Lifting body aerodynamics: chord length/airfoil area - how do I find it?

[Pipcard]

I barely know anything about aerodynamics, so does anyone here know about it? I would guess so considering the existence of Ferram Aerospace Research (although I've never used that before).

I'm trying to develop an add-on for the Orbiter space flight sim using a development plug-in called "spacecraft3." It has less functionality than an actual compiled C++ module, but it only requires the creation and modification of .cfg files.

In these .cfg files, some of the parameters describe the aerodynamics of the vehicle, especially the chord length and wing area of an airfoil.

The issue is that I'm developing a lifting body (without any actual wings) based on a JAXA aircraft, and it's not quite clear what the chord and area of the airfoil should be.

Note that the length of the fuselage is about 8-9 meters, and the girl (Hatsune Miku) is 1.58 m tall. Does the fuselage count as one big airfoil (that isn't as efficient as a true wing)?

Edited November 30, 2014 by Pipcard

[EzinX]

I suspect you need to take your model and run it through a CFD solver and have it calculate the lift and drag coefficients numerically. I highly doubt an analytical solution to a complex, arbitrary shape like this exists. I don't think analytical solutions exist even for wings as complex as the ones used on real world aircraft - pretty sure they have to use numerical solvers and wind tunnel tests to determine their actual performance. Let me know if you find a good solver, I've been curious about this myself.

Once you have those coefficients, you could then solve for an equivalent airfoil, if one exists, and feed that to orbiter.

[Pipcard]

I actually do have the lift and drag coefficients for LIFLEX (the experimental JAXA aircraft mentioned earlier, but it's only for angles of attack ranging from -10 to 30 degrees). They're from page 11 of this 86-page .pdf. I do not actually know Japanese so I had to do a little digging around.

And here's the lift-drag ratio graph (it's from some other .pdf, but I'm not sure which page).

(ALFLEX being another experimental Japanese aircraft with delta wings.)

Now how would I "solve for an equivalent airfoil"?

Edited November 30, 2014 by Pipcard

[Pipcard]

Does Ferram calculate lift/drag based on the shape of the mesh?

[steve_v]

Not yet Spoilers...

[Pipcard]

I've seen a video in which Ferram calculates the lift-drag ratios of a craft in a graph. Isn't that because it's actually calculating the airflow over the geometry of the parts?

[Jonfliesgoats]

This is why I think a wind tunnel mode in KSP would be cool.  IRL we still have to do wind tunnel testing because we still can't always predict exactly how a craft will behave as various parts interfere with each other.  We get pretty dang close with computers, as others have said.

With a wind tunnel mode in KSP, you'd get some numbers to calculate lift and drag coefficients at different angles of attack, yaw and with different control deflections.  From there your could get a decent idea of the Amax, qmax and even estimate maximum reentry heating of your vehicle based on a given, reentry angle.

[DrLicor]

Here's a 'short' explanation how lift works:

AFAIK lift is created by many factors. As you know it's basicly the force that pushes the plane upwards.

Lift is calculated as follows:

where

• L is lift force,
• ρ is air density,
• v is true airspeed,
• A is the wing area, and
•  is the lift coefficient at the desired angle of attack

 

You can see, it's impossible to calculate for KSP the constant lift, because there is no constant lift. 

You have to calculate the lift under certain circumstances.. For instance, on an altitude of 20km (for air density), airspeed of 900km, a wing area of 50m2 and the angle of your wings 5°degree,

I'm not going to calculate that, but if plane flights straight under the circumstances stated above, and the answer is positive above 1, you know your plane creates lift and will go upwards, so you need to put your nose down. Is it negative less than 1, you have more drag dan lift, you need to put your nose a but up for remaining the same altitude.

When playing KSP with FAR, I believe you can fill a formula and it calculates it for you. 

Hope this helped you  

 

ofcourse, there much more behind the lift-thing (no pun intended), but this is the basic explaination.

 

Pff I keep editting this post but:

When flying a plane, and you want to maintain the same altitude, find the angle of attack you need so the vertical velocity is zero, than press 'alt+one' of your movement keys (AWSD)'and your plane will maintain this angle.

Notice, when the vertical velocity is positive, your angle of attack will produce more lift the longer you fly. (untill you reach angle of stalling)

 

some extra things, when a plane doesn't create enough lift to maintain altitude, they're a few thing to do

change the angle of attack:

• can be done by trimming the flaps
• can also be done by angle the wings more

or change the size of the wings, but with extra size you can extra weight and construction.

 

And indead, if you want the right lift, it has to go with FAR.. For stock you will have to do some test runs and just tweak it 

 

[Jonfliesgoats]

So what, specifically, are you trying to calculate?  Many times we are talking about averages, (mean chord, mean camber, etc.).  You can probably get within 10% of reality of those numbers with the data you have in front of you.

You have rough dimensions of the JAXA craft in front of you via these drawings.  Without actual technical data, you could eyeball this to get close enough.  It looks like this thing will stall around 30-40 degrees AOA which is actually typical of delta wings.  You can even use the data to figure roughly how steep this thing should glide once it's below the speed of sound.

With regard to mass, you know what rockets the Japanese have and have planned.  If this thing is going to ride one of those to orbit, it's not going to be much heavier than the other payloads they launch.

I'll bet you imaginary donuts that you can google the relevant equations, make some rough estimates and get within ten percent of reality.  You'll surprise yourself with how well you'll do.  There's a famous story of an American engineer doing tins with satellite photos to estimate the performance of Soviet ground effect vehicles.

 

I just saw this was two years old!  Oops!

[wizzlebippi]

The fuselage is one awkward airfoil. On conventional aircraft, the fuselage is usually assumed to produce as much lift as the wing area it covers.

Assume the chord is the length of the craft, and the area is what you see in your top down view. The easiest way to calculate area on that is to break it down into simpler shapes.

[K^2]

On 11/30/2014 at 0:25 PM, Pipcard said:

I actually do have the lift and drag coefficients for LIFLEX (the experimental JAXA aircraft mentioned earlier, but it's only for angles of attack ranging from -10 to 30 degrees).

That's quite normal, since it's starting to stall at 30° already. Most airfoils stall at even lower AoA, but narrow lifting bodies tend to have high critical AoA. In the configurations in red and green, you can clearly see the onset. On the blue one, the actual critical AoA is a bit further out, but L/D ratio is well on its way down, so you aren't likely to try and fly like that anyhow.

If you need to plug this into a simpler model, this airfoil is amazingly close to thin airfoil theory. For a thin airfoil, drag coefficient is quadratic in AoA, and lift coefficient is linear. And I see a very neat set of parabolas and almost perfect straight lines there. Since the vertical axis isn't labeled on these graphs, it's hard to extract effective wing area, though. Do you happen to have a table with actual numbers represented by these graphs?