Question about physics.

Hi, I'm interested in flight dynamics and simulation, and I need some help.

Wing area of MiG-29 is about 38m^2. Is this just the wings or all areas that creating lift (elevators, LERX, space between engines)? In modern lift equation, did you use only wing area or sum off all lifting surfaces?

http://imageshack.us/photo/my-images/32/94000957.jpg/

Here is the lift coefficient of MiG. I presume that this is result in wind tunel testing. In modern lft equation, you need to know either lift or Cl. Is there any math function that represents the curve on te diagram, or did you use cases ( like, if velocity is 0,6M then Cl is 0,071 etc.)?

And, is ther any way to obtain L or Cl without kowing either of the two?

Maybe I'm asking to much, but thanks in advance.

On most conventional aircraft flying in the sub-sonic area with out computer assistance, ie most propeller and turboprops the horizontal stabilizer and elevator do not develop lift in straight and level flight, They create a negative lift in order to have longitudenal stability.

The graph you linked is a graph of lift versus velocity measured in kmhx1000.

Yeah, the cL of the MiG is the entire lifting surface, not just the wings.

And if you are trying the find the Lift, you need the coefficient of lift, which should stay stable for a given airframe under most circumstances. The cL does NOT change with velocity, total lift for an airframe is calculated using this formula L = (1/2) d((Air Density)) v^2((Velocity)) s((Surface Area of Lifting Surfaces)) cL((Coefficient of Lift))

http://www.grc.nasa.gov/WWW/k-12/WindTunnel/Activities/lift_formula.html

This equation is good for any values that fit into the formula. it does not matter what particular object, or part of an object is being measured, just note that you cannot say use it for finding the lift of a single MiG wing if the cL is obtained using the entire airframe.

Regards,

Pyro

Edit: As with any math equation, as long as you know the other values, it is always possible to reverse-engineer the formula to find any one missing value. For example, lets say I know lift, velocity, density, and the cL, but not the SA. Well I can transform the equation using Algebra into S = ((1/2)*v^2*cL)/L. If however, you are lacking more than one value, solution of the equation becomes impossible.

Edit2: except in chemistry, for some reason that particular field doesn't like to follow the normal rules.

Actually to correct one thing Pyroflash said... The CL does not change MUCH with velocity as long as we stay in the subsonic region. Going supersonic, it's another story and you get lots of effects such as supersonic pockets on the upper side of a wing and shock effects so the behavior of a given airfoil changes a lot.

Basically, CL is a function of Shape, Mach and Angle of Attack (or Alpha). Usually Airfoil/Airplane shape is the most important thing, after it's Alpha and then it's Mach.

Lift is simply the resulting force, kind of the result if you wish. In wind tunnels, a way to find the CL is with an aerodynamic scale by measuring the Lift. Then as Pyroflash pointed out, you can find the CL and caracterise the behavior for this given mach and AOA. You do this for lots of cases and you get nice plots. Then the results can be used on a model to find the lift as equations work in all directions like he said :thumbup:.

Your graph is a bit out of context and I don't read russian but it seems to represent the CL/alpha with Mach. Think of it as the slope on the CL to Alpha Curve which Pyroflash posted.

Look for xfoil if you want to have some fun looking at the CL for different wing shapes in 2D. (3D results vary a lot)

Also here is a good introduction to models used in simulation as it takes into account Aspect Ratio in it's formulas : http://www.aerospaceweb.org/question/aerodynamics/q0184.shtml. Your graph would give ou the Clα value for a given Mach in there.

Enjoy !

Sorry, I should have mentioned that the formula assumes uniform Alpha and mach, in addition to the fact that velocity is measured in TAS, not IAS, so trying to determine lift in the cockpit without correcting for density altitude will most likely lead to to the wrong answer :thumbup:

Thank you for your answers.

On the first graph on the curve it says (without rockets, with rockets) but that's not very important right now.

On this graph taken from MiG29 aerodynamics manual, i see that Cl vary a lot for different velocities.

http://imageshack.us/photo/my-images/5/cl2qs.jpg/

Anyway, now I have enough to research. Thank you all.

( In Russian, Cy = Cl ).

FYI that graph measures cL/AoA, not velocity.

So, what M means. Is it Mach number?

Yes, the three curves show different cL 's(Y Axis) for any given AoA(X Axis). The top curve shows cL for a given AoA recorded in transsonic velocities, the second curve shows sub-sonic, and the bottom curve shows cL recorded for supersonic velocities.

Edit: If you take a look at the last picture you linked, you can kind of see a larger picture of the Delta cL versus Mach as it goes from sub-, to trans-, to supersonic. It kind of gives you a feel for how the cL changes with the different mach regions.

Edit Edit: I'm going to start referring to the coefficient of lift as sue(Cy in Ruskie is pronounced sue). At least I find it comical.

