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Optimized Induction Airbox for Reciprocating Engines

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Toobuilder

Well-Known Member
Supporting Member
Joined
Jan 19, 2010
Messages
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Location
Mojave, Ca
PART I – GENERAL INFORMATION

System under Test

2005 Harmon Rocket II / IO-540-D4A5 (260 HP Nominal). The engine is a parallel valve model in compliance with the Lycoming TCDS with the exception of the addition of piston cooling oil nozzles and a SDS EFI and ignition system.

Problem Statement/Opportunity for Improvement

Existing induction air inlet and airbox configuration is sub optimal for pressure recovery (Manifold Pressure (MP)) as well as filtration of airborne particulates. Airbox provides poor aerodynamic shape for effective pressure recovery and is of minimal volume to feed a 540 CI engine. Additionally, induction air inlet is close to boundary layer of spinner and within the arc of the propeller root, compromising airflow into inlet. Finally, the plenum housing is sealed to the servo via a spring loaded flat plate seal which will break seal with engine articulation. Airbox system uses a cone filter, presenting media in a concentrated footprint that increases the risk of a single contaminant event (such as snow, ice or FOD ingestion) fouling the media.

Description of Change

The existing airbox, servo, and 90 degree servo adapter will be removed, and a large capacity, aerodynamically optimized filter airbox housing will be fabricated and installed. The servo/throttle body will be reoriented to vertical and rigidly mounted to the sump inlet. The airbox will be attached to the servo inlet throat directly, sealed with an O ring. The O ring will allow the airbox to articulate in pitch, yaw and roll as well as translate vertically in relation to the throttle body – all while maintaining an airtight seal. Support for the airbox is provided by a semi-rigid tension element between the airbox and sump as well as a foam doughnut seal between the inlet of the cowl and end of the airbox. The design of this system allows articulation of the engine in all expected maneuvers and conditions while providing seal integrity from cowl inlet to combustion chamber.

Verification Methodology

Pressure Recovery – In situ flight test using direct comparison with similar make/model aircraft with original induction configuration, data comparison with historical flight test data direct measurement of pressure.

Filtration – Analysis and observation.

Expected Behavior

Primary – The optimized configuration will produce higher observed MP than the original configuration in all measured flight conditions.
The additional MP will enable a higher TAS, overcoming the additional frontal and wetted area of the cowling changes.

Secondary – The effective inlet pressure of the new configuration will significantly exceed that found within the cooling plenum. Verification or disproval will be used to determine if the cooling plenum is a viable source of air for the induction system.

PART II – FABRICATION AND INTEGRATION

Plan view of original 90 degree sump adapter, servo and filter housing inside “new” filter housing mold for size comparison.

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Plug, mold and airbox half shell: Classic shallow angle divergent duct shape results in minimal internal turbulence and pressure recovery with the reduction of velocity.

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Internal configuration shown. Bellmouth entry helps the airflow “turn the corner” Not shown is a deflector wall which has been installed to prevent high velocity debris from entering inlet and impacting the filter media directly.

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Controlled articulation airbox hanger. Rod end, bolt and spring.
Airbox neck "floats" on the O ring seal on the throttle body, so needs something to keep the airbox from falling off and dropping into lower cowl.

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Tension element supports weight of airbox

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Installed airbox shows large capacity and divergent shape to advantage

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