Cowl inlet vs exit openinings

Roger Lee wrote:

If 240F CHT and oil is only 220F is only on climb then that's fairly normal for hot summer climate. What rpm are you cruising at and are these cruise temps or climb temps? 

Some have opened the front of their cowling opening more. Some people have added a small dam down at the bottom exit on their cowl to help bring more air flow through their cowl. It went all the way across the exit opening and protruded back and down about 2.5 inches. I don't have any pictures, but I have seen a number of them on forums and maybe even here quite a while ago,

The 240 is cruise temps at 5200 to 5400 rpm.

I don't think I want to open up the inlets anymore because I have more inlet area then exit.

Don't we need more exit area to compensate for the hot expanding air because of the heat of the engine?

Is it possible for you to measure the inlet vs exit area on one of your planes?

Thx!

Tyler Hathaway wrote:

You say WOT in cruise is 5700... but you are not actually cruising at those rpms, are you?

What engine are you running?

Correct, running at 5200 to 5400 in cruise.

I'm assuming Roger asked because he was thinking the prop was pitched to much?

912s

Can you measure your inlet and exit area?

Thx

Aaron, I use a pusher configuration and no cowling, so can't help on measurements.  :)

Roger...thx anyway

This is why it is critical to conduct hot weather testing when designing an engine cowling. Many aircraft manufacturers skip this step OR not in the environment that would allow them to conduct this type of testing. Designing a cowling is not as simple as making holes larger or smaller, as there's a science to it. A bigger hole/opening in one area may take airflow away from another location and not allowing sufficient exiting airflow may not allow the hot air to escape and create a buffeting zone at the entrance (no airflow).

It can take many hours and multiple flight tests with temperature probes to get it right. It's important to not only worry about coolant and oil temperatures but other components such as fuel lines (fuel) and electronic temperature limitations as well! (ignition modules max temp are 80 °C (176 °F)

The Rotax installation and operator's manual will provide you good reference when determining the proper temperature, but the ultimate design of the cowl is up to the builder/manufacture.