Hi Mike,

 

The theory indicates that once you get the inlet/diffuser combination worked out – the inlet size should be in the range of from 25 – 40% of core frontal area.  After than, it’s actually the exit conditions that dominated the air flow through the core/cowl.  A streamline duct diffuser (K&W) is the most efficient practical diffuser (at least that I have come across) – the theory behind it is to keep the air flow energy (velocity) high until just before you expand the duct area in front of the core to convert the dynamic energy to a localized pressure increase.  From what I have read, it appears that smoothness and preventing air flow separation from the duct walls near the inlet is of major importance.  Any disturbance there creates an expanding “shadow” of disturbed air which impinges on the core and reduces the effectiveness of that area.  So prevent flow separation is one of the key challenges. 

 

I developed my “Pinched” ducts for short run ducts (the streamline duct requires something like 16” for cores our nominal size for optimum performance).  The ideal behind my “Pinched” ducts is to speed up the airflow through the pinched area giving the boundary layer of the flow more energy to stay attached to the duct walls as it makes the curve just before the core.  Any separation that does happened is much closer to the core and generally up near the edges and corners – where the core is not particularly effective in the first place.  Been flying with them for over 5 years and they do the job for my installation – however, just about any, smooth flowing duct will add to cooling effectiveness – sharp discontinuities generally do not help.

 

There is no question that the exhaust augmentation can work if done properly, the question in my mind is whether there are easier ways to accomplish the desired results.  It’s my opinion (have not tried one) is that if it were easy to achieve success and there were major benefits, we would be seeing them on many more installations.  The one’s I have read about that seem to be successful were not what I would call simply installations.  Most have a long tunnel of some sort in which the exhaust is directed out the end causing airflow inside the tunnel to be accelerated and  “dragged” along and out from under the cowl.  In many cowls that presents a considerable challenge to fabricate - as such things as motor mounts and hardware have a way of getting in the way.{:>) 

 

I can see that Exhaust augmentation might be advantageous for promoting airflow during taxi and other low speed operations such as ground run up – if other ways can not be found to do the job.  But, if an installation is not cooling at cruise – then the  cooling system needs work

 

Just an opinion

 

Ed.

 

 

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

 

 

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.