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Old Jan 21, 2013, 10:02 PM
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United States, NV, Gardnerville
Joined Jan 2013
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Discussion
OS 4 Strokes

I am thinking of using either an OS 72-a or 81-a in a Top Flite P039 Airacobra that i"m building. anyone having any experiance with these engines I would appreciate your comments.

thanks

J
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Old Jan 21, 2013, 10:56 PM
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Joined Jun 2011
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I had this kit with an OS FS90 on it. It was a rocket and weighed a ton (12 pounds)
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Old Jan 22, 2013, 12:38 AM
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United States, MI, Clarkston
Joined Mar 2007
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I had a OS72 in the TF P39 and found it was very marginal for power especially on takeoff. I replaced it with an older OS90 like Datsunguy used and found that it had plenty of power. I believe that the old OS90 rear cam motor was more powerful than the newer OS 91's. I do not think you will be happy with the 72 or 81.
My opinion only!
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Old Jan 23, 2013, 09:51 PM
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United States, NV, Gardnerville
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Thanks guys for your opinions. Guess I'll have to rethink my power plant.

thanks again

J
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Old Jan 27, 2013, 03:17 PM
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USA
Joined Feb 2005
408 Posts
Yep... I have that P-39 and I put an OS 91 four stroke in it
I installed the engine on its side, that way the exhaust blew
down and away from the fuselage.
.
Mine has the air retracts too.

The 91 flew it pretty good but the plane was so heavy that
you have to land it really hot or it will stall.

It sure is pretty though...

Bob
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Old Jan 27, 2013, 03:25 PM
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DarZeelon's Avatar
Israel, Ramat HaSharon
Joined Nov 2003
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Video...

The real plane flew pretty easy and stalled at ~100 mph...
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Old Jan 28, 2013, 08:31 PM
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Joined Jun 2011
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Quote:
Originally Posted by AirmanBob View Post
Yep... I have that P-39 and I put an OS 91 four stroke in it
I installed the engine on its side, that way the exhaust blew
down and away from the fuselage.
.
Mine has the air retracts too.

The 91 flew it pretty good but the plane was so heavy that
you have to land it really hot or it will stall.

It sure is pretty though...

Bob

That was exactly the problem with mine. It would stall and snap roll at the drop of a hat at low speeds. You had to land going lickity split. Always exciting. The old 90 was more than enough power though.
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Old Jan 30, 2013, 04:32 PM
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United States, NY, Ithaca
Joined Sep 2007
914 Posts
I think you'd be fine with the .81. I don't understand why the OS 72 even exists. The 81 is essentially the same but more powerful. It is only down from an OS 91 Surpass by a couple hundred rpm on standard props (I cannot believe the old 90 FS was more powerful than the Surpass; If it seemed so then the prop on the Surpass was not optimal).

So just be careful about weight as you build, always a good idea; if the old 90 FS will fly it then I'm sure the 81 would also.

Jim
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