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Old Oct 26, 2012, 03:10 PM
Complete RC Idiot Savant
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I have fiddled around a bit with all needles, but they ended up pretty much the same as they were.
Main needle is maybe one or two clicks richer now, just in an attempt to get the temperature down when WOT, the rest is more or less unchanged.

I have the impression that midrange is slightly on the rich side,but it runs pretty good when cruising around, so I will probably keep it like that, as high midrange is most used (throttle opening is approx 75% most of the time).

I have noticed, that the engine is performing the best in the first 2~3 minutes after starting, which would indicate the engine is running pretty hot.
With the glow ignition, it would typically perform like it does now when hot, but not perform extra when heating up like it is now
But it is running fairly consistent, and it still seems to climb slightly better than with glow, even when hot.

It has never been a power house anyway....

Brgds, Bert
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Old Oct 26, 2012, 07:00 PM
I HATE GLOW PLUGS!
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Quote:
Originally Posted by Brutus1967 View Post
I have fiddled around a bit with all needles, but they ended up pretty much the same as they were.
Main needle is maybe one or two clicks richer now, just in an attempt to get the temperature down when WOT, the rest is more or less unchanged.


It has never been a power house anyway....

Brgds, Bert
If you have not leaned the mixture out you are probably running pretty rich & not making peak power. That is why you are not observing the usual power increase & improved fuel economy.

It would be to your advantage to get your mentality out of the glow ignition mindset. Leaning out the mixture for peak power W/CDI will not raise temperatures as much as it would W/glow ignition.

Also, your ignition timing is now "fixed" by the spark timing. It is not altered by mixture or engine temperature.

The engine will run hotter W/CDI compared to GI, but the engine temperture will not result in volitile ignition timing issues as W/GI for the reasons stated above.

If there was a way for you to mount the engine on a test stand, you could put a large prop on it to load the engine down to the expected RPM operating range.

Blocking some of the the prop wash from cooling the engine to simulate conditions in the heli would allow you to do some real world testing of timing, mixture, engine cylinder head temperature relationships under load.

It is going to be very difficult for you to tune your engine on the heli.

High performance water cooled automotive 4-stroke engines typically make peak HP @ the top of the RPM ranges W/around 36* BTDC ignition timing on unleaded pump gasoline.

W/methanol in a small aircooled 4-stroke, the peak HP should be developed @ similar ignition advance. On gasoline, that timing advance needs to be backed of to about 28* BTDC unless the compression ratio has been reduced to prevent detonation..
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Old Oct 27, 2012, 01:08 AM
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The first tests, a couple of days ago, I went to the point of leaning out where power went down. That was still with the original 10% Nitro, 10% oil fuel.
When changine over to 2% Nitro, I very carefully tried leaning out 2 clicks, but that did not anything, noticed no difference
The original 10/10 contains 80% by volume methanol, the 2/15 contains of course 83%, but should have a slightly higher viscosity, so I figure at the same needlesetting, volumetric flow of the fuel would be less anyway.

Therefore I did not lean out further with the 2% nitro fuel, because it contains more oil and I was worried that it would be leaner on the same needle setting due to the extra oil content. So I backed out to the original setting of the 10/10 mix and left it there.
It is hard to say without measuring, but it seems, volumetric fuel consumption is lower with the 2/15 blend compared to the 10/10 blend on the same needle settings.

I know, that the fixed ignition changes a lot, but as it was, the engine was allready running pretty hot to begin with when it was still glow. I would say, there is probably not much room for even more increased temperatures.

The testing on a stand is not as simple as one would think. It is not only the limited cooling, but also the fact that the engine stays at constant RPM (and thus constant cooling air flow) over the top half of the power output, and whatever it does, it hast to run reliable and smooth over that range.
I would like the extra power, but main priority is a reliable and constant running engine, because the engine developing a constant torque is one of the main factors in the handling of the helicopter (tail rotor contol is highly affected if the engine is not running smooth). So I would gladly trade of max power for constant running.

As I have seen now, invariably, the engine keeps running OK over the duration of the flight, but seems to loose a bit RPM when steady operating temp is reached. It tends to run better while it is still heating up.
That is not because the mixture leans out at lower tank levels, I have tested with nearly empty tank and unchanged needles, and it shows the same performance while still heating up, and the same performance loss after it is at operating temperature. It also, after heating up, does not change performance from full tank to empty tank. It only changes performance during the process of heating up from cold to operating temperature. It did that also when still running GI.
I checked the valve clearance, and it is OK at 0.04 mm, and behaviour is not improving with slightly larger clearances.

