Archive for the ‘ aircraft configurations ’ Category

Airplane design from structural efficiency point of view combined with aerodynamics point of view – multi-domain optimization

So far I have been looking only the aerodynamics side, but it is quite evident that compromises are needed on the aerodynamics side to achieve the best structural efficiency. I think one good example is Virgin Global Flyer (Scaled Composites model 311). I have not analysed yet the structure, but common sense says that trimaran has weight placed more evenly along the wing span avoiding a very large point load in the middle where the single fuselage would normally exist. The trimaran may have more wetted area than a single fuselage, but on the other hand, weight savings in the very high aspect ratio wing and space gains for the extra fuel are in this concept very important factors.

I find the trimaran configuration quite interesting – several different engine placement configurations for example can be used with this configuration without changing the aerodynamic shape of the concept very much. It is also interesting because it allows placement of the main gear away from the center fuselage and thus provides greater stability on the ground when the aspect ratio is high even if there is fuel placed to the wings very far away from the center of gravity. And as can be seen the same design suits several different missions: Global Flyer is very much like White Knight 2 with SpaceShipTwo under it on the center. Almost the same configuration, adapted to different kind of mission for very different kind of parameters (Global Flyer = long range cruise, White Knight 2 = optimized for climb).

Global flyer drawing Google found from some site
Wikipedia has another great photo, this is from front

The configuration is not really so new and not so unproven either, as people might expect, here is one example where a similar configuration has been used a long time ago:
Northrop Widow
The only difference here is that the Northrop Widow was optimized for different mission than either of the abovementioned and that it had piston engines in front of the outer “fuselages” which were interconnected from the tail section similarly than in Adam A500 whereas the Global Flyer and White Knight Two have two separate tails. It is quite apparent why the tails are separate in these aircraft – because the outer fuselages are placed so widely apart from each other, connecting the tails would have made the tail unnecessarily large which would have caused negative effect for the drag despite it would have had fewer intersections. On the other hand, I have been looking different HALE concepts, and it is quite apparent that the number of intersections is not the major drag source in high altitude aircraft, but the induced drag is, and to minimize induced drag, more intersections can be allowed as the penalty from them is lesser than limiting the aspect ratio would be. This is why there are even some concepts considered at the moment which have wing struts – even if everybody knows that they produce drag, in some concepts, the significance of that drag can be proportionally small whereas the increased aspect ratio has major effect on minimizing the total drag of the aircraft. HALE aircraft have to be quite different than those which are designed to cruise at low altitude, the drag percentages of each contributors are quite different and “one size does not fit all”.

It is quite interesting area to explore when the structural efficiency is added to the equation in addition to the aerodynamics and the result is a compromise on both structures and aerodynamics instead of being optimized for either aerodynamics or for structures. The mission parameters tend to heavily affect both and best suited results can be achieved by combining these two and by knowing the intended use exactly, potentially bigger gains can be realized than in a concept that is a general purpose in everything (GA = GENERAL aviation).

New variant of the shape I have been thinking about

Here is my today’s result from iRhino:

The idea is that the fuselage center section blends into wings like on blended wing body, but it only forms a minor portion of the shape, high aspect ratio wings continue from the blended part and there is a tail in the rear. I have not drawn this as I was thinking because I have been thinking either V-tail or T-tail. This picture doesn’t yet have a rudder.

Now the difficulty is that I have hard time on getting the Rhino do what I think. The loft is challenging, because it follows airfoil shape, it follows the configuration and contour from the top I was thinking, but the problem is to vary the airfoil shape in the center section so that the transition from the right side to the left side is smooth and more circular than in this thing where it is pretty sharp (the sharpness there is completely unintentional and will go away as soon as I figure how to loft this thing properly).

The wing tips did not loft as I planned, and also the elevator has wrong airfoil shape in the tip, the scale2D produced results I was not planning to get. There is still something to learn in Rhino. I need to ask from maybe Jani tomorrow how to do this right.

New variant of the shape I have been thinking about

Here is my today’s result from iRhino:

The idea is that the fuselage center section blends into wings like on blended wing body, but it only forms a minor portion of the shape, high aspect ratio wings continue from the blended part and there is a tail in the rear. I have not drawn this as I was thinking because I have been thinking either V-tail or T-tail. This picture doesn’t yet have a rudder.

Now the difficulty is that I have hard time on getting the Rhino do what I think. The loft is challenging, because it follows airfoil shape, it follows the configuration and contour from the top I was thinking, but the problem is to vary the airfoil shape in the center section so that the transition from the right side to the left side is smooth and more circular than in this thing where it is pretty sharp (the sharpness there is completely unintentional and will go away as soon as I figure how to loft this thing properly).

The wing tips did not loft as I planned, and also the elevator has wrong airfoil shape in the tip, the scale2D produced results I was not planning to get. There is still something to learn in Rhino. I need to ask from maybe Jani tomorrow how to do this right.

Comparing different configurations and plotting fuselage cross sections

Each configuration is a different compromise. I have been thinking hard which would work out the best. This may need to be proven to do a design for all the different alternatives as follows:

1. Laminar body fuselage with prop in rear. Boom tail. Front free of protruding elements until the laminar-turbulent transition point. Rotax 914 might fit into the rear of a rotated NACA 66-030 with no (or at least not long) extension shaft needed.
2. Laminar body fuselage shape with prop in the front, potential for laminar flow lost because of the prop disturbing air in the front. Like Stemme S6.
3. Laminar body fuselage with prop in the rear of the tail. Requires extension shaft which is structurally challenging.

