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High speed roll rates of Russian aircraft (with wooden wings) too high?


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#1 Holtzauge

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Posted 25 March 2017 - 16:24

Usually, a lot of forum roll rate discussions end up being about stick forces but there is another phenomena that comes into play as well, namely the aileron reversal speed. I won’t go into the details here but for those who are interested but unfamiliar with the term it can as most things in life nowadays be googled with success.

 

Comparing IRL data, the two German fighter plane types we have in BoX are quite different in this respect with the Fw-190 being one of the best with around 750 mph while the Me-109 (like the Spitfire!) is only a moderate performer in the order of 500-600 mph: Note that these are ballpark numbers for reference only and what I’m after here is how the Russian wooden winged aircraft fared so please avoid derailing this thread by nitpicking the other numbers!. Note that the aileron reversal speed is the theoretical limit and that roll performance is seriously degraded long before that: As an example, the Spitfire only retains circa 35% of its stiff wing rolling capability at 400 mph due to wing twist!

 

Now while wood is an excellent material for building airplanes with a good strength to weight ratio and excellent fatigue properties it does have a serious limitation: The resistance to bending and torsion in terms of deflection is miles away from that of metal. At lower speeds this is not much of an issue but at higher speeds it can become a serious problem.

 

Anyway, I have since I started flying in BoX tried to find some info on the aileron reversal speeds for the Russian wooden winged fighter as well. However I have come up short and even asking around has produced nothing.

 

Therefore, I have done some ballpark calculations to try to get a "feeling" for how a wooden wing compares to one in aluminium:

 

Russian Yak and LaGG plywood wing skin: Estimated average E modulus circa 9000 N/mm**2 (See data on Russian wood attached).

 

Dural Aluminium assumptions: E modulus 72000 N/mm**2, G modulus 27000 N/mm**2

 

For a ballpark estimate I rely on the so-called Bredt-Batho formula:

 

Twist derivate = T/(4xA**2) * Integral(ds/(G*t)

 

Where T is torsional load, A enclosed area, G the torsion modulus and t the skin thickness. Now ds integrated is the circumference of the enclosed area but in order to do a ballpark comparison assuming a wing of the same cross section we can conclude that it’s only the G modulus and the wing skin thickness we need to compare.

 

For the Russian plywood wing skin we as per above only have the E modulus which is in the order 9000 N/mm**2 but what we need is the G modulus which is not in the attachment but as far as I have been able to find out depending on the wood grain direction this is in the order of 12-15 times less than the E modulus. So my very rough estimate for the G modulus for the Russian wing skin is in the order of 600-750 N/mm**2.

 

Further, while the wing skin varies (circa 0.5 to 1.3 mm) on the German fighters I’m assuming its 0.8 on average and I’m assuming the wing plywood at 6 mm on the Russian planes.

So for a similar resistance to torsional deflection we need the G*t for wood to be the same. However, calculating G*t we get:

 

Aluminium: 27000*0.8= 21600 N/mm

 

Wood: 675*6= 4050 N/mm

 

If we factor these we can see that the aluminium wing is more than 5 times better at resisting torsional deflection than the wooden one.

 

My personal conclusion from this is twofold: One being that this may well be one of the reasons for the low Vne for the Russian fighters and two, that the Russian wooden winged fighters IRL probably had a low aileron reversal speed and consequently rolled very poorly at higher speeds, even worse than the Spitfire and Me-109 which were not known to do very well in this respect which is a bit at odds with what we see in BoX today I think.

 

Note my use of the words “may”, “probably” and “I think” above! I’m not saying I’m right and seeing some solid data would be lovely but until then I’m leaning towards the high speed roll rates of the BoX Russian wooden winged fighters being a bit on the high side.

 

And before someone lynches me for being a “Luftwhiner”, please read this and this post I made earlier on concerning my view of Russian wooden aircraft structural engineering!

 

So, what I’m hoping for here is a civil discussion with lots of nice input so that after a few pages we have a better understanding of how the Russian wooden winged fighters performed in this respect, and who knows, based on the high level of knowledge in this community, maybe even some constructive input to the developers. :)

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#2 JG13_opcode

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Posted 26 March 2017 - 03:18

Discussions like this make me wish I worked for the developers.  Getting paid to puzzle this stuff out would be so fun.  If we could get a decent set of blueprints and someone had the time, we could run a finite-element analysis and get some pretty decent numbers to work with.

