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DCS P-51D Flight Model Overview


MaverickF22
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Hi,

 

I'm not starting this thread with the reason of finding any flight model problems or whatsoever, because i've flown the P-51D pretty well with ALL aids and helps off (included using the simulation mode and not game), and this plane can turn quite well, having a respectfully low turn radius at the same time(especially when using flaps) if it is flown as it should! So i've got used to the aircraft's limitations of not allowing Maverick style pulls on the stick and use correct and fine rudder inputs..., and btw, i don't have a movable stick, i have a Satiek X65F which is a pressure control stick (doesn't move), but i don't find it difficult to know how much should i pull in order to move the real stick to a given position (this joystick, even if it's pressure based, has the highest input accuracy i've ever experienced), thus i'm not finding it difficult to pass under very low bridges while flying inverted also..., if i'm gentile with the pressure, but that's another thing!

 

So i'm mostly trying to explain what happens and why things behave in a way or another to the regular or experienced P-51D pilots in DCS, because these pilots aren't many not due to the fact that the aircraft wasn't done right in this sim..., but because they can't explain "what a heck is going on" in some areas, where they knew from other simulators, that things were a lot easier and you seemed like a pilot after one day! Well it isn't so...! For all who believe that DCS didn't, at a high level of accuracy, manage to replicate the exact behaving of the real aircraft..., they are very wrong!

 

In other words, i'd like to use this thread as a school for the flight behaviour of the P-51D Mustang, for myself and for others who wish to express their opinions and try to understand the basics of it!

 

I also have some questions myself that i'd like to ask, for the sake of my knowledge:

 

1. Why does the normal G load seem to continue inceasing (positively for positive alpha, and slightly negatively for negative alpha respectively) after you've pulled the stick up to a level and held it there (just locked the stick and did not continue to pull further), a level which is behind the stall limit, so the aircraft remains within it's flight envelope...! Why does the alpha continue to increase by itself to a higher limit (which in turn leads to a higher G level than the one desired) over the one corresponding to my input, which is held for a constant negative (negative because it creates downward lift) elevator deflection or a back stick input? The same thing happens for negative alpha and negative G load, where again, after i apply, for let's say a 10% forward stick input which positively deflects the elevator (now the elevator creates positive lift, giving a pitch down momentum)..., the negative alpha will continue to go more negative, thus increasing the negative G load..., but this self-increase for the negative alpha is softer (as i've already mentioned) than what happens for the positive alphas!

 

For short..., the back and forth movement of the stick while in flight, will be a some sort of combination between an initial pitch rate (something usual and normal) given by the input and a pitch rate acceleration which develops for a short time after the input has been given(this acceleration onsets quickly and diminishes within 0.5 to 1 seconds after applying and holding a constant input) for both negative and positive alpha angles or G values!

 

As i could see during various testings (visually) within the sim..., this effect is inversely proportional (let's say linear, but it might be a function of airspeed) to IAS (Indicated Air Speed, and not to be confused with the TAS (true airspeed) which is indicated on the external view)! In other words..., the faster you go, the less this effect will be felt, and vice-versa for lower speeds where you have to be very careful, especially for takeoffs and landings, but more for takeoffs because there, the high torque effects play a huge role for directional and longitudinal stability (that's why you may use the takeoff assist feature within the game, but which i hate because i want the real thing)! I've been playing IL-2 Sturmovik 1946 for many years (which was considered the most realistic for FM(flight model) at least for props) and i also own IL-2 Cliffs of Dover, but i've never seen this kind of behaviour at any prop plane in any of the two IL-2 sims, not just for the P-51D which also was flyable in the first one! So, ok..., they might've been wrong in these places where DCS simulates something different..., but this still has to be explainable in a decent manner in order to be realistic!

 

My own first answer for the first question would be that the Cm(momentum coefficient around the pitch axis) derivative versus alpha derivative has a steeper function for the laminar airfoils than for non-laminar ones, so it tends to go positive (unstable) much sooner before the flow separates (boundary layer thickens and separates = stall) and as alpha increases...! This would be one reason for the pitch rate and angle of attack accelerations which develop after i've rapidly set an aft stick of about 10% holding it there, accelerations which diminish after 0.5-1 seconds as i've said, gaining me an additional pitch rate, alpha and G load than the input i've set for, because the center of pressure continues to travel a lot to the front of the plane during that period, pitching the aircraft further up until it settles at a momentum equilibrium between the main wing's Cm, fuselage Cm and the up/down lift generated on the elevator (otherwise the aircraft would pitch over and depart if it's unstable)! For short..., a very common aspect of a laminar airfoil is that it provides a better lift/drag ratio up to a given AoA, greater top speed, but a sudden/violent stall..., the visually (in an aerodynamic tunnel) softest disturbance in the boundary layer can lead to a violent flow separation from the trailing edge towards the leading edge of that airfoil's section, completely stalling that wing if the airfoil is laminar from root to tip (which is the case for the P-51), and because usually a wing stalls before the other, a high roll rate + some yaw rate are rapidly induced, so everything is correct in this sim!

