Calculation question: for a wide cockpit is it correct to consider the wingspan only the sum of the wings lenght?

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Arnaldojrbr

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My question raised after many spreadsheet calculations considering both. I'm drawing a WIG craft with capabilites of flying up to 150 metres high. The cockpit will be built around a fletcher 12ft speedboat hull wich is 1.55 meters wide (beam). I'm calculating the wing load with just the wing lenght solely. For a 8 meters wingspan (without cockpit and 1.8m chord) I'm achieving 50.59 Kg/M² which is my limit for wing loading, but if I include the cockpit in the wingspan this value falls to 42 kg/m², but does nt seems correct to include 1.55m of non lifting surface in the calculations. What I'm trying to do is to make the wingspan as short as possible but keeping the load under 50kg/m² to permit the takeoff from the water. Any consideration in the above will be appreciated.
 

Riggerrob

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At risk of over-simplification, aerodynamic engineers calculate wing-span and wing area by ignoring the fuselage.
Wing area calculations pretend that the wing is constant-chord through the fuselage.

IOW Measure the distance between the wing-tip vortices. Count the vortices as centered on the physical wing-tips.
 

Arnaldojrbr

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Yes! I know 150m is far off WIG effect, what I meant is the craft will be capable of sustained flight way beyond WIG effect, but I'm not willing to fly up to 1.000 feet. The normal "cruise altitude will be around 1 to 2 meters.
 

Arnaldojrbr

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At risk of over-simplification, aerodynamic engineers calculate wing-span and wing area by ignoring the fuselage.
Wing area calculations pretend that the wing is constant-chord through the fuselage.

IOW Measure the distance between the wing-tip vortices. Count the vortices as centered on the physical wing-tips.
So all the calculations we see in aircraft specs are made in this way? No one calculates the REAL area of hte wings?
 

addaon

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The so-called area of the wings is just a reference number, used to aid calculations. If you want to use 3D lift, moment, and drag coefficients obtained by similarity to existing designs, you'll do well to calculate the reference area the same way they do -- including the fuselage crossing area. If you're deriving those coefficients in other ways, you can use whatever reference area you want, as long as the two are consistent.
 

TFF

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Weight and speed is your problem. Flying out of ground effect means it’s an airplane no matter what you call it. How fast do you want to do this and at what load. The heavier and smaller the wing the faster you will go just to WIG and then out of it will be that that speed.
 

wsimpso1

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So all the calculations we see in aircraft specs are made in this way? No one calculates the REAL area of hte wings?

You are getting good advice, but I will get into the why of including wing buried within the fuselage. Wings influence the air a long ways up and down from the wing. If the air influenced were all influenced equally, it would be a cylinder one wingspan in diameter. The extent is actually much larger, but the amount of influence drops off with vertical distance. Since air pressure can only change slowly as you move from place to place, this influence of the wings extends over the fuselage too.

If you could see the airpressure applied to the skin of an airplane that is flying, you find the pressure on the top of wings (which is lower than ambient) wraps over the fuselage, is similar but not quite as low as on the wings adjacent to the fuselage, and spreads out a fore and aft of the wing along the fuselage. Some airplanes are far worse than others. The lift from reduced pressure above (the majority of the lift) looks to be about the same as if the fuselage were not there... Looking at the bottom, the pressure increase on the wing bottom also crosses the fuselage, with only a small reduction and extends fore and aft a ways. Again, a suitable fuselage makes it look like the wing behaves almost like the fuselage is not there. Fuselages that are poorly shaped can really mess this up...

Look up a series of articles in Sport Aviation by David Lednicer, he had modeled several WWII fighters and a sportplane or two, and published the pressure plots for review. Have fun.

I have it on good authority of a couple lifelong aerodynamicists that the way to build a low drag airplane is to pick good wings for the job, then shape the fuselage to interfere with the wings as little as possible. This commonly means, from ahead of the wing to behind the wing:
  • Vertical fuselage walls, and;
  • Constant or very slightly increasing width.
Some folks believe bulges like canopies should be aft of max wing thickness. In combination these two or three features look like the P-51. Kind of slab sided and straight through the wing. Making a fuselage that looks low drag, like the P-40 or Spitfire, with the fuselage getting narrower as you go aft along the wing results in all sorts of interference with the wing, and much higher drag.

Modern airliners are all about min fuel burn for the mission. Look at them. Straight fuselages through the wing. Big fairing from in front of the wing to behind the wing that gives vertical fuselage walls and constant section through the wing. All designed to make the air just go straight back through the wing. Want a little more proof. The Mustang and Spitfire used the same engine. The Mustang was bigger and heavier, significantly more wing and other wetted areas, carried much more internal fuel, flew much longer missions, and was faster. The Spitfire looked low drag, the Mustang, with that deep straight fuselage, was lower drag than the Spitfire's sexy tapered fuselage.

So, if you are building a blimp, make the fuselage look low drag. If you are getting lift from wings, make the fuselage, nacelles, etc interfere with the wing as little as you can.

Does this mean a parasol wing is better? Remember, the air is bulging out as it passes over the wing for a long ways up and down, so the presence of a fuselage is not avoided by standing it off by any reasonable distance.

When you take the span and the area of the wing including the part buried in the fuselage, that is "the REAL area of the wings". Believe it...

Billski
 
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jedi

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What is the reason for the 50kg/m>2 limit. Regulation or self imposed.
If it is regulatory use the specified procedures if any. If not do as you please. Use full span if the over 50 is ok or use the area of the two wings and be happy.
 

Arnaldojrbr

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jedi

Self imposed, I compiled data from 20 seaplanes and found this "optimal number" but don't have sure about the procedures of calculation from the manufacturers.
 

wsimpso1

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We should all pay Billski tuition for the education he’s doled out to us over the years.

I’ll have to pay you in beer though. :) cheers to repeated condensation of complex for the consumption of the lay!
I am planning on snowboarding in the Cottonwood Canyons again sometime... Could make for an entertaining rest day...
 

Norman

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Self imposed, I compiled data from 20 seaplanes and found this "optimal number" but don't have sure about the procedures of calculation from the manufacturers.
Speed, CLmax of the airfoil (with and without flaps) plus planform corrections, and CL at minimum drag are considered when deciding on the wing loading. If your benchmark seaplanes all land and cruise at the same speeds as your proposed design that method is sound. However monkey see monkey do can get you into problems if you don't know why the guy you're copying from made certain choices. Lift and drag characteristics (of both the wing and tail) are quite different in and out of ground effect and there are 2 phases of ground effect. If your machine can fly stably at 150 meters it's an airplane and calling it a WIG doesn't change that.
 
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