Best fuselage shape

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Eugene

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Can someone explain to me which shape will work better? A or B?

With A I am trying to avoid separation and I have smallest angle possible. But from what I see almost every composite airplane shaped like B.

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Eugene

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It is all done. I call this one “model C”. I fixed it by making tail arm 2 feet longer. Now picture looks much better, I think. Tail arm was increased from 2.2 to about 2.6 MAC. Horizontal tail volume increased from 0.34 to 0.41.

So, long tail arm is our friend in many ways. Thank you, I can sleep better now!

D43563B1-50A3-4E5D-B681-87345F1BD696.jpeg
 
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rtfm

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The most efficient (to date that I'm aware of) is the Questair Venture, also dubbed the "Flying Egg". It all has to do with the fineness ratio. From the web (don't remember where...)
One of the primary forms of drag is skin friction. As the name implies, this is drag caused by the interaction of the airflow with the aircraft's skin. To minimize this drag, the aircraft should be designed to minimize the exposed skin area, or "wetted surface", which generally implies the fuselage should be somewhat "egg shaped", with a fineness ratio about 4.5. A good example of such a design is the Questair Venture.
 

rotax618

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In your case it is the shape from the bulkhead at the seat back to the upright just in front of the prop that is critical to drag and airflow into the prop and over the tail surfaces. A side by side pusher is the worst possible arrangement and I don’t think that there is any magic shape that will produce a smooth airflow around that steep taper.
 

Pilot-34

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Drag is more than just skin friction or planes would be perfect Spheres
The faster you go the more elongated the body should be in the axis of flight.


It seems like there should be a simple graph describing the shape based on the maximum width and speed requirements.
Alternatively the minimum enclosed volume and the maximum speed.

Not being an expert on these things I can’t say for sure but I really suspect there will be no concave curves in the optimum shape.
 

wsimpso1

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There were studies done before WWII to determine the ideal shapes for things like struts and nacelles and fuselages. This did not include laminar flow shapes. IIRC, the ideal shape for struts and fuselages with a given frontal area was a length about three times the thickness and a roughly teardrop shape - less taper aft than forward. This is the basis for all of our streamline tubing and the shape of many radial engine nacelles, etc.

The next step in fuselage shape evolution is the understanding that if the fuselage interferes with the wing, it costs a lot of drag. Lots of work on how to fillet the wing were done, and the expanding wing root fillets were figured out because folks were trying to make the fuselage low drag, then add wings and tails to it. Each of the shapes was pulling on the air, and where they both make pressure reductions, there was separation. Then they figured out something that is counter intuitive: If you make the fuselage (or nacelle) constant width and vertical walled through the wing, the wing hardly knows the fuselage is there. Examples abound, but the North American A-36/P-51, DeHaviland Mosquito, and the Douglas A-26 are kind of obvious and no one can fault each's speed for its class. This has been taken farther, with slight expansion of the fuselage through the wing to lengthen laminar flow on the wing.

Some other folks feel that maximum cross section of the fuselage should not coincide with maximum wing thickness, and will shift canopy forward or aft to reinforce this.

Letting foils not intefere with each other helps too. I will go back to the A-36/P-51 with the horizontal tail offset forward of vertical tail by the chord of the rudder. This offsets the max thicknesses of the vertical and horizontal tails and greatly reduces separation drag of these foils.

Billski
 

wsimpso1

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Now Eugene, on to your problem. You have a strong taper going into the prop inherent to the layout. This tends to cause separation that is not only draggy by lowers prop efficiency a bunch too. Ouch, Ouch! A number of other experimenters have found that VG's applied to this region tends to lower drag and bring prop efficiency into nominal range. Some of the VG were shaped like the usual two blade type, but proportioned big, like 2" above the base surface.

The rest of your shape problem is the shape below the engine feeding into the boom. I do feel that expansion beyond what is needed to get around internal hardware is both more frontal area and more wetted area and without purpose - excess drag. Usually, each cross section as you move aft should be a smooth shape, and the shapes of these cross sections should look like they are related... The shape should look like it progresses smoothly. Abrupt drops in area are to be avoided.

You have found a bunch of separation low and aft on the fuselage. The usual fixes are to fill in some of the radical taper / sudden cross section changes, either with raw shape or with VG's. You have played with the shape approach, and went with a rather radical shape, then tuft tested it. I still do not understand the purpose, but you found it to be an improvement, so I have not previously objected in print... Your cross sections are lobed, and usually that has varying separation and interference as AOA changes. I can not help but wonder if it would work better with a little smoothing forward and a more gentle and conventional fairing to the boom.

The scheme with high engine in the wing root is inherently draggy and tough to get smooth air to the prop. Making that work well would be my first priority, followed by a smooth transition of flow from the cockpit aft. Beyond that, I suspect you are largely working in rapidly diminishing returns.

Billski
 

cluttonfred

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I have always wondered about the Questair Venture since fuselage length/weight/drag is an inverse trade-off with tail surface area/weight/drag not to mention the stability concerns. Since we don't see many aircraft with the proportions of the Venture, it appears that most designers think the practical sweet spot is substantially more than 3:1 or 4.5:1.
 

TFF

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The Venture is a handful airplane. It is cool. It does the job. It’s not a finesse airplane. It’s a prop Lear with all the issues that comes with flying something that high performance. Is why a lot have been damaged over the years.
 
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