 

A second reason for this..., and basically because it has a higher effect for positive alpha than for negative..., is that the horizontal empennage (stabilizer + elevator) are aerodynamically shadowed by the wing at angles between 7 to 10 degrees AoA (angle of attack), and this shadowing of the H tail is dependent on airspeed also..., and the slower you fly, the lower this angle where the shadowing effect occurs due to the boundary layer which thickens and separates earlier at lower speeds, creating more buffet on the H (horizontal) and V (vertical) tail! So the slower you go, the lower the stability of the plane (it isn't unstable like an F-16, but with a reduced longitudinal (or pitch) stability) will be! The shadowing never occurs for negative alpha, and it's logic why, therefore this effect is reduced when pushing the stick!

 

***Something similar to the pitch's Cm variation with alpha, happens to the fuselage's Cn (yaw momentum coefficient) variation with beta, but the fuselage alone is aerodynamically unstable around both pitch and yaw axis, always tending to swing around + or around - 90 degrees alpha and or beta! Just imagine a steel pipe (let's say it has a length of 10 times it's width) falling from the sky that always has the tendency to settle at a 90 deg AoA, or like a falling sheet of paper if it wouldn't have the vertical and horizontal stabilizers (you must've encountered this tendency of the plane yawing without control to the left or to the right when your vertical fin/stab is completely blown off)! So when you apply left or right rudder, you increase the yaw (directional) instability generated aerodynamically by the fuselage..., and the final beta angle or yaw angle will be a bit higher than the angle you should get according to your rudder deflection! Thus when you have applied full left or right rudder let's say and the aircraft has settled at a given yaw angle, the fuselage will tend to yaw further in order to reach 90 degrees where it likes to settle, but the the hole vertical empennage/fin, even if you have deflected in the direction you want it to yaw, will generate lift slightly opposite to your command which is not as high as the beta/sideslip angle that the hole plane sits at! So at those high angles of yaw/beta your rudder is generating opposite lift in order to hold you there, otherwise the plane would overyaw or depart from yaw and spin uncontrollably! Moreover, this is the only simulator for which, let's say that i'm between the few to actually have observed this effect, the rudder or vertical fin also simulates a stall! You can tell it when you apply rudder inputs in order to make it swing from one side to another, you'll notice where and how do the accelerations around the Z (yaw) axis vary! So this is the only simulator i've played so far which simulates the stall on all the 3 elements (wing, elevator, fin/rudder), so i may say: CONGRATULATIONS DCS for this kind of performance..., this seriously is a simulator, aerodynamically speaking! Only for the yaw there is no further acceleration occurring like it did to pitch after applying and holding a constant rudder input..., but for the pitch i think i have already answered correctly (if not, someone at ED should please tell us why this happens in order to know that it is realistic)!

*** This is why we have the feeling that the P-51D in DCS yaws so much and this is more a reply for the guy called "Rodd" who wrote:

Ok so I bought the P51 when It came out..& I've used it maybe 10 times since then. Mainly because of the flight dynamics I thought it would bee more stable like the A2A P51 in FSX...but this thing is just to hard to fly, to sensitive on the controls, stalls all the time, yaws like crazy when u try to shoot....not many multiplayers either.....I hope I can get the love back for it some day in DCS, but now I'm just disappointed

 

2. Why does the G level and alpha come back so slowly (like for the F-16) when i reduce the elevator/stick position (let's say null position) and it maintains me (not 100%, but higher than 70% i'd say) the attitude angle i have at the moment when i release the stick or bring it to null? The pitch rate should become opposite for a split second as alpha rapidly decreases! This thing seems to work in some manner when releasing the stick after a high negative alpha, when the nose has a slight pitch up, because the alpha rapidly decreases, but not the same thing happens from positive alpha, and i can't find the reason! Now it shouldn't necessarily oscillate as it would do when recovering from beta or sideslip angle which tends to accelerate towards 0 (zero) around the Z (yaw) axis in a tendency to stabilize in a position after the rudder has been set to null, which is simulated well, but it seems that the elevator doesn't provide the lift required for it's corresponding position (null), but a less lift which increases to the normal value in time! So it's like after i've rapidly released the stick, the elevator slowly comes to the desired position at a given rate just enough to hold me at a constant attitude, which is something that only happens on FBW (Fly By Wire) or ACS (Augment control system, like the A-10C) aircrafts!

