I have made it a point to examine just about every model of GA aircraft with slats on them, and read up as much as I can digest on the design and placement of slats. Conclusion for the CH 801: Unless doing STOL competition, take them off and install VGS. The fixed slats on the CH801 only reduced stall by 1.5 knots compared to without (but with VGs installed)and I picked up a whopping big 12 KTAS in cruise at same power setting and added about 200 fpm ROC. I have done this on both of my 801s with the same results. And you get a gentler stall.

Fixed slats are by their nature a compromise. The 801 slat bottom edge is about flush with the bottom edged of the wing instead of about 1 to 1.5 inches below the bottom edge of the wing as they are on all the retractable ones I have looked at and as all the design literature indicates. Thus you really don't get the radical STOL improvement that you could be getting from properly deployed slats. And you get a lot of parasitic drag in cruise. Also the retracted slat designs I have seen present a sharper, lower drag leading edge profile. They cruise faster with retracted slats than the same wing with no slats.

On my second 801, the slats were rigged just a fraction low. STOL performance noticeably better than in factory spec location. BUT, the drag in cruise was noticeably higher, and as you approached cruising speed it got unstable in pitch, wanting to "tuck" the nose and was very vague on the stick (like a plane flying too far aft of CG limit). Decidedly not the way to fly it.

This summer I plan to re-mount my slats in several locations from full back and tight against the leading edge and sealed, to forward and down to 1" or more below the bottom surface of the wing. I will share the flight test data. If it works as expected the challenge is going to be to come up with the simplest way possible to modify the stock slats to retract and deploy. Not going to rest until the 801 approaches the performance of the SuperSTOL. It has the potential to be really great not just good.

Comments and suggestions?

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I am all for seeing the results!!! Keep us posted.

looking forward to hearing the results too.

Great project. I can't wait to see the data.
I took my stock 801 with no strut covers, carbureted, low compression piston (170 hp) O360 that cruised at 82 knots at 75% and about 8.75 to 9 gph (high flow to keep CHTs down). Did the following and cruised at 101 kTAS at 65% power LOP at 7.5 gph. At 4 to 6 k DA. Please note that the individual. mod don't all add up one on top of the other because increased speed effect. But th total gain is dead nuts accurate carefully verified and compensated for load and DA using calibrated AS with DA compenstion to read TAS and cross verified by flying triangular timed route with GPS cross checks too.

-Added one PMag ignition unit. Allowed me to run the carbureted engine at LOP after oil is up to temp. It lowered CHT by 50+ 'F made starting even in the cold sooo easy. (No primer)A wonderful mod and you only need to replace 1 mag to get 90% of th benefit.

-Installed wing strut covers. Mine were my own design. Way Cheaper than factory kit, easily removable for repair or inspection. My design had longer cord and sharper leading edge than factory. Don't forget to fair the ends of the struts where they meet the wing. The factory fairing is like a barn door in the wind. Adds around 10 knots and 200 fpm ROC at +5 knots faster VY. Easily the single best thing beside removing slats that you can do for cruise.

-Added the small fairings at the root of the H Stabilier and empennage. Be careful of how you do it or they become air brakes rather than adding about a knot and much improving elevator authority. Very easy, simple to make weigh about 2 ounces for the pair including the rivnuts and screws. Added VGs on elevator the same time but VGs have no effect on cruise, but really helps get th nose wheel up early and hold it up longer on landing. Good for safety on rough fields.

- Instlled a simple strut cover on the nose gear strut. Like the wing struts, easily removable, extends from top of fork to just below the strut stop ring. But with my cowl that put the unfaired part behind the "turbulence strip" on the front of my cowl exit opening. So the entire unfaired strut tube is out of the air flow. Adds 2.5 knots because the prop air flow is both straighted some and is in the highest velocity air flow on the plane. Anything you do to reduce nose strut and front wheel drag profile produces huge results. Again simple to make adds about 2 ounces. (Thanks to tests done years ago by Cessna Pilots Association).

-Removed the slats and put on VGs, 12 KTAS gain (strut fairings were off at the time) and only increased stall by 1.5 KTAS. Gentler stall. Slip if you want a steep descent. Biggest single thing you can do to improve speed, improve stability, and make more fly able. Ok so I gave up not quite 50 feet in landing distance. Picked up another 150 fpm ROC at 2 knots faster VY. Still under 300 feet total.over 50 ft obstacle. Started to study on how to make slats retractable.

