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Author Topic: Wing Flex  (Read 10233 times)

bomber

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Wing Flex
« on: August 12, 2014, 02:11:41 pm »

I've been asked how I'd go about modelling wingflex into a JSBsim flight model ?

So first off we have to put aside the animation of it within Outerra as that's just a 'red herring'. We're after understanding how the wing flex effects the 3 force directions LIFT, SIDE, DRAG or  X, Y, Z and the 3 moments ROLL, PITCH, YAW.

I'll start the ball rolling with my thoughts on what wingflex does and doesn't do....

It doesn't change the incidence to the fuselage body or any twist angle along the wing.... so It doesn't change the AoA
It's does change the dihedral of the wing and as such the LIFT and SIDE vectors.

What I can't get my head around as yet is what does it do to induced drag ?

Simon



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PytonPago

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Re: Wing Flex
« Reply #1 on: August 12, 2014, 03:10:00 pm »

 As for animating, you could make a function for a wing-bone deformation.



It's does change the dihedral of the wing and as such the LIFT and SIDE vectors.

What I can't get my head around as yet is what does it do to induced drag ?


 ... it doesnt affect the side vectors ? ... well maybe its too less of a effect to be seen (till the wings get ripped at critical G´s). It would doe slightly drop the lift (and all the stuff being connected to it) as the local lift vectors on the edge would be tilted to the hull direction.

... unless the wing twists alongside its length (getting front of the wing higher than the back and vice verse whyle hull keeping its vector ) - and, as i know, they should build planes to not do such stuff, cause it would be a pain in the ass for material-wear and micro-tears in the Aluminum/composite wing structure rods.

If it would happen, it should change the elevation angle, and the roll angle according to the deformation and difference between both wings. Still, the construction and weight of the plane shouldn't let such things happen in a much "visible" way.

 Straight wings in this should be a clear no, but tilted :



 ... could do that.  ??? ... you probably should calculate the wing-area behind the "tilt" and depending on the flight status take away a split of the lift on both wings according to the alongside wing axis deformation (left to right) + wingflex (the same in its axis of deformation) (front to back). (still, dont thing it would be more than 1-3% of the wing areas lift, if a heavy airliner plane pilot can feel the difference under normal flight turbulence).
« Last Edit: August 12, 2014, 03:17:25 pm by PytonPago »
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twizzstyle

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Re: Wing Flex
« Reply #2 on: August 13, 2014, 09:17:39 pm »

I just want to make the small point that wing flex most definitely DOES change wing twist along the span.  It's probably not worth the effort incorporating that into a flight model you're working on for Outerra, but since you explicitly mentioned that I thought it was worth pointing out.  (I'm a flight test engineer, so these things matter to me).

With a normally swept wing, as the wing flexes up the wing tip twists leading edge down.  With a forward swept wing (think X-29) as the wing flexes up, the wing tip twists leading edge up!

As for how induced drag changes with wing flex - a very interesting question!  For a given airplane, a wing flexes up because the wing is being loaded, generating more lift (either pulling g's, or just supporting a heavier aircraft).  There is more induced drag due to the increase in lift, but that's not a result of the wing flex.  For a given airplane, at a given weight, comparing a straight wing vs a flexed wing, I don't believe there is a change in induced drag (but I am scratching my head about it now)...  If there is any TINY amount of reduction in induced drag due to the wing shape, it's far out-weighed by the increase in induced drag due to the increase in lift.
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twizzstyle

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Re: Wing Flex
« Reply #3 on: August 13, 2014, 09:23:49 pm »

Also wing flex massively effects rolling moment due to sideslip (Cl_beta) because of the change in dihedral.  This is probably the most notable effect from a dynamics perspective.

Also if you want to get REAL nit-picky, any time the wings are asymmetrically loaded (any time you are rolling the airplane) the wings are flex slightly differently (the up-going wing is generating more lift, and therefore flexing more).

Large commercial airplanes have very very flexy wings, and they are designed so that at 1g flight they have the optimal dihedral and twist (again, they do twist!).
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PytonPago

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Re: Wing Flex
« Reply #4 on: August 14, 2014, 02:05:56 am »

Wow ... that must be some material-wear on them then ... would like to see the computation way for the optimization there. Do they pick special materials to make it twist a certain way (different mixes of composites in front and back of the wing) or just try to make the wing profile and shape to achieve it ?

