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  #21  
Old 11-21-2016, 10:38 PM
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Originally Posted by paul330 View Post
Might be a bit tricky around here - at least until I do a trip to Cape Town

In any case, surely that only affects the climb CHTs? If you lean to a fuel flow in the cruise then surely it makes no difference what the TO fuel flow is......
True. Which is why I think your cooling is not optimal because even LOP your temps are higher than most others.

But if you have insufficient flow on takeoff, you have climb limitations you otherwise should not have, and where you live that can be a limit you do not want.
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  #22  
Old 11-22-2016, 12:47 AM
paul330 paul330 is offline
 
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Idle mixture is easy to adjust. Presumably, there is a screw or something on the injection unit (Precision) which can adjust the max flow......? Or is this something that should not be messed with?
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  #23  
Old 11-22-2016, 07:15 AM
BillL BillL is offline
 
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Paul, Check this post from Bob, it has a link for downloading the manual for your servo with all the proper instructions you will need.

http://www.vansairforce.com/communit...11&postcount=4
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  #24  
Old 11-24-2016, 01:48 AM
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Originally Posted by paul330 View Post
Idle mixture is easy to adjust. Presumably, there is a screw or something on the injection unit (Precision) which can adjust the max flow......? Or is this something that should not be messed with?
Paul, if your FCU is not flowing properly, you need to remove it and send it to a facility with a proper test bench, and insist they set it to the upper limit of spec.

My partner in the Advanced Pilot Seminars Andrew Denyer does these often at his engine facility in Adelaide. If you can't get it done in RSA, Fedexit to Riverina Airmotive in Adelaide.

Your will not be the first.

And no, there is no simple screw you can tweak.
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  #25  
Old 07-13-2017, 10:19 PM
paul330 paul330 is offline
 
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Just to keep this thread going as I haven't totally solved the issue......

The ol' "washer between the baffle and cylinder" trick worked beautifully on the No5 and took 20F off the temperature. It won't work for the No6 because it is 180 out and doesn't have the narrow fins at the back.

I believe that the issue is the big 4" hole for the oil cooler just above the cylinder taking too much air. Unfortunately, I can't block off any of it as the oil temperature is where I want it. I don't think it is a baffling issue as I can't find any obvious faults with it and the other cylinders are fine.

I have just come back from an air race in Botswana (see www.airrace.co.bw - it's awesome ). At 21gph, max MAP (about 26" at 4000' DA) and 2600RPM, the CHTs were around 370F except No 6 which hovered around 400F. Oil temperature was 200-205. I feel I could have leaned a bit more for best power and an extra couple of knots but didn't want to because of the No6 temp. I made my handicap speed (173.5kts TAS) but racing is all about those extra knots (Oh, but staying on track and finding the turn points helps.....)

My plan is to fit a 17 row oil cooler instead of the stock 13 row and use a 3" duct. My hope is that the lower airflow through a bigger cooler will reduce the CHT whilst maintaining the oil temp. Before embarking on that expense, I intend to check a couple of other things:

- accuracy of probe - don't want to be chasing ghosts
- injection jet - am I getting the correct fuel into the cylinder? Doubt this is the problem because otherwise wouldn't I expect to see No6 EGT peak first?

The engine now has 100 hours on it so I don't believe that a "tight" cylinder can any longer be a possibility. Since the cylinder stays below 435F in climb and 400F in cruise, I am not overly worried but I really would like to get 30F off it so it more closely matches the others.

Question for the engine gurus: can I lean to peak at full power so I can then go to +100F? Doesn't sound like a good idea so I was using the engine power charts to estimate the required fuel flow.
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  #26  
Old 07-13-2017, 11:41 PM
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Originally Posted by paul330 View Post
My plan is to fit a 17 row oil cooler instead of the stock 13 row and use a 3" duct. My hope is that the lower airflow through a bigger cooler will reduce the CHT whilst maintaining the oil temp.
Don't think that plan will work. You'll cut flow area almost in half, but you won't double cooler area.

As for CHT reduction, I'm pretty sure you won't see it go down. However, one good test is worth 1000 opinions, including mine. Make some simple conical adapters (sheet metal, a pop rivet or two, and duct tape) so you can run a length of 3" duct for one flight. Yes, oil temp will be warmer than usual, but you should be able to determine if CHT changes in relation to the other cylinders.

Quote:
- injection jet - am I getting the correct fuel into the cylinder? Doubt this is the problem because otherwise wouldn't I expect to see No6 EGT peak first?
Correct. If it was restricted that cylinder would peak first when pulling rich to lean.

Quote:
can I lean to peak at full power so I can then go to +100F?
Answer is "maybe". A lot depends on CHT and intake air temperature. An FAA-standard detonation sweep is done with one cylinder at redline CHT, the remainder within 50F of redline, oil at max temp, and intake air at 103F. At full rated power under these conditions, the hot cylinder will probably suffer detonation at only a bit leaner than best power mixture...still well rich of peak EGT.

However, when racing at 4000 ft (26" and maybe 2650), with lower CHT and intake air temp, the risk is reduced. The EAB wildcard is ignition timing, fixed (usually magnetos) or variable advance. More advance increases detonation risk.