 

I don't have that kind of time, sadly  :(

 

edit:    

 

torsion modulus

 

I believe this should be the shear modulus aka modulus of rigidity.  Being anisotropic there's a lot of variation in wood's mechanical properties but for MIL-STD compliant S2S sitka spruce that we use up here in Canada you might expect G to fall in the range of 500-600 MPa or thereabouts, so I'd actually suggest your figure of 600-750 MPa is a decent estimate for their fancy deltawood.


Edited by JG13_opcode, 26 March 2017 - 03:58.

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#3 Holtzauge

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Posted 26 March 2017 - 07:49

I see you use MPa which means you have a more fresh engineering background than mine. We used N/mm2 and the generation before Kp/cm2. ;)

 

Anyway, nice that you come to the same conclusion concerning a ballpark figure for the shear modulus G for deltawood.

 

Also, I just found some new data:

 

It seems (which is logical when you think about it) that the G modulus is dependent on how the force is applied in relation to the grain. Found the following numbers for Swedish spruce:

 

G modulus: 600 MPa parallel and 40 MPa perpendicular to the grain.

 

Now the deltaplywood in the wing skin on the Russian planes is made of sheets probably with the grain in 0, 45 and 90 deg plys so thinking some more about this I believe this means that the figure I used (675 MPa) is optimistic since it is probably closer to the plys parallel to the torsion load, i.e. the 45 deg ply while the other plys don’t contribute as much as that.

 

About the FEM analysis that would for sure be nice because then you could figure out exactly how much the different wings twisted. However, for the purposes of a comparison on a higher level I think comparing the shear modulus and wing skin thickness like above gives you a good idea that wing twist will be a big problem for wooden winged fighters at higher speeds.

 

This is why I’m starting to think that the Russian wooden winged fighters had a very low aileron reversal speed. And taking that one step further, not much use in going faster if you can’t roll so therefore a low Vne.

 

But I think the implication for the modelling in BoX go further: Ze-Hairys roll rate tests imply that the Russian wooden winged fighters in-game are no more affected by aileron reversal effects than the German Dural aluminium ones but based on the above reasoning, the Russian wooden winged fighters should have their rolling capabilities severely impacted at higher speeds.


Edited by Holtzauge, 26 March 2017 - 07:50.

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#4 Hutzlipuh

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Posted 26 March 2017 - 08:23


This is why I’m starting to think that the Russian wooden winged fighters had a very low aileron reversal speed. And taking that one step further, not much use in going faster if you can’t roll so therefore a low Vne.

 

So the 650 km/h dive limit in the manual may be justified ? ;)

 

found some nice pictures of a in-restoration of a yak 7 ....wing construction and main spar are nicely visible on a few of them : http://www.russianae...yak-7b-project/


Edited by Hutzlipuh, 26 March 2017 - 08:25.

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#5 VO101Kurfurst

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Posted 26 March 2017 - 09:06

Dunno about the Wulf but the 109's aileron reversal speed was in the order of 800 mph, as demostrated by the relevant historical reports.

Also I suggest you should take a close look at its wing sheet thickness when you have a chance, since assuming 0,8 mm thickness for a box spar design is quite an amusing statement.

Edited by VO101Kurfurst, 26 March 2017 - 09:08.

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#6 Holtzauge

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Posted 26 March 2017 - 09:16

So the 650 km/h dive limit in the manual may be justified ? ;)

 

found some nice pictures of a in-restoration of a yak 7 ....wing construction and main spar are nicely visible on a few of them : http://www.russianae...yak-7b-project/

 

Well based on that the wooden wings were much more prone to twisting, the question is how much aileron authority is left at 650 km/h for a Yak? Remember that the aluminium winged Spitfire only retained about 35% of its rolling capability at 644 Km/h (400 mph).....

 

Nice pictures BTW, will be nice to see that one flying! Really like the Yak: check out this cockpit video from a Yak-3. Yummy!


Edited by Holtzauge, 26 March 2017 - 09:17.

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#7 Holtzauge

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Posted 26 March 2017 - 09:47

Dunno about the Wulf but the 109's aileron reversal speed was in the order of 800 mph, as demostrated by the relevant historical reports.

