Jump to content

Questions about SFM aerodynamic parameters


CptSmiley
 Share

Recommended Posts

Hey all, here is what the commented version of the Wunderluft looks like for the SFM parameters:

 

aerodynamics = -- Cx = Cx_0 + Cy^2*B2 +Cy^4*B4
	{
		Cy0			=	0,      -- zero AoA lift coefficient
		Mzalfa		=	4.355,  -- coefficients for pitch agility
		Mzalfadt	=	0.8,    -- coefficients for pitch agility
		kjx			=	2.3,    
		kjz			=	0.0011, 
		Czbe		=	-0.014, -- coefficient, along Z axis (perpendicular), affects yaw, negative value means force orientation in FC coordinate system
		cx_gear		=	0.3,    -- coefficient, drag, gear
		cx_flap		=	0.08,   -- coefficient, drag, full flaps
		cy_flap		=	0.4,    -- coefficient, normal force, lift, flaps
		cx_brk		=	0.08,   -- coefficient, drag, breaks
		table_data  = 
		{	--      M		Cx0		 	Cya			B		 	B4	    	Omxmax		Aldop		Cymax
			[1] = 	{0,	0.024,	0.07,	0.075,	0.12,	0.5,											30					,	1.2},
			[2] = 	{0.2,	0.024,	0.07,	0.075,	0.12,	1.5,										30					,	1.2},
			[3] = 	{0.4,	0.024,	0.07,	0.075,	0.12,	2.5,										30					,	1.2},
			[4] = 	{0.6,	0.0239,	0.073,	0.075,	0.12,	3.5,										30					,	1.2},
			[5] = 	{0.7,	0.024,	0.076,	0.075,	0.12,	3.5,										28.666666666667		,	1.18},
			[6] = 	{0.8,	0.0235,	0.079,	0.075,	0.12,	3.5,										27.333333333333		,	1.16},
			[7] = 	{0.9,	0.025,	0.083,	0.075,	0.125,	3.5,										26					,	1.14},
			[8] = 	{1	,	0.044,	0.085,	0.14,	0.1,	3.5,										24.666666666667		,	1.12},
			[9] = 	{1.05,	0.0465,	0.0855,	0.1775,	0.125,	3.5,										24					,	1.11},
			[10] = 	{1.1,	0.049,	0.086,	0.215,	0.15,	3.15,										18			,	1.1},
			[11] = 	{1.2,	0.049,	0.083,	0.228,	0.17,	2.45,										17,	1.05},
			[12] = 	{1.3,	0.049,	0.077,	0.237,	0.2,	1.75,										16,	1},
			[13] = 	{1.5,	0.0475,	0.062,	0.251,	0.2,	1.5,										13,	0.9},
			[14] = 	{1.7,	0.045166666666667,	0.051333333333333,	0.24366666666667,	0.32,	0.9,	12,	0.7},
			[15] = 	{1.8,	0.044,	0.046,	0.24,	0.38,	0.86,										11.4,	0.64},
			[16] = 	{2,	0.043,	0.039,	0.222,	2.5,	0.78,											10.2,	0.52},
			[17] = 	{2.2,	0.041,	0.034,	0.227,	3.2,	0.7,										9,	0.4},
			[18] = 	{2.5,	0.04,	0.033,	0.25,	4.5,	0.7,										9,	0.4},
			[19] = 	{3.9,	0.035,	0.033,	0.35,	6,		0.7,										9,	0.4},
		}, -- end of table_data
		-- M - Mach number
		-- Cx0 - Coefficient, drag, profile, of the airplane
		-- Cya - Normal force coefficient of the wing and body of the aircraft in the normal direction to that of flight. Inversely proportional to the available G-loading at any Mach value. (lower the Cya value, higher G available) per 1 degree AOA
		-- B - Polar quad coeff
		-- B4 - Polar 4th power coeff
		-- Omxmax - roll rate, rad/s
		-- Aldop - Alfadop Max AOA at current M - departure threshold
		-- Cymax - Coefficient, lift, maximum possible (ignores other calculations if current Cy > Cymax)
	}, -- end of aerodynamics

 

For another project, not the F-16, I'm utilizing the SFM. I am not seeing in these paramaters how control coefficients are set (CmdelElev, CldelAil, etc...) any ideas?

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

Link to comment
Share on other sites

Thanks Sevas, yeah I saw that thread earlier. I could not find anything in there describing how stability and control coefficients are defined (only aerodynamic ones)

 

For example, where is Cm-delE (pitch moment coefficient change with incremental elevator change) or Cl-delAil (roll moment coefficient change with incremental aileron change) or Cn_delRud (yaw moment coefficient change with incremental rudder change) amongst others...

 

Does it use the known geometry of the aircraft you are flying and performs simple analysis using DATCOM or some other method to estimate it that way?

 

Thanks again Sevas..

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

Link to comment
Share on other sites

Has nothing to do with geometry. It most likely uses surface area reference numbers and a generic equation... tune the dimensionless coefficient until you see behavior similar to what you want. That's the best it gets.

 

A little contradicting...how is "surface area reference numbers" not geometry?

 

Stability and control coefficients have everything to do with geometry (location, planform, airfoil, min/max deflection, etc...) or else aircraft could be shaped whatever they want. Hence why you use AVL or some analysis tool derive control coefficients simply by only having to describe aircraft geometry.

 

I'm just really confused on how, when utilizing the SFM, that it knows that when I pull back on the stick it means that I am telling the elevator to go leading edge down which induces a pitching moment of some amount. There is nothing in the Wunderluft example that describes how elevator, aileron, and rudder deflections work..they just do. Is it somewhere in the cockpit or input lua files? How does the 3d model even know to animate the surfaces a given deflection? I am clear withing the external flight model DLL how to do this but looking through the LUA stuff I can't really find correlating stuff, it just happens magically or I'm looking in the wrong place...

 

I'm clear on the static aerodynamic coefficient definitions within the SFM configurations...just how control theory works within DCS.

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

Link to comment
Share on other sites

A little contradicting...how is "surface area reference numbers" not geometry?

Perhaps. I was referring to the total frontal and vertical surface areas as defined for SFMs. The position, size, deflection, and shape of control surfaces are not defined. It is a simplified flight model.

 

you use AVL or some analysis tool derive control coefficients simply by only having to describe aircraft geometry.

Don't get hung up on this, I'm warning you now. :smilewink:

 

I'm just really confused on how, when utilizing the SFM, that it knows that when I pull back on the stick it means that I am telling the elevator to go leading edge down which induces a pitching moment of some amount.

It's simplified. You have CyA, which essentially defines the slope of the CL vs A curve, and CyMax, which defines a maximum lift value. If you want to further limit pitch rate, consider limiting maximum angle of attack. Novice aerodynamic theory suggests a near-linear angle of attack change per unit of control deflection. I would suggest that SFM allows a linear angle of attack change proportional to CyMax, or something of that nature. You have no control over it.

 

 

There is nothing in the Wunderluft example that describes how elevator, aileron, and rudder deflections work..they just do. Is it somewhere in the cockpit or input lua files?

Nope.

 

How does the 3d model even know to animate the surfaces a given deflection?

That's kind of a "why is the sky blue" question. The animation is created by the modeler. He can give it a range of deflection. The SFM dlls will make use of that entire animation range. (-1 to 1)

 

I'm clear on the static aerodynamic coefficient definitions within the SFM configurations...just how control theory works within DCS.

You just get what you get.

Link to comment
Share on other sites

It's simplified. You have CyA, which essentially defines the slope of the CL vs A curve, and CyMax, which defines a maximum lift value. If you want to further limit pitch rate, consider limiting maximum angle of attack. Novice aerodynamic theory suggests a near-linear angle of attack change per unit of control deflection. I would suggest that SFM allows a linear angle of attack change proportional to CyMax, or something of that nature. You have no control over it.

 

I would imagine that is the only possible way this works in the SFM world unless someone from ED can clarify. If that is really the case, then wouldn't all SFM aircraft essentially be controlled and flown very similarly. The only difference in their angular acceleration response would be the differences in their moments of inertia. From what I understand is you are basically stating that for a given elevator control deflection, you're getting the same "power" to affect change in moment for all SFM aircraft. It's a good idea you have, I'll mess with the static aerodynamic values and see if the control reponse changes at all and report back, thanks for the suggestions.

 

That's kind of a "why is the sky blue" question. The animation is created by the modeler. He can give it a range of deflection. The SFM dlls will make use of that entire animation range. (-1 to 1)

 

Maybe for you it is, but for someone who is just beginning to figure out how the an aircraft is configured in LUA, it is not, that is obviously why I asked the question in the first place. What you are saying is that internally, a 3D modeler defines a part as "elevator_part" with a given animation range, then DCS internally knows that when I pull back on the stick to animate "elevator_part" proportional to how much I pull back on the stick?

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

Link to comment
Share on other sites

If that is really the case, then wouldn't all SFM aircraft essentially be controlled and flown very similarly.

Yes. Of course, all real aircraft share handling qualities, but SFMs are decidedly homogeneous.

 

 

Maybe for you it is, but for someone who is just beginning to figure out how the an aircraft is configured in LUA, it is not, that is obviously why I asked the question in the first place.

You have a point. My lenses are somewhat tinted.

 

What you are saying is that internally, a 3D modeler defines a part as "elevator_part" with a given animation range, then DCS internally knows that when I pull back on the stick to animate "elevator_part" proportional to how much I pull back on the stick?

Exactly. For now.

Link to comment
Share on other sites

Yes. Of course, all real aircraft share handling qualities, but SFMs are decidedly homogeneous.

 

 

 

You have a point. My lenses are somewhat tinted.

 

 

Exactly. For now.

 

Thanks for the responses, I think I have a better feel for how the SFM works. I'm messing with it tonight to see how altering the static parameters changes the control response. Will report back..

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

Link to comment
Share on other sites

What you are saying is that internally, a 3D modeler defines a part as "elevator_part" with a given animation range, then DCS internally knows that when I pull back on the stick to animate "elevator_part" proportional to how much I pull back on the stick?

 

Smiley, that`s what you need I think, cool animation tutorial for EDM models, probably you saw it?

 

http://forums.eagle.ru/showpost.php?p=1522334&postcount=185

Link to comment
Share on other sites

Thanks Sevas! I think I got things figured out and figured out how to manipulate the SFM to get the control authority I want...Mzalfa and Mzalfadt where the big ones for the pitch control and Omxmax was the big one for the roll control. Made a huge difference. Thanks for the help guys!

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

Link to comment
Share on other sites

 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...