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Cox .049 max RPM?

Old 02-11-2013, 01:48 AM
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mick1404
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Default Cox .049 max RPM?

Hi everyone,
I have always loved Cox engines, (the small ones and the tiny ones) and I'm just curious, what is the max RPM anyone has had their engine up to without getting to the point of detonation or something letting go, and what mods have been done to achieve high RPM's.I know this has probably been discussed before somewhere butcuriosity has got the better of me.
Thanks.
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Old 02-11-2013, 01:09 PM
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Default RE: Cox .049 max RPM?

I used to run TD .049s with Cox 5x3 rubber props at 23-24,000 rpm all day, every day on 30-40% Powermaster fuel. They needed frequent ballsocket resets and they used to gobble up glow plugs. The venturis had been opened up to 5/32" and the cranks were polished.
I tried to keep 3 or 4 of them running at this power level all the years I flew 1/2A combat in competition. If the engine couldn't hit 23,000..it didn't get to "make" the road trip.
The bigger venturi requires backplate pressure or a latex bladder "tank".
Fast forward to 2003-4 and there is information given here [at RCU] about how to shave excess weight from the insides of the piston. This improves revving ability and takes stress away from the reciprocating parts. Now you can expect to see upwards of 28,000 rpm from TDs without rapid self destruction...with a 4.2x4 prop or a FG combat prop.
The only KillerBee engine I ever examined already came with a lighter piston and huge intake.
It was ready to turn a APC 4.2x4 at 28,000+ right out of the box.
The ball sockets will crumble/erode if you allow them to get any slop.
25% oil content fuel is a good idea.
As far as I know, the KillerBee is the most powerful .049 they ever made.
This forum ought to have stickies posted at the top that link to all the great articles that have ever been published about how to modify these engines.
The North American Speed Society is made up of the AMA control line speed competitors. They are also a valuable resource if you look them up.
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Old 02-11-2013, 01:48 PM
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Default RE: Cox .049 max RPM?

I've had an absolutely standard TD 049 up to just over 25,000 on 30% nitro on a DC 5-1/4 x 3-1/2 prop [which are anything but what they're supposed to be, pitch wise] which are very flexible-I'd imagine the blades had washed out to very little actual pitch. It sounded fantastic-and had heads turning-but it didn't fly the model [1/2A Witch Hawk] worth a knob of proverbial on that prop-and I went back to a Cox 5x3 black. Always remember-THRUST is what flies models, not revs..........

....as for modding-well the sky is the limit I suppose-once upon a time people even used to get their own cylinder blanks from Cox and do their own (usually schneurle) bespoke cylinder porting, but that avenue has dried up. As CP has said-dozens of articles have been published over the years on hotting up Cox engines. Some of it requires access to precision machine tools, some can be done with simple tools. Blueprinting, using cylinder shims, and playing around with head clearance and alternative heads are the start points, as well as opening up shaft timing and the carb bore, plus improving balance, lightening the piston and flow contouring. Done well, a lot of gain is possible-done ineptly, you now have an interesting paperweight for your desk..................
ChrisM
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Old 02-12-2013, 12:06 AM
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Default RE: Cox .049 max RPM?

awesome pointers there guys, i just got to get some 'guinea pigs' from Cox International to modify and see what RPM's I can achieve.
thanks heaps.
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Old 02-12-2013, 02:42 PM
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Default RE: Cox .049 max RPM?

Hey Mick,
If you havn't allready, check this out.
http://coxengines.ca/files/MRP.pdf
Have fun,
Bob
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Old 02-13-2013, 02:51 AM
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Default RE: Cox .049 max RPM?

hey thanks Bob, very informative, interestingand quite handy, thanks heaps.
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Old 02-13-2013, 01:07 PM
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Default RE: Cox .049 max RPM?

