Rocket CDT and IAT

[PJClark]

Need some perspective from the Rocket drivers.

Yesterday climbing out on my way to 16,000 my JPI alerted with CDT at about 250 (when I noticed it). It slowly climbed up about 275 when I'd had enough and reduced power.

As near as I can tell there is no CDT limit in the Rocket.  Anyone disagree?  I see an IAT limit (called a Cylinder Intake Temperature in the Rocket POH insert) and the limit is 235...which I suspec is where JPI set the alert on the CDT. Just a guess.

The questions are:

1. What does your Rocket run for CDT while climbing at full power in the mid teens?

2. What does your IAT run in cruise?

Now a theory.

When I flew mine for prebuy last April, on the way back to SAT and AAA I ran it at 30", 2200, 17.3 gph, 1500 TIT, 10,500 MSL. Temp was 8C. It ran there at 195 KTAS and the cylinders were all comfortably below 380. That flight sold the airplane to me.

Closed and the airplane went straight to the paint shop and I picked it up in late August. On the flights from SAT to KGXY for Avionics with a stop in KABQ to drop of Bob Cabe, I ran the same 30", 2200  rpm at 14,000 feet, and I could not lean below about 20 gph and had to leave the cowl flap full open to maintain CHT below 380. If I recall I was also only running about 185 KTAS. It was hotter in August for sure...but TAS should increase and the mighty Rocket should still have been making sea level power at that altitude. And it wasn't running cool as I've heard so many do... @aviatoreb and @Cody Stallings

Fast forward to November.  I picked it up from avionics and flew 3 hops home to ohio at 15,000.  OATs below 0F most of the way. Still had to have cowl flap open and at least 19 gph to keep #5 in particular below 380 CHT. Dec I had the fuel system checked for rough idle...and this is the point...the mechanic said the air filter was clogged with paint. So he replaced it. Also did some baffle work and installed GAMIs at that time.

Now this morning I described my experience yesterday for one of our Rocket friends, and he wondered if the alternate air door might be open and feeding hot cowling air into the compressor.  Higher CDT.  Hotter IAT. I looked at some of my Savvy uploads this afternoon, and my IAT usually runs around 70 degrees...less than 100 anyway.  Yesterday it was at least 107 as I recall.  That hotter induction air would likely heat up the CHT a little? and the hotter induction air would produce less power (less dense?--K model POH seems to indicate less power produced with alt air open due to temp).

Anyway the big question:  Can maybe the alternate air door be stuck at least partially open because it got fouled with some paint overspray, and I've been running hot and slow since the paint shop because of it?

thanks in advance for the patience reading this lengthy tome and any experience or knowledge that can point me and my mechanic toward a cooler, faster Rocket!

[carusoam]

Do you have the Rocket STC documents?
 

That would be the place to look for guidance...

We know the intercooler exit temp is the important temp for avoiding detonation...same as the OAT for a NA engine...

 

If you have extra TCs... you would want to know what the temp drop is going across the intercooler... an inlet and outlet temp for just the intercooler...

 

If you are not getting the Temp drop expected...first thing to look at is what is getting in the way of the cooling air that cools the intercooler... it may need getting something cleaned on the outside... like a radiator would get...

Cleaning On the inside is not usually required, because it only sees filtered air... unlike an oil cooler where sludge can get all over the inside...

Sure, check your Alt air gate as well... can’t hurt... if Alt air is getting in you want to change some rubber seal....

Increased OAT is tough for meeting CHTs, when compared to winter weather... you have warmer stuff going into the engine... and warmer air trying to cool the engine... it is a double hit...

PP thoughts only, not a Rocket pilot...

Best regards,

-a-

[jlunseth]

I went into the CDT/IAT issue with several experts when I first got my 231, which has an aftermarket intercooler. The 231 has a published CDT limit, but that is for the factory engine which has no intercooler. What I learned is that if there is no intercooler, then CDT is important. In that type of engine configuration, CDT is IAT or thereabouts. A CDT limit is set to protect the engine from pre-detonation or actual detonation because the induction air is too hot.  

