PJClark

Rocket CDT and IAT

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"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."

I never had a gap there until at last annual my A&P/IA said to me: "hey, your air filter box was rattling around loose, so I got it reattached properly".  Now there's a gap between the filter housing and the cowl, exactly as you describe.  I choose to believe that the gap is intentional and gives dense things (rain, ice, bugs) a chance to fly past the face of the filter and exit to the rear instead of impacting the filter.

I don't think ram air would be important in this (turbocharged) application.  Certainly I'm not having any trouble reaching near-maximum MAP at altitude.  The inlet is a NACA shape which isn't typically used for building ram air pressure.

"the mechanic said the air filter was clogged with paint"

With a clogged air filter, to reach the same MAP the turbo would have to spin faster at a given altitude, increasing CDT due to the increased pressure ratio and efficiency decrease.  The intercooler would moderate the effects of this increase in CDT, but I would still expect the CIT to be higher, which yes would increase CHT as well.  If the air filter were clogged enough, the alternate air door would open, but note that the alternate air door is not especially big.  You'd still have some delta P as you try to suck 200 hp worth of air through it, and again that means you need a higher pressure ratio out of the turbo, so it's going to have to spin faster, increasing CDT, plus your inlet air is now warmer, also increasing CDT.  And if it's also warm out, then you aren't getting as much cooling from the ambient air, and the intercoolers aren't as effective.  So yes, I vote for higher CHT with a clogged air filter and other conditions being equal.  If your air filter is not clogged but the alt air door is open, then I guess you're just going to get somewhat warmer IAT as under-cowl air mixes with air coming in through the filter.  I do find needing cowl flaps open and also 20 GPH surprising to manage CHTs at the 65% operating point, but I guess it depends how clogged/how hot.

"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"

That's not a surprise.  CDT is going to increase with increasing altitude at constant MAP, even with an unclogged air filter.  What were your climb and cruise operating points and CHTs, and what was OAT?

"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?"

"It works automatically only. No way to test it on the ground or anywhere else"

I don't think the alt air door is getting stuck open due to paint overspray, but even if it did, you should see a light come on in the panel.  In my Rocket, the alt air door presses against a limit switch when it is closed, and when the door opens, a light illuminates in the panel.  I've had to have this switch replaced.  You can reach up under the cowl and manually open the door by pressing on it, then ask someone in the cockpit if the light turns on.  Check to see that the magnet will actually pull the door closed.  If it doesn't pull the door closed, or if it takes very little force to push open the door, and you don't see a light on the panel, then perhaps you are unknowingly flying with your alt air door open all the time.

BTW, IAT to me means Inlet Air Temperature, i.e. the temperature of the air coming into the single air inlet after the air filter/alt air door.  This is related to but different than Outside Air Temperature, and not the same thing as Compressor Discharge Temperature (before the intercooler) or Cylinder Intake Temperature (after the intercooler), but I don't mean to argue over the appropriate initialisms, only to clarify the terms I am using.  In an automotive context, IAT can also refer to the temperature of the air in the intake manifold, practically a synonym for CIT for engines that have manifolds with short runners, but in this case since we're talking about air entering the system that can be at different temperatures depending the position of the alt air door, I'm using IAT for that and CIT for the post-intercooler temperature.  With that distinction in mind, I don't have an IAT gauge.  I have the stock CDT/TIT gauge, and I assume but I guess I don't actually know that the CDT needle now reads CIT.  I've never seen this needle move off the bottom of the scale, which starts at 100F.  Maybe that means it's INOP, but if PJClark's 107F IAT was actually CIT, then that could explain why my needle never budges.

High enough CIT will induce detonation, but CIT/CDT monitoring would be far more critical for the non-intercooled engine my aircraft originally came with and for which the analog CDT gauge was intended.  I imagine but cannot confirm that the intercoolers would have been sized to keep CIT below detonation limits at maximum boost and high ambient temps during maximum climb conditions with rated full-rich fuel flow.

FWIW, I commonly run 2300 RPM/30"/17.5GPH in cruise and have no trouble keeping CHTs below 380F with the cowl flaps closed.  (I don't use 2200 RPM anymore after reading Continental CSB09-11A).  My EDM-700 reads about 1550F TIT at this operating point, but I should mention that I don't trust the absolute reading of either my analog TIT gauge or my EDM-700.  They disagree by about 100F.  Instead, I've learned to follow the "Lean XX-YYF rich of peak" instructions in the 305 Rocket power settings table.  When I do that, the fuel flows are right in line with the power settings table, so I'm starting to trust simply leaning to a specific fuel flow.  The lowest cruise operating point I have dialed in is 2300 RPM/22"/11.0 GPH (about 45% power), increasing 1.5 GPH per 2" of MAP to 30".  To this I then add an arbitrary 0.5+ GPH to keep me off of the absolute minimum fuel flows in the table, in the interest of transition tube and turbine blade life (which is very sensitive to temperature; going from 1550F to 1750F drops blade life by ~75%).  I've tried lean of peak operation, and it does run smoothly, but the power drops off quickly and I get no net MPG benefit.  Also, lean of peak provides an oxygen-rich environment that would be expected to accelerate oxidation and mass loss of the turbine blades and transition tube.

I did have trouble keeping CHTs below 400F in climb, especially on hot days.  I've since learned to make extended climbs only at 100% power, or less than 78% power, full rich, and 140-150 MPH indicated, ignore the VSI, just fly constant indicated airspeed.  The fuel controller delivers extra fuel at wide-open-throttle; mine pushes another 10-15% more GPH per BHP at WOT than at 88% power.  Back off to the 88% or even 80% power point and temps will start rising in the climb, even with mixture full rich.

Now if I could only figure out what the correct probes are for the analog CDT/TIT gauge so I could change them out and see if that affects the readings...

Edited by SpamPilot
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