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Oct 19, 2017, 12:02 PM
West Coast Director, CPA
Vertigo II's Avatar
Discussion

Wing Area- Fuse or No Fuse


Howdy all,

Wing area is typically measured inclusive of the area occupied by the fuselage. This seems stupid to me. I realize that some fuselages may contibute to "lift", but in my experience, flying a fuselage without wings didn't work out so well
Any wing structure that is in between the fuselage sides does not see airflow, so it seems to me that including it in the wing area total is disingenuous.

How do you do it, and WHY?

Thanks in advance,
Robert
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Oct 19, 2017, 01:11 PM
Registered User
Quote:
Originally Posted by Vertigo II
How do you do it, and WHY?
Aerodynamic coefficients like CL and CD are defined according to a "reference area". It doesn't really matter what area you use for a particular project, as long as you consistently use the same one.

By convention, CL and CD are usually defined according to wing "planform area", which includes the part of the wing that goes through the fuselage. Probably just because it's simplest to calculate that way.

If you're estimating "skin friction drag", calculations will be based on "wetted area" which only includes exposed surfaces.
Oct 19, 2017, 03:09 PM
B for Bruce
BMatthews's Avatar
For smaller fuselages it's "close enough". If I were building a model where it has a REALLY fat fuselage such as THIS ONE I would likely just add up the wing areas on each side. For things like new jet fighters where the central body is able to contribute to the lift in a large way we go the other way and include all or a good portion of that area. But in that case how much do we include again? All of the area not including the forward "pod"? Part of the area where the section is fairly thin like the area between the engines on an F14 or 15 but not the pods? Or all of it on something "blended" like the YF-23? The mid body on the F-22 certainly provides some lift. But since it's got engines inlets and outlets at each end is it fully effective or is there some derating number to use for this plane and some others with this same style of design for lift contributed by the wide central area?

With some of the more extreme examples it can become pretty much a guessing game to figure out the effective wing area. But I'd suggest that if the area bounded by a simple fuselage is 10% or less that the convention of including the area bounded by the fuselage is "close enough".

Then there's also the idea that to some extent the wings will control the air around the fuselage well enough that it connects the air of each side across that gap to maintain the low or high pressure areas of the wings to each side depending on a high or low wing.... although I'm not sure where that leaves us with a mid wing....

Then there's the fact that it's also simply the convention.
Oct 19, 2017, 04:25 PM
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ShoeDLG's Avatar
Quote:
Originally Posted by jruley
Aerodynamic coefficients like CL and CD are defined according to a "reference area". It doesn't really matter what area you use for a particular project, as long as you consistently use the same one.

By convention, CL and CD are usually defined according to wing "planform area", which includes the part of the wing that goes through the fuselage. Probably just because it's simplest to calculate that way.
Certainly you can define your reference area any way you want, but I think the question is more along the lines of: If I want to know how a plane will fly (i.e. its effective wing area/loading), should I include the area where the wing "carries through" the fuselage.

The OP's observation that a fuselage without a wing wouldn't be good at generating lift is valid. What's interesting is that the presence of the wing makes the fuselage significantly better at generating lift.

Suppose the fuselage carried no lift. If that were the case, you would not really have a single wing, but two wings flying side by side, separated by the fuselage. Rather than having just two tip vortices, you would now have four tip vortices (the part of the wing that joins the fuselage would have to behave as a wingtip if the fuselage carried no lift). Not only is this extremely inefficient (increased drag), but the airplane would also generate less lift for a given angle of attack (you've effectively reduced the aspect ratio).

The full-scale Citabria has a fairing that covers the wing-fuselage joint. Without that fairing, there is a good-sized gap that air can flow through. There have been cases where pilots thought flying without the fairing would cause a minor increase in friction drag. They took off without the fairing and saw a massive increase in induced drag, to the point where some could barely climb out of ground effect. This illustrates that if the wing were to behave as separate left and right wings, you would know it.

The good news is that you don't have to do too much to get the fuselage to carry the "right" amount of lift. Without significant design gymnastics, fuselages naturally tend to behave almost as if you simply extended the wing through the fuselage. Where efficiency really matters (airliners), designers do try to minimize any "slump" in the lift to preserve a near optimal spanwise distribution. If you look closely at an airliner, you don't see evidence of great effort to make the fuselage behave like an extension of the wing... it just does (mid-wing or otherwise).

There is good reason to include the fuselage "carry through" area in your wing reference area, because the fuselage does tend to carry the lift through from one wing to the other.
Last edited by ShoeDLG; Oct 19, 2017 at 04:31 PM.
Oct 21, 2017, 09:54 AM
Registered User
Convention is the deciding factor in my opinion. It's simply the way the aircraft industry has historically come to calculate wing area. It also keeps points of reference standardized and simpler.

In the P-51 wing cube thread. I was thinking about how NAA apparently never makes a distinction with extra wing area when they added an "Expanded Leading Edge", first with Mustang I, and a much much larger root feature on P-51D. Engineering calcs never mention it.
Oct 21, 2017, 10:16 AM
Registered User
Quote:
Originally Posted by packardpursuit
NAA apparently never makes a distinction with extra wing area when they added an "Expanded Leading Edge", first with Mustang I, and a much much larger root feature on P-51D. Engineering calcs never mention it.
Wind tunnel balances measure forces and moments. To reduce these to force and moment coefficients, it is necessary to divide by dynamic pressure and reference area.

"Freestream" dynamic pressure is commonly used, even though this isn't "felt" on any part of the airplane, except maybe the wing leading edges.

Reference area is also arbitrary. Once a project decides on a value, it's simplest to keep using the same one even as minor configuration changes occur.


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