Wing slats for STOL

[dusty IV]

I'm making a model of a CH 701 STOL. You can see the real thing at www.zenithair.com

I have the fus and tail features done and that was easy. The slats in the wing are a bear. I'm trying to make a balsa covered foam D tube that will incorporate the slats. The CH 701 has a venturi shaped slat that is extremely hard to cut and even harder to cover with balsa. I could probally use fiber glass here however.

So I'm thinking , simplify. KISS. Rather then use the complex cuts the original calls out, just make a standard foam D tube and after it finished run the wing through the table saw to make the slat( slot) opening. The slat will then have internal flat sides instead of the airfoil shaped interior.

I wonder if this will function properly. Has anybody fooled around with slats and what was your experience?

[JetPlaneFlyer]

The wing should have a relatively ordinary rounded nose airfoil shape. The slat then just sits in front of this (the 'Slat' by the way is the bit that sits in front of the wing, not the 'slot'). A wing with a wedge shape leading edge as you appear to be suggesting is a bad idea.
I cant see any reason to try to build both the wing and slat in one piece. Why not just build a conventional 'slatless' wing then, after covering and finishing, add the slat. The slat could just be made from a strip of balsa carved and sanded to shape, or whatever is your building preference.

[vintage1]

You know I think the easiest way to do this would be to make the wing first. then cover it with 'clingfilm'..and lay up the slat over it with glass tape (or even CF cloth) and epoxy. Then sand the whole lot smooth before separating them.

If weight is an issue use one layer of glass/epoxy, then balsa, sand that lot down, then glass over the top..

[dusty IV]

Thanks for the replies. My poor verbiage didn't get across my question so I'll try again. This time with a pic. One pic is worth a thousand words thingy.

Looking at airfoil 701 note the curved inlet for the slat. This complicates fabrication. Next airfoil KISS. The inlet sides are changed to flat instead of curved. An entire wing could be fabricated then the leading edge cut off and then the section behind the leading edge. The wing pieces are then reassembled with the necessary gap.

My question is do you think using flat sides instead of the stock curved 701 would screw the air flow up to point it wouldn't work? I don't understand stand the theory of what going on here.

One more item, Airfoil # Droop shows a small section added to the front leading edge. A stock airfoil like a Clark Y has what I think is called a Phillips Entry. The 701 airfoil has deleted most of the Phillips. I assume since Zenith has worked this out this is the way to go.

My model has a 80" wing span and 15" chord. Electric of course.


I have never seen a model with slats for some reason. I don't know why it hasn't been used. I made a FF's years ago using slats. They had a big problem looping. Very unstable in the climb and glide because of the wide speed changes. I added slats in the front canard wing and it solved the problem. One model I made had a most unique climb. The nose rose sightly then the plane climbed flat like a Helicopter. A real crowd pleaser.

[JetPlaneFlyer]

Dusty,
I don’t think any of the details you have drawn are ideal... number one would work to a point but the other two would be no good at all.

I think you are approaching this from the wrong stance... Don’t think of it as a wing with a ‘slot’ cut through... Think of it as an ordinary wing with a turning vane (aka ‘slat) mounted on the front.

The wing section (minus slat) should be a relatively conventional (if slightly round nosed) wing section. This should be built just like any regular wing. The slat is than mounted on the front of it.

Something like this:
http://i3.photobucket.com/albums/y65/Rapier_HP/ARA-D13.jpg

The purpose of the slat is to turn the airflow around the leading edge of the wing at high angles of attack. This helps hold the air onto the upper surface of the wing and delays the stall. To work the airflow must flow smoothly through the gap in between the slat and the wing and be directed across the upper surface. The square edges of the second and third sketch would promote separation and probably would result in earlier stall than a conventional wing.

Read about slats here: http://www.zenithair.com/stolch801/design-wing.html

[John O'Sullivan]

If you were doing this experimentation on a 40" model I would go ahead with your KISS "quickslat" option.
However, an 80", 15" chord model is a relatively major undertaking. As such you should not compromise the whole project by imposing an untested simplification on it.
If you are comitted to the larger model, the project deserves no shortcuts - go with the proper slats.

I am considering slats on a variant of my Eyespy Photoplane and found the CH701 STOL article and the various links to be an inspiration.

I think you have a good thing going, but don't short-change yourself on the details.
John

Do a bit of research on potentially suitable model sized airfoil sections. The full size stuff is not always optimum for model sized airfoils.

[dusty IV]

Jet, My posted sketches didn't come out clear. They were scanned from templates that I made up for foam cutting. Sorry for that.

My airfoil labeled 701 is identical to yours. It just has lines connecting the wing with the slat. This came off the zenith site.

So we are in agreement here. I understand the turning vane aspect. What I don't understand is why it has to be in that curved shape. The airfoil is less then 2" thick at that point and flat vanes should work almost as well as the curved vanes as shown in airfoil #2 KISS. (Keep in simple stupid.) I hate to have to model this in a wind tunnel to find out. Did a lot of searching on the net and nada on this subject that I uncovered including Hintz.

Airfoil # 3 has the dropped leading edge which jets use except it's extreme. Their slat is movable mine is fixed because of KISS again.

I also changed the tail feathers out the 701. They used an inverted airfoil mounted half way up the rudder. To complex for my model. Conventional is lighter and stronger. See pic attached. Based on Sig prints, kinda.

So I want to move on this wing. I'm going to cut the foam blanks out tonight. I have 2" foam but need 3" so I have to split a 2" piece and 3M 77 it to a 2" piece of foam to give me 3". I vacuum bag it to make sure it's together.

