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OS FS26S-C 4-stroke development

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Old 11-12-2017, 11:48 AM
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More progress.
I sat here thinking how I could add flywheel weight to the engine. Also, what if I DO use the buku clutch? It certainly weighs less than the big brass flywheel I made before so if anything I'll lose flywheel weight.
Here is what I came up with. I machined down the propeller hub that came with the engine. Up until now it's just been taking up space. It's pretty much hollow and doesn't add any weight. I machined it down because all I want is the center hub of it, which is keyed to the crankshaft. Now I can machine a large brass piece of metal to press that hub into.
This has a bunch of benefits.

1)Tons of flywheel weight.

2)A place to bump start the engine from easier.

3)I can use any normal flywheel now, standard aluminum or the Buku brass (if my clutch doesn't work out)




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Old 11-12-2017, 12:27 PM
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Originally Posted by timjs View Post
I installed the weights.
With these el-cheapo shoes, no extra weight, and .7mm springs I get engagement at 6,000rpm almost exactly.

With the weights added, it engages at 4,500rpm.

With the weights and .9mm springs, it engages at 8,500 rpm.

Obviously, I like 4,500rpm the best. IF I can get the engine to idle low enough. I'm sure it's simply a matter of having enough flywheel weight, but we will see.
Regardless, I just put in a Buku clutch order this morning and I'll use that instead if mine doesn't work. I always wanted one anyways.

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Cool, nice info..be interesting to see how you get that buku flywheel working, as I couldn't get my engine low enough to use a standard 34mm fly wheel. Think mine is somewhere around 38 or so. Also can't get engine to idel low enough for .7 springs , it's even tough for .9 springs as they still grab a little, could be my carb setup, but if messed around with it a lot. As Of now I like the 1.1 springs in the truggy.
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Old 11-12-2017, 12:42 PM
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Originally Posted by speedworks View Post
Cool, nice info..be interesting to see how you get that buku flywheel working, as I couldn't get my engine low enough to use a standard 34mm fly wheel. Think mine is somewhere around 38 or so. Also can't get engine to idel low enough for .7 springs , it's even tough for .9 springs as they still grab a little, could be my carb setup, but if messed around with it a lot. As Of now I like the 1.1 springs in the truggy.
Not sure if you saw my post right after that one (it started the next page in the thread so you might have missed it).
My prop hub modification solves both of those problems (starting, idle).
I'm actually on the lathe right now trying to finish the part. I'm ready to just drive this thing.
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Old 11-12-2017, 01:38 PM
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Originally Posted by timjs View Post
Not sure if you saw my post right after that one (it started the next page in the thread so you might have missed it).
My prop hub modification solves both of those problems (starting, idle).
I'm actually on the lathe right now trying to finish the part. I'm ready to just drive this thing.
Crap completely missed your other post lol. Now I see what your doing. Cool build, looks like it's coming along nicely. Definitely looking forward to track day!
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Old 11-12-2017, 03:39 PM
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Okay, here's the propeller hub so far. I machined it down so I can use the keyed hub portion. I made the brass section a VERY close slip fit on the hub. Any tighter and it would be a press fit.

Loctite 603 retaining compound curing now.

Once cured, I'll bolt it to the naked crankshaft, mount in the lathe between centers, and machine the O.D. concentric with the crank.

Then knurl it, since this is where the starter wheel will engage now.
The engine side has a small amount of stick out to give clearance to the bearing, and the flywheel side is flush.


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Old 11-12-2017, 04:32 PM
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Originally Posted by timjs View Post
Okay, here's the propeller hub so far. I machined it down so I can use the keyed hub portion. I made the brass section a VERY close slip fit on the hub. Any tighter and it would be a press fit.

Loctite 603 retaining compound curing now.

Once cured, I'll bolt it to the naked crankshaft, mount in the lathe between centers, and machine the O.D. concentric with the crank.

Then knurl it, since this is where the starter wheel will engage now.
The engine side has a small amount of stick out to give clearance to the bearing, and the flywheel side is flush.


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Looking good, wonder how much it will weigh compared to steel setup. Guessing you'll have more weight and will be able to get lower rpm for idle.
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Old 11-12-2017, 05:13 PM
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Originally Posted by speedworks View Post
Looking good, wonder how much it will weigh compared to steel setup. Guessing you'll have more weight and will be able to get lower rpm for idle.
It's definitely heavier.
I'm thinking I can get a reliable 3-4k idle, making the 4500rpm or 6000rpm clutch arrangement work well.
These engines will idle down to 1500ish with s propeller.. it just shows it's all about flyweight.
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Old 11-13-2017, 04:21 AM
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Originally Posted by timjs View Post
It's definitely heavier.
I'm thinking I can get a reliable 3-4k idle, making the 4500rpm or 6000rpm clutch arrangement work well.
These engines will idle down to 1500ish with s propeller.. it just shows it's all about flyweight.
Sounds like it will work out great for ya. Wonder if the extra weight will cause slower revs. And if so, will it even be noticed. Looking forward to seeing how it runs!
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Old 12-07-2017, 05:42 PM
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Okay so I've been tinkering some more.
My propeller hub that I turned into a flyweight turned out nice. I got it mounted and did some test fires on a spare chassis as a bench test. Unfortunately, the D shaped key section stripped out and the flyweight was able to spin on the crank. Bummer.





