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GH_Klingstroem
12-25-2007, 08:28 AM
When flying straight and level perfectly trimmed. Try this
Push the nose down quickly and watch not only downwards movement but for some reason the planes yaws slightly to the left as well! I did this many times and Im sure im using the elevator only and no rudder! I even tried
with the buttons too to make sure I was using the elevators only. it becomes even more obvious if u enable the K14 gunsight, you will see the gunsight moving not only vertically but also horizontally! So the conclusion is that for the P51 the use of elevators (pushing negative G) will throw off ur aim slightly to the left! Thats a serious issue for the p51 in game since its got the stupid point convergence which requires its pilots to have the best aiming in the game!
Now, I dont see this in other aircraft, or the few other ones I tried that is...
The p51 was considered only an average gun platform IRL, would this be the reason? Anyone with more info on this??
The only reason i can find is that the torque changes with an increase in angle of attack. Its a while ago since I studied for my traffic pilot licence so if anyone would the reason for this that would be great...

BSS_Sniper
12-25-2007, 08:59 AM
I'm just throwing out a quick guess without thinking about it...lol gyroscopic precession as modeled in game.

rnzoli
12-25-2007, 09:00 AM
I noticed this also, but I think this is also with other planes... Never really knew the theory behind it, I was (wishfully) thinking that only the "ball" is moving without real side-slip.. http://forums.ubi.com/images/smilies/halo.gif

Ratsack
12-25-2007, 09:05 AM
I think this happens in all planes. At least, I notice when pulling the nose up or pushing it down that I have to apply rudder to keep the ball centered. I assume it's gyroscopic effect.

cheers,
Ratsack

FoolTrottel
12-25-2007, 06:05 PM
Or could it be P-factor?

I'm not sure...

Maybe read this: The 4.0x flight model...some hints & tips (http://forums.ubi.com/groupee/forums/a/tpc/f/23110283/m/5111047273/r/5111047273#5111047273) written by Tully

Jaws2002
12-25-2007, 06:14 PM
IAR-80/81 has this too. and more torque then a corsair. http://forums.ubi.com/images/smilies/compsmash.gif

Viper2005_
12-25-2007, 06:15 PM
Gyroscopic precession as suggested by BSS_Sniper.

It isn't unique to the P-51.

But you notice more in a P-51 than in for example a 190 because of the importance of marksmanship when shooting HMGs instead of cannon.

It isn't P-factor because it is a function of pitch rate rather than alpha.

Ratsack
12-25-2007, 06:46 PM
Originally posted by Viper2005_:
Gyroscopic precession as suggested by BSS_Sniper.

It isn't unique to the P-51.

But you notice more in a P-51 than in for example a 190...

Actually, I notice it most strongly in those planes that need a lot of trimming anyway. The Tempest and P-51 are high on that list.

cheers,
Ratsack

Viper2005_
12-25-2007, 10:19 PM
The need for constant re-trimming is a symptom of a lack of stability.

Many WWII fighters suffered from this throughout their development lives.

The P-51 was no exception; hence the fillet and the eventual adoption of a taller fin for the P-51H.

The Typhoon suffered from similar issues; hence the larger fin on the Tempest. It is worth recording that the Sea Fury also ended up with a taller fin.

It is likely that this general process of fin enlargement would have continued were it not for the interruption of the jet age which dramatically changed the rules of the game...

M_Gunz
12-26-2007, 01:39 PM
How wide is the lane between slightly unstable and too slow to initiate maneuver to be a fighter?

K_Freddie
12-26-2007, 02:31 PM
It's big on the FW190 (radial engine). The yaw/gyro effect in very pronounced in the FW190 (or any radial) when pulling high-g in that the turn and bank indicator 'centres' slightly to the left.
If you try to centre it propper via rudder, you go into a opposite spin http://forums.ubi.com/groupee_common/emoticons/icon_cool.gif

Choctaw111
12-26-2007, 03:20 PM
It's gotta be the torque.

