I found your address in http://mail.lancaironline.net and some e-mails of you concerning air-flow for cooling.

Would you mind to discuss this problem with me?

 

I live in Freising close to Munich, Germany and build a Lancair 320 (IO-320 brand new engine) D-ESWS with first flight in Oct. 1995 and logged about 420 hours.

Since the beginning of flight I found my CHTs close to the recommedet limits (using EPI800 with 4 probes):

During climbing (120 MHP) all 4 CHT about up to 450°F

During crouse (75%) all 4 CHT about  400°F.

Oil Temperature is perfect about 180-210°F.

My target it to reduce CHT for best engine operating life time!

Main reason: I already had to change two cylinder heads because of low compression: 35PSI of 80PSI and 50PSI of 80PSI. I don´t know if exactly this was the reason but I think a lower CHT couldn´t be wrong! 

 

Here are some information about my "cooling problem" and what I did  until now: 

1. I still use the cowling air inlet cut out diameter of the original Lancair kit  (about 3.7")

2. I made a pressure cowling:

   Bevore                                                                                                  After

 

I made the pressure cowling out of carbon fibre sandwich using a 360°F prepreg system

Result: no siginficant change!!!!  Beleve me: I was frustrated!

Positive: the weight of the pressure cap is about 0,4 lbs which is about the weight of the removed silicone rubber! And: it looks like "high tech" (don´t ask about the hours  I spend  to make it) 

 

 3. I tryed to make more professional investigations by checking the pressure situation above and be low the cylinder head  (see red circle) :

I run two thin tube s (green)  form "above cylinder head" and "below cylinder head" into the cockpit to a Airspeed Indicator to compare it with the aircraft air speed during flight.

I expected: having a good pressure cowling, the "speed above cylinder head" should be about or less of the value of aircraft speed; the "speed below cylinder head" sould be about zero (ideal below zero-> suction)

Result: "speed above cylinder head" is about 75% form indicated aircraft speed which is pretty good i think!

But: "speed below cylinder head" is about  40% form indicated aircraft speed which is much to high!!!!  There is only 35% pressure difference left form a maximum of (theoretical)possible 100%!!!!

 

Why is below the cowling a  high  positive pressure  when there should be a slight suction?

My opinion: there is a perfect (and sufficient) pressure "above the cowling" but very bad conditions (positive pressure) "below the cowling".  Target is it to reduce this positive  pressure below the cylinder head ...

 

 4. My exlanation: there might be turbulences under my cowling which prevent air form escaping! I suspected my nose gear cut out to cause turbulences:

 

Cut out cause turbulences?                                                                                                                     Small door close cut out when gear retrected

 

Now I modified the cowling and added a small door which closed the nose gear cut out after retrection of the nose gear.

In a new flight I measured the pressure again "speed below the cowling"

Result: no significant change!!!  (Frustration) 

 

5. In a earlier modification I enlarged my lower cowling outlet: to get space for my muffler and more space for air outlet:

 

This is situation right now and I think about other sulutions to reduce positive pressure below the cowling... 

What I do not understand is: I thought the passing air under the cowling (blue curve) is acting like a venturi effect to suck the cooling air (red curve) out of the cowling.

Obviously I´m wrong!

 

If you neet more detail information don´t hesitate to ask! Would be great to discuss this problem with other people...

 

Any idea?

 

Johannes,

 

I have removed the pictures to shorten the reply.

 

I am sending this and my reply to the LML - maybe there will be more ideas.

 

I had been to Munich many times in the early 1980s and even started a subsidiary there (no longer in existence).  I used to take morning runs in Englischer Garten.  Ahhh, fond memories of the past.........

 

All of the replies given on the LML are quite good, but let's try to tie everything together for your situation.

 

1. Measurements:

a. CHT.  There is some question as to the correctness of the CHTs.  As was suggested, dip a probe in boiling water and note the temperature for calibration purposes.  Remember that the boiling point changes downward if the water is not pure and you are located where the conditions are not standard sea level (i.e. higher altitude).  Old style CHT probes used special resistant wire to connect to the display device - If I remember correctly, if the wire was shortened from its' original length, the temperature indication would be artificially higher.

b. Cowling air pressure differentials.  Generally the standard used for reporting such pressure differentials has been inches of water (" H2O). Thru the rest of this reply I am going to use English measurements and let you convert to metric where necessary.  Airspeed indicators have been plumbed in (pitot/static) because they are easy to use and airspeeds (pitot ram air pressure differential from static) can easily be converted to inches of mercury (" Hg) or " H2O.   In order to get good cowl pressure readings, the openings must not be affected by air flow.  As others have mentioned, some sort of filter or mesh could be used.  Another more common way is to create a picolo tube - a metal tube pinched close at one end and many holes drilled into the tube that is then inserted into the tubing leading to the sensor.  Placement should be in a non turbulent location.  5 to 7 " H2O difference between the upper and lower plenum is considered sufficient pressure for cooling.

