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TELEDYNMCS(at)aol.com
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 Posted: Wed Sep 26, 2007 7:06 am Post subject: ASI vs. Manometer |
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In a message dated 9/26/2007 2:58:38 AM Eastern Daylight Time, europa-list(at)matronics.com writes:
| Quote: | thanks again! btw, why not use an ASI connected wrong way round to
measure the pressure differential inside the cowl, easier and more
convenient than flying with water filled tubes in the cockpit?
There is a table somewhere of indicated airspeed versus delta P |
Hey Graham,
Somebody posted a conversion table of airspeed to inches of water column on the Yahoo Jabiru list a while back. I have that table, but I don't have a spare ASI that works to try it. I've been testing with a "calibrated" manometer I got from a friend who owns a heating and air conditioning supply house. These manometers list for over $50, but he only wanted $12, so I couldn't resist. The testing I've been doing with the manometer has all been done in the morning when it's smooth. So, no real issues with water filled tubes in the cockpit. I have the manometer secured with masking tape to the panel, so no problems yet with it moving around and making a mess. I also have the manometer set up so that I can switch between the right and left intakes by switching the feed tubes and thereby compare the results. The static source is common to both ducts.
I sure hope I don't make a mess! The wife would kill me for sure if I spilled green dyed water on the upholstery she made for N245E! I think the courts would rule that grounds for "justifiable homicide" after she fought my old sewing machine for 3 weeks before I finally broke down and bought a new machine so she could finish the seat cushions. Death would surely be slow and painful if I stain that upholstery!
I did two brief flight tests with the Sonex duct yesterday and it was terrible, i.e, much, much worse than what I have been seeing with the old, original left duct, but I don't necessarily think at this point it's the Sonex duct's fault. I played with a fan and a piece of sewing thread tied to a piece of welding rod in front of the ducts near the intake and found some rather interesting behavior in the airflow around the left intake. I used a piece of welding rod to hold a 6" piece of thread in the airflow of the fan so I could keep my hands and arms out of the way and not skew the results.
As the thread approaches the center of the left intake hole the intake rejects the thread and the thread actually blows away from the intake duct and towards the fan! This left me puzzled, so I first made a couple of vortex generators out of some thin, scrap aluminum and taped them on the outside of the cowl near the intake where I seemed to have a particularly low pressure area that was sucking the thread out of the intake. The thread showed remarkable improvement in flow around the intake hole with the vortex generators installed and I observed much less tendency for the intake to reject the thread. I removed the vortex generators and put zigzag tape in various places around the intake and tested it again and again with the thread and even more improvement in flow was observed. I found what I think is a sweet spot for the zigzag tape and clearly it is helping the flow enter the duct. Whether or not this stays constant at flying speeds remains to be seen, but I don't know of any reason why it wouldn't. Unfortunately, I broke one of my CHT rings when I had to remove the plugs to get the Sonex duct off and it'll be Friday before I can get a replacement and do a test flight. It's going to be a long two days waiting, for sure. I'm thinking I had a "Eureka" moment yesterday. Flight testing will ultimately reveal whether I'm onto something or not.
Something is causing a high pressure bubble to form just outside the mouth of the intake and the thread is showing that bubble is preventing air flow from entering the intake. In fact, it appears to be rejecting flow without the zigzag or vortex generators on about half the diameter of the intake on the left, particularly on the outboard edge. The thread is essentially showing me exactly what my manometer testing has shown, duct pressure is not an indication of flow, but rather it is an indication of the absence of flow. The right side does not show this tendency to reject the thread at all, even though the intakes are identical, mirror images in shape and the aluminum intake rings are identical in size. Some of this almost certainly has to do with the direction of rotation of the prop, or in this case the fan. Beyond that, why this rejection occurs on one side and not the other is a real mystery. In fact, the right side sucks the thread into the hole from more than an inch away laterally. The thread also shows good laminar flow all around the outside of the intake on both sides, but even with no internal duct installed, the left intake hole rejects my thread unless I have the vortex generators installed on the outside of the cowl near the intake or the zigzag tape along the lip of the intake. I think I've found the missing piece to the puzzle. I'll let ya'll know more once I've had a chance to do some flight testing on Friday or Saturday.
Regards,
John Lawton
Whitwell, TN (TN89)
N245E - Grounded until Friday
See what's new="_blank">Make AOL Your Homepage.
[quote][b]
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astills
Joined: 24 Apr 2007
Posts: 6
Location: Phoenix,Ariz
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| Posted: Wed Sep 26, 2007 7:17 am Post subject: ASI vs. Manometer |
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John,
I’m following your testing with baited interest as I was thing of the Sonex style duct also. I’ve gotten my temps within limits but haven’t been able to balance them
as I would like.
Al Stills
N625AZ [quote] [b]
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