FLOPS Wing Weight Buildup

[WillCEOS]

Hi there,

I've seen that the example aircraft models provided have used the input CHORD_PER_SEMISPAN_DIST to specify the stations for load determination. How do we determine where the appropriate values/positions for our own aircraft configurations would be? I've attempted looking at the FLOPS literature on this but little light is shared in the document on selection/calculation of these positions.

Should this be at engine locations for the moment about the engine inertial relief?

Thanks.

[tozorosk]

First part of the answer: Chord per semi-span is a non-dimensional way to define the chord lengths at various stations on the semi-span of the wing. The stations themselves are usually expressed non-dimensionally as 'eta', the Y-distance from the fuselage divided by the semi-span (eta=2y/b), where b is the wingspan. This allows for geometrically scaling the wing during the re-sizing process.

[WillCEOS]

Thanks for the response - unfortunately that's the portion of this that I'm familiar with already.

Calculating the correct values of eta to use, that's what I can't seem to find info on. Values of 0.01 to 0.3 are used in the example models in Aviary, but I cannot find how this was determined.

[tozorosk]

So, the spanwise positioning of the engine depends on the one engine inoperable moment and the ability for the vertical tail to counteract the moment generated by one engine, at takeoff speeds. The positioning also relates to the wing on fuselage location, the height of the wing, and the height of the main gear, so they are all factored into the design together.

[tozorosk]

I also think 0.01 is kind of impossible, but it might be a test of sorts. The fuselage wall on the B737-800 is about y=6.0 ft or 0.1 eta.

[tozorosk]

Second parts of the answer: generally, the wing geometry is parametrically defined using a trapezoidal baseline or reference area. The trailing edge of the wing inboard section is parallel to the fuselage to allow for flap integration, though there are flaps outboard too. This adds planform area that exceeds the reference trapezoidal area. That inboard section is modeled as beginning at the center of the fuselage (Y=0) to the wing break point, where the outboard wing sections match the trapezoid. Other sections may be used to model winglets outboard, or non-perpendicular junctions with the fuselage. For the N3CC, the breakpoint is set to be eta=0.30 to 0.33, and the engine pylon is usually a bit closer to the fuselage at roughly eta=0.27 to 0.30. Outboard sections would most reasonably be set according to the desired flap and aileron extents. For many basic reference models like the N3CC, I have set an outboard break of about 85-90% semi-span, but it isn't really necessary for our FLOPS analysis.

[tozorosk]

One break point and two sections is good enough for most work. That leaves wing area> trapezoidal area, which can be a stumbling block. The other main point is that there are two non-dimensional quantities relevant to your question: 2c(y)/b and 2y/b.