 ... also, was it a good thing in the X-29 acrobatics/handling ? Sounds, like it should increase the lift in a maneuver when getting upwards, but at a fighter-jet speeds, it would probably want to tear the wings off.
« Last Edit: August 14, 2014, 02:22:40 am by PytonPago »
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bomber

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Re: Wing Flex
« Reply #5 on: August 29, 2014, 05:33:31 pm »

Levi
   
Code: [Select]
<function name="T4T/forces/wing_normal_left"> <!-- normal force generated by the wing -->
<description>force normal left wing</description>
<sum>
<value> 0.000001 </value>
<property>T4T/forces/wing_normal_left_8</property>
<property>T4T/forces/wing_normal_left_7</property>
<property>T4T/forces/wing_normal_left_6</property>
<property>T4T/forces/wing_normal_left_5</property>
<property>T4T/forces/wing_normal_left_4</property>
<property>T4T/forces/wing_normal_left_3</property>
<property>T4T/forces/wing_normal_left_2</property>
<property>T4T/forces/wing_normal_left_1</property>
</sum>
</function>

This is the force normal on the left wing... Animation of wing flex can be using the function to rotate each section based on 5° Max 150% wing loading.

Just a thought....
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Levi

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Re: Wing Flex
« Reply #6 on: August 30, 2014, 03:13:17 am »

Levi
   
Code: [Select]
<function name="T4T/forces/wing_normal_left"> <!-- normal force generated by the wing -->
<description>force normal left wing</description>
<sum>
<value> 0.000001 </value>
<property>T4T/forces/wing_normal_left_8</property>
<property>T4T/forces/wing_normal_left_7</property>
<property>T4T/forces/wing_normal_left_6</property>
<property>T4T/forces/wing_normal_left_5</property>
<property>T4T/forces/wing_normal_left_4</property>
<property>T4T/forces/wing_normal_left_3</property>
<property>T4T/forces/wing_normal_left_2</property>
<property>T4T/forces/wing_normal_left_1</property>
</sum>
</function>

This is the force normal on the left wing... Animation of wing flex can be using the function to rotate each section based on 5° Max 150% wing loading.

Just a thought....
Nice, I'll start doing the ASK-13's animation based on those properties. And the other ones for the right wing of course.

Also, the section number 8 is the one closer to the fuselage, right?
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Levi

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Re: Wing Flex
« Reply #7 on: August 30, 2014, 04:07:48 am »

Bomber,
Here you have the .js file attached with the animations based on the new properties to test it out.

1. I considered that wing_normal_left_1 and wing_normal_right_1 are the sections closer to the fuselage.
2. Inside the .js file (3rd line), I added a variable called wingflex, which multiplies the overall flexing of both wings. You can toy with that to see how much flexing you want.

Tell me what you think, and what to change :)

« Last Edit: August 30, 2014, 04:10:28 am by Levi »
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bomber

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Re: Wing Flex
« Reply #8 on: August 30, 2014, 05:04:41 am »

OK but I'm driving to Cornwall today, so be a while
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Levi

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Re: Wing Flex
« Reply #9 on: August 30, 2014, 05:06:37 am »

OK but I'm driving to Cornwall today, so be a while
No real rush there. Be safe on the road ;)
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bomber

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Re: Wing Flex
« Reply #10 on: September 12, 2014, 05:08:30 pm »

Now that just looks silly :)

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ZeosPantera

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Re: Wing Flex
« Reply #11 on: September 12, 2014, 09:29:18 pm »

Looks perfect to me...

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Levi

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Re: Wing Flex
« Reply #12 on: September 14, 2014, 04:12:12 am »

Now that just looks silly :)


Did you used the script I attached last time, or you just modified the older one?
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bomber

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Re: Wing Flex
« Reply #13 on: September 14, 2014, 06:24:02 am »

It's not your fault Levi, I looked at your script and as I needed to calculate the wing flex within the the flight model, I did so in JSBSim and simply pass the value to your animation code... However I needed to use a -1 in the rotation code. It's all fixed now I must get it loaded for you, but busy today taking my daughter to uni for the first time.
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Levi

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Re: Wing Flex
« Reply #14 on: September 14, 2014, 07:02:11 am »

It's not your fault Levi, I looked at your script and as I needed to calculate the wing flex within the the flight model, I did so in JSBSim and simply pass the value to your animation code... However I needed to use a -1 in the rotation code. It's all fixed now I must get it loaded for you, but busy today taking my daughter to uni for the first time.
Alright. Take your time, no rush.
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