Frontal area vs wet area on LE

[DaveGherardini]

Ive read some where that a wing that has a D cell LE is better than a wing that has no sheeting to keep the covering from saging between the ribs on the forward section. Ive built a large motor glider(110" ws) without sheeting on the LE to conserve weight. The covering sags between the ribs wich reduces the frontal area alot considering the total length of the wing. My thinking was to have a wing that cuts thru the wind better so i can fly better on a winy day. But the total surface area(wet area?) is greater than if i used a Dcell style LE. I would like some input on this if anyone is interested. I guess im confused wheather its better or more effeicent to use a Dcell because the wing cross section gets smaller where it sags thus producing less lift. Thanks for any input on the subject

Dave G

[DaveGherardini]

top view

[Ollie]

The problem with covering sag over the front of an airfoil is that the mean line camber is higher at the rib and lower between ribs. The difference in mean line camber causes differences is coefficient of lift and in turn, differences in lift distribution along the wing. When the lift distribution isn't smooth and elliptical the induced drag due to vortices increases. This vortex drag is a much bigger effect than drag due to skin friction or frontal area. The spanwise flow hits the bump in spanwise contour caused by the ribs and sag. If this bump is too big it can result in premature seperation of the flow, early stall and generally greater profile drag. Full scale planes in the thirties and models, before leading edge sheeting became popular, tried to minimize the problem by putting one or two subribs between the full length ribs to reduce the sag and minimize the bad effects of the sag. The closer the rib spacing the smaller the problem with sag. The smoother the wing chordwise and spanwise the better the wing.

BTW, you can get better wind penetration by selecting a thinner airfoil to start with and fully sheeting the wing. You will add weight but there is a trade off between minimum sinking speed due to low weight and good wind penetration with a low drag (smooth) wing. Trying to avoid this trade off just leads to worse performance.

[DaveGherardini]

Hello Ollie, Thanks for your reply. That actually does make some sense to me. One other thing i wondered was that will it create the same effect that polydihedral does in a wing. That is to stabilize a wing better so you can turn with out aleroins. That is not exactly the same effect but creating different angels of lift betweent the ribs might cause weird stuff to happen. And as you have said that it messes with the vortices at the te. Or makes bad vortices at the te. Even possiby curving the boundry layer and plum breaking it up. I can see now in a turn how the flow would start to cross the the ribs and agin possibly breakup the boundry layer.( that would be bad as you have pointed out) Im still studying the vortice thing and am learning more buy the day. I definately apreiciate your advice on this one. . I hope these questions are not to silly. Thanks agin!
Dave G

This plane does fly nicely, very stable. AUW is 48 oz with 6 cell 3000 Nimh pack. (very lite for the size)But im working up a bigger one and thought i would seek advice on the issue this time.

[Sparky Paul]

Another method other than full sheeting is to add intermediate spars between the leading edge and the high point spar. This reduces the sag, and adds a bit of stiffness.
If you're a fan of building, and can fly well enough to evaluate such changes, it can be educational to try the added spars, and also the full-sheeted D-tube wings.
You'll find something that works for you, which is what counts.
You know what your goals are better than anyone.

[Salto]

Dave,

As well as the aerodynamic considerations already mentioned, there is an important structural one. The D tube wing with top and bottom LE sheeting bonded to the spar is considerably more torsionally stiff than the open design you have pictured. This is particularly important for a glider wing where span to thickness ratios are typically much higher than a powered plane. The spar in the wing in your photo does not appear to have much chord-wise thichness so it will not be contributing much to the torsional stiffness, leaving only the covering to do this job. Therefore, for your larger model I would suggest using LE sheeting.

In practice, the added torsional stiffness will better resist wing flutter which can easily occur in a dive when you are trying to get out of that monster thermal. Also, the whole wing will perform aerodynamically better if it is not twisting all over the place.

To get a gut feeling for how closed tubes contribute to torsional stiffness, try getting a cardboard tube and twisting it from each end. Now make a length-wise cut through the tube and note how it has lost most of its torsional stiffness. A wing design like the one in your photo may well have LE, spar, sub spars, and TE, but if none of these components are connected by shear resisting structures (like the skins of a D tube) they contribute little to torsional stiffness.

Another consideration; the material in the top and bottom sheeting of a D-tube structure is in a good position to contribute to spanwise bending strength and stiffness, so you could reduce the size and weight of material in your spar, and end up not adding too much extra weight.

Having said all the above, I have recently finished a motor glider myself with open wing design (no D-tube sheeting) and sub ribs along the LE. This wing is very torsionally stiff because it has a carbon fibre/end grain balsa built-up spar with + and - 45 deg. woven glass shear webbing. I would like to attach a jpg of this, if someone can tell me how from my iMac. I tried, but got a message saying the file size was too big.

Graham.

[Sparky Paul]

I've found that d-tubing the trailing edge as well makes a VERY stiff wing, nearly impossible to distort.
Example:
http://www.angelfire.com/indie/aerostuff/dtubes.htm

[DaveGherardini]

Ha guys, I completly understand the structure issue. This particular wing was built in the lightest possible config so that i could keep the auw under 50oz. Its plenty strong for the application. I do thank you for your structural advice and i understand it completely. Its obiously not as strong as d-tube or if it had stringers. The idea was to lower the mean rib height to make a better penetrator. The rib is real close to the Gental Lady or Electra. I ve built both of theses planes and enjoy them to. The rib is pretty thin on these designs and the mean rib height on this bird is lower. (understanding it alot longer to though). THe airflow issue is realy what im interested in.
Thanks for all the good advice.

