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rickofudall
Joined: 19 Sep 2009
Posts: 1392
Location: Udall, KS, USA
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 Posted: Mon Mar 10, 2014 8:40 pm Post subject: Drag reduction |
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Dennis, Have you ever heard of the Pareto Principle? To generalize using it we could say that 80% of the drag of a Kolb is caused by 20% of the drag producers. So, we start by listing all the things that produce drag on the Kolb and tackle the big hitters first, those 20%er's. Speculation, don't yell at me or cry HERESY, please, on the big hitters of drag that keep the Mk III from going faster.
1. The pod shape. When Kolb created the IIIX based upon suggestions from Barnaby Wainfan (see Kitplanes Magazine, "Windtunnel" monthly column to get his bonafides as to why he might know a thing or two about aerodynamics) where did they make modifications? They tried to reduce the included angles between the forward and aft surfaces of the pod, the idea being that when air is forced to change its direction, if you change it more than 14 to 16 degrees the flow can't follow and separation occurs. What's the stall angle of attack of almost all airfoils? Yep, 14 to 16 degrees. On YouTube there's a video I made a few years ago where I tufted up the aft side surface of my Mk III's pod. Look under Rick51076. Watch what the tufts do at all air speeds. Surprised by what you see? I was. Notice the tuft right behind the strut intersection. That little guy had a big story to tell.
2. The wing. There's a reason why NACA pretty much stopped investigating airfoil sections with flat bottoms along about 1925. They are high drag. You don't have to go crazy and jump to the ultra modern natural laminar flow (NLF) airfoil or any of the laminar flow airfoil sections for that matter. IMHO you can look at what are called the five digit series. A great variety of airplanes have flown with airfoils from this series. My friend, Dick, who retired from Cessna's engineering department always points me there when we start talking about airfoils. I have a NACA report about this series of airfoil sections, TR 460, from 1935. Matronics keeps regurgitating it due to size. I'll be happy to send it to anyone interested or you can get it from the NASA technical report server.
3. The empennage. There are big and small drag producers here, but one that really gets me is the open boom tube. I once put an extra air speed indicator with a long pitot tube into the boom tube facing aft. The airplane was going about 70 mph south and that airspeed indicator was reading 28 mph from the north. So the air was making a 98 mph change in direction. Someday I'd like to tuft up the upper and lower vertical stabilizer and the rudder and video what the flow looks like around the end of the boom tube. I'll bet that they will tell a very interesting story, but for now it's just speculation. Putting a symmetrical airfoil section on the horizontal and vertical stabilizers might be interesting, too, but I suspect we're getting into the 80% of drag producers that only produce 20% of the drag. The things to tackle after 1 and 2 are dealt with.
Just some things to ponder. I love my Mk III and really miss flying it, but since I have an opportunity to try some things when I rebuild her.....I'll report back when I know more.
Rick Girard
do not archive
--
Zulu Delta
Mk IIIC
Thanks, Homer GBYM
It isn't necessary to have relatives in Kansas City in order to be unhappy.
- Groucho Marx
[quote][b]
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John Hauck
Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)
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| Posted: Mon Mar 10, 2014 8:58 pm Post subject: Drag reduction |
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Let me add a couple bits of info.
First, you don't need tufts to see the dead air. Simply fly in the rain. The area between the leading edge of the center section and the windshield is dead. Easy to see air flowing forward when it hits the rear half of the cockpit. I call them rear quarter windows.
Back when Brother Jim and I were much younger we decided to fabricate a fairing similar to the rear fuselage fairing of the OH-58 or Bell Jet Ranger helicopters. I believe that was what those fairings were for, to straighten up the air flow. We never got around to it. Now I have been flying the MKIII so long like it is, I'm too lazy to experiment. Having the fairing right by the rear of the door opening was put it in a position to get knocked around and would probably be a pain in the butt. Like washing a wing with VG's. 
Another note on the bottom of the wing. Everyone says the bottom of the Kolb wing is flat. It is flat on sitting on the ground. Soon as it gets loaded up in flight, it becomes a concave bottom. That is the reason I shrink the fabric as tight as I think I can get away with to keep the bottom of the wing as flat as possible. A lot of airspeed is lost the more concave the bottom of the wing.
If I am patient I can go anywhere I desire at 80 mph. My airplane is draggy, but I love to fly it.
Oh yea…there is a lot of air coming forward in the boom tube. One notices it much more in the winter when that cold air hits the back of your neck.
john h
mkIII
Titus, Alabama<![if !supportLineBreakNewLine]><![endif]>
From: owner-kolb-list-server(at)matronics.com [mailto:owner-kolb-list-server(at)matronics.com] On Behalf Of Richard Girard
Sent: Monday, March 10, 2014 11:40 PM
To: kolb-list(at)matronics.com
Subject: Drag reduction
Dennis, Have you ever heard of the Pareto Principle? To generalize using it we could say that 80% of the drag of a Kolb is caused by 20% of the drag producers. So, we start by listing all the things that produce drag on the Kolb and tackle the big hitters first, those 20%er's.