However, when there is an intercooler, CDT is not important.  The IAT will be somewhere in the vicinity of -50 to -125 degrees from the CDT depending on the day conditions, airspeed, etc. The CDT limit you might see in a nonintercooled engine has nothing to do with protecting the turbo, TIT is the temperature for that. What is relevant is the temperature of the induction air. 

Most engine gauge set-ups do not show both CDT and IAT. You must have the JPI 930, which does have that capability. But you can safely ignore CDT for an intercooled engine. 

In theory, I need both CDT and IAT in my 231 because of the aftermarket intercooler. It comes with a table that tells you what MP to use that would correspond to what is in the power tables in the POH. That MP is determined in the STC table using the differential temperature -  the difference between CDT and IAT. But I don’t think that is an issue in your Rocket, is it? The Rocket engine is intercooled from the factory and the power settings with the STC  should already take into account the differential temp.

When the alternate air door is closed there is a slight power drop. The intake air is hotter. So if your door is jammed shut by paint that would make some difference in power output. I have never noticed it makes much difference though. 

My first guess with your issue would have been that something happened with the baffling when the cowling went back on at the paint shops.

Edited June 30, 2020 by jlunseth

[jlunseth]

PS on your JPI alerting on the CDT temp. When JPI sets up the unit to deliver it to you, they program in two types of alarm limits.  The first are the alarm limits that are in the POH for your engine and are required to be present. For example, my RPM limit in the POH is 2700 so that limit is programmed into the 930. The second set are limits that are not in your POH and do not apply to your aircraft, but JPI puts them in anyway. These are purely arbitrary limits set by JPI. As an example, there is no fuel pressure limit in my POH (M20K 231). JPI sets at least a lower limit (I have never seen an alert on an upper limit so don’t know if one is programmed in). However, the lower limit appears to be set with cruise power in mind.  The result is that when I am taxiing normally, I get a flashing red alert that fuel pressure is low.  In this instance, fuel pressure is not abnormally low, the JPI just has no way of distinguishing between cruise and idle operations. It only “knows” that FP has fallen below the arbitrary limit programmed in by JPI. Just about every Mooney instructor, and every DPE who has ever flown with me asks why the fuel pressure is low during taxi, so now I go out of my way to explain how the JPI system works and what kind of irrelevant (and relevant) alerts they may see.

The low FP alert is not a bad idea. If I were flying along at a cruise power setting and my fuel pressure were to suddenly drop to 3.7 GPH I would want to know about it, undoubtedly the engine would already be letting me know there is a problem. However, when I taxi I just ignore the alert, 3.7 GPH is normal for that regime of flight.

How and why JPI sets these alerts is known only to JPI. I surmise that since they sell units used in aircraft such as the factory 231 (nonintercooled TSIO360GB/LB), the unit programming requires that a CDT be programmed in and they just pick one that makes sense to them. In this case, unless there is a CDT limit stated in your POH you can ignore the JPI set limit.  TIT is the limit to watch, and if in your POH, then IAT.

The alerts you should observe are the limitations in your POH. The rest of them you will just need to figure out when you can ignore them.

The other thing worth mentioning about the JPI is that its instrumentation is more sensitive than the factory analog stuff, so it will read variations and may alert on variations that the factory instrumentation would not even have “seen.” For example, I will occasionally see 2710 or 2720 for RPMs. A variation of 10 or 20 rpms would have been barely visible if at all on the factory gauges (which are now gone).

I don’t actually know how the JPI is programmed, that is proprietary, what I am telling you comes from 10 years of having and using one on my panel, and sometimes when I had questions (like the CDT/IAT thing), asking some people who are engine experts so I understand.

