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DCS P-51D Flight Model Overview


MaverickF22

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If the bank-and-turn indicator is placed at the cg of the aircraft, the ball shall not move due to the increasing of the yaw rate. But it is before the cg I think.

 

Doesn't even matter where the CG is, in relation with the -slide indicator-, even be it right within the indicator as you exemplified, the ball should slide/move opposite to the CG's movement along the plane's Y axis or if that CG is within a centripetal force and the plane has a constant yaw rate, because that's a circular acceleration of the CG..., which everyone knows as V^2/R (R=radius) or V*angular velocity (in radians) and that's what the ball actually does..., it measures an acceleration around the plane's Z axis OR an acceleration along the plane's Y axis, so if the yaw rate would increase as you've said, therefore you should have a yaw rate acceleration which is even more, it's an increasing acceleration of the CG and that will stick the ball to one end of it's tube, doesn't just move it..., so think again of what you're saying!

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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.

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@MaverickF22, under the risk of sounding really ignorant :-/ I will ask if the oscillation could be caused by fuel imbalance forces and the correspondic aerodynamic damping forces counteracting it (?)

 

Hi there jcomm..., nope i definitely doubt that it has something to do with fuel imbalance or with corresponding aerodynamic damping forces...!

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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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.

 

Ok, by this you mean about the P-factor again, but this time it won't take the lift or thrust center into account, but drag one, because the blades are stopped and let's say they are stopped in cross position as you've said, having two blades vertical and two horizontal, your explanation is good, for instance, as the P-51 that has a clockwise rotating blade from cockpit view..., and the plane is gaining a positive alpha let's say, no matter the blade pitch, the left horizontal blade would have it's own alpha (AoA) increasing towards -90 deg, while the right one will have it more positive/decreased from -90, let's say -45...(which depends on pitch), therefore the left blade will create more drag than the right blade, and now this imbalance which turns as a drag center that is shifted left of the aircraft's longitudinal axis, will create a left yawing momentum, and the negative beta created due to this will trigger a drag imbalance between the vertical blades now, having in our case the lower blade's alpha closer to -90 deg and the upper blade's alpha more positive also, so the lower blade will create more drag and pull the nose down and is reducing our initially increasing plane AoA, therefore the beta will start to reduce itself too and we now have created a swirling effect which sustains just due to our initial input that started the imbalances...! But even on a more than 1 square meter prop area..., would you have that amount (that much) of effect on an aerodynamically stable airframe in pitch and yaw, even if you'd have the prop pitch at the angle where this effect would have the highest value (that is when the blades make a 90 deg angle between their chords)?

 

So the theory here is fine.., but the amount seems a bit exaggerated, that's all!

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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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.

 

Make me understand what are you trying to say with the "right flat turn" or what would it mean to you for making the ball jump a little opposite to the direction were it will naturally go! Probably you might want to say about the side force created by the fin only...! Are you trying to tell that the hole airframe is initially "pulled" to the side of the force created by the fin deflection (which is opposite to the fin direction) and that the airframe is rotating around an imaginary center which is in front of the plane's CG and in front of the slide indicator..., and until the CG will actually start to accelerate along the plane's Y axis due to the centripetal force which is aerodynamically created (for which the ball will slide in the correct direction), the CG will slide with the force created initially by the fin and around that imaginary center of rotation and that would make the ball listen to the initial opposite movement of the CG? Or would you mean the the CG itself is in front of the slide indicator (which can't be that much in front, cause it would be ridiculous) and that's why, if the rotation is around the CG this time, the ball will initially listen to acceleration around the Z axis of the CG and then to the side force Y axis of the CG? Cause otherwise there is no other explanation than these 2, and i think the first one is more appropriate!

 

The weird thing is that this happens only with the P-51..., not with the A-10, nor with the Su-25's or with the KA-50, that's why it is so intriguing...!


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 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...

 

First of all, landing or flying with the throttle to idle isn't our subject here..., as the gyro effects are normal and there are no problems whatsoever in that area because you won't experience that swirl phenomena in that area as the propeller is still spinning..., then you say that you didn't experience "any torque or gyroscopic effect when the propeller is still"..., well that's obvious within itself don't you think? You can't have gyro effects on a non-rotating body/object! It's that pitch and yaw motion combination that occurs ONLY when the blades are fully stopped and that's due to the P-factor that i've talked earlier about and which Yo-Yo has firstly defined!