This is a doppler "report" of a speed pass made by a TD .049. The engine was running on 10-12% nitro and 22-25% oil.
4.2x4 APC prop pulling a fairly small [150 sq inch wing] and a pretty thin airplane made mostly from balsa sheet.
The engine has a shaved [lightened] piston, fine thread needle valve, 5/32" venturi, crankcase pressure nipple type backplate and uses Nelson glow plugs.
The engine's mechanical condition is nothing special, actually it is well past it's prime.
The Thrust HP chart shows what the inflight rpm needed to be to acheive 115 mph and 1/4 HP.
I've found that it's possible to build small models that can acheive the mathematical pitch speed..by using 3 seperate ways to record speed that end up in nearly perfect agreement with each other.
Building "small, thin & light" is how you make total use of the .049's power potential.
Yes, you can also use one of these engines to power an Erector Set draw bridge, but that isn't what the quest for maximum usable power is about here.
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Old 02-13-2013, 03:14 PM
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Default RE: Cox .049 max RPM?

Should be able to calculate inflight rpm from the Wav-o-scope trace more accurately than thrust calc:

2987Hz receding + 4067 Hz approaching = (4067+2987)/2 Hz = average 3527Hz

You used the 8th harmonic, which is 8 times the base frequency

3527 cycles per second x 60 seconds = 211620 cycles per minute / 8 = 26,453 cycles per second = 26,453 rpm. Does this sound about right?

I have to get my head around if it would read low from parallax error (distance from sound source at center stage) or if that only affects the speed calculated from the frequency shift. I believe the latter is true.

The actual aerodynamic pitch is always higher than pitch measured the classical way from the lower surface datum line, unless the prop airfoil is a flat plate. The difference is often several degrees. So a prop with 4 inch pitch measured the classic way (which AFAIK is standard for prop makers, someone correct me if I am mistaken) is higher than 4 inch aerodynamic pitch. An efficient aircraft could achieve the theoretical pitch speed or perhaps even a bit more, because at that point the prop blades are still at a positive angle of attack relative to the zero lift line.
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Old 02-13-2013, 04:09 PM
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Default RE: Cox .049 max RPM?

I don't really understand it. 500 hz is actually 30,000 rpm...so I'm not sure what those frequency numbers mean on Wavoscope.
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Old 02-13-2013, 05:00 PM
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Default RE: Cox .049 max RPM?

The left hand trace is the fundamental - lowest - frequency detected. Each line to the right is an integer multiple of that base frequency - so from the left, 1, 2, 3, 4x etc. These are all overtones of the base frequency. The prop for example has two blades and will create two pulses every revolution which will sound double the frequency (one octave higher) than the engine.

You can see this by clicking on the peak of each trace as you move left to right - if you are accurate with the mouse the numbers should follow the sequence above.

So if you have a good clean trace on the very left, chances are it is the exhaust note and you could directly read the apparent frequencies - cycles per second - produced by the engine exhaust. If the prop noise was dominant you might instead pick up the prop sound at twice that, but no matter, the math and result is the same. For Doppler shift measurement, it is the ratio between the advancing and the receding frequencies that matters - not which one of the traces you use. The average of the advancing and retreating frequencies represents the frequency when the model is neither advancing or retreating relative to the microphone - so if you average the raw exhaust sound trace you will get a direct read on rps (not rpm of course). There is a slope between the advancing and retreating because it is not possible to fly the model directly through the microphone (though I know you have come close, so have I ) - otherwise the shift would be virtually instantaneous.

On your graph, you chose the 8th trace from the left, which is the 8th harmonic of the fundamental frequency and is therefore 8 times the frequency of the fundamental. That's why I averaged the advancing and receding frequencies and divided by 8. This shows btw that the left hand trace is indeed the exhaust tone. Otherwise the result would be off by 2x or 3x or more.

You are absolutely right - 500Hz is equivalent to 30,000 rpm. So with a one bladed prop, an engine running at 30k right in front of you and not moving would sound at 500Hz (6 Hz above the "B" on a piano tuned to A=440). With a two blade prop you will probably hear 1000Hz - maybe stronger than the exhaust note, maybe not (which is the B one octave above by the way, since each successive octave ascending is double the previous frequency).