 

I've also provided a video that i've done using FRAPS, in order to review these effects:

 

http://www.youtube.com/watch?v=BBZ2UQKPStY

 

Now i can see that the aircraft, as the way it has been built by it's engineers, won't need a stick travel of more than 15-20% back from it's null position in order to reach critical or stall alpha on the wings..., at least this is what DCS shows, i haven't flown that plane for real, so i just trust what i have so far! This baby uses a laminar airfoil, close to NACA 66-415, (although roughly rounded at the leading edge) and having the laminar layer being driven up to almost 60% of the chord back, before it becomes turbulent, at low AoA (angle of attack). I don't know why did the engineers

of that time..., let the P-51D, which has a lower critical alpha (with flaps raised) due to that kind of airfoil and it's overall stall danger, have so much amount of unneeded pitch input (let's say: so much amount of positive(aft input) for so much negative elevator deflection (negative because it produces negative lift), because we mostly pull the stick and that concerns us the most), because even during takeoff or landing it becomes dangerous to apply a 20% back stick..., so let's say you'd need a short 30% back stick input for whatever reason, but why are 70% still available..., when they could've made the stick travel the same as much, no problem with that, but for a lower amount of elevator travel, giving a less sensitive or less violent pitch control! Anyway, this might be an off-topic question of why did they do that..., but i'm confident that DCS simulates the real aircraft!

 

I'm becoming an aircraft engineer myself the next year (2013), so i'm pretty confident with my flight mechanics and aerodynamics knowledge that i have so far and i'm also an active pilot, so i might say that i know enough, but i'm always willing to learn more!

 

Here's a useful link which talks about laminar airfoils and their advantage/disadvantage characteristics:

 

http://www.dreesecode.com/primer/airfoil5.html

 

Sorry for this long and possibly boring introduction to aerodynamics and flight mechanics story, but i hope that all which i had to say are for the purpose of realizing the complexity (which the people at ED already know more than i do) of flight dynamics, yet what i've said here isn't enough for understanding all that happens with the aircraft for some given cases, but let's hope that time will reveal all the aspects that are necessary to be addressed in order for these things to be sorted out...!

 

Thank you!


Edited by MaverickF22

Mistakes, obviously, show us what needs improving. Without mistakes, how would we know what we had to work on!











Making DCS a better place for realism.

Let it be, ED!



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I feel like printing your thread :D

AWAITING ED NEW DAMAGE MODEL IMPLEMENTATION FOR WW2 BIRDS

 

Fat T is above, thin T is below. Long T is faster, Short T is slower. Open triangle is AWACS, closed triangle is your own sensors. Double dash is friendly, Single dash is enemy. Circle is friendly. Strobe is jammer. Strobe to dash is under 35 km. HDD is 7 times range key. Radar to 160 km, IRST to 10 km. Stay low, but never slow.

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I was wondering if somebody could go into the P51 stall and spin characteristics, as I am not learned in this area. I was very surprised by how hard it is to properly spin this aircraft (not snap spins, they're really easy to do). For starters it only spins in one direction, and exiting the spins is as simple as full throttle and letting the stick go.

<VAAF>

Virtual Australian Air-Force :thumbup::joystick::pilotfly:

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Why does the normal G load seem to continue inceasing (positively for positive alpha, and slightly negatively for negative alpha respectively) after you've pulled the stick up to a level and held it there (just locked the stick and did not continue to pull further), a level which is behind the stall limit, so the aircraft remains within it's flight envelope...! Why does the alpha continue to increase by itself to a higher limit (which in turn leads to a higher G level than the one desired) over the one corresponding to my input, which is held for a constant negative (negative because it creates downward lift) elevator deflection or a back stick input? The same thing happens for negative alpha and negative G load, where again, after i apply, for let's say a 10% forward stick input which positively deflects the elevator (now the elevator creates positive lift, giving a pitch down momentum)..., the negative alpha will continue to go more negative, thus increasing the negative G load..., but this self-increase for the negative alpha is softer (as i've already mentioned) than what happens for the positive alphas!

 

I don't really understand why it does this, but I recall reading someone from E.D. explaining that this is a feature of the real P-51D and has to do with, I think, C.o.G. and other factors. Maybe C.o.L.? Elevator booster tabs? I don't remember the explanation. It might be in the manual; I just don't remember where I read this.

