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Old Oct 30, 2012, 07:15 AM
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The area is not the problem or these engines would be overheating in model aircraft, no? Maybe a more effective fan is in order.

Greg
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Old Oct 30, 2012, 07:34 AM
Complete RC Idiot Savant
The Netherlands
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Well.... the load is different (probably higher) and the problem is a little bit that with airplane use the amount of cooling air is virtually unlimited and the speed at which it is flowing past the engine is load dependent.

In a helicopter it is running at a constant RPM over the whole range, so cooling effect does not change, but load does.

The engine is also not designed to be in a duct, so the cooling fin arrangement is not optimal as well.
I guess it is a bit equal to having the engine in a too restrictive cowling.

2 strokes usually work OK in the same duct (the original design is for a 2 stroke) but those are usually fitted with a heat-sink, and the fan is running 14000~15000 RPM. No real good possibilities to fit a heat-sink on a fourstroke, and the fan is only doing 11000 RPM....

The biggest factor however, is that the fan is drawing air over the engine, and the air then tends to be less turbulent compared to a fan blowing over the engine, thus heat absorption is most likely not as effective as well

But I am looking into possibilities to improve cooling without spending power on a bigger fan, as it seems that is the way to go now.

It is all a bit on the limit of what's possible, actually....

Brgds, Bert
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Old Oct 30, 2012, 08:03 AM
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I understand that the engine doesn't have a large heatsink head for low volume cooling, but this was a given at the time of installation.

I had to look up the manual for the Skyfox to see the cooling system. That axial fan arrangement appears less than optimal for an engine without the typical heli cooling design.

Greg
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Old Oct 30, 2012, 08:18 AM
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It is....
For a 2 stroke it is very efficient (we have 2-stroke engines over 2.5 HP running absolutely reliable in exact the same configuration) but for a fourstroke it is minimal.
In this case, Vario recommended to give the engine a very thorough break in, reduce oil content to minimum and keep the engine as close as possible to 11000 RPM. But this version was discontinued already 10 years ago and was only around for maybe 2 or 3 years.

In the basic trainer it is working OK, because the engine is not loaded so much continuous (required power is lower in hoover/cruise, compared to my Bell 47G). The engine will handle a longer climb-out as the engine more or less works as its own heatsink; you end the climb, before the engine gets too hot, or you will be too high anyway. Not so in my over 1 kilo heavier Bell 47G.... It runs hotter in hoover, and climbs slower so you have to maintain climb longer to get altitude....

This helicopter has always been just able to hoover and maybe climb with a few feet per second. Heat has always been a bit of a problem, usually I left it home when it was over 20 degrees outside.

But it is what it is, and I am just trying to optimize it. The running behaviour has at least improved considerably, and during that phase that is is still heating up, power seems to have improved.
Needles seem a lot less critical.
Now I am trying to "unleash all that power" by keeping the engine cooler, as there is definitely a fair amount of power extra at lower engine temperatures. That was never there in such amounts: when the engine was still glow, the difference between hot and cold was a lot less and the mixture range over which the engine ran good was a lot smaller.

But it is just.... I have had the fortune to see Bell 47 G's with the open pipes on that 6 cylinder Lycoming, and the sound of the OS , the flight performance of the helicopter is so very realistic, I can't just decommision it....

Bgds, Bert
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Old Oct 30, 2012, 09:00 AM
I HATE GLOW PLUGS!
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United States, NY, St Lawrence
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Quote:
Originally Posted by Reginald View Post
Never had any trouble with my onboard glowdrivers and the idle of my OS 4cyl 240 is magnificent WITHOUT an onboard. As for fuel economy that's an issue I have never understood and the very least of my modelling expenses, but I suppose you guys do fly day and night ? Fourteen litres in my 911 when driving moderately, now tht's fuel consumption. But please do not let me stop you guys fiddling. But when I want to fly ignition I go for Zenoah or DA or Seidelwhatever purpose made engine.
All but 1 of my converted engines is 1.50 or larger uncluding 2.00 & 3.00 twins.

When fuel consumption is approaching 3oz per minute W/GI on the larger engines, 23% longer run times on a tank of fuel can add up.

As far as 2-stroke gas engines? They don't sound anything like full scale engines. The large single gas ebgines vibrate a LOT & make less power per displacemnt/weight than a 4-stroke running methanol on CDI.

My high compression 180 W/methanol/CDI makes 3.4 HP @ 8400 RPM W/an 18 x 8 prop. It weighs just over 2# including the ignition system.
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Old Oct 30, 2012, 09:06 AM
I HATE GLOW PLUGS!
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Quote:
Originally Posted by Brutus1967 View Post
Now I am trying to "unleash all that power" by keeping the engine cooler, as there is definitely a fair amount of power extra at lower engine temperatures. That was never there in such amounts: when the engine was still glow, the difference between hot and cold was a lot less and the mixture range over which the engine ran good was a lot smaller.

But it is just.... I have had the fortune to see Bell 47 G's with the open pipes on that 6 cylinder Lycoming, and the sound of the OS , the flight performance of the helicopter is so very realistic, I can't just decommision it....

Bgds, Bert
If you want to unleash the power & reduce operating temperture, set the timing to @ least 34* BTDC, 36* might be even better.

