ok guys, some of you know im looking for a engine for my skybolt. i have a 70 surpass, but some have told me that really wasnt enough. how about these engines? GMS 1.20/Magnum 180 AR/ST2000(25cc)/ST 3000(30cc) Or Super Tiger G-90 with Standard and Pitts Style muffler. Which one would you go for and is any of these engines a good trade for a 70 surpass brand new? TIA

I wouldn't trade a NIB 70 Surpass for my first born... Well, ok, MAYBE then.

A 91 Surpass is good power. A 120 - 125 4-stroke is about the most you would want.

I can't comment on putting a 2-stroke in it, cause I'm pretty sure that's illegal in this state.

minn, y would it be illegal? would any of these engines work well with the plane?

Assuming you've got the larger GP Skybolt, I'd vote for the cheaper but still good quality/power Magnum 91 4c or the Surpass 91. I flew my older and heavier Sig Skybolt with the now extinct Magnum 80 4c. It had plenty of power for scale flight at half or 3/4 throttle. Most biplanes don't hover or harrier that well to warrant overkill on engine size.
Main rule of RC -- only 4 strokes in Cubs or Bipes and fly at scale speed.

Illegal..... as in a 4-stroke is a perfect match for the Skybolt because the engine note gives it that awesome scale like sound that was just meant to be there. There's no laws stopping you from running a 2-stroke, just some purists that might object.

The reccomended motor is the OS91 4-stroke and by all reports they fly very well with combination.
A decent 2-stroke in the range of 61-91 would also work. I'm building a Skybolt now and i'm going to use an OS .75AX 2-stroke with a Jett Engineering Turbo-Jett muffler. So far that muffler is giving me 500-600rpms more than the stock muffler and it's spinning the same 14x6 prop 1000rpm faster than an OS or Saito .91 4-stroke can.

The only motor in your list above that would be suitable is the Supretigre 90 with a Pitts muffler. That would be a great match for the plane (except for that 2-stroke sound).

Mine flies excellently well on the OS61FX I stuck in it on day one. The originals flew with 60s and they were often 9-10lb airplanes. And they flew so well, that the model was picked to make a comeback as an ARF. And THE ARF COMES IN AT ABOUT 7 AND A HALF POUNDS!!!!!! Mine is 7lb 4oz with the 61FX on it.

BTW, if you plan to keep the airplane awhile, and especially if you plan to put a 4stroke on it, and ESPECIALLY if you plane to put an oversized 4stroke on it............... The 2nd picture shows what the firewall is made of and what supports it. The firewall is two pieces of LITEPLY (no kidding) and the upper 1/3 is supported only by that white foam and it's 1/16" balsa sheeting. No kidding. The damage done in the picture was from what one guy called a hard landing. The only other damage was a broken prop. Even the cowling wasn't really hurt. Matter of fact, it's flying today on the repaired bird (and is on that repaired bird and shown in the 1st picture). One thing the repair included was a layer of real plywood over the entire firewall. And you should consider that no matter what engine you choose.

Mine has a Saito 1.00 and if it had any thing smaller , I would be disappointed . It will hang on the prop with this engine.
If I had to go with a 2 stroke , the Supertigre .90 with pitts muffler would be my choice . Also don`t be afraid of Magnum or ASP 4 strokes . I own a .Magnum 4c .52 an a ASP 4c 1.20 , both are great engines.
If you can hang on to the O.S .70 , you`ll find it a perfect match for most .40 size planes. Which there are many.
MinnFlyer and I were at a Funfly a few weeks ago, where a guy had a Skybolt with a Saito 1.25 . It was pretty impressive ,but at the upper limits for the airframe.
One of the things I like most about the 4c, is clean up . It takes very little effort to keep the Skybolt clean.

da Rock , that looks just like mine with the firewall pulled out. Lite ply with lots of holes drilled in it.. I made a new one with aircraft ply and did`nt cut any lighting holes in it. I have 2 Skybolts and the second one I used fiberglass and 30 min epoxy to reinforce it. Hope it holds.
I also went with the ARF because of the weight and have`nt regret it .

The firewall is two layers of liteply. As you know, the outer layer hasn't got any holes through it except for the pushrod and the bolts. What is amazing is that the inner layer has lightening holes. Amazing. The 1st picture shows how cleanly the two layer LITEPLY can be punched out. The fact there are lightening holes in LITEPLY and that stuff is being used in a firewall tells you something about the designer. Amazing. LITEPLY is probably the least uniform construction material we have. It's basically one layer of indifferent wood with a veneer on each side. The only real effect the veneers have is to hide the flaws in the inner crap. It's actually magic as fuselage sides and bulkheads with sensible lightening holes placed logically, but it's really not somthing that should be expected to adequately carry an engine, even one sized to the job, and certainly should not be expected to stand up to even the motor mount being tightened securely with 4 bolts. whatever...........

As for Skybolts needing 4strokes to return from the wars with little exhaust residue............... another myth.
It takes 50cents worth of tubing and 2 minutes time to bust that myth. There are no rules that say you can't do something to cure a problem. And when the cure is braindead simple................ hhhmmmmmmmmmm

Yeah, 2strokes put out exhaust residue. No laws say you can't blow it away from the airplane. 2nd picture....... Look down where the muffler pipes come out. 50Cents and 2 minutes. And they might even have a positive aerodynamic effect. [sm=wink_smile.gif] Matter of fact, I'm not sure they don't give the airplane maybe 20-25mph more top end.

Ouch. The first thing I now do with any ARF is to epoxy 2" fiberglass reenforcement tape inside the firewall on both inside verticals, paint the tank compartment with thinned epoxy and then use an acid brush to apply Titebond II along every corner in the cabin and especially the frames and landing gear blocks/pads. Getting the glass in can be like building a ship in a bottle but It's cheap insurance. So far I've never lost a firewall, though I had one "hinge" before I started the process of adding the glass and that's why I started. Luckily, I noticed it when taxiing. "Why the heck is it pulling right so hard?" Basically, it was being retained by the tube around the throttle rod! And that was a Big Stik converted to a tail-dragger with an OS 70II Surpass on the nose, coincidentally.

Probably a good idea if you have an ARF to yank on the prop occasionally to see if there is any seperation at the firewall starting. Better you should find such flaws on the ground. I tug on every control surface as I pre-flight when I get the model assembled and ready at the field. I guess I trust CA hinges, but I still check.

No problem with the Surpass .91 four stroke in the ARF version.

It pulls mine around with authority and has no problems pulling the plane out of a hover or accelerating straight up.

I `ve seen one with a .91 and it does a nice job . Probably more SkyBolts with an OS .91 than any other engine . Nice thing about this airframe, it will fly well on many different combinations.

Those are great ideas for most models.

The inside of the firewall on this Skybolt ARF doesn't have anything behind the top of the firewall but white foam.

All good ideas for sure, and you might also think about adding a layer of real plywood over the front layer of Liteply. Even thin "real plywood" is made of decently strong wood and glue. And it stands up to crushing way better than Liteply. And won't add much weight. I've started doing that on all the ARFs I care about. Like the H9 Corsair I'm about finished. I plan on having all my airplanes long enough to wear out an engine or two, and don't plan on wearing out the firewall before the engines.

One of the guys at the field flies one with an OS .61 FX. While it doesn't have unlimited vertical, it's quite fast and does just fine. The FX .61 seems to be plenty of power and it performs like a good sport bipe. Remember this is not a 3D plane and the full scale doesn't hover either.

Hogflyer