Regards,

Pyro

Aha! Thanks for posting the meaning of those symbols Yo-Yo. Confirms our guess about the variables in the Simple Flight Model files.

Just one thing, I think "inductive drag vs non-inductive" would probably be rendered as "induced drag vs non-induced drag" by English speakers - just in case anyone who knows a little aerodynamics (me!) gets a disconnect there.

... and Yo-Yo, your flight model for the A-10C is fantastic, very nice job there!

Now this is a marginally off-topic but I'll try and make the most of Yo-Yo being on this thread with some questions for him. Please indulge me ..

The Simple Flight Model for aircraft is described in Scripts/Aircrafts/_Common/SFM_Aerod.lua

I often get asked questions on what parameters mean what. So far I've made semi-educated guesses on each, but the change from 'Russian' symbols to 'Western' symbols leaves me scratching my head sometimes.

For each aircraft there are the following parameters that characterise it:

kjx, kjz, Czbe, cx_gear, cy_gear, cx_brk and then there aerodynamics table values of:

M, Cx0, Cya, B, B4, 0mxmax, Aldop, Cymax.

From these I guess:

cx_gear = drag coefficient when gear extension is 100%

cx_flap = drag coefficient when flap extension is 100%

cy_flap = lift coefficient when flap extension is 100%

cx_brk = drag coefficient when airbrake extension is 100%

M = Mach Number

Cx0 = non-induced drag as a function of Mach Number.

Now here are some guesses for quantities (could well be wrong):

Cya = lift cofficient as a function of angle-of-attack and Mach Number?

Aldop = critical angle-of-attack

Cymax = maximum lift coefficient at critical angle-of-attack

So that leaves the following quantities as unknown (to me at least):

kjx, kjz, Czbe, B. B4, 0mxmax

It would also be very nice to see the SFM equation for lift, assuming it has a linear section and then a roll off at the critical angle-of-attack, eg:

When angle-of-attack (a) is below Aldop:

Cy(M,a) = Cya(M)*a

and then something else when above the critical angle

Then

Cx(M,a) = Cx0(M) + B(M)*a + B4(M)*a^4

These are probably very wrong guesses, but it would be very instructive to have them written out for us - for the quantities we can adjust at least (I guess the inertial coefficients are not user modifiable?)

Yo-Yo I would be very grateful if would take the time to defines these quantities (or point to a reference/thread/post) where these quantities are defined. I would like to write definitions up for the modders out there (might save you and I getting asked some of the easier questions). Many thanks.

I'm glad you're still at it Moa. I wanted to create this full reference with a link to the equations and how each variable affected flight profiles, but became frustrated with the instability of my custom flight models (likely due to insufficient knowledge of the actual meaning of each variable). While I had the aerodynamics knowledge, I only learned flight dynamics when I was trying to mess with these files and had never had any formal training before.

Fantastic Yo-Yo. Thanks for taking the time to answer and put me straight.

I had missed that SFM page - it is just what I need.

Hi, I'm interested in flight dynamics and simulation, and I need some help.

Wing area of MiG-29 is about 38m^2. Is this just the wings or all areas that creating lift (elevators, LERX, space between engines)? In modern lift equation, did you use only wing area or sum off all lifting surfaces?

 

http://imageshack.us/photo/my-images/32/94000957.jpg/

 

Here is the lift coefficient of MiG. I presume that this is result in wind tunel testing. In modern lft equation, you need to know either lift or Cl. Is there any math function that represents the curve on te diagram, or did you use cases ( like, if velocity is 0,6M then Cl is 0,071 etc.)?

And, is ther any way to obtain L or Cl without kowing either of the two?

 

Maybe I'm asking to much, but thanks in advance.

Based on your questions I presume you're just starting. If that's the case you'll benefit more by getting the big picture and not the details. For that purpose I recommend the following book.

You can find all the details (and generalities!) there. By studying the book you'll learn the basics of flight mechanics rather than flight dynamics. After you deal with the Corke you can look for a flight dynamics course on the web. NASA has good, simply written courses... on everything re flight mechanics and dynamics.

The equations that matter directly to FM were linked by Yo-Yo http://lockon.co.uk/dev_journal/technology/?PAGEN_2=3#559 You can also refer to the How simulators work topic http://forums.eagle.ru/showthread.php?p=919468#post919468

On to your questions:

1. In all calculations I've seen and done a reference areas are used, e.g. a reference wing area is

Yes, in case of MiG-29 it would include LERX.*

2. The well known lift force equation is for a singular surfaces only e.g. wings (see above), horizontal empennage etc. Total aircraft lift force equation is basically a sum of equations for comprising elements plus some extras and looks nothing like the equation above.

3. The linked graph shows C_l for constant alpha in relation to speed (Mach number) and most probably comes from wind tunnel testing, not from calculations. Note that it looks simillar to a case for a single aero surface...

4. It's simple math - one equation can be solved if there's only one variable (unknown value).

General advice - if you want to learn, don't try to learn on cases like contemporary fighters :D They can't be designed solely using the classical methods.