Edited June 30, 2020 by jlunseth

[PJClark]

Savvy confirmed my suspicion that the CDT doesn't  matter.  My IAT on the flight was about 107.  Good to go.

The CDT is clearly an optional alert programmed by JPI for whatever reason...probably they set that as the same IAT limit in the Rocket POH but that's a guess.  The alert didn't  turn on the RAL, I just looked down in my scan and saw it steady reading CDT.  To be forever disregarded.

Now...anyone have thoughts about the idea of alt air stuck partly open and allowing warmer cowling air into the compressor? Thus increasing CDT and IAT by about 30-50 degrees and resulting in hotter everything?

[aviatoreb]

2 hours ago, PJClark said:

Savvy confirmed my suspicion that the CDT doesn't  matter.  My IAT on the flight was about 107.  Good to go.

The CDT is clearly an optional alert programmed by JPI for whatever reason...probably they set that as the same IAT limit in the Rocket POH but that's a guess.  The alert didn't  turn on the RAL, I just looked down in my scan and saw it steady reading CDT.  To be forever disregarded.

Now...anyone have thoughts about the idea of alt air stuck partly open and allowing warmer cowling air into the compressor? Thus increasing CDT and IAT by about 30-50 degrees and resulting in hotter everything?

I don't seem to have much to add.  My EDM is an 830, so not a instrument replacement STC but just a extra information instrument.  I just checked my records and its as I thought - I don't have CDT on it.  I do have the old CDT gauge which I cannot even remember worry about the units since it is always easily in the green and low green, so not something that has been on my radar.  Which EDM do you have that reads CDT?

[Phil EF]

CDT is an engine limitation for the 231 so JPI is required to program. 
Don’t know about the Lycoming but your POH will tell you. 

[PJClark]

I have a 900.  My Rocket is a 252--always intercooled--never had a CDT guage before the 900.  No op limit on that temp, obviously.

JPI just programs some weird stuff.

 

[jlunseth]

Ah, a thought for you on the alt air door. I wonder if you haven’t solved that problem but you just don’t know it yet.

First, I was trying to puzzle out in my head how painting could jam the alt air door and I just don’t think that would do it.  I can see how, if the paint shop failed to cover the induction intake and then sprayed into it, the filter could get clogged.  But the alt air intake is inside the cowling so it would take some doing to hit it with paint.

In the original 231’s pilots would sometimes get into trouble with induction icing, fly into a cloud of snow or ice crystals and the air filter would clog, starving the engine for air and bringing the plane down unless the pilot figured out what was happening and pulled the manual air door knob, admitting warm intake air from inside the engine compartment. So the door was converted to an automatic door.  If the air filter starts to plug, the door activates.  The automatic system was installed in all the subsequent turbo models.

I think that is your answer. The automatic system detects the plugged filter and opens the alt air door, feeding warm air in from the engine compartment.

If you have cleared up the plugged intake that should have eliminated the problem, but if you have a manual alternate air knob you might work it back and forth a few times to see if that makes a difference. I suppose it is also possible there is a spring or something that holds the door in the normal position, and that has failed. Worth checking.

But you might just try a couple of flights now that the air filter has been replaced and see if the problem has gone away on its own.  I don’t think there is any doubt you were sucking in engine compartment air when the plugged filter was there, that may have been the only way the engine could operate.

[carusoam]

Air Temp And pressure change along the way....

What matters is the temp and pressure being fed in the intake plenum, final control of MP is the throttle plate...

1) Hot OAT... will be just a starting condition...

2) Alternate air open... even warmer air will be entering the system...  (testing the alternator air door is a pre-flight step)

3) This air now enters the compressor... and gets even hotter... while being compressed...

4) CDT... only important if you don’t have an intercooler... (still important if you want to know everything you’re engine is doing...)

5) Intercooler efficiency is important... the hot OAT is used as the coolant... cold OAT is better at cooling...

6) IAT... is the final reading on the adjusted temp of the air heading to the engine...

7) pre-ignition is exacerbated by high IAT

8) JPI seems to only know what is in the POH you send them... they are not analyzing anything that you have for accuracy...

9) If you have a Rocket STC... this is where the good data comes from...

10) If you add an after market intercooler... there is data that comes with that too... we have a guy around here that excels at building intercoolers... 

11) If you are not sure about your hardware while purchasing a JPI... pay the extra to get Good guidance... try to avoid having to do this step twice...
 

13) If you know the temps in and out of the intercooler... you will know how well this device is working... 

14) It takes being a process engineer or thermodynamicist to make this interesting while flying... 
 

15) Some adiabatic cooling will occur as the air expands and pressure drops down stream of the compressor... yet nobody is measuring pressure drop anywhere... Just MP...

16) We can assume the air temp is always getting hotter, until it is in the intercooler...

17) We can assume the air pressure is always going lower as it exits the compressor...

18) The only thing that matters to the operation of the engine is the IAT and MP...

19) never use the original power charts from a plane that has been modified with a different compressor and intercooler set-up...  A new power chart comes with the STC for these things... the old one won’t be very helpful....
 

PP thoughts only,  Ideas that arise around MS when discussing turbo mods.... and intercoolers...

Best regards,

-a-

[PJClark]

I have a Rocket.  The manual Alternate Air control is disabled in the Rocket. It works automatically only. No way to test it on the ground or anywhere else.

On the Rocket the induction inlet is a NACA duct on the bottom left side of the cowling, not where it is on a 252 or 231.  The interior of that duct is painted to match the rest of the cowling, so the filter almost certainly got some overspray no matter how well masked it was.  My concern was that with the cowling also masked, or the engine masked while fuselage was being painted, if some overspray also got to the Alt Air mechanism, and it's now stuck open. But I've not seen the mechanism. Some mechanic is about to see it for sure.

Don't even get me started on JPI. There appears (to me) to be a fair amount of randomness in their process.  My 252 came with a mandatory FF instrument. The ROCKET has a max power limit/requirement to get 31-33 gph at takeoff power. They set up the 900 without the FF as primary--and we sent them the same POH insert I sent to @kortopates yesterday morning.  I had to twist their arm to get a new software load and pay a shop $100 to install it. Great instrument. Programming guys a little iffy/idiosynchratic.

[Phil EF]

Even for the Rocket, IAT is a LIMITATION .  
JPI normally requires all stc poh inserts before programming. 
They did miss the Monroy fuel tank stc when they set mine up. 

[Yooper Rocketman]

Your alternate air door is on the back side of your Bracket air filter assy.  It is a hinged door with a magnet on it.  My recollection (I sold my Rocket a 18 months ago) is once it opens you need to manually close it.  It's either open or closed.  I don't think you can get a look at it without removing the bottom cowl but it's possible you could view it through the nose gear well, looking at the backside of it to see if it's open.

I posted a picture of the interior of the Bracket air filter assy on this post, Page 3, last post on this thread.  I am amazed our resident genius / reveled leader on memory of past posts ( @carusoam ) did't beat me to this. 

 

[carusoam]

Wow time flys at a Turbine Mooney Rocket speeds!

Memories of some really good people in that thread... 

Best regards,

-a-

[PJClark]

@Yooper Rocketman is that the door on the right in this photo?

 

[Cody Stallings]

25 minutes ago, PJClark said:

@Yooper Rocketman is that the door on the right in this photo?

 

26 minutes ago, PJClark said:

That’s it.

[donshapansky]

I flew  a Rocket for 5 years and 1000 hrs. I was told to run the 520 at more rpm than you are running. My numbers were 2450 @ 33 MAP LOP at 15.5 - 16.5 gph. TIT was always around 1560 - 1580 F I don't recall my CDT I will check some panel shots to see. I trued 195 Kts at 10 - 11 and 200 at 12.5 to 13.5.

