mgnostic

I had my first twin engine flight with my first twin engine plane this weekend. The plane was a 4Star Twin with a pair of OS LA40’s. The plane started its life in 2002 as a Sig 4Star60. After the tail was knocked off in my first mid-air in 16 years of flying I repaired it with an enlarged rudder. Later that year it crashed on the shore of the lake behind the local flying field (Wichita Falls RC club). The crash ripped off the nose just in front of the wing. The wing and the back half of the fuselage hung around the shop until this years funfly was looming on the horizon. With that for inspiration I raided the hobby shop for a couple of sheets of lite ply and carbon fiber tape (great stuff). I faired in the nose and made a couple of nacelles. The nacelles were made to hook over the wing spar. I wish I had made them a little longer or at least the hatches a little bigger. As it was I ended up putting the servos behind the spar. No big deal really, there is a former in the nacelle that acts as a doubler for the spar web. I also ended up making shims to give the engines the proper thrust line (2-3 degrees down). There is no left or right thrust. The wings are clipped one bay for faster rolls. A little weight was required for proper balance but I don’t know the all up weight yet. I had to wait until the fun fly was over to test fly the bird (safety first) but I taxied out to the runway with hands shaking and toes curled with anxiety. The ground handling with the 4Star120 main gear was great. The plane took off without a hitch. Aside from having too much elevator dialed in the plane flew just like a 4Star. I tuned the engines for reliable idle and was happy with reasonably close RPM at full throttle. It must have been close because you could hear the engines going in and out of sync as it flew around the pattern. Landing was a non event and the next two flights went well. So far the 4Star seems like a great candidate for conversion.

AmishWarlord

My Feedback: (5)

Looks great!

William Robison

My Feedback: (3)

Matt:

Welcome to the twin asylum. Excuse me. I mean, of course welcome to the twin club.

Haw.

Bill.

BillHarris

Not a bad job on the 4*60. Or, should we say, 4*40^2 ?

I have a 4*60 that I've started flying again this year and I've toyed with the idea of converting it to a twin. I have a couple of TT GP 42 engines packratted away and I looking for an excuse to use them.

I presume that the fuel tanks are in the engine nacelles. I wonder if this would not be a good application for bubble-less fuel tanks?

--Bill

mgnostic

The tanks are in the nacelles. They're just standard Sullivan 8oz. tanks. I wish I had planned a little better for the throttle servos. There wasn't room under the opening of the hatch with the tanks installed. I ended up mounting the servos on a removable mount to the top of the nacelle extension behind the wing spar.

BillHarris

It sounds like you're having a great time with the conversion.

This 4*60 is the second plane I built when I got back into toy aeroplanes in the mid-1990's and the fin is not well-aligned. Since "twins have to have" more fin/rudder, this'll be a good excuse to do this repair when I enlarge the vertical feathers.

The 4*60 is adequately powered by a .60 2-stroke or a .90 4-stroke, so I'll assume that a pair of .40 to .45 2-strokes will be fine. Weight is in the 7-8 pound range with the TT 91 four stroke single.

Good flying plane, but I was itching to build one of Bruce Tharpe's Venture 60's.

Still like a kid in the candy store after all these years...

--Bill

BillHarris

Tossing some initial (re)design ideas around...

I fly from a rough field so I use 120-size main gear anyway. There is a generous 4"-5" of prop clearance with the single engine and I think I can get get that by mounting the twin engines above the wing. Mounting the twin engines so that the prop hubs are 9"-10" off the ground will give the desired clearance and the thrust lines of the twin engines will be within an inch of the thrust line of the single engine.

I have a JR 8103 Tx and can use an elevon mixer to synch the engines. I've flown this plane for about 5 years and it ought to be good to learn how to set up and fly twin engines.

--Bill

mgnostic

I put the 120 size gear on mine when it was still a single. It helps with the ground clearance. I also mounted the engine as low in the fusalage as low as possible to get the carb down to the fuel tank midline. It gives plenty of ground clearance with the Ten inch props that I am using on the LA 40's

Here are pics of the original plane with an Irvine Q72, Just after the first crash(midair) and after the first test run of both engines

BillHarris

Here is where I'm going with this 4*60 twin conversion:

I'm going to fly and enjoy the plane with a TT 91 four-stroke til the Xmas-New Year's Holidays, at which point I'll do the conversion.

