Lower cowl scoop shape

So....since my last post I have added ramps in front of both cyl #1 & #2, added a "top ramp" to help direct the air DOWN from the top of my baffle plenum, this seems to have helped at cruise speed, 2400rpm and 85Kts TAS, I am seeing CHT's of 350, Oil of 180 and I can EVEN lean about 150º of EGT.... wow..... but..... at slow flight (below 40kts IAS she still ends up running over 400º CHT and creeping over 200J Oil.


here you can see the first "top ramp" and the ramp in front of cyl #1, both seemed to help so I went ahead and completed the "top ramps" and built another front ramp for cyl #2


here you can see my second "top ramp", after this I closed the gap with another "top ramp"

I have really tried to find a spot in the front of the cowl for the oil cooler but this BCSC cowl is SO tight up front and SO tight around the cylinder heads and intake manifold and exhaust branches that I came up with another idea that I would like to bounce off the forum members before trying out.....
WHAT ABOUT trying a NACA duct on the right hand side of the cowl, just below the cylinder heads out of the way of the intake and exhaust -> this NACA duct is then ducted to the 13 row oil cooler mounted on the firewall behind cyl #3 (more space this side) and the exit air is SCAT tubed down to a separate exit scoop on the lower right side of the cowl... see pic below for rough location



please try and ignore the strap, we were fitting the new lower scoop, which I will share if it makes a difference...
under the strap you can see the rough dimensions (cryptic... I know) for the NACA duct..... any opinions on whether it will work? think there will be enough air to draw in from the side of the cowl?

I like the ideal of the NACA duct.
DENNY

bodumatau said:

So....since my last post I have added ramps in front of both cyl #1 & #2, added a "top ramp" to help direct the air DOWN from the top of my baffle plenum, this seems to have helped at cruise speed, 2400rpm and 85Kts TAS, I am seeing CHT's of 350, Oil of 180 and I can EVEN lean about 150º of EGT.... wow..... but..... at slow flight (below 40kts IAS she still ends up running over 400º CHT and creeping over 200J Oil.

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That portion of the baffle which is in front of the cylinder fins (not the cylinder head) ought to be shortened and aligned with the center (up & down) of the fins for the cooling of the steel barrel. You have no temperature measurements of this section. Overheating here can damage the piston rings. You could also cut the section in front of the aluminum head back to the center to increase the cooling on the head fins.

The ducting on the side of the lower cowl works on Huskys and the Carbon Cubs, both of which tend to run warm. The Huskys use a cowl vent like this:



Here's a picture of one of the newer Carbon Cubs:



Which might not be efficient enough for your needs. But, either this or an outward opening duct like the earlier CC might help:


This would be the first thing I'd try-it's pretty well documented to help cylinder head cooling. The latter could be made so it's adjustable. There's a similar kit for the Cessna 185.

MTV

skywagon8a said:

That portion of the baffle which is in front of the cylinder fins (not the cylinder head) ought to be shortened and aligned with the center (up & down) of the fins for the cooling of the steel barrel. You have no temperature measurements of this section. Overheating here can damage the piston rings. You could also cut the section in front of the aluminum head back to the center to increase the cooling on the head fins.

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copy that NP1A, #1 has never been a problem though only #3, but I hear you, I will trim it back a little.

mvivion said:

The ducting on the side of the lower cowl works on Huskys and the Carbon Cubs, both of which tend to run warm. The Huskys use a cowl vent like this:

View attachment 46644

Here's a picture of one of the newer Carbon Cubs:

Which might not be efficient enough for your needs. But, either this or an outward opening duct like the earlier CC might help:

This would be the first thing I'd try-it's pretty well documented to help cylinder head cooling. The latter could be made so it's adjustable. There's a similar kit for the Cessna 185.

MTV

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These are all sucking air OUT of the cowl, do you think there will be enough positive pressure along the side of the cowl to let enough air INTO the NACA duct?

pictures are worth a 1000 words, so here goes trying to explain a bit better what I want to do:

RED would be the NACA duct INLET
YELLOW the ducting to and from the oil cooler
BLACK is the oil cooler on an ali frame mounted on the firewall
BLUE is the separate outlet scoop to suck air through the system.

bodumatau said:

I am seeing CHT's of 350, Oil of 180 and I can EVEN lean about 150º of EGT.... wow..... but..... at slow flight (below 40kts IAS she still ends up running over 400º CHT and creeping over 200J Oil.

