Cyclic Hardover

My Feedback: (3)

I was up late last night and reading through some of the RCU when the Engine Temp thing came up again for the 50th time. Since there are no published specs on it all as far as glow goes, my opinon still stands of a guy pointing a temp gun at the area of an engine shows[sm=bananahead.gif]. It was something that W8ye said that got me thinking about it but i was tired and blew it off until this morning.I still like to think I know a bit about turbines. Not from RC but from the real world and I am now curious if we can associate between turbine theory and glow. Now I don't get serious very often so you guys need to take advantage of this and maybe log it.

Many full size turbines to include are own RC versions use EGT or Exhaust Gas Temp. Nothing difficult about this as it simply does as it says. It measures the temperature of the exhaust. No more difficult to understand here than sticking a thermometer inside the exhaust of your car. However the temp reading you get determines three areas, "fine, not so fine and something s wrong here."

Now lets change direction and go to the glows. You point a temp gun at the glow engine but are you pointing it in the right place? For the sake of argument, lets say there are specs for operating temps on a glow. Where you point the temp gun determines an accurate reading. W8ye tells me the glow plug is generally the hottest area. I honestly don't know but sounds good to me, so lets go with that.

It's safe to say that the temp decrease's the further it is away from the source, so the temp on a fin is going to be cooler than the temp on the plug or at the base of the fins. So with this, taking a temp from an engine will not give an accurate reading of whats going on inside-UNLESS the temps at certain locations onthe engine are based on the inside of it.

So with this, it seems the best location is to shoot is at the glow plug or thermocouple attach to it. Where would the exact location be to get the most accurate reading. Simply pointing a temp gauge at the head doesn't work with me. My twist on this whole thing is would the measurement of the exhaust on a glow be as an effective reading as the plug, more or less?

Phew! all that gave me a headache.

proptop

My Feedback: (8)

Just fiddle-fartin' around with one of those little Duratrax things, and IIRC the highest readings I got were at the back of the cyl. right at the exhaust stack? (side exhaust engine )
The air flow is less there than at the top of the head, and maybe there are eddy(sp?) currents there, or no direct impingment anyway.

(what happened to spell check? )

saramos

Since there are no specs to go by, where you take the temprature may be less important than consistantly taking the temp at the same location each time. With the small size and quick heat transfer on these engines, seems to me that the temp at any given spot will vary at an equivilent ratio. On the other hand, if on day you take the temp at the back of the cylinder with the cowl off, and then try to compare it to a temp of the front of the engine with the cowl on, then next time at the exhaust, there can be no meaningfull comparisons.

Scott

Cyclic Hardover

My Feedback: (3)

I dunno, if I can't spell a word, I make one up!

BillS

Cyclic,

In an automotive engine exhaust gas temperature also varies all over the map depending upon where you read it.

Nevertheless I thing reading the temperature provides valuable information. As with any other measurement readings need to be consistent to provide information. Clarence Lee stated many years ago that the most consistent readings were thermocouple under the glow plug. Most will not have a thermocouple but pyrometers are inexpensive and available. Any consistent reading somewhere on the head is probably valuable for the relative temperature values.


And yes someone can screw up any measurement.

Bill

w8ye

My Feedback: (16)

Get a spell checker here.....

http://www.google.com/tools/firefox/toolbar/index.html

BarracudaHockey

My Feedback: (11)

There's other issues at hand that saramos pointed out. I use a Venom temp gauge with the sensor mounted directly under the top head fin on the back of the case of my helicopter engines. The advantage to this is the Venom reads the highest temp attained in flight. This often varies by 15 or more degrees from what is showing up by the time you idle it down to land, shut the motor off and spool down the blades and take a reading with a temp gun. Also the case acts like a heat sink and pulls the head down and away from the head after a short time so after the initial cool down you can watch the temp climb back up but I like the max temp while loading the engine rather than the temp taken after its slowed down, landed and checked.

proptop

My Feedback: (8)

Cool[8D] or would that be hot?
Just out of curiosity, what sort of temp. are you seeing with the Venom (what is it, a data recorder? )

Sport_Pilot

In full scale aircraft the EGT is taken right at the exhaust valve. Not all over the place there. Infrared guns do not actually measure temperature. They measure infrared heat; the temperature they give is an educated guess. If you have a powder coated black engine and white engine running at the same temperature the gun will give a higher temp for the black engine because black radiates infrared light better. Likely the black engine would actually be cooler because black radiates heat better.

