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Building Tips and technical articles. => 1/2 A building. => Topic started by: John Rist on March 20, 2017, 10:33:01 PM
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Saw this on eBay. Wounder if it really works? ???
http://www.ebay.com/itm/Very-Nice-RC-Tech-Cox-098-V-Twin-Nitro-Control-Line-Model-Airplane-Engine-/371894732813?hash=item5696a7480d:g:tPUAAOSwWxNYy00d
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I don't know if it would work or not,but it sure looks cool!!!
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Looks well done,should work.
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Looks well done,should work.
No reason why it wont work. There was an opposed twin cox 098 sold in the late 60's early 70's for perhaps a year or two. Memory is short anymore but the price was probably too high for success in that time frame.
Dennis
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I think the two cylinders would need to both fire at the same time, or else the reed valve would not be able to function correctly.
If the were 90 degrees out of phase the first cylinder would hog the incoming charge.
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I think the two cylinders would need to both fire at the same time, or else the reed valve would not be able to function correctly.
If the were 90 degrees out of phase the first cylinder would hog the incoming charge.
Saito made a twin 90 4 cycle with the cylinders not staggered and alternate firing .It originally had one carb but eventually went to 2 to cure idle and transition problems. I had one of those and they were serviceable but not the most potent engines .They had a really odd timing over 500 degrees to get it to work and it was replaced with a larger version but that also did not survive.
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Four stroke is altogether different.
With 2-stroke the crankcase undergoes alternating pressure and vaccum.
Previous 2-stroke multi-cylinder engines had separate crankcases.
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Looks like it never has been run. There may be a good reason. y1
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Looks like it never has been run. There may be a good reason. y1
Looks like Valentines creations
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I doubt Valentine would release a non-functioning engine in any form.
I doubt it runs. Both cylinders would need to be very near the same timing or it won't draw in enough fuel. The offset of the cylinders is pretty small for this to work out.
Phil
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It's up to a $172. n~
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It's up to a $172. n~
$710 final bid. Think I should start making Cox V-twins. LL~ LL~ LL~ LL~ LL~
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I have seen several home-brew 3-cylinder and 5-cylinder radials made with Cox reedies. The only way to make it work is separate engines, all gear-connected around the propeller gear. Only problem, they are way too heavy for any practical use (except maybe a scale-model tank.)
(try to get them all needled at the same time..on the same day!)
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Non runner.
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Non runner.
Why bother building it?
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Because 7 hours of work with $30 or parts gets over $700 from someone who wants a cool display piece
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Because 7 hours of work with $30 or parts gets over $700 from someone who wants a cool display piece
Exactly, do a search on these, not one says that they actually work and its probably because the primary compression ratio is so poor that scavenging would be non existant.
If they were reported to work the first thing you see is something like "pulls 10 000rpm with 7x4 prop."
(Please, some one prove me wrong here!)
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I also do model engineering and have built both steam and internal combustion model engines, so have done a fair amount of research in these areas.
Theoretically, the V configuration as in this engine should produce some pumping action in the crankcase. The big issue here is scavenging. The leading cylinder would be over scavenged while the other would be starved. This engine with the 90 degree V might run but it is doubtful if the second cylinder would have enough fresh mixture to fire.
Now if the V were smaller and in the range of 30 to 45 degrees, this configuration might have a better chance of performing since the pumping action would be greater. In order to balance up the scavenging one could use a single bypass cylinder for the leading one and a double bypass cylinder for the trailing one. Of course this engine would have to run in only one direction.
This is an interesting idea and I might just make one to satisfy my curiosity.
Orv.
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There are instructions with the tooth count and diameters for making geared twin Cox engines. Those are actual runners suitable for scale airplanes.
Phil
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I also do model engineering and have built both steam and internal combustion model engines, so have done a fair amount of research in these areas.
Theoretically, the V configuration as in this engine should produce some pumping action in the crankcase. The big issue here is scavenging. The leading cylinder would be over scavenged while the other would be starved. This engine with the 90 degree V might run but it is doubtful if the second cylinder would have enough fresh mixture to fire.
Now if the V were smaller and in the range of 30 to 45 degrees, this configuration might have a better chance of performing since the pumping action would be greater. In order to balance up the scavenging one could use a single bypass cylinder for the leading one and a double bypass cylinder for the trailing one. Of course this engine would have to run in only one direction.
This is an interesting idea and I might just make one to satisfy my curiosity.
Orv.
Hi Orv,
I agree with what you are saying, a common crankpin twin cylinder two stroke set at 0 degrees (or cylinders parallel) is the best primary pump configuration and when set at 180 degrees it simply doesn't work since one pistons motion cancels out the other (one rises as the other falls).
So logically any angle that approaches 180 simply gets worse as the cancellation effect becomes more prominent.
But what gets me here is two things -
1/. The common crank pin length and the massive primary volume that it lives in, please see the supplied pic and visualise where the back plate must finish at inside the lower case. That lower primary volume must be huge when expressed as a ratio against the secondary or swept volume.
2/. Also when the lead piston starts on its compression stroke the primary volume is supposed to be lowering its pressure in preparation for the intake but the following piston is still descending mid stroke and negating part of that ...... and since its a reed valve engine there is no getting around the pressure demands.
Give it a crack, I am curious too to see if you can get a runner out of this configuration.
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I can see how it can be done if the piston connecting rod bottoms don't ride on the same crank pin, but keyed in a way that both pistons achieve TDC and BDC in unison. However, to do that would require possibly another say disk that rides off the first piston's crank pin that the con rod bottom rides in, and has a separate pin that the second piston's con rod bottom rides in. Doable but adds another level of complexity when considering bearing surfaces and ability to remain true in alignment.
Personally, I prefer our cheaper massed produced engines to power my toy aeroplanes, simplicity at its best. ;D
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Hi Chris,
I dug out a production engine, did some measuring and calculating:
1. Using the existing piston/con-rod set-up, one would only have to add .120" to the distance between the crank pin and the carb. This works out to an approx. volume increase of .03 cubic inches. We now have an extra piston/cylinder swept volume of .049 cu. inches. Since the extra swept volume is a fair amount greater than the increase in crankcase volume (.049 to .03), we should see better pumping action.
2. Now you are correct in the theory that some of the volume change is compromised by the "V" configuration and the greater the angle, the greater the inefficiency. This is why it is vital that the "V" angle be made as small as possible otherwise any gain in pumping action is quickly cancelled out.
3. Balancing the bypass flow is also vital so that both cylinders operate at equal efficiency... lots of fun experimenting with this problem!
In the old days, the CL stunt guys using the ubiquitous Fox .35 would put a wood block in the "too large" bypass port in order to prevent the burp at the top of a loop. Some JB Weld may be needed in the leading Cox cylinder bypass to do the balancing, carving it away with a Dremel until a good balance is achieved.
Orv.
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Us Fox users still JB weld blockage in the bypass if flyimg on a profile model, but it doesn't seem to be needed if upright or inverted.