 
Well Kurfurst, I think Me-109's with the 800 mph aileron reversal speed you refer to is not the one modeled in BoX because they were built in such limited numbers due to the scarcity of the special Hitlerinium alloy they required.
 

Also I suggest you should take a close look at its wing sheet thickness when you have a chance, since assuming 0,8 mm thickness for a box spar design is quite an amusing statement.

 

Glad you find it amusing! However, I suggest you google Bredt-Batho and you may actually learn something: The "box spar" contribution to resisting wing torsion is close to negligible, the reason the wing skin is used in the formula is because the main contribution to torsion resistance in the wing comes from the closed box formed by the upper and lower wing skins joining the the forward and rear spar and added to that the so-called D-cell forward of the main spar.


Edited by Holtzauge, 26 March 2017 - 09:56.

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#8 VO101Kurfurst

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Posted 26 March 2017 - 10:11

 
Well Kurfurst, I think Me-109's with the 800 mph aileron reversal speed you refer to is not the one modeled in BoX because they were built in such limited numbers due to the scarcity of the special Hitlerinium alloy they required.

 

Well Holtzeuge, it is simply not my problem that you are[Edited] (which I find the more plausible explanation) [Edited] about the Me 109 aileron reversal speeds and instead propagate these completely baseless aileron reversal figures you have conjured up from nowhere, I have merely pointed out the actual, correct figures for the rest of the audience, lest they would be mislead by your fantasies. That's how myth are born and its best the squash them at their infancy.

 

Now, I recognize you will keep sticking to your baseless opinion piece, as you always had when you were being caught using wrong base data to arrive at your desired conclusion, so kindly consider the above a simple correction of your error for the rest of the audience and as a form of pre-emptive mythbusting.

 

 
Glad you find it amusing! However, I suggest you google Bredt-Batho and you may actually learn something: The "box spar" contribution to resisting wing torsion is close to negligible, the reason the wing skin is used in the formula is because the main contribution to torsion resistance in the wing comes from the closed box formed by the upper and lower wing skins joining the the forward and rear spar and added to that the so-called D-cell forward of the main spar. So the "box spar" us used to carry bending loads while the wing skin carries the torsional loads.

 

 

As we both know, the skin's ability in the box spar to resits torsional loads is why I pointed out that you have made yet another false assumption with regards of the actual wing skin thickness, irrespective of this smokescreen you cover your retreat with.

 

So, irrespective of the fantasies put forth in this thread, the actual historical figure for aileron reversal for the Me 109 is 850 mph (now that I cross checked the wartime  source,  I recalled it a bit lower than it actually was)

 

Edit - rechecked source for aileron reversal.

 

Please refrain from name calling when posting on these boards. 


Edited by Bearcat, 07 April 2017 - 02:19.

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#9 Holtzauge

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Posted 26 March 2017 - 10:41

Well Holtzeuge, it is simply not my problem that you are either completely ignorant (which I find the more plausible explanation) or lying about the Me 109 aileron reversal speeds and instead propagate these completely baseless aileron reversal figures you have conjured up from nowhere, I have merely pointed out the actual, correct figures for the rest of the audience, lest they would be mislead by your fantasies. That's how myth are born and its best the squash them at their infafncy.
 
Now, I recognize you will keep sticking to your baseless opinion piece, as you always had when you were being caught using wrong base data to arrive at your desired conclusion, so kindly consider the above a simple correction of your error for the rest of the audience and as a form of pre-emptive mythbusting.


I take that as a rather long winded way of saying you want the audience to know that the Me-109 aileron reversal speed is 800 mph? If so its been duly noted. Now can we please return to the question of the high speed roll performance of Russian wooden winged planes which this thread is about?
 

As we both know, the skin's ability in the box spar to resits torsional loads is why I pointed out that you have made yet another false assumption with regards of the actual wing skin thickness, irrespective of this smokescreen you cover your retreat with.

 

Well AFAIK the wing skin thickness on the Me-109E varies between about 0.5 and 1.2 mm and most likely the latter models had thicker skins but you are missing the point: I have made no attempt to actually calculate an aileron reversal speed, I'm merely doing ballpark comparisons as to the RELATIVE behaviour of wooden and aluminium wings and if I were to assume a thicker aluminium wing skin then the relative wing twist would look even worse for the Russian fighters.

