Help with measuring cowl pressure differential

[kreidljj]

Dan thanks for the reply! Some more data to help out here, our 4000' data was at 156kts TAS and the 8000' data was gathered at 158kts TAS. TAS was read off the GRT box, not as a three leg run - so take it for what it is worth.

We are using a Dwyer Magnehelic gauge with a range of -10 to +10 inches of water. Our piccolo tubes are rather crude; I just punched .040 holes in 1/4" OD plastic tubing at random over a 4" length. I am connecting the gauge in the cockpit to the piccolo tubes using plastic tubing with a 5/32" OD. The plastic tubing is the same stuff that is used for pitot static installations, but obviously a different size. In the 10 it is difficult to route through the heater vents, so I am instead running the lines up through the top of the cowl where it meets the firewall then running the lines through the side vent of course all help in place with some speed tape.

The top piccolo tube is located at the engine spine, the lower piccolo tube is tied to the fuel line the runs between the servo and spider. The injector pressure is taken by disconnecting one of the cylinders and connecting the line directly to the line from the turbo nozzle manifold.

I would be happy to document a 'standard test' to contribute meaningful data, as long as we can live with the smaller lines running to the cockpit.
One of the things I have been trying to understand is what we want the value of the lower cowl pressure to be relative to static. We are seeing 2" to 3" is that a good number, should that number be higher or lower, should it be negative? I think that would tell me which end needs to be attacked right? My intuition tells me that I want it to be as low as possible since it is directly opposing the flow of air from the upper to the lower deck. However, I wonder if that number can ever get to zero since in theory we should have a vacuum sucking the air out of the exit. Great - now my brain hurts again!

Thanks again for the help and meaningful input!

Jason Kreidler ? 4 Partner Build ? Sheboygan Falls, WI
RV-10 - N44YH - #40617
Flying ~450 hours

[DanH]

Originally Posted by kreidljj

...our 4000' data was at 156kts TAS

Ok, that confirms being in the same ballpark. Your slightly higher test altitude shouldn't lower available dynamic pressure more than 1.5%, maybe 2%, yet you have 7.5" plenum pressure at 156 knots and I'd have over 12". The maximum available dynamic pressure (Q) at this velocity and altitude would be a little over 14". Measuring the delta between upper plenum and aircraft static tells you what percentage of that available Q is being converting to increased plenum static pressure.

We are using a Dwyer Magnehelic gauge with a range of -10 to +10 inches of water. Our piccolo tubes are rather crude; I just punched .040 holes in 1/4" OD plastic tubing at random over a 4" length.....The top piccolo tube is located at the engine spine, the lower piccolo tube is tied to the fuel line the runs between the servo and spider.

You'll need a manometer with more range if you get your cowl working right. Switching to the "standard" piccolo tube setup which Sonny, Ken, and I are using would be nice.

The injector pressure is taken by disconnecting one of the cylinders and connecting the line directly to the line from the turbo nozzle manifold.

Can't comment. Perhaps a new thread for the injector discussion?

One of the things I have been trying to understand is what we want the value of the lower cowl pressure to be relative to static. We are seeing 2" to 3" is that a good number, should that number be higher or lower, should it be negative?

As high as possible while still maintaining the necessary delta between upper and lower.

I think that would tell me which end needs to be attacked right?

It's quite clear which end needs work. 7.5 / 14 = 53%...a rather miserable conversion of available Q to upper plenum pressure.

My intuition tells me that I want it to be as low as possible since it is directly opposing the flow of air from the upper to the lower deck.

Only if you can't improve the upper plenum pressure.

However, I wonder if that number can ever get to zero since in theory we should have a vacuum sucking the air out of the exit.

In theory we want an exit jet with a velocity as close to freestream as we can get. That requires pressure.

Try this example....if you have 7 in the upper plenum and lose 4 across the fins you have 3 available to push air out the exit. If you have 14 in the upper plenum and lose the same 4 across the fins you have 10 available to push air out the exit. Same cooling, but the second example has far less cooling drag.

Lets suppose you can't increase upper plenum pressure....you're stuck with only 7". Opening the exit area will indeed decrease lower cowl pressure. Let's say it is now down to 2". The result will will a 5" delta across the fins, so you get more mass flow. Cooling capacity increases, but the decreased lower cowl pressure slows exit velocity. You get a double whammy as punishment for taking the lazy man's path (a big exit). Cooling drag is mass x loss of momentum, and you increased both.

[artrose]

Wow has this discussion gone off the deep end. Still a couple questions come to mind. If we're measuring differential pressure, it really doesn't matter if we're using total pressure or static pressure, differential is differential, but how do we verify the accuracy of those readings under the cowl? If you use single point probes, proper orientation of the pick up tube is critical. To accurately measure total pressure the probe must face directly into a laminar airstream. If we're measuring static, the probe must be exactly perpendicular to the laminar airstream. Inside a cowling there is the potential for very high airflows, much turbulence, as well as areas of stagnation, and most importantly airflow striation. And this is keeping it simple. The use of a picollo tube, (multiple samples in a linear area) might help some with accuracy, but you might also find that the orientation of this tool will yield widely varying results. The most accurate method my simple mind can come up with to approach any relative accuracy would be to install some sort of vel grid device over the inlet and outlet of the area in question. I'd speculate it's already been done, who knows, but in reality, this is really not such an easy project to get right, numbers is numbers though, and I guess even if they're not accurate, what's it really matter anyhow?

[kreidljj]

You're making good progress at educating me!

