Simple aircraft physics question
the relative airspeed to an airplane goes also in 3d ,-genius-. is not an scalar/angle or "up/down" as the simpler equations tend to suggest.
the airfoil/wing model should cope with relative airspeed in 3D , i.e (not perpendicular to the wing ). Airplanes skid sideways , slip , etc.. most of the time. The relative airspeed can be in any axis .
how do your simple equations cope with non-perpendicular to the wing length airspeed ?
Therefore to define the lift/drag behaviour of a airfoil/wing when the relative air direction can come in ANY DIRECTION you need something much more complicated that the 2 equations you just wrote (and assume everyone knew).
For a wing whose area is defined by lenght , mayor chord , and minor chord (a ticpical trapezoidal wing ) you need serious math to compute the lift/drag if the Airspeed is not perpendicular to the length.
how would you do with your "angle of attack" ? ;)
Fortunately there are good approximations , all of them involving a 3x3 Matrix
-having said that , I agree that for a "simplistic flight model" , as for the original thread , your approach will do.
Cheers
J
oi oi .. I just realized that I forgot to explain why a Matrix is better than a Vector .
For a non lift-generating body (i.e. a bycicle) the aerodynamics can be defined with a Vector (expressing the equivalent drag-areas/coefficient) in any of the axis.
however, my friend , when we are interested also in the lift (as in a wing ) we need to express :
->the drag of that wing in any of the axis (as the with the previous bicycle example )
->the LIFT of that wing , for EACH axis , and this for EACH direction of relative airspeed .
i.e. (the lift of the wing on the Z axis when airspeed come in the Z Axis -this is your Lift Coeficient)
but also ... the lift on the wing in the X axis when airspeed come in the Y Axis ... and so-on
An airpliane moving sideways (x axis ) Also generates some sort of lift (and definitely lots of drag and rot-moment ). How your 1-dimension approach cope with this ?
but real planes (in 3 dimensions) move relative to the air in -also- 3 and hence have ccoefficients in 3 d.
Drag.x Momment.x Lift.x
Drag.y Momment.y Lift.y
Drag.z Momment.z Lift.z
whereas Drag.z is your "Drag Coefficient when air comes in z. direciton )
but Drag.x is the drag coefficient when air comes in the x direction (left-to right )
I am sure you start to see the need for the Matrix. Whereas you put those 9 coefficient values in a Matrix or in a sandwitch is up to the implementer. A matrix is handy as you multiply the relative airspeed ( as a VECTOR , not as a scalar ! ) to the matrix , etc..etc.. and you probabbly figure out the rest.
Cheeers
Javier
[Edited by - jcarrion on January 29, 2009 4:04:12 PM]
For a non lift-generating body (i.e. a bycicle) the aerodynamics can be defined with a Vector (expressing the equivalent drag-areas/coefficient) in any of the axis.
however, my friend , when we are interested also in the lift (as in a wing ) we need to express :
->the drag of that wing in any of the axis (as the with the previous bicycle example )
->the LIFT of that wing , for EACH axis , and this for EACH direction of relative airspeed .
i.e. (the lift of the wing on the Z axis when airspeed come in the Z Axis -this is your Lift Coeficient)
but also ... the lift on the wing in the X axis when airspeed come in the Y Axis ... and so-on
An airpliane moving sideways (x axis ) Also generates some sort of lift (and definitely lots of drag and rot-moment ). How your 1-dimension approach cope with this ?
but real planes (in 3 dimensions) move relative to the air in -also- 3 and hence have ccoefficients in 3 d.
Drag.x Momment.x Lift.x
Drag.y Momment.y Lift.y
Drag.z Momment.z Lift.z
whereas Drag.z is your "Drag Coefficient when air comes in z. direciton )
but Drag.x is the drag coefficient when air comes in the x direction (left-to right )
I am sure you start to see the need for the Matrix. Whereas you put those 9 coefficient values in a Matrix or in a sandwitch is up to the implementer. A matrix is handy as you multiply the relative airspeed ( as a VECTOR , not as a scalar ! ) to the matrix , etc..etc.. and you probabbly figure out the rest.