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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In my opinion, the value of the blade's CD (total drag coefficient) seems a little bit high, because that's why all this phenomenon occurs in such an amount that literally oscillates the hole heavy airframe around pitch and yaw axis like that! So i'm only guessing that the CD is to blame..., yet otherwise i don't know if it might be correct and then this whole phenomenon is completely realistic and it may only have been known by the pilots that actually fly/have flown a dead engine (prop fully stopped) P-51D, to tell: "YES, that kind of swirl really is noticed, but i didn't tell anyone"! I also don't know what kind of CFD (computational fluid dynamics) software have ED used to determine the CL and CD values for the P-51D's blades, or have they simply taken them from a wind tunnel test, which would be better in any case than trusting the CFD! So i personally don't know if that value really isn't too high, which creates this amount of oscillation or it would really be confirmed/correct! But i'd like to know that...!

 

 

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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  • ED Team
Make me understand what are you trying to say with the "right flat turn" or what would it mean to you for making the ball jump a little opposite to the direction were it will naturally go! Probably you might want to say about the side force created by the fin only...!

Yes, that's the point.

Are you trying to tell that the hole airframe is initially "pulled" to the side of the force created by the fin deflection (which is opposite to the fin direction)

 

Yes, it's right.

and that the airframe is rotating around an imaginary center which is in front of the plane's CG and in front of the slide indicator..., and until the CG will actually start to accelerate along the plane's Y axis due to the centripetal force which is aerodynamically created (for which the ball will slide in the correct direction),

 

There is no use here to take the short-period movement in account. If you can see, the ball (In P-51. But not in A-10, Su-25 and Ka-50! ). is placed right at the CG (plus-minus several % of MAC) and the plane does rotate around its CG.

 

The only thing I am trying to explain is that THERE ARE TWO AD FORCES ACTING.

THe first one produced with the fin itself is applied to the fin causing both CG movement to right (ball goes left) and right flat turn. The second one is applied near plane CG and is produced with all airframe. THis force is much more than the first (moments equilibrum at very different arms). But this force IS EQUAL to ZERO at the initial moment as the yaw =0. It grows with the yaw overriding the first force.

But before it overrides the resulting force is to the right causing CG to curve its path RIGHT - the ball goes left.

 

The opposite ball movement will not be seen in the planes having cockpits far forward from the CG because of plane rotational acceleration that pushes the ball RIGHT as yaw movement begins. THis acceleration obviously higher than the acceleration from the initial right turn.

 

 

the CG will slide with the force created initially by the fin and around that imaginary center of rotation and that would make the ball listen to the initial opposite movement of the CG? Or would you mean the the CG itself is in front of the slide indicator (which can't be that much in front, cause it would be ridiculous) and that's why, if the rotation is around the CG this time, the ball will initially listen to acceleration around the Z axis of the CG and then to the side force Y axis of the CG? Cause otherwise there is no other explanation than these 2, and i think the first one is more appropriate!

 

The weird thing is that this happens only with the P-51..., not with the A-10, nor with the Su-25's or with the KA-50, that's why it is so intriguing...!

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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
In my opinion, the value of the blade's CD (total drag coefficient) seems a little bit high, because that's why all this phenomenon occurs in such an amount that literally oscillates the hole heavy airframe around pitch and yaw axis like that! So i'm only guessing that the CD is to blame..., yet otherwise i don't know if it might be correct and then this whole phenomenon is completely realistic and it may only have been known by the pilots that actually fly/have flown a dead engine (prop fully stopped) P-51D, to tell: "YES, that kind of swirl really is noticed, but i didn't tell anyone"! I also don't know what kind of CFD (computational fluid dynamics) software have ED used to determine the CL and CD values for the P-51D's blades, or have they simply taken them from a wind tunnel test, which would be better in any case than trusting the CFD! So i personally don't know if that value really isn't too high, which creates this amount of oscillation or it would really be confirmed/correct! But i'd like to know that...!

 

 

With honest respect,

Maverick

 

If you have aeroengineering background you can estimate this effect yourself. You can take generic stability derivations for the plane itself then take a 360 degree wind tunnel data for any airfoil at suitable Re because at 90 AoA they all similar. Then you can estimate the possibility of the auto-oscillation using any criterion you prefer....

By the way, this effect appears only at high IAS. Any sane pilot having its engine dead will get 160-170 mph IAS as soon as possible. :)

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 have some question about not stricte flight model but overheating modeling.

 

I tried some dogfight in quick mission against AI. I used military power - 61Hg and 3000 RPMs (dont even touch WEP) and i found that in many case i got overheat and engine blow even below 5 minutes since mission start ( Military power theoriticaly could be used for 15 minutes). It happend many times. I notice that coolant and oil temperature rise very very quickly at slow speeds or in vertical manouvers ( which AI is constanly doing). I made for example 2 vertical zooms ending at near stall speed and temperatures during these manouvers raised imidietly from normal to maximum above red line. I wonder if really temperatures should rise so quickly?


