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Guest pecrowther

Adding Lag to Rudder Response

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I know this may seem like a step backwards, but: is there a way to add a lag to the rudder response?Thanks,Phil

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>I know this may seem like a step backwards, but: is there a>way to add a lag to the rudder response?>Thanks,>PhilIncrease Moment of Inertia for yaw ?

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Thanks for the response. If I had my way the wings would be broken - since I am modeling a ship. I calculated the MOI based on the dimensions of the ship, so that should already be pretty high. In the ship I am modeling, the rudder takes about 15 secs. to swing from side to side. I thought it would be "fun" too add that feature to introduce virtual sailors to the concept of pilot-induced oscillations.Phil

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>Thanks for the response. If I had my way the wings would be>broken - since I am modeling a ship. I calculated the MOI>based on the dimensions of the ship, so that should already be>pretty high. In the ship I am modeling, the rudder takes>about 15 secs. to swing from side to side. I thought it would>be "fun" too add that feature to introduce virtual sailors to>the concept of pilot-induced oscillations.>Phil"Break the wings" is actually a part of my signature.I was just blind-guessing about the MOI. I would also like to incorporate lags between control and response along the other axes.

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Heh, no wonder the reply didn't quote "break the wings".I found an article praising the designer of the fs2004 Vickers Vimy for incorporating a lag in the control response. And some of the messages on this board indicate that there may be a value in the air file that can be changed to cause a lag.I was just hoping that someone might know the answer right off.Your MOI idea might help. Also I might be able to change the yaw rate by distributing the weight at the front and back. For example, I assume that hanging engines far out on the wings will decrease the roll rate (as in the P38). Perhaps hanging weight (e.g. engines or fuel) far to the front and back will help slow down the yaw rate (although it might adversely affect the pitch rate as well).Thanks,Phil

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>some of the messages on this board indicate that there may be>a value in the air file that can be changed to cause a lag.>I was just hoping that someone might know the answer right>off.>Your MOI idea might help. Also I might be able to change the>yaw rate by distributing the weight at the front and back. >For example, I assume that hanging engines far out on the>wings will decrease the roll rate (as in the P38). Perhaps>hanging weight (e.g. engines or fuel) far to the front and>back will help slow down the yaw rate (although it might>adversely affect the pitch rate as well).>Thanks,>Phil Cn_beta (Weathervane Stability) Cn_r (Yaw Damping) Yaw MoI are the primary Yaw related components. While Cn_dr scales the rudder moment. Since dynamic pressure, q, is due to water rather than Air, they would have to be much higher for a ship than for an aircraft (at the same speed). To correct for the fact the flight model assumes the vehicle is moving through air, not water. OTOH, setting wing area high might let one use more typical values. One still has to set wing area, S, since all the Stability Derivatives are scaled from it. Actually from S*q. Perhaps setting it especially large would compensate for the water vs air difference. For air, q = 1/2 rho * V^2. I'd guess it's the same formula for water, but density, rho is much higher. For air, rho is 0.00237 slug/ft^2. I think rho for water is about 2.0. Since S*q is a major scaling factor, it seems making 'wing area' S, 1000 X what one would figure for an AC would be worth trying. However, FS messes up the 'equations of flight' when these variables are too far from what typical AC have. The MoI's are NOT affected by where the engines, etc. are. These MoI's should include the effect of such mass distributions. Variations in fuel tank positions and fuel weight are accounted for. But, are likely not that significant for ships. I don't know how a ship's rudder affects its Roll, but I'd expect ships to also roll some when turning. One would probably be best to set the Yaw-Roll components to zero, at least for a start. Position of the 'vertical tail' also affects rudder to roll couplings. Note the rudder of a ship is below the vertical center of gravity. This aids in turning, since any roll moment is in the appropriate direction for a turn. The reason most AC have vertical fins above the fuselage is to keep them clear of the ground. Ron

