The manual of Four star said that four stroke 70 size engines would be thw most suitable engines for this plane.
So, I put on os 70 on it when I also had os 91.
It barely flew and I couldn't get much out of it.
Even accepting it as a basic low wing trainer, it was very underpowered.
When I read the forums here and compare the suggested engines and the ones people really use, ther is a big difference.
Some people even use 90 size engines for 40 size airplanes.
Is there any guide lines to follow for the right size engines?

The O.S. .72 FS SHOLD fly the Four Star 60 with authority... you have somehting wrong with how you are operating the engine. Wrong prop, wrong fuel, bad adjustments....

A .61 2-stroke with the right prop and the plane will do any maneuver it was DESIGNED to do... thats about everything in Sportsmand and Intermediate Pattern competition schedules.... and more. It may not be good for precision at some of them, but it will do them. (infinite inside-outside vertical 8's which requires GOOD power... {thats stacking an inside loop on top of an outside loop.... now do 10 of them without stopping.})

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Where this trend to stick excessively oversize engines on every model came from... got no clue. It is often harmful to the performance of the aircraft in the speed range it was DESIGNED to fly.

Part of it is the fault of scale warbirds.. when you build a Top Flite Gold edition warbird by the plans and cover it with MonoKote, the .61 2-stroke is PLENTY. when you fiberglass finish it, add a DETAILED scale cockpit and put 16 coats of assorted color paints on, then hang the thing with all kinds of detail items... you need a 1.2 to get the lead sled off the ground.

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The proper guide is... the listed engine on the kit instructions. Build LIGHT if you want it to be fun to fly. If you want to compete in scale contests, don't expect the model to be fun to fly.

Edit - I had a little spiel about altitude and engine performance here until I looked up the altitude in Seoul - 86 meters - so I guess that doesn't really apply.

I'd delete the post but that doesn't seem to be an option so just ignore me[sm=bananahead.gif]

-Jinks

A 70 4-stroke won't be enough for a 60 size airplane, but it would be perfect for a 40 size. You never did mention which 4* you have (40 or 60) but if you have the 60, then a 91 4-stroke is what you would want.

What about two strokes, would it fly with a .75? Y do they say it calls for a .60 if it wont fly with a .60??

yes, a 4* 60 will fly nicely with a 75 2 stroke. I had a Super Tigre .75 on a 4* 60 which was a nice combination.

As for the 4 strokes, I have no experience with those so will keep my comments to myself.

Just plain WRONG.

A .72 4-stroke puts out as much (or more) power as a .61 2-stroke of equal quality. The 4-stroke will have MORE static thrust with the CORRECT PROPELLER.

A 4*60 can hover on a .72 4-stroke.

Wooo, I'm just plain wrong?

Fine FHH, you put a 72 in your 60 size plane. I'll stick with a 91.

91 for me too in the 4*60 and 4*40 for all that matters.

I would have to agree also with the 4-stroke on the 4*60. I have a kitted 4*60 with a YS .91FZ up front. This motor has the power of a 1.20 and this plane is a blast with this setup. Hovering and pulling out with plenty of power in reserve. I am spinning a APC 15x6 at 10,000 rpm. I need to put a 15x8 or 16x4 to get the RPM down some to around 9000 rpm. I modified the rudder and elevator by adding 1" extra to both, so the plane now can Knife edge better, hover, waterfalls, hammerhead, flat spin, etc... I added a front hatch for easy access and battery placement for proper CG, pull-pull system. Each elevator half has it's own servo and placed a 14 oz fuel tank plus some extra mods I can't remember right now. I firmly believe this plane flys better with the extra power as long as the plane was well built if it is a kit. An ARF may not handle the extra stress as well. I flew a 4*60 with a Saito 61 which was too sluggish for me. The YS .91FZ has vertical unlimited power for a 8 Lbs bird.

Here is my 4*60 from another post on RCU with PHOTOS
http://www.rcuniverse.com/forum/fb.a...538&tostyle=tm

DTB

I seriously doubt that a 4* 60 will "hover" with a 72 four stroke, but I'm sure it would fly okay. It probably wouldn't have the performance that most people are looking for today though.
Dennis-

I disagree with FHH. A good rule of thumb is that a 4-stroke engine needs to have 1.5X the displacement of a 2-stroke for the same power. Go to the product guide here in RCU and compare the specs of a Saito .72 4-stroke to a O.S.Max .61 FX and you'll see that the .72 puts out 1.0 HP@10,200rpm and the .61 puts out 1.9HP@16,000rpm. Now, the 4-stroke will swing a bigger prop so it has a better grab on the airstream, but a .72 4-stroke is not a good substitute for a .60 size 2-stroke. The .72 4-stroke is an excellent alternative for a ~.45-.50 2-stroke. Again, this is a general rule since some engines (YS) are powerhouses for their size.

That's an old rule from the days when 4 strokes weren't as powerful as they are now. Yup, most people still use this rule, but then most people seem to like to overpower their planes

My Saito 72 will swing an APC 13x6 at around the speed you mentioned ie 10,2000rpm, thus generating its peak horsepower. My 65LA (allegedly more powerful than the 61FX) will swing an APC 12x6 at around 12,000rpm, nowhere near the 16,000rpm where the maximum HP is generated. Where we're really talking about here, is a 2,000rpm gain against a smaller disk area. Someone else can do the actual thrust calculations, but I'd say there's not a lot of difference.... Bear in mind also that the Saito 72 is about the weight of a 46FX and will be a fair bit lighter than a 61 2 stroke...

When I get my Tiger 60 together, I'll probably do a comparison between the 65LA and the OSFS70. Should be interesting...