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jcomm

Where's the problem with ground handling in X-Plane 10?

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We all know that under, even mild, crosswind components, aircraft, from small GA to airliners, are a nightmare to taxi or control during takeoff & landing.  What's goin on here?

 

I've asked this question to myself several times.

 

The pictures bellow were all taken with a 38 knot crosswind component from 3 o'clock, the first with the default C172, taxiing at around at around 12 knot...

 

Default C172 with collapsed tail, taxiing, wind 38 knot from 3 o'clock... (sideforce vector only at the rear fuselage):

 

 

I was able to taxi it and take the screenshot, and it didn't slide sideways accross the rw like aircraft do in MSFS with the default sideways friction coef!  What turned this possible?

 

Well, first, look at the sideforces. You will notice two vectors on the rear part of the fuselage, but no vector at all ahead of the CoG !!! It's like if there was no wind , from the left pushing the nose of the aircraft, actually the front part of the fuselage as a whole.

 

The reason why this C172 was so easy to control under this extreme conditions was the fact that I reduced, in Plane Maker, the size of the tail ( there are actually two tail components ) to almost 0 ft. ) Both were set with a semi-length of only 0.05 ft !!!  Without a tail, the big contributors were the rear prts of the fuselage and the tiny ( but existent ) tail.

 

What do I think is missing here? The calculation of the wind component and associated forces ahead of the CoG, to the front of the fuselage.

 

On the other two screens you can see the 744, where I applied the same "tail shortage technique". With the aircraft fully ground braked, and engines at full thrust ( you can see the thrust vectors ) there are no sideforce vectors. They only appear the instant you release the brakes!!! And... again, only in the rear part of the fuselage, as if the wind didn't play any effect on the rest of the fuselage, up to the nose!!!!

 

Default B744 parked, with engines at full thrust, wind at 38 knot from 3 o'clock, (no sideforce vectors while parking brakes not released):

 

 

Default B744 during takeoff run, at around 160 KIAS, under same conditions (sideforce vectors only at the rear of the fuselage):

 

 

How can such a calculation give as a realistic ground behavior? Well, I'll prefer not to think about how things are calculated when the aircraft is flying :blush: ...

 

So:

 

1) Apparently X-Plane only starts calculating the sideforces due to the wind components the moment brakes are released!

2) It forgets about a considerable part of the fuselage, namely all of it ahead of the CoG!!!

 

BTW, see what happens when I restore the tail to the C172 :-)   A lot of sideforce, behind the CoG, and nothing at all to compensate ahead of it, including the whole of the passengers cabin, the engine cowl, etc...

 

Poor C172 with it's tail back in good shape :-)

 

 

 

Austin? Heeeeeeeeeeeeeeeelp!

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Where I used to live winds in the high teens were pretty much " normal" and taxiing in them was no nightmare unless you did not use proper control input while taxiing. In a heavier aircraft like a Baron really not much notice till you got to the mid/ high twenties and of course gusty winds a whole other story. I got so I really preferred higher winds-with an final landing speed of 100kts. Nice to cut your groundspeed down by 20. Even so my wife who was never a nervous flier confessed she never liked flying in the Baron cause the speed always felt "too fast" on landing.

 

I cannot tell from your screenshot but it appears you are not using any control input to counter the wind?

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Where I used to live winds in the high teens were pretty much " normal" and taxiing in them was no nightmare unless you did not use proper control input while taxiing. In a heavier aircraft like a Baron really not much notice till you got to the mid/ high twenties and of course gusty winds a whole other story. I got so I really preferred higher winds-with an final landing speed of 100kts. Nice to cut your groundspeed down by 20. Even so my wife who was never a nervous flier confessed she never liked flying in the Baron cause the speed always felt "too fast" on landing.

 

I cannot tell from your screenshot but it appears you are not using any control input to counter the wind?

 

Yes, I wasn't... wel, just a bit on the B744 at 160 KIAS, but that's why I shrinked their tails to a mere 0.10 feet :-)

 

The main purpose was to show that IMO what's wrong is the fact that the side forces due to the wash, which has to be affected by the wind, are calculated only near the rear part of the fuselage, hence well behind the CoG, with no compensation ahead from the mid/front fuselage! This way, any aircraft will necessarily have to weathervane madly.

 

It was also a way of showing that I believe the main problem is not in the sideways friction coeficients being used.

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Was the engine idle? Your going to get prop wash and torque factor that's going to exhibit force on the aircraft. forces that your not going to get with the 747

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Was the engine idle? Your going to get prop wash and torque factor that's going to exhibit force on the aircraft. forces that your not going to get with the 747

 

Iddle in one shot, not iddle in the other, when on the takeoff run, but notice that the 744 also shows the vectors as it starts moving and the 2nd shot is taken at around 160 KIAS!

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Well, discard the above tetst :-/  My fault...

 

The 744 has a yaw damper... It'll come into play and that's what was causing the sideforces... Duh!

And, btw, I wouldn't say a b744 could fly so well without it's tail... :blink:

 

The sidevectors obviously only display for aerodynamic forces, although I still think that the Cessna propwash should give some components near the front of the fuselage and not only at the tail...

 

Ah! As I was testing the new Game-Ready Nvidia drivers, I switched HDR on, and the vectors are more difficult to see in this mode.

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Thx Morten...

 

know what? I love to read Ben's sometimes indcredably phylosophical posts, and really appreciate it, but then again, I close X-Plane 10, and fsx, many times, and go flying DCS World (or even Aerofly FS now that I installed it to test v1.2.0.1 ) because those guys, I don't know how ( are they geniuses ??? ) simply nail it down to what we expect and want!

 

Is it really that computationally complex????? It certainly can be, if taken to the limits, but I'd rather have a relaxed math / physics for this sort of modelling than have an "ultra-realistic" platform where sometimes things feel so unrealistic... :-/

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