 

Now i can see that the aircraft, as the way it has been built by it's engineers, won't need a stick travel of more than 15-20% back from it's null position in order to reach critical or stall alpha on the wings..., at least this is what DCS shows, i haven't flown that plane for real, so i just trust what i have so far! This baby uses a laminar airfoil, close to NACA 66-415, (although roughly rounded at the leading edge) and having the laminar layer being driven up to almost 60% of the chord back, before it becomes turbulent, at low AoA (angle of attack). I don't know why did the engineers of that time..., let the P-51D, which has a lower critical alpha (with flaps raised) due to that kind of airfoil and it's overall stall danger, have so much amount of unneeded pitch input (let's say: so much amount of positive(aft input) for so much negative elevator deflection (negative because it produces negative lift), because we mostly pull the stick and that concerns us the most), because even during takeoff or landing it becomes dangerous to apply a 20% back stick..., so let's say you'd need a short 30% back stick input for whatever reason, but why are 70% still available..., when they could've made the stick travel the same as much, no problem with that, but for a lower amount of elevator travel, giving a less sensitive or less violent pitch control!

 

This is a feature of real fighters. The reason why they make it this way is, having the option of more elevator can only be a good thing, for a trained pilot. You know, "It's better to have it and not need it, than to need it and not have it." If you don't need all the elevator (which is true almost all of the time), then you simply refrain from pulling it all the way. But if you do wind up in a situation where you need to pull full elevator, well, then you have it, because the engineers gave it to you. One example of a situation where full elevator might be needed is when deliberately stalling the airplane, or during spin recovery. In a flat spin, it may help certain aircraft under certain circumstances to pump the stick fully forward and backward in time with the revolutions. The principle is similar to that of the F-16's deep stall recovery procedure, I believe; it's sort of like how a child on a swing gradually rocks back and forth, with his motion increasing each time.


Edited by Echo38
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I don't really understand why it does this, but I recall reading someone from E.D. explaining that this is a feature of the real P-51D and has to do with, I think, C.o.G. and other factors.

 

It depends on the fuel load of the center tank. If the center tank is fully filled, the Mustang develops negative stability characteristics in pitch because of the far back center of gravity..

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  • ED Team

This is a feature of real fighters. The reason why they make it this way is, having the option of more elevator can only be a good thing, for a trained pilot. You know, "It's better to have it and not need it, than to need it and not have it." If you don't need all the elevator (which is true almost all of the time), then you simply refrain from pulling it all the way. But if you do wind up in a situation where you need to pull full elevator, well, then you have it, because the engineers gave it to you. One example of a situation where full elevator might be needed is when deliberately stalling the airplane, or during spin recovery. In a flat spin, it may help certain aircraft under certain circumstances to pump the stick fully forward and backward in time with the revolutions. The principle is similar to that of the F-16's deep stall recovery procedure, I believe; it's sort of like how a child on a swing gradually rocks back and forth, with his motion increasing each time.

 

You forget the situation when the elevator is stripped or even lose one half.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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I was wondering if somebody could go into the P51 stall and spin characteristics, as I am not learned in this area. For starters it only spins in one direction, and exiting the spins is as simple as full throttle and letting the stick go.

 

You may also exit the spins, i forgot to do that in my video..., by simply working with the throttle only as you've said..., and depending on the side of the spin, you may have to apply throttle, or to reduce it, and the torque effect changing to it's opposite direction when throttling back may also take you out of the spin, which is something normal! If other simulators prove a more persistent spin for the P-51D, that's because they don't have an accurate data or FM(flight model) leading to an exaggerated spin, from which is also harder to recover! If you'd look at a real flat spin for most of the prop palnes (search youtube), you'll see the same as in DCS, that you may exit it quite fast if you apply the proper inputs, even if the spin rate is initially high! So, DCS boys and girls..., has broke the ice!:thumbup:

 

Cheers!

Mistakes, obviously, show us what needs improving. Without mistakes, how would we know what we had to work on!











Making DCS a better place for realism.

Let it be, ED!



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I don't really understand why it does this, but I recall reading someone from E.D. explaining that this is a feature of the real P-51D and has to do with, I think, C.o.G. and other factors. Maybe C.o.L.? Elevator booster tabs? I don't remember the explanation. It might be in the manual; I just don't remember where I read this.

 

 

 

This is a feature of real fighters. The reason why they make it this way is, having the option of more elevator can only be a good thing, for a trained pilot. You know, "It's better to have it and not need it, than to need it and not have it." If you don't need all the elevator (which is true almost all of the time), then you simply refrain from pulling it all the way. But if you do wind up in a situation where you need to pull full elevator, well, then you have it, because the engineers gave it to you. One example of a situation where full elevator might be needed is when deliberately stalling the airplane, or during spin recovery. In a flat spin, it may help certain aircraft under certain circumstances to pump the stick fully forward and backward in time with the revolutions. The principle is similar to that of the F-16's deep stall recovery procedure, I believe; it's sort of like how a child on a swing gradually rocks back and forth, with his motion increasing each time.

