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landing gear toe-in

Good sims require good FDMs (the "thing" that makes an aircraft behave like an aircraft).

Re: landing gear toe-in

Postby OO ZVY » Sun Apr 11, 2021 1:59 pm

test with a " light " . not perfect but visible improvement.
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Re: landing gear toe-in

Postby danielHL » Sun Apr 11, 2021 2:46 pm

Ysop wrote in Sun Apr 11, 2021 1:24 pm:The trick would be to model the airplane as it is and then everything should fall into place correctly. At least this is ONE philosophy.

There is a bit of discussion on controlability of JSBsim-Taildraggers. Maybe there is a bit room for improvement for those as well.

On a tricycle gear it would be a bit unnatural. But as a YASIM-"hack": Why not.

Why would it be unnatural? It seems regular road vehicles also do this on the driven axis.
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Re: landing gear toe-in

Postby Ysop » Sun Apr 11, 2021 6:22 pm

Why would it be unnatural? It seems regular road vehicles also do this on the driven axis.


Always happy to learn more, but in tricycle configuration there are no stability problems.
Structure department would not like it.
Is there a reason to have toe in/out on maingear?
If this should compensate over-/understear, then maybe reducing taxi speed is an idea ;-)
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Re: landing gear toe-in

Postby OO ZVY » Mon Apr 12, 2021 1:28 am

" but in tricycle configuration there are no stability problems. ".

this isn't totally true. Trajectory is maybe less problem but there is still balance issues :

think about turn with a tricycle car like this while braking ...

Image

Not a problem for Take off but ... after landing, during deceleration, the aircraft experience a weight transfer from main gear to the 'single alone isolated' nose wheel. :-)
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Re: landing gear toe-in

Postby portreekid » Mon Apr 12, 2021 4:45 am

Yes I have done a few Mr Beans in tricycle configuration by turning too fast
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Re: landing gear toe-in

Postby S&J » Mon Apr 12, 2021 1:28 pm

I can only assume this thread exists because people know for a fact that propeller Inertia, moment and p-factor are all correct for the planes being complained about.

That being so please explain how you know this, and explain your working out ?
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Re: landing gear toe-in

Postby OO ZVY » Mon Apr 12, 2021 6:17 pm

Good afternoon S&J,

The principle behind this toe out stability is describe in page 1 of this post.

When the plane experience a right turn, due to weight shift the weight is mainly on the left wheel. ( momentum due to centrifugal force and position of aircraft CG above the wheels )
For the same ( opposite ) reason, the weight on the right wheel is reduced ( the more yaw the less weight ) .
If we estimate a proportional relation between weight and friction, the friction on the right wheels is reduced while the one on the left increase.
Assuming extreme yaw, all weight will be on the left wheels with no weight on the right. ( no weight = no friction = no effect form the right wheel )
A wheel toe-out produce a straight or 'more to the left' path in this example to counter the right turn and stabilise the path. .

The remark about other physical laws is off course relevant, but not the present focus.
This post is intended to use a 'real world' setting in FG for improvement like a mechanic would do to fine tune a real aircraft ( within manufacturer limits off course ). The toe in/out is indeed a setting affecting real cars and planes stability and small angle of it may be a huge importance.

Regarding the personal question how I know this, it's from experience, practice and learning :

Image

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Re: landing gear toe-in

Postby S&J » Mon Apr 12, 2021 7:14 pm

Good day to you sir.

I understand the principle of toeing out, however the issue I'm raising, which you seem to have missed is that you're addressing the symptoms and not the root cause of the issue ie. the reason that this weight shift is occuring.

If the amount of 'weight shift' is correct then the amount of toe out needed would also be correct. However if forces from the prop 'weight shift' is excessive, you may well solve the ground handling but the planes performance will still be wrong. And others will complain of the excessive dive speed, refusal to lose speed, excessive non uniformal roll rate. Etc.

That is all I'm saying, are you sure the forces acting are fine ?
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Re: landing gear toe-in

Postby OO ZVY » Tue Apr 13, 2021 11:09 am

well well....
Root causes :
- The root cause of 'weight shift' or ' weight transfer ' is the law of physics. Second law of newton if I remember..... Unless we build up a ' antigravity field startrek style ', we can't escape but luckily we may still deal with it. :-) This is real and quite well simulated on FG.

are you sure the forces acting are fine :
- I am assuming YASIM FDM coding is right regarding the law of physics. ( I still have a doubt about yaw axes but the matrix mathematics are very complex and I need more time and energy to fully understand and conclude on this )
- I am also assuming that YASIM aircraft design ( frame, engine, propeller,... ) has been made right.

to say my core hypothesis is that the aircraft simulation has been made as close as possible to what a real aircraft is. This is a thinking at a mechanics level, not computer coding level.

Based on this, little toe out on main gear taildragger FG plane ( just like on real plane ) should improve ground stability and flying experience. This is the only little objective of this research.

And yes, like in real world, tittle toe out stabilise, little toe in unsabilise FG tailplanes.

As you says, a deep study of YASIM coding law of physics compliance should be amazing but I think very very few people have the knowledge to do so. ( I am far yet from being good enough for this job ... )



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