Cheeers
Javier
[Edited by - jcarrion on January 29, 2009 4:04:12 PM]
well, J... it is obvious from your last post that you have a lot of homework to do and probably should not be arguing this point as if you know what you're talking about, no offense. I hate to be argumentative, but I don't like it when bad information is spread. So, I'll humor you.
The L, D and V in "my" equations (you said MY equations? do you actually think I came up with those?) are vector quantities, not scalar. The "sideslip" angle you are referring to is typically called beta, and a more complicated model would use that component. Yes, a 3d flight model uses those equations, don't believe me??
What exactly do you think you're simulating here, a Harrier? a helicopter? maybe the JSF? When exactly was the last time you saw a conventional airplane flying backwards or sideways?
Perhaps you want to discuss the finer details with me (a commercial pilot with single and multiengine land instrument ratings, certified flight instructor, CFI instrument and multi-engine instructor with 1000+ hours and 5 years experience teaching aerodynamics to students, not to mention a BS in Aeronautical Science including several engineering courses) through another means, so as not to spam these boards with arguments.
BTW I was offering my help to OP because I have already created both a 2d and 3d flight simulator, and the only 3x3 matrix used in the flight model is the inertia tensor. Everything else is vector and quaternion based. If I didn't know what I was talking about, I would not have posted.
The L, D and V in "my" equations (you said MY equations? do you actually think I came up with those?) are vector quantities, not scalar. The "sideslip" angle you are referring to is typically called beta, and a more complicated model would use that component. Yes, a 3d flight model uses those equations, don't believe me??
What exactly do you think you're simulating here, a Harrier? a helicopter? maybe the JSF? When exactly was the last time you saw a conventional airplane flying backwards or sideways?
Perhaps you want to discuss the finer details with me (a commercial pilot with single and multiengine land instrument ratings, certified flight instructor, CFI instrument and multi-engine instructor with 1000+ hours and 5 years experience teaching aerodynamics to students, not to mention a BS in Aeronautical Science including several engineering courses) through another means, so as not to spam these boards with arguments.
BTW I was offering my help to OP because I have already created both a 2d and 3d flight simulator, and the only 3x3 matrix used in the flight model is the inertia tensor. Everything else is vector and quaternion based. If I didn't know what I was talking about, I would not have posted.
hi there
>>The L, D and V in "my" equations (you said MY equations? do you actually think >>I came up with those?) are vector quantities, not scalar. The "sideslip" angle >>you are referring to is typically called beta, and a more complicated model >>would use that component. Yes, a 3d flight model uses those equations, don't >>believe me??
>>What exactly do you think you're simulating here, a Harrier? a helicopter? >>maybe the JSF? When exactly was the last time you saw a conventional airplane >>flying backwards or sideways?
I ll try again...
your L , D , and V are computed for airspeed normal to X=0. I assume you understand what a plane , a normal to a plane is .
you have not mentioned yet what happens with the airspeed in the plane z =0. If you are a pilot you should know better than anyone that airplanes skid , slip . either by leaning, or using the rudder , asymetric thrust , etc.. -I do believe you are a pilot so I assume you know what I mean- Airplanes experience lateral airspeed . Not much , but certainly not negligible . and has tremendous consecuences in drag , and rotation torque.
yet "your simplified model" doesnt take this into acconut at all.
My point is that you should have another set of equations (with its lift , drag and Torque ) for the airspeed moving in X=0. Airplanes have drag , and lift , and torque also when applied lateral airspeed .
That this airspeed = 0? fantastic ! we are fliying straight or turning coordinately ., call it a day . But It may not be . If you are doing a flight simulator you SHOULD cope with the aircraft moving backwards , forward , or whichever position/ attitude/relative airspeed ( aerobatics ?.)
I don't say that 1 plane model is wrong . For most simple flight simulators is ok .It is just too simplistic.
(I am diying you see your 2D (???????) flight simulator! )
On a absolutely unrelated front :
do you have any idea where can I find data for turboshafts ? Mainly Rotational mass , diameter , total mass , specific fuel consumtion etc ? I am really lost , and can't find anything .We are indeed reverse-engineering values from Microsoft FS ! which is dodgy as hell . Any idea where can we find manufacturer's data ?
oi oi
Jc
>>The L, D and V in "my" equations (you said MY equations? do you actually think >>I came up with those?) are vector quantities, not scalar. The "sideslip" angle >>you are referring to is typically called beta, and a more complicated model >>would use that component. Yes, a 3d flight model uses those equations, don't >>believe me??
