What size engine? (Started Komander build... finally) - RCCanada - Canada Radio Controlled Hobby Forum
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Old 08-31-2016, 08:54 PM   #1
HAL9KPCM
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What size engine? (Started Komander build... finally)


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I'm more of a helicopter guy, but I'm finally getting around to building my Sig Komander now that I finally have everything. I've been toying with the idea of going with a 4 stroke instead of a two stroke. The kit calls for a .40-.50 size 2 stroke, so what would be a reasonable sized 4 stroke?
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Old 08-31-2016, 09:42 PM   #2
chuebner
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Re: What size engine?

Hey Dave...... A .65 to an .82 4 stroke would work fine if you don't want to go 2 stroke.
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Old 08-31-2016, 09:52 PM   #3
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Re: What size engine?

The cowling on that model may be an issue in regards to a four stroke Dave.
Try and get some measurements from a Saito 65 or 62 and see if that will work.

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Old 09-01-2016, 12:32 AM   #4
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Re: What size engine?

Thanks guys, that was kind of the range I was thinking it would be. And yes, I would do some measuring first in case I need to do some modifying to make it fit.
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Old 09-01-2016, 01:35 AM   #5
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Re: What size engine?

OK, looking into it a bit more, I think I'll be staying with a two stroke. A four stroke will cram in with little modification especially with a spinner with an offset backplate, but I think prop clearances become the bigger issue. It looks like most of the engines in that range would like around a 12" prop. Using the suggested wheel size of 2 1/4" on the front, that only leaves 1 1/8".
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Old 09-01-2016, 06:55 AM   #6
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Re: What size engine?

You can opt for a 3-blade prop for ground clearance. Unlike 2-smokers 4-strokes can easily work with them.

In attempts made over 3 decades I was never able to get a 2-stroke to work that way. The extra drag always prevented them from reaching the rather narrow peak power band, (regardless of the "Conversion" rules). The much wider torque curve on 4-strokes makes best use of that extra blade.

For example, with the kit built Saito 91 powered Sig Skybolt and the G/P Super ARF with the Saito 100 in the pics you can see 3-blade props. I found the 13/8 Master Airscrew to be a good fit and got me back the ground clearance I needed for grass ops.

Yes, you lose some performance, particularly in vertical. However, I can work with that and in fact the Super has virtually a 1:1 power/weight so can climb forever if I want to. Makes for some impressive take offs at gatherings.

NOTE: Just thought I would add this bit extra: I found when testing a larger diameter 2-blade that can make use of the power also shows greater torque effects on uplines, to the point I could not counter fuse rotation at very low climb rates. Imagine that could come back to bite if doing a show routine.

Beyond the torque curve the 4-strokes sound much better, especially with scale. Their fuel consumption is also MUCH better. If any negative it is that the lack of an internal fuel/air charge makes adequate cooling more critical. If cowled you may need to do some mods to ensure good flow.

If you are comparing the recommended 11/7-8 range of a 45-50 2-stroke you could stay with that diameter and start with an 11/7 3-blade and work up if still too low. The reasons for lack of specific recommendations are different makes of props can generate different performance. Although I usually try to stay with wood for the first little bit till the bugs are out I use M/A more for availability, (but really prefer Graupner for their wide chord and absolute rigidity).

ps. If memory serves they recommended a spar be inserted into the foam wing when engines pushed the upper edge of their range, (45-50). There were also some significant changes to the fuse between Kommander V1 and 2, with the latter having thicker skins and a ply lower. If you have the V1 would strongly suggest you incorporate those changes.
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Old 09-01-2016, 07:20 PM   #7
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Re: What size engine?

I have a few partial kits actually. 2x V2 bodies, 1x V1. I'm using the V2's. Now you have me thinking again. A Saito 62 will squeeze in quite nicely. I'm just looking for casual/sport flying, nothing too wild. I might need to make a slight cutout in the firewall for a little more carb clearance, but I think we're talking millimeters here which is where a spinner with an offset backplate might give me the little extra required. Actually, when I think about it, I could glue a balsa or ply ring to the front of the cowl for a few extra mm as well. I do like the sound of the 4-strokes.
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Old 09-01-2016, 10:31 PM   #8
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Re: What size engine?

Do you think an O.S. FS-52 would be too small?
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Old 09-01-2016, 11:06 PM   #9
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Re: What size engine?

Quote:
Originally Posted by HAL9KPCM View Post
Do you think an O.S. FS-52 would be too small?
She will fly it but won't be a rocket. Some have used a .40 2 stroke and it flew fine, so the .52 4 stroke would work but it would be at the low end of the engine range.
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Old 09-01-2016, 11:11 PM   #10
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Re: What size engine?

Will be great on fuel.

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