Cowl inlet vs exit openinings

Rotax-Owner wrote:

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. 

Thx for that and I agree.

Still hoping someone will measure there inlet and exit opening for me to give me a rough idea if where to start

Anyone with a rotax 912s cowl with some rough openings to get me started?

Any help would be appreciated!

Hi Aaron

The exit total air must always be more than the inlet.  You must establish low pressure on the exit to have air flow over your coolers.  Adding more and more inlets simply will not work.  Air will reach a point in the cowl area while in flight that if it cant escape it simple flows around the outside of the cowl without going in.  

On relatively low speed aircraft, such as Ultralight and Light Sport, the 3 to 1 rule of thumb works well.  (3 out to 1 in) To test your engine a simple test can be done with an altimeter.  take a spare altimeter into the cockpit with you and place the static port hose in your cowl near say the carburetor area.  You can do a test on the ground but you really need to test it inflight to see what the effects are.  If the altimeter agrees with your onboard one things are great.  if however it reads lower your pressurising the cowl and this suggests you don't have enough outlet area.  

Be very careful on where you add inlets and exists.  Most aircraft will have dead zones of low pressure, such as the sides of cowls typically, that do little to help you get air in and out.  The old way to check this is to use wool strings attached to the cowl and video them while in flight.  Typically with a wing camera or another aircraft flying with you.  

Be aware that in climb a lot of other influences come into play, such as the angle of attack in climb to your inlet ports.  Most OEM will work out the correct cowl lip to direct air into the coolers in climb.  You can find information online on such testing but there is no magic formula.

Cheers

You may find this worth a read

http://customflightcreations.com/wp-content/uploads/2017/11/Aircraft-Cooling-101-for-Tractor-Engine-Installation2.pdf 

Thx rotax wizard!

Using an altimeter is a great idea.

Thx mike!

I'm heading on vacation tomorrow and will read it then, but it look like exactly what I've been looking for. Which is just some direction of what's needed.