Dave G

[Sparky Paul]

The airfoil on the GL or Electra isn't a really good one for any amount of speed performance. It's a very good floater, sink rate is quite low in dead air.
The Selig 3021 is a better choice if you want more speed at the high end.

[Salto]

Dave,

But I suggest you could build it even lighter by optimising the structure, which means getting all the materials towards the outside surface of the wing and connecting them together with shear resisting bits. Maybe I am on to something here..... if you used a lightweight material, and shaped it to an airfoil.... and skinned it with something very stiff and strong.... ????

For reference, my motor glider is 1.5 meter span (60 inches), and weighs 20 oz. Here is the break-up, sorry about the units:-

All weights are for finished and painted/covered items ready to fly:
Wing 157g
7x760mAh NiMH 96g
V tails x 2 8g
GWS 280 motor, gbox, prop, spinner 88g
ESC 5g
Hardware/wire 10g
2 x HS55 servos 16g
Receiver without case 17g
Fuse, boom, glass,epoxy, paint etc. 148g

Total flying weight 545g
Airfoil used; Selig 3021
Covered with Solarfilm Lite


Let us know how you get on with the larger one.

Graham.

[Salto]

I'll try the photo again.

Graham.

[DaveGherardini]

Well i finally got an opertunity to test fly this scalloped wing. And it was a pleasure. The hand launch was easy. The wing was like i expected, very fast in dive and would float nicely and didnt mind the moderate wind(5-10). Winds calmed after an hour and it was very relaxing. I was suprised at the climb rate with the little S400 motor. Plenty of power. The plane makes a different sound when flying than my other sheeted le glider. Must be the vorticies. Il try to post a pic so you guys can see the experimental plane upclose

Dave G

[DaveGherardini]

After studying up a little i found that the wet area coef is much less efected buy the small amount of extra skin in the droupy area between the ribs. But the Frontal area differenc makes a big difference in total flat plate area(did i get that rite). Now the vortice issue still has me puzzled. Trying to measure or find the little devels has become very difficult.




I definatly learned alot and enjoyed the process. Now im having a blast flying this big bird.


Dave G

[DaveGherardini]

Ha Graham,

Nice plane. very sleek. Thanks for the info to. Vtails...
Do you prefer them or just like to build them to? That is an impressively lite setup for that big of a plane.

[Salto]

Dave,

Nice plane. From your post, I gather you designed it yourself. What airfoil did you use? The 48oz weight seems good for that size plane. What battery pack is in there? Is there any sign of wing flutter at high speed?

The scalloping appears very mild, and at that minimal level I wouldn't be too concerned about the distortions to the airfoil. My guess is the performance would be somewhere between that of your "full" airfoil and one of the same family slightly thinner. The original choice of airfoil is usually a bit of a guess anyway.

Thanks for your comments on my plane. I do prefer V tails for this sort of model, the connection to the tail boom is real tidy and easy to make strong, and the tails are up out of the way for those inevitable landings in the rough. But I have to admit this plane has a V tail primarily because it is roughly based on another model "Lowatt". See http://www.klneumann.com.au/

I was happy with the weight, up until I painted it which added about 40 - 50 grams. I first flew it bare at 500 grams, which was much better. Oh well, some lessons are never learned.

Graham.

[DaveGherardini]

Thanks Graham. Yes i built it from scratch. I have gave it a name now since it fly so well. The air foil is kinda like a elongated CY. its 10.25" x .9". thats about 28cm x 23mm. 113" (287cm) tip to tip. cut from 3/32 sheet. The inside panels are 48" and the outside are 18". total lg is 60" motor is promax cobalt 4011 with a psru of 2.5:1 swinging a 10 x 6 graup folder. Drawing 18 amp with 6 cell 3000Nmh car battery (Racers Edge). Ihave ordered a 12x 6 since the motor will handle 25amps. I used every idea possible to keep the weight down but still using all wood. Struts keep wing from bending and so far after 3 filghts no sign of wierd resonance or flutter at all. I havnt pushed it yet but so far everything is as planned and i will try a loop next time out.

I will have to say that the wing is a beauty. There someting about scallops that give it a classical look.

Ive always wanted to try and build a vtail ship. I curiuos how you determined what angle for the fins. Is there a formula? Or have you used you experiance to improve them. Thats the next thing to making a lighter fuse. Im planing to build a bigger ship. Around 200" ws. Im considering vtail to save weight.

Heres a pic of my ugly mug with the plane. Its hard to see the big grin on my face. You can see the scallops a little better. It sure have a classic look

[DaveGherardini]

Heres a pic from the yellow page.
http://www.mh-aerotools.de/airfoils/index.htm

Its explaining the pros and cons of covering droop.

[DaveGherardini]

That example pic shows the change. But its a sheeted le wing. So i would think that on a scaloped wing(like mine) That the differential would be more.. Thus making the lower end of the envelope better. Now that conclusion is from what the yellow page advice would be. I still believe from study that scallops widen the flight envelope, but i only have 2 cents. Heres another pic from the yellow page.