Weight shouldn't be a problem: the two 40 two-strokes will tip the scales below the single four-stroke. I'm planning to clip one or two bays off each wing tip anyway, so I'll do that ahead of time and get that modification de-bugged.

I've decided to use a pair of Magnum XL-40A engines instead of 46's. The plane flies like the proverbial "bat-outta-" with the TT 91 and the prop swept area of two 5" props is the same as one 14" prop; seems that the pair od 40's would be more than adequate and a pair of 46's would verge on overkill.

Another consideration: the XL-40's are fuel sippers and reportedly get 20 minute runs from 8 ounce tanks. This is critical since I am using 8 ounce tanks in the wing nacelles.

Twins need more rudder, so I'm going to increase the height of the fin and rudder by some 3.5". This will take the fin area from 35 in^2 to 47 in^2 and the rudder area from 28 in^2 to 34 in^2; the total vertical area will go from 63 in^2 to 80 in^2, or an increase of abt 30%. I may or may not add that as an aerodynamic counterbalance tab.

Engines will be mounted above the wing chord to give adequate prop clearance. Although I may end up with 2-3 deg of downthrust, I'll set the engine mounting points at -0-, -0- initially and shim em' up.

I like Matt's recycling of the 4*....

--Bill

William Robison

My Feedback: (3)

Bill:

Since the plane is already flying as a single, I'd not clip the wings until after having flown it as a twin. You may well find it is nice at stock span after the conversion.

If you have not already bought the engines, get the XLS 40 engines, the XL 40s are out of production, and the XLS 40s are a lot stronger than the XLs, too.

Bill.

BillHarris

>I'd not clip the wings until after having flown it as a twin.

Gotcha.


>...get the XLS 40 engines, the XL 40s are out of production...

Good point, I suppose that is why the XL 40 engines had that old-timey look.

Oh well, I'm not a cast in stone Luddite, I can change with the aeons...

--Bill

mgnostic

Update:I had my first experience with a lost engine on takeoff. [X(] I was at six or eight feet altitude when I heard the Bzzzrp of a dead engine. The port engine windmilled to a stop. I chopped the throttle and glided to a strait ahead landing just about fifty feet from the treeline at the edge of the field. The pucker factor was high but the cloe in thrustlines and large fin kept anything drastic from happening.

William Robison

My Feedback: (3)

Matt:

Glad you (and the airplane) survived.

Now. What have you discovered to have caused one engine to quit while the other purred happily?

Bill.

mgnostic

Fuel foaming may be an issue. At certain RPMs (mostly just above the lowest idle, but possible at other settings) there is significant vibration even though the props are well balanced. I'm thinking about trying to brace the nacelles a little better. They are just liteply boxes with square stock in the corners and an aircraft ply firewall. Alternatively the weather changed and the needle needed resetting.

twinman

My Feedback: (2)

My two cents, ALWAYS do the vertical test at full power for 10 seconds before every flight. If you worry about fuel foam, then cycle the engines with the plane vertical. No change of rpm is allowed or DO NOT TAKE OFF!!
This is really a must for multi's.
Twinman

mgnostic

After a few rounds of fiddling with the engines and and two flameouts on the other engine I finally got the needle settings sorted out. Luckily loosing an engine on takeoff is not the disaster it is with some planes. Chop the throttle, put the nose down and land in the back 40. I gotten a few more flights in and have some impressions. I tried out the flaperons. One needs to carry a little more throttle but landing speed is slower. If I were going to start from scratch I wouldn't clip the wings. Takeoffs with flaps down allow for a very steep rate of climb. I didn't notice any significant pitch change with flap deployment. I'd say on the whole that the plane seems slower, probably due to weight gain. A longer nose and nacelles with less nose weight would probably help. The loops seem to get a little skinny across the top. I would like to see some more torsional rigidity in the engine nacelles. I may add some internal bracing as the engines shake at high idle or a little above.