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Slats and VG's
I recently had my 750's completion published in EAA's Sport Aviation magazine. Accompanying this was a picture of my first flight. Many sharp-eyed readers noticed that I didn't have the slats installed, and I've received a number of inquiries from all over the country about "how does the 750 fly without slats and do you have VG's installed (on the main wing)?" Since I've had to compose my thoughts a number of times to respond to these requests for information, I thought it might be of general interest to the forum, too:
I selected the 750 to build and fly as I already had a 2100' turf strip situated in a most picturesque place, the Sequatchie Valley in mid-east Tennessee. Although I did not need the extreme STOL capability, I wanted a plane that was Light Sport, large/comfortable cabin, great visibility, and designed for "low and slow." I purchased a partially-built kit with slats mostly built and the brackets already installed in the wings. As I approached first flight, Russell Calhoun generously let me fly his 750 with and without slats ... it had never occurred to me not to install the slats! He did have the Stolspeed VG's installed on the main wing, too.
His plane flew so well without slats, I was very confident that mine would, too. My DAR wanted to move up my inspection, and the slats were still in the paint booth, so I certified the plane without slats and without VG's on the main wings. I later added the slats during Phase I to get the testing out of the way, and did weight-and-balances for both configurations. I later removed the slats after testing. I've always had Zenith's VG's on the elevator and just recently added the Stolspeed VG's on the wings.
What I've found is this:
1) I gained 8 kts at cruise (2900 rpm, Jab 3300) and 14 lbs of useful load without the slats.
2) Take-off and landing rolls are probably a bit longer without slats. However, with the Stolspeed VG's installed and no slats, if there is any difference in lengths of rolls (vs slats), I can't detect it. The VG's for the main wing have no effect on cruise speed, cost about $100, and take about a half-day to install.
3) Obviously, the slats allow steeper approach and ascent angles. As I accelerate through 25 kts indicated, I can haul back on the stick and you can feel the plane sort of jump and "hang" on the slats and then accelerate quickly and steeply! I couldn't quite do that without slats, but once the VG's were added, it's very similar to the slats except perhaps the initial acceleration is a little slower and the angle a little less aggressive, but I can still get lift-off going through 25 kts.
4) If you're a little high on final and pull the power back and push the nose down, the slats act like speed brakes - you don't seem to pick up a lot of excess airspeed and reach a "terminal velocity" of about 55-60 kts indicated. However, if one misjudges the flare and runs out of airspeed/energy, it sinks abruptly and is probably the cause of a lot of the nosegear collapses we hear about.
5)With the slats off, the plane is relatively resistant to slowing down (thought you'd never hear that! HA!) and I find myself using flaps more. But, the increased gliding and penetration ability, plus the low-speed control authority with the VG's makes it easy to do "squeaker" landings.
6) Practicing departure stalls, I found that with the slats, I never could really say it "stalled" - just mushed along completely controllable with a higher and higher descent rate. Without the slats and without VG's on the wing, it pretty much classically stalls at 25 kts indicated with plenty of pre-stall buffeting and warning and then the left wing drops at the stall, but easily picked up With the VG's, the stall delayed to nearly 20 kts indicated and the buffeting gave plenty of warning prior to the stall.
7) If I had know then what I know now, would I even install the slat brackets? I think that would be a pretty gutsy move for the first time builder. After all, it only takes about 20 min to bolt the slats on or off. But I do think the plane looks odd with "naked" slat brackets. I made some leading-edge shaped trim pieces to attach to the brackets, but I'm undecided whether that really looks better or not! I believe someone on the forum mentioned developing a hidden slat bracket that would only leave a bolt head showing? Might be a good alternative but esthetic way to keep the slat capability in the event of having to sell the airplane later.
Just my $.02!
Replies to This Discussion
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Permalink Reply by Lawrence Van Egmond on
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Thanks for the information as I have built my wings without slats and planned on using VG's from the very beginning. I have VG's on the plane I am flying now and they made such a difference I could not see why they would not work as well on the 750. The added cruise is a bonus but I was mainly looking for a smoother transition on landing as well as the weight savings.