My FA91S made about the same power on CDI compared to GI when timing was 28* BTDC. It responde very well & increased RPM up to 35* BTDC & remained the same @ 37* BTDC so I backed of to 35*.
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Old Oct 30, 2012, 09:25 AM
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Originally Posted by SrTelemaster View Post
If you want to unleash the power & reduce operating temperture, set the timing to @ least 34* BTDC, 36* might be even better.

My FA91S made about the same power on CDI compared to GI when timing was 28* BTDC. It responde very well & increased RPM up to 35* BTDC & remained the same @ 37* BTDC so I backed of to 35*.
It is at 34 degrees....

Did not see too much difference in operating temperature between 28 and 34 degrees.

Need to improve cooling.

Brgds, Bert
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Old Oct 30, 2012, 09:30 AM
The Prez....... again
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I looked at the Vario Skyfox mechanics you are using. The look very high quality and well designed. I think the whole problem is the fact you are 6000 RPM shy of the 2 stroke RPM. The extra RPM of the two stroke will move a considerable amount more air.

I was concerned at first the engine would be breathing hot air but the design puts the carb on the cold side so there should be no issue there.

If this were mine, I would make sure the fan is very tight in the shroud to reduce tip recirculation to an absolute minimum. Seal all the gaps that will allow air to leak in after the engine and block any opening that does not directly let air to the cylinder of the engine. Maybe direct more air to the cylinder head if possible?? If the crankcase is open to air can you block it off to maximize the air to the cylinder?

I'm sure you have done all this already but I thought I'd throw it out there anyway.

Having the fan suck hot air is less efficient that blowing cold air. But you already know this. Too bad you can't rig some sort of water mist upstream of the engine to make use of some evaporative cooling???? Just what you need, more complication

Ken
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Old Oct 30, 2012, 10:29 AM
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As for durability, the SkyFox is one of the first helicopters ever that really had some. Most of its design has not changed a bit since introduction some 20 years ago, and they are still at the top of my list of favourites for their reliability, parts interchangeability (you can repair a SkyFox from the first production run, with the parts nowadays sold for it), and easy flight characteristics. Bit oldfashioned, but it is still a fantastic mechanics for scalers though.

You mention some points that I have considered but not yet tried: closing tipclearance of the fan for example does a lot. That I will try next, should not be too hard.

I have thought about changing the cooling fan for a shortened prop and reversing the airflow, but the downside is that on the ground the helicopter will suck up lots of dirt.
Not keen on doing that.

Watermist requires a water tank, and water is heavy.... But it is sure an interesting thought, only how to prevent thermal stresses, you're dripping water only on one side....

Brgds, Bert
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Old Oct 30, 2012, 10:45 AM
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Quote:
Originally Posted by SrTelemaster View Post
All but 1 of my converted engines is 1.50 or larger uncluding 2.00 & 3.00 twins.

When fuel consumption is approaching 3oz per minute W/GI on the larger engines, 23% longer run times on a tank of fuel can add up.

As far as 2-stroke gas engines? They don't sound anything like full scale engines. The large single gas ebgines vibrate a LOT & make less power per displacemnt/weight than a 4-stroke running methanol on CDI.

My high compression 180 W/methanol/CDI makes 3.4 HP @ 8400 RPM W/an 18 x 8 prop. It weighs just over 2# including the ignition system.
I can assure you that the sound of our Corsair or Thunderbolt or Spifire large scale models make the right sound, despite db restrictions. I agree entirely on the vibration. That is why I prefer twins or radials. And we have agreed that glow engines converted to ignition are sounding just like the full sized ones. Learning every day.
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Old Nov 13, 2012, 04:54 AM
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Just to give some feedback to all those who contributed:

I have finally managed to get myself 5 litres of clear Methanol and 3/4 litre of Carbulin oil, and started testing.

In glow configuration, the engine needed 10% Nitro, and 10% Carbulin.
Tests with my normal 2-stroke fuel (2% Nitro and 15% oil) showed the engine kept running, but was overheating somehow.

I made 5 times 1 liter of 0% Nitro fuel with alternately 14, 13, 12, 11 and 10% Oil and started testing. I leaned it out on each fuel until power reduced again, than two clicks back.

It came out clear, that 12% performed the best, and the differences between 13 and 11 % Oil were pretty clearly noticeable.

It strikes me as odd, that if fuel consumption goes down with approx 20%, and I need to increase oil by about that same amount. Apparently the 10% lubrication I was using in the past 10 years, was pretty much spot-on optimal.

During these tests I closely monitored the oil coming from the crankcase breather, and I noticed that even on the 11 and 10% fuel, the oil stayed clear, Slightly discoloured, no metal particles visible no "burnt smell" and even the quantity per minute of flight did not noticeably change....

I still like to keep the engine a bit cooler, but on the 12% it kept running consistent.
On the higher oil content the engine noticeably lost power after one or two minutes of flying, on the lower oil content, the engine did not develop power from the start.

At the moment I'll leave it at this, and try to find ways to improve cooling.

Brgds, Bert
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Old Nov 19, 2012, 02:10 PM
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As a spin-off from the heat problems with the fourstroke, I started experimenting with liquid cooling.

The posts about that are in the thread linked below, but I'd thought it would be interesitng in the engine departement as well....

(the experiment starts from post 143)

http://www.rcgroups.com/forums/showt...5#post23309341

Brgds, Bert
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