>>What exactly do you think you're simulating here, a Harrier? a helicopter? >>maybe the JSF? When exactly was the last time you saw a conventional airplane >>flying backwards or sideways?
I ll try again...
your L , D , and V are computed for airspeed normal to X=0. I assume you understand what a plane , a normal to a plane is .
you have not mentioned yet what happens with the airspeed in the plane z =0. If you are a pilot you should know better than anyone that airplanes skid , slip . either by leaning, or using the rudder , asymetric thrust , etc.. -I do believe you are a pilot so I assume you know what I mean- Airplanes experience lateral airspeed . Not much , but certainly not negligible . and has tremendous consecuences in drag , and rotation torque.
yet "your simplified model" doesnt take this into acconut at all.
My point is that you should have another set of equations (with its lift , drag and Torque ) for the airspeed moving in X=0. Airplanes have drag , and lift , and torque also when applied lateral airspeed .
That this airspeed = 0? fantastic ! we are fliying straight or turning coordinately ., call it a day . But It may not be . If you are doing a flight simulator you SHOULD cope with the aircraft moving backwards , forward , or whichever position/ attitude/relative airspeed ( aerobatics ?.)
I don't say that 1 plane model is wrong . For most simple flight simulators is ok .It is just too simplistic.
(I am diying you see your 2D (???????) flight simulator! )
On a absolutely unrelated front :
do you have any idea where can I find data for turboshafts ? Mainly Rotational mass , diameter , total mass , specific fuel consumtion etc ? I am really lost , and can't find anything .We are indeed reverse-engineering values from Microsoft FS ! which is dodgy as hell . Any idea where can we find manufacturer's data ?
oi oi
Jc
I really don't want us to hijack this thread, but I guess we're the only ones keeping it alive now so it's ok.
but I did not present the entire model... only a small piece of the puzzle. I never stated that the lift and drag equations represent the entire flight model.
Secondly, it is not my model. My name is not Bernoulli and it is certainly not Newton.
You are absolutely right. For directions of travel outside of the normal flight envelope, with high alpha beyond your CL and CD curve/table range you can extrapolate for a while, beyond that use a simple linear model for CL and CD. Weathervaning tendency will give negative static stability to a conventional airplane in sideways or backward flight anyway, and that direction of travel cannot be maintained for long - that is, moments generated by the flight surfaces will tend to realign the longitudinal axis with the direction of travel, and back within the normal flight envelope.
You are taking a series of operations to determine the drag forces acting upon an aircraft and over-simplifying it based on the premise that a vector-matrix multiplication yields another vector.
Right, and the model does not negate it, it is all incorporated.
Why do I have to explain this again? Those are vector quantities. That means 3 dimensions dude! The equations give you the magnitude of the vector. I've already stated the direction in a previous post. It's not more simplistic.
It is actually called a vertical plane simulator because it simulates exactly that, the vertical plane. It is based on a VPS used in a Flight Technique Analysis class that I took. I will get it posted and provide a link so you may download it. It's been about 9 years since I looked at it, and is in no way a finished product, my crude proof-of-concept before delving into the full 3d model, just integrates linear motion and not rotational motion. It is meant to be an analysis tool more than a typical flight sim, so control inputs involve setting the pitch and thrust explicitly, and seeing how that impacts your angle of attack, airspeed, etc. and watching the forces on the airplane eventually settle back into equilibrium.
Turboshafts? for rotercraft? I'm a fixed-wing guy myself and don't know where free data may be found on the internet. I image you should get it from the manufacturer. You could also pull some useful data out of flight or maintenance manuals from specific models. I wouldn't necessarily say pulling values from MSFS is dodgy, they probably got their data from the manufacturer.
[Edited by - y2kiah on January 30, 2009 7:36:30 AM]
Quote:yet "your simplified model" doesnt take this into acconut at all.
but I did not present the entire model... only a small piece of the puzzle. I never stated that the lift and drag equations represent the entire flight model.
