Plenty of Plenums
 2005.10.28
 

Plenum Construction
The plenums were fabricated using three layers of RAF BID and EZ Poxy. No Gemini space suit was donned for protection during fabrication. They were step-cured at 50 degree F increments. Otherwise I’m guessing that they would deform while taxiing in after the first flight. Several hundred hours on the previous set showed no effects from heat.

The upper plenum shape was molded by placing a 2 inch thick layer of foam over the top of the cylinders, rounded on the corners, and covered with gray tape. The glass is formed around the push rod tubes and just lays onto the flat areas around the valve covers.

First the NACA internal inlet/tunnel area was glassed on to the spar and then the snout from the plenum was layered over that to make a movement joint. In the picture to the right you can see the forward element of the right tunnel that is glassed to the top of the spar.

It’s probably best to carry the plenum shape over the top of the engine area so cool air is blown on the case for better oil cooling.

However, my plenums enclose only the cylinder heads. The necks, or finned barrels, each have their own two-layer epoxy/BID wraps that clamp around them. There are one-inch slots on top and bottom for an air ramps coming from the main plenum. On the bottom of each barrel there is a one inch slot and a specific ramp leading into the lower plenum. The air ramps can be seen on some of the engine shots in the Featured Canards and the lower right. The air ramps and the barrel wraps are held in place on the barrels with safety wire. The upper plenums are easily removed, removing the pins and lifting straight up.

Shaping the lower plenums included placing flat half-inch thick foam pieces around the top and bottom and outside of the exhausts, covered with gray tape. The augmentation area around the aft exhaust tubes are metal, pivoting, three-inch heat ducts from Home Depot. They are wrapped with fiberfrax. To protect the cowls, on three sides of the exhausts a layer of fiberfrax is trapped with thin aluminum shields riveted inside the plenums.

The glass under the cylinders was shaped to fit against the inside of the cylinder heads. There is a (fairly complex) triangular wall enclosing that open lower triangular area between the heads. The lower spark plug and sender wires, and the valve cover oil drain tubes pass through this triangular wall. This allows the lower plenums to be simply removed without dealing with the wires.

There are any number of ways the plenums can fit to the engine. While mine have a lot of detail around the cylinder heads, the plenums could probably be mated more easily against the engine case. With the fore and aft attaching pins in place the plenums can move up and down on the cylinders a little. For normal engine movement, there is an overlapping lip segment in the tunnels between the engine and the spar. Again see the eleventh picture in the Featured Canards.

With the plenums off you can see the internal cylinder-head wraps between the cylinders. I tried to enhance the top air flow between the fore and aft cylinders a little and included airfoil flow guides.

 

Operational Observations
For those that have installed stock engine baffling with good results, good on ya. The 0-235 was initially installed with the LongEZ baffling that came on it, slightly trimmed, using updraft armpit inlets.

Cooling was unacceptable. And I needed something to keep me off the streets and out of trouble anyway. The first plenums were fabricated fairly quickly and provided good cooling from the start. In reaching for that ethereal extra five knots it is pleasing to have reduced the frontal area by losing the eighteen square inch armpit scoops, and to have significantly streamlined the cowls. The dd NACA inlets fit in the existing frontal area and worked great. The quickcowl attachment was also more easily accomplished in conjunction with the cooling enhancements, since I was doing them anyway.
No over-rationalization there, I’m sure.

 

Other EZs with plenums
In our intense summer heat we have seen a significant operational improvement with the plenums here. Dave Perry in our hangar, aka Dancin’ Dave, will be glad to give a report, in boisterous ringing Russian accent if you let him. He routinely idles outside our hanger running his engine for extended periods making sure everyone sees his strobe lights and new pin stripes, with no CHT problems at all. This is a big change. He has short exhausts and does incorporate exhaust augmentation. It will also suck a string into the NACA inlet idling on the ground.

Dancin’ Dave’s oil cooler exits out the top of the firewall. Since his cowl is not pressurized he uses two cooling tubes from the NACA inlets to feed the oil cooler. It works well but also reaches the upper comfort zone in hot weather.

Another friend may tell you of creating an exquisite pair of plenums that are so beautiful that they must be seen to be believed. We spoke on the phone several times discussing how to fabricate them. They didn’t work. Then we spoke on the phone several times trying to figure out how to make them work. He had a strong resistance to changing them, because they looked so good.

