AutoMike

Hey fellas, I'm starting the bipe project this weekend. A good friend of mine and I are both building one each from the original Ace kits. We are going to use gas engines instead of glow four strokes, I'll be using a Ryobi 31cc converted with Wacker Engine parts and with CH ignition to keep the weight very close to a glow four stroke. My friend is using a U.S. 25cc stock, but since I hate to add lead to any airplane, I cautioned him that the engines weight might require him to consider a few mods such as moving servos to the tail, moving the firewall back up to 2" or adding a modest amount of lead to the tail and there by increasing the wing loading even more over what his aprox. 4lb engine will add. He seems to think I'm full of it because his kit is a few years newer than mine and the engine recommendation shown on his kit states a range from a 1.20 glow FS up to a G-38 and maybe a G-45. His box has some damage to the label and another engine is listed beyond the G-38 but can't be made out. His way of thinking is that "Doc" is making this statement so no consideration should be given to making any large mods to the airframe so long as an engine within the recommended range is used. Well, he makes a good point in general I tell him, its hard to believe that a 2lb 1.20 FS can be replaced with a 4lb+ gasser and a 1500mah ni-cad flight battery alone can be moved reasonably far enough to provide a correct CG.

I've built a number of Doc's planes over the past 20-25 years and they have always turned out wonderfully when built as he originally intended and I've told my friend that anything more that the 120 is over what his design goal was originally and we are on our own with regard to heavy gas engine installs and any mods to make it work. I've read as many post about the 4-120 as I could find, which wasn't all that many, and only one or two made mention that the plane came out tail heavy, but they didn't qualify that by listing what engine they used or if they used a heavy construction method such as using epoxy perhaps in the tail or an aftermarket tailwheel assembly that might be much heavier that the simple stock unit.

I'm hoping that some of you might offer some incite on your personal experience with this kit or if any of you might have an old mag with the original construction article, circa 1986, or a kit review and might be willing to part with it or make a copy of it for a fair price plus shipping.

Thanks for any help. Mike E.

MinnFlyer

My Feedback: (4)

While I'm not a big fan of over-powering planes, The 4-120 has a wing the size of Baltimore, so I think it will easily handle adding a bowling ball or two.

I wouldn't worry about it

thebigguy

AUTOMIKE, could you tell me the cg for the 4-120? I got one from a friend,did some repairs,but know idea of cg. thanks

AutoMike

Sure, the CG range given on the plans is 4 1/2 to 5 1/4 inches back from the leading edge as measured on the top wing.

tater1

Back in the old days RC Modeler did a kit bash article on the plane with some reasonable mods to make it look more like an ultimate,the result was pretty impressive,be great if you could find the 2 sheets of plans they had to do these mods

99999999

hello, i have the plans for this conversion still on a back burner however. the picture on the cover of rcm had me in cold sweats for some time. there was a kit to add to the 4-120 that contained a new canopy, l.gear (longer/heavier) and heavy duty wheel pants. sorry but not a clue where they were obtained but it may be mentioned in the article. i wouldnt bother on obtaining it as the gear are heavy enough to moor my 22' catalina sailboat, the rest can be seen on plans and ordered. with the new lightweight gassers this may get moved up real soon. patience and some cash could get plan copies for cost.

Waco

My Feedback: (265)

you can still get the plans for a ultimate conversion free from outerzone , https://outerzone.co.uk/plan_details.asp?ID=6958

99999999

i was rearranging the clutter in the shop for the 100th time and found accessories box to this kit and plansbash. got them and re put em in rafters. later that day came across the first post in this series. so added my .02 and went on about my own business,,,,,,,,, then your post came. i took a minute to look it up and there were pictures toooooo! i had forgotten just how good this bird looked or how argry the first mrs. k. was at the boxes that happened afterwards. no this wasn't the reason for the (2nd.) but didn't help. we are still friends to this day 25 years after the boxes arrived. i had to go to the shop to look for the plans just to see if they were still there. yup! took the boxes down, touched all the contents, you know the scene all to well. got a few birds on the table to complete but this moved up many spots. heh tx waco for trouble you have caused me!!!

carlgrover

I didn't build the 120 but I did do the 60 bipe. I recommend adding functional interplane struts. When I did snap rolls with mine you could see the wings warping. Did a heck of a snap, but seeing those wings going way out of parallel isn't pretty. I flew mine so much I wore it out. I also recommend beefing up the area behind the firewall. My firewall eventually came loose. Get some fiberglass, any size you have, and chop it up into little pieces until you can fill a medicine cup. This chopped up glass is known as "bunny fur" or fiberglass rovings. You can also buy it pre chopped. Mix up some epoxy and add the chopped glass until you have a thick, goopy mess in the cup. Now spread that around the back of the firewall in a big fillet on the sides and bottom (before you glue the top down). That firewall will absolutely not go anywhere. Bunny fur sets up like concrete. The landing gear on my 4-60 was a one piece fiberglass type of unit. My plane was squirrely on the ground because it was situated just a tad too far forward. I rotated it around 180 degrees and ground handling vastly improved. Watch out for warping of the top wing trailing edge when you cover it. I covered mine with Koverall and dope and the top wing warped. It is flimsy at the trailing edge. Monokote might not be an issue. I also don't recommend ailerons on both wings. Mine didn't need it. If you want snappier performance, seal the aileron gap with tape and sneak your CG rearward until it's just a tad forward of soiling your shorts.

Post up some pics

carl

99999999

thanks carl; your points have not been mentioned before now. the wings have been discussed as a bit wimpy but no specifics were given. going with a moki 210 upfront so firewall reinforcement will be implimented. the interplane struts were on rcm kit bash article as necessary to avoid the mentioned shortcomming.. will be building a copy of the red, yellow and white example in the article. if you were sharp enough to not view it in the article disregard looking before you do! 25 years on my to do list.