So if your engine is indeed running at 30k as it flies by, the left hand trace should average out to 500Hz. Each successive trace to the right will average out to 1000Hz, 1500Hz, 2000Hz, and so on.

If you've ever listened to an ultralight going overhead, with a gear or belt drive that is not exactly 2:1 or 3:1 or 4:1 etc. you will hear two distinctly different pitches at the same time - one from the engine, the other from the prop. IN the case of direct drive, it does not mess up our ears because all you hear are layered octaves which don't clash to our ears.

)
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Old 02-13-2013, 05:33 PM
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Default RE: Cox .049 max RPM?

Thanks for that explanation. There was an animated tutorial that explained doppler that I need to revisit. It did a good job of illustrating how the frequency seems to change depending where you are standing.
I usually present to Wavoscope a 10 second sound bite to analyze and I'm not sure what the lines on the chart are showing. Are the middle lines the part of the recording when the plane is directly overhead or nearest to me...?
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Old 02-14-2013, 04:24 AM
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Default RE: Cox .049 max RPM?


ORIGINAL: MJD

Should be able to calculate inflight rpm from the Wav-o-scope trace more accurately than thrust calc:

2987Hz receding + 4067 Hz approaching = (4067+2987)/2 Hz = average 3527Hz

You used the 8th harmonic, which is 8 times the base frequency

3527 cycles per second x 60 seconds = 211620 cycles per minute / 8 = 26,453 cycles per second = 26,453 rpm. Does this sound about right?

I have to get my head around if it would read low from parallax error (distance from sound source at center stage) or if that only affects the speed calculated from the frequency shift. I believe the latter is true.

The actual aerodynamic pitch is always higher than pitch measured the classical way from the lower surface datum line, unless the prop airfoil is a flat plate. The difference is often several degrees. So a prop with 4 inch pitch measured the classic way (which AFAIK is standard for prop makers, someone correct me if I am mistaken) is higher than 4 inch aerodynamic pitch. An efficient aircraft could achieve the theoretical pitch speed or perhaps even a bit more, because at that point the prop blades are still at a positive angle of attack relative to the zero lift line.
correct me if I'm wrong, but I thought it would be impossible to achieve aspeed faster than the theoretical pitch speed asthe element of drag comesinto play? Not only thedrag of the prop but the drag of the airframe as well?Forgive me if I'm wrong, but it has been a while since I have done any aerodynamical theory.
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Old 02-14-2013, 05:10 AM
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Default RE: Cox .049 max RPM?

forgive me, I'm thinking of something else entirely, not the frequency.
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Old 02-14-2013, 06:06 AM
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Default RE: Cox .049 max RPM?


ORIGINAL: mick1404
correct me if I'm wrong, but I thought it would be impossible to achieve a speed faster than the theoretical pitch speed as the element of drag comes into play? Not only the drag of the prop but the drag of the airframe as well? Forgive me if I'm wrong, but it has been a while since I have done any aerodynamical theory.

[/quote]

You are correct, but you have to be clear which pitch you are talking about. Classical prop pitch measurement runs a datum line across the bottom of the prop blade. This would be end of story, if the prop was slicing through solid matter, it would act like a screw and carve a helix based on that lower surface angle.

But.. this line is not the line of zero lift for the prop blade airfoil. The zero lift angle of the airfoil is specific to the airfoil and is at least the chord line between the LE and TE apexes, but is often a hair more negative than that depending on the specific airfoil. For argument's sake just think of the chord line of the airfoil - LE to TE apex. That angle is always steeper than the line drawn across the bottom surface of the airfoil.

If you think about how the angle of attack of the prop blades, at a specific rpm, decreases with increasing forward speed (relative wind), you can form a mental picture of how the system will reach equilibrium somewhere below aerodynamic pitch speed.. AoA of the prop blade decreases, forward speed and drag increase.. eventually they find that point where thrust equals drag.

This all assumes that the prop pitch as molded into the hub is bang on.. many props have been measured as slightly more than stated. So add that factor.

That's aboot the limit of what I think I know about this..

