Adding fixed slats to wing #2

[Bill Harris]

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.

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, please...

Other discussions in this forum on Slats include:


http://www.rcgroups.com/forums/showthread.php?t=622119

http://www.rcgroups.com/forums/showthread.php?t=449664

http://www.rcgroups.com/forums/showthread.php?t=637654



--Bill

[Brandano]

Why not scratch build a new wing instead? That way you get the safe slat-less wing for flying, and a scale wing for show that might even be used for flying if the thing works out. I am not sure if slats are really beneficial at small scales, but birds use them so there must be something to them at lower speeds.

[Bill Harris]

The R&D saga continues...

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.

Now to find someone in north Alabama (B'ham-Cullman-Huntsville-Tuscaloosa) with a foam cutter...

--Bill