 

Thank you very much man..., so that explains it..., no more for the questions! The answer lied where i least expected, but it clarifies it well!:doh:

 

I appreciate it! Thank you!

Mistakes, obviously, show us what needs improving. Without mistakes, how would we know what we had to work on!











Making DCS a better place for realism.

Let it be, ED!



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  • 3 months later...

I have found some issues related to the flight dynamics of the P-51 and these are as follows:

 

-Opposite roll rates due to beta at negative G-loads and negative AoA (alpha) are non-existent

 

-Strange whirl phenomenon occurs when the flaps are fully lowered and the engine is throttled up..., or when the engine is cut dead and the propeller is fully stopped!

 

 

As any pilot and aircraft engineer should know..., there is a roll rate that you will attain at a given angle of sideslip (beta), angle of attack (alpha) and airspeed even if the stick is fully centered, due to lift force imbalance between the wings...! For instance, if you fly at 1G in normal steady flight and at a given airspeed, it means that you have a given positive alpha angle for that, and if you apply left or right rudder and try to maintain 1G (thus maintaining the alpha angle) without changing the position of the stick in lateral direction, you will get a roll rate on the same side for which you apply the rudder or to the opposite side of slip! But this thing only applies when you have a positive G-load and/or positive alpha..., because when going high negative on alpha or just negative enough in order to created negative lift or negative G-load..., then the roll will be on the opposite side of the rudder, and the rate of roll will be according to how much negative alpha (and G-load), how much sideslip (beta) and airspeed you have, but again..., on the opposite side of the rudder or on the same side of the slip this time!

 

The idea is that i haven't seen this opposite roll due to negative G-load occurring on the P-51 if i don't apply lateral stick input to that side..., and this doesn't smell good! So yes, it will do an opposite roll due to rudder at negative G-load, but only if i help it with lateral stick, while the roll should occur even if the stick doesn't have lateral input, so something isn't right!

 

The second strange phenomenon is the whirl motion, which is something common to the propellers due to the gyroscopic effects which increase with rpm..., but tendency of the aircraft's propeller to perform a whirl type motion around the Y (pitch) and Z (yaw) axis should be diminished by the aerodynamic forces which tend to keep the airplane's direction straight! Yet this phenomenon may not be completely understood for why is it happening when the flaps are fully lowered for our P-51 in the sim, but when the engine is gone/cut off and the propeller is either stuck or it stopped spinning..., why does this phenomenon still occur, and it occurs even more intense than in the previous case?

 

So i really think there's something wrong with the P-51D's flight dynamics behaviour here!

 

If someone needs a proof, i'll make a video in no time and show it, but i hope the devs understand the phenomena and will fix this nonsense behaviour!

 

 

Any answer is appreciated

 

Thank you,

Maverick!

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Making DCS a better place for realism.

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Oh, and i forgot to mention another weird behaviour of the yaw rate (side slip) ball indicator..., as it has an initial wrong side tendency to move when you quickly apply rudder inputs. So, for example if you fly at positive G-loads and apply a right rudder fast enough..., the ball will initially move to the same side of the rudder input, just to change it's movement to the correct direction at around 0,5 seconds since you've applied the input! So it moves to the right with the rudder for the first half of a second and then it starts moving to the opposite direction as it normally should!

 

This is a small issue, but i'm confident the devs will fix it!

 

 

Have a good day,

Maverick!

Mistakes, obviously, show us what needs improving. Without mistakes, how would we know what we had to work on!











Making DCS a better place for realism.

Let it be, ED!



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The second strange phenomenon is the whirl motion, which is something common to the propellers due to the gyroscopic effects which increase with rpm..., but tendency of the aircraft's propeller to perform a whirl type motion around the Y (pitch) and Z (yaw) axis should be diminished by the aerodynamic forces which tend to keep the airplane's direction straight! Yet this phenomenon may not be completely understood for why is it happening when the flaps are fully lowered for our P-51 in the sim, but when the engine is gone/cut off and the propeller is either stuck or it stopped spinning..., why does this phenomenon still occur, and it occurs even more intense than in the previous case?

 

Are you saying it is performing a Lomcovak with the engine stopped? A track would be helpful to demonstrate the behaviour.

Good, fast, cheap. Choose any two.