 

Right now I have assumed 6 mm on the Russian planes and 0.8 on the German. I used 6 mm because I have a vague memory that that was what they used on the Russian planes but I may be wrong. This gave a result that the wooden wing twists more than 5 times as much as the aluminium one.

 

If you are unhappy about that then you can use the above factor to calculate any other relationship because the relationship is linear. So, if we assume instead a 1.5 mm aluminium skin then the 6 mm wooden wing would twist 1.5/0.8*5 or circa 10 times more.

 

Hope this explanation makes it clearer for you! ;)


Edited by Holtzauge, 26 March 2017 - 10:42.

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#10 JtD

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Posted 26 March 2017 - 12:25

An interesting opening post. I would however, like to point out that in the overall construction the wing of a Fw190 or Bf109 differed from the wing of a Yak-1 or LaGG-3 like day and night. The Yak and LaGG both come with two full wing spars, which are meant to absorb both bending and torsional stress, whereas the Bf and Fw come with a single spar structure, supported by an auxiliary spar, which are meant to absorb bending, but only limited torsion. The torsion would mostly be taken by the load bearing shell, on the Fw more than on the Bf, but for both more than on both Soviet fighters.

So while a direct comparison might not be apples and oranges, it certainly is two different kind of apples. If you factor in the stiffness of the spars, you'll probably find it will compensate a lot, for the price of a higher weight. It might reduce the factor 5 to a factor 2, or something along these lines.

I also think that the ailerons on Yak and LaGG are closer to the centre of the wing than they are on Bf and Fw, meaning they'd cause less torsion for the same deflection.

Edited by JtD, 26 March 2017 - 12:25.

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#11 Holtzauge

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Posted 26 March 2017 - 13:53

An interesting opening post. I would however, like to point out that in the overall construction the wing of a Fw190 or Bf109 differed from the wing of a Yak-1 or LaGG-3 like day and night. The Yak and LaGG both come with two full wing spars, which are meant to absorb both bending and torsional stress, whereas the Bf and Fw come with a single spar structure, supported by an auxiliary spar, which are meant to absorb bending, but only limited torsion. The torsion would mostly be taken by the load bearing shell, on the Fw more than on the Bf, but for both more than on both Soviet fighters.

So while a direct comparison might not be apples and oranges, it certainly is two different kind of apples. If you factor in the stiffness of the spars, you'll probably find it will compensate a lot, for the price of a higher weight. It might reduce the factor 5 to a factor 2, or something along these lines.

I also think that the ailerons on Yak and LaGG are closer to the centre of the wing than they are on Bf and Fw, meaning they'd cause less torsion for the same deflection.

 

Sure the wings are totally different which is why I don't attempt to calculate any absolutes since this would as JG13_opcode pointed out earlier require a FEM analysis to produce anything useful. However, in the OP I'm actually comparing apples with apples since the conclusion is that a wing in deltaplywood would flex around 5 times more compared to a wing in aluminium assuming that they were geometrically the same.

 

In fact it's very difficult to even compare the Me-109 and the Fw-190 which are both in aluminium so those could be the apple and the pear and then we add the wooden La-5 and LaGG as the peach and banana and we have a fruit sallad but seriously, sure the rear spar on the Russian fighters will definitely improve the situation, but when it comes to bearing torsional loads, the best way to do that is to enclose as large as possible cross section into a closed section and using a material with a high G modulus. Concerning how much a rear spar will contribute, a reduction from 5 to two sounds just as arbitrary as going from 5 to 4 or 1.5. Also, please note that the initial deltawood G modulus estimate of 675 Mpa was probably optimistic and that a wooden spar will flex much more than an aluminium one due to a E modulus that is in the order of a 1/10 of an aluminium one so maybe starting with a factor closer to 10 would be more appropriate than 5.

 

However, IMHO there is no going around the fact that wood is not a suitable material when it comes to building high speed aircraft when you are looking at trying to minimize deflection. Wood is a superb aircraft building material if you can live with deflections like many aircraft can, but for pushing the envelope in terms of aileron reversal speeds in a fighter? Not so much. A La-5 made out of carbonfibre/expoxi laminate OTOH, now that would have been a sight for sore eyes! ;)


Edited by Holtzauge, 26 March 2017 - 14:05.

