I am building a 1/4 scale PZL Wilga kitted by Alexander Frisch. The wings are obeche-sheeted EPS foam with balsa LE and TE caps and a 35mm aluminum tube spar/wing joiner. Wingspan is 108", each wing half being 48" in length.

http://www.frisch.flugmodellbau.de/wilga1engl.htm

The full-scale Wilga has fixed LE slats on the wing but this kit does not have slats. One question that I have wrestled with from Day One was whether or not to add the fixed slats to my build. Although functionality would be a plus, I am adding them for scale appearance and would be more concerned about a negative effect; ie, causing adverse changes in the aerodymanics of the wing or weakening the structure.

I have carefully examined the profiles of the model and full-scale wing and find that overall shape of the model winf is very close to that of the full-scale wing plus slats. In other words, the profile is close to the same, but the model wing doesn't have the "slots" cut.

It would be easy to add the fixed slats to the model wing. Attached is a drawing based on a traced outline of the modeled wing. I think that I can carefully cut the obeche sheeting and with a hot-wire cutter cut the "rear edge" of the slat (with the LE balsa cap previously attached), and then secondly cut the new "nose" of the wing panel. I would make root-and-tip templates for each operation. After cutting, I would add proper reinforcements and obeche (or balsa) sheeting to the cut areas of the new slats and wing panels, along with ply standoffs to mount the slats to the wing.

Scanning and resizing a section of the wing profile from a 1967 PZL 3-view of the Wilga 104, I compared the full-scale slats to my proposed modification.

There are substantial differences. However, I think that it will still work well at the model-scale. The "nose" of my wing modification corresponds in shape and location to the fuselage profile at the wing root, so this shape is a "given". I also suspect that Frisch worked on scale slats on his Wilga design, so I'll agree with his hint.

The other difference is the shape of the slat and the slot. Going back over mt notes, I derived my slat shape and location based on Andy Lennon's work in Basics of Model Aircraft Design ISBN 0-911295-40-2, which ought to be appropriate for use at our model scale.

Given that these wings would be replaceable only with great difficulty and greater expense, I need to consider if adding the slats would reasonably do-able and not create problems. I don't know if the plane will ever do "Scale Masters", but I'd like to decide before too long.

Feedback, anyone?

--Bill

I foresee no structural problems, things to note when flying; it will have more drag, but then the real thing is hardly a Spitfire, to effectively use them you will need plenty of elevator to keep the AOA up when flying slow, lots of rudder too, as you can't use aileron to pick up a wing at the speeds it will be capable of. And an engine capable of turning a large prop of reasonably fine pitch, to be an efficient match to the speed profile of the model. You may find that some sort of flap arrangement might be useful too.
Evan, WB #12.

Yep, the Wilga can do all that. It will use a Brison 4.8 Twin, so she ain't lacking on power. It has huge rudder and elevator, and yes, flaps.

--Bill

At the larger sizes like this model I can't imagine that you'd get anything but pluses. And you've got the right idea for the cuts. Although you may find that it's easier to skin the rear of the slat with glass cloth and epoxy instead of veneer. And if you can rig up a way to slit the skin and then templates to hotwire the front off then you could use mylar and foam to push it back against the wing and keep it lined up while the back skin cures.

I think it'll work out well, Bruce. Lots of figuring out the details of exactly how I'm going to do things. I agree with your idea about 'glasing the rear of the slat and pressing it against the "first-cut nose" of the wing while the epoxy cures. I also need to think about how I'm going to approach mounting the slat to the wing (it'll mimic the stock mounting) and what sort of reinforcement I'll need at the attachment points. I've read some Storch build logs sp I can more or less use what worked there.

I need to ask around and find an experienced foam-cutter in my area to do the cutting; I could build a hot-wire cutter and do it myself, but I'd be hesitant to learn on this wing (although I do plan to buld a foam cutter some day).

--Bill

I may be able to help here as I have flown Wilga's with and without slats. With that amount of power the added drag of slats will not be noticable IMO. The Wilga lands very slowly without slats & uncannily slowly with them. There is a slat (inverted) on the tailplane if memory serves me correctly, and the wing slat extends across the top of the windscreen. The model I flew with slats appeared to have an unmodified wing with the slats just added on.
Regards,

John

Thanks for the feedback, John. I think that the wing slats will work well and be a good scale detail. I've already got the inverted tailplane slat worked out, it shouldn't be that much of a job to add it.

--Bill