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I have found some issues related to the flight dynamics of the P-51 and these are as follows:

 

-Opposite roll rates due to beta at negative G-loads and negative AoA (alpha) are non-existent

 

Maverick!

 

Curious you mention this because last week I had suggested such a test for X-Plane10, because I no longer have it installed - just DCS World :-)

 

I myself tried to get into such a situation but was unable to in the limited time I had to be at the PC, but I was willing to test if indeed the flight model will exhibit control reversal under such circumstances...

Flight Simulation is the Virtual Materialization of a Dream...

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Oh, and i forgot to mention another weird behaviour of the yaw rate (side slip) ball indicator..., as it has an initial wrong side tendency to move when you quickly apply rudder inputs. So, for example if you fly at positive G-loads and apply a right rudder fast enough..., the ball will initially move to the same side of the rudder input, just to change it's movement to the correct direction at around 0,5 seconds since you've applied the input! So it moves to the right with the rudder for the first half of a second and then it starts moving to the opposite direction as it normally should!

 

This is a small issue, but i'm confident the devs will fix it!

 

 

Have a good day,

Maverick!

 

Have a good day, too.

 

It's not weird. It's only a physics... what is the reason of ball decentering? Yes, the acceleration of the point the ball indicator is placed. If we take a flat turn it is centrifugal. Left pedal, right wing forward, right sideslip and left flat turn. The ball stays right as it should. But the point is, that steady left flat turn is due to the side force of the whole airframe having balanced angle of slip though at the first moment as you just apply left rudder there is NO SIDESLIP ANGLE and the only force applied to the plane is the force at its fin. This force, as you can see, causes RIGHT FLAT turn but only for a moment... that is sufficient to the ball that goes left a little indicating the force produced by the fin.

Moreover, there is another reason that forces the ball to move right at the initial moment of yawing - its inertion. Left pedal, nose left - the ball goes right. But this component is less than the centrifugal component, so the overall movement is to left.

 

 

To be continued...


Edited by Yo-Yo

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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  • ED Team

 

The second strange phenomenon is the whirl motion, which is something common to the propellers due to the gyroscopic effects which increase with rpm..., but tendency of the aircraft's propeller to perform a whirl type motion around the Y (pitch) and Z (yaw) axis should be diminished by the aerodynamic forces which tend to keep the airplane's direction straight! Yet this phenomenon may not be completely understood for why is it happening when the flaps are fully lowered for our P-51 in the sim, but when the engine is gone/cut off and the propeller is either stuck or it stopped spinning..., why does this phenomenon still occur, and it occurs even more intense than in the previous case?

 

So i really think there's something wrong with the P-51D's flight dynamics behaviour here!

 

 

Do you know the main moments and forces due to so called P-factor? If yes, you can easily draw the picture and explain the reasons of this movent yourself. By the way, you are absolutely right mentioning that aerodynamic forces must diminish this effect but as you can see this effect becomes noticable at low speed where P-factor increases due to high AoA and the aerodynamic forces decrease due to low dynamic pressure.

Do not forget prop gyroscopic effect, too.


Edited by Yo-Yo

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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Well, it's allways a delight to read your explanations Yo-Yo :-) The sometimes not so evident "phenomena" turn crystal clear :-) and of course, Maverick's posts are also challenging and focus on details that we usually do not even try to consider on other simulation platforms.

 

Regarding control reversal, I believe that when I managed to get the p51d into a tailslide (not an easy task, at least mantaining it as aligned as possible when it starts "moving back") exhibited at least rudder and elevator reversal, and I believe aileron too, but I would have to confirm and try saving a track.


Edited by jcomm

Flight Simulation is the Virtual Materialization of a Dream...

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Do you know the main moments and forces due to so called P-factor?.

 

Of course i do, but this only applies if that propeller is spinning, yet i was explicit when i've said that the engine was fully stopped and the prop wasn't rotating at all..., i'll put a video right away covering both the still not understood wrong way initial ball movement and also this abnormal oscilation!

 

By the way, you are absolutely right mentioning that aerodynamic forces must diminish this effect but as you can see this effect becomes noticable at low speed where P-factor increases due to high AoA and the aerodynamic forces decrease due to low dynamic pressure.

Do not forget prop gyroscopic effect, too.

 

I know what you are saying, but it's not the case here, so i'll apply a video!

 

Many people have encountered this at least once when their engine had stopped as well as the propeller, but none dared to ask about it or didn't pay attention it, yet i'm not like that and i take all of these things seriously even for the tiniest issue..., not because i'd be mad, but because i want you guys to do good things, so i'm with you all the way even if i fight GGTharos arguing with him on other threads about various stuff...! I like and want to support ED for what they are doing, but all these things have their effort too!

