Exhaust Eductors


This is how I built eductors on to N123LE existing cowls.  Actually the process was very quick and easy to do and resulted in significant a improvement in cooling on the ground.  With a heat soaked engine, I could idle for 15+ minutes without overheating the engine and only see a max CHT temps of 395F

The next step was to wrap the flow guide with sheet foam to allow the construction of a circular ring around the prop.

The duct taped foam spacer allowed me to create the extension of the cowl between the top and bottom cowls to give a form to allow a circular ring to be created between the top/bottom. I glassed 2 layers of BID between the top to the bottom cowl and extended the cowl lip for  about 2 inches

A 3/4” piece of foam was cut to the shape of the cowl and placed just aft of the exhaust pipes to act as a spacer.  This have me a way of creating a 3/4” distance between the end of the exhaust pipe and the beginning of the pour foam eductor.   I then used 5 min epoxy to attach the spacer foam on the cowl

Pour foam was the used to fill the space between the 3/4” foam spacer and the end of the cowl. 

The foam was then cut and carved to the shape of the eductor I wanted to create.  The spacer was completely dug out.    Here you can see  one side carved and the other side not carved.

I also wanted to change the shape of the cowl to give it a more unique look.  Note where the cowl extends near at the prop flange.  The added length of the cowl at the prop allowed me to only trim about 1/2” off the cowl in the at the center of the eductor.  The outside edges remained the same overall length with no trimming.  So overall there was no real shortening of the cowl, just a more swoopie curved shape.

After glassing in the eductors I then added some foam and glassed in the ring on the inside of the circular opening to stiffen it.

After splitting the cowl and removing the peel ply, all the pour foam was dug out from around the eductor and the front and back lips of the eductor were glassed onto the cowl to add additional attachment strength to hold the eductor in place.

Now that all the glassing has been done, the cowl was mounted back on to the plane, a level line was drawn and I cut the cowl back apart.  **Be sure to add a tab and nut plate near the prop hub to keep the cowl from spreading due to cowl pressure during the flight. 

The finished look

A little paint and I am finished.  I really like the new look of the plane and  the improved efficiency of exhaust system.  It is most noticeable when on the ground.  I would get a -60mph differential pressure (static to cowl) on the ground at 2200 rpm. 

I decided to start testing and building eductors after seeing the build up of exhaust in the cowl.  I reasoned that if the exhaust was not efficiently being directed out of the cowl then the cooling air couldnt get out of the cowl either.

Below you see the crude ones I fabricated on N29TM which work very well

For testing purposes I first build an Al one for N123LE.  I then blocked off the rest of the cowl and flew in this configuration for a a while.  The mod resulted in a significant reduction in cowl pressure. until the mounts on the stbd side broke.  Fortunately the eductor didn't go through the prop.  I then decided that fiberglass eductors would be best for our purpose.

This is the process of how I built the final eductors on N123LE.  I had two purposes with the rework.  To build in the eductors and to extend the cowl for a more pleasing look and improved efficiency of the air going into the prop by extending the cowl near the hub. 

I wanted to extend the cowl to within 1 inch of the prop at the hub area.  **BE SURE to allow 3/4” clearance between the flow guide and the cowl to allow for engine movement.  Some alum was cleco on to the cowl for the extension layup.