Question about physics.

[lubey]

glad I made sense for once:smilewink:

[Moa]

To be accurate, "raketa" in Russian means both rocket and missile. We have to use "unguided" and "guided" before "rocket" to define the certain type.

 

Thanks for the expansion Yo-Yo. I didn't say this because I was talking about the English (and I realise there are Russian speakers like yourself who are far more qualified than myself to talk from the Russian perspective).

 

Would you mind giving us the two uses (in Cyrillic please, IMHO it is easier to transliterate from Cyrillic to Latin than start with the Latin and get to correct Cyrillic). Thanks.

Edited June 21, 2011 by Moa

[aaron886]

Correct, Mach and TAS aren't the same thing. However for a given speed of sound, they are directly proportional.

 

@Pyro - perhaps more clearly... They aren't the same thing because Mach number is technically a ratio, and TAS is a quantity. (Therefore obviously not interchangeable in an equation.) However, TAS is corrected for temperature and pressure, and therefore essentially velocity through an airmass. Mach number is the ratio of velocity through an airmass to the speed of sound in that airmass, so the two will relate pretty closely. (Hence "proportional.") Divergence in the two, I think, is due to compressibility error at high airspeeds.

 

Cl is affected by both the Reynolds number which incorpotates TAS, and the Mach number.

 

AFAIK, RE can be found with V*DLE/KV, where V is TAS, DLE is distance from leading edge, and KV is kinematic viscosity... no recollection of Mach number in there. Correct me if I'm wrong!

 

I didn't know that CL was calculated with Reynolds number... I only knew it as a number based on the shape/design of the lifting device, changing with AOA. It seems right, though, since turbulent flow at high Reynolds numbers would spoil lift production. (At most angles of attack.) So for all I know you're an aerodynamicist, and I'm just a dope with a couple college classes in aerodynamics for pilots. :doh:

Edited June 21, 2011 by aaron886

[Yo-Yo]

Thanks for the expansion Yo-Yo. I didn't say this because I was talking about the English (and I realise there are Russian speakers like yourself who are far more qualified than myself to talk from the Russian perspective).

 

Would you mind giving us the two uses (in Cyrillic please, IMHO it is easier to transliterate from Cyrillic to Latin than start with the Latin and get to correct Cyrillic). Thanks.

 

ok.

УР is an abbreviation of управляемая ракета or MISSILE

НАР is an abbreviation of неуправляемая авиационная ракета or ROCKET

in older sources you can find НУРС for the same item , where РС means ROCKET SHELL

[Gundark]

Man, I'm lost reading all this. Just one thing to confirm. ED team used values of CL in the form of table, and not calculating it as math function?

Edited June 23, 2011 by Gundark

[Moa]

Man, I'm lost reading all this. Just one thing to confirm. ED team used values of CL in the form of table, and not calculating it as math function?

 

The Simple Flight Model (SFM) aerodynamics and engine values are tabular and interpolated.

 

What is not known are the forms of the:

* Advanced Flight Model (Su-25A, Su-25T)

* Blackshark Flight Model

* Warthog Flight Model.

 

Any of these models are sufficient, tabular or not. What matters is how many terms the equations have (how many effects they cater for) - which is balanced against the available time to compute for all aircraft and missiles currently in-game.

 

People don't generally notice if tabular values are a little off (unless they are way off by a significant factor, eg the roll rates for unloaded aircraft until the FC2 patches). They notice when equation terms are missing or unbalanced (excessive roll in stall, poor recovery time, handling with missing outer wings etc).

[aaron886]

People don't generally notice if tabular values are a little off (unless they are way off by a significant factor, eg the roll rates for unloaded aircraft until the FC2 patches). They notice when equation terms are missing or unbalanced (excessive roll in stall, poor recovery time, handling with missing outer wings etc).

 

Definitely.

[lubey]

I didn't know that CL was calculated with Reynolds number... I only knew it as a number based on the shape/design of the lifting device, changing with AOA. It seems right, though, since turbulent flow at high Reynolds numbers would spoil lift production. (At most angles of attack.) So for all I know you're an aerodynamicist, and I'm just a dope with a couple college classes in aerodynamics for pilots. :doh:

 

Well I'm no professional aerodynamicist, just have a knowledge of fluid mechanics from being a mechanical engineer. But from my knowledge, turbulent flow can actually increase lift production at high AoA. This is due to an effect similar to that which occurs on a golf ball - in this case the dimples on the ball create turbulence even at a lower Reynolds number in order to reduce the drag.

 

But either way the Reynolds number usually only has a small impact on the lift coefficient.

[Moa]

Ah, lubey, they covered this in my Air Force flight theory two decades ago (yep, I'm getting on).

 

The turbulent boundary layer creates more drag than laminar flow, which would ordinarily cause a shorter distance for the golf ball.

 

However, the turbulent layer has more energy (as well as more drag) so it follows the surface for a longer distance before separating. The laminar flow is less energetic and separates sooner.

 

So while the portion of laminar flow has less drag than turbulent flow the earlier separation results in a higher overall drag for the smooth surface (golf ball in this case).

 

The golf ball is dimpled to introduce turbulent flow and delay separation, reducing drag and giving a longer flight.

 

You'll see a similar trick on some aircraft (the A4 Skyhawk is the only one I saw close up). Where little strakes are put around the middle of the wing chord (as the earlier part of the chord is in laminar flow and produced most lift). The strakes also help prevent sideways movement of air, reducing wingtip 'spillover' (which modern commerical jets now have winglets for).

 

So for a wing you want a very smooth leading edge back to around 1/3 chord (hence the "no step" signs) and after that the surface roughness doesn't matter as much.

[aaron886]

I must have missed this before, but yes... turbulent flow (high Re) is both an increase in drag and loss of lift, but because turbulent air is more energized, as Moa said, it will stay attached to the wing surface more readily. (In simple terms.) By doing so, it helps hold off the stall. It's a trade-off.

 

...from my knowledge, turbulent flow can actually increase lift production at high AoA.

 

So sort of. It doesn't increase lift production, it merely allows the wing to continue producing lift at higher angles of attack.

 

The little strakes on the A-4 you mentioned, Moa, are called "vortex generators," and can be spotted on many aircraft. Their primary function is not to stop "spanwise flow," but rather to energize the boundary layer and delay certain stall characteristics. You might see much larger vertical surfaces on the wings of other aircraft, like the MiG-15, called "fences." Their job is to stop that spanwise flow, mainly to improve stalling characteristics, particularly control loss at the wingtips with certain swept planforms.

 

(MiG-15 wing fences: http://wio.ru/korea/mig15.jpg)

Edited June 27, 2011 by aaron886

[Gundark]

Eh, I feel little embarressed... But, i forgot one important thing. In drag equation ( D= 0.5*Cd*R*V^2*A) where A is referance area. I could not find value for A on Mig-29. It's not the same area from Lift equation as i understand. Does anyone know this value for Mig?

Edited January 15, 2012 by Gundark

[effte]

Can be any old area, really. Frontal area, wing planform area, the area of the tray in a catering trolley... as long as you specify the area. Cd will be change in unison, for the same net result. Planform area is convenient to use though.

 

Without the area specified, there's no way to convert the Cd (or Cl) into actual drag generated.

 

Cheers,

Fred

[Gundark]

Will it be a mistake if i calculate drag area by calculating cross section of an airplane based on the blueprins? What value for drag area did ED team use ( if it's not classified )?

[effte]

What drag/lift coefficients are you using? You need to use the same area which was used to establish those coefficients. If you have the coefficients but not the area, you are stuck with guesstimating until your performance matches data from other sources.

 

For all you know, the area used could have been the surface area of the pilot's helmet. It wouldn't make sense, but it'd be valid. :)

 

Cheers,

Fred

[aaron886]

effte says you can use any area for the drag equation reference area (A), which is true, but if you're going to use the same reference for both lift and drag equations (which you must,) and you should be selecting the wing surface area for the lift equation (it matters,) then you ought to be using the wing area used in the lift equation.

 

I think.

 

Correct me if I'm wrong, effte.

[effte]

Aaron,

you got close? ;)

 

There's no need per se to use the same area for the lift and the drag equation. Double the area for one and the net result is simply that the corresponding coefficients are halved. It makes sense and reduces the chances of screwing up later, but there's no technical need.

 

Using the wing planform area is conventional, but again - there's no direct need. The planform area to use for a given design can also be discussed. What do you do about the wing box, for starters? Is that planform area or not?

 

Cheers,

Fred

[Gundark]

Reffering to the pic D0, it seems that the refferenca area is indeed the wing area from Lift equation ( on D0 it is S ). But it still is not logicall for me, becouse graphs shows Cd (Cx on graphs) of the whole airframe with different loads ( Missiles, bombs etc. ). But, trying to translate from Russian is hard enough, so I'll post pages from Aerodynamic manual, maybe someone could find out what area is used there.

[Gundark]

And more...

[Gundark]

Hope someone could menage this. Thanks.

[RedBeard2]

Sorry for raising this old thread again, but I'd like to clarify some things with the SFM.

 

The SFM_Data[table_data] includes terms B and B4 by Mach, which appear to be used in the equation Cx = Cx_0 + Cy^2*B2 + Cy^4*B4. While this appears to be an equation for drag, I have yet to see any drag equation that uses a coefficient raised to the 4th power. My understanding is that CD (coefficient of drag) = CDmin (coefficient of drag for minimum lift) + CDi (coefficient of drag for induced drag) + CDv (coefficient of drag for viscous drag or drag due to skin friction). CDi = K'CLa(coefficient of lift per AoA)^2 and CDv = K''(CLa-CLmin)^2 where K' and K'' are constants based on wing efficiency, leading edge radius, and taper ratio. Note that none of the terms are raised to the 4th power and that K does not vary with Mach.

 

So, what am I confusing in interpreting the drag formula and terms?

 

Also, there seem to be two pitch coefficients, Mzalfa and Mzalfadt. The torque around the center of gravity will be based on lift from the wing, down force from the tail and offset thrust from the engine, though engine thrust torque doesn't vary by AoA. Pressure from the airstream hitting the engine air intakes and other parts of the body at higher AoA can also have an effect, though that may be overkill for the SFM. However, Mzalfa and MZalfadt are constants and do not vary by Mach, so I can't figure how these relate.

 

Lastly, I cannot find any reference anywhere to kjx and kjz either in the LOMAC equations or in any flight dynamics equations.

 

Can someone help educate me?

 

Thanks

[SilentEagle]

I have seen this equation: Cx = Cx_0 + Cy^2*B2 + Cy^4*B4 is Russian aerodynamics books, but nowhere else. Perhaps it is a Russian convention.

[RedBeard2]

Then what do B2 and B4 mean if they vary with Mach number and are not the K' and K'' constants?

[RedBeard2]

Ok, I went back through my aerodynamics text books and while K' and K'' are calculated as above for subsonic flows, there is a different calculation for K for supersonic flows, so I'm going to have to rethink a little bit about K being a constant, but I still cannot figure how Cy^4*B4 is equivalent to (CLa-CLmin)^2*K''.

[RedBeard2]

Ok, here's my latest set of unknowns that I'm looking for answers to:

 

What is Mzalfa and Mzalfadt? They are described as coefficients for pitch agility, but that isn't making any sense to me. Pitch is going to be based on a sum of moments about the center of gravity such as wing lift and tail plane down force and you would need to know the distance of each from the CG. There's not much in the way of variability for these either. For Mzalfa, 33 aircraft use 4.355, 2 used 6, and 26 use 6.6. Mzalfadt is similar in that 33 aircraft use 0.8 and 29 use a value of 1. Also, the sign and magnitude look wrong to be coefficients as most aerodynamic coefficients are negative and rarely are they greater than 1.

 

What is kjx and kjz? Kjz appears to be a constant of 0.00125 as it never varies for any of the aircraft. I can't figure what kjx means.

 

Czbe appears to be the coefficient for directional (yaw) stability (C sub n sub beta), but I can't figure out how the side slip angle beta is determined unless it is hard coded to the rudder pedal controller value. In this case, 23 planes use a value of -0.012 and 39 use a value of -0.016.

 

I'd appreciate any help in understanding how these parameters are used.