Wing sections and structures

[Dereck]

Okay folks. On one hand, my beloved CAP - D box symetrical wing, film hinged ailerons that match the wing section.

OTOH - what could loosely be described as a "Got-At Four Star 40". This has what I think might be Clark Y - flat on the bottom aft of the mainspars, with turbulator spars between LE and mainspar - hence the section is mostly a bunch of flat areas of Monokote. Hanging on the back of this are ailerons whose only virtue is being film hinged - they are flat plate 1/4" sheet balsa with sharp square edges.

Power? The CAP is highly powered, the Four Star even more so! Let's say 'adequate', as Rolls Royce describe the power of their car engines.

I've given up on why the 4*40 out-knife edges the CAP, apart from the odd things I've done to her over the years. Lowered, larger tailplane, bigger rudder, shorter wing, minor odd changes like that.

What baffles me is how the 4*40 acts like it has a completely symmetrical wing when everything is telling me it's closer to a Kadet with the wing on the bottom.

I've done so much to this model that the only resemblance to the kit is the wing section. It's getting a little time-worn, so I'm going to roll all the 'improvements' into a new one for next year, and am wondering if I should build it with my usual beloved D box wing with nicely matched ailerons etc and call it my design - or admit defeat and copy what I have now. A copy would be lighter and better looking, and incorporate all the minor internal details I've figured out over the last four years of flying her.

Any comments welcome - opinions, hard sums from folk who can spell "aerodynamics" (I have trouble!), experience, WHY?

Please,

Pretty please?

Regards

Dereck

PS - to those who saw her fly at RAMS, you should see what 16 cells does to the performance, over the 14 I flew her on there :eek:

[dtroyer]

I think the airfoil has less camber than a Clark Y, but it idoes fly inverted very well. Seems like mine needs more down elevator than a full symetrical wing would. I think tapering the aileron stock might slow it down around neutral.

[Essex boy]

Hi Dereck, If it is a true Clark Y then it is far from a 'flat bottom' section just happens to be easy to built because it is flat after about 30% chord. If you draw the chord line in you will see what I mean. As you have found out it does a good immitation of a symmetrical section. The other thing you mention is the use of turbulator spars from my observation at the speeds we fly they seem to produce a very effective wing. Might not be what the book says what but what the hell its real life that matters. Like wise the square edge to control surfaces seems to give a crisper response and cut out flutter. Why, don't know, but it seems to work. Just do your own thing, look at all the models around with 1/4 sheet wings, they are not suposed to work either. Perhaps when 'they' work out how a bee can fly, we might gets some explanations to the things we do and know work.

Cheers

Andrew.

[Looooeeee!]

Dereck,

Howzabouts the Eppler 205 that Sig uses on their venerable Riser 100? I realize that this section is from snoarerland, but my flying buddy has a Riser 100 that gets some pretty awesome wringing out when the our inland slope is running warm.

Have you ever seen a 100" WS poly gasbag do inverted speed runs? Well they're not all that fast, (or long) but his ship makes my Oly II look narcoleptic

Anyways the E205 is still somewhat of a semisymetrical foil, pretty close to the Clark Y in cross section. I think that it just starts the flat part on the bottom around the 35% point.

Looee

[Dereck]

Hi Andrew - yes, understand about the Clark Y thing. Anyway, with the five spar wing, the nice rounded bit in front of the mainspar is actually a collection of flat plates mostly, so the little curve of a true Y's bottom going missing is unlikely to affect matters one way or t'other.

Your comment about turbulator spars - am forming an idea to inflict on my present 10 cell ship (D box wing, Sorta Eppler 205, see below). Basically, eyeball where a spar would be if it had a Four Star 40 wing and stick a couple of thicknesses of sticky tape there, about 1/8" wide. Then go fly and see wotinell happens after she takes off. Sounds easy, if it messes things up, I land and remove them.

E205 - I think is very close to the Aveox Embat wing section, given that it too developed a flat bottom in the usual way of such things. If not Mr E's 205, it's some kind of "similar to a Clark Y" Eppler .

The Embat has even caused people to suggest that it's short winged for a snoarerplane, but were soon disabused of this idea after launch ;). Its section is a tad more pointy at the front than "Clark Y" and its innumerable close cousins possibly.

This is more on the lines of being infuriating. I could admit defeat of course, and live with it. After all, I don't think that much about my MaxCim BL motors beyond that they propel these models almost obscenely well - they do, great, why buy anything more complicated?

So why am I fretting over a wing section that I know works!

Heck, I feel like I need to understand something every now and again...

Regards

Dereck

[Neil Stainton]

Dereck could you post a picture of the wing section? I had 4*40 kit but unfortunatley sold it - I've been regretting it ever since.

From my memory it was closer to being symetrical than Clark Y. I think the two questions are:

Q1) Does slight camber always hurt aerobatic performance.

Q2) Are the "turbulator" spars actually helping, or would it fly just as well with a conventional D box.

My uninformed guess at the answers are:

A1) No camber doesn't always hurt. It will cause slighty lower max lift when inverted (bigger bunts), and slightly more when upright (better low speed handling). I don't think the fact that the max lift in both directions is different matters during aerobatics. Pilots have to correct constantly anyway when flying aeros because of gravity, and a bigger down max elevator deflection would minimuse the difference.

A2) No I don't think the forward spars are helping simply because if that was the case for a 4*40 it would apply for all similar models and would have passed into common knowlege by now.

Regards,

Neil.