From: Finn Lassen

To: Rotary motors in aircraft

Sent: Wednesday, January 28, 2004 10:12 PM

Subject: [FlyRotary] Re: Streamline Ducts

I found that going from a bell shape curve to a straight wedge to my oil cooler the oil temps dropped somewhat.

If I understand it correctly, the 7.5 degree max rule is to ensure that the airflow stay attached to the duct walls (no turbulence). According to Bernie Kerr's experiments, that is true for an open duct (e.g. a wind tunnel). However, as soon as you put a restriction in there (a screen or a radiator) that rule no longer holds true and you can have a much steeper angle with the flow still staying attached. He observed this by tufting the walls of a test duct. An experiment anybody could do.

Shamelessly copied from http://www.rotaryaviation.com/bernie_kerr.htm (see pictures there):

That Bernie is some guy.   From what I understand about the topic , I agree with Bernie regarding the 7.5 deg angle and wind tunnels.  The characteristics of the core of the radiator does indeed have an effect on the diffusion process through its resistance to air flow.  Seeing the tuffs from Bernie's radiator would appear to indicate he is getting decent air flow (at least at low velocities).  Again, as I have mentioned several times, if you have sufficient air mass flow through your core, you WILL cool.  However, you could at the same time also incurr large cooling drag. 

 

The difference between an adequate cooling system and a great cooling system (both keep the engine from meltdown) is the amount of cooling drag.  That is where careful attention to ducts, diffusers, exit and cooling core characteristcs comes to play. 

 

Clearly a system that fails to cool the engine is inadequate.  Keep in mind you could have the best duct design, diffuser pressure recovery,  etc, all optimized - BUT, if you radiator is too small for the BTUs you need to reject then the system will fail to cool you engine. 

 

Ed Anderson