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These oil temperatures are perfect. I would leave the oil cooler as it is.

Is this 400º CHT on more than one cylinder or for a considerable period of time? If so perhaps just increase the fuel flow a bit during that time with the mixture control.

at FULL RICH during slow flight my CHT's creep up toward 425 and my oil also creeps up to 210, so I definitely want to get my oil cooler to have its own air supply, I think that would give me some more leeway on the CHT's if I give the cylinders the air back that the oil cooler is taking.

my mission profile rarely is cruise at 2400rpm, maybe for 30 min max to get somewhere interesting, the majority of my time I want to spend on my first notch of flap at between 25&40Kts and 2000rpm, so I need that slow flight cooling to work properly.

Ducting those NACA inlets to the cooler? That might work but I'm not sure that's a high pressure area of the cowl so inlet pressure may not be what you need it to be. Louvers increase air exchange through the cylinders by increasing outlet flow because that area is in negative pressure. I'd think you'll need to move those ducts forward to an area of impact air. In the Carbon Cub examples they're using plenums. I know of a couple of SQs that have recently done plenums to improve cooling. Hot rodded parallel valve engines are hard to cool in Cubs. You aren't the only one searching for solutions. Most of those guys are moving coolers to the nose, by the way. Robbing air from the top to blow on an aft mounted cooler worked on O-320s. It isn't working as well with higher output engines.

bodumatau said:

These are all sucking air OUT of the cowl, do you think there will be enough positive pressure along the side of the cowl to let enough air INTO the NACA duct?

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it seemed to me the biggest issue is high CHTs. These cowl mods increase air flow through the cowling, and should help reduce CHTs. And, they’re relatively easy to install, and proven on a number of platforms.

As Pete said, I wouldn’t be that concerned about the oil temps.

MTV

Of the examples shown, I like the location of this one the best. It is in a location where the airflow will be the most effective.



When I had a turbo in my 185, it ran hot. Cowl vents similar to these dropped the temperatures considerably. Also bringing down the CHTs will in turn bring down the oil temps. Fix the CHTs and the oil temps will fall in line.

stewartb said:

Ducting those NACA inlets to the cooler? That might work but I'm not sure that's a high pressure area of the cowl so inlet pressure may not be what you need it to be. Louvers increase air exchange through the cylinders by increasing outlet flow because that area is in negative pressure. I'd think you'll need to move those ducts forward to an area of impact air. In the Carbon Cub examples they're using plenums. I know of a couple of SQs that have recently done plenums to improve cooling. Hot rodded parallel valve engines are hard to cool in Cubs. You aren't the only one searching for solutions. Most of those guys are moving coolers to the nose, by the way. Robbing air from the top to blow on an aft mounted cooler worked on O-320s. It isn't working as well with higher output engines.

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yes exactly Stewart, this is what I am worrying about

NACA Scoop LOCATION

bodumatau said:

These are all sucking air OUT of the cowl, do you think there will be enough positive pressure along the side of the cowl to let enough air INTO the NACA duct?

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Your drawing locates the NACA duct at the point, on the side of the cowl, where that maximum suction is trying to lift the paint off the cowling. Trying to overpower that suction and make air enter the cowling is not going to work. If the NACA duct was installed 10 inches forward; where the side walls of the duct would be able to generate two strong vortexes would be more viable. The 2 vortexes are what makes the suction, in the other direction, to draw the air into the duct.

A short walk at Oshkosh would provide a hundred examples of where not to install a NACA duct. Remember, even on a Formula 1 or Nascar vehicle the duct is probably in a compromised place because of other more important considerations. Race cars are also optimized for much higher Reynolds numbers. Airflow starts to behave radically differently below 50 miles per hour. Narrow your search for data to the mid 1950s on the NASA Technical Reports Server. The reports from that era have excellent nuts and bolts data and discussions. The more modern stuff is not very helpful.

Jonny

Those guys are right, I doubt a NACA duct will work well there. The sides of cowlings are generally a low pressure region. On the other hand, a regular inlet with a scoop behind it will probably recover all the pressure you need to feed that oil cooler.

It’ll add drag, but I’d put money down that you’d never notice it.

