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Right, Pilotasso, but something most forum users probably don't know. You obviously do. The F-16 wing / fuselage joint reference is at Buttline 50, meaning 50 inches from the centerline. Lift outboard of that line is wing, inboard is fuselage - very clear distinction.

Thank you for your comment.

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WHere the heck did that come from? o_O
My statement was realy harmeless. And yours is way off target. Dont know why are you so touchy about this. As far as I know the air flow doesnt "care" about imaginary borderlines where you give different names to things. It just flows. So where you say its there a clear distintion by names it isnt so straight dorward aerodynamicaly, because body/wing is blended. Thats what I meant, it never crossed my mind to see the thing the way you did. Air flow lines dont come at a screeching halt by hiting onto an imaginary arbitrated wall.

And please dont get offended by opinions of others next time.

Elite 1K Joined: Aug 19, 2004 Posts: 1092

I understand what johnwill is saying just by looking at a F-16 model sitting in my room. You can clearly make out where the wing ends and the fuselage begins. The wing and fuselage on the F-16 are fairly distinct.

Pilotasso,
Hey, let's step back here and try again. I am not offended by anything. I try to avoid conflict and be as polite as I know how. If I said anything offensive to you, I apologize. See, I even said thank you for your comment. Your statement was indeed harmless, but I don't understand why you said mine was off-target. Please explain that to me. I am never offended by someone disagreeing, but I will speak up, because I know I am absolutely right.

Thank you again for your comment.

Elite 1K Joined: Aug 01, 2006 Posts: 1003 Status: Offline

I think that for any flight condition and for any aircraft, one can definitely draw a distinction between lift which occurs over a wing area and a fuselage area. Basically, lift is simply a set of pressure differences between the upper and lower sides of an object. If you define the wing as a certain area, then you can say that a certain amount of lift occurs over this area. The same can be said about the fuselage.

What is more difficult to determine is whether an unattached wing or fuselage has the same amount of lift in the same flight condition. For instance, vortexes generated by fuselage features may end up adding lift to a wing -- lift which would have been completely absent if the wing was detached. Similarly the wing prevents air from spilling over the sides of the fuselage over a large percentage of its length, this definitely affects the lifting properties of the fuselage.

However, when the two are attached and taken as a whole, with all the interactions taken into account, we can still say that of the total lift how much of it is occurring over the wing area and how much of it is occurring over the fuselage area. The F-35 appears to generate a significant amount of lift over its fuselage area and the center of the total amount of lift forces over the fuselage appears to be well forward of the main wing during cruise. That is the hypothesis behind the origination of this thread.

dwightlooi,
You are entirely correct about the wing increasing the fuselage lift by preventing spillage. However, the opposite is also true - the fuselage increases the wing lift exactly the same way. That increment of lift is called carry-over. Carry-over is also present at the fuselage / horizontal tail intersection and fuselage / ventral intersection.

You are also correct about the fuselage lift AC (aerodynamic center) being forward of the wing. Not surprising, because the lift on any surface is roughly at 25% of its chord, depending on mach no. Of course, deflected flaps and ailerons change that for a wing. Actually fuselage lift does not depend much on fuselage shape, mostly angle of attack and planform area for a given speed and altitude. That is also true for wings, believe it or not. The fancy airfoil shapes of wings are primarily to reduce drag, control pitch moment, and increase the useful range of angle of attack. I am talking about airfoil effects only, not planform or elastic effects.

These carry-over effects are there, but it is not important determine what they are, becuse they don't matter. Wind tunnel pressure models and CFD models are complete airplane models, not components. As you say, the pressure differences between the top and bottom are all that matter.

Elite 1K Joined: Aug 19, 2004 Posts: 1092

This whole discussion should remind people why simple wing loading calculations based on the wing reference area (especially on an advanced fighter like the F-35) are of little use when trying to evaluate an aircraft's performance.

Quote:

This whole discussion should remind people why simple wing loading calculations based on the wing reference area (especially on an advanced fighter like the F-35) are of little use when trying to evaluate an aircraft's performance.

I have to agree,evaluating an aircraft's performance levels is determined in flight testing. Yes you can get some fairly close to the target predictions, But hey are just that, Predictions. As Grandma always says.... "it will all come out in the wash"
,

Elite 1K Joined: Aug 01, 2006 Posts: 1003 Status: Offline

Raptor_One wrote:

This whole discussion should remind people why simple wing loading calculations based on the wing reference area (especially on an advanced fighter like the F-35) are of little use when trying to evaluate an aircraft's performance.

Well, that hasn't stopped people from saying that the F-35 has a 460 sq-ft wing and weighs whatever, so it is better or worse than an F-16 with a 300 sq-ft wing or a Rafale with a 492 sq-ft wing with their corresponding weights. And other things of that nature.

Keep in mind that wing reference area is defined by extending the wing leading edge and trailing edge to the airplane centerline. So a large portion of wing reference area is not wing area at all; it is fuselage area. People saying something does not make it correct.

johnwill wrote:

Pilotasso,
Hey, let's step back here and try again. I am not offended by anything. I try to avoid conflict and be as polite as I know how. If I said anything offensive to you, I apologize. See, I even said thank you for your comment. Your statement was indeed harmless, but I don't understand why you said mine was off-target. Please explain that to me. I am never offended by someone disagreeing, but I will speak up, because I know I am absolutely right.

Thank you again for your comment.

My bad I interpreted your message as ironical, I iterpreted as "your a forum smart a$$" Very Happy

Forget it. Its cool.