Lt_Letum
12-26-2007, 03:48 PM
This isn't a actual increace in yaw.
It only affects the instruments. The Yaw instrument is G dependant, so low G makes it hyper sensative to tiny amounts of yaw.

Urufu_Shinjiro
12-26-2007, 03:54 PM
Originally posted by Lt_Letum:
This isn't a actual increace in yaw.
It only affects the instruments. The Yaw instrument is G dependant, so low G makes it hyper sensative to tiny amounts of yaw.

No, it's actual, google gyroscopic precession.

VW-IceFire
12-26-2007, 09:06 PM
I've noticed this allot with the Tempest, Spitfire, P-51, and Yak in particular. The effect is less so for the FW190 or 109 but neither of them respond as quickly to negative pitch movements anyways so its probably just the effect being exaggerated on the more sensitive aircraft in this regard.

I didn't know the theory behind it but its definitely not specific to the 51 and I think the strongest is probably the Spit or Tempest actually.

The best in this regard are the P-38 and Do-335...amongst the prop fighters anyways. I guess its another effect of those counter rotating propellers.

The-Pizza-Man
12-26-2007, 10:04 PM
I notice it alot on takeoff as well, when you move the nose up and down it yaws left and right, and when you yaw left and right to stay on the centerline it will pitch up and down.

GH_Klingstroem
12-26-2007, 11:30 PM
Ok so almost all AC have it! However I still think the p51 is affected the most from it since the stupid point convergence makes it extremly difficult to hit ur target when its moving.
Heck I hit ALOT more with the 4 0.303 MG of the spitfire than I do with the 6 .50s of the p51. The reason is that the Spits MGs have slightly more spread!
My hit percentage usually is around 10-15% after a hours of flying online in the p51. So I do know how to aim!

So the yaw effect is realistic then, problem is that for all other AC that small yaw to the left is not too serious but for the p51 that yaw will make all ur rounds miss its target!

WTE_Ibis
12-27-2007, 05:01 AM
Viper2005_ Posted Tue December 25 2007 21:19 Hide Post
The need for constant re-trimming is a symptom of a lack of stability.

Many WWII fighters suffered from this throughout their development lives.

============================================



Instability is required for a manouverable WW11 fighter, without it you lose manouverability.
As far as I am aware only thrust vectoring may be the exception.


.

K_Freddie
12-27-2007, 07:54 AM
Originally posted by VW-IceFire:
The effect is less so for the FW190 or 109 but neither of them respond as quickly to negative pitch movements anyways so its probably just the effect being exaggerated on the more sensitive aircraft in this regard.

Odd.. I see more of this in an FW190 and ME109 than the others, but I have all my JStick settings at 100%. This makes everything more sensitive and unstable... the way I like it. http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

M_Gunz
12-27-2007, 10:43 AM
You want to see p-factor at work then do a steep climb at best climb speed with neutral rudder.

Viper2005_
12-27-2007, 11:34 AM
Originally posted by WTE_Ibis:
Instability is required for a manouverable WW11 fighter, without it you lose manouverability.
As far as I am aware only thrust vectoring may be the exception.


Instability implies divergence.

There are no unstable aeroplanes in IL2 (the Lerche may be an exception if the auto stabiliser isn't enabled in the hover by one of the 3 people who actually attempt to land it...).

If we neglect that possible exception, the closest that any aeroplane in IL2 comes to instability is that it may exhibit poor damping in one or more of the short period modes.

Genuine instability is a very different, far more dangerous and ferocious animal.

***

Actually if you want to get into the stability & control debate, it is perhaps worth observing that in an ideal world you want quite a lot of both.

- Lots of stability so that the nose stays where you have put it, facilitating accurate gunnery.

- Lots of control authority and fidelity so that you can rapidly and accurately point the nose.

However, in the absence of some kind of active control system, these two requirements tend to be in conflict.

In addition, various limitations make themselves felt:

- Excessive control authority is likely to induce ham-fisted pilots to break their aeroplanes or themselves.

