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Old Dec 29, 2012, 02:07 PM
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east central ohio
Joined Jan 2009
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Question
Gasoline Engine and Static thrust

I have a 27.2cc Stihl gasoline engine that is 1.66 Cubic inch. I am told that it produces an APC 16 x 8 prop at 8200 rpms, which is right at 14 lbs. of static thrust. With an 18 x 6 prop, which it would probably turn at around 7800 rpms more or less, which would be about 16 lbs. of thrust.

The engine weighs 3 lbs. 12 oz.

So, what does all this mean? Will this determine speed, torque etc? I have included a few pic's of the engine.

Thanks,
-Gary
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Old Dec 29, 2012, 05:24 PM
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earlwb's Avatar
USA, TX, Grapevine
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There are lots of factors that come into play. The prop pitch determines the air speed. The size of the prop can determine the thrust or pull too. if you are using it in an air boat, mount that sucker up and see what it does with the different props on your airboat. It usually all winds up being a compromise with choosing a prop that allows for good pull and also good airspeed too. Sometimes the prop that gives the most speed, doesn't have any pull to accelerate out of the turns well and vice versa with a prop that has lots of pull doesn't have enough speed.
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Old Dec 30, 2012, 01:55 AM
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Israel, Ramat HaSharon
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With the current existence of so many purpose-built CDI engines; the validity of this rather heavy flywheel/magneto engine, is quite doubtful.
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Old Dec 30, 2012, 12:05 PM
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east central ohio
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Quote:
Originally Posted by DarZeelon View Post
With the current existence of so many purpose-built CDI engines; the validity of this rather heavy flywheel/magneto engine, is quite doubtful.
In what way?
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Old Dec 30, 2012, 10:42 PM
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Israel, Ramat HaSharon
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RCAB,

Engines with built-in flywheel fans are designed for applications, where a prop is not used.
They need it both too cool the engine and for inertia to keep it running.

When a prop is installed, neither the fan's cooling, nor the flywheel effect are needed.

With the auto-advance control and low weight of current CDI ignitions, running the heavy magneto is just keeping things 20 years in the past.
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Old Dec 31, 2012, 07:25 AM
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Traverse City, Michigan
Joined Dec 2005
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Quote:
Originally Posted by DarZeelon View Post
With the current existence of so many purpose-built CDI engines; the validity of this rather heavy flywheel/magneto engine, is quite doubtful.
Can you back this statement up with facts, or is this your opinion ?

Many of us like the simplicity of the magneto system. There is no battery that needs to be mounted, or maintained.

Many of our airframes need the nose weight. To me, it seems illogical to mount a lighter engine, and then add nose weight.
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Old Dec 31, 2012, 09:02 AM
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United States, MI, Waterford Charter Township
Joined Aug 2008
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Mags are generally not used much anymore, where power to weight ratios are optimized to the max. I doubt you'll find one in a 3D plane for instance....

But, that doesn't mean that they've all been tossed out as obsolete. As mentioned, there are still applications where they work pretty darn good!

Recognizing there's quite a difference in size and weight between them is a valid point though...
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Old Dec 31, 2012, 10:49 AM
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east central ohio
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I am using this engine on a Airboat project I'm doing. And yes I do like the simplicity of the mag. Seems to me that by the time I add cdi and battery and the wires their isn't much weight different anyway.
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Old Dec 31, 2012, 11:40 AM
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USA, TX, Grapevine
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Quite a few people still prefer magneto ignition systems. otherwise they would not be selling them anymore (Zenoah is one example). The added weight at least is useful weight versus having to add lead weight to some airplanes to get the CG to come out OK. But even with electronic ignition, the CDI unit and battery pack could add up to a significant amount of weight negating its benefits.

Actually people used to use the extra weight argument with four stroke model airplane engines too. That didn't work out either.
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Old Dec 31, 2012, 11:50 AM
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United States, MD, Elkton
Joined Oct 2011
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One guy got it right!!
Mount that sucker and drive it like the cops are after you......change to a different prop of your choice,and try your getaway again..
Magneto vs.ignition is an on-going battle,and neither side will ever agree......some people take it as a personal affront,if some one disagrees with their philosophy.
I personally use all three IC types (mag,ign,glo),and each serves it's purpose.
One ignition can't be better than another,if all three engines operate well...
Magnetos last 25 years...-glo as long as the plug lasts-I have 5 failed ignitions,all less than 6 years old..Which is better ?
If an ignition system fails,it's usually difficult to diagnose a problem without-(you guessed it) -an expensive diagnostic tool !
Some guys buy a complete "back-up" ign system,so when it fails,they can replace parts until they find the part at fault.,then order a replacement..-Not my cup of tea.....
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