Some progress...... We have a new lower cowl outlet, for now a fibreglass one and if it works we will make it out of carbon, the greybeard who made it for me insisted on extending past the firewall by about 4 inches to get better air flow at the outlet, I am not convinced it is better, it SURE LOOKS A LOT BETTER [emoji23] than our original added on to add on

Our old one

Temps unfortunately stayed about the same, still fine in a high powered cruise (2400rpm) but need to watch the temps when below 40KIAS.

then I tried an "eyebrow", not the prettiest but it seems to have given me about 10°F better temps in both my CHT's and Oil temp


This tells me that I need more air for my cylinders (as many of you have already kindly pointed out) and the best solution would be to move my oil cooler out of my cylinder plenum which I haven't done cause it would mean down time for the plane till I get it finished.....

I hear what all of you are saying about my NACA DUCT idea, why I chose that is because my lower cowl is so tight that I hardly have any space on the bottom and lower corners because of my vetterman exhaust really filling the cowl up, I need to figure out what kind of scoop I can put on the lower front of the cowl and how big it needs to be to satisfy a 13 row oil cooler, can anyone help me with this? Oil cooler is about 140mmx140mm (5.5"x5.5") and I want it to cool at 30KIAS, is there data about how many CFM these coolers need to work at which temp and is there a formula to work out how much a scoop will scoop at certain speeds?

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You are fighting two different temperatures (oil & CHT) at the same time. CHTs effect the oil temp so when you do one, the other is also effected leaving a mystery in your mind. Separate the two. Temporarily remove the oil cooler from the engine compartment and mount it on the landing gear cabane V. Close the hole in the baffle and tackle the CHTs. Once the CHTs are solved you can then address the oil temps. Who knows, perhaps just moving the oil cooler outside will solve the whole issue? Some of the banner towing Cubs mount their oil coolers outside.

I’ve got similar cooling problems at full power Vy climb, and have been talking through it with a guy on the RV forums. Here’s some pointers he’s given me:

At that low of airspeed, you don’t have enough total pressure (static plus airspeed) to cool much of anything...

You might need to do some major cowling work to get there.

At 1000’ and 70 KIAS, the available air pressure is only around 3.3-ish inches of water. Assuming you had no losses in the lower cowl, that’s still not enough to cool your motor if you’re applying power.

You’ll have to take advantage of the prop wash as much as you can, which means air inlets further away from the spinner.

This is a NASA data collect from an Aztec, using a pressure rake.



Note the reduced pressure region near the spinner. The main takeaway here is that if part of your inlet is close to the spinner you could set up a spanwise flow from outboard to inboard which does nothing for your cooling. You would need to open your inlets up and move them away from the spinner.

Here’s some carbon cub inlets. They do a good job of this.







How do yours measure up? Mine need to grow and shift outward some. Until I get the time to do that I just fly faster.

Move the cooler back to the nose, duct the outlet air so it doesn't pressurize the lower cowl, close the big hole behind #3, and 95% of the battle will be won.

stewartb said:

Move the cooler back to the nose, duct the outlet air so it doesn't pressurize the lower cowl, close the big hole behind #3, and 95% of the battle will be won.

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we have tried Stewart, the previous 10 row cooler was fitted under cyl #2, it rubbed on: lower cowl, starter motor, fuel pipe, induction box and stole air from cyl#2, the 13 row won't even come near to fitting in there without making a big block that sticks out the front of the cowl, so I would rather move it underneath where I can take the intake further away from the prop and let the air exit in a controlled way.

Two coolers. Both have outlet ducts. These are 9 row. I had excess cooling capacity and thought about switching to 7s but the new motor will make more heat so not now. That cowl you have has been a struggle for many other guys before you. Especially with parallel valve motors.




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if a 13 row oil cooler has a 5,5X5,5inch "face" its effective orifice is less than that because of the fins and cooler rows, does anyone know what this factor is for a 13 row aero classic oil cooler? my rough guess is about 30% of the area is "covered" up by the fins?

Not familiar with Aero Classic but Niagara provides air pressure and heat rejection specs for all their coolers. Yours would compare to a 20006A.

https://niagaraairparts.com/products/

bodumatau said:

copy that NP1A, #1 has never been a problem though only #3, but I hear you, I will trim it back a little.






Ramps in front cylinders 1 & 2 that end halfway up the cylinder are good practice. However, the left edge of the ramp in front of cylinder 1 should have a lower angle, so that the fins next to that exhaust valve are not get blocked off. Bending down the corner of the ramp down at a 45° works.

Jonny

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