So I decided to make a new flyweight but instead of machining down another prop hub to steal its keyway, I wanted to use a tapered collet system that we're used to using on our 2 strokes to secure the flywheel.

I machined the collet first. (I still need to slit it) It had to be custom made, because the section of the crankshaft by the front bearing is much larger than the typical flywheel collets you have in your pile of RC parts...
Then I made a new flyweight.

The flywheel itself is secured by being tightened up against the flyweight with a diamond friction disc. These discs are used in automotive engines to lock cam sprockets to the cams, and other high torque applications. It will not slip.

Testing soon first with my DIY weighted shoes, then with my Buku setup if the DIY don't work out.



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Old 12-07-2017, 10:53 PM
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Looking good! Iv never heard of those disc before, but that should definitely hold it in place. Looks like that lathe is giving you a great surface finish!.
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Old 12-09-2017, 10:16 AM
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More bench testing. Trying to get an idle below 4,500 RPM because that's when my clutch engages. It seems that once I idle just a little too low, the engine actually starts running backwards at really high RPM and shoots a huge plume out of the carburetor. Has anyone else encountered this? It did not do this when I had the engine in my HPI bullet. It seems that the engine is bouncing off of the compression stroke right before it stalls and then starts running backwards. But I have 105 grams of flywheel weight at the moment.
Here is a video. It doesn't show the backwards thing, it's just a running video.
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ERROR: If you can see this, then YouTube is down or you don't have Flash installed.
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Old 12-09-2017, 11:57 AM
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Success!!

The rpm is as low as it can go reliably. So I went to a larger clutch spring.

Here's a video of the engine idling (which it was for two entire tanks prior to shooting the video) and I can grab the clutch bell.

This is with el-cheapo aluminum shoes, with one tungsten-copper slug per shoe added for weight, along with .8 springs. The .7 springs in the last video were engaging at idle. They would likely work great with standard shoes, which are usually less than 2 grams. My shoes weigh 3 grams each.
Of course I still have Buku on the sidelines, just in case.

Once I get my chassis back from my buddy who is milling the engine mount slots ect, I'll put it all together and go for a test run!

Im excited! Sharkey, where ya at ???

I wish more people were active here still!

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Old 12-15-2017, 02:24 PM
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So, I finally broke down and bought a new-in-box FS-20 on eBay. I'll probably just fiddle with it, unless I can find parts to attach a pullstart to it. I'm one of those annoying nitro hobbyists who prefers convenience vs. performance. If anyone happens to have pullstart parts for a FS-26 (it should fit), let me know; I'll be happy to trade my non-pullstart crankshaft and backplate for those parts, plus some cash if necessary to make things even. Otherwise, I've got myself a(nother) cool desk toy to play with.
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Old 12-16-2017, 01:01 AM
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ive havent been paying attention here for a while, been busy with my 1/1 scale cars.

im not surprised to see the aluminum hub in the flywheel strip, hpi tried a double D flywheel on the 15fe many years ago and it would strip in no time. good to see the collet flywheel it working.

when you idle the engine too low whats happening is the flywheel cant quite carry the engine past top dead center, but the engine builds enough compression to ignite the mixture and it will kick the engine backwards. at this point it may have enough momentum to keep running backwards and sucking in any unburnt fuel in the exhaust.
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Old 12-16-2017, 03:51 AM
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Originally Posted by Sharkey_t View Post
ive havent been paying attention here for a while, been busy with my 1/1 scale cars.

im not surprised to see the aluminum hub in the flywheel strip, hpi tried a double D flywheel on the 15fe many years ago and it would strip in no time. good to see the collet flywheel it working.

when you idle the engine too low whats happening is the flywheel cant quite carry the engine past top dead center, but the engine builds enough compression to ignite the mixture and it will kick the engine backwards. at this point it may have enough momentum to keep running backwards and sucking in any unburnt fuel in the exhaust.
Hey man, how's that Bonneville speed record going? Or the Porsche? I think I'm remembering those correctly.
Yeah, you're right on what's happening. And that's with 105g of total flyweight. A REALLY low idle would require a huge slug of metal I guess.
Are you still going to put that wankel in the tc3?
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