buzzard bait

I got an OS 81a FS in a club raffle and figured I'd use it to replace my old OS 70 FS on my Four Star 60. It's not quite that simple, so I thought I'd let people know.

My main problem is that the prop stud is longer by about 3/16th inch. I guess they wanted to accommodate higher pitch props, but I think they overdid it. My aluminum spinner won't fit, and I'm already using the shortest adapter nut, so I can't see how this can be done. I carved the cowling to fit the curves of an FAI type, so no way to use the engine without spoiling the lines.

The distance from the mounting bolts to drive plate face is the same. But it is about 1/4 inch longer behind the mounting bolts. The needle valve is actually a little forward of where it is on the 70. The throttle arm sticks out farther and it's plastic; the old 70 had it kind of tucked back behind the cylinder and it was metal.

In another thread I think someone said that the throttle arm travel distance from full throttle to idle was very short. But it is actually the same as on the 70.

I can carve out the cowl in places to fit the new engine, and I can probably get the throttle cable to reach, but the prop stud length really has me stumped. I'd love to use the new engine, but it would look awful on this plane without the spinner.

Jim

buzzard bait

Wait a minute, that doesn't make sense...so I went back and puzzled over it. It turns out the new prop nut washer is very slightly thicker than the old one, and that seems to take it from just fitting to not quite fitting. So obviously I can just use the old one. Unfortunately, I think I really do want a bit more pitch with this engine, and then it won't fit, but it's not the fault of the design change.

rcdude7

My Feedback: (264)

I think they only mean to imply that the 81 has the same mount pattern than the 70, the other engine dimensions are going to be off.

Did you build a cowl for your 4*60? It would be great to have a picture to drool over

buzzard bait

You're right, I'm not really criticizing so much as giving a heads up.

One more thing, it weighs about an ounce more: 21.1 oz vs. 20.1 oz. That's including muffler, prop nut and washer, on each.

Thanks for the interest in the cowl. It's sort of a semi-cowl...not removable, so there's a big cutout. This is an old picture with a red plastic spinner from Hobby Lobby. The plastic deformed when I tightened the prop nut, so I went to a metal Vortech of almost exactly the same shape before the first flight.

More pics of the Four Star in my Gallery.

Jim

Campgems

Jim, first nice looking 4*

is the inside of the spinner hitting the adaptor nut? OR is the spinner scrwew to long and bottoming on the crankshaft before tightening up the spinner. If that isthe case, just get a shorter screw.

Don

rcdude7

My Feedback: (264)

That nose looks nice, you did get creative with that kit. I was interested because I have built, and destroyed a few 4*60 kit birds that I have built over the years. My planes were only modified with larger control surfaces and stronger klett gear. After trying a few engines, I had settled on the OS .91fs and Enya .90 4C as my prefered engines on this plane. I have two more of these kits socked away, I have not found another plane that is as forgiving and yet fully aerobatic, I need to build another one soon.

noveldoc

My Feedback: (2)

How did it fly with the .70 engine? Had an old one that did pretty well with an OS 50. (I fly slow...) Rebuilding after a crash and I have a .72.

Tom

rcdude7

My Feedback: (264)

I have never flown one with a .70 four cycle. The smallest engines I have tried were a .61 supertigre and OS .61fsr. They were not enough for my type of flying.

A OS .91 four cycle will give a long vertical climb, but even it will still run out of steam.

buzzard bait

Guys, thanks much for the compliments on my Four Star.

Don, it's the inside of the spinner hitting the adapter nut when it is screwed all the way in. I have a friend who can do some light metal working, so I'm thinking of asking him to grind down the adapter nut by 1/8 inch. That would easily do the trick, and I think it would still leave enough meat for the screw to attach securely. What do you think?

RCD7 and Noveldoc, the OS .70 flies it very well using a 14x4 prop, but I know not everyone would be satisfied with the vertical. It will easily do vertical eights, either with an outside loop on the top or a half roll at the center. Remember, it's lighter than most 4 Stars, at 7 lbs even. Also, with the 14x4 it lands very nicely without the excessive float that some people complain about. But it runs out of steam fairly soon if you just point the nose straight up. I was satisfied with it for my style of flying, but once I started thinking about getting more power with an engine swap I started liking the idea.

I think that grinding down the adapter nut would do the job. Let me know if any of you have any thoughts on this. Meantime I'm going to clean up the 70 a bit and put it back in. I agree, 4 Stars are really nice flying planes.