 

  I have also used a spare altimeter open to the cockpit for making other measurements (after corrections) against ambient (static) pressure without tapping into my aircraft static/pitot system.  For example, in my LNC2, the cockpit is .1"Hg (1.36" H2O) less at 120 KIAS to .3"Hg (4.1" H2O) at 190 KIAS.  My upper plenum has varied from 9 to 13 (or more) " H2O higher the lower, depending on speed, and the lower plenum is about 2" H2O higher than ambient (static) although I don't remember the change for speeds.  This means the lower cowl is about 5-6" H2O pressure higher than the cockpit at high speeds. 

 

In your chart example, At 180 KIAS, the upper/lower cowl indicated about 135 KIAS or about 11.9" H2O.  The reason you cannot see the same speed as your aircraft is because the aircraft pitot tube is measuring ram air pressure in a closed chamber - the cowling is not closed as the air is moving thru it.  The numbers you report (if correct) indicate more than adequate pressure for cooling.  The lower cowl/ambient showed about 65 KIAS or 2.8" H2O - also OK.  Indeed, your numbers compare to mine quite favorably.  Remember that the incoming air expands from heating.  I don't think there is any problem with some pressure in the lower cowl as long as the upper/lower difference is sufficient to get air to flow.

 

The chart below is useful for converting KIAS to inches of water.  The left 2 columns are examples of my IO 320 in different parts of flight. "Cowl Kts" is airspeed as measured by pitot/static differences between upper and lower cowl.

 

	Aircraft	Cowl	Delta	Delta	Delta
Flight	IAS Kts	Knots	"H2O	"Hg	PSI
		34.8	0.79	0.06	0.028
		39.1	1.00	0.07	0.036
		43.4	1.23	0.09	0.044
		47.8	1.49	0.11	0.054
		52.1	1.77	0.13	0.064
		56.5	2.08	0.15	0.075
		60.8	2.41	0.18	0.087
		65.2	2.77	0.20	0.100
		69.5	3.15	0.23	0.114
		73.9	3.56	0.26	0.129
		78.2	4.00	0.29	0.144
		82.6	4.45	0.33	0.161
Climb	140	86.9	4.94	0.36	0.178
Climb	150	91.2	5.45	0.40	0.197
		95.6	5.98	0.44	0.216
Level	165	99.9	6.54	0.48	0.236
Climb	160	104.3	7.12	0.52	0.257
		108.6	7.73	0.57	0.279
Level	176	113.0	8.37	0.62	0.302
Level	185	117.3	9.03	0.66	0.326
		121.7	9.72	0.71	0.351
Level	200	126.0	10.43	0.77	0.377
		130.3	11.17	0.82	0.403
		134.7	11.93	0.88	0.431
		139.0	12.72	0.94	0.460
		143.4	13.54	1.00	0.489
		147.7	14.39	1.06	0.520
		152.1	15.26	1.12	0.551
		156.4	16.15	1.19	0.583
		160.8	17.08	1.26	0.617
		165.1	18.03	1.33	0.651
		169.5	19.00	1.40	0.686
		173.8	20.01	1.47	0.722
		178.1	21.04	1.55	0.760
		182.5	22.10	1.62	0.798
		186.8	23.18	1.70	0.837
		191.2	24.29	1.79	0.877
		195.5	25.44	1.87	0.919
		199.9	26.60	1.96	0.961

 

Here is a partial list of some of the causes for high CHTs. 

 

Airflow:

 

The ideal cooling condition is to slow down the incoming air, direct it to the right places and speed up the air for exit.

 

Slowing the air - If the air is not slowed, it can produce high pressure blockage on the fins.  Some have found that a short slowly expanding tube at the entry (less than 7 degrees divergence) will slow the air down and retain its' pressure.  There is an excellent article in the March, 2006 issue of EAA Sport Aviation on cooling (Control the Flow). 

 

Directing the air - The cooling air is best utilized by passing it thru all the upper cylinder head fins where most of the heat is.  I noted that you have two large air exits at the back of the baffling - can these be causing the air to pass over the cylinders rather than down thru them?  Possibly creating a low pressure area where you need the flow the most. 

 

Speeding up the air for exit -  Not much we (LNC2) can do here because of tight cowling.

 

Timing:

 

You don't mention whether you are using magnetos or some other device.  Timing advance can raise CHTs.  You don't mention what caused the low compression readings - burnt exhaust valves, bad rings?  I see you have nitrided cylinders (blue paint), are you using high compression pistons?

 

Oil:

 

Oil accounts for a good part of the cooling. Although your temps sound reasonable, it is when they occur that is important.  I am using a separate NACA duct for my very small oil cooler.  The oil temps are 180F in cruise at warm OATs but rise to 200F in the pattern and after landing because of poor airflow thru the NACA at slow speeds.  Is this pattern similar to yours or do you see 200F+ oil temps in cruise?

 

It would be helpful to see a collection of numbers such as

Altitude, Baro, OAT and airspeed

RPM, MAP and fuel flow

CHTs and EGTs for each cylinder.

 

It seems to me that your CHTs should not be as high as they are in cruise.  It will be interesting to find out why.  After all your work to date, it is very puzzling.

 

Again, the pressure seems good, but is the air slowed enough and getting to the right places?  Are your probes calibrated?  How different are the CHTs between cylinders?

 

I hope this helps.