I've had an Akromaster for a couple of decades.

It weighs 17.5 ozs and is a flying mini elephant. It also does wide radius 90 degree direction

turns.  The answer is simple, the wing loading is too high.   It is powered by a Chinese Yin Yan

plain bearing 5oz diesel engine. This same engine has flown a Dumas Tomahawk on 60' X .015"

lines which weighs the same 17.5 ozs.  Yet, it fly's a much better stunt pattern. The reason is

the Akromaster has a 10.5 oz/sq' compared with the Tomahawk's 7.6 oz/sq' wing loading.  

About seven years ago, I went a bit crazy and scratch built another Akromaster, shortening the nose moment for

 the MP-Jet 1.0cc diesel engine and much smaller fuel tank required. It has a 6.65 oz/sq' wing loading and

 pops square turns like a Combat ship.  I changed nothing but the nose moment.

Tony :)

Offline Tim Wescott

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Re: Sig Acromaster spearmint
« Reply #4 on: January 14, 2017, 03:04:48 PM »
If you're not too far along on the replacement plane, try extending the rear fuselage.  You have more than enough elevator, but the tail moment arm looks a bit short.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Paul Smith

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Re: Sig Acromaster spearmint
« Reply #5 on: January 15, 2017, 08:40:44 AM »
That would be another approach.  But more work than I had in mind.  I upped the tailplane span from 13" to 15".  I'll check the weight when I get it back together.
Paul Smith

Offline Paul Smith

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Re: Sig Acromaster spearmint
« Reply #6 on: January 15, 2017, 02:34:32 PM »
I measured it up:

Wing area: 282 sqin
Old tailplane: 42 sqin
New tailplane: 52 sqin, a 25% increase.

I'm know if it works, probably some time in April.

I doubt that a 15-size stunter with this size wing will ever be great, but I hope it's better.
Paul Smith

Offline Tim Wescott

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Re: Sig Acromaster spearmint
« Reply #7 on: January 15, 2017, 03:34:40 PM »
That would be another approach.  But more work than I had in mind.  I upped the tailplane span from 13" to 15".  I'll check the weight when I get it back together.

It may work better if you move the elevator hinge line back.  All of my flapless stunters have about 1/5 to 1/3 of the total horizontal area in elevator: contrast that to 1/2 or so on a flapped stunter.  You just don't seem to need as much, and if you have too much then you'll hit the Netzband wall.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Paul Smith

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Re: Sig Acromaster spearmint
« Reply #8 on: January 16, 2017, 10:16:03 AM »
It's ready to fly.  A little under 20 ounces with the LA 15.  I might be able a save a few tenths by going to a Max III, if it runs.
Paul Smith


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