BarracudaHockey

My Feedback: (11)

The venom shows current, highest and lowest temps. Its nothing fancy, I run my OS 50's so that that they are lean enough to put them at 180 with peaks under 200, most motors will tollerate 220 so I'm leaving a safety margin.

Not24

My Feedback: (1)

Back to the original post.

I know very little about turbine engines, but I do know about recips used in airplanes. The more powerful engines use an exhaust gas temperature gauge as the primary instrument for adjusting the mixture in flight. Some airplanes have one probe on one cylinder, usually a rear cylinder, and others have probes on all cylinders that read individually on the gauge. It's interesting to note that there is no real temperature setting. The engine is peaked out, and then richened for best power by going to 50 degrees rich of peak. Conversely, for best economy, it is set to 50 degrees lean of peak. On an engine with no EGT, the tach is used to get close, but it's not as accurate.

On model engines, we are basically doing the same thing. Finding the peak rpm, and then richening from there for best performance. Since the temperature is not necessarily required to be in a certain range, the temp it runs at is basically useless. (of course, this assumes the engine is running right and not overheating).

It would be interesting to learn if model engines act like full scale engines. Does peak rpm = peak temperature? Does the temp start to decline as you go to the lean side of max rpm? Are two stroke engines totally different than full scale engines? How about the four strokes? They are lubed the same as the two strokes.

It doesn't really matter. So long as it's not running too hot, and it makes good power.

Cyclic Hardover

My Feedback: (3)

I guess the only way for that is some form of inflight data that can be relayed to you to constantly monitor a flight and see how things change.

downunder

Gas turbines have thermocouples placed just downstream from the last stage of turbines but before the afterburner (if it has one). Their purpose is to give an indication of the turbine blade temperatures because it's a bit hard to put a thermocouple directly on the blade . Gas turbines are run very close to their destruct parameters so a little too much heat weakens the blades beyond the point they can resist centrifugal forces. It's been so long now since I've worked on them that I can't even remember now what temps we were allowed.

With model engines the temp won't decline going lean because by doing that you've reduced the oil flow so friction heat builds up and there's less fuel for internal cooling. Infrared guns rely on emissivity (the black or white that Sport_Pilot mentioned) but apparently the highest rate of heat rejection by radiation comes from a natural aluminium with a satin or cast finish. Even anodising gives a heat barrier.

I use a contact type thermocouple that plugs into my tacho/multimeter and put the probe at the base of the plug. This is almost exactly the same distance from the centre of the combustion chamber for all engines and away from any cooling fins so it's likely to give a more comparative reading between different engines. I only take temperatures for my own interest and I'd never try tuning an engine by them.

Not24

My Feedback: (1)

Oh really? I put all sorts of sensors on turbine blades. If we're lucky, they stay put for a while and actually give good data. Check out www.tasnn.com.

Sport_Pilot

The peak RPM of our models is a combination of mixture and timing advancement. Rich mixtures cools the plug and retards the ignition and lean mixtures advance the mixture. So peak RPM is not usually peak temp.

BillS

I can believe you may be correct. But have you measured to confirm the above statement?

Bill

Sport_Pilot

No, but the fact that the engine gets hotter as it goes lean of full peak confirms it. In a full scale engine the exhaust get cooler lean of peak.

BB Modss

Always read head temp at the glow or spark plug

asmund

I`m also into rc-cars and tempguns is widely used when tuning car engines. Everybody is aiming at the plug when measuring the temp. Most buggy/car engines will be fine from 200-260 fahrenheit, I suspect ABC aero engines will run fine at these temps also. Once I had an airleak I got my 21 buggy engine up to 375 fahrenheit without any damage, still runs strong

yallaair

Temperature measuring on RC engines is very difficult due to the fact that you can't get the really interesting measurement: online EGT at various engine loads. What's interesting to know is the EGT since this is a direct measure on the AVERAGE oil outlet temp. If this temp gets higher than the max design temp for the oil, a large part of the oil will start to decompose or polymerize. This will give varnish / carbon deposits / total oil film breakdown, which can lead to engine failure......
In daily life it is best to use a calibrated IR-gun on the engine plug at the highest load you operate in!