So for now, I am more focused in settings that keep it a bit cool, and I am trying to think of ways to improve cooling efficiency. I have thought of liquid cooling, but have no way of fabricating that: I have a workable design of the pump incorporated in the backplate capable of giving sufficient flow for a 10 degree temperature difference between inlet and outlet @ 80 degrees outlet, piping with air-free expansion vessel, radiator (about the size of a fliptop pack of marlboros), even a possibility to incorporate a small and light thermostat in the radiator, but no way of fabricating a decent cooling jacket around cyclinder and head, and that's where it all comes to a grinding halt.

I did not so much design that cooling system to keep temperatures in check, but to get rid of the power loss of the cooling fan, which I believe to be in the range of 100W (which rougly translates to additional 700~800 gram weight increase with the same performance or the performance increase as if weight was reduced by that, the cooling system would roughly add only 150~200 grammes). But without possibilities to fabricate, that will stay a beautiful dream only.
I still keep thinking about it though, as the engine runs (and always has been running) at the limits....

Brgds, Bert
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Old Oct 27, 2012, 12:01 PM
I HATE GLOW PLUGS!
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Could the engine be making more power when below operating temperture be due to a rich mixture being more suited to the cooler operating temperture?

Not saying that you should neccessarily lean out the mixture for more power @ normal (higher) operating temperatures, rather that you not be overly concerned.

Power loss can also be attributed to a higher intake charge temperture (less dense intake charge) as the engine compartment temperture heats up the induction components. (carburetor, manifold, port runner, etc)

While drag racing @ high temeprtures in Ohio, I once experinecd a .45 second ET reduction just by icing down the intake manifold & getting ECT @ staging down to 156*F from 179*F on the previous pass..

The point is, that a hot induction system can significantly reduce power output.
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Old Oct 27, 2012, 12:08 PM
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How long does it take for the power output to stabilize at the lower power while warming up? I think this is just normal and I have observed similar with engines with minimal cooling (130C CHT). Improving cooling could improve output as suggested.

Greg
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Old Oct 27, 2012, 12:26 PM
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Usually when started from cold, I let the engine idle for approx 30 seconds, than spool up and take off typically it will take some 2 or 3 minutes for the engine to get constant, and power drops after 1.5 to 2 minutes.

If I set her down and idle for lets say, 30 seconds, the first minute after take off I have that "cold" power, after that is goes down again. But once it is heated through, it stays stable, and the helicopter is flyable with it, as long as you plan a little bit ahead and make use of speed/altitude conversion and v.v.

There is not much power left for a vertical climb: something like 8~10 ft/sec is about all the old lady can manage.... When cold, it will do maybe 12~13 ft/sec.
But the engine is in both cases labouring audibly.

If I look at the 110 deg C I observed, approx 15~20 sec after touchdown, it might well be that 130 deg C is a realistic value in flight.
According Vario, the guys that once thougt an OS FS 91 in a helicopter might be an acceptable plan, the engine temperature rises 1.5 degree for every % of additional oil over 10%. That would mean, that I could gain a few degrees by reducing oil content to let's say, 12% (compensating for the lower fuel consumption with spark ignition), but than we're talking maybe 5 degrees max, I do not know if that makes the difference.

Improving cooling.... I wish, I had some more ideas as to how, without modifying to liquid cooling....

Brgds, Bert

Brgds, Bert
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Old Oct 27, 2012, 12:44 PM
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Quote:
Originally Posted by SrTelemaster View Post
Could the engine be making more power when below operating temperture be due to a rich mixture being more suited to the cooler operating temperture?.
I think not: it does it the same, whether I set it a few clicks fatter or leaner. The mixture does not really seem to change, and the motor does not really seem to need a different mixture when hot

Quote:
Originally Posted by SrTelemaster View Post
Power loss can also be attributed to a higher intake charge temperture (less dense intake charge) as the engine compartment temperture heats up the induction components. (carburetor, manifold, port runner, etc).

While drag racing @ high temeprtures in Ohio, I once experinecd a .45 second ET reduction just by icing down the intake manifold & getting ECT @ staging down to 156*F from 179*F on the previous pass..

The point is, that a hot induction system can significantly reduce power output.
The engine is fitted with the crankshaft pointing vertical down. Air is flowing vertical down through the cooling duct.
The carburettor is sticking out vertical upwards, with a spoolpiece of appr 1" between head and carb. So the carb is drawing air from even before the entry into the cooling duct
The carb is literally icing up on the outside at anything below 10 degrees ambient (degrees centigrade that is).
Has done that always with both the original and the 7H carb, and this has never infuenced the needle settings.
I have flown it with the original FS carb for over 5 years, and approx 1 year with the 7H carb, in temperatures ranging from 30+ till subzero, and the biggest change I needed to make due weather conditions was 2 clicks....
The 7H carb is equipped with a "power bell", basically an aluminium cap that catches the blowback fuel. This fuel evaporates in this bell, effectively cooling down the charge air before it enters the carb, while at the same time re-introducing fuel that would otherwise have been lost.
The effect of this bell was clearly noticeable when I installed it.
I have no idea how fast the air heats up while underway from carb to cylinder, but it should be pretty cold when it leaves the carburettor. So personally, I don't think heating of the charge air is an issue here.