Prior to going to GAMI I ran 2450 @ 29 MAP at 22 gph In most cases running ROP or LOP I could usually close the cowl flaps completely, loved the one piece electric control. My hottest was always No. 5 never let it get above 380 F. I figure I paid for a complete engine OH saving roughly 7 gph.

[donshapansky]

Just checked a few panel shots, lowest one I found was 16,500 2450 @ 31 MAP with CDT 93 F some other shots at lower altitudes were all 103 - 105 F.

[PJClark]

Thanks.

 

Just got back from the airport.

I confirmed that there is no "closure" inside my inlet duct between the duct and the air filter housing.  There's  a gap between the front edge of the filter housing and the inside of the cowling/inlet duct.  I can easily fit my fingers into the gap and feel the inside of the cowling

Should there be a gap there? Seems to me the intent is some ram air pressure at the filter and I doubt there's any without that sealed up. I figure also some warmer cowling air is being sucked into the induction system

Anyone know if there should be a seal or something between duct and air filter? I've looked thru both the rocket service manual and install instructions and I can't see anything that would answer this question.

Thx

[N231BN]

Thanks.
 
Just got back from the airport.
I confirmed that there is no "closure" inside my inlet duct between the duct and the air filter housing.  There's  a gap between the front edge of the filter housing and the inside of the cowling/inlet duct.  I can easily fit my fingers into the gap and feel the inside of the cowling
Should there be a gap there? Seems to me the intent is some ram air pressure at the filter and I doubt there's any without that sealed up. I figure also some warmer cowling air is being sucked into the induction system
Anyone know if there should be a seal or something between duct and air filter? I've looked thru both the rocket service manual and install instructions and I can't see anything that would answer this question.
Thx

Do you have any pictures?

[PJClark]

I tried with my phone, but it's too tight in there to get anything but a face full of filter. If any Rocket men are flying this weekend though, put your hand into the induction inlet and tell me if you find a gap or some kind of seal and no gap.

I did do a quick ONR wash on all leading edges and cowling and took this pic...

[FloridaMan]

On 7/1/2020 at 5:20 PM, Phil EF said:

Even for the Rocket, IAT is a LIMITATION .  
JPI normally requires all stc poh inserts before programming. 
They did miss the Monroy fuel tank stc when they set mine up. 

You can program your fuel limits yourself. No need for JPI to do those from the factory. 
 

As for CDT, it’s a dead fish on Rocket installs, but because of JPI limitation requirements, it’s gotta be there. Most people just put the probe as far from the compressor discharge as possible and use it to measure intercooler efficiency. 

[Phil EF]

14 hours ago, FloridaMan said:

You can program your fuel limits yourself. No need for JPI to do those from the factory. 
 

As for CDT, it’s a dead fish on Rocket installs, but because of JPI limitation requirements, it’s gotta be there. Most people just put the probe as far from the compressor discharge as possible and use it to measure intercooler efficiency. 

You are correct on the fuel quantity. JPI sent me the procedure (3 pages worth) to edit. 
What is meant by IAT being a dead fish?

[aviatoreb]

15 hours ago, FloridaMan said:

You can program your fuel limits yourself. No need for JPI to do those from the factory. 
 

As for CDT, it’s a dead fish on Rocket installs, but because of JPI limitation requirements, it’s gotta be there. Most people just put the probe as far from the compressor discharge as possible and use it to measure intercooler efficiency. 

Which JPI do you have?  I have the 830, and as for fuel capacity, I can only show 2 digits, so max 99.  When I had the Monroy installed when I did tank reseal about 3 or 4 years ago, I had the fuel capacity in the edm set to 99 instead of the official 102 (and I believe that if you are patient refueling you can get the tanks to take on a lot more than 102).  I don't mind showing 99 as capacity instead of 102 since that is a sort of safety margin.

[FloridaMan]

I have the EDM900. It’s an ok unit and does what I need it to. I download the data and use it to build my logbooks.