Secondly, it is not my model. My name is not Bernoulli and it is certainly not Newton.
Quote:If you are doing a flight simulator you SHOULD cope with the aircraft moving backwards , forward , or whichever position/ attitude/relative airspeed ( aerobatics ?.)
You are absolutely right. For directions of travel outside of the normal flight envelope, with high alpha beyond your CL and CD curve/table range you can extrapolate for a while, beyond that use a simple linear model for CL and CD. Weathervaning tendency will give negative static stability to a conventional airplane in sideways or backward flight anyway, and that direction of travel cannot be maintained for long - that is, moments generated by the flight surfaces will tend to realign the longitudinal axis with the direction of travel, and back within the normal flight envelope.
You are taking a series of operations to determine the drag forces acting upon an aircraft and over-simplifying it based on the premise that a vector-matrix multiplication yields another vector.
Quote:Not much , but certainly not negligible . and has tremendous consecuences in drag , and rotation torque.
Right, and the model does not negate it, it is all incorporated.
Quote:I don't say that 1 plane model is wrong . For most simple flight simulators is ok .It is just too simplistic.
Why do I have to explain this again? Those are vector quantities. That means 3 dimensions dude! The equations give you the magnitude of the vector. I've already stated the direction in a previous post. It's not more simplistic.
Quote:(I am diying you see your 2D (???????) flight simulator! )
It is actually called a vertical plane simulator because it simulates exactly that, the vertical plane. It is based on a VPS used in a Flight Technique Analysis class that I took. I will get it posted and provide a link so you may download it. It's been about 9 years since I looked at it, and is in no way a finished product, my crude proof-of-concept before delving into the full 3d model, just integrates linear motion and not rotational motion. It is meant to be an analysis tool more than a typical flight sim, so control inputs involve setting the pitch and thrust explicitly, and seeing how that impacts your angle of attack, airspeed, etc. and watching the forces on the airplane eventually settle back into equilibrium.
Quote:do you have any idea where can I find data for turboshafts ? Mainly Rotational mass , diameter , total mass , specific fuel consumtion etc ? I am really lost , and can't find anything .We are indeed reverse-engineering values from Microsoft FS ! which is dodgy as hell . Any idea where can we find manufacturer's data ?
Turboshafts? for rotercraft? I'm a fixed-wing guy myself and don't know where free data may be found on the internet. I image you should get it from the manufacturer. You could also pull some useful data out of flight or maintenance manuals from specific models. I wouldn't necessarily say pulling values from MSFS is dodgy, they probably got their data from the manufacturer.
[Edited by - y2kiah on January 30, 2009 7:36:30 AM]
Author
Quote:Original post by y2kiah
For the lifting surfaces
------------------------
Lift: L = (CL)(1/2)(rho)(V^2)(S) or L = (CL)(q)(S)
Drag: D = (CD)(1/2)(rho)(V^2)(S) or D = (CD)(q)(S)
Where:
CL = coefficient of lift (varies with angle of attack)
CD = coefficient of drag (varies with angle of attack)
rho = density of fluid (air)
V = velocity of free-stream air
S = wing planform area
q = expressed as "dynamic pressure", is equal to (1/2)(rho)(V^2)
Thanks, I'll give that a try.
Do you know of any good resources where I could find the coefficients for lift and drag for current aircraft (I'm trying to simulate a few military fighters)? Or is there a good way to generate them?
you can go here to find out which wing type your airplane of interest uses
then start searching for good locations of coefficient data. I found a good source here for the NACA families of airfoils, and many other sources more detailed but not quite as convenient
then start searching for good locations of coefficient data. I found a good source here for the NACA families of airfoils, and many other sources more detailed but not quite as convenient
Author
Quote:Original post by y2kiahThanks for the links.
you can go here to find out which wing type your airplane of interest uses
then start searching for good locations of coefficient data. I found a good source here for the NACA families of airfoils, and many other sources more detailed but not quite as convenient
How would I convert the values on the NACA profiles to lift and drag coefficients? For example I am looking at the F-15 (which has a 'NACA 64A006.6' wing), so this would be the matching profile, not sure how to get the coefficients from the data (google hasn't turned up any good results yet).
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