Meter range - I will search for the same instrument the rest of the team is using and try to pick one up. I would be really pumped to need range above 10" - it means I am making progress towards the solution.

Piccolo tubes - I can either make a set from your plans or if you would be so kind to supply I will send you my information.

I am worried that the 'fix' will not be as simple as just increasing the inlet diameter but will also include a significant increase in plenum volume. But we are a bit away from knowing for sure what the fix is until we have good data gathered.

Thanks, Jason

[DanH]

Originally Posted by artrose

Wow has this discussion gone off the deep end.

Would you care to elaborate?

how do we verify the accuracy of those readings under the cowl?

At the moment we can't claim global accuracy between Jason's airplane and the airplanes in the test group. However, we have taken some pains to standardize the large area piccolo installation on all the test group airplanes. We even use the same manometer model. Jason is welcome to make the exact same installation. Identical installs and test flight plans go a long way toward making the data comparable within that fleet.

BTW, CR3405 explores several measurement methods and relative accuracy.

[artrose]

One of the more accurate methods for measuring a ducted airflow is a duct traverse. You take velocity pressure readings at equidistant points in the duct where the airflow is likely to be laminar, average the Vp readings, then calculate the total airflow. If you simply pick a spot, take one Vp reading at one point somewhere in the duct, you'll essentially have nothing reliable. You'll have varying results if you take a reading of the airflow along the walls of the duct because of friction losses, read air moving around corners, flowing around obstacles, entering or leaving a transition, turbulent air, etc., worthless data. True, velocity and pressure are inversely proportional, but how do you take reliable readings in turbulence? That's why you take readings at equidistant points in a clean straight piece of ductwork, and average the readings. The one positive thing to look at is that in a constant state of flow in a sealed duct, total and static pressure in the duct will remain relatively stable while the quantity of air is moving through the duct, but the challenge is to find the spot that is undisturbed that reliably represents true conditions. That brings us back to measuring differential pressures under a cowl. Unless you can find a way to reliably measure the airflow before it is disturbed, as well as measure it relatively undisturbed as it's being discharged, you'll have an issue with reliable readings. You could try installing pick ups at exactly the same positions under each of the cowls, but unlike laboratory conditions, exact placement would be near impossible because no two installations are exact duplicates. Garbage in, skewed results. There is a possibility? One might try a couple automotive style mass flow sensors (hot wire annometers), but once again? Who knows. We'd be trying to measure a huge amount of airflow with a device that was not designed for the job. I do know that trying to install single point pick ups, especially if you're tryting to duplicate installations for multiple data acquisition is a little bit of futility if you're looking for any kind of accuracy. The piccolo route might be as accurate as needed? You would need multiple locations for any real accuracy, and be very precise in the duplicate installs.

[zav6a]

Dan

Have you noticed any change in plenum pressure as you modified the outlet? Thinking about one of the objectives of testing, determining pressure recovery characteristics of various cowl configs, it would seem that how much air is bled out through the outlet would be a factor. That is, seal the outlet (and spinner) and almost any inlet would have great pressure recovery.

Conversely, if the bottom cowl is full of big holes, you might not ever see decent pressure recovery. At least not without a maw like a gray whale.

[DanH]

Originally Posted by zav6a

Dan, Have you noticed any change in plenum pressure as you modified the outlet?

It has been an item of interest.

Originally Posted by artrose

The piccolo route might be as accurate as needed?

We think so. Thank you for your comments.

[RV8R999]

Originally Posted by artrose

Wow has this discussion gone off the deep end. Still a couple questions come to mind. If we're measuring differential pressure, it really doesn't matter if we're using total pressure or static pressure, differential is differential, but how do we verify the accuracy of those readings under the cowl? If you use single point probes, proper orientation of the pick up tube is critical. To accurately measure total pressure the probe must face directly into a laminar airstream. If we're measuring static, the probe must be exactly perpendicular to the laminar airstream. Inside a cowling there is the potential for very high airflows, much turbulence, as well as areas of stagnation, and most importantly airflow striation. And this is keeping it simple. The use of a picollo tube, (multiple samples in a linear area) might help some with accuracy, but you might also find that the orientation of this tool will yield widely varying results. The most accurate method my simple mind can come up with to approach any relative accuracy would be to install some sort of vel grid device over the inlet and outlet of the area in question. I'd speculate it's already been done, who knows, but in reality, this is really not such an easy project to get right, numbers is numbers though, and I guess even if they're not accurate, what's it really matter anyhow?

If we were trying to meet a pressure recovery spec or some other critical desgin criteria I would agree having ABSOLUTE accuracy would be important. Our testing (Dan, Sonny and I) was concerned more about consistent and relative data. We each used the same measuring device, picollo tubes (Dan fabricated for us) and located them in the same locations. Our standardized testing methodiligy resulted in consistent results from test-to-test providing credibility to the "relativistic" nature of our testing. Once we were convinced we could develop repeatable data throughout our various flight regimes (airspeed mainly) we investigated configuration changes (mostly exit area) and the relative affect of those changes to the pressures in question.

Within the scope of our budget (both $ and time) it was very informative and I'd stand-by our data as being as credible as required to understand, in general, what is going on. Could it be better - you bet and even with $2 million and two years of testing we could still have questions needing answers... This isn't NASA and we are not solving problems to the 12th decimal place... but you are welcome to do so and share your data with us!!

[tx_jayhawk]

Just curious...has anyone considered making the ramp out of fiberglass? Seems like that might be easier given the curves and interference.

Scott