I suggested that if he wanted to admire them and show them off so much that he should just put them on his mantle. Later I saw him at Oshkosh flying with dd inlets. He said that he was flying good with a new set of plenums with the detail I had suggested. I inquired about the first set and he said they are at home on his mantle. Good for him. I can understand that, kinda like having your cake and eating it too. Congrats to him for pushing though and getting it working.

Don't try this at home!

Cooling drag
Since converting to dd plenums several years and 500 hours ago there has been an intent to get the CHTs to come up just a little by reducing the size of the NACA inlets over the spar, expecting this would indicate that the cooling drag had been minimized and optimized. The intake area with the 0-235 had been reduced several times, from the old male gorilla 2x9 inlets with eighteen-square-inches per side (see Featured Canards), down to the female NACA 1x6s with six inches per side.

There was no CHT temp bump up on the 0-235. Now the 0-290 temps appear about the same, even with the smaller 1x5 square inch NACA inlets per side. The testing at cruise is not complete. The only firm conclusion so far is that a 60% reduction in cooling inlet size does not yet result in 300 knots.

Inlet Size
Depends on cowling internal resistance.
At Burt’s birthday last summer I walked the ramp and found 7 downdraft installations out of the 102 EZs. Talking with two owners that used exhaust augmentation indicated that their plane worked well. I expect that several do not use augmentation but still get the cylinders to cool somehow. I am not familiar with how to make dd cooling work without exhaust augmentation. Neither do I know how some updraft, per plans EZs cool with so much intestinal ductage. So, augmentation use and cowl stagnation skews the inlet size discussion.

The thing that triggered my interest in developing low pressure aft of the cylinders was seeing a fellow at a fly-in taxi in with a very hot engine. He got out complaining about the high CHTs.
A bystander suggested that his two 9x9 inch armpit inlets were too large. The pilot told the bystander that he was crazy and that he was going to make them even bigger. The crazy bystander was Gary Hertzler.
If you’re really good you can demonstrate oil flow evidence corroborating that if the inlet is too large and if there is enough internal stagnation, an air bubble can form in the inlet and the airflow will take the easier path, around the inlet. Ram air pressure can be stymied.

So I decided to aim early at internal simplicity and augmentation. Does anyone need help in getting a good mental image of how the exhaust augmentation works?

What is good cooling?
To me, good cylinder cooling means normal temps at full throttle. The first downdraft plenums were installed with significant attention to detail and successfully ran wide open at 2000 feet over the hot ranchland with excellent cooling from day one.

In our 100 degree temperatures, good CHTs to me with the 0-235 meant a little over 400 degrees after liftoff to 2000 ft. Staying wide open at 2000 feet at around 200 mph they were about 390. When the power was pulled back a little they went to 350 or lower.

In 130-140 mph climbs to 10K the 0-235 CHTs started around 400, gradually reducing during the climb, to 340-350 by reaching 10K. During cruise they further reduced to 320-340. Some cruise temps were even lower.

I did some instrument error testing, putting all four CHT washer senders on the top and bottom of two cylinders. The top dd cooler-side temps were about 60 degrees lower than the hot / bottom side temps. Some of the digging I did recommended that downdraft installations (spam cans) should have the washer type senders on the top while bayonet senders should be in the bottom fitting provided for them. These readings are from washer senders on the lower side, with a little fudge factor to the good.

This 0-290 testing thus far includes installing the senders on different cylinders for further comparison. A bayonet sender has been installed on a couple of cylinders and the readings are the same as the bottom washer senders.

Attention to detail
It may sound improbable, but, it is fully within the realm of possibility –no further comment for now- that this can be screwed up. Proper cylinder head and barrel (neck) airflow could be missed or dismissed or just totally overlooked. The fins could be completely blocked with RTV globs-with updraft or downdraft cooling. Or the space between the cylinder necks could be left open, with the air going through that wide open space rather than the fins. I don’t know what to say about these omissions, other than another set of eyes might be beneficial.

A friend has a beautiful stock Glastar. His experience is useful when comparing EZ temps with the best of other types. For some time I have been envious of his stock cooling setup with good cylinder and oil temps. However he recently mentioned that like the rest of us, this summer his temps are higher than desired and even borderline.

Enough for now. All of these cooling and cowling tweaks fit well into my overall interest to simplify the internal cowl airflow and finesse the outer shape. This is the easy stuff. Later, some oil cooling torture.

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