From: Ed Anderson

Sent: Friday, March 25, 2011 8:27 AM

Subject: [FlyRotary] Radiator Caps [FlyRotary] Re: On the subject of installations...Coolant Pressure

 

One thing to keep in mind about the pressure rating of the radiator cap you are using.  All are rated relative to a standard sea level ambinent pressure.  This means that he air pressure itself is contributing 14.7 psi at sea level. 

 

So at sea level when your differential pressure coolant gauge is reading 10 psi - the absolute pressure in your coolant system is then  14.7 + 10 = 24.7 psi.  Now if you are flying at 8000 MSL your ambient pressure is approx 1/2 at sea level.  So your 14.7 psi CAP now has the lesser capacity of 7.3 + 10 = 17.3 psi absolute pressure capacity - somewhat less than the 24.7 psi absolute it had at sea level.

 

When you increase in altitude this component naturally decreases.  So while a 14.7 psi CAP may work fine at sea level, more than one person found that at altitude  that rating was  insufficient and some coolant was lost.

 

I personally would not fly with less than a 21 psi cap and currently fly with a 24 psi radiator cap.

 

There is another factor that occurred to me.  Once a hot coolant system blows the relief valve on the cap, that lowers the pressure inside the system, superheated (>212F) coolant can quickly flash to steam and further cause lost of coolant.  So a Higher PSI cap can lessen the chance of that happening. 

 

FWIW

 

Ed

 

Lynn,

 

You have hit my quandary squarely on the head.  Assuming that the pressure was zero at room temp and rose to say 6-7 pounds at 200 degrees, and assume that the flow restriction in the system caused a pressure rise of about 2 pounds at 2000 rpm and a pressure rise of, say, 6-7 pounds at 6000 rpm.  Under that scenario, you would have a pressure that ran at between 7-9 pounds at 2000 and rose to 12-14 pounds at 6000.  Those pressures would be easily contained with the stock cap of 14 pounds.  And the pressure would be constantly changing with rpm and possibly temp of the system as you were under power or not.

 

I have a 21 pound cap.  My system climbs smartly to the top and stays there.  No fluctuation with rpm, no real fluctuation at temp because it has already hit the 21 pounds before the engine is actually hot.  (around 190)

 

Something is wrong and I don’t understand what it is.

 

Bobby

 

I am going to insert answers to your questions in your msg below.

 

Bill B

 

 

Bill,

 

This is all I can think of at the moment.

 

Are you running a thermostat or did you plug the hole between the inlets and outlets of the water pump?

 

No thermostat.  The Renesis has a thermostat tower which is too tall to fit under my cowl, so I removed it and fabricated a flat plate with two 1.25 aluminum tubes for the inlet and outlet of the water pump.  That hole you are referencing exists in the tower, but not in my plate.

 

If plugged did you drill a small hole in the plug to allow air a place to escape? I used a #30 drill.

 

Any air in my system would have to be carried to the highest point, (radiator cap) then be forced by pressure to the bottom of the swirl tank where it would be trapped.  This seems to be working because I have found that after I open the system for some reason, the level in the swirl tank will go down and there is never any air at the top of the radiator.

 

After an engine run is the radiator the same temperature at the inlet / outlet / bottom and top? If I have trapped air only part of my radiator gets hot to the touch. Dual pass barrier leaking?

 

Seems to be.  I have mistakenly laid my arm on the top of the radiator after a run…usually I manage to get off pretty fast!  :>)  I can not see into the radiator tank on the barrier side.  It is possible it could be leaking.  I suppose if there was a rag in the thing it could be trapped here also.  I have no way to see in there even if the hose was removed.  I would need some kind of flexible camera or something like that.

 

Use an extra CHT channel and clamp it to the inlet of your radiator to measure Delta T.

 

Good idea.  I will have to rig one up.  I don’t have an extra now.

 

My water pressure does not vary that much in operation. But it does hold a little pressure for a few days after shutdown.  1-2 psi.

 

What is your pressure during operation?  What is the pressure cap rating?  Does it vary at all with temp and/or rpm?  Mine is maxed out..always!

 

What size hoses? I’m running 1in.

 

The RX-8 uses 1.25 hoses.  I stuck with that size as well.

 

Where are you measuring water pressure?  

 

The hose that comes off the top of the rear iron goes to the top of the radiator, just below the cap. A tee in this line has the water pressure sender in it.  The radiator cap is just a cap, not a pressure cap.  The outlet in the radiator neck goes to the bottom of the swirl tank.

 

Is your pressure cap on the bleed / swirl tank?

 

Yes.

 

Any pictures of the plumbing?

 

Not of the current setup.

 

Bobby

 

I need some help with figuring out my cooling problems.

 

Just so it doesn’t get lost in the following BS, I need to state that I think the problem is related to the coolant pressure.  It seems to me to be way too high.  I am beginning to think that the coolant is somehow partially blocked.

 

Now the rest of the story….

 

After I did the flight described in the attached email, I installed an opening in the bottom of the cowl that would work something like a cowl flap that is permanently open.  The new opening is 9 inches by 5 inches or 54 sq in..  There is a flare in front of the opening that is at an angle of 45 degrees and extends down into the airstream about 4 inches.  (it is 6 inches long.)  This opening is in addition to two 6 X 6 openings, one of which has a 3 inch diameter exhaust pipe in it.

 

This helped a little, but nothing like I expected it would.  The pressure inside the radiator inlet dropped from 10 inches of water to 8 inches of water.  (by the way, Steve was right about it being inches of water and not inches of Hg.)

The pressure after the radiator and inside the cowl dropped to 8 inches as well.  The temperature of the water dropped to 208 and the oil a couple of degrees.  Neither of these had the result that I expected.

 

My water pressure cap is rated at 21 lbs.  I have not calibrated the sender that came with my EM-2 and it shows 3 lbs when the engine is cold.  I assume that this is zero.  When in flight the pressure climbs to 24 lbs and stays there.  I assume that at this point, the cap is at 21 lbs and is bleeding off air.  There is about a pint to a quart of air above the coolant in the bleed tank.

 

This morning I ran the engine on the ground for about 10-15 minutes at an rpm of 3000 to 3200.  The OAT was 75 degrees.

 

I read the temps and pressures every minute or two during the run.  I hope this doesn’t get jumbled during transmission.

 

Oil temp –                      80      103      130              139             144           155          162         165

Air temp after cooler -               97       109                                122            129          133         134

 

Water temp -                  86       126      161              173             178           190          198          199

Air temp after rad -                    109      133                                157           169          179           183

 

Water pressure -             2          11        17               20               21                            24     

 

I noticed that the water pressure could be brought from 2 lbs to 9 lbs by changing the rpm while the engine was still relatively cool

 

What should I expect for water pressure at lower temps and how could I go about making a determination that the water passages are clear?

 

I am beginning to think that a rag has been left either in the engine or the radiator.  I have never torn the engine down and I sent the radiator out to have a leak repair a couple of years ago.  It is a double pass radiator.  I can look into the end that has both sides connected thru the radiator cap neck, but not into the inlet and outlet end.

 

Suggestions???

 

Thanks,

 

Bill B