Replies to This Discussion

Permalink Reply by John Austin on August 24, 2016 at 6:01pm

Sam,

I remember looking at your fairings at Sun 'n Fun last year and that's what partially inspired me to do something similar!  I did have structural concerns about extending the elevator span and that is why I kept the tips at 4" each/8" total span increase. Not being an engineer, I felt that was still pretty conservative but long enough to "pick up" the enhanced flow off the tip fences. Roger (Zenith) was interested and asked for pics and I sent them and received no comments, so I "hope" that means it didn't scare him structurally ... although he's not an engineer, either!  Can't say that it added stability but I did not have any "tail wag" before the mod, either.  It might be a hair more sensitive in pitch, but nothing alarming.  I do have a gut feeling that the fences probably provide some drag reduction since it eliminates the tip vortices spilling over the end of HT tips.  After slat removal, fairing the HT to the fuselage, tip fences and elevator tips, my 750 seems to be getting incrementally faster and faster in cruise.  Used to be a 72 kt airplane in cruise and now at the same power settings it's an 82 kt airplane.  ;>)

John

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Permalink Reply by Lonny M. Kirk on August 25, 2016 at 6:52am

John,

What benefits would you attribute directly to the tabs, increased cruise, approach stability, increased elevator effectiveness, etc.?

Do you note any effect in a crosswind landing?

I would be very interested to see some more detailed photos of the tip mods as well as the fence installation if you have them. You could email them directly to me at lkirk@jingoli.com

Thanks

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Permalink Reply by John Austin on August 25, 2016 at 9:37am

If by "tabs" you mean the tip fences, they prevent air spilling off the ends of the HT, which likely slightly reduces drag. However, the primary reason for them is two-fold - inboard, they keep more air flowing over the HT which then means more air flowing over the elevator for increased elevator effectiveness and outboard, they prevent air from flowing around the forward/inboard face of the tip extensions, making them even more effective.  The fences alone without extensions should help somewhat, but the increased elevator effectiveness is really noticeable when used together with tip extensions.  I've noticed no increased yaw, etc. in crosswinds.  In cruise, if you encounter turbulence, there might be slightly more pitch sensitivity, but it's slight if at all. There's not much difference on approach except at the last moments of flare when you're getting really slow, then you really notice the improved, positive, linear response of the elevator!

I've got a detailed tutorial with plenty of pictures here: Elevator Mod - HT Fences, Tip Extensions.

Finally, my usual disclaimer: I'm not advocating or recommending that anyone do these mods, I'm simply relating my experiences!  You're on your own!  ;>)

John

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Permalink Reply by Lonny M. Kirk on August 25, 2016 at 10:47am

Thanks John

the "tabs" thing was an editing screw-up and I did mean the fences. Thanks for the answer and the link.

I can send you a Hold Harmless Agreement if you like :-)

Thanks for the invaluable input.

Permalink Reply by Samuel McNair on August 25, 2016 at 5:45pm

I am in the 801 group but not the 750 group. So I can't see it. How does one get into the other sites?

Permalink Reply by John Austin on August 25, 2016 at 6:13pm

I am in the 801 group but not the 750 group. So I can't see it. How does one get into the other sites?

You're not restricted to the 801 group - you can join any of the other groups including the 750 group. 

John

Permalink Reply by Ken Duncan on December 20, 2016 at 11:08pm

Following this thread a little late.  I own N801S which I did not build, rather be flying than building.  Previously I owned a Savannah VG 100HP Rotax with a 26MPH stall and, about a 50ft roll with two on board.  It is a step brother to the Z-701, long convoluted story there of the family.  It came from the factory with elevator and wing VG's from www.stolspeed.com in Australia.  I suggest going to his web site and take a look, he belongs to a crazy group of very intelligent STOL aviation guys.  I would call them Practical Flying Engineers.

Later I needed more interior space and found an 801 to purchase with about 200hrs.  Early with the 801 I hated the landings, felt like I needed more elevator, and the slats were like an on/off switch.  Because of my Savannah VG background I got to checking and I ordered the STOL speed elevator and wing kit for the 801 from STOLspeed and glued the elevator kit on first.  Wow, power steering is back.  I can wheelie down the runway for fun.  Big difference.

A couple weeks later I had time to remove the slats and do the wings.  You can find some of my earlier posts on this site elsewhere, but I will NEVER bolt those slats back on.  If I sell this plane and someone wants the slats, I will ship them to them and they can put them on as they wish.  I would never send anyone new off in this plane with slats on for a first flight if they had never flow them before.  Way too many nose gears and props bent for no reason. Why learn to land a total different animal to shorten 20 ft if you are not in high competition.  Yes, you can get used to the slats and hanging on the prop flying on the edge on a regular basis for competition, and bouncing off the runway.   Not what I am looking for.

I personally like to have fun without stress. Me, I can stand to add an extra 20 -25 ft in and out roll to have a Cessna type handling mushy landing on spot every time.

If you went to the VG's and had a feel you needed more excitement in your life you can always bolt the slats back on.  I know a person with a Z-750 who did that one time.  I asked him how he liked it, his answer was it was back to the normal slat performance.  "Never again"  He found someone building to sell his slats to.

Now my disclaimer:  Zenith builds a great kit airplane.  I have helped a little on a 750 and an 801 build.  I am totally comfortable with the structure and design of my 801, What an exciting aircraft to fly.  The 701 is the Sky Jeep.  My 801 is the Sky Suburban, actually 4 inches wider than my Cherokee 235. 