Speculation, don't yell at me or cry HERESY, please, on the big hitters of drag that keep the Mk III from going faster.
1. The pod shape. When Kolb created the IIIX based upon suggestions from Barnaby Wainfan (see Kitplanes Magazine, "Windtunnel" monthly column to get his bonafides as to why he might know a thing or two about aerodynamics) where did they make modifications? They tried to reduce the included angles between the forward and aft surfaces of the pod, the idea being that when air is forced to change its direction, if you change it more than 14 to 16 degrees the flow can't follow and separation occurs. What's the stall angle of attack of almost all airfoils? Yep, 14 to 16 degrees. On YouTube there's a video I made a few years ago where I tufted up the aft side surface of my Mk III's pod. Look under Rick51076. Watch what the tufts do at all air speeds. Surprised by what you see? I was. Notice the tuft right behind the strut intersection. That little guy had a big story to tell.
2. The wing. There's a reason why NACA pretty much stopped investigating airfoil sections with flat bottoms along about 1925. They are high drag. You don't have to go crazy and jump to the ultra modern natural laminar flow (NLF) airfoil or any of the laminar flow airfoil sections for that matter. IMHO you can look at what are called the five digit series. A great variety of airplanes have flown with airfoils from this series. My friend, Dick, who retired from Cessna's engineering department always points me there when we start talking about airfoils. I have a NACA report about this series of airfoil sections, TR 460, from 1935. Matronics keeps regurgitating it due to size. I'll be happy to send it to anyone interested or you can get it from the NASA technical report server.
3. The empennage. There are big and small drag producers here, but one that really gets me is the open boom tube. I once put an extra air speed indicator with a long pitot tube into the boom tube facing aft. The airplane was going about 70 mph south and that airspeed indicator was reading 28 mph from the north. So the air was making a 98 mph change in direction. Someday I'd like to tuft up the upper and lower vertical stabilizer and the rudder and video what the flow looks like around the end of the boom tube. I'll bet that they will tell a very interesting story, but for now it's just speculation. Putting a symmetrical airfoil section on the horizontal and vertical stabilizers might be interesting, too, but I suspect we're getting into the 80% of drag producers that only produce 20% of the drag. The things to tackle after 1 and 2 are dealt with.
Just some things to ponder. I love my Mk III and really miss flying it, but since I have an opportunity to try some things when I rebuild her.....I'll report back when I know more.
Rick Girard
do not archive
--
Zulu Delta
Mk IIIC
Thanks, Homer GBYM
It isn't necessary to have relatives in Kansas City in order to be unhappy.
- Groucho Marx
01234567890123
[quote][b]
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John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama |
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Dennis Kirby
Joined: 05 Dec 2013
Posts: 35
Location: Albuquerque, NM
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| Posted: Wed Mar 12, 2014 6:43 am Post subject: Drag reduction |
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Hauck wrote: << Loose fabric makes a lot different airfoil than tight fabric. As tight as the fabric is on my wing, I still get a concave lower wing surface in flight - The more concave that surface, the slower the airplane will fly, but probably kill a lot of top and cruise speed. >>
?! I did not know this! Will hafta check it out the underside of my wing, next time I fly.
Rick Girard wrote: << Pareto Principle? To generalize, we could say that 80% of the drag of a Kolb is caused by 20% of the drag producers. >>
Rick is correct - I have also seen it written (by Barnaby Wainfan, in one of his Kitplanes columns) that to reduce drag, the most benefit will come from tackling the biggies first: Streamline any struts in the airflow (like wing struts and landing gear legs), use gap seals for all the moving control surfaces, close up any open areas on the fuselage (Kolbs pods are notorious for this), and try to avoid sharp bends along the path of the airflow. On my Mark-3, this happens as air flows past the aft edges of the doors - it's a pretty sharp corner for the air to flow around.
My stock Mark-3 had open sides on the upper-rear area of the pod. I covered this area with Lexan sheet, and that helped. Hauck was right about the windshield-to-wing junction being a source of stagnation - lots of drag there. I made an aluminum fairing to smooth the airflow at that area. That helped too.
Boyd suggested that now, with all my clean-ups, I actually CAN get that extra knot or two added to my top airspeed if I now dial in an additional degree to my prop pitch. At the same power settings as before, I should now see a bit more airspeed. Didn't think of that - I might try it.
Dennis Kirby
Tackling the drag on my Kolb, one molecule at a time
do not archive
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byoungplumbing(at)gmail.c
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| Posted: Wed Mar 12, 2014 7:08 am Post subject: Drag reduction |
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maybe a quarter to half degree.... a full degree may be a bit much....
boyd
| Quote: | >>>>>>>>>>>>>>>>>>>>>>>>>
Boyd suggested that now, with all my clean-ups, I actually CAN get that
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extra knot or two added to my top airspeed if I now dial in an additional
degree to my prop pitch. At the same power settings as before, I should now
see a bit more airspeed. Didn't think of that - I might try it.
Dennis Kirby
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This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com
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