Tomorrow I'll think about wire cutting. John says whoa, but what the hey, eh?

Where's Vintage when you need him?

[JetPlaneFlyer]

Dusty,
The air needs to be ducted through the 'venturi' formed between the wing leading edge and the slat than turned smoothly backward accross the upper surface of the wing. On your KISS design the air would exit the 'venturi' going straight up rather than backward, it would then have to do a 90 Deg turn round a sharp edged corner. The resultis that the airflow would become turbulent and seperate from the wings surface as it tried to get around the corner... The whole point of a slat is to keep the air attached to the wings upper surface.

Remember the bit behind the slat is the wing... Would you make a wing with a square/blunt leading edge? If you did would you expect it to fly very well?

Just cut an ordinary wing then make the slat from a seperate piece... You cant get much simpler than that.

[dusty IV]

Dusty,
The air needs to be ducted through the 'venturi' formed between the wing leading edge and the slat than turned smoothly backward across the upper surface of the wing. On your KISS design the air would exit the 'venturi' going straight up rather than backward, it would then have to do a 90 Deg turn round a sharp edged corner. The result is that the airflow would become turbulent and separate from the wings surface as it tried to get around the corner... The whole point of a slat is to keep the air attached to the wings upper surface.

Remember the bit behind the slat is the wing... Would you make a wing with a square/blunt leading edge? If you did would you expect it to fly very well?

Just cut an ordinary wing then make the slat from a separate piece... You cant get much simpler than that.

Good advice. I was thinking the same thing about the air exiting straight up last night. One of the big problems is the strength of the slat. This is on the leading edge of course and acting like a bumper. I, er, uh, run into things and I usually beef parts up that I'm liable to break. The ideal way to make the slat would be out of carbon. I'm trying to avoid this. Now I see why I have never, ever, seen a plane with slats or slots or what they are called.

The slat is very small and hard to make. I tried foam cutting but that's hopeless. I'm now thinking maybe I can make the vertical saw cuts and then add small balsa parts like a piece of molding to it. That could turn the air toward the back and would be easy to make.

Small planes like foamies use blunt or square edges all the time. I've read to avoid flutter on trailing edges don't round them off. I think Dr. Kamm wrote a paper on this. Never ran test though myself. Sometime I round off and sometimes I don't Never had a problem either way. A sharp leading edge will sure get you in trouble fast, however.

So I'm going to wait till tomorrow. When in doubt procrastinate and ponder.

[N74463]

Dusty,

There are a couple of build threads floating around RCGroups on the E*Star Wilga and Storch. I've not built either one, but IIRC both have slats. The wings are built up balsa with film covering. You might take a look and see how E*Star did it. Just a thought...

Joe

[Mister UHU]

I would have thought carbon strip would be excellent for KISS slats,
as would thin plywood, or you could try slicing a rounded balsa LE strip inside a sort-of "aerofoil" shape ?

As other have posted I don't think this is the best forum for these tips and tricks.
SEARCH or scanning other forums for earlier answers is probably the fastest way. :-)

[dusty IV]

Good idea , I'll poke around some of the other sites. Any ones in particular?


I like this site because it seems to have some mad scientist types that can get a real fast take on problems. Jet has been a big help, I kid you not. Thanks.

I'm having a real hard time visualizing what the air is doing. Guessing isn't worth much with air as it's not intuitive as to how it acts.

So I decided to run a crude wind tunnel test. I just cut a full size 15" chord , Clark Y, 12" wide section of the wing out of bead board. I'll have the tunnel made it in a hour or so. This is gonna be crude, man , but I think it will show be what's going on with smoke.

Now I got to figure out how to generate smoke , CHEAP. I looked up smoke candles but they are major money. Got any ideas? Wish I had some fire works left over.

[dusty IV]

I seem to be talking to myself on this but I found some more info. Searching the net I just couldn't come up with enough info to improve the learning curve so I decided to mock up a wind tunnel and do some smoke tests.

So I typed in "Wind Tunnel tests of fixed slot airfoils". A paper popped up from NACA dated 1937. Eureka, it has wind tunnel tests of a Clark Y with Maxwell leading edge slots and comments on the Handley Page design. Perfect! I got an order of magnitude gain on knowledge on this paper.

They tested two sizes of slots on the Maxwell. Turns out, if you believe the data, larger is better. Their Slot looks a lot like a Clark Y with a bit of undercamber on the bottom trailing edge. The test Chord was 10" with the slot airfoils 1.75 and 3" chord tested.

The wing actually looked a bit like my Kiss Design except it was laid back at a very steep angle.

So with my 15" chord wing I can use a 4" chord slat instead of the very small one modeled on the CH701 design. This allows me to build in enough mass where it won't get broken during accidental tip strikes.

The 701 has a bit more lift and a more sophisticated design but I can accept the trade off.

The next thing to work out is the main wing's leading edge. The Maxwell doesn't use a Phillips entry. I think I will modify it so it does like the 701. The Maxwell also doesn't use a droop like the 701 on the leading edge of the slot. I think I will move this around a bit also.

Change subject

While searching on the net for more info last night I ran across a paper by NASA on "discontinous droop" wings to prevent spins from the 70's. They added on the wing ends an airfoil extension so the chord is wider at the tips and has a slight droop. I see Hobbico Nexstar trainer and some others have picked this up and have a clip on airfoil extender for new flyers to avoid spins.

I have never flown one so have no idea if it works, but it sounds like a good idea since we know that tapered tips tend to stall faster then wide tips.

[Bill Harris]

The standard reference for STOL flight is Andy Lennon's Basics of Model Aircraft Design ISBN 0-911295-40-2.

--Bill