A bit of mild detonation won't hurt anything, so sneak up on it. Set desired power while rich, let everything stabilize, then lean a little bit at a time while watching CHT carefully. It will rise as you lean, but if at some point the CHT digits start flipping like a gas pump, richen up or pull off some MP; you just found the detonation limit.

Alternate plan; We know best power mixture will be about 125 rich of peak. Settle in at the expected flight conditions, set 24/2400 or similar, find peak, go to 125 ~150 ROP, and note the actual EGT. Now put power where you want it and readjust mixture if necessary to achieve the same target EGT. The EGT which equates to 125~150 ROP at 70 or 75% will be the same EGT at a higher power setting.
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  #27  
Old 07-14-2017, 12:11 AM
paul330 paul330 is offline
 
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Thanks for the input Dan.

If I simply temporarily cover a portion of the duct inlet with AL tape, would that validate the experiment? Why do you think that restricting the oil cooler flow won't help the No6 CHT?

Finding +120F at 75% sounds like a good plan. Since the engine ran really sweetly at 2600RPM, I think I will run 2650 next year. If I can get the CHT down by 30F, then with a little leaner mixture I could possibly get 2-3kts extra. Over a near 2 hour race, that can make around 1:30m difference or 3:00m over the 2 days......
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  #28  
Old 07-14-2017, 07:07 AM
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If I simply temporarily cover a portion of the duct inlet with AL tape, would that validate the experiment? Why do you think that restricting the oil cooler flow won't help the No6 CHT?
The corrugated interior wall suggests a thick boundary layer, so true flow area is probably less than the nominal diameter, and it would get proportionally worse as the nominal diameter is reduced. A simple intake restriction wouldn't incorporate the effect of the duct walls, which is why I suggested inserting a length of actual 3" scat hose.

As to CHT, well, been there, done that. It started when measurement found a 16F to 17F difference between OAT and the face of the oil cooler, just down a short 4" duct from the #3 baffle wall. A little bit is compression heating, but most is heat picked up from the upper cylinder fins.

One way to lower oil temperature was to duct the oil cooler air supply all the way from the front of the plenum, just behind the cowl intakes, delivering lower temperature air to the oil cooler. It would also isolate cylinder #3, which would no longer have any of its cooling air supposedly "stolen" by the oil cooler duct inlet. So I built this insulated duct system.






Results: As expected, dropping delivered air temperature improved oil temperatures, even though I'm sure it reduced mass flow slightly (long flat ducts, not an optimum shape).

However, I noted no change to #3 CHT when I installed the above duct system, nor when I later removed it, returning to a flared intake hole in the rear baffle wall. A little thought tells why. The key factor driving air through the cylinder fins is simple pressure delta between the upper and lower cowl volumes. An oil cooler intake near the cylinder may flow a lot of mass, but if it does not reduce pressure in the vicinity of that cylinder, the cylinder's cooling performance will not change. Setting aside frictional duct losses, pressure should be pretty much the same in the oil cooler duct as it is in the upper cowl volume. It has to be. The upper/lower cowl pressure delta is what drives air through the cooler face, just like the cylinder fins. The pressure drop comes after the cooler.

Admittedly, it's an observation and a theory. Confirmation or denial would be to install two piccolo tubes, one over each rear cylinder. One would be near the oil cooler inlet of course. Compare in-flight pressures. If the piccolo near the oil cooler duct inlet is lower by some significant amount, then (and only then) is the duct stealing from the cylinder.

Quote:
Finding +120F at 75% sounds like a good plan. Since the engine ran really sweetly at 2600RPM, I think I will run 2650 next year. If I can get the CHT down by 30F, then with a little leaner mixture I could possibly get 2-3kts extra. Over a near 2 hour race, that can make around 1:30m difference or 3:00m over the 2 days......
The mixture vs power curve is fairly flat in the ROP region, but there is a little power to gain with careful knob management. Here's an example, a 540K on the FAA dyno at Hughes, 26" and 2700, pretty close to what you mentioned as your race conditions:



Max power is about 245, at 125 ROP (1590 EGT vs 1465). Detonation onset is about 60 ROP, with full development around 50~45 ROP. The difference between heaven and **** is a spread of about 75F.

Remember, this data was taken when real hot, and the K is considered to be a worse case example. You might have a bit more leeway, or less, depending on compression and ignition advance.

First determine if you can run at best power mixture with the CHTs you have now. If so, work on CHT reduction. When you get it down by 30F or so, install a variable cowl exit to reduce mass flow at high airspeed. The restricted exit area will drive the CHTs back up to where you started, but you'll get the 2 knots.
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Last edited by DanH : 07-14-2017 at 08:16 AM.
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  #29  
Old 07-14-2017, 09:42 AM
paul330 paul330 is offline
 
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Wow! - I'll get back to you...........
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  #30  
Old 07-14-2017, 06:45 PM
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N941WR N941WR is offline
 
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How is your exit opening on the bottom cowl? (Forgive me if I missed it.)

It should be even withe the firewall, even a little forward. This will help draw air out of the bottom cowl.
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