Interestingly, if I correctly interpret the extract on exit area I took from a study on aircraft liquid engine cooling, it would seem to imply that a “free floating” cowl flap would seek its own/best position with regard to minimizing cooling drag.  The flap would open more if the air pressure on the cowl side exceeds the free air pressure at its exit into the airstream and would be closed by the airstream until the pressure on both sides of the cowl flap equalized.

 

  At least that is what it appears to say to me.  However, minimum cooling drag may not/will not always be conducive to maximum cooling.  In some cases, like in climb, you may not mind more drag if your cooling is better by opening the flap more than optimum for minimum drag.

 

So I imagine, you would still want a control level/servo motor to control the extension.  There is more pressure on the flap than you might think.  I think it was Bill Eslick that tried a cowl flap and believe he eventually abandoned it – but, not certain if he did or exactly why (I think I knew at one time {:>) – Bill, speak up!

 

Ed

 

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

http://www.dmack.net/mazda/index.html

http://www.flyrotary.com/

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.rotaryaviation.com/Rotorhead%20Truth.htm

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From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Tracy Crook
Sent: Thursday, August 20, 2009 10:14 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Cowl Flap/Exit was [FlyRotary] Re: back in the air

 

Count on Ed to supply the technical background :-)  That's another good rule of thumb on the maximum inlet area too  (.3 *  core area).   More doesn't do you any good.

Main point is, You don't have to be a rocket scientist to make this stuff work if you follow a few basic rules of thumb.  (The right ones that is, there are a lot of bogus ones floating around as well :>)  

Tracy

On Thu, Aug 20, 2009 at 9:49 AM, Ed Anderson <eanderson@carolina.rr.com> wrote:

Past a specific inlet area to core frontal area (this ratio in the vicinity of 0.30, no additional flow is achieved by further opening of inlets – the exit area becomes the controlling factor. As Tracy stated, probably better to look to a cowl flap than mess with your inlets if you are getting good cooling with your present inlets.

 

Here is an extract out of a study of cooling of small aircraft engines that points out the two functions of your exit area. 

 

Exits

 

The exit has two basic functions: to regulate the cooling air

flow and to exhaust the cooling flow into the external flow so

as to result in minimal drag penalty. To adequately perform

the regulation, the exit must act as both a throttle and a pump.

Throttling is necessary in cruising flight to minimize the cooling

drag by reducing the cooling flow to that sufficient to meet

cooling requirements.

 

 In ground operation and in climbing

flight, the exit must act as a pump to induce sufficient cooling

flow through the system. Both of these functions can be performed

by a hinged flap. The fundamental principle here is

that for any subsonic flow system, the flow rate through the

system will always adjust itself so that the static pressure at the

exit will match the local external flow static pressure surrounding

the exit. The static pressure at the exit is controlled by the

exit area.

 

 Thus, regulation is obtained by varying this area.

Opening the flap beyond the contour of the airframe creates a

low-pressure region that induces additional flow through the

system. It became common design practice during World War

II to serve the exit flap to a coolant temperature sensor to optimize

the system operation.

 

 

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

http://www.dmack.net/mazda/index.html

http://www.flyrotary.com/

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.rotaryaviation.com/Rotorhead%20Truth.htm

---

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Tracy Crook
Sent: Thursday, August 20, 2009 8:49 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: back in the air

 

"So far its looking like I could stand to shrink both inlets a little - maybe a lot. "

Mike,
Glad your EC2 & cooling issues are under control.  I'd go with a cowl flap to restrict air flow (and increase exit velocity) rather than reduce inlet size.   Probably less fiberglass work for the cowl flap as well.

Won't go into the mind numbing formulas and laws that govern the principal but here is what I have boiled them down to:

It is not the absolute SIZE of the cooling inlet that determines drag but the amount of air that goes through it.

  Main thing is to make sure the air that is diverted around the inlet has a clean path.  (you've done a good job there).  Restricting the inlet will also restrict the flow but why reduce your options for other conditions (hot weather, max climb, etc).

Tracy

On Wed, Aug 19, 2009 at 11:30 PM, Mike Wills <rv-4mike@cox.net> wrote:

I seriously considered a water oil exchanger. Fluidyne also made some of those that looked really good. Dont know if they still do.

 

Ultimately decided not to go that route. So many people were having cooling issues when I was designing my system and it seemed that using seperate air oil exchangers would ease potential debug hassles.

 

I'm mostly happy with my choice. Its working well so far. And using a $25 wrecking yard RX-7 oil cooler was about as cheap as it gets. My only complaints so far are that I am not real happy with the looks of the extra scoop I added for the oil cooler, and my braided stainless oil cooler lines are fairly long and heavy.

 

I probably will redo the cowl somewhere down the road after I get a good handle on just how well the cooling is on hot days. So far its looking like I could stand to shrink both inlets a little - maybe a lot. On my flight the other day coolant temp never went over 180. And that included taxi from the extreme west end of the airport to the other end of an 8000' runway.

 

Mike Wills

RV-4 N144MW