- Large fixed and moving control surfaces impose a large penalty in weight and drag.

Generally speaking, high speed aeroplanes are designed around a roll and pull control strategy, which means that yaw inevitably tends to ends up being the poor relation as regards stability & control*.

In the context of WWII piston engined fighters, this was made worse by the process of development, which resulted in considerable weight & power growth, usually without corresponding growth in vertical stabiliser volume or rudder authority.

In the case of the P-51, things were made especially bad by the long moment between the prop and the CoG. This was even worse at aft CoG.

(Contrary to popular belief, the P-51 as modelled in IL2 does not exhibit the stability and control characteristics associated with aft CoG conditions.)

Remember that the P-51 originally started out with a 3 bladed prop, and a much lighter radiator installation, so it is unsurprising that seems especially afflicted by gyroscopic effects.

As for gunnery, my recent experience flying the P-51 and Mustang III online has only strengthened my view that the problem here is not so much the guns as the situation in which they are used:

- Virtual pilots tend to be brave. They won't generally abandon their aircraft after sustaining damage, especially whilst over enemy territory, unless that damage is likely to be immediately fatal (fire, loss of wings, loss of elevators).

- Virtual skies tend to be busy.

- Because of the first point, wounded aeroplanes are quite common, and because of the second, they are disproportionately likely to be finished off by some cannon armed fighter.

I have found that I can be quite an effective fighter pilot in a .50 armed aircraft, but that this will not score me many points.

Alternatively, I could attempt to score points by parking myself behind my target and firing until the outcome was in no doubt; but doing so would be reckless in the extreme since it would force me to be Co-E with my target, and therefore at great risk of retribution from his comrades.

I believe that it is therefore the situation and the perception of the results obtained that are the root cause of the "problem" here, though in a sense that's a different debate.

*Of course, we know a lot more now than they did then about aircraft stability & control, though we face somewhat different challenges; for example, inertia coupling tends to drive the optimisation towards big fins if you're designing supersonic fighters, and this may be beneficial elsewhere in the envelope...

WTE_Ibis
12-28-2007, 01:29 AM
Thanks for your reply Viper2005, a very interesting and thoughtfull response.
I know little about the subject really, only what I have read which seems to indicate that
a very stable aircraft is loath to leave that state thereby being slow to manouver.
Cheers, Ibis.

M_Gunz
12-28-2007, 06:31 AM
Originally posted by Viper2005_:
Actually if you want to get into the stability & control debate, it is perhaps worth observing that in an ideal world you want quite a lot of both.

Quite a lot is still relative. I don't see many fighters with 10 degrees of dihedral for example. http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif

Viper2005_
12-28-2007, 09:10 AM
Roll stability is rather a special case.

If you're flying a fast aeroplane, a small disturbance in roll will not significantly affect your gunnery since the turn rate associated with any given bank angle at 1 g (which is presumably what the aeroplane is trimmed for) will be small. This was not the case with slow WWI aeroplanes, which is one explanation for the relatively large amounts of dihedral they exhibited.

Also, there is no roll limit which if exceeded will lead to departure from controlled flight because roll is an absolute quantity rather than an angle of attack*.

This second point means that the concept of roll stability cannot directly be compared with pitch or yaw stability, since both pitch and yaw have a much more direct connection to angles of attack (usually termed alpha and beta respectively).

If you were to consider stability with respect to an angle of attack:

C = (alpha at left wingtip)-(alpha at right wing tip)

Then you would be able to make a fair comparison, and it should be obvious that it is necessary for C to exhibit positive stability if the aeroplane is to have nice handling characteristics.

Since C is proportional to (roll rate)/(velocity), assuming constant velocity, we may see that positive stability of C implies positive stability of the derivative of roll. Cue much fun with laplace transforms if you want to launch a theoretical attack.

Since dihedral affects roll stability rather than roll rate stability, it isn't hugely useful in this regard, and instead attention is generally paid to ensuring that the control surfaces are well behaved (ie well balanced with positive gradients of force vs displacement).

