Cooling air inlet plugs

[Bob Axsom]

I would Love to see it

We are not going into the big show this year. We are going to fly to Fond Du Lac on Sunday then return home from there on Monday. Be sure you sign up for the judging, from the photos I've seen you should come away a winner. I saw your diffuser tubes in the earlier photo and that is another thing I have to work on. It is not easy without the prop extension but I could do something.

Bob Axsom

 

[RV8RIVETER]

Thank you Tom for telling me about the article "Cool it! Engine Cooling and Drag Reduction". It is written without technical pretense, as if Neal Willford was your neighbor and he didn't have to prove anything to you. He describes every element of the cooling system and gives hard numbers and formulas for you to use in designing or improving your system. I have frozen my configuration for the summer but I will try to improve my airplane's system using this information over the winter. This should be required reading for anyone thinking about improvement of their cooling system and reducing the associated drag. The inlet lip shape and the defuser function and shape are included as well as the interaction of the inlet and outlet on the mass flow. Very good information.


Bob Axsom

Great example of the EAA responding to members. Believe it or not, that article came about by me writing an email requesting one on the subject. So, if you can't find any previous articles on a subject you are really interested in, be sure and ask for it.

 

[Bob Axsom]

New Idea

This falls in the category of wonderful ideas that come in the night. I sent an e-mail to the EAA about the "Cool it" article in August 2003's Sport Aviation - no response at all - I hear they are working on something big south of the city of Oshkosh, WI right now and they may be distracted. I will call them later. Anyway, in my sleep last night I thought about a significant modification to the front end of the cooling system that seems very good.

1- Take the plugs and sand out the last web I added over the depression between the spinner and the inlet lip.

2- Determine the size needed for the inlet based on the "Cool it" article spread sheet and extend the plug over the entire inlet to provide a round inlet of the proper size with an eliptical shaped lip leading into the defuser area.

3- Develop a plate to extend across the entire front of the plenum (the standard baffle with the upper cowl providing the closure surface) with non-matted (that cloth reinforcement removes metal like a buffer with engine vibration) hi-temp baffle seals all around and a round flange extending toward each inlet. This plate has no hard contact and is supported on all sides by the rubber.

4- Install a smooth walled rubber hose/tube between the round flange of the inlet plug and the new forward plenum closure plate flange.

What do you think?

Bob Axsom

 

[kentb]

Bob, if I understand your new idea...

Cooling wise it sounds like a good plan. But will it only be attached by the three screws as your current plug? If so I think that I would be concerned that some force (air pressure from inside the cowl, wind gust, something that I don't know about) might cause the outboard edge to move forward and hit the prop.

Would there be some other attachment to make sure the bigger piece of plug doesn't come loose?

When you get this all figured out I may have to change my plane to take advantage of all you work. Keep it up.

Kent

 

[Bob Axsom]

Two Items

First - I have and attach point already at the outboard edge of each inlet and my initial thought is to take advantage of that but as with any of these ideas the actual implementation may dictate something else. The three inboard screws are not sufficient for the full front plug idea.

Second - I called the EAA and found out how to get the missing information from the August 2003 Sport Aviation article. Log in to the EAA website; scroll down the icons on the right side of the page until you get to the one showing the current issue Sport Aviation and click on it; scroll down the page until you get to the August 2003 entry; there you will find links to the missing figures and the spread sheet and most importantly a link to an excel form of the spreadsheet that you can download and save for your own work.

Bob Axsom

 

[Bob Axsom]

Implementation Idea

I gave the idea of implementing defuser tubes in the inlet without hard topped plenum within the cowl a lot of thought while on vacation the past three weeks and came up with a couple of reasonable ideas. Both involve a closure plate across the inlet back near the cylinders 1 & 2 and the are only different in the way the rear end is configured.

The plate would be mounted with #8 screws and dimpled platenuts on (1) the lower inlet ramp, (2) the outboard baffle and (3) the baffle behind the spinner. This plate would have rubber across the upper edge to seal against the upper cowl just like the rest of the vertical baffle members.

This new baffle element (closure plate) will have a hole cut in it of a size determined by the defuser tube size and configuration. The defuser tube itself will consist of three elements (1) front end, (2) middle and (3) rear end with a size determined by the August 2003 Sport Aviation article.

The front end will be a further development of the existing inlet plugs that will extend outboard to include the entire inlet. The front end of each defuser tube will be an integral part of the inlet plugs extending back inside the stock cooling air inlets. This defuser tube will have an elliptic taper lead-in and it will end after a suitable clamping length has been established, maybe an inch or so.

The rear end of the defuser tubes will be an integral part of the two new baffle plates with an elliptic outward taper into the plenum (with the upper cowl still being used to close the stock plenum). I have two ideas on the defuser tube rear end configuration at this point: (1) aluminum extending back through the baffle plate and (2) fiberglass.

The aluminum rear end configuration would be formed by rolling flat sheet aluminum into the required diameter tube and riveting the seam. It would be riveted to the front of the baffle using small aluminum angle segments between the outer surface of the tube and the baffle. The portion extending through the plate would be cut from the rear edge to the junction with the baffle plate every so many degrees (say 10 degrees for a start) and they would be formed in an elliptical shape so that they come back to the rear surface of the baffle plate with an outward bent tab on the end. They are attached individually with 3/32 rivets through tabs at the ends of the of the elliptically shaped members or collectively with a ring that is riveted to the baffle sandwiching the tabs. Fiberglass would be laid up over the "spider legs" to provide a smooth surface for the air flow.

The fiberglass configuration will be laid up over clay on the front surface of the baffle plate. The inside surface will flare out elliptically so that the end of the approximately 1/4 ellipse cross section will be the end of the semi-major axis blended into the rear surface of the baffle plate.

The middle section of the defuser tube will be rubber at this point - probably sheet rubber rolled to a diameter or shape conforming to the shape and size of the front end and rear end flanges. The seam will be sealed with hi-temp RTV and the ends will be clamped to the front and rear end flanges with hose or custom clamps. I'm concerned with the surface roughness at the joint transitions but tapering the inner surface of the flanges is all I have in mind at the moment.

Installing the new defusers would be done with the upper cowl off with full access to the clamps. Removing this mod would only require removing it and plugging the holes with #8 screws to return to the stock cowl inlet configuration.

Bob Axsom