• Exhaust Augmentation RSS Feed

    by Published on 06/05/2007 09:55 PM
    1. Categories:
    2. l Chapter 12 - Engine/Propeller,
    3. n Chapter 14 - Final Assembly, Finishing

    We've decided to create an augmented exhaust system for our plane (see 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.

    Forming exhaust augmentors from 4" PVC piping
    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.

    Augmentor piping with annular ring to hold it concentric with exhaust pipe
    Prepared Support
    Augmentor support glued into place
    Both Supports Glued into Place
    Augmentor supports and forms in place with lower cowling
    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.

    Augmentor form cover in duct tape and ready for fiberglass
    Form with Heat Shield and Release Agent
    Preparing augmentor glass form
    Gutter Guard ready for Bondo
    Preparing augmentor glass form
    Bondo has been waxed
    First set of augmentor layups completed
    Bondo and First Layer of Augmenter Layups Complete
    Final set of augmentor layups completed
    Final Layups
    Augmentor layups completed
    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.

    Building cowling outlet cover flanges
    Cowling cover flanges
    Cowling outlet covers in place
    Cowl outlet covers
    Coating lower cowling with raw epoxy to make it more oil resistant
    Cowl Coating
    Forming cowling rear flanges
    Cowling Center Flange
    Smoothing cowling finish with microballoons
    Cowling Smoothing with Micro-Balloons

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