willsonman
Well-known member
As I have been able to play around with RF6 lately I have noticed several issues with custom content that have been, well,... frustrating. As it is a new feature I would like to discuss, perhaps ad nausea, the topic of center of pressure (COP) modifications.
Ground rules:
RF's description:
"Moves the nominal center of pressure of the fuselage, for side wind, in percent of half-chord forward (positive) and backward (negative) from the center of area.
Most of the time you can leave it at the default value of 0%, and let RealFlight calculate it for you.
If you wish to override the RealFlight calculation, move it backwards by entering in percentages less than 0%. Move it forward by entering in values greater than 0%. A value of 100% places the center of pressure at the front, while a value of -100% places it at the rear."
Definition (Wikipedia for simplicity sake):
The center of pressure is the point on a body where the total sum of a pressure field acts, causing a force and no moment about that point. The total force vector acting at the center of pressure is the value of the integrated vectorial pressure field. The resultant force and center of pressure location produce equivalent force and moment on the body as the original pressure field. Pressure fields occur in both static (hydrostatic) and dynamic (aerodynamic) fluid mechanics. Specification of the center of pressure, the reference point from which the center of pressure is referenced, and the associated force vector allows the moment generated about any point to be computed by a translation from the reference point to the desired new point.
...
A stable configuration is not only desirable in sailing, but in aircraft design as well. Aircraft design therefore borrowed the term center of pressure. But unlike a sail, a rigid non-symmetrical airfoil not only produces lift, but a moment. The center of pressure of an aircraft is the point where all of the aerodynamic pressure field may be represented by a single force vector with no moment. A similar idea is the aerodynamic center which is the point on an airfoil where the pitching moment produced by the aerodynamic forces is constant with angle of attack. The aerodynamic center plays an important role in analysis of the longitudinal static stability of aircraft and other flying vehicles. It is desirable that when the pitch angle and angle of attack of an aircraft are disturbed (by, for example turbulence) that the aircraft returns to its original trimmed pitch angle and angle of attack without a pilot or autopilot changing the control surface deflection. For an aircraft to return towards its trimmed attitude, without input from a pilot or autopilot, it must have positive longitudinal static stability.
The first example I'd like to discuss is the TF P-51 provided by DHK
https://forums.realflight.com/downloads.php?do=file&id=10144
This can be directly compared to the Gold edition aircraft provided by EP5.
The first thing I would like to point out is that in my findings it is more critical to adjust this value when using non-symmetrical airfoils for the fuselage. Second, switch the airfoils for the fuselage on DHK's models. I've uninstalled G4.5 at this point but the side vs top seems to be switched from the logical standpoint.
I noticed that the EP5 version flys very stable and has predictable flying. DHK's version has a very difficult time with lateral stability and is heavily sensitive to torque from the engine. So much so that it is beyond realistic measures. What is interesting to note is that when DHK's fuselage is modified to have the same NACA0016 airfoils this issue immediately dampens. Flying is much better but torque is still a factor and... back torque factor adjustment is now removed from the advanced editing of the motor. This all being accounted for the airfoils selected by DHK for his version are more accurately shaped for the physics model. Then herald in the COP adjustment. The COP of the average airfoil is generally toward the 25-33%MAC. This would indicate that COP would need to be +50% or so. In fact when you adjust this you find that stability gets worse and -50% is more ideal. This is something that I am not as familiar with so feel free to chime in. Once adjusted to -50% for both parameters the plane flies very well... however you need to trim the roll channel.
At this point I would like to emphasize two more things.
1) It is now mandatory to actually look at sensitive physics properties when setting up custom content.
2) Ratings will be lower on the swaps by amateurs because they will be required to trim a model once downloaded from the swaps unless trim is programmed into the throws of control surfaces. As I understand trims are user-custom and do not transfer with uploading.
Now I want to address the subject of lift.
Study subject: P-47 Razorback by pplace
https://forums.realflight.com/downloads.php?do=file&id=13605
I know this subject well. The physics model was fashioned after an actual model so I know weights and incidences are correct. However, the lift seems to be tremendous upon download and there seems to be slight lateral instability. The fuselage uses two symmetrical airfoils so the above principle should not be as critical right? WRONG! Without changing incidence on the main wing if you adjust the COP to the average -75% for both values on the fuselage she flies perfect again... after only a few clicks of roll trim.
These are some of my findings and I hope to see some of this taken to heart. I notice that a lot of folks do not spend as much time with the physics as perhaps they should and I am hoping that some of this will open the eyes of others to really analyzing they methods they use for setup.
I would like to see the back torque factor adjustment come back.
On a closing note I want to make it perfectly clear that KE is to be commended for these changes as they reflect the real world more and add to the realism of designing a custom model. I am thrilled about this as I always thought it strange to download a plane and never have to trim it. I've never had a maiden when something needed to be trimmed or adjusted to some degree and this is more like it!