Jim

Jim

Campgems

Jim, how many nuts are you using. I've got a few engines in the 70 to 90 range and with the stock main nut and then the Adaptor nut for the spinner, all have at least 3/4" from the top nut to the spinnner. How about a photo of the nuts on the engine and another of the spinner. Something is not adding up, maybe me.

I would not grind down the nut. It will probably be unbalanced in the end. There is somethign else going on here.

Balance is a funny thing. I just finished working on a friends 1.20 O.S. surpass III and i found an interesting thing. On most engines it would never be noticable. As the engine dropped down in RPMto and idle, at 2000rpm, there was a shutter on the engine, that smoothed out at about 1850 and was OKdown to the low of 1500 that the engine would idle to.. It happened several time. I know the prop and spinner and back plate were balanced, but it was really obvious. This was on a test stand, It was like it got into a harominic viberation then out of it.

Send us some phots of the prop mounted and the spinner you are using.

Don

buzzard bait

OK Don, I'll post pics tomorrow, turning in for the night now. I appreciate the help.

Jim

mike109

G'day The one in the Avatar pic on the left is my Four Star 60 and at the time that picture was taken it had a Saito 72 in it and flew well. It now has a Saito 100 in it and files better but mainly when it is going up. The 72 would do all the normal aerobatics but no go vertical forever and was not all that fast. My son flew it (he was flying it in the photo) and liked it but said more would be better for him. For me it did not really matter. Depends on what you want.

blw

My Feedback: (3)

Sometimes, harmonic resonance is impossible to design out of an engine or airframe. Full scale planes with this problem have caution zones somewhere on the instruments. You can get them with multiple gearboxes too.

http://www.youtube.com/watch?v=RihcJR0zvfM

http://www.youtube.com/watch?v=D2tHA7KmRME

That's ground resonance. Somewhere, I have a short video on VHS of an OH-58 test pilot intentionally inducing several resonances in the air that causes the transmission to start coming out of the airframe, and the tailboom buckles.

Campgems

My first helicopter ride was in a piston powered Korean war vintage one that would hold about a squad of solders. When the pilot started winding up for take off, I though the thing was gong to come apart, it was shaking so bad, but just before liftoff, it smoothed out and what a ride.

Don

buzzard bait

I'd better rescue this thread in a hurry!

Here is the issue: if I use the prop nut plus the adapter nut, the combination is too long. The second picture shows that it's not by much, maybe 1/16 inch or a tad more. If I use only the adapter nut, then the prop stud is too long and you can't tighten down the prop.

So, it makes sense to me to grind down the adapter nut. I don't see any reason why there should be vibration problems. Why is that any different from using different spinners, or different spinner nuts, or even different props?

Jim

Campgems

Jim, what size spinner are you using. I could be wrong,it's been known to happen more than once, but it looks like the spinner could be another inch in diameter with no problems. That would also give you more clearance behind the spinner as it will be longer. If you stick a nail into the center hole on the adaptor, how deep is it before the screw bottoms on the crank? Then measure the point of the spinner to the crank with the adaptor off. Allowing for the center hub in the spinner cone, that is how long your adaptor can be. I would be tempted to find someone with a lathe and make the trim.

One other thing, it looks as if it is not a O.S adaptor but an aftermarket one. The ones that fit the first nut as a jam nut are the best. The person with the lathe could cut the taper on the back of the adaptor nut to fit the stock nut. Keep the grinder away from all things on an engine. The last thing you want is some grit trpped in the threads that eats the crank and nut each time you put the nut on or take it off. An old machinest and mechanic's recomendations.

Last stay away from the crome plated brass spinner adaptors. They are soft and they break easilly. Your looks like a good old blued steel one.But for the next one.

Don

buzzard bait

Don, changing spinner sizes or shapes is not an option in this case, because of the way I shaped the cowl to fit the spinner.

The adapter nut is Dave Brown and it fits into the indent on the OS nut perfectly. I've checked out the inside threads on the adapter nut and I'm pretty sure it will work.

In the meantime I've got the 70 FS re-mounted and I'm going flying as soon as the weather clears. I'm dying to get this bird back in the air, and now that the engine switch is not so easy it will have to wait.

I'll post again when I get the nut shortened and let everyone know if it works. Thanks for all the interest and help.

Jim

Skeezix4me

Thank you for posting this information - I was considering the same swap you have done!