Edited by Kwiatek
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  • ED Team
I have some question about not stricte flight model but overheating modeling.

 

I tried some dogfight in quick mission against AI. I used military power - 61Hg and 3000 RPMs (dont even touch WEP) and i found that in many case i got overheat and engine blow even below 5 minutes since mission start ( Military power theoriticaly could be used for 15 minutes). It happend many times. I notice that coolant and oil temperature rise very very quickly at slow speeds or in vertical manouvers ( which AI is constanly doing). I made for example 2 vertical zooms ending at near stall speed and temperatures during these manouvers raised imidietly from normal to maximum above red line. I wonder if really temperatures should rise so quickly?

 

If you have a car - disconnect the radiator fan driving at full steam on the test rolls in local service .... :)

And, by the way, there is no use to perform near stall maneuvers to kill AI.

 

To be serious - it's ok with the heatsink model. It uses true calculations for the radiator heatsink effectiveness and was calibrated using some test results for the Mustang.


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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then you say that you didn't experience "any torque or gyroscopic effect when the propeller is still"..., well that's obvious within itself don't you think? You can't have gyro effects on a non-rotating body/object!

 

Of course. I was under the impression that you were saying that you experienced a gyro effect with engine off.

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Ok i just follow AI in vertical manouvers and i found that if during near vertical zoom ending at stall speed temperature rise very quickly so if you made 2 or 3 such manouvers you could killed your engine even in 1-2 minutes not even using WEP so i just wonder if it is correct :)

 

But if you say that it should be that way i have to belive :)

 

Sry for off topic but i wonder if ED Team know about frequently huge fps drop during P-51 guns shoting and plan to fix? These is really problem in dogfights when fps drop even below 10 fps and there are heavy stutters even then when other P-51 is shoting and i dont see it.

 

Edit.

 

I checked again temperature rising in vertical zooms and i found that one vertical zoom from high speed till stall cause temperature rising in coolant from green zone up to end of scale --- im really concerned if it is realistic. Im sure that in sustained climb below recomended speed or very slow speed turn which is doing by some time a few minutes coolant and oil tempeature should rise more quickly but i seriously doubt that will rise such quickly during ab. 10 second vertical zoom?


Edited by Kwiatek
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I'm sure someone will correct my reasoning, but off the top of my head - If a merlin has an output of 770 kW (not peak output) - say it's 40 % efficient, doesn't that mean there's 1900 KW of heat being generated inside the block ? (equivalent of 1,900 single bar heaters running inside the motor).

Now obviously a lot of that energy leaves with the exhaust gasses, but even say 15 % ends up absorbed into the block - that's still the best part of 300KW of heat to be dumped through the cooling system. I'd have thought that would drive the engine temp up pretty fast once the airflow over the radiators dropped away...


Edited by Weta43

Cheers.

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Well if we will take vertical zoom which all take ab. 10 seconds till stall so we get about 5 seconds flying with slow to stall speed ( it could be measured more detailed with stoper) when airflow over radiator is seriously dropped. So i consider if 5 second should be really enough to rise coolant temperature from green zone to above red line close to the end of scale expecially if plane is still moving in the air ( speed>0)

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  • ED Team
Well if we will take vertical zoom which all take ab. 10 seconds till stall so we get about 5 seconds flying with slow to stall speed ( it could be measured more detailed with stoper) when airflow over radiator is seriously dropped. So i consider if 5 second should be really enough to rise coolant temperature from green zone to above red line close to the end of scale expecially if plane is still moving in the air ( speed>0)

 

If you perform usual vertical maneuvers (300 - 150 mph at bottom and top) the radiator barely keep coolant near red label. When you perform stalls heatsink capacity drops proprtionally the average speed and you gain 10-15C easily.

 

5 minutes of warming at 1000 rpm (very low power comparing to MIL!) gives 70C increasing. of the coolant t


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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I dont know if i understand you correctly with these 1000 rpm and rise 70 C coolant temperature but in P-51 manual is note that to keep engine cool at the ground operations is recomended to not exceed 1000 RPMs ( in second manual i found not exceed 1500 RPMs).

 

Still i cant belive that during one zoom from ex. 350 mph till 50 mph coolant temperature could rise from 100-110 C till even 150 C - so we got about 40-50 C increase during 5-10 seconds with still some speed ( >0).

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