Improving cowl Pressure differential

[Bcone1381]

I am curious if anyone can help me understand part of A Dave Anders article in Nov 2023 Kitplanes p.47 titled "Build a Custom Plenum That Will Work."

The focus of my question is creating a pressure differential between upper cylinder plenum area and the lower area where the air flows to after it passes by the cylinder fins.

Dave says.....

"If you don't have any help decreasing the (air) pressure under the cylinders from your exhaust your Delta P will be about 7 inches of H2O....."

"However with exhaust assistance lowering pressure in the lower plenum the delta p can be 11.5 inches of water."

Can you show me how folks building exhaust augmentation into their lower cowl to improve the pressure differential?
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My Original post is above this line.

Edit Oct 11th
My dad, a high school teacher,/coach from the 1950's - 80's once told me "Definition and nomenclature understanding is essential to learning. If the kiddies don't understand the words they cant learn the topic." I'm not too smart, but I'm curious. I took some terms used in posts below by the smart guys and included them here to help facilitate us slow guys to learn and keep up.

Mass airflow: Weight of Airflow per unit of time, like Pounds per Second. (we must understand that Air has mass....it weighs something and has inertia)
deltaP: Change in Pressure, or pressure ratio, or a pressure drop across a heat exchanger.
Coefficient: A multiplier. Like a better design has a better multiplier (coefficient) than a less efficient design.
Entrain: To draw along with or after.
Exhaust Augmenter: An outer tube (commonly a tube but also another shape or an area) that an exhaust pipe exits into. Air entrainment in the tube (area) by the moving exhaust air is a strong motive force that increases air flow through the augmenter tube.
Freestream: The air speed at the start.
Goal of a cooling System: Dan's key words "twin design goals are reducing the quantity of air and making it exit with velocity close to freestream.
Fineness ration: The ratio of the length of a body to its max width. Like an arrow has a large fineness ratio. So a smaller cowl inlet gives a larger fineness ratio.

 

[DanH]

I am curious if anyone can help me understand part of A Dave Anders article in Nov 2023 Kitplanes p.47 titled "Build a Custom Plenum That Will Work."

The focus of my question is creating a pressure differential between upper cylinder plenum area and the lower area where the air flows to after it passes by the cylinder fins.

Dave says.....

"If you don't have any help decreasing the (air) pressure under the cylinders from your exhaust your Delta P will be about 7 inches of H2O....."

I'm sure Dave was thinking of some specific aircraft velocity and cowl configuration. Reality is deltaP varies significantly with change in velocity.

I've attached a plot below with cowl pressure measurements from three different RV's. The three vary in coefficient of pressure, the efficiency at which they convert dynamic to static, resulting in different upper plenum pressures. They also have different exit areas, blue being a small exit. The end result is roughly the same deltaP for all three, ranging from 4" to over 8" across a 50 knot speed range.

In any case, the value of interest is mass flow in pounds per second. For any given deltaP, mass flow varies with density, i.e. altitude.

Point is, there is no one number.

"However with exhaust assistance lowering pressure in the lower plenum the delta p can be 11.5 inches of water."

Or less, or more.

Can you show me how folks building exhaust augmentation into their lower cowl to improve the pressure differential?

Very few do, because it tends to require a lot of fabrication, and introduce less desirable issues like noise, vibration, and heat. That said, it works. At the basic level, extend the cowl and shorten the tailpipe, so the exhaust stream can entrain the cooling exit stream.

Photos below are exits from Dave (at some point in his experiments!) and Brian Schmidtbauer. The most radical I recall were from the late Paul Lipps. He wrapped the cylinders in shrouds sealed to individual augmenter tubes. The exhaust ports blasted into the augmenters just below the cylinders.
.

 

[Bcone1381]

Thanks Dan! I wanted to understand the principle and your description of "Exhaust Stream Entraining (drawing or pulling along) the cooling exit stream" helped a lot.

Vibration noise and heat.....ahhh! Heat!! i'm building a non-RV and i would wonder about heat in this application. One Would be depending on the cooling exit air to surround the hot exhaust enough to keep the nearby airframe from overheating.

I like to understand the big picture and this helps.

 

[DanH]

Vibration noise and heat.....ahhh! Heat!! i'm building a non-RV and i would wonder about heat in this application. One Would be depending on the cooling exit air to surround the hot exhaust enough to keep the nearby airframe from overheating.