 

 

With honest respect,

Maverick!

Mistakes, obviously, show us what needs improving. Without mistakes, how would we know what we had to work on!











Making DCS a better place for realism.

Let it be, ED!



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Have a good day, too.

It's not weird. It's only a physics... what is the reason of ball decentering? Yes, the acceleration of the point the ball indicator is placed. If we take a flat turn it is centrifugal. Left pedal, right wing forward, right sideslip and left flat turn. The ball stays right as it should. But the point is, that steady left flat turn is due to the side force of the whole airframe having balanced angle of slip though at the first moment as you just apply left rudder there is NO SIDESLIP ANGLE and the only force applied to the plane is the force at its fin. This force, as you can see, causes RIGHT FLAT turn but only for a moment... that is sufficient to the ball that goes left a little indicating the force produced by the fin.

Moreover, there is another reason that forces the ball to move right at the initial moment of yawing - its inertion. Left pedal, nose left - the ball goes right. But this component is less than the centrifugal component, so the overall movement is to left.

To be continued...

 

I guess i was misunderstood, i know how the ball works and what type of liquid it has inside it's tube and it doesn't apply complicated physics..., it should only work as a pendulum (is what they were using in the early days to measure the sideslip or centrifugal force around the plane's Z (vertical) axis), so now they use this ball which does exactly the same by using it's static inertia to slide along the tube..., but still there is no explanation nor any reason why would that ball in reality slide with the force, in the direction of the force for 0.5 seconds and then go where it should..., just like it would be grabbed by that force..., there's simply no reason for it to act like that, here's the video;):

 

http://www.youtube.com/watch?v=ISSmzNIlA9w

 

There is no P-factor applying to a stopped propeller, or to not lie..., it is very small due to the very small airspeed component which tends to create differential lift between the blades when they are stopped..., yes there is a difference, as each blade encounters the airflow from different directions if you have any alpha or beta angles and that creates different lift forces on the blades, but that ammount is almost null as the propeller is fully stopped!

 

Take a look at the video and notice the behavior!

 

 

Cheers,

Mav.!


Edited by MaverickF22

Mistakes, obviously, show us what needs improving. Without mistakes, how would we know what we had to work on!











Making DCS a better place for realism.

Let it be, ED!



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Of course i do, but this only applies if that propeller is spinning, yet i was explicit when i've said that the engine was fully stopped and the prop wasn't rotating at all..., i'll put a video right away covering both the still not understood wrong way initial ball movement and also this abnormal oscilation!

 

 

 

I know what you are saying, but it's not the case here, so i'll apply a video!

 

Many people have encountered this at least once when their engine had stopped as well as the propeller, but none dared to ask about it or didn't pay attention it, yet i'm not like that and i take all of these things seriously even for the tiniest issue..., not because i'd be mad, but because i want you guys to do good things, so i'm with you all the way even if i fight GGTharos arguing with him on other threads about various stuff...! I like and want to support ED for what they are doing, but all these things have their effort too!

 

 

With honest respect,

Maverick!

Once again - it's only a physics...

 

Stopped prop is not less than 1 sq. m of a WING placed before the CG. If you ever sea a full range wind tunnel test of a wing you could remember that CL is not constant even within the area 90 deg of AoA.

Then, let's imagine that the prop stopped as a straight cross so one pair of the blades is horisontal.

 

When you change AoA of the plane you change AoA of right and left blades - one of them gets closer to 90 deg AoA and increases its drag and the opposite blade gets farther from 90 deg AoA, so its drag decreases. The result is a yaw moment. Progressing yaw causes the same effect for the vertical pair of blades causing pitch movement. Depending on the IAS and together with the plane inertia and stability it produces swirling damped or continous oscillation.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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I guess i was misunderstood, i know how the ball works and what type of liquid it has inside it's tube and it doesn't apply complicated physics..., it should only work as a pendulum (is what they were using in the early days to measure the sideslip or centrifugal force around the plane's Z (vertical) axis), so now they use this ball which does exactly the same by using it's static inertia to slide along the tube..., but still there is no explanation nor any reason why would that ball in reality slide with the force, in the direction of the force for 0.5 seconds and then go where it should..., just like it would be grabbed by that force..., there's simply no reason for it to act like that, here's the video;):

 

 

 

 

Cheers,

Mav.!

