What size engine?
 

I have a Goldberg Skylark 70. It flys great with my OS .75 AX engine, however, I've been toying with the idea of putting a good four stroke engine on that plane.

Based on your experiences, what engine would you guys recommend I use for this plane? Don't forget, the OS .75 AX is a power house engine and I enjoy the performance of this plane with this engine. I just want to put a good four stroke on it to gain some experience with four strokes.

As you can see from the below list, I have several with OS two stroke engines from .46 to 1.20 (well I didn't list the two .46's and one .50 but they are in my "fleet" and the 50 is going on the Flying Thingz Skycutter flying mower).

Thanks for your suggestions.

CGr.

RE: What size engine?
 

For that plane id definitely consider a saito 125, at 24.76 oz (with muffler) its lighter than your 75ax and i would think it would match it power wise just at a different rpm range.

The os 120 is an option but its 33.3 oz for a 120

Steven

RE: What size engine?
 

The thing flys great with the .75 so a 1.20, I believe, would ba a tad much for that plane. I was thinking more on the order of the OS 91 Surpass II looks like a pretty good match.. the output power is about the same.. I don't know. Still hoping for some good recommendations. Although the Saito 1.25 looks like a good engine too. Thanks for the recommendation.

CGr.

RE: What size engine?
 

I recently replaced a .75 2-stroke in a plane of mine with a .91 4-stroke with great results. It has very similiar power and should work out great for you. I agree a 1.20 would be overkill.

RE: What size engine?
 

Saito 100 ... light and versatile for many other planes ... :)

RE: What size engine?
 

The saito 100 would work, but its too light!

The 125 matches the wight of the 75ax more closely, not sure if you can move stuff forward for the cg?

Don't forget you can never have too much power;)

RE: What size engine?
 

You're right on track with the 91 Surpass. That should provide comparable power output. I'm personally a Saito guy, but the OS engines are really good quality engines! Take a look at the Saito 91. It's over three ounces lighter than the OS (about the same weight as your current engine) and has a little more power output that the OS 91.

RE: What size engine?
 

Uh, to my knowledge the Saito 100 is heavier than the 75ax; however, the Saito 100 is a little lighter than the OS 91.

RE: What size engine?
 

The O.S. .75 AX weighs in at a none-to-svelt 26.46 ounces. I'd recommend the outstanding Thunder Tiger F-91s 4-stroke, which weighs in at 25.7 ounces. It's very easy to set up and tune, very reliable, and a stump-puller of a 4-stroke that will only get stronger and stronger over the years as you continue to fly it.

Ironically, it's actually a less expensive engine than your current O.S. .75 AX. It's available for $194.99 with free shipping:

http://www.thundertiger4u.com/thunde...s-p-13121.html

The folks at Radical RC have it listed for $184.00 plus $3 USPS shipping:

http://www.radicalrc.com/shop/?shop=1&cat=6

And last but not least, Infinite Hobbies shows the F-91s at $184.99 w/ free shipping:

http://www.infinitehobbies.com/morei...p?part=TTR9801

This is a great engine at a great price. It will also come within .75 of an ounce of balancing your plane exactly like your .75 AX does. If you are wearing your O.S. Max/Saito blinders, hower, my apologies for wasting your time.

http://www.thundertiger-europe.com/u.../509801_01.jpg

RE: What size engine?
 

Big Ed.. I wear no blinders.... except maybe for the Spektrum DX7 and Airtronics vs just about everything else.. :D and somehow, I just can't figure out where to mount the prop on either one of those.

So, I am looking for a viable replacement for the OS .75 AX. Not that I don't like the OS, quite the contrary. I love it. But I am interested in spreading my wings, broaden my horizons.. yeah yeah yeah... :D and trying a four stroke.

I had an OS 52 Surpass, and oddly enough, it was on my Goldberg Skylark 56 (RIP). I sold the engine to someone that wanted it more than I had a home for, so I sold it to him. To my knowledge, he has not put the engine in anything yet, and that was four or so years ago.

Anyway, as I said, no blinders here. I am pretty much game to try anything within reason.

CGr.

RE: What size engine?
 

engine w/ Muffler

75ax = 26.46 oz
saito 100 = 20.9 oz
saito 125 = 24.67 oz

Info comes from tower and horizon.

RE: What size engine?
 

I must tell you guys this. I permanantly mounted about 6 ounces of lead in the nose using Epoxy thinking that this would be the only engine for that plane. I added an additional 3 ounces using screws so they will come out easy. The epoxy'ied ones will NOT come out. Period. So, I have to watch the weight up there. So, lighter is better, I can add weight easy enough. But heavier, by 3 ounces or more is not.

CGr.

RE: What size engine?
 

No problem. Get your heat gun out, heat up the epoxy, and scrape out the weight.

RE: What size engine?
 

I will second the TT .91. OS quality at a Magnum price, but a bit heavy. If you have 6 ounces of lead in the nose then you could put in a 1.2 four stroke, if you have enough prop clearance.

RE: What size engine?
 

Well, again, I recommended the Thunder Tiger F-91s originally because it is very, very close to the .75 AX with regard to weight. Changing out the engines shouldn't require rebalancing the airframe. Just swap engines and go fly. At worst, swap engines and motor mounts then go fly.

CGr. should enjoy longer flying times as well; I would expect the TT F-91s to be less thirsty than his .75 AX.

Please give the F-91s serious consideration, CGRetired. Thunder Tiger four-strokes are wonderful engines and should be more popular than they currently seem to be.