Trouble is, a really efficient cooling system heats the cooling air as much as possible. Remember, cooling drag = mass x loss of velocity, so the twin design goals are reducing the quantity of air and making it exit with velocity close to freestream. If we reduce exit area, velocity goes up and mass flow goes down. If the reduced mass is to carry away the same heat energy, the mass must be heated to some higher temperature.

Augmenting is about increasing mass flow, but offsets some or all of that loss by increasing exit velocity and allowing better fineness ratio at the nose.

I think augmentation could be more useful on a STOL aircraft. The very low forward airspeed means they suffer from poor dynamic pressure, a function of velocity squared. Sucking down the lower cowl pressure to increase deltaP would be a good thing. Since there is no interest in low drag, extending an augmenter down away from the belly could do a lot toward keeping it cool.

 

[N661DJ]

Dan,
I am finishing the build of an RV-6A. Plan is to install a NACA scoop in the right side of the bottom cowl, this will supply air to the oil cooler, thus not robbing air from the upper plenum. The exhausted air from the cooler exit thru louvers in the aft area of the lower cowl, the louvered exit area will be the same dimension, sized the same as the cooler. This will result in no increase pressure in the lower cowling. My hope is that this will help with the cylinder cooling here with the Florida summer temps. This will involve a fair amount of work, do you feel I might be wasting my time with this mod? I will be using a 7row cooler.
I did a similar mod on a previous 6 build only difference was I did take the cooling air from the upper plenum, ducted thru a 3" scat hose to the cooler that was exhausted thru the louvers on the side of the lower cowling with excellent results in oil cooling, CHT's were also well within acceptable limits.
Any thoughts would be appreciated.

 

[Marc Bourget]

Is a NACA scoop the best choice for an accessory that depends on Delta P?

The energy transfer from the exhaust to the exiting cooling air stream is poor.

An article* by John Thorp in S/A explained the length of the Augmenter Tube must be long enough to always have one pulse in transit to avoid reverse flow cycles. John developed exhaust augmenters in a Navion STC.

Lipps' approach relied upon the Coanda effect. After looking at it closer, I formed the opinion that Lipps' approach allowed smaller cooling intake size and not a net reduction in cooling size.

*I'll try to remember to post the S/A issue date when I get to the office.

 

[DanH]

Dan,
I am finishing the build of an RV-6A. Plan is to install a NACA scoop in the right side of the bottom cowl, this will supply air to the oil cooler, thus not robbing air from the upper plenum. The exhausted air from the cooler exit thru louvers in the aft area of the lower cowl, the louvered exit area will be the same dimension, sized the same as the cooler. This will result in no increase pressure in the lower cowling.

There is some merit IF the system is well sealed, i.e. no leakage into the lower cowl volume. Practical result will depend on inlet Cp and local pressure at the exit.

 

[Bcone1381]

I just added some definitions of terms used above to the bottom of post #1.

 

[David Paule]

For folks unfamiliar with the topic, those definitions are very helpful. In fact, it might be worth posting a sticky thread someplace that is a repository for these and other necessary definitions.

Dave

 

[rv6ejguy]

A NACA duct may not be the best choice for feeding the oil cooler. Pressure recovery is always less than a a well designed ram type duct.

 

[Marc Bourget]

As to the definitions, IIRC, referencing your father's advice,

Mass and weight are related but not equal.

Mass is constant for the material, but weight varies inversely to the distance of the mass from the center of the gravity source.

 

[DanH]

I just added some definitions of terms used above to the bottom of post #1.

Well done. And your dad was right.

Even with a good grasp of the basics, terminology can be tough. We all tend to use terms which are incorrect, but communicate just fine. For example, everyone uses "shock absorber" rather than the technically correct "damper". Well, everyone but the Brits...

Consider the term "pressure recovery" as Ross uses it above. I know what he means, because I know the context, that being conversion of available dynamic pressure to increased static pressure. No communication problem, so perfectly reasonable usage.

However, technically that would be Cp, coefficient of pressure, defined in CR3405 and elsewhere as...

(subject static pressure - freestream static pressure)/ freestream dynamic pressure = Cp

A CFD or wind tunnel guy might see it differently...

total pressure at end of duct / freestream total pressure = pressure recovery

In other words, a measure of loss.

My own aero mentor would take issue when I used "pressure recovery" to describe cooling plenum static pressure rise, but for a different reason. When wearing his teacher's hat, he would remind me that...

static pressure + dynamic pressure = total pressure

...and given static and dynamic can trade back and forth without loss, technically there is nothing to "recover".

He was also sensitive about "radiator" for heat exchanger. No surprise