Tracy,

Like yourself I have been considering the benefits of an Eductor for some time, but can't understand why it would cause extra drag.

 

As a matter of fact I thought it might decrease drag by speeding up the rad exit air back up to outside air speed as it exits the cowl.

 

Can you explain the reasons behind the increased drag issues.

George (down under)

"Since the draw of air via low pressure on the output side seems to be key, I wonder if an eductor type of scenario would work."

Sometimes I doubt my ability to get a point across clearly :-)  An eductor can be made to help (but very hard to do as Ed pointed out)  but the point I was trying to make in my original post is that the draw of air on the low side is NOT key.   You will never get a fraction of the pressure delta with low side help (even with an eductor) that you can with the proper inlet and diffuser.   This is especially true on faster airplanes.  A Pietenpol might be an exception. 

The inlet is what fixed my problem.  This is an extreme example but when I used low side help, it did cool but the drag caused the fuel consumption to increase by 50 - 60% ! at the test speed of 130 mph.   That's not a price you want to pay.

Tracy

On Wed, Apr 28, 2010 at 9:19 AM, Chris Owens - Rotary <rotary@cmowens.com> wrote:

You know, I don't know if this has been discussed, but the whole pressure differential thing got me thinking of something that I'm surprised I hadn't thought of earlier.  Since the draw of air via low pressure on the output side seems to be key, I wonder if an eductor type of scenario would work.

Back in my Navy days, we used to use a device called an in-line eductor for dewatering flooded spaces.  Similar to a venturi, more or less, you pumped water through it, it created a suction, and it was designed to suck as much water through it as you put into it.  100 gallons per minute input would dewater at 100 gallons per minute with 200 gallons per minute flowing through the output.  A representative device is here (perhaps not for fluid use, but the concept is similar):

http://www.1877eductors.com/eductor_gas_dimensions.htm

I presume a similar approach could be taken with a radiator setup, would you think?  I imagine it would work well for a center mounted radiator with a centerline, below-the-nose scoop, so one could utilize the cheek inlets to provide source air for the outlet side.

~Chris

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From: "Ed Anderson" <eanderson@carolina.rr.com>
Sent: Wednesday, April 28, 2010 7:08 AM
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: 20B RV-8 cooling results

Hi George,

 

As you know, taking heat away from your radiator cores requires sufficient air mass flow - a number of factors affect this - one of the principle factors is pressure differential across your core.  No pressure differential = no flow.  The primary positive pressure on the front side of the core comes from converting dynamic energy of the moving air into a local static pressure increase in front of the core.  This is basically limited by your airspeed and efficiency of your duct/diffuser.  The back side of your core air flow (in most installations) exits inside the cowl.  Therefore any positive pressure above ambient under the cowl is going to reduce the pressure differential across your core.  So once you have the best duct/diffuser you can achieve on the front side of the core - the only thing left to increase the pressure differential is to reduce the pressure under the cowl.

 

An extreme example is someone who flies with an opening (such as one of the typical inlet holes beside the prop) exposed to the air flow.  In effect this hole with little/no resistance to airflow can "pressurize" the cowl and raise the air pressure behind the radiator cores reducing the pressure differential and therefore the cooling.  Exhaust augmentation is theoretically a way to reduce the under the cowl pressure by using the exhaust pulse to "pump" air from under the cowl, thereby improving the Dp across the core and therefore your cooling.

 

While exhaust augmentation can apparently work - there was a KITPLANE issue back several years ago on the topic showing several installations where this was used.  However, from what I read (and think I understand), it takes some carefully planning to get an installation to work correct and the effort is not trivial.  Give the challenges you may encounter (such as motor mount struts, etc), fabrication of the augmentation exit,  the need to have the exhaust pulse exit at or inside the cowl (or construct an extended bottom cowl tunnel) means you would have the bark of a rotary in front of your feet.  Also, to gain maximum advantage of these techniques, it is desirable to have the exhaust velocity at the maximum - which implies little/no muffling.  Having had my muffler back out one time (at the cowl exit), I can tell you that you do not want to position the pilot behind the exhaust outlet (in my opinion).  It is much quieter when you have the exhaust exit behind the position of the pilot {:>).

 

Some few people seem to have been able to achieve some degree of success, but even in aircraft where you have an engine without the aggressive bark of the rotary, you seldom see it used.  The basic reason (in my opinion), is that it offers few advantages (cooling wise) that can not be achieved easier and more reliability by other methods.  For an all out racer where noise and discomfort is secondary, it may have some benefit.

 

Having said that, it's clear that in some installations it appears to work well (see KITPLANE issue), but if it were the magic solution, I think many more folks would be employing it - but, again, just my opinion.

 

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 George Lendich
Sent: Tuesday, April 27, 2010 9:41 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: 20B RV-8 cooling results

 

Ed/ Tracy,

Can't say as I understand Tracy's set- up completely, other than it's toward the lower end of Rad sizes. I was thinking to myself how I could create a -ve pressure in the rad outlet to create a suction on the Rad. We all know how the exhaust augmentation works and I was wondering why we can't do the same thing with the rad outlets by running the rad outlets inro a larger outlet fed by outside air. At idle the air is fed by the prop air stream and at level fight it is fed by outside air stream.

The outside air could be could controlled by a butterfly - simple enough. I know there emphasis on using shutter /flaps to control the cowl outlet and I believe their good at restricting air flow, but I don't know if this equates to a good -ve pressure behind the Rad. This presupposes the Rads are completely enclosed for both inlet and outlet air.

George ( down under)

75% of my cooling problems were solved with the oil cooler change I did but still needed more margin for hot weather climbs.   Made the decision to not change or enlarge the cooling outlet (that adds drag)  so went ahead and butchered the pretty inlets I made. 
Ed Anderson's spreadsheet on BTUs & CFM cooling air required was instrumental in deciding to go this way.   It showed that without negative pressure on the back side of the rads, there would never be enough cfm to do the job during climb at full throttle.  Negative pressure is what I had when I flew without the cowl on but oh what a draggy condition that was.

The old inlets were 4.5" diameter for the radiator and 4.125" diameter for oil cooler.
New inlets are        5.190" for the rad,  and   4.875" dia for the oil.

This may not sound like a lot but it represents a 36% increase in inlet area.

Results were excellent.  Oil temp went down 19 degrees at the test speed (130) and water temp dropped 9 degrees.  On 80 degree day and 500 ft msl the oil temp maxed out at 194F at 210 mph which is way faster than I would normally go at this altitude.  Temp was around 175 at 130.    Oil Temp in climb remained below redline (210) but the temperature lapse rate today made results not very meaningful.  OAT was dropping 14 degrees a minute at 3000 fpm climb rate.

now back to that nasty composite work to pretty up the inlets again.  They look like large stubby pitot tubes now.

I hadn't thought of a good name for the RV-8 but a friend in California recently came up with the winning idea which fit it well. "Euphoriac"  It's a term from a  Sci Fi book (Vintage Season)  meaning something which induces euphoria.