Zenith Aircraft Builders and Flyers
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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 John Austin on
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Chris Heintz in his article said he had no problems with removing the slats and the brackets. I really don't know why - the brackets don't seem to affect the wing or flutter, etc. I primarily shaved them back for aesthetic reasons - people would look at the brackets and say "What goes there?" Shaving the brackets back looks better and makes the plane looked "finished" vs brackets hanging out there with empty holes.
John
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Permalink Reply by Wayne Tyson Jr on
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I know that you know what you're doing, but I'm one of those guys who always wants to know "why," like an irritating 3-year-old. So, we've got brackets intended to hold the slats on; the factory designed them that way for some reason. We take the slats off. We shave them back to 20 mm and give them a nice sexy shape. Now we put the slats back on. How do we know there's enough "meat" left on the bracket?
Best,
Wayne
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Permalink Reply by Ram on
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A quick (or not) question,
I own a 701 and looking into building a 750.
About 50% of my flying is bush flying, but I could use the extra cursing speed.
Is there any noticable difference in performance between the 750 without slats and with vg to the cruzer with vg?
Any thought?
Many thanks
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Ram,
The STOL 750 has very little performance difference without slats and with VG's on the main wings as far as take off and landing ground roll (based on personally flying two STOl 750's - each with and then without slats. However, the slats are going to give you steeper approach and departure angles, so that's going to get you into shorter runways with obstructions to landing or take off.
There are very few CruZers flying at this point, and I've heard of no reports of adding VG's. However, the CruZer has a completely different internal wing structure, airfoil and tail and is not a STOL aircraft and has smaller wheels/tires that are not intended for off-airport operation. I think some are planning to delete the wheel fairings and upgrade the wheel size, add bubble doors, etc., but undoubtedly that will cost cruise speed.
John
N750A
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Permalink Reply by Joe Spencer on
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I don't know about the 750 but my 701 w/o slats will descend just as steep in a side slip as it would with the slats. I would think the 750 would do as well
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Thanks for the info. However, I wonder how to depart a short strip I've slipped into. Has anyone done any flight tests that indicate that one can get out of any spot they can get into; that is, how much flatter is the angle of climb without slats and do the VGs make any difference in angle of climb without and with slats?
W
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Maybe I missed it, but I wonder what the final count is for the number of "plastic" VGs required for the 750, and how many extras I should carry in case there's a fuel spill and some of them dissolve.
Wayne
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Permalink Reply by Harold Bickford on
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Hi John,
Thanks for the reply yesterday to my query on VGs for the 750 STOL. Glad you mentioned to include the flaperons as part of the chord measurement in determining placement of the VGs; that could easily be overlooked. Just need to keep on building!
thanks again,
Harold and Edi
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