We’ve decided to create an augmented exhaust system for our plane (seeĀ Buffalo Forge paper on augmenters, also under “Downloads”). I foresee a couple of benefits from this. First, we don’t have the ‘rooster-tail’ that the factory exhaust creates by dumping out directly down from the cowling near the firewall. This creates a major disturbance to the airflow, that many believe slows the plane down significantly. By creating an augmenter, firstly the exhaust exits directly aft, and gives some jet thrust to increase speed. Secondly, the exhaust creates a slight vacuum on the inside of the cowling by exhausting into a venturi section where the high speed exhaust expands and lowers the pressure, increasing the cooling flow through the engine plenum.

To create the augmenter, I first cut slots out of the cowling to allow the exhaust headers to poke through. The exhaust pipes are about 2″ diameter, so I then acquired some 4″ PVC from the Depot to use to form the outer ring of the venturi section. You can see that here.

4″ Pipe used to form the augmenter

Below you can see how I mounted this tube. First I created two supports to fit just inside of the larger tube, and around the exhaust to support the 4″ pipe in place. The aft support has a cut in the top so it can slide under the exhaust pipe in the forward area.

Prepared Support

Both Supports Glued into Place

Both Forms in place

Once both forms were ready, they were wrapped in aluminum flashing, to simulate the thickness of the stainless steel heat shield that will be installed later, and then covered in duct tape and waxed as a release agent. The next step was to close up the large slots where the exhaust came through, leaving only the area where the augmenter will be bonded into place. This was done by gluing gutter guard inside the cowling, duct taping it, then bondoing the area smooth, and finally waxing it. Then this area was sealed with TRIAX cloth.

Form with Heat Shield and Release Agent

Gutter Guard ready for Bondo

Bondo has been waxed

Bondo and First Layer of Augmenter Layups Complete

Final Layups

Layups Completed

Ok, so here we finish up our cowling modifications, by closing up the normal cooling openings. We’re doing this with a flange and screws holding on a 2 Triax cover in case we find that we’re running too hot with just the augmentors. Also, we’ve started some finishing work so that we can smooth the shape out in preparation for priming the cowling. Also, the lower cowling was coated with epoxy to make it more grease and grime resistant.

Cowling cover flanges

Cowl outlet covers

Cowl Coating

Cowling Center Flange

Cowling Smoothing with Micro-Balloons


Posted By: Brett Ferrell
Tuesday June 5th, 2007 at 12:37 PM

Categories: Exhaust Augmentation
Tags: Augmentation Building Engine Exhaust

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