Wing Surface Finish

[Strawbalebob]

Assuming a wing with fully-sheeted D-tube construction and non-laminar section, which has lower drag and/or "better performance"? 1) fully covered with plastic film or 2) covered only over the open bays so that the D-tube sheeting is exposed (sealed with polyurethane and sanded with 220 grit).

Considering that a dimpled golf ball flies farther than a smooth one, and that turbulator strips trip the boundary layer and help keep it attached farther back on the wing, I'd think that option 2 is lower drag. Anyone have data or experience?

If you really want to know, the plane is a 2m Spirit with full D-tube sheeting and a few other mods. Finished it last night and made the first few test flights.

Thanks.

[Red Baron]

I'v never flown a spirit, but by looking at pics of one id guess it's quite a slow flying plane anyway, so for ease covering the whole wing in film may be the better bet, as I don't think (open to critisism) that it would make any noticable difference.

Even so, the aerodynamics is interesting (I'm not a great knowledge on the subject so any1 feel free to tell me what iv said wrong!).

As far as I know, a turbulator strip works by have a lower pressure behind the strip, causing a vortex to "roll" over the wing. By adding the covering to the rib bays, there is no area of lower pressure, so there isn't the rolling vorticy effect, so I don't think it would cause the effect you mention. A few layers of covering on the D-box may, as there will be an area of lower pressure immediately behind the D-box. I personally wouldn't bother, it could go the wrong way and cause the boundary layer to seperate from the wing altogether!!

If I was building it, I'd cover the whole wing with film. It's easier and less messy.

Whatever you do, enjoy flying it!! :D
Conor

[Robglover]

It's a Spirit, drag is no big deal. In any case, the wind tunnel studies that have attempted to prove the smooth versus rough finish drag reduction have not ever shown conclusive results, so far as I know. I'd cover it with Monokote and park it in a thermal.

happy trails - Rob Glover

[xtc]

It's a Spirit, drag is no big deal. In any case, the wind tunnel studies that have attempted to prove the smooth versus rough finish drag reduction have not ever shown conclusive results, so far as I know. I'd cover it with Monokote and park it in a thermal.

happy trails - Rob Glover


i gotta totally agree with rob`s comments ,the spirit does not perform at high enough airspeeds to fully take advandage of laminar flow and i doubt that you will ever notice a differance.

thermalling a glider that is designed mostly for sink rate does not require a very advanced airfoil ,although the increase in wing load for sheeting etc will effect the sink rate,,long story short,;;;for the type of flying that the spirit was designed for,its hard to inprove on

xtc

[Strawbalebob]

I guess I meant "tape strip" rather than turbulator strip. I understand your description, RB, and that makes sense. But you are correct that it doesn't really apply to a sheeted D-tube.

I got it flying last night, with only the rib bays covered, and it seems to fly really quite nicely! This is the first plane I've had with spoilers, and they seem to be pretty effective (and fun!). I'm surprised by the amount of up-elevator needed to compensate, and I think I need more elevator authority. I'll either increase the throw or increase the elevator chord (not stab, just elevator). I increased the vertical tail area quite a bit, keeping in line with mods recommended on other websites.

I'm also pretty concerned about the longevity of the stab - I have to land in DEEP weeds, and I don't think the stab will be there for long. I could switch to V-tail, but I'm a big fan of T-tails. Maybe I'll build one up and have it ready to install when the stab gets torn off.

Anyway, I'll keep the D-tube uncovered - saves weight and money, and I like the look anyway. If there is conclusive evidence one way or another I'd like to know, not just for this plane but for future projects.

[schrederman]

Generally, non-laminar sections are "turbulent flow" sections and do not depend on smooth airflow attachment like laminar sections. They turbulate the airflow on their own and though this flow is attached, it isn't smooth. Separation is actually achieved only in stall condition. Laminar flow sections sometimes use a turbulator in the aft areas to turbulate the flow instead of having a separation bubble, which means it's still laminar, but detached from the wing surface. Both situations create more drag than laminar flow all the way back, but the separation bubble is the greater of the two evils, hence the use of the turbulator at 70% or so. On your Spirit, I'd be surprised if anything other than a good Monokote job would make any difference. I'll be smearing oil on my wings soon to find out where the turbulator needs to go... better yet, I'l go back a few years in my Soaring Magazines to find the flight test and see where they recommend to put the zig-zag tape...

Get High and Stay High...on LIFE!!

Jack Womack

[Strawbalebob]

Alright, I guess it doesn't matter so much for this design. Based on the arguements above and my other data point: a 2-hour flight on Saturday.

Much of the flight was at speck-height - I was just steering a dot left and right across the bottoms of the clouds! I lost it once for about about 20-30 seconds and managed to find it while spiral-diving to get a glint off the covering. Lift was so good that I had to thermal with the spoilers out just to keep from going up too bloomin' fast. What a day!!

I've only been flying for a couple of months. I've got a Lil' Bird hand launch and this modified Spirit. Both fly great for me, and I'm having a blast. I think the next step is flaps - might just modify the Spirit wing for that. Maybe not the most efficient wing for such a feature, but it flies very well for my skill level and I'm sure it'll keep me entertained nicely for the rest of this year's flying season.

I D-tube sheeted the entire leading edge, and can turn the thing on a dime without tip stalling. Yeah, it added weight, but I think it was worth it!

[bigjohn]

The only way to figure out if turbulating the glider would help is by first taking a look at the polars for the airfoil at various Re. If the drag balloons along a increased alpha segment of the curve it often indicates the laminar bubble. If the curves don't show this, the turbulation does no good.

The next step would be to try the tape (or whatever). Monokote is not thick enough to be a turbulator. You need a heavy tape or multiple layers to make a trip. I use Dymo tape - the stuff you run through a little labelling machine that prints letters into it by turning a dial and squeezing a trigger. If you still don't know what I'm talking about, google "dymo". Anyway, that tape is .015-.018 thick(if I remember right) and is available in multiple colors.

Anyway, I had results with it on an RG-14 airfoil on a scale B-4 glider. I had done the figuring and wanted to test the effect. I put the tape only down one wing. I did multiple throw glides. Repeatedly, as the angle of attack increased, the plane would yaw with the taped (lower drag) wing swinging forward slightly. Once I was satisfied it was a real effect, I taped the other wing.