Elite 1K Joined: Aug 01, 2006 Posts: 1003 Status: Offline

johnwill wrote:

Keep in mind that wing reference area is defined by extending the wing leading edge and trailing edge to the airplane centerline. So a large portion of wing reference area is not wing area at all; it is fuselage area. People saying something does not make it correct.

That is rather odd. By this method, deltas will have much larger "wing areas" because the leading edge when extended to the centerline will cover more fuselage area. The same thing goes for wings with a reverse swept trailing edge. On the other hand, straight wings and/or parallel swept wings will have much less fuselage area counted as wing area even if the body itself is very broad and may contribute a lot to the total lift.

Wouldn't this make the wing area referenced even less representative that if only the wings themselves are counted?

Elite 1K Joined: Aug 19, 2004 Posts: 1092

It's not odd. A wing in the absense of any fuselage is simply some 3D shape. When viewed from above, you have the 2D wing planform from which you calculate the wing reference area. When 3D finite wings are tested in a wind tunnel or via CFD simulations, the basic aerodynamic coefficients (lift, drag, and pitching moment coefficients) are calculated based on the wing reference area. In the end you're really just concerned about how well an aircraft (including its wing) performs throughout its flight envelope, but the use of appropriate non-dimensional coefficients enables engineers and the like to evaluate specific aircraft characteristics. These coefficients by themselves tell a lot about how well an aircraft will perform (on paper at least), but they're also convenient for performing many flight performance calculations.

This post is already too wordy, so I'll try to sum things up as best I can. The wing reference area is just that... the area to be referenced when calculating things like the lift/drag/pitching moment coefficients. Alternatively, it's the area one must reference when using the various aerodynamic coefficients to calculate things like total lift, drag, pitching moment, etc. To determine how fast an aircraft can fly in level flight (theoretically) at a certain altitude, you simply determine where total drag = total thrust. You estimate the installed thrust of your aircraft for a range of speeds at the altitude of interest and you calculate the total drag for the same range of speeds. How do you calculate the drag at a given altitude and airspeed? Well... as long as you know the drag coefficient and its associated reference area, you just "plug and chug". D = 0.5 * rho * V^2 * Sref * CD where rho is the density (of air at some altitude in this case). V is velocity of course. The reference area, Sref, and the aircraft's drag coefficient CD must be consistent with each other.

In other words, if you determine CD based on some sort of simulation (wind tunnel or computational), it's based on a reference area which is, by convention, the wing planform area. Any future calculations of the aircraft's performance based on this CD you calculated experimentally must be used in conjunction with the appropriate Sref. In a scaled down wind tunnel model, Sref won't be the same as the full sized aircraft. But the geometry of the full size aircraft including its wing planform area will be almost identical to the scale model's. If you calculate Sref for the full size aircraft the same way as you did for the model (based on the wing planform area), you can safely use the nondimensional aerodynamic coefficients you obtained from the model for approximate performance calculations.

The only thing that comes into play with wind tunnel testing on scale models is Reynolds number. The smaller the scale of the model with respect to the real thing, the larger the disparity in Reynolds number for a given flight/test conditions. CL, CD, and CM are dependent on the Reynolds number, so you need to make sure your wind tunnel model is... umm... big enough. It's a tricky business and can involve using fluids other than air in the wind tunnel to achieve an equivalent Reynolds number with respect to the full size aircraft. While computer simulations don't suffer from this problem (you don't have to scale down your computer model), they suffer from a host of others which can affect the accuracy of the solution. That's why flight testing is often done to determine the "true" aircraft stability derivatives (including the aerodynamic stability derivatives such as CL, CD, etc). Of course this is never the first step in the process. These days one goes from numerical simulations to wind tunnel simulations (which hopefully verify the numerical simulations) to flight testing.

Damn... I couldn't manage to keep it short and to the point. Oh well. All one has to keep in mind is that wing reference area need only be suitable for non-dimensional analysis purposes. The whole concept of combining a bunch of variables into "god-like" non-dimensional parameters is a subject in and of itself. As long as you use appropriate length scales (i.e. of the right order), you're good to go. I think it would be difficult to build an aircraft whose wing planform area wasn't suitable for calculating CL, CD, CM, and so on. I don't know what the deal is at the microscopic level, but it should work fine under almost any circumstance at the macroscopic level.

Raptor One,
Good on you! Good explanation all around. There is also a reference length used in moment coefficients. Normally MAC (mean aerodynamic chord) is it. For example, pitch moment = .5 * rho * v^2 * Sref * MAC * Cm. Note that v is true airspeed, not calibrated or indicated.

Good point about using other gasses in wind tunnels. NASA had a freon tunnel that was very good at matching full-scale Reynolds Number, but it had to be shut down due to environmental effects on the ozone layer. Inevitably some freon escaped into the atmosphere.

Pilotasso,
Fine, no problem. In written communication, it is very easy to mis-interpret feelings.

Elite 1K Joined: Aug 19, 2004 Posts: 1092

Johnwill,

With all the non-dimensional stability derivatives out there, I took one of the simplest ones as an example. I guess it's worth noting that you have to use the right combination of reference dimensions and "physical quanities" (velocity, density, etc) to cancel out the units of measure for the dimensional quanity you're trying to non-dimensionalize. This isn't for your benefit obviously. Smile

I'm sure you could teach me a thing or twenty.

On a side note, I've actually done a lot of research on the F-16 over the years and eventually collected enough data from technical reports and the like to put together a set of high fidelity flight models for the PC flight sim Falcon 4.0. Perhaps I used data from some tech report you helped write. Who knows. You can check out the manual I helped put together for these F-16 flight models if you're interested by going here:

http://www.checksix-fr.com/downloads/fa ... manual.pdf

Take care,

Raptor One

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Body lift on the F-35