 

I think you misunderstood me. Let's count the side forces after you push the left pedal, for example. At the first moment we have no angle of slip but we have a force acting to RIGHT at the fin. This force applied to the plane creates RIGHT flat turn (the ball jumps left a little) and a moment that causes right yaw. As the plane gets AoS, the side force from the whole airframe increases and becomes higher than the side force from the fin. Then the plane began to turn left, so the ball goes right.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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Let's count the side forces after you push the left pedal, for example. At the first moment we have no angle of slip but we have a force acting to RIGHT at the fin. This force applied to the plane creates RIGHT flat turn (the ball jumps left a little) and a moment that causes right yaw. As the plane gets AoS, the side force from the whole airframe increases and becomes higher than the side force from the fin. Then the plane began to turn left, so the ball goes right.

 

Hmm. I can't see what could cause right yaw, at any point, in a situation where left pedal is the only acting force (that is, barring things like gyroscopic effect from pitch changes, and adverse yaw from aileron input). The rudder is well behind the center of mass, so the right-ward energy applied to the area of the rudder hinge as a result of the deflected airflow should push the rudder-hinge area exclusively to the right (resulting in left yaw only). No?

 

Now, the rudder itself does create "equal and opposite reaction" as it moves toward the left--shifting the rudder's mass to the left of the aircraft's CoG--before the airflow starts to direct the tail to the right. But since the rudder is moving left, the reaction would also be to the right--thus further resulting in left yaw (the tail being behind the CoG).

 

Everything I can see here points only to left-ward yaw (that is, right-ward motion of the tail section). I can't see a single thing that can cause a left-ward motion of the tail during this process. Can anyone explain what I'm missing? (Or maybe even draw a diagram?)

 

Many people have encountered this at least once when their engine had stopped as well as the propeller, but none dared to ask about it or didn't pay attention it

 

I don't quite think you're being fair here. Since I perform all of my landings without using power at any point during the approach, I'm rather familiar with the power-off handling of our P-51D. I've never experienced any torque or gyroscopic effect when the propeller is still, and I've put her into spins and everything with the engine dead (in addition to many normal dead-stick landings). Heck, I've even tried to continue dogfights with my engine dead, albeit without much success!

 

So, if you're not seeing the majority of the community complaining about this gyro-effect bug/error you're describing, I would suggest that the lack of complaints is due to the bug not occurring for the majority of us, rather than for lack of us experimenting or caring. I've thrown her through damn near everything and I've never seen anything like what you're describing, unless we're greatly misunderstanding each other. (I suppose it's also possible that I've experienced it and not noticed, but I find that unlikely; when I was a beta tester for a different sim-game, I found & reported dozens of bugs & errors which everyone else missed.)


Edited by Echo38
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This is an excellent subject, very interesting and complex.

 

After left rudder the only moment I could think of that gives right yaw effect is when the right side vertical stab and starboard side of the fusalage hit the wind and gives counter force to the side slip, this might cause a right yaw effect.

ofcourse this is all depending on COG of the plane at that moment, weight and speed.

 

Also, in left rudder we get left roll and that is because the right wing goes forwards and gets lift before the left wing which makes for left roll.

 

 

There are probebly numerous tests that we could do to test all of the forces, but the real question is if ED used data from a wind tunnel to build the FM, if so the FM would ofcourse be much more accurate, but that only ED can answer.

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There is one of the main theorems in mechanics. It stated that all forces acting to the rigid body can be replaced with only one force acting to the given point and one fprce pair (pure torque or pure moment) acting relatively this point. It's very convenient to use CG as the point.

 

So, as I wrote above, the force, applied to the fin causing the adequate acceleration of the CG of the plane as well as a yawing moment.

The same effect will be if you abruptly pull the stick - before G starts increasing it will drop a little. Generally, all empennage surfaces create opposite force (for static stable planes). This effect is well known as trim or balance losses.

Though even some static stable planes can eliminate this losses having CG aft. They have positive lift at elevators. But generally, the best way for contemporary planes is to be static unstable as Su-27, for example.

 

P.S.

Everyone can perform a test - put a brick of wood on the water surface. Then fix a line over its CoG. Pull the line slowly and see CoG movement. Then shift the fix point along the longer axis of the brick and pull perpendicularly to this axis. Notice the overall movement.


Edited by Yo-Yo

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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P.S.

Everyone can perform a test - put a brick of wood on the water surface. Then fix a line over its CoG. Pull the line slowly and see CoG movement. Then shift the fix point along the longer axis of the brick and pull perpendicularly to this axis. Notice the overall movement.

 

Ok, it's been more than 40 yrs since I remember having played with the water in the bathtub, but you got it!!!! Next job for the weekend, and I'll invite wifey too... maybe we can come up with yet another explanation for the whirl type motion!... ;-)


Edited by jcomm

Flight Simulation is the Virtual Materialization of a Dream...

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