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Permalink Reply by John Austin on
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You're on the right track, Lawrence - they'll definitely smooth the transition to landing! By the way, here's a pic of the trim pieces (also known as useless bits of aluminum!) I made to give the slat brackets something to attach to. Made them of .063 - heavy enough they don't flutter. I kept the rear edge to just floating above the true leading edge so it wouldn't chafe or fret the paint. They sort of look like stall fences ... sort of! I can't decide if they make it look better or worse. If anyone wants to chime in with an opinion, you won't hurt my feelings! LOL!
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Permalink Reply by joseph hanz on
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Thank You John,
This is something that I have been wondering about. You submitted a very nice report. I very well may end up doing what you have done.
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I think the tail VG's definitely help with landings - I put mine on during construction - never flown a 750 without them. I've never seen an argument to not install the tail VG's. Whether slats AND wing VG's work even better together is waaaay beyond my pay grade! Maybe we have an aerodynamicist lurking out there who can discuss this! All I can say is I have flown Russell's 750 with the slats and Stolspeed VG's and it flew (to me) about the same as slats-only - there certainly were no adverse effects. Of course, he normally flies slats-off and VG's-on!
By the way, just to be clear - I'm not advocating everyone go slatless, just relaying my impressions.
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I have had a request for some pics of the VG's, so I thought I'd append them to this thread, too. The Stolspeed VG's are Lexan - pretty much indestructible except for avgas - so you need to be very careful when refueling. They are paintable, but the clear Lexan makes them seem to blend with the wing and they're sometimes almost invisible. Supposedly, they contain an "extra dose" of UV inhibitor and Russell told me his have held up well so far. I haven't had any come off - they now come with a laser-cut sheet of 3M adhesive - you just peel the paper off (already laser cut in the shape of the VG's base), stick the VG on the adhesive, then peel the adhesive off the backing and stick it on the wing, guided by a template and string-line. Per Stolspeed's instructions, I put the tip of the VG at 7% chord so as to put the top of the "sail" of the VG at about 8-9% chord. One thing mentioned but not emphasized in the instructions: chord is measured from leading edge to the trailing edge of the flaperon. On the 750's wing, the 7% chord line for aligning the tips of the VG's worked out to measuring 155mm directly on the skin forward of the main spar rivet line. If you make a mistake and need to reposition the VG, the adhesive will come loose with steady pressure and doesn't harm the paint - you just rub the adhesive and it balls up. I haven't had one come loose otherwise yet!
By the way, if you look closely in the background, you'll see the slats stored on the hangar wall, feeling quite dejected. : (
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Permalink Reply by Michael A Housewert on
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John . . . I am ready to start the VG installation process. I have removed the slats, cleaned the wings, and stretched the string per your suggestion. Now my question is which spacing? The VG's came with 2 templates . . . . see the attached picture. I don't want to assume anything . . . 30 mm or 60 mm ? Your recommendation, please?
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20230504_112428.jpg, 5.1 MB
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Normally the closer spacing is used on the tips so they don't stall first and that helps you retain flaperon authority when the wing is stalling inboard.
John
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John,
I am ready to start. Here are a few pics showing the setup of the 1st VG using scotch tape, ruler, line placement, etc. Do you see anything that says "STOP" . . . ??
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I couldn't open the attached photos, but if you're asking where to stop placing VG's, basically take them inboard all the way to the outboard edge of the wing root skin. I had plenty of VG's left over, so I actually continued them down the leading edge of the wing root skin, too. That's only a few VG's, so to be honest I just "eyeballed" their position using the "TLAR" method (That Looks About Right!" LOL!} rather than try to calculate the exact position on the constantly changing curve of the wing root skin leading edge.
John
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John, My apologies. When I try to send a group of pictures within one attachment, it frequently fails. I was asking if my setup looked correct.
I have completed one wing, so with this message, I have only 2 pictures - Before and After. What do you think?
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20230504_165048.jpg, 3.7 MB
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20230504_195452.jpg, 3.2 MB
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Your string is in the wrong location ... it is supposed to be ON the main spar rivet line, not between the two rivet lines (you were supposed to locate a centerline between two of the rivets in the main spar center line and place your string on that), which makes your VG's a little further forward. Probably it's no big deal - I'd go ahead and fly it and see if your stall speeds are about 5 kts lower and if they are, you're good.
John
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John . . . Bummer !!
I just looked at the string placement and what you are saying is that my string is about 1/2" too far forward . . . is that what you are saying?
Would that make the tip VG placement closer to 5% of the chord?
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