Webra .36
05-01-2009, 07:00 PM
#7
RE: Webra .36
I've been running FOX .36 COMBAT SPECIALS that way for the last 24 years, [8x6s at 22,000 rpm]. The 8x6 is a good compromise between acceleration and flat out speed, but at 22,000 rpm a 6 pitch prop becomes an air brake at 120 mph.
05-03-2009, 09:14 PM
#9
RE: Webra .36
It all depends on the airframe and RTF weight. The 8x6 on a plane like the Vampire might be the best all around choice. There probably isn't any "off the shelf" prop that is optimum, but the only thing to do is try different props that are in the ball park with the 8x6. I hope Weston doesn't raise the exhaust timing to the point where the engine can't tolerate more load than a 8x6. I don't have any experience running this engine, but I'll bet a 7.5 x 7 is worth trying. Keep trimming diameter until the plane slows down, just beware of increased launch difficulties if the plane is heavy. Too bad the old maple pylon props are so scarce. Believe it or not, Master Airscrew has a thin blade series that respond well to trimming and shaving.
The prop that my OS .32 makes the best speed with is a 8x8 at 18,000 rpm...right about 130 mph. I've run the same engine with 7.5x6s to compete against the Foxes and Nelsons in AMA Fast Combat and it holds it's own....I would expect the Webra .36 to be an upgrade.
I hope it is, because the OS .32 is "no more" and will need to be replaced someday.
The prop that my OS .32 makes the best speed with is a 8x8 at 18,000 rpm...right about 130 mph. I've run the same engine with 7.5x6s to compete against the Foxes and Nelsons in AMA Fast Combat and it holds it's own....I would expect the Webra .36 to be an upgrade.
I hope it is, because the OS .32 is "no more" and will need to be replaced someday.
05-04-2009, 04:37 AM
#10
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RE: Webra .36
ok, cool.
Inoticed the 7x7 apc, the hub becomes very small. It wouldn't suprise me they have played with exhaust timing. I've been blowing alot of plugs recently. My header is very short, could this be an issue.?
Inoticed the 7x7 apc, the hub becomes very small. It wouldn't suprise me they have played with exhaust timing. I've been blowing alot of plugs recently. My header is very short, could this be an issue.?
05-04-2009, 09:19 AM
#11
RE: Webra .36
I'm not sure. If this is a "factory set" system, your tuning options are:
1. Make sure the fuel isn't foaming. I would use a bubbless tank.
2. See to all the other basic needs like tight bolts, no pipe pressure leaks, fuel system leaks, loose needle, etc.
3. OK, hate to mention...but basic needle setting should be done with the nose pointed up and it should be just rich enough to pass a pinch test.
4. Shim head, increase combustion chamber volume. This is needed if the plug elements are being obliterated. You might consider less nitro and more oil.
5. Decrease prop load.....shave those blades, scrape them with a hobby knife, make the tips lighter. Decrease diameter. Don't mess around too much with modifying APC, but they can be shaved. I can't see how a 8x6 could be any problem. I've had highly timed .36s that would tear it up on the flat and just stop suddenly during a violent manuever....high timing kills torque. changed to less prop and the problem went away.
6. It's pretty hard to keep the fuel from foaming at 22,000 rpm, a Jett or Tettra tank is cheap insurance it will never happen.
7. Mess around with plug brands and heat ranges....I have seen miraculous results but have also seen not much improvement. I just get concerned when the engine is blowing away plug elements often because that ups the odds of dragging tungsten into the cylinder and ruining an engine. The cold plugs with the heavy wire are considered for heavy duty. It is possible to have a combination of problems that contribute to blown plugs. At this rpm level, I'm happy if the elements get used up every few runs, just not blown completely out the exhaust.
8. Weston should be able to "walk you" through a check list that they have on paper, I would imagine. Good luck and have fun. I was teasing you about the 22,000 @ 6 pitch, I'm sure it's quite a crowd pleaser. You might see a significant improvement in speed with a 7x7, but be ready to shave blade material if the pipe refuses to stage. My Webra .50 is finicky, it does best with just an open header.
1. Make sure the fuel isn't foaming. I would use a bubbless tank.
2. See to all the other basic needs like tight bolts, no pipe pressure leaks, fuel system leaks, loose needle, etc.
3. OK, hate to mention...but basic needle setting should be done with the nose pointed up and it should be just rich enough to pass a pinch test.
4. Shim head, increase combustion chamber volume. This is needed if the plug elements are being obliterated. You might consider less nitro and more oil.
5. Decrease prop load.....shave those blades, scrape them with a hobby knife, make the tips lighter. Decrease diameter. Don't mess around too much with modifying APC, but they can be shaved. I can't see how a 8x6 could be any problem. I've had highly timed .36s that would tear it up on the flat and just stop suddenly during a violent manuever....high timing kills torque. changed to less prop and the problem went away.
6. It's pretty hard to keep the fuel from foaming at 22,000 rpm, a Jett or Tettra tank is cheap insurance it will never happen.
7. Mess around with plug brands and heat ranges....I have seen miraculous results but have also seen not much improvement. I just get concerned when the engine is blowing away plug elements often because that ups the odds of dragging tungsten into the cylinder and ruining an engine. The cold plugs with the heavy wire are considered for heavy duty. It is possible to have a combination of problems that contribute to blown plugs. At this rpm level, I'm happy if the elements get used up every few runs, just not blown completely out the exhaust.
8. Weston should be able to "walk you" through a check list that they have on paper, I would imagine. Good luck and have fun. I was teasing you about the 22,000 @ 6 pitch, I'm sure it's quite a crowd pleaser. You might see a significant improvement in speed with a 7x7, but be ready to shave blade material if the pipe refuses to stage. My Webra .50 is finicky, it does best with just an open header.