Brgds, Bert
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Old Oct 27, 2012, 12:56 PM
The Prez....... again
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I'm kind of thinking out loud here

Try setting the valve clearance down to .001 or .0015. Maybe the extra heat is causing the valve clearance to open up more which would in effect reduce the maximum opening of the valve. I know it is a minute amount but just maybe......

Ken
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Old Oct 27, 2012, 01:19 PM
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Valve clearance is currently set at 0.04 mm, which would be 0.016", which is the minimum limit (and recommended setting) OS states.
Usually valve clearance gets smaller when an engine heats up, that is why it is there in the first place: especially the exhaust valve is the part that heats up the most, and partly it is the elongation of the valve stem that reduces the clearance, but Lube oil film thickness (mainly the film between cam lobe and cam follower and to a lesser extent the film between rocker and valvestem) plays a role here too.

The nasty thing is: if you would set the clearance on a cold engine to zero, there would be measurable clearance on a hot, stopped engine. That can lead people to making the wrong deduction that hot engines have greater clearances.
But when you run it, the oilfilm reduces the clearance to zero again, the elongation pushes the valve open, and from that moment on the exhaust valve is uncooled (exhaust valves cool when in contact with the seat) and will burn quite quickly.

NEVER reduce valve clearances....

I did not notice any difference in performance before and after valve clearance adjustment (adjusted the day before yesterday).
Clearances before adjustment were smaller than 0.1 mm (max limit) but noticeable bigger than 0.04 mm.

Brgds, Bert
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Old Oct 27, 2012, 01:29 PM
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I would say what you're observing is normal for the installation.

Valve clearances in model engine open up as temperature increases. The crankcase is aluminum while the rest of the valve train is steel. The valve itself runs hotter, but how much does it really expand when it's only 26.4mm long?

Greg
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Old Oct 27, 2012, 01:43 PM
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Measurable, and in the range of 0.01~0.02 mm....
The valve typically gets to 350~400 degrees, give or take.

But like I said, with a hot engine at standstill, the valve clearance opens up. The valve will be the same temp as the engine. When it is running, the valve gets MUCH hotter than the rest of the engine, and the clearance closes again, more than it opened from the engine being hot.
A lube oil film will easily reach a couple of hundreths of mm, and then there's always the rocker ratio.
The oil film thickness will cause the valve to "float" allready, while the mechanical clearance (the clearance that would be measured if the engine was stopped at that moment but the individual parts stay at their individual operating temperature) is still positive.

But.... valve clearances ar not likely to be the problem. It takes less than the 36minutes I ran the engine yesterday, to ruin an exhaust valve when the clearances are too small.

Brgds, Bert
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Old Oct 27, 2012, 07:19 PM
The Prez....... again
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Quote:
Originally Posted by Brutus1967 View Post
Valve clearance is currently set at 0.04 mm, which would be 0.016", which is the minimum limit (and recommended setting) OS states.


Brgds, Bert
MY bad. I was reading it as inches, not millimeters. You clearly stated MM

Ken
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Old Oct 28, 2012, 10:06 AM
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.04mm is .0016"

Bert, what coefficient of expansion did you use for the exhaust valve calculation?

Greg
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Old Oct 28, 2012, 01:21 PM
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Not really an expansion coefficient, but an experience factor. I know, that exhaust valves with a length of 50 cm (20") typically expand some 0.3 to 0.4 mm when at operating temperatures.
The length you mentioned is close to 1/20 of that....
Should not be too far off. It is measurable anyway:
If I am to do liner measurements, I need to measure to an accuracy of 0.01 mm, and from liner diametres of 50 cm and over, I am not allowed more than 10 deg C temperature difference between liner and rod, in order to prevent mis-measurements.
I have checked this, and indeed, with bigger temperature differences you will get repeatable deviations in the order of 0.01 mm per 10 degrees.
And the bigger the engine, the more carefull you have to equalize temperatures
(very inconvenient if you're time pressured, the liner is 70 degrees and you have to wait until the rod is the same temperature, apart from having to wear gloves when handling a micrometer....)

Brgds, Bert
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Old Oct 28, 2012, 01:55 PM
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How did you measure that at operating temperatures? A half meter of steel heated to 400C will expand by ~3mm.

Greg
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