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Old 02-14-2013, 06:18 AM
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Default RE: Cox .049 max RPM?

I go by the RPM X pitch and divide by 1000.  I can usually do that in my head.  It is 1056 or something.  The ground rpm will pick up a couple of thousand in the air.  On a clean plane it is fairly accurate with the ground rpm.  I guess there is a loss of 10 or 20% but the rpm goes up as the motor unloads in the air. I worked out the cycles per second x60 sec. per min. and all that (inches per mile - seconds per hour... for another chart) once and made a chart.  starting at middle C and working on an octave being double the pitch (or rpm)  I don't have the value right now. but  think it may be 60 cps.  I  scratched the rpm on a pitch pipe like the music teachers use and can tell the rpm fairly close with that.  As soon as the plane is flying, the doppler effect makes it impossible to check.  With a control line plane it can be checked from inside the circle.  That is what I did with it. A piped motor will unload much differently than an unpiped one.  The pipe length  will either speed things up or slow them down and the rpm will be half or full rpm, and will actually slow the motor down if it is too long etc. The fancy chart just makes my brain spin, but it looks like the tighter lines are a higher pitch, so likely the the straightaway speed, and the looser lines are when it turns around.  I noticed when CP flies, he does the straightaway flight after the turnaround and a dive.  Likely this is helping the speeds a bit.  I can just get my Sig Wonder up to full speed at this point, and will have to work on the dive at the turn around.  Oh, and I need a throttle too.  Control line is no problem and easier to time.
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Old 02-14-2013, 06:21 AM
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Default RE: Cox .049 max RPM?


ORIGINAL: combatpigg

Thanks for that explanation. There was an animated tutorial that explained doppler that I need to revisit. It did a good job of illustrating how the frequency seems to change depending where you are standing.
I usually present to Wavoscope a 10 second sound bite to analyze and I'm not sure what the lines on the chart are showing. Are the middle lines the part of the recording when the plane is directly overhead or nearest to me...?
No - every line from left to right, is happening at the same time. Time is along the Y axis, and frequency along the X-axis. The pass starts at the bottom and moves up the graph with time. Each line to the right is like a different part of the sound wave detected by the software - all different harmonics, it is a complex sound wave. Lowest frequency trace on the left (usually the engine) and highest frequency on the right.

The high frequency at the bottom, before it slopes down, is the apparent frequency as the aircraft approaches. The lowest frequency towards the top is when it is receding. The angle of the slope between the high and the low is determined by the distance between the aircraft and the microphone. Think of dropping a ball through a hole in a board - if you could fly the aircraft right through the microphone, the effect would be the same - constant speed advancing towards the microphone, a split second as it passes by/through, then almost instantly constant speed receding. The slope between the high frequency and low would be almost a straight line, and the Doppler analysis more accurate. The farther away you are from the microphone, the more gradual the change between advancing and receding and the shallower the slope between the high and the low frequency. And you have to pick your high and low points farther and farther from "center" for accuracy.

The Doppler effect is like walking towards a Super Soaker that fires a stream of water droplets evenly spaced. If you run towards the nozzle, you will hit the drops at a higher rate than if you are standing still. If you walk away, they will hit you at a lower rate. With sound, each drop is like one complete cycle of the sound wave.
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Old 02-14-2013, 07:02 AM
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Default RE: Cox .049 max RPM?

O_O [&:]
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Old 02-14-2013, 08:50 AM
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Default RE: Cox .049 max RPM?


ORIGINAL: combatpigg
Are the middle lines the part of the recording when the plane is directly overhead or nearest to me...?
On the trace you posted, the point where the aircraft passes over the microphone and changes from coming to going, is the steep sloped part that is directly in line (vertically) with the text on the left that says "Air Temp.." etc. Below that is coming, above that is going. The closer you are to the mike, the steeper that line and the more the line will flatten out on either side.

If you put a ruler horizontally between the lines of the "=" sign before "(15.6)" you pretty much intersect the point where it is directly in front of the mike.
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Old 02-14-2013, 10:09 AM
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Default RE: Cox .049 max RPM?


