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Mike Welch
Joined: 13 Feb 2011
Posts: 272
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 Posted: Wed Aug 27, 2014 6:24 am Post subject: AOA-parts, etc |
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Russ, others interested,
For those interested in the subject;
I'd like to mention there is a subtle difference between AOA and lift reserve indicator, although they perform essentially the same basic function, which is point out a pending stall.
An AOA is predicated upon a specific angle that a person has found that would create a stall situation on a given wing shape. Let's say 15 degrees. Then, when flying along, no matter what the airspeed is, when THAT particular wing is tilted such that the angle of attack is 15 degrees, it will stall. This angle is not variable. Whether the plane is lightly loaded, or not, or flying fast or not, THAT angle, when crossed will stall the wing.
A "lift reserve indicator" is sort of similar to the AOA, but rather than using a wing's angle of attack to determine a pending stall,
a LRI uses air pressure. In normal flying mode, air above a wing is always lower than air below a wing. (that's why they call it lift....it lifts the wing and everybody is happy). As the angle of incidence is raised, the air above the wing will begin to decrease and as it gets lower, and lower, and lower, eventually the wing will get to a point where the difference in pressure is unacceptable to remain flying. You will have super high pressure (relatively speaking) BELOW the wing and virtually no pressure above the wing.
If we put two sensors at strategic locations on our wing in question, and we go out and fly around, we could watch the differences in air pressure as we raise and lower the nose of the plane. That would be one way of doing it.
Or.....we could make a pitot tube shaped such that a leading edge hole would experience high pressure ram air, and a trailing hole
(hidden from the ram air airstream) would experience low(er) pressure. If we hooked a pressure differential gage up to our "ram air and non-ram air" ports, we could see the differences visually on our gage. All we'd need at that point is to go flying around the skies and determine what angle on our pitot mast made the gage colors correspond with what we want to see. ( I would think
entering the red zone would be getting mighty close to stall)
At any rate, a properly built and adjusted lift reserve indicator is an extremely accurate way of sensing a pending stall.
Russ,
The following pictures features LRI, Corp's lift reserve indicator pitot mast, along side my version. Their's is the aluminum one and is longer than the one I made. It is also a fair amount heavier. I made mine out of 1/2" thick Lexan that I had a large piece of. The reason mine is shorter is because I only had a 6" drill bit. I suppose I could have come up with an 8" long (1/8") drill bit, but I didn't think being 2" shorter needed to be so critical if the pitot mast is positioned properly (far forward).
The gage is a Dwyer Minihelic 2-5002, 0-2" W.C. But like I said, Dwyer has that exact same pressure version in metric, they just
label it with Pascals or whatever. eBay usually has a fairly decent selection of the range we'd want.
If someone wants to make their own AOA/lift reserve indicator, I'd suggest studying this article;
http://www.ch601.org/resources/aoa/aoa.htm
I would also recommend that if you really want to build one, and you have questions on how to fabricate any of the parts, give me a call. It would be far easier to explain over the phone how to do something than for me spend 4 hours trying to explain something that doesn't need explaining.
In a nutshell, here is some facts on the LRI set-up;
a) the pitot mast has two ports, the front and the bottom. Their position is shown in the drawing in the above link.
b) Aircraft Spruce sells the yellow/red/green stick-on arches. I recommend that, unless you have some sophisticated
way of making vivid color arches.
c) the pitot sticks out through the circular round mounting plate. You need to allow enough clearance around the mast itself for
adjustment for proper calibration.
d) a hairdryer and a Hall wind meter works pretty well for getting the angle figured out "approximately". You have to make a jig to hold the hair dryer, and a jig to hold the pitot mast. Or you could use a car and a digital level if someone else can drive the car for you.
d) it is a fun project, and shouldn't cost but a few bucks. I think if I were to build another mast, I'd think about trying to make a
fiberglass one. Not that the Lexan one is weak, but for ease of shaping it and all, I'd try the fiberglass next time.
e) the factory uses threaded tube fittings. I thought that wasn't necessary in my version, so I epoxied alum. tubes. VERY strong!!!
f) there's a bit of a trick to drilling 8" in 1/2" material and not poking out on the side. Ask if you're interested.
If anyone wants more information, call me. Easier to talk than type.
Oh, one more thing. I saw yesterday where Boyd mentioned an audio/visual alarm for pending stall like Cessna uses. On my Cessnas, I found those audio alarms to incredibly valuable. For fun, sometimes I would fly right on the very edge of stall, stall horn blaring away, doing slow wide turns!!!
