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Old Oct 04, 2012, 12:56 PM
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Joined Jul 2011
72 Posts
Question
Prop for a .12 engine

I tried a converted O.S. CZ-2 today with a prop and Iīm not sure if the readings I got is good or bad.
First of, my tach was giving me problems as always, but I got one reading from that and then I recorded the sound and did a check in my computer.

The prop used was a JXF 8x6 and the fuel is 20% with 17% oil.
Tach; 13400-Peak
Sound; 13200-Peak

Would a "lighter" prop be better, or will it unload in the air? I will be hand launching the plane, so good static thrust is desirable.
Is the numbers OK for a .12?


Thank you in advance!
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Old Oct 04, 2012, 02:08 PM
Airplane Gearhead
Central N.Y.
Joined Jan 2005
1,144 Posts
After reading many r/c car engine dyno test results over the years, I have noticed that a lot of "sport" type r/c car engines make their peak power at around 25K (give or take a couple)...and torque at a several thousand below that...maybe 17 to 20K.

I think you are over-propped a bit...I would suggest a 7x4 or maybe 8x3 if you need more diameter. Try a few different combinations, and try and get the r.p.m. up closer to 20K.

What sort of exhaust system are you using?
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Old Oct 04, 2012, 02:43 PM
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Ok, I kind of thought it was a bit over-propped.
I could try to take the prop down a bit first to see how it reacts to that.

I just looked at the specs and they state practical rpm range to 3-24K, so... smaller prop it is!

The exhaust is a semi-custom built.
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Old Oct 04, 2012, 10:18 PM
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My old Fox 15 liked an 8 x 4 best.

It was rated at 1/4hp.
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Old Oct 05, 2012, 01:52 AM
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I have no experience of hand-launching, so my priority is static thrust until I get some more practice.
So a 8x4 would probably be the first choice to try or a 8x5, depending on what my LHS has in itīs inventory.

Later a smaller (diameter) prop would probably be better as I will be belly-landing the plane.
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Old Oct 05, 2012, 02:09 AM
Complete RC Idiot Savant
The Netherlands
Joined Nov 2009
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Quote:
Originally Posted by NitroVein View Post
I have no experience of hand-launching, so my priority is static thrust until I get some more practice.
So a 8x4 would probably be the first choice to try or a 8x5, depending on what my LHS has in itīs inventory.

Later a smaller (diameter) prop would probably be better as I will be belly-landing the plane.
Unless really extreme props are used, static thrust is not really an issue when handlaunching: your typical handlaunch will allready provide you normally with sufficient speed to build up thrust.

If the prop is fitted properly (prop horizontal at closing point of exhaust port) the prop size should not really matter for belly landing.

Brgds, Bert
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Old Oct 05, 2012, 02:22 AM
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surfer_kris's Avatar
Sweden
Joined Jan 2008
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For modern glow ABC engines I would go with the following props;
.12 7x4
.15 8x4
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Old Oct 05, 2012, 07:10 AM
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I have fitted the prop so that i will be standing against compression when itīs horizontal.
But... things have a tendency to "go south" when you least expect it, maybe a smaller prop would give some margins then.

Iīm going to try a different glow plug also, Iīm using Enya-3 right now and that might be a little to hot for 20% fuel. I found a O.S. A5 that I will try, but that might be too cold....

Does anyone know of a tach that works good? I have a Jamara RC200 and that thing has never work well with any prop.


Thank you all for your help!
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Old Oct 05, 2012, 07:46 AM
Airplane Gearhead
Central N.Y.
Joined Jan 2005
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NitroVein...please correct me if I'm wrong...your converted O.S. .12 was/is a car engine, correct? If so, it's power band is considerably different from a typical r/c airplane engine. (the port timing(s) are for higher r.p.m.

Consequently, you should use a somewhat smaller prop than typical for an "airplane" .12 to allow it to get up into it's power band.

If I'm wrong about it being a converted car engine, then....nevermind...
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Old Oct 05, 2012, 08:22 AM
mjd
MikesRC
Canada, ON, Orangeville
Joined Aug 2003
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Quote:
Originally Posted by NitroVein View Post
I tried a converted O.S. CZ-2 today with a prop and Iīm not sure if the readings I got is good or bad.
First of, my tach was giving me problems as always, but I got one reading from that and then I recorded the sound and did a check in my computer.

The prop used was a JXF 8x6 and the fuel is 20% with 17% oil.
Tach; 13400-Peak
Sound; 13200-Peak

Would a "lighter" prop be better, or will it unload in the air? I will be hand launching the plane, so good static thrust is desirable.
Is the numbers OK for a .12?


Thank you in advance!
I agree with surfer's prop recommendations.

