Cooling air inlet plugs

[Bob Axsom]

Here is a summary of how I reached the current leading edge shape.

Phase 1 - When I first started working on the idea of plugging the inlets as part of the evolving effort to reduce cooling drag the leading edge was determined by the height of the opening and the distance from the front of the inlets to the front of the cowl. This was quite blunt. On my cowl the leading edge of the left cooling air inlet is farther back from the leading edge of the cowl behind the spinner than the right indicating that there is some side thrust in the design. Thus the right side plug is more blunt than the left. However, testing with increments of 1/4" width closure of the openings from the inboard edge indicated there was some increase in speed to be had from this form of reduction in cooling air inlet size. There is also an increase in CHT. After the plugs width reached 1.75" the speed appeared to drop but the CHTs continued to increase. I interpreted this to indicate the parasitic drag of the plugs were exceeding the reduction in cooling drag due to the reduction in air mass flow through the cowl after the plug width exceeded 1.75" and the CHTs were getting into the 400 F range. Further study at a much later time with the NTPS spreadsheet indicates that the nice curve correlating the number of 1/4" test plug plates with airspeed indicated by the USAR Handicap Procedure was illegitimate but I chose to press on anyway using the 1.75" plug width.

Phase 2 - I glued seven of the test plates together for each inlet to obtain the desired plug width then I extended the maximum protrusion of the leading edge by gluing on seven crescent shaped extensions. I could see by looking at the light contact mark on the rear spinner plate that the blades rotate all the way back to this plate at high pitch. The extensions could extend past the front of the cowl and the back of the spinner but I had to leave room for the propeller blade trailing edge when rotated to high pitch. The maximum protrusion of the leading edge of the plug near the hub is aft of the front of the spinner back plate with the crank shaft pushed in and the outboard edge of the plug tapers back to allow for the broadening of the propeller blade as it extends outward. The first test was made with the lightly tapered leading edge and the last one had the leading edge tapered sharply back into the cooling air inlet opening. The ~straight plug was faster than the sharply tapered plug. That is all I concluded from this phase.

Phase 3 - This is the current phase and other than construction the leading edge is the important difference. The inboard leading edge extends as far forward as it is safe to go without a prop shaft extension. Then it dips back smoothly to through a depression then returns forward to form a smooth protruding lip isolating the inlet from the air inboard of the opening. I hope this will result in only the air directly in front of the opening, entering the cowl.

This is definitely a "work in process" and no performance improvement may come of it. I suspect I will have to install the rear baffle hole cover plate to hold down the CHT of cylinder #3 at least. We shall see. It will take a week or more for me to finish these plugs.

Bob Axsom

[Ironflight]

Cool Bob - Thanks for the insight into the process!

We could use more of your kind of engineer back in the program these days BTW....)

Paul

[Bob Axsom]

You have no idea how good that makes me feel. I see Dawn is launching soon. I know how the people are going to bed late, sleeping little, waking up early and bonding into a tightly knit team in anticipation of the launch after years of development, reviews and operational planning and simulations. There is nothing else like it. I hope the young folks realize it and are aspiring to the exploration of space. I played my small part as well as I could and I loved every agonizing minute of it.

Bob Axsom

Originally Posted by Ironflight

Cool Bob - Thanks for the insight into the process!

We could use more of your kind of engineer back in the program these days BTW....)

Paul

[Bob Axsom]

I have a 24 hour cure cycle to complete, sand, add another layer on the leading edge, wait 24 hours, sand, fill, sand, redrill and countersink the mounting holes and it will be ready to test. We will be out of town for a few days beginning Friday so the test flight will probably be around the 4th of July.

Bob Axsom

[osxuser]

So... what resin did you use? I just have to ask because of the color... vinyl ester?

[JohnR]

Looking good Bob. I'm anxious to hear the results.

[Bob Axsom]

The part A material is clear but the 24 hour hardner is a very dark amber color (100:44 mix by weight). It is epoxy from Aircraft Spruce.

Bob Axsom

[osxuser]

Originally Posted by Bob Axsom

The part A material is clear but the 24 hour hardner is a very dark amber color (100:44 mix by weight). It is epoxy from Aircraft Spruce.

Bob Axsom

Hmm, I have some sitting around in my hanger, maybe I'll give it a try, you like how it works?

[TShort]

Bob,

Could you explain to this non-aerodynamic type what the thought process is regarding the unusual forward facing shape of the plugs? There appears to be two "humps", one near the spinner and one near the cowl inlet. I'm interested to know what this should do.

Thanks!

Thomas

edit - oops! I must have missed your prior post ... never mind

[Bob Axsom]

Just in case someone else tries this:

1 - The top view photo of the clay molding is the early shape which is still similar but I did trim them back after the photograph to provide more clearance from the trailing edge of the prop at high pitch. That is explained earlier in this thread.

2 - The forward protrusion not only has to be clear of the prop trailing edge but the spinner as well in its worst side displacement. My test in Phase 2 had no problem but the temptation to get closer and conform to the contour of the spinner could be a big problem.

Bob Axsom