ORIGINAL: mick1404
correct me if I'm wrong, but I thought it would be impossible to achieve a speed faster than the theoretical pitch speed as the element of drag comes into play? Not only the drag of the prop but the drag of the airframe as well? Forgive me if I'm wrong, but it has been a while since I have done any aerodynamical theory.

[/quote]

The advertised pitch multiplied over the course of a 300 foot run at a high rpm will give you one mathematical speed result.
The true, effective pitch of that same prop with all the other real world factors like unloading, drag, borrowing stored energy during a dive.....will give the only result that matters, which is what you see on the stop watch, radar gun, onboard speed sensor and the doppler chart.
With these measuring tools I suppose you can work backwards from the measured speed and recorded rpm to try to make sense of the aerodynamic theory you were taught.
These little racers are so naturally fast it is amazing how little power it takes for them to get up to an impressive speed with just a mediocre engine run.
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Old 02-14-2013, 11:30 AM
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Default RE: Cox .049 max RPM?

You is right about fast. My 25" Profi .061 jobbie scared the crap out of me on the first flight, and second too I guess. I had never flown a 1/2A (1cc, close enough) with that kind of moxie. Tail heavy, excessive throws, and a bit of built in negative incidence didn't help matters much. Mental note: always take a change of shorts to the field.

Hope to get radar or at least Wav-o-scope on it come spring.
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Old 02-14-2013, 12:43 PM
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Default RE: Cox .049 max RPM?

The 116 mph chart that shows 26,452 rpm with the TD.049 works out to a 4.6 "effective" pitch, even though the prop is sold as a 4.2x4.
That rpm figure sounds feasible, I just watched the video of that run again.

The same plane was equipped with a Cyclon.061 and Wavoscoped 149 mph at 35,767 rpm [calculated like you showed me]. This works out to an effective pitch of 4.4 with the same prop.
Does this mean that the 4.2x4 has various efficiency at different rpms and air speeds with this particular plane..? I'll bet it has something to do with that.


Both runs were with the same plane...the plane is some what heavier with the Cyclon vs the TD and the tests were done several weeks or months apart..

Get your 1/2A racer a real tall control horn if you can work that in. I set the pushrod at just about an inch above the control surface on these racers for the smoothest control feel.
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Old 02-14-2013, 05:37 PM
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Default RE: Cox .049 max RPM?


ORIGINAL: combatpigg

The same plane was equipped with a Cyclon.061 and Wavoscoped 149 mph at 35,767 rpm [calculated like you showed me]. This works out to an effective pitch of 4.4 with the same prop.
Does this mean that the 4.2x4 has various efficiency at different rpms and air speeds with this particular plane..? I'll bet it has something to do with that.
That in-air rpm sounds about right doesn't it. What does that combo tach on the ground?

I'd be cautious to draw too many conclusions from the 4.4 versus 4.2, that is a small difference and it might be splitting hairs. Drag is increasing as the square of the airspeed, and I don't believe it is reasonable to expect the same slippage factor from the same prop at vastly different rpm and airspeeds. It might even be correct to expect a difference as input power changes.. interesting thought, wonder about that one.
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Old 02-14-2013, 06:33 PM
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Default RE: Cox .049 max RPM?

I don't recall what that TD.049 tached on the ground..but that inflight rpm "song" was what I heard the very 1st time I started up a Norvel AME .061 with a 5x3 prop and tached it on the ground.
It ran at 26,500 right out of the box and it made my jaw drop.....[X(]!!
This would have been about 20 years ago..[how time flys].
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Old 02-14-2013, 08:02 PM
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Default RE: Cox .049 max RPM?

also, there could be the possibility of a prop changing pitch from coarse to finein flightas well,alsothere must be apoint where the prop wouldeventually lose efficiencylike on a full sized aircraft.
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Old 02-14-2013, 08:12 PM
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Default RE: Cox .049 max RPM?

Then too, a prop may say it is a 4" pitch and be over or under that if checked with a pitch guage.
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