Optimized Induction Airbox for Reciprocating Engines

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Toobuilder

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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.



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.



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.



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.



Tension element supports weight of airbox



Installed airbox shows large capacity and divergent shape to advantage

 
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Toobuilder

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Just wanted to remind everyone that this section (unlike the member project logs) is open for comment by anyone. Got a question, idea or critical review? Bring it!

To be continued...
 

12notes

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You mention disadvantages to the cone filter. What are you using for the new filter?
 

Mad MAC

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Can't quite tell from the last pic if you have a drain hole to let any accumulated moisture exit.

I am not sure ether way, are you tempted to have a suck in valve for inlet / filter blockage case (the Apex Robin R21XX with a birdstrike in the air intake comes to mind)
 

Toobuilder

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You mention disadvantages to the cone filter. What are you using for the new filter?
The new filter is an oval K&N measuring 14x7x2. As a big oval, it should be less susceptible to a localized FOD event clogging the whole thing, and it also has more filter area overall than the cone that was in it before.
 

Toobuilder

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Can't quite tell from the last pic if you have a drain hole to let any accumulated moisture exit.

I am not sure ether way, are you tempted to have a suck in valve for inlet / filter blockage case (the Apex Robin R21XX with a birdstrike in the air intake comes to mind)
There will be a drain hole in a lower corner before it is complete, yes.

There are provisions in the mold for an alternate air door. I left it sealed up for the test period so that I can evaluate how this thing works with a "perfect" airtight seal. If implemented it will open (via manual cable) in front of the filter and block the main inlet, injesting lower cowl air. Wont help if the filter is already fouled, but I think that is a fairly low risk considering this is a VFR airplane for now.
 

pictsidhe

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What about a sprung bird door? Just enough spring to resist vibration and it will open automatically should something plug the regular intake. If you need to know it has opened, a microswitch and lamp.
 

Toobuilder

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Considered the spring loaded door too. The original had that, in fact. But if I decide to add the door, the addition of a cable allows the door to act as a blocker in inclement weather and act as a positive lock to keep it sealed when stowed. In other words, the addition of a cable adds a bunch of utility (good ROI).
 

TFF

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I think it is nicely done. The only thing that would have been nice would be more room around the filter from the walls on the back side. Probably not an issue in action. I like the shape and volume available.
 

Toobuilder

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Thanks.

I completely get what you are saying but I dont think there will be any significant flow restriction through any particular area of the filter. It does not look right at first glance, but due to the large circumference (relative to the throttle body), the air should not even "feel" the media because it's moving so slowly. So no sharp corners to turn through the filter. If I had more room to go wider on the airbox, I would have gone with an even bigger filter.

I do intend to run a test with the filter element removed to see if I get a MP change. I suspect (hope) I don't, but we will all know soon enough.
 

TFF

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The only reason for more room would be so more air would be scrubbed over more of the filter all the time. The volume as is has got to be great already. It’s still going to be hard to get that filter dirty.
 

fly2kads

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This is a great project! Aircraft intake systems seem to be designed more for operational and packaging considerations than performance, especially on carburetor-equipped engines. Your project is fairly similar in concept to the intake system I have been sketching for my little Supervee concept, so I am glad I'm not off in left field. (Or if I am, at least I have company!) I'm looking forward to your updates and conclusions.

Question: did you calculate a specific target volume for the plenum, or was this driven by the space available?
 

Toobuilder

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Big filter means low loss. I understand that there becomes a point of diminishing returns, but the goal is to have a full time filter that the engine cant even see.

The plenum volume was not targeted - it was driven by space available and geometry and that's as big as I could get.
 

dtnelson

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Awesome! As a retired engineer, I particularly appreciate the use of the good 'ol scientific method. I'll be very interested in your specific measurement techniques! And, Timely! I'm in the process of putting in an updated induction air system in my Long-eze project (starting with a Vans induction box).

One comment; and I know this is highly debatable; I've never had an air filter on my Velocity (IO-360, 1500+ hours). I'm not bragging about this... but in my installation I've had no issues (and yes, I know it only takes one...). My induction air comes in via the roof mounted NACAs which helps to limit the normal runway debris. Also, FWIW, I've always done regular oil analysis, and I've never had high silicas (i.e., not much dirt in the oil). Again, I'm not making a recommendation here, only an observation.

Looking forward to your next installment!

Dave
 

Toobuilder

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Thanks Dave. I know plenty of people get by with no filtration but considering my environment I just couldn't let myself go down that path. Part of this experiment will determine if I really have achieved "no loss" filtration.
 

Victor Bravo

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Really impressive looking to my amateur eyes. Questions:

Is that the textbook 7 degree divergent angle from the inlet to the pressure recovery plenum? Would this duct benefit from flow straignteners or "splitter" baffles to reduce the angle of divergence that the air "feels" as it travels through the duct?

Other than ease of mfg. the plug and mold, is there an aerodynamic reason you made the cross section of the duct rectangular instead of oval or matching the inside contour of the cowling? Don't the corners create a little disturbance or change in velocity?

Did you "clock" the inlet and propeller to time the pressure wave off the back of the propeller into the intake port?
 

Toobuilder

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The divergence is actually less than 7 degrees so no splitters are required. The square corners are for manufacturing ease as well as provisions for a possible alt air door. The square corners only exist at the rear portion of the airbox where the local velocity is low. The high velocity forward section is circular in cross section.

There is no attempt to time the blade passage with an individual cylinder aspiration event. The large volume of the airbox (and the future installation of the 300 HP sump) should serve to average out any prop pulse.
 

dtnelson

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Just as a point of interest, in the pusher world, there have been some measurements looking for a pressure wave caused by the prop. None was found. Instead, surprisingly, the air is mostly static right behind the prop (especially inboard). This is one of the reasons that on the faster 'ez's, you'll usually see the exhaust kept completely inside the cowl, and cut off 3 or more inches inside the cowl edge. The exhaust obviously creates a strong flow, which augments the flow out of the back of the cowl, resulting in better cooling.

Dave
 
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