A rocket specific issue - re calibrating a new TIT probe

[Warren]

Another option might be to try an alternate temperature probe and compare readings at the same engine operating settings.  They external temp, speed and external conditions might not be exact, the engine settings might not be exact but I think if there was a temperature probe accuracy of 50 F you should be able to see the difference.

Could be a quick, relatively inexpensive test.

[Guest]

On 9/2/2020 at 7:33 AM, aviatoreb said:

Thanks Clarence,

What is your opinion though of going with that rocket POH calibration concept?

If when the probe itself is calibrated, the temp it senses will be surely dependent on the exact placement in the hot gases so the reading of even a calibrated probe would vary with placement.  Is that right?  So there is some sense with that in flight calibration that my POH offers, if the test itself is valid.

I found the unit for calibrating EGT/TIT.  A little pricey though.

Clarence

[aviatoreb]

1 hour ago, M20Doc said:

I found the unit for calibrating EGT/TIT.  A little pricey though.

Clarence

Do you have one?

If yes maybe I’ll need to visit you when the pandemic is over and they reopen the border?  I’ll come south to cross the border and come visit finally.

E

[carusoam]

If you bring an engineer along...

It could qualify as a maintenance flight...   

Of course taking it to a maintenance shop probably does that too... 

Best regards,

-a-

[Guest]

8 hours ago, aviatoreb said:

Do you have one?

If yes maybe I’ll need to visit you when the pandemic is over and they reopen the border?  I’ll come south to cross the border and come visit finally.

E

I have access to an older version of it.

Clarence

[aviatoreb]

3 hours ago, M20Doc said:

I have access to an older version of it.

Clarence

Great!  Now if you could just end the pandemic and open the border, Ill be right over.  :-)

[aviatoreb]

8 hours ago, carusoam said:

If you bring an engineer along...

It could qualify as a maintenance flight...   

Of course taking it to a maintenance shop probably does that too... 

Best regards,

-a-

Do I count as an engineer?

Which makes me laugh because in GA we are all engineers - since like half of our instrumentation on the panel is for managing the operations of our engines.  In fact that word, engineer, I believe began with describing the guy who manages an engine, like on a train, and a steam engine, who needed to understand some basic principles of engines, physics, heat baths, and all that as it was understood at that time when the word started trending, my guess in the 1800s.  And before long the word started morphing to include people like me (in my work setting that included ECE) who does "engineer" things that include worrying about feature maps in infinite dimensional Hilbert spaces, or countability and cardinality of strange attractors.  No where an engine in sight.  But still - when I sit N314EB I enjoy that the word "engineer" fits me and what I am doing in that left seat as true as the day as when it was coined.

[Austintatious]

FWIW, I have read up on, contemplated and performed this test in both my rockets.

The principal is this... at a known power setting (MP/RPM) 1700 degrees is the max temp the TIT should reach.  Since we can do that for up to 1 min, the POH says to perform this procedure to identify the inaccuracy of our TIT probe.  As I understand it, this is to be done every flight.

I have a GEM and the stock TIT probe, so what I do personally is use whichever one is higher and run just under 1600 degrees as per the rocket power settings. (1600/1650 max).  So I lean to get my highest reading TIT probe to about 1590.

Someone said that thermocouples are usually accurate to a "few degrees"... I am not sure if I agree with that.  In performing the Rocket TIT test I have seen both my probes vary in their error by about +/- 25 to +/- 50 degrees on different days.  I have no idea why this might be the case. It could be caused by minor variances in exhaust flow or some other variable.  It could just be the inaccuracy of the probe.  Perhaps thermocoupels ARE quite accurate, but in practice I have seen variations in the readings from one day to the next IE: day one probe A hits 1600 for peak and probe B hits 1650.  Day 2 probe A hits 1650 and probe B hits 1700.

I don't do the test every flight.  At this point I am convinced that by going by my hottest probe (which can change one day to the next) and keeping it at 1590 or lower that I should not ever be exceeding the 1650 max in the rocket power setting chart.  On some days I cant even lean that far or my cylinder #5 starts to get too warm.  So on those days I may have my hottest probe at 1550.

I love the Rocket.  The only disappointment is how TIT limited it is.  Running LOP is pretty much impossible at anything above 55% power.  I have tried leaning quickly to try to get past the TIT peak, but it seems I run into rough engine running at that point.  I am fine with this, I am still moving around at 200 knots burning 17 gallons an hour and that is pretty damned impressive.

[carusoam]

Technically, the TC is very accurate... if everything is ideal...

The instrument doing the reading... has a few challenges with each connection... and extension...

Location is always a challenge... (more so for open ended exhaust pipes)

including how deep in the exhaust stream it sits...

Is the a Rocket TIT instrument getting adjusted to make the reading where it is, try to estimate what the turbine blades are seeing, further downstream..?

Best regards,

-a-

[aviatoreb]

5 hours ago, Austintatious said:

Someone said that thermocouples are usually accurate to a "few degrees"... I am not sure if I agree with that.  In performing the Rocket TIT test I have seen both my probes vary in their error by about +/- 25 to +/- 50 degrees on different days.  ....

I don't do the test every flight.  At this point I am convinced that by going by my hottest probe (which can change one day to the next) and keeping it at 1590 or lower that I should not ever be exceeding the 1650 max in the rocket power setting chart.  On some days I cant even lean that far or my cylinder #5 starts to get too warm.  So on those days I may have my hottest probe at 1550.

So you are saying one day your hottest might be your classic probe by say 25 deg and then another day it might be your new digital device probe by say 50?

[Austintatious]

1 hour ago, aviatoreb said:

So you are saying one day your hottest might be your classic probe by say 25 deg and then another day it might be your new digital device probe by say 50?

I have not seen them get that far off, but I have seen them be pretty much equal on some days.  Typically my GEM probe is reading 50 degrees higher.  but not always.

[aviatoreb]

38 minutes ago, Austintatious said:

I have not seen them get that far off, but I have seen them be pretty much equal on some days.  Typically my GEM probe is reading 50 degrees higher.  but not always.

Do they switch roles which is higher from day to day?