Yes the slats are a great invention and allow tremendous lift at low speed to a point.  My self, I can not justify that little bit extra lift with the on/off slat over the VG's vs the smooth mushy stall safety of the VS's for about a 20ft roll difference.  Just me

Just my opinion, but this is a cheap, safe, and easily reversible experiment.  I highly advise trying it.  Most likely the easiest, least expensive modification you can make that will make this much positive result unless you are in regular competition.  Your plane will look weird for a month or two, then you will get over it.  I have not found CG to feel any different, but the low speed drag feel is much less to me and I can keep the nose lower.

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Permalink Reply by Samuel McNair on December 21, 2016 at 8:18am

Ken
Just 2 weeks ago I had time to install the STOL SPEED VG's on the wing and tail of my newest 801. I did not yet have the strut fairings on yet nor the H Stab to empennage fairings on yet either. Haven't had time to run an entire test series. At first glance I picked up ~ 8 to 10 knots about 100 Fpm more ROC, the more gentle stall, and modest changes in TO & L. That vague "light stick" (feels like a too far aft CG, but isn't) feeling has gone away and the stick forces are much more linear. With 1 on board I can have the nose off the ground slightly before the Dynon airspeed comes alive at indicated 25 knots. Whatever the true rotation speed is, it is good enough for me. I operate almost exclusively off of grass, dirt, farm and duster strips, and some built for ultralights with my 801. And the performance is good enough for me. I am keeping my slats for a someday later experiment with making them retractable but when extended further out and down. But that will have to wait for retirement. Working now on strut covers and a new Dynon/Garmin IFR panel. I'll never go fast, but I plan to go RREAL FAR in this plane. Its kicks flying an ILS at 50 knots with the doors off - reach out and touch a cloud. After panel comes a belly pod.

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Permalink Reply by Walt Snyder on December 22, 2016 at 11:46am

Good points on several issues, though like most, I want the slats for their performance in the real world, competition is the furthest thing from my mind. It doesn't land like a Cessna with my slats, but, just like I reaped the rewards from learning to land my tailwheel aircraft, with the possibility of ground loops, and damage like you describe in landing a Zenith with slats, the rewards far, far, exceed the risk and the downside of limiting myself to just landing like a "normal" aircraft.  

Case in point, last weekend we had several fly into a 800 ft backcountry mountain strip we have under construction in the Sierras, and we had two limited to circling overhead because they did not have the sink rate and STOL to clear trees at one end. Who landed? Carbon Cub and a Zenith 750. You can see the Carbon cub coming in a little hot threading the trees, he has several lifetimes of experience in backcountry flying and short strips.

https://youtu.be/GWi7dylpvdo

The reward of being able to fully use your experience and slats? No competition trophy, just sitting on the cabin porch high in the Sierras enjoying your morning coffee, wondering what the golden trout are biting on, and maybe listening to your buddies circling overhead whining they can't clear the trees and land  :)   (soon to be 1,800 ft for "normal" aircraft)

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Permalink Reply by Samuel McNair on August 24, 2016 at 8:54am

I really do not know. I speculate that if their test arrangement does not have unsafe stall characteristics that either one of 2 things are going on:
1) Somehow the outboard end of the wing does not stall first which means that the function of the slats is to create more drag than lift and create a steeper but not necessarily slower approach.

2) They have some unusual type of aileron or possibly spoilers on it so that somehow it retains roll control even if the outboard wing is stalled. I haven't figured out how yet, though. Maybe they have increased rudder size and dihedral, and can "leg muscle" it into rolling.

There is a 3rd possibility and that is that the plane does have unsafe stall characteristics (and that is a place like "coffin corner" on an older Lear that once you go there there is no way back.) and that is why it is still a prototype. Even the best of us occasionally have a "Duh" moment.

It is experimental and that means that it does not have to meet current safety standards.

You are right to be cautious. Every little mod that you make, makes you a test pilot the next flight.

I used to do a lot of industrial accident and equipment failure investigations and taught courses on the subject. It is rare that the cause is true incompetence, but it is very often the result of an unintended consequences of an area that was just outside the person's area of competence or experience base. And very often what was changed was changed to make an "improvement" in performance.

That is what makes these forums so valuable. We get to learn about our individual areas of ignorance as well a share our collective knowledge.

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Permalink Reply by Samuel McNair on August 24, 2016 at 8:34pm

Good to hear from you.I got my 801 to cruisee at 101KTAS before a bounce and bad bungee caused it to be totalled. Got another one I am working on now. It has long range tanks so except for economy, the increased spred isn't all that critical. But it was operating at a solid 18 knots faster (21%) than before I had started. And it still had amazing short field capabilty.

I have seen others with end plates. But never thought about them. You see them mostly on floatplanes. Those extended tips give you "lift" from both down wash and bernouli effect. It makes them act more like a stabilator and most stabilators have anti servo tabs to reduce sensitivty in pitch. You were right to be careful. That 4" extra is probably as effective as twice that much if the H stabilizer extended in front of them.

I am going to check the parts on the old narrow tail 801 compared to the 1 foot wider tail of the later ones and see if they had to beef it up any. If not, I just might make a set of removable tips with aluminum extensions that I can increase in small increments for testing.

You did a super nice job on them!

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Permalink Reply by Lonny M. Kirk on August 25, 2016 at 1:19pm

Ken,

I have looked at retractable slats for a while now. At one time you could buy a Zenith 701 with the Pegazair wing with retractable slats. They are rare now and I tried to by a set 6 months ago but the wanted too much for me.

I would love to see a retractable slat retrofit for the plane.

Best of both worlds.

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