Each design would need to be identical (fuselage pod length in Reynolds number should be equal) and the objective would be to investigate which one produces best compromise for low drag and is structurally the best solution (without unacceptable risk of in-flight failing parts (extension shaft in any circumstances must not fail)).

Measuring the difference actually is quite difficult because of the difference in the Reynolds number of a model aircraft and a full size aircraft because it affects quite heavily the laminar low drag area and where the transition to turbulent flow occurs. Also airfoil which is proper for full size aircraft would not work on a model. The NLF414F I discovered earlier does not work with low Reynolds number, it has nasty stall characteristics with low Reynolds number.

What interests me most in this is that how much drag the two tail booms would add. Would the penalty be more than the benefit of achieving laminar flow in the forward fuselage? Is the extension shaft the only way to achieve laminar flow without sacrificing the benefit?

I have been thinking possible concept for a model: try out the boom tail configuration as specified above. Fuselage would be rotated NACA 66-030 with propeller in the rear. Wortmann FX38-153 profile might work with the target Reynolds number range (the wing span and fuselage length would be determined by the interior size of our car, must be able to be disassembled to a size that fits inside for transportation, using a trailer for moving a model aircraft would be overkill). Target aspect ratio could be around 12-14 for main wing. I haven’t done any calculations yet though.

I want to also create a plotting program for the fuselage. Martin Hollman’s book has a Basic language program listing for a such thing. I am not sure if it is useful actually, I have been thinking how to parametrize a fuselage cross section (often it is not circular but rather boxy with rounded corners or it might have entirely different airfoil shape in horizontal and vertical axis), how to modify the shape of the centerline where the fuselage cross sections are referenced to and how to make the cross section follow a airfoil coordinates, possibly using the same data files that work with X-foil. Making circular or elliptical (LH-10 cross section for example seems to be elliptical) cross section plots from nose to tail for a rotated airfoil wouldn’t be that impossible task to do and visualization could be even quite reasonable to do with OpenGL. Before doing the visualization, I however, need to determine how to parametrize it, in other words, how to make it easy to produce differently shaped fuselages. Rhino3D does all this, but I don’t have Rhino3D, and this task is not that complicated, it should be doable with some little C++ work.

Any advise on the math and how to make the fuselage design easy would be great, feel free to add comments if you invent something or know something already.

>Comparing different configurations and plotting fuselage cross sections

>Each configuration is a different compromise. I have been thinking hard which would work out the best. This may need to be proven to do a design for all the different alternatives as follows:

1. Laminar body fuselage with prop in rear. Boom tail. Front free of protruding elements until the laminar-turbulent transition point. Rotax 914 might fit into the rear of a rotated NACA 66-030 with no (or at least not long) extension shaft needed.
2. Laminar body fuselage shape with prop in the front, potential for laminar flow lost because of the prop disturbing air in the front. Like Stemme S6.
3. Laminar body fuselage with prop in the rear of the tail. Requires extension shaft which is structurally challenging.

Each design would need to be identical (fuselage pod length in Reynolds number should be equal) and the objective would be to investigate which one produces best compromise for low drag and is structurally the best solution (without unacceptable risk of in-flight failing parts (extension shaft in any circumstances must not fail)).

Measuring the difference actually is quite difficult because of the difference in the Reynolds number of a model aircraft and a full size aircraft because it affects quite heavily the laminar low drag area and where the transition to turbulent flow occurs. Also airfoil which is proper for full size aircraft would not work on a model. The NLF414F I discovered earlier does not work with low Reynolds number, it has nasty stall characteristics with low Reynolds number.

What interests me most in this is that how much drag the two tail booms would add. Would the penalty be more than the benefit of achieving laminar flow in the forward fuselage? Is the extension shaft the only way to achieve laminar flow without sacrificing the benefit?

I have been thinking possible concept for a model: try out the boom tail configuration as specified above. Fuselage would be rotated NACA 66-030 with propeller in the rear. Wortmann FX38-153 profile might work with the target Reynolds number range (the wing span and fuselage length would be determined by the interior size of our car, must be able to be disassembled to a size that fits inside for transportation, using a trailer for moving a model aircraft would be overkill). Target aspect ratio could be around 12-14 for main wing. I haven’t done any calculations yet though.

I want to also create a plotting program for the fuselage. Martin Hollman’s book has a Basic language program listing for a such thing. I am not sure if it is useful actually, I have been thinking how to parametrize a fuselage cross section (often it is not circular but rather boxy with rounded corners or it might have entirely different airfoil shape in horizontal and vertical axis), how to modify the shape of the centerline where the fuselage cross sections are referenced to and how to make the cross section follow a airfoil coordinates, possibly using the same data files that work with X-foil. Making circular or elliptical (LH-10 cross section for example seems to be elliptical) cross section plots from nose to tail for a rotated airfoil wouldn’t be that impossible task to do and visualization could be even quite reasonable to do with OpenGL. Before doing the visualization, I however, need to determine how to parametrize it, in other words, how to make it easy to produce differently shaped fuselages. Rhino3D does all this, but I don’t have Rhino3D, and this task is not that complicated, it should be doable with some little C++ work.

Any advise on the math and how to make the fuselage design easy would be great, feel free to add comments if you invent something or know something already.