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Ron,At one time, I had tried removing all the non ship parameters from the air and cfg files. But I could never get it to handle properly. I increased parasitic drag, so that it took quite awhile for the ship to start moving. But, once it started moving, it just kept accelerating. Obviously parasitic drag was not doing its part. So I have to assume that induced drag plays a big role in fs2004, which is why I have pretty much left the aerodynamics of the 737 air and cfg files alone.The ship still has some significant oscillations in speed. While this may be due to the low speeds that I am using (about taxi speed for an aircraft), I notice that the Wright Flyer can hobble along forever without any significant oscillations in speed.I think ships do roll a bit when turning, leaning away from the turn. Might be worth fooling around with the location of the rudder to simulate this effect. At one time, I tried to model the thrust of a propeller in water, but that was not very successful.I did have some success redistributing the weight, by moving the fuel from the center to about 100 feet fore and aft. Although that did not cause the rudder to lag, it did cause the ship to lag in response to the rudder.That's what I get for trying to turn a flight simulator into a ship simulator! And no matter how good it gets, I will still miss the sensation of bouncing around in the waves.Phil

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>Ron,>>At one time, I had tried removing all the non ship parameters>from the air and cfg files. But I could never get it to>handle properly. I increased parasitic drag, so that it took>quite awhile for the ship to start moving. But, once it>started moving, it just kept accelerating. Obviously>parasitic drag was not doing its part. So I have to assume>that induced drag plays a big role in fs2004, which is why I>have pretty much left the aerodynamics of the 737 air and cfg>files alone. Induced Drag can be set to approach zero. A high Wing Aspect ratio and low lift slope (TBL 404) would reduce it. Since Lift is also low or zero at zero AoA. I assume your ship is on floats. Those are all messed up in MSFS. They give a high drag until speed gets above a certain amount; I'm not sure what the drag function is. Regardless, aerodynamic effects will add to any effect of the floats.>The ship still has some significant oscillations in speed. >While this may be due to the low speeds that I am using (about>taxi speed for an aircraft), I notice that the Wright Flyer>can hobble along forever without any significant oscillations>in speed. Once the C208 Amphibian is moving, speed can be controlled fairly well.>I think ships do roll a bit when turning, leaning away from>the turn. Might be worth fooling around with the location of>the rudder to simulate this effect. At one time, I tried to>model the thrust of a propeller in water, but that was not>very successful. Once I took a DL'ed auto and modified it to make it more realistic. I set a high prop Ct slope and a large prop. To emulate an automatic transmission. I'd have had to control Prop Beta directly to model a manual transmission. BTW, I set the Cefficiency table near 0.95 at all combinations of J and Beta. With the large prop and prop table adjustments, Engine RPM was almost directly proportional to speed and Prop Beta. IOW, very little slip. One needs some 'slip' at low RPM's or he can't start the engine. Since it would be locked to the wheels. >I did have some success redistributing the weight, by moving>the fuel from the center to about 100 feet fore and aft. >Although that did not cause the rudder to lag, it did cause>the ship to lag in response to the rudder.>>That's what I get for trying to turn a flight simulator into a>ship simulator! And no matter how good it gets, I will still>miss the sensation of bouncing around in the waves.>Phil Float AC bounce some, though the waves don't appear to depend on surface winds. Remember, float AC can have water rudders. I had to set at least the Roll MoI higher than realistic for the auto to stay stable. I left the rudder, etc. effects; the ground handling still needed some aerodynamic effects to stay stable. For the Auto, I set the wing to the dimensions of top surface of the vehicle. And, probably made CL vs AoA in TBL 404 appropriate for the limited lift one gets from an auto's body. Cd for autos is normally based on frontal area, while it's referenced to Wing Area for an AC. I probably adjusted Cdo to give an appropriate drag relative to S rather than Afront. Though, would have tweaked it manually to get an appropriate top speed.Ron

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Ron,I hadn't even thought about the way FS models floats, but you are right. The drag for the float would drop dramatically when it reaches the speed where it gets "on the step". I wonder is that is "hard-wired" into fs2004? If so, it really increases the difficulty of modeling drag on a ship in fs2004 (not that fs2004 was designed for modeling ships).Also, after staring at the airfile, I realized that I have zeroed out most everything in the airfile relating to the rudder. Presumably, the steering is done entirely by the floats (although I have not deployed the water rudder).Perhaps I need to reactivate the rudder, set the turning radius of the floats to zero and fiddle around with yaw damping in the air file.Thanks for all the insights. Did you ever upload your auto cfg/air files?Phil

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