* as for the center fuselage between the wings - it would probably have to be taken as a separate part due to complicated shape and intereferences. Not to mention that it has not a typical airfoil crossection. Also some contemporary designs have variable profile along the wing span.

_________________

you need the coefficient of lift, which should stay stable for a given airframe under most circumstances.

Incorrect.

The cL does NOT change with velocity

Incorrect.

total lift for an airframe is calculated using this formula L = (1/2) d((Air Density)) v^2((Velocity)) s((Surface Area of Lifting Surfaces)) cL((Coefficient of Lift))

... aaaaand incorrect.

Tell NASA that

L = (1/2) d v2 s CL[/Quote]

• L = Lift, which must equal the airplane's weight in pounds
  
• d = density of the air. This will change due to altitude. These values can be found in a I.C.A.O. Standard Atmosphere Table.
  
• v = velocity of an aircraft expressed in feet per second
  
• s = the wing area of an aircraft in square feet
  
• CL = Coefficient of lift , which is determined by the type of airfoil and angle of attack.
  

[/Quote]

 

 

And then tell yourself that, because you just stated the EXACT same equation :doh:

2. The well known lift force equation is for a singular surfaces only e.g. wings (see above), horizontal empennage etc. Total aircraft lift force equation is basically a sum of equations for comprising elements plus some extras and looks nothing like the equation above.[/Quote]Also, allow me to generalize, if you will, because Alpha and Mach aren't in the Lift equation and I was trying to simplify for his sake so he wouldn't flip a brick about the easy stuff.

Tell NASA that

NASA didn't say that you can calculate the total lift for an airframe from that equation, like you said.

And then tell yourself that, because you just stated the EXACT same equation :doh:[/size][/font]

But [see above].

I re-posted the equation for two reasons:

1. To correct your statement about it being applicable to whole airframes.

2. To present it in a clean form.

Alpha and Mach aren't in the Lift equation

You're very eager to dish out 'doh's' for a person who say such things.

Also, allow me to generalize, if you will (...)

 

(...) I was trying to simplify for his sake so he wouldn't flip a brick about the easy stuff.

Generalization is one thing, and by all means desirable. Incorrect statements are the other. You've dropped quite a few so at least, I'm not having a go with you, be more careful and relay more on linking to whole articles rather than posting your interpretations of them.

I have a question. AFAIK mach and TAS aren't the same thing, correct me if I am wrong on that however(about that as well, what I attempted to say was that alpha and mach influences on the cL didn't fit directly into the lift equation(you can't have on variable in an equation actively influencing another variable that has already been defined)).

Also, the cL IS NOT applicable for whole airframes? Hmm... my professor and I might have something to talk about then(No seriously, if I am being taught something wrong it doesn't make me feel happy that I am spending $2000 on tuition for a class that isn't teaching me anything)...

And I do admit that many of the things I say don't exactly come out very clean. But your comment kind of came off as trying to call the equation wrong(Math is always right :smartass:).

Correct, Mach and TAS aren't the same thing. However for a given speed of sound, they are directly proportional. The speed of sound is not constant over the entire flight envelope however, it varies with temperature and hence altitude.

Cl is affected by both the Reynolds number which incorpotates TAS, and the Mach number. As I think it was mentioned earlier in the thread, Cl is only a weak function of both of these variables, alpha has a much stronger influence. Why is it affected by both variables when they are both 'speeds' you ask? Well the Reynolds number is an indicator of the amount of flow turbulence, while the mach number is about flow compressibility and how pressure can propogate (sound is just a pressure wave). While the aircraft's speed will increase both, they are still independent and produce separate effects.

So Cl = f (geometry, alpha, M, Re)

Regarding the reference area, the documentation that gave the Cl data really should say what it is. It doesn't matter what reference area is used, as long as it is kept consistent for a given geometry. I would guess that it would use whatever the "wing area" of a MiG-29 is officially specified to be.

Hard so say seeing as we've only been given one graph, but those lift coefficients are for the full aircraft in my opinion, evidenced by the fact that is has a "with rockets" and "without rockets" curves (I dont speak russian but someone said thats what it said). I'm not saying the reference area is the whole aircraft. Just that they stuck the whole thing in the wind tunnel, not just the wing by itself. Reference area and the geometry are not related, you could make the reference area the pilot's head and it would only scale the graphs, it doesn't change the actual overall lift force.

Thanks lubey! That was all I needed to add. I simply refrained because I wanted to cross-check it all before posting and TBH didn't feel like it recently...

@lubey, thanks for your comments. My Russian is rudimentary (as in, equivalent to an autistic chimpanzee) but "raketa" (ракета) is often transliterated to English directly as "rocket", but the true English translation is often "missile".

The curves would probably be thought on of in English as "clean" and "[air-to-air] combat load".

Thanks lubey, +1 :thumbup: