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johngilpin
Joined: 23 Mar 2011
Posts: 93
Location: 004
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 Posted: Wed Apr 17, 2013 2:48 pm Post subject: Slats |
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Gday,
As the guy who sparked of all this slats/no slats controversy, and manufacturer of VGs, I usually don't post to the forum, in order not to be thought to be trying to sell VGs. The portion of the VG market that goes to replacing slats on 701's is so small that it makes no difference at all to me.
But I do feel a need to balance various opinions with actual experience, and state some of the facts that I now know by real-life experiment. I still do have a keen interest in the aerodynamics of all this, and many hundreds of hours of careful flight testing experience with published results. There's lots of theory-based, and just plain second-hand, opinion floating around, but none that I've seen backed by careful real-life test results. I don't make any claims that aren't backed by numbers from real-life flight testing.
- I would caution to be careful about just filling in the gaps on the existing slats. I did try that early on, and didn't like the results, at least not on the Savannah aircraft that I used. It moved the center of lift forward, such that it felt like the aircraft was then flying near the limit of of aft CofG. Very light and sensitive in pitch control, which feels great but I know it to be a first symptom of an aft CofG..... Also, the trim had changed, and now needed some nose-down trim to stay balanced, not a lot but noticeable..... Most 701's seem to start off with a CofG more toward the forward end of the range than the Savannahs, so this move could then be beneficial, especially for float-equipped aircraft, but still, caution advised......
- We have found many times, that removing the slats doesn't change the trim at all. Which would indicate that the slats don't really provide any significant lift at cruise attitude. This is contrary to what Zenith and many books of theory claim, but our real-life testing shows otherwise. This would also be borne out by flying aircraft with the retractable slats, such as the Pegastol wing on the 701, and the Helio Courier, both of which I have some experience. On those aircraft the slats self-deploy as the aircraft slows to a slow cruise speed. Flying right around this critical speed causes those slats to deploy and retract intermittently, sometimes one wing at a time, but this causes no change in pitch of the aircraft. Which would indicate to me that those slats when deployed aren't providing any significant lift at cruise either, just lower stall speeds.
- It's not the smaller wing area after removing slats that gives increased speed, it's removal of all that drag caused by the slats. Even if they're positioned so there's little flow through the slot (and I don't think that ideal is really attained in practice), there's still considerable drag due to all the disrupting edges and gaps. The Savannah 'VG' model, with the leading edge extended about 65mm to a profile more like the original NACA 65018 from which this wing was originally derived, increased cruise speed by about 1.5kts, compared to the more blunt leading edge with just the slats removed. The more blunt leading edge gave a stall speed 2kts lower, and more gentle.
I've found the 701 to be a really fascinating aircraft to experiment with. The construction is brilliant - very basic and easy to work with. Various mods have been very tempting to try, and easy to do, and some gave dramatic improvements - very satisfying to an experimenter. Hans just got back from a 40hr flight to Tasmania and back in his much modified 701. Averaged 16.5 litre/hr (4.3 usgal/hr) at cruise speed about 84kts (96mph), fully loaded. And did STOL landings at a couple of grass strips, one quite short and behind powerlines. Just the greatest little aircraft for adventure flying!
JG
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johngilpin
Joined: 23 Mar 2011
Posts: 93
Location: 004
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| Posted: Thu Apr 18, 2013 1:04 am Post subject: Slats |
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Gday Ernest,
Please don't sell your aircraft until you've given it a chance to prove itself.
Take those slats off and try it that way before you give up. Hans flew about 3500 miles on this last trip. Three days down to Tassie and three back; that's pretty good x-country cruisin'.... We don't feel the need to go any faster, cause we want a good look at the country below. And besides, at our age can't think any faster than that..... And wouldn't want to give up short field capability to be able to land easily on short private strips. How much STOL do you really need??..... And we want good wild weather capability just in case, which the 701 does really well. Hans and mates ran into 40kt winds with mountain waves, but rode it out and landed safely in wild conditions no problem, due to those excellent flying ailerons and that forgiving wing. All were flying aircraft with the original wing with VGs instead of slats, nice and simple.
You can "...have your cake and eat it too..."
JG
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JohnDRead(at)aol.com
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| Posted: Thu Apr 18, 2013 6:30 am Post subject: Slats |
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Filling in the slot with tape just makes the airfoil back to the basic airfoil that has about a 1.3 coefficient of lift. It's the slats that make the difference. I am at a loss to understand why any of the "Slat Removal Group" bothered to build the 701 in the first place. Didn't you know it had slats and what they do?? Slats double the lift of a given airfoil VGs do not they only make a poor performing airfoil work better. Taking the slats off of the 701 wing make a completely different airfoil so you have become test pilots not experimental pilots. The waiver that you signed with Zenith when you undertook the 701 construction could really bite you in the butt in the event of a incident! Have fun. All intelligent replies will be answered. Please read Abbott and Von Doenhoff chapter on slotted airfoils before you start typing. Happy flying.
