flyrotary@lancaironline.net Mailing List Archive

Hi Jeff. I’ve done lot’s of experimenting with cooling sys. Tough to explain all this, but I’ll give it a try:

Pretty clear you saw 6 psi only because you took off with 3+ cups of air in the system. If you’d taken off with 2 cups of air, then pressure would have been 9 psi. 1 cup, 14 psi. No air in system, then you would have seen rated cap pressure (16 psi in your case).

Sorry to say, the pressure you see has no significant effect on cooling efficiency (heat transfer). The next time you fly, since you’ve removed most of that air, you will still see 210F. Just like before. There’s one huge exception to that statement, that’s if you have air in sys that can’t move to a high point out of the flow. In that case air in sys has huge negative effect. Causes local boiling when it passes hot areas and inflates cooling temp.

You don’t have to fly to prove these concepts. Ground running at idle is all that’s needed. Let’s assume all air is removed. Then letting engine warm up to 180 F will result in rapid pressure rise to 16 psi (rated cap pressure). Fluid will exit system. With 2 cups of air in sys, that same 180F will yield slow rise in pressure to only 9 psi. No fluid will leave sys.

A good cooling design pretends air is stuck in block, so you add a path for that air to rise out of the block coolant flow. This is called a dynamic bleed. Air is automatically removed from engine coolant flow. Super low risk way to fly as you no longer care if air is in system. It’s can’t affect cooling.

It’s a bit higher risk to fly without dynamic air bleed, you just make darn sure you purge all air from block sys prior to flight. Applying vacuum to rad cap is great way to remove air.

One of the ironies about cooling design is that air that resides above engine flow is a safety asset. For example, your cap is highest point in sys and you have 2 cups of air under cap. Big safety advantage simply because your pressure gage is then a great predictor of how well your sys is doing. A leak will be detected long before overheating. A bunch of other assets to this design.

Conversely, air in engine flow has overwhelming negative affect. Temps soar and risks boil over.

Clear as mud eh?

Fwiw

-al wick

From: Jeff Whaley

Sent: Wednesday, May 06, 2015 4:54 AM

To: Rotary motors in aircraft

Subject: [FlyRotary] Re: Return to Flight

Dave, I can’t argue with your adequate theory; I have that for my oil cooling system, along with the temperature-controlled Mazda oil cooler, I never worry about oil temperature.

It’s the practical application of that theory to the radiator installation that has plagued me from the beginning … after the next flight I’ll know where I stand and if there’s still a problem, then radical change may have to be the recipe.

Jeff

From: David Leonard [mailto:flyrotary@lancaironline.net]
Sent: Tuesday, May 05, 2015 9:31 PM
Subject: Re: [FlyRotary] Re: Return to Flight

I agree with Bobby, this may (hopefully) be the cause, but if so it just means that you didn't design in an adequate way to fill the coolant without trapping air. You would hate to have to do a coolant-prep flight every time you wanted to drain and refill the coolant. (but not the end of the world).

I originally designed my system like Ed's. Minimal drag (just the 2 stock RV-6 inlets) but cooling was like you and Ed describe... OK, but you always have to keep and eye on it and often reduce power or AOA in order to keep coolant temps in check. Then I made a major change to provide TONS of air to an adequately sized radiator. Since, I never even think about cooling. Yes, I do probably burn a little more fuel but it is well worth it to never worry about cooling again. Its not rocket science. Adequate air to an adequate radiator equals adequate cooling. Change adequate to plenty in that sentence and now your are rocking!

Dave Leonard

On Mon, May 4, 2015 at 12:37 PM, Bobby J. Hughes <flyrotary@lancaironline.net> wrote:

Jeff,

Congratulations on getting back in the air. That ½ quart of coolant may have been trapped air that purged during flight.

Bobby Hughes

N416AS-

RV10 223 hrs

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]
Sent: Monday, May 04, 2015 7:53 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Return to Flight

Yesterday 3-May, my bird got airborne once again on its maiden flight after 2+ years rebuilding. I had a friend test-pilot fly it while I was flight engineer watching the gauges. Flight controls and rigging were reported to be good and the engine performed flawlessly. The oil temperature was stable around 160F (Mazda oil cooler in series with a Fluidyne) but again the water temperature was high reaching 210F by circuit height so we backed off the throttle, did a touch and go with a 500’ circuit and parked it. One unusual result was the cooling system pressure was only at 5-6 psi; with the high temperature it should have been 15-16 psi. Removed the cowlings and found no leaks of either oil or glycol. I checked the coolant level and it was down, requiring 1/2 quart to top it up. I also looked at my pinched duct and think it needs more pinching, so am already starting to modify that – hope to get another test flight in before the weekend.

Jeff

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