Magnehelic pressure differential gauge

[Mark Wright]

Is it a straightforward process to measure the pressure differential in the cowling using a magnehelic gauge by connecting hoses to the top and bottom cowl, and is the positioning of the hose end within the cowl, as well as its direction, a crucial factor?

 

[DanH]

Is it a straightforward process to measure the pressure differential in the cowling using a magnehelic gauge by connecting hoses to the top and bottom cowl, and is the positioning of the hose end within the cowl, as well as its direction, a crucial factor?

If the goal is simple deltaP for one particular airplane, a hose routed to the center of each space is probably good enough. It would let you see if a modification raised or lowered deltaP.

The data in CR3405 suggests there is significant pressure variation between different locations in the upper plenum. A single point measurement would miss those variations.

Dynamic pressure plus static pressure = total pressure. If significant air movement (i.e. available dynamic pressure) exists around the end of a hose, it can affect a static pressure measurement. If only static is desired, you'll need some sort of diffuser on the end of the hose to cancel the effect of air movement. Lots of ways to skin that cat; an aquarium bubble rock has been popular.

There is no "official" standard. At one time Lycoming published a guide to how they did it, but I've never seen a copy. CR3405 used a variety of methods, including baffle buttons, which probably reflected some dynamic pressure in the results.

I have a moderate database of pressure measurements made by other builders using a specific set of piccolo tubes mounted in specific locations, the idea being to all use the same setup. If we all fly the same profile with the same setup, the measurements should be comparable between different airplanes. The long piccolo tubes mechanically average much of the volume while ignoring dynamic pressure...or anyway, that's the idea. If you want to play, let me know.
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[krwalsh]

Piccolo Tube

You should use a piccolo tube at the ends so that they do not end up measuring ram air pressure:

https://photos.app.goo.gl/UXH2qT24EoVwYmJP7

This is on the oil cooler exit on a Berkut that we were testing. It is basically just a metallic tube with a series of cross-drilled holes.

Make sure the tubes are well secured so they don't come loose and get tangled up in something.

 

[Mark Wright]

Thank you

I appreciate your responses, and I'll be conducting tests over the upcoming weekend using the piccolo tube. This is intended for my cub, for which I recently redesigned and entirely remade the baffling during the past weekend. While I have similar plans for my -14 (using a plenum), that project is still a couple of months away from completion and first flight. I'd be more than happy to share that information and findings with you, Dan. Thank you again.

 

[DanH]

I appreciate your responses, and I'll be conducting tests over the upcoming weekend using the piccolo tube. This is intended for my cub, for which I recently redesigned and entirely remade the baffling during the past weekend. While I have similar plans for my -14 (using a plenum), that project is still a couple of months away from completion and first flight. I'd be more than happy to share that information and findings with you, Dan. Thank you again.

Download the white paper here:
https://www.danhorton.net/Misc/COWL PRESSURE AND TEMPERATURE MEASUREMENT.doc

Note upper and lower pressures are measured against the aircraft's static system pressure, not each other, and the method uses true airspeed computed from a three leg flown while recording the pressures. With temperature, pressure altitude, and exact velocity, it's easy to compute available dynamic pressure. Compare that to the upper plenum/freestream deltaP and it's easy to see how well your inlets convert dynamic to static, the coefficient of pressure defined in CR3405.

For a slow bird like a Cub, you'll also want to make a full throttle climb measurement, as cooling in the climb condition is highly dependent on harvesting the dynamic pressure in the propeller outflow.

 

[Mark Wright]

Thanks Dan for providing the test instructions! I've ordered the required parts for conducting the tests on both the cub and the -14. It's gonna be awesome to finally have some test data for tweaking the cowling and baffles, instead of just going by conventional lore and tradition. The CR3405 research paper is very eye opening! I was unaware that investigations had been conducted concerning the airflow dynamics around air-cooled engines. The element that stood out to me was the SIGNIFICANT air leakage that occurs with traditional baffle configurations. Having read this, I can't imagine NOT doing a well designed plenum.

 

[DanH]

The element that stood out to me was the SIGNIFICANT air leakage that occurs with traditional baffle configurations. Having read this, I can't imagine NOT doing a well designed plenum.

Traditional GA flap sealing tends to be awful, and the fit of baffle wraps is generally also poor.

It is entirely possible to build good sealing without a plenum lid.

Measuring cowl exit air temperature gives us a yardstick for cooling efficiency. If the flaps leak and the wraps don't keep the air between the fins all the way to the lower spark plug, exit temperature will be low...much of the air had no opportunity to pick up heat.

The yardstick value is...

efficiency = (exit temperature - OAT) / (heat exchanger temperature - OAT)

...with heat exchanger temperature here being CHT.

Our lower cowls enclose a lot of hot exhaust pipe, so the above isn't strictly valid for air cooled cylinders. However, it's a reasonable apples-to-apples comparison within a group of RV's.

Download the white paper again. I've added a test card worksheet for your cockpit clipboard.