Regards, John
CH701 - Colorado - Jabiru 3300
Cell: 719-494-4567
Home: 303-648-3261
[quote][b]
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d.shrader
Joined: 21 Nov 2008
Posts: 38
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| Posted: Tue May 28, 2013 9:57 pm Post subject: Re: Slats |
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Are you flying yet John?
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kmccune
Joined: 22 Sep 2007
Posts: 577
Location: Wisconsin, USA
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| Posted: Sat Jul 13, 2013 6:12 am Post subject: Re: Slats |
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Actually it does not, the slat may be cut out of the front of the original airfoil, but it is extended and I believe rotated slightly to create the slot. If you tape it up, you will have an extended nose shaped differently the the original airfoil.
Are you frying the 701 with the 3300 yet?
Kevin
[quote="JohnDRead(at)aol.com"]Filling in the slot with tape just makes the airfoil back to the basic airfoil that has about a 1.3 coefficient of lift. It's the slats that make the difference. I am at a loss to understand why any of the "Slat Removal Group" bothered to build the 701 in the first place. Didn't you know it had slats and what they do?? Slats double the lift of a given airfoil VGs do not they only make a poor performing airfoil work better. Taking the slats off of the 701 wing make a completely different airfoil so you have become test pilots not experimental pilots. The waiver that you signed with Zenith when you undertook the 701 construction could really bite you in the butt in the event of a incident! Have fun. All intelligent replies will be answered. Please read Abbott and Von Doenhoff chapter on slotted airfoils before you start typing. Happy flying.
Regards, John
CH701 - Colorado - Jabiru 3300
Cell: 719-494-4567
Home: 303-648-3261
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_________________
“Always do what you are afraid to do.â€
R.W. Emerson (1803-1882)
"Real freedom is the sustained act of being an individual." WW - 2009
"Life is a good deal...it's worth it" Feb 1969
Dorothy McCune |
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robert.eli(at)comcast.net
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| Posted: Sat Jul 13, 2013 9:18 am Post subject: Slats |
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All the slats do is to permit the angle of attack to increase beyond the point where stall would normally occur.. The coefficient of lift increases with increasing angle of attack up to the stall point, and it is the same with, or without, slats.... The slats permit the increase to continue beyond what would be possible without slats by delaying the stall to a higher angle of attack.. The slats do not increase the lift coefficient in level flight.. And they do not improve the lift coefficient at moderate angles of attack.. The slats allow a greater increase in angle of attack as compared to VG's.. but the effect is the same, which is to delay the stall point to a greater angle of attack.. They both delay the stall point to greater angles of attack than would otherwise be possible.. There is a good plot in this behavior in the zenith document below, with regard to the Zenith slats: PS: The biggest difference between the two is that the VG's have less drag as compared to the slats up to the stall point.
http://www.zenithair.com/stolch801/design/design.html
Robert Eli
CH701
From: "kmccune" <kmccune(at)somtel.net>
To: zenith701801-list(at)matronics.com
Sent: Saturday, July 13, 2013 10:12:46 AM
Subject: Re: Slats
--> Zenith701801-List message posted by: "kmccune" <kmccune(at)somtel.net>
Actually it does not, the slat may be cut out of the front of the original airfoil, but it is extended and I believe rotated slightly to create the slot. If you tape it up, you will have an extended nose shaped differently the the original airfoil.
Kevin
[quote="JohnDRead(at)aol.com"]Filling in the slot with tape just makes the airfoil back to the basic airfoil that has about a 1.3 coefficient of lift. It's the slats that make the difference. I am at a loss to understand why any of the "Slat Removal Group" bothered to build the 701 in the first place. Didn't you know it had slats and what they do?? Slats double the lift of a given airfoil VGs do not they only make a poor performing airfoil work better. Taking the slats off of the 701 wing make a completely different airfoil so you have become test pilots not experimental pilots. The waiver that you signed with Zenith when you undertook the 701 construction could really bite you in the butt in the event of a incident! Have fun. All intelligent replies will be answered. Please read Abbott and Von Doenhoff chapter on slotted airfoils before you start typing. Happy flying.
Regards, John
CH701 - Colorado - Jabiru 3300
Cell: 719-494-4567
Home: 303-648-3261
--------
“Always do what you are afraid to do.�
R.W. Emerson (1803-1882)
"Real freedom is the sustained act of being an individual." WW - 2009
"Life is a good deal...it's worth it" Feb 1969
Dorothy McCune
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=404595#404595
bsp; -================
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