Why gullwings?

[LedKitty]

Hi gang!

Well, I don't visit this portion of RCGroups much, but as I've been getting closer to designing my own aircraft, the questions in my mind have been pushing me in this direction ;) That and the rust covering the knowledge I gained during my aircraft maintenance days needs some clearing off :D

I'm a very visual and aesthetics based creature, so for me, style is an important factor when it comes to creating an appealing looking aicraft.

My specific question is: why do some aircraft make use of gullwings?

Most instances of inverted gullwings like on the Corsair and such are obvious, ie: prop clearance when on the ground.

The only reasons I can think of for normal gullwing application is land/water clearance for twin/multi engines.

The reason I ask, is that I've been toying with a high-wing flying boat design with an pusher-puller engine pod along the centerline like on the Dornier 18. My choices of wing mounting are:

1) a relatively flat solid wing mounted atop the engine pod pylon and below/inline with the engines

2) gullwings mounted at the base of the engine mount pylon going out in a vee to a certain point with the rest of the wings extending straight out from there.

Are there any reasons beyond structural strength I should consider? Is there any aerodynamic reasons why one is prefereable to the other? Stylistically I'm leaning towards the gullwing design, just because. It will also go well with the dihedral I'm going to be using on the modified Do24-like twin rudder tail arrangement I'm envisioning.

But if opinions all lean in the other direction, well.. I guess I'll be doing things the other way! ;)

I'll try and throw up some of my napkin sketches so y'all have *some* clue as to what I'm talking about!

Kitty :)

[Ollie]

Gull dihedral wings are very, very poor additional dihedral function. Put the dihedral near the wing tips for dihedral function.
From Dr. Drela:
"You should be aware that on a rudder/elevator glider, the ideal
dihedral distribution is more like a shallow circular arc or parabola, not an ellipse. The arc has a minimum induced drag penalty from the sideslip during an established turn. In contrast, the elliptical dihedral in a sideslip has the near-vertical tips loaded and unloaded too much. This not only increases induced drag, but can also easily result in local stalling on the inside tip. The outside tip can easily go to negative cl's, which will likely increase drag there as well."

http://members.cox.net/evdesign/pages/aircraft_design.html
http://members.cox.net/evdesign/pages/plane_geometry.html

[Tom Harper]

Gull wings are strictly for structure or cosmetics. In sport models the penalty is not worth worrying about. Gull away!

[Sparky Paul]

The '30s gliders certainly look great with their gull-wings.. but it's only appearance, no functional advantage.

[Andy W]

Visibility, I think.. at least in the Minimoa.. allowed the pilot to see to the rear beneath the wings..
..a

[Kiwi]

The secondary reason for inverted gull wings on the Corsair is that drag is lessened when the wing meets the round fuselage at right angles (as it were) i.e if a radius continue in a straight line through the center line of the wing (notice there are no fuselage/wing fairings on a Corsair?)

[LedKitty]

Sweet! Thanks gang! That's what I wanted to know! Yeah, 30s, 40s.. all that golden age stuff is what I love! ;) That being said, I'm probably going to to with the gull wing roots in the top front view of the illustration below.

This is what I had in mind! A "fantasy Dornier" if you will. I wanted a Dornier type flying boat that looked fast but still kept the relative look and feel. This is partially based on the awesome designs that IAD emailed me, especially for the planform of the wing. I'm calling the design a "blackback" as in a blackball gull.

And yes, I do seem to have drawn my avatar as would a four year old :P talk about rust.. well at least I can still draw planes! ^_^

Now that that's out of the way, I can draw some decent sized 3views and work on those sexy lines. You really have no idea how small these napkin sketches of mine are! :D

Thanks again!

Kitty ;)

[vintage1]

I would go gull on that Kitty: Fow one reasin - it puts the drag component up higher which should stop yiu needing so much upthrust.

Flying boats wer terrible for digging the noses in when the throttles were opened up, and also nosing down under power - thats the propblem with a high mounted engne.
By gulling the wing you at least get the wing closer to the thrust line.

Consider aking the pod adjustable so you can add upthrust too.

[LedKitty]

Hi Vintage! Long time no see! ^_^ Great point about the thrustline! Keeping the nose proud of the drink is definitely a must and apparently a Dornier strongpoint! From what I've heard, Catalina crews had to have the copilot standing up in his seat with his head out the top to see over the over the surf on takeoff when operating from rough seas while the Do24 crews suffered no such problems, the Do24 having excellent rough seas performance!

As I get closer to workable drawings, I'm going to be playing with proportions and placement of things to get her to fly as good as (I think) she looks! :D Next I'm going to need to figure out MAC, CG placement, tail area and moment arm and all that jazz and proper placement for the double step too.. I believe I've got an issue of FSM around with all the forumulas I need! :D

Thanks!

K ;)