Dihedral is useful for controlling the coupling between roll and yaw. This is rather more complicated than simple stability with respect to a single axis. It is conventional to arrange for there to be some coupling between yaw and roll, such that left rudder will result in a rolling moment to the left. This is what dihedral is for.

In swept wing aircraft, the sweep itself produces considerable effective dihedral, so it is not uncommon for anhedral to be used in order to reduce the degree of coupling so that dutch roll may be avoided or reduced. But now we're heading towards inertia coupling, and there be dragons!

Of course, the idea that we should use a direct control strategy in pitch and yaw, and what NASA would call a dot strategy in roll is simply a convention. In space there is no damping of inertia, and so it is natural to use a double dot control strategy in pitch, roll and yaw. Indeed it is natural to use such a strategy in translations as well.

Humans aren't generally very good at working double dot strategies since two sets of integrations are required in order to actually get where you want to go. Therefore it is advantageous to use computer controls to generate artificial stability and allow for the use of a single dot or direct law.

It would be quite possible using such a system to produce an aeroplane which was controlled by direct law in pitch, roll and yaw at all speeds, though now pilots are used to the way aeroplanes have handled historically, it is questionable whether such an approach would gain acceptance. Then again, console gamers often happily fly arcade flightsims which use these kind of control laws...

*If you exceed a certain helix angle then it is possible to depart, but by definition you can't do this by using the ailerons. Which is why flick entry relies upon crossed controls (with aileron against the direction of roll) and bags of alpha.

M_Gunz
12-28-2007, 02:07 PM
Okay but Sopwith Camel upper wing is flat and according to Brittanica about 40 years ago the
SPAD's had about 2 degree anhedral to the wings. ;^) The SPADs had maybe the highest trainee
loss rate of them all unless the Camel did for more.

I look at a lot of dihedral as something you'd have to fight in order to roll. I forgot about
the slip coupling being from the dihedral, it would kind of make up at least for turning.

K_Freddie
12-28-2007, 04:17 PM
Physics is good.. but you know what they say about the theorists and those who know.. http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

Forests, worth of trees have been written about, this and that, and it still comes down to, who reacts faster.
http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

A little knowledge in the wrong hands.. is dangerous...
Too much knowledge in the right hands.. is useless...

Viper2005_
12-28-2007, 07:43 PM
Originally posted by M_Gunz:
Okay but Sopwith Camel upper wing is flat and according to Brittanica about 40 years ago the
SPAD's had about 2 degree anhedral to the wings. ;^) The SPADs had maybe the highest trainee
loss rate of them all unless the Camel did for more.

I look at a lot of dihedral as something you'd have to fight in order to roll. I forgot about
the slip coupling being from the dihedral, it would kind of make up at least for turning.

The Camel's upper wing was probably flat for structural reasons, with the lower wing exhibiting high dihedral to provide the necessary mean dihedral. You can see the same sort of strategy applied to the outer panels of the F-4's wing, and of course to the late lamented TSR.2...

Of course, the Camel was not known for excessive stability... http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

***

K_Freddie, "theorists" like me build your aeroplanes, so if you want improvements, I suggest you ask nicely... http://forums.ubi.com/images/smilies/mockface.gif

And if you're another one of those "it's the pilot not the plane" guys, you can take a 109, and I'll take a Eurofighter Typhoon... http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

Bremspropeller
12-29-2007, 03:46 PM
So the Rhino's 12 degs of dihedral were put there in order to reduce roll-yaw coupling?

Gotta imagine how bad the Rhino must have flown (r/y coupling-wise) with 0? of dihedral across the whole span http://forums.ubi.com/images/smilies/35.gif

Maybe that behaviour was caused by it's strange downward deflecting aileron/ spoiler combo.

Viper2005_
12-29-2007, 06:45 PM
http://oea.larc.nasa.gov/PAIS/Partners/F_4.html

Bremspropeller
12-29-2007, 07:22 PM
http://forums.ubi.com/images/smilies/11.gif

Thx a lot!