I concur with Boyd completely. If all a guy wanted was a loud alarm, I'd seriously consider looking at that idea.
There are a slew of easy-to-build alarm circuits. (using a 555 timer Integrated circuit, as an example) There are also tons of super low PSI switches. (0-2 psi, etc, etc) It wouldn't take a lot to combine the two of them into a nice, neat audio stall warning alarm.
I think this concept could be explored on the Aeroelectric list extremely well. In fact, I'd be willing to bet you could build the attached Lift Reserve Indicator and tee it into the gage's rubber tubes, to combine it with the appropriate low pressure switch to activate an alarm when the needle approaches the red.
We should run this concept past the Aeroelectric group.
Mike Welch
352-206-6867
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John Hauck
Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)
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| Posted: Wed Aug 27, 2014 8:03 am Post subject: AOA-parts, etc |
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Personally, I doubt there is any advantage in additional stall warning on a Kolb aircraft other than the airspeed indicator. Monitoring the Kolb List indicates most folks that inadvertently stall their Kolb had no idea how fast they were flying THROUGH THE AIR at the time of their inadvertent stall.
Make it a practice to hold at least 10 mph above stall speed when flying low and making approaches and landings. If the air is lumpy, add a little more airspeed (forget about ground speed). Check the stall speed any time the weight and configuration of the Kolb changes, i.e., fuel, cargo, passenger. Know that to prevent accelerated stalls, carry a little more airspeed. Very seldom, back in the old days at Lakeland and Oshkosh, while demonstrating the MKIII could I get it into an accelerated stall. At low altitudes if you let the Kolb stall, you probably will not have enough altitude to recover, depending on how low you are. The low level stall comes as a complete surprise and you are on the ground before you figure out what you did wrong and decide to attempt to correct it. Most folks pull the stick back while aggravates the situation.
Before you load your airplane up with a lot of extra instrumentation, really learn to fly the Kolb, especially with the ASI. Get out there and wring it out. Gets used to how it flies.
If you don't change the attitude of the aircraft, probably will not change airspeed unknowingly. If you get into the habit of making a good cross check of airspeed, you may not inadvertently stall your Kolb.
In 2009, on takeoff from Grants, NM, field elevation 6,460 feet, D/A probably out of sight (I failed to check D/A prior to takeoff), direct cross wind 15 gusting to 30 mph, visible and invisible dust devils in close proximity to runway, I stalled my fully loaded mkIII. I had no idea what my airspeed was. Unconsciously, made a takeoff like I would normally at home at 400 feet ASL. I broke one of my cardinal rules, did not know what my airspeed was, busted my butt, but did not do any more than bend my steel gear a little. The mkIII quit flying about 30 feet, the wind blew me to the left, just off the tarmac, smacking the sandy desert, which may have saved the mkIII from more serious damage. Two mkIII's took off before me, had a lot of trouble overcoming the undesirable conditions, but I did not realize their problem until we landed at Gallup, NM. I remember waiting a little longer to take off to insure I did not get into their wake turbulence. Don't know if I did or did not. All three of us were into our second day at fields with elevations of more than 5,000 feet. Muscle memory took over.
Remember, at the beginning of this post I said "personally". That means what I said above, is my own personal opinion and experience after building, breaking, and flying Kolb aircraft for more than 30 years.
john h
mkIII
Titus, Alabama
From: owner-kolb-list-server(at)matronics.com [mailto:owner-kolb-list-server(at)matronics.com] On Behalf Of Mike
Sent: Wednesday, August 27, 2014 9:23 AM
To: kolb-list(at)matronics.com
Subject: AOA-parts, etc
Russ, others interested,
For those interested in the subject;
I'd like to mention there is a subtle difference between AOA and lift reserve indicator, although they perform essentially the same basic function, which is point out a pending stall.
An AOA is predicated upon a specific angle that a person has found that would create a stall situation on a given wing shape. Let's say 15 degrees. Then, when flying along, no matter what the airspeed is, when THAT particular wing is tilted such that the angle of attack is 15 degrees, it will stall. This angle is not variable. Whether the plane is lightly loaded, or not, or flying fast or not, THAT angle, when crossed will stall the wing.
| | - The Matronics Kolb-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?Kolb-List |
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_________________
John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama |
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