flyrotary@lancaironline.net Mailing List Archive

Interesting thread.

There are several figures throughout the thread which are very suspect. The most glaring is the airspeed being measured through (or behind) the radiator and oil cooler. Numbers like 60 to 100 mph in this area are definitely due to instrumentation error or misunderstanding. This just doesn't happen.

I think Mark S. mentioned his reference point as the aircraft static pressure port. If so, this is one big error producer. In this instance, the proper reference would be under cowl static pressure in the same area as the pitot probe. Take a look at some older combination pitot / static probes. The static probe is an identical tube right under the pitot tube. It has the front of the tube blocked off and has several holes drilled into the sides to measure local static pressure. This is the type of probe needed to measure airspeed in this application.

Wish I could find it in my files but I think it is in the book "Aerodynamics of Propulsion" there is a chart which takes all the relevant factors and charts factors including airspeed of aircraft (A) vs airspeed through heat exchangers (a) and other factors and shows that the ideal liquid cooling system would have (a) to (A) ratio of .1. i.e, at 200 mph the airspeed through heat exchangers would be 20 mph (10% of aircraft airspeed). This might give some idea of what to expect. In my own experiments I found that it It is darn hard to push high speed air through a rad due to the very turbulent airflow (which is a good thing).

As several have already pointed out, in practical terms, you can't have the water going through the rad too fast. In General, the faster the better.

Bobby H asked about delta T of air through Rads and if 70 - 80 F is possible. I commonly see 70 deg F Delta in OAT to after rad air temps. Sometimes more.

Tracy

On 9/30/07, Bobby J. Hughes <bhughes@qnsi.net> wrote:

Snip

That's exactly what I HAD thought, until I was told that the air could pass through too fast and not pick up as much heat.

Mark... this is what I was trying to communicate. It could be totally wrong so let's get more opinions. Ed??

With respect to exit area size only.

If the volume of air through the coolant radiator was moving faster than optimum the air delta T would be lower than it would be with slower air. Slower air at a higher delta T means less air needs to exit the cowl. What is optimum? Every install is unique so it needs to be viewed \ identified on each installation. Ed uses a 30% duct air speed as a reference point. If I understand this correctly the diffuser play a role in duct airspeed. If the air is not being diffused optimally the airspeed could be much higher than 30% through the part of the core. Dennis H. and I observed airspeeds through a test core at 50% + without a diffuser.

Your inlets are 72sqin with 306sqin core face for water and 24.75 sqin with 102 sqin for oil. Both are competing for the same exit area. IF this is an exit area problem only then enlarging it should improve both water and oil proportionally. Water should realize 2/3 of the improvement and oil 1/3. 306 sqin vs 102 sqin. Your improvements after opening the exit seem to track this very close. So increasing your exit area further should show more improvements in both oil and water to a point. But you do not need more water cooling improvements right?

Core 306 sqin at an airspeed of 115mph.

40% or 46 mph = 8602 cfm   8602 cfm at a 50 deg air delta T = 7741 btu's \ min

30% or 34.5 = 6451 cfm        6451 cfm at a 80 deg air delta T = 7741 btu's \ min

In this example 2151 cfm less air needs to flow through the water radiator to produce the same btu rejection. So what effect would 2151 cfm less air through the water radiator and exit area have on the oil cooler's ability to flow more air? Would it improve the oil cooler air flow by 1/3 or 717 cfm? Not sure.

Can your radiator produce a 80 deg air delta T? It may only produce 60, 70 or??

Am I in left field here?

Bobby

(flow testing new radiator ducts prototype today)

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net ] On Behalf Of Mark Steitle
Sent: Sunday, September 30, 2007 4:28 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Another cooling question

ED wrote:

<snip>

Mark, if you really had excess air flowing through your radiators the coolant would drop more than 4 Deg F. In fact, the more air flow the more coolant Delta T you would drop through the radiator.

<snip>

That's exactly what I HAD thought, until I was told that the air could pass through too fast and not pick up as much heat. This didn't make sense to me. Maybe I wasn't listening closely and missed the point altogether (wouldn't be the first time).

What I DO know is that the air is flowing faster through the water radiator than the oil radiator. (I'm not sure I have the ASI's hooked up correctly, but they're both hooked up the same). I have a pitot behind each radiator hooked up to two separate ASI's. In slow cruise, say 125-130 kts, the water radiator ASI will read about 110knts and the oil ASI will read about 90 kts. The way it was behaving before I opened up the exit, it appeared that the air from the water radiator was trying to exit backwards through the oil inlet. I say this because of how high the oil temps were reading. I enlarged the cowl exit, and both the water and oil temps dropped significantly.

The ASI's are referencing the static port for these readings; should they be referencing cowl or cabin pressure instead? Airspeeds readings seem awfully high to me.

Mark

(Going to the airport today to recalibrate temp sensors)