- Here is the big surprise! I added a square cross section belly pod 16" deep and as wide as the aircraft lower fuselage perimeter. Pretty blunt with only slightly rounded corners. Guess what? despite adding over 4 square feet of frontal area the darn thing ADDS 2.5 KTAS to cruise. Took pictures, did some reading on drug effects between surfaces (thank NASCAR). Two things happening. (Small effect)- when main gear legs flex down in flight they "hug" the pod and air goes around the whole mess including the clunky strut to MLG attach bracket reducing drag. But the huge gain is moving air around the bottom of the MLG cross over section than trying to squeeze it through that gap. The drag caused by the air trying to channel through the gap and turn the corner to rejoin the upturn of the fuselage behind it is more than if you had a flat plate across the belly and as wide as the distance between the bottom of th cross member and the fuselage. Think spoilers on a glider. The pod was huge, big enough for my 250 lb son to crawl up inside it to hold the attach bolts. Despite this and added weight cruise increased!

I think that a very shallow pod or false belly about 6 inches deep, which would be real handy for tow bars, the doors when flying without them, fuel bladders and camping gear, with a much shallower taper at thefront (mine was sharper than 45 degrees) would conceivably add another 5+ knots and add utility to th plane. Or a simple tapered fairing could do more.

I will be building a new pod for this 801 as the last one got crunched when N801G was destroyed in an accident (upgrade that front gear bungee!). It will be bit more rounded and only 12" deep. Just enough for duffle bags, camping gear and 4 removable ferry tanks I have from my old 801.

-I have seen partial wheel fairings for Cubs that are a closed teardrop shape that just fits snugly behind the tires. That's a later project. Keep in mind that for parasitic drag all of the drag is in how the air leaves the object not how it impacts the object.

So 101 KTAS at 65% power with a 170 Hp O360 and 350fpm ROC improvement at 7 knots higher VY WITH a honking huge 78 lb (empty)belly pod on it. It used to be that when you cut power all that parasitic drag caused so much deceleration that it pushed you forward against the shoulder harness. Now when you chop power it glides with a reasonable sink rate, like an airplane instead of a brick. Good for safety as it improves your emergency landing options due to better glide ratio. With the humongous rudder it has you can slip it in just as tight as with the slats on if you need to. And without slats and with VGs you don't have to drag it in on the prop and when it does stall the whole bottom doesn't drop out.

But removing the slats also removes some serious cool factor.
One thing I forgot. The 750 and 701 owners that have installed the "beanie mod" (airfoil shaped cabin roof instead of flat) report better low speed handling and reduced stall speeds and small increase in cruise. Look at later 750 Cruisers and you will see that they quietly slipped that into the design. better for headroom too. That is also on the list of future mods to check out. All good reason to keep it all raw aluminum for now.


Airfoil covers on the vertical mid strut braces even as small as they are would probably net around a knot. If I had the extra cash I would toss my entire struts and build them from airfoil tubing. But well over $1000 in just materials and a lot of work. My strut covers cost under $200 including some sheet metal work as I did not have acces to a wide brake at that time. And except for screws I had enough material left over to make another set.

All my extra bucks for a while going into the all glass IFR panel. Ever fly an ILS at 45 knots with no doors? Reach out and touch the clouds.

Any images/dimensions of the strut fairings and the horizontal stabiliser fairings?

Dan, 

Agreed, a remodel similar to the 750 cruiser (think 850 cruiser) would attract a whole new crowd of purchasers as they would be getting up into C182 territory if done correctly.

I made up a set of instructions tonight for you and others. Sorry no pictures from the wrecked 801 exist of this detail.  The stabilizer fairings are knock offs of the ones shown in the 2  after market mod videos, except that I made mine out of a flat sheet of aluminum that did not require any tools more sophisticated than a pair of snips. All the curves are made by bend and twist from a flat piece of metal. Don't have a set on my new one yet.  Will document and share when I get them on. First priority is to get new panel in and go flying.

Attachments:
Mhmmmm data.

Since the title is retractable slats, what other aircraft have had retractable slats?

  • Fieseler Storch
  • Helio Courier
  • Tapanee Pegazair
  • Zenith CH-701 with Pegazair wing option

I like the Just Super STOL arrangement – or at least I find it interesting…  The Peg retraction system is also easy to understand…  Will be interested to see how your experiments turn out… 

Attachments:

Now this is really great practical design information. It seems to support hypothesis that best low end lift is with the slat forward and below the bottom surface.  I looked at the SuperSTOL design in person at the factory and agree it is the simplest.

Some design objectives are to:

1) If possible not change the existing slat cross section ( I don't want to build entirely new slats) but small add on extensions to trailing edge, or build up to the nose is OK.

2) Simple and robust. I don't want it breaking or hanging up.

3)  I don't mind cutting into the slat to make mounting modifications, but I definitely do not want to be cutting up my leading edge. So limited to using the existing attach brackets in some modified form. I don't want my aircraft down for weeks resurfacing the leading edge.

4) Add on profile changes (as in a mini-cuff) to change the nose profile are ok, as I would make them removable to facilitate experiment.

 I  recognize that it might compromise the results a little. But if I can get most of the benefit of a properly deployed slat and not decrease cruise. I will call that a success.

Thanks very much for contributing this.

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