Please comment and post your findings here as you experiment with this setting.
Ground rules:
RF's description:
"Moves the nominal center of pressure of the fuselage, for side wind, in percent of half-chord forward (positive) and backward (negative) from the center of area.
Most of the time you can leave it at the default value of 0%, and let RealFlight calculate it for you.
If you wish to override the RealFlight calculation, move it backwards by entering in percentages less than 0%. Move it forward by entering in values greater than 0%. A value of 100% places the center of pressure at the front, while a value of -100% places it at the rear."
Definition (Wikipedia for simplicity sake):
The center of pressure is the point on a body where the total sum of a pressure field acts, causing a force and no moment about that point. The total force vector acting at the center of pressure is the value of the integrated vectorial pressure field. The resultant force and center of pressure location produce equivalent force and moment on the body as the original pressure field. Pressure fields occur in both static (hydrostatic) and dynamic (aerodynamic) fluid mechanics. Specification of the center of pressure, the reference point from which the center of pressure is referenced, and the associated force vector allows the moment generated about any point to be computed by a translation from the reference point to the desired new point.
...
A stable configuration is not only desirable in sailing, but in aircraft design as well. Aircraft design therefore borrowed the term center of pressure. But unlike a sail, a rigid non-symmetrical airfoil not only produces lift, but a moment. The center of pressure of an aircraft is the point where all of the aerodynamic pressure field may be represented by a single force vector with no moment. A similar idea is the aerodynamic center which is the point on an airfoil where the pitching moment produced by the aerodynamic forces is constant with angle of attack. The aerodynamic center plays an important role in analysis of the longitudinal static stability of aircraft and other flying vehicles. It is desirable that when the pitch angle and angle of attack of an aircraft are disturbed (by, for example turbulence) that the aircraft returns to its original trimmed pitch angle and angle of attack without a pilot or autopilot changing the control surface deflection. For an aircraft to return towards its trimmed attitude, without input from a pilot or autopilot, it must have positive longitudinal static stability.
The first example I'd like to discuss is the TF P-51 provided by DHK
https://forums.realflight.com/downloads.php?do=file&id=10144
This can be directly compared to the Gold edition aircraft provided by EP5.
The first thing I would like to point out is that in my findings it is more critical to adjust this value when using non-symmetrical airfoils for the fuselage. Second, switch the airfoils for the fuselage on DHK's models. I've uninstalled G4.5 at this point but the side vs top seems to be switched from the logical standpoint.
I noticed that the EP5 version flys very stable and has predictable flying. DHK's version has a very difficult time with lateral stability and is heavily sensitive to torque from the engine. So much so that it is beyond realistic measures. What is interesting to note is that when DHK's fuselage is modified to have the same NACA0016 airfoils this issue immediately dampens. Flying is much better but torque is still a factor and... back torque factor adjustment is now removed from the advanced editing of the motor. This all being accounted for the airfoils selected by DHK for his version are more accurately shaped for the physics model. Then herald in the COP adjustment. The COP of the average airfoil is generally toward the 25-33%MAC. This would indicate that COP would need to be +50% or so. In fact when you adjust this you find that stability gets worse and -50% is more ideal. This is something that I am not as familiar with so feel free to chime in. Once adjusted to -50% for both parameters the plane flies very well... however you need to trim the roll channel.
At this point I would like to emphasize two more things.
1) It is now mandatory to actually look at sensitive physics properties when setting up custom content.
2) Ratings will be lower on the swaps by amateurs because they will be required to trim a model once downloaded from the swaps unless trim is programmed into the throws of control surfaces. As I understand trims are user-custom and do not transfer with uploading.
Now I want to address the subject of lift.
Study subject: P-47 Razorback by pplace
https://forums.realflight.com/downloads.php?do=file&id=13605
I know this subject well. The physics model was fashioned after an actual model so I know weights and incidences are correct. However, the lift seems to be tremendous upon download and there seems to be slight lateral instability. The fuselage uses two symmetrical airfoils so the above principle should not be as critical right? WRONG! Without changing incidence on the main wing if you adjust the COP to the average -75% for both values on the fuselage she flies perfect again... after only a few clicks of roll trim.
These are some of my findings and I hope to see some of this taken to heart. I notice that a lot of folks do not spend as much time with the physics as perhaps they should and I am hoping that some of this will open the eyes of others to really analyzing they methods they use for setup.
I would like to see the back torque factor adjustment come back.
On a closing note I want to make it perfectly clear that KE is to be commended for these changes as they reflect the real world more and add to the realism of designing a custom model. I am thrilled about this as I always thought it strange to download a plane and never have to trim it. I've never had a maiden when something needed to be trimmed or adjusted to some degree and this is more like it!
Please comment and post your findings here as you experiment with this setting.
