stuntflyr

Hi Guys,
I'm restoring an old, not so nicely built Sterling King Cobra to be SPA legal.
It had a Magnum 75 two stroke in it and will be easy to gut the nose and convert it to any power so... I am in the fortunate position to having been given both an OS 61 side exhaust and a Magnum 91 FS as my father flies only giants now and these are surplus.
I am interested in the pros and cons of the two engines for the model, which is about 70 inches and probably will weigh 7 pounds by the time it is finished.
Any huge weight penalty in practical terms for the 4S, or huge weight benefits by using the 61?
Is the power increase (or thrust) of the big 4S prop the overwhelming benefit?
Being new to this and having only minimal experience so far I am greatful for any help.
Thanks,
Chris...

dhal22

despite my love for fast and loud, i watched an ARF Intruder powered by a .91 4s fly recently that really impressed me. of course the pilot had a lot to do with it but the intruder was smooth, fast and quiet. plenty of power to hold an even speed throughout all parts of the maneuvers. not a bad way to go.

david

PatternPilot

My Feedback: (58)

Chris,

The big push several years ago was the sound, some of the clubs in the south east where SPA started had some sound issues. Plus to be honest it was monkey see monkey do thing, amazing to sit and watch. I started to break the mold when I did a Thunder Panzer and not the norm of a Daddy Rabbit.. Now it is the World Models intruder.. Well I'm making Deceptions and Peppermint Patties for people and most of us including me have gone back to a 2 stroke OS61 FX less money, less to break and if you know how to build light and strong, no problems..

Just my 2 cents.. Yes I will be doing a electric SPA plane also.

Scott Anderson
SPA 239
Team Castle Creations
Team tanicpacks.com
www.rcfoamy.com

8178

My Feedback: (17)

Not counting the weight problems with the FS, the cost deferential is pretty big. The OS FS-91II Surpass that is/was the SPA standard is $300 ($70 more for the pumped version) compared to $100 to $120 for a strong 61 2S.

flywilly

My Feedback: (121)

Propeller ground clearance is a factor, too. All the old designs were for 60s and 11 - 12" diameter props (mostly 11" props). That is why the majority of the 4-stroke powered versions of the old designs are tail draggers (often with shorter nose moments which IMHO unfavorably distorts the appearance of the original design).
You can put a Rossi 61 with tuned muffler (SPA legal) and have a honkin', ballistic pattern ship. Not that a 4-stroke can't generate some speed - they can as the above post attests.
Check out the new Curare with the Jett 90 - as Jiim Carey said in The Mask "SMOKIN!!" Unfortunately, not SPA legal (BOO) but completely BPA legal! Definitely my preference!!!
By the way, I own a new OS 91 surpass (for reference). Nice engine (I haven't run it), but bigger and heavier than a conventional 60.
I vote 2-stroke and second what Scott said above!
Will BArtlett

Skylane

My Feedback: (3)

I've been very happy with the OS55AX in my Cutlass Supreme. The older pattern designs (this one is from 1970) had less room up front for the engine since the engines back then were smaller than is often found now. I started with an OS61FX and moved to the lighter 55AX. The speed is fine for the SPA pattern and the vertical is excellent with an APC 11x8 and 20% Pro Pattern. I am also building a Deception which is somewhat larger than the Cutlass and the 61 FX will go in that. I have a new 55AX for the Compensator I am working on. As you can tell, I am a two-stroke fan. Now, I have watched, judged, and flown against a number of 4-strokes in SPA competition. In a plane with the right moments they do an excellent job. Smooth, great vertical, good braking on the down lines, relatively quiet, and they give a good presentation. But I can do that with my 2-strokes at half the price. I think that it is more the pilot and the setup that matters - especially the practice part! I personally like to use a bit higher pitch today than I did back with the piped 60s. The 61 FX liked a 12x7, but the Cutlass has short gear so, as mentioned, the 55AX with an 11x8 is a better fit.

Jeff
SPA 243

8178

My Feedback: (17)

I prefer two strokes too and don’t care for single cylinder four strokes. I only take my four strokes in the V8 configuration!

stuntflyr

Thanks for all of the great replies, guys. I am to receive said engines soon, and will make the choice once I fully survey this hulking Sterling built-up kit. Lots of lumber built this baby!
Chris...

bob27s

My Feedback: (19)

Two cylinders at least sound nice. Its not something I'd put in a pattern plane, but it does a nice job with the 18x5 and a big yellow plane.

(gemini 120 - engine is about 20 years old)


As for the original post .... i think the key here is that the classic aircraft were designed for 2C .60 engines that were about the same size (physical size and weight) as current era .46/.50/.55 engines. And for example an OS .55AX will keep up with the power a Webra .60, bluehead ST60, old Enya .60 III so it is possible to keep things fitting well and still have the power. About when you get to the Compensator (1975/1976 ish) is where the first heavier block OS, Rossi and schnurle engines -showed up.

