drksky1056

I've been looking at two different planes, the Goldberg Giant Chipmunk and the Great Planes Giant Super Sportster, but I'm wondering why there's such a difference in the engine recommendations between the two even though the airframes are fairly similar. Wingspan is almost the same, one being 81, the other being 82, although the Chipmunk only has 1000 sq. in. are vs. the Sportsters almost 1300. Wing loading is similar with the Chipmunk being a little bit higher and the weight is similar 14lbs max for the Sportster, 13lbs for the Chipmunk. With such similar airframes, why are the engine recommendations so different. The Sportster recommends up to a 32cc 4-stroke and the Chipmunk recommends only up to 19.5cc.

I've got a Brillelli 40GT (40cc) on order for the Sportster, but I've been contemplating getting the Chipmunk instead. Is this too much engine? Will it even fit in the cowl of the Chipmunk?

Any insights appreciated.

Here's the Sportster:
http://www2.towerhobbies.com/cgi-bin...&I=LXLYC5&P=ML

...and the Chipmunk:
http://www2.towerhobbies.com/cgi-bin...&I=LXGLK2&P=ML


Thanks

Robby368

ANY engine will fit As for the engine sizes on those two, it would make more sense if they were reversed. You would think a plane with 1000in wing area would take a smaller engine than a 1300in. I'm betting a 32-40cc would fit either one just fine. If I can cram a .90 into a .46 size extra 300, you can stuff a 40cc into a 32cc firewall.

drksky1056

It's not so much the firewall I'm worried about as it is the cowl. I'd hate to spend $400 on the Chipmunk only to discover that the cown won't fit over the engine. I've never had a plane with a cowl on it, so I'm kind flyin' blind as to how much room there is for an engine other than they recommend.

Robby368

Is the engine inverted or sideways? Normally inverted there's no problems mounting a larger engine. Mounting sideways though, even the recommended size engine sticks out the side. Alot of gasoline mounted engines and planes are designed for inverted mounting. A good way to find out is to get the plane first and measure how much room is in the cowl. All engine specs give you pretty detalied mounting dimensions. Then you can decide which engine will fit the best.

carrellh

Maybe someone who has a Chipmunk will be able to provide some measurements of their plane.

The Chipmunk manual is online but it doesn’t show a lot.
http://www.greatplanes.com/manuals/gpma1303-manual.pdf
On page 23 it says they added 20 ounces (yes, 1.25 POUNDS) of nose weight to balance the prototype with a 1.20 four stroke.

Page 14 shows the engine mount on the firewall but doesn’t show enough with the engine mounted to get a great perspective. Pages 16-18 show mounting the cowl, and only the valve cover sticks out.

The glow engine mounts sideways. It seems that ‘everyone’ mounts gasoline engines inverted. That might help. The photo of the firewall looks sort of tall and skinny. Muffler clearance is probably the biggest question.

drksky1056

I was flipping through the manual a noticed that the motor mount is off-center on the firewall, is that because of the sideways mounting?

Going by the specs on the two planes, it looks like the 40 would be a good performance match for either of them. What confounds me is that GP is recommending a 1.2 4-stroke for the Chipmunk which is very close in terms of specs to the Sportster for which they recommend a 2.0. I would think that with the smaller wing area and higher loading that the Chipmunk would do better with a bigger engine than the Sportster. My biggest reservation is that this is going to be my first gasser and don't want to get caught up is heavy modifications. It looks like the 40 would drop into the Sportster by just drilling holes to match the stand-offs on the Brillelli, but with the Chippy, I'm not so sure.

Is there anyone out there reading that's put a gasser into the GP Chipmunk?

jrf

My Feedback: (551)

I'm surprised no one has answered you yet. There were several active threads on the GP Chipmunk a little while back. (Do a search) I guess since you didn't mention the Chipmunk in the title, they haven't clicked in.

Anyway to answer your questions (some of them anyway), in spite of the similar wingspans, the Sportster is a much larger and sturdier airplane. It is designed to handle the extra power and absorb the stronger vibration of a gasoline engine. The Chipmunk has a brittle fiberglass fuselage and light duty construction that wouldn't last very long behind a 40cc gasser.

Oh yeah, the Chipmunk cowl is very tight. If you put that gasser in there, you wouldn't have any cowl left.

Jim

jrf

My Feedback: (551)

No. It is because of the right thrust angle built into the firewall. The engine is moved to the left on the firewall so that the spinner will wind up in the center of the cowl. I doesn't matter whether the engine is mounted upright, inverted or sideways, it still has to be moved to the left by 3/16" or so.

Jim

daveopam

My Feedback: (9)

The performance of the two planes is quite a bit different. What kind of flying do you want to do? The Supersporter is more of a sport plane. It will do some aerobatic stuff but the Chipmunk leans more toward an Extra or Cap. I cant help you on the fit of the motor but maybe the flying style can narrow down your decision. I personally would not worry about the reccomended motor size. This is only a starting point. For example Goldberg says a 60 two stroke is right for their ULT. Sure it will fly it but not very well. I run a 90 in mine and it could use more power.


David

drksky1056

Thanks for all the information, folks. Since this is going tobe my first big plane (and first low-wing) I'm looking for more sport performance. Something that can do some aerobatics, but isn't a handful to fly otherwise. From what I've read here, it's looking like I'm going to stick with my initial decision and go with the Sportster. Maybe I'll go with the Chipmunk a little farther down the line and put a 26 in it.

tIANci

Check this thread out on the Chipmunk

http://www.rcuniverse.com/forum/m_22...tm.htm#2223570

Its a great flying plane and I feel the Supersportster will be a 'lesser' plane. I wish I could get one here but the sales of the Chippie is for US only. You may not want a gasser for the Chippie as the colwing may need to be butchered.

The Chipmunk is a slim plane with a narrow fuse (compared with Extras, Sukhois and YAKs) and the SS may be much fatter, hence more drag (increased wing area too of 300 sq in)