8-6 is too big IMHO even for a sport .15. I always say 8-4 max for most .15's. That said the numbers aren't bad for that much shaft load. As mentioned, this engine will be much happier if you let it spin some more. The OS literature "rates" it at up to 24k, indicating that in comparison to many car engines, it is more mildly timed and designed for easier handling.

Not sure what you have it bolted to - this plays an important role in prop selection - but the engine should do a lot more for you if you do not lug it. My suggestion would be 7-4 as well, and go from there. Maybe 7-5 tops if the aircraft is not draggy.
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Old Oct 05, 2012, 08:58 AM
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Quote:
Originally Posted by proptop View Post
NitroVein...please correct me if I'm wrong...your converted O.S. .12 was/is a car engine, correct? If so, it's power band is considerably different from a typical r/c airplane engine. (the port timing(s) are for higher r.p.m.

Consequently, you should use a somewhat smaller prop than typical for an "airplane" .12 to allow it to get up into it's power band.

If I'm wrong about it being a converted car engine, then....nevermind...
Yes this is a soft car engine or a more powerful plane engine, depending on how you look at it. Much like the "CV-range" they make today.
Itīs also slightly modified, but not much.

Quote:
Originally Posted by mjd View Post
I agree with surfer's prop recommendations.

8-6 is too big IMHO even for a sport .15. I always say 8-4 max for most .15's. That said the numbers aren't bad for that much shaft load. As mentioned, this engine will be much happier if you let it spin some more. The OS literature "rates" it at up to 24k, indicating that in comparison to many car engines, it is more mildly timed and designed for easier handling.

Not sure what you have it bolted to - this plays an important role in prop selection - but the engine should do a lot more for you if you do not lug it. My suggestion would be 7-4 as well, and go from there. Maybe 7-5 tops if the aircraft is not draggy.
I tried this prop as I already had it, but I will buy some other prop to try now that I know a little better what range of prop to use.

The plane is a clone of a "SIG-Wonder", not the same, but close enough!
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Old Oct 05, 2012, 01:15 PM
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Well, the A5-plug didn't do much in performance, but it ran smoother and had better throttle response.

I decided to try the plane as the weather was just perfect.
The plane took to the sky without hesitation and the engine soon got a mean sound to it and unloaded quite a bit!
BUT, the plane was terribly out of trim so I honestly thought I had seen the plane for the last time. Ones I got that sorted out I got a dead stick when I least expected it...
Long story short, I have some wing dowels to re-glue and a ego to repair.

I think I will try both 7x5 and 8x4 to see which I like the most.

Again, thank you for your help!
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Old Oct 05, 2012, 01:19 PM
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Canada, ON, Cottam
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7-5, 7-4, 6-6, or 8-3 would be my choice in that order for that plane. The motor may have piped timing, If so, then a 6-5 would honk out good. You can tell roughly if it is piped if the prop goes about 180 degrees from the non-compressed half of the stroke, then it is piped timing. If there is about 150 degrees of uncompressed exhaust timing then it is unpiped. Just rotate the prop back and forth and if it is a fair bit less than half a rev, it is non piped and won't want to rev. real fast like 25,000 rpm.
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Old Oct 05, 2012, 02:50 PM
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Originally Posted by aspeed View Post
7-5, 7-4, 6-6, or 8-3 would be my choice in that order for that plane. The motor may have piped timing, If so, then a 6-5 would honk out good. You can tell roughly if it is piped if the prop goes about 180 degrees from the non-compressed half of the stroke, then it is piped timing. If there is about 150 degrees of uncompressed exhaust timing then it is unpiped. Just rotate the prop back and forth and if it is a fair bit less than half a rev, it is non piped and won't want to rev. real fast like 25,000 rpm.
I didn't do any accurate measurement of this engine so I only have some notes saved and I donīt remember if/what I changed.

Timing numbers;
Exhaust 150 degree (total) with an area of about 41mm2
Transfers 108 degree (total) no area written down
Crank Opens 40 ABDC Closes 50 ATDC =190 total (I think I increased this or maybe it was the area I increased... )
Carb is a twin needle 5,2mm
Muffler about 20cc with a 6x30mm stinger
I know I increased the compression, but donīt remember by how much...
I think I increased the crank bore and did some other "touching-up" in the crankcase etc.

As I said, I didn't do anything special enough to make a full blueprint to look back at.

I will see what the LHS has on the shelf and take it from there, at least I know now that I need a smaller prop thanks to some good advice.
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Old Oct 05, 2012, 06:41 PM
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I ran a 7X4 on my .10 so I think a 7X5 would be a good match. Sounds like you have a handle on it already.
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