4C engines are fine for SPA stuff. But those that have been effective and successful with 4C power have used designs that lend themselves well to the 4C, or have in someway been "tweeked" to use the 4c 91 engine.

Besides, you will have a limited number of 4C engines available for SPA shortly.

The new OS 110 is coming out. I would imagine it will replace the OS .91 in the product line. OS 91 will be no-more.
http://www.os-engines.co.jp/english/.../110/index.htm

Bob

flywilly

My Feedback: (121)

Whatever will the SPA do then??
I've heard that OS will, sometime soon (if they haven't already), discontinue production of the 61FX since they now have the 55AX and the 75AX. the 10cc (.61) sized engine resulted from a rules change by the FAI back in the '60s. Hopefully, the SPA will rethink their engine rules. I wonder how much the OS 1.10 wil cost?
-Will

PatternPilot

My Feedback: (58)

I don't think the SPA will have problems with the 55 or if need be re-adjusting the rules to the 75.. heck they have made provisions for electrics so far...

scott spa 239

rainedave

My Feedback: (1)

The new OS 82a 4-stroke is supposed to be close enough to the .91 Surpass in power that it can be used. But, it costs $40 more.[sm=bananahead.gif]

David

stuntflyr

Tell me this about these two engines;

Is the difference in performance between a free Magnum 91 4S and a 300 dollar OS 91 4S worth the payout for the OS?

(Remember this is for a beginner flying a legal SPA Novice Sterling P-63 rebuild, not rockets to the moon.)

Chris...

flywilly

My Feedback: (121)

NO!

bob27s

My Feedback: (19)

Free engines always produce more power

The Magnum is a decent engine. Pay attention to details, run it a bit.... nice engine.

KLXMASTER14

In the same context, is the difference in performance between a $100.00 Magnum .61 2S and a $180.00 O.S. 61FX worth the payout for the O.S? Your opinions please. This would be using them with mufflers per SPA usage, not piped.

-Robert

stuntflyr

Thanks Flywilly and bob27s,

I have decided to go 4S to try it out. I'm glad to hear good things about the Magnum.
My Kaos (ARF) has a 46 2S, so I should have a nice comparison between the two types. I have never had a 4S before so it should be educational.

Attention 2S guys, I plan on getting two kits from Carolina Aircraft and have a YS61LS and an OS61RF, both pumped and piped.
I already have a glass UFO with a piped and pumped K&B61, but that stuff is too hot for me at this stage. But I'm gonna do it soon.

Thanks again,
Chris...

P.S. Bob, I have a nice RoJett BSE40RE I use in CLPA, nice engines.

bob27s

My Feedback: (19)

Mag 61 is a good engine too. It performs well. Same goes for the TT61, but that one cost a few $$ more.

You can add a Jett-stream or ultra-thrust muffler to it, and remain SPA legal.

Set the Magnum 61 up with the jett muffler and an 11x7 prop ..... and it will pull any SPA ship with authority.

I would expect the 61FX to disappear as the new AX line takes over in the 10cc size range. Plus the 91FX seems to be more popular for the sport pilots.


(Stuntflyr - Good news on the rojett - the stunt engines really are amazing products. Dub and Richard put in the hours on them. That tiny BSE40L engine is a real jewel)

Ed Lyerly

Chris,
There is an excellent article by Bill Collins comparing these engines in the March-April 2007 SPA newsletter (page 5) .... you can download it here:
http://www.seniorpattern.com/newsletter.asp
Ed

Sport_Pilot

I think the differances are minor, the OS can deliver more power, especially with more nitro. I think the SPA site has a side by side review, even comparing parts. OS paid more attention to detail, things such as valve and port design, polishing some mold lines, a bit better machining of the crank. As I recall it wasn't anything that would help a lot but added up caused the OS to have a bit more power, but not by a great margin. I would use the free Magnum, at least till my skills are competitive. I might even pay for one for the differance in cost, till the small differance in power matters.

Sport_Pilot

Mickey Walker has been using a TT Pro .61. I would consider that as well as the Tower, Fox, K&B, and Supertigre .61.

Sport_Pilot

KLXMASTER14

Thank you, Bob. The Mag .61 w/Jett muffler will be my SPA powerplant setup.

Best regards,
Robert

stuntflyr

Hi Ed,
Thanks for the link. Great little article that gets to the heart of the matter. I've started reading all of theose back issues now. Appreciate the help.
Chris...

beppeVRCS

Guys,
I think that a 60 2C still has a little edge on a 90 4C, as far as speed is concerned. A 90 4C may have an edge on torque at a reasonable RPM; if you like 3D and if you have enough clearance for a big prop, go for it!
What I consider a common MISTAKE is to think that 4C are heavier than 2C! Remember that the muffler (or pipe) MUST be taken into account!!! Please use the scale again and let's compare the results...
Beppe