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#12 unreasonable

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Posted 26 March 2017 - 14:19

 

So, irrespective of the fantasies put forth in this thread, the actual historical figure for aileron reversal for the Me 109 is 850 mph (now that I cross checked the wartime  source,  I recalled it a bit lower than it actually was)

 

 

Trying to follow this thread but it is really beyond me. In particular I had no idea that the Me 109 could do Mach 1.1, let alone use ailerons at that speed - Chuck Yeager obviously a typical Yank braggart. 


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#13 Holtzauge

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Posted 26 March 2017 - 14:51

Here is a new idea for calculating Russian deltaplywood wing skin G modulus:

 

Swedish spruce G modulus is about 600 Mpa parallel to the grain and 40 Mpa perpendicular to the grain. Russians probably used birch though.

 

Assume deltaplywood made up of sheets bonded in resin with 0, +45, -45, and 90 degree direction. Further, assume 45 degree ply G modulus (600+40)/2=320 Mpa.

 

Deltawood G modulus for a 0, -45, +45 and 90 deg ply composite= (600+320+320+40)/4=320 Mpa

 

Original G modulus estimate of 675 Mpa in the OP gave a factor of 5.33. Here is a revised estimate based on 320 Mpa:

 

Aluminium: 27000*0.8= 21600 N/mm

Wood: 320*6= 1920 N/mm

21600/1920=11.25 more deflection

 

Of course this only applies to a comparison for wings that are geometrically identical and as JtD rightfully points out does not take the different layout, construction and the rear spar in the Russian planes into accounts but it still shows the uphill struggle you face with wood compared to aluminium…. :)

 

EDIT: Found some data on coniferous and birch plywood from a Finnish source: So commercial birch plywood is pretty strong: G modulus for 6.5 mm birch plywood 620 Mpa. This means that the original OP estimate of 675 Mpa for deltawood is probably not too bad after all…..


Edited by Holtzauge, 26 March 2017 - 15:03.

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#14 Y-29.Layin_Scunion

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Posted 26 March 2017 - 16:40

Trying to follow this thread but it is really beyond me. In particular I had no idea that the Me 109 could do Mach 1.1, let alone use ailerons at that speed - Chuck Yeager obviously a typical Yank braggart.


Yes. Apparently the BF 109 could go as fast as a Vietnam era jet's top speed....

I'm sure he means kilometers per hour. That said, the aircraft he's talking about was specifically fitted for a high speed test in June/July '44. And if I recall correctly, the test consisted of the pilot beginning his pull out of the dive at ~5000m altitude.

So let's not think every 109 F should be pulling 850kph dives easily and pulling up at 1500m. I know some people like take numbers without context...
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#15 JG13_opcode

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Posted 27 March 2017 - 02:37

I see you use MPa which means you have a more fresh engineering background than mine. We used N/mm2 and the generation before Kp/cm2. ;)

It's a habit ;) We used both, but rarely kiloponds.

EDIT: Found some data on coniferous and birch plywood from a Finnish source: So commercial birch plywood is pretty strong: G modulus for 6.5 mm birch plywood 620 Mpa. This means that the original OP estimate of 675 Mpa for deltawood is probably not too bad after all…..


Very nice find. I myself was digging around the NTRS and in NASA Technical Memorandum 76535 there is a reference to aviation plywood and to a 1954 Soviet publication called "Reference book to design airplanes for strength", which I believe should be translated as Справочная книжка Пo расчету самолета на прочность, which can be found here:

http://airspot.ru/bo...a_prochnost.pdf

My Russian isn't very good but I'm going try this week, maybe they've got some mechanical properties tables listed.

Edit: accidentally a word

Edited by JG13_opcode, 27 March 2017 - 02:50.

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#16 19//Moach

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Posted 27 March 2017 - 02:40

actual historical figure for aileron reversal for the Me 109 is 850 mph

that's gotta be a typo... kph would make a bit more sense, since it's not above the speed of sound and all...

 

 

but gets me thinking about it in game... at that speed, aileron reversal doesn't apply:  you have no ailerons  :joy:


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#17 JtD

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Posted 27 March 2017 - 07:48

Thank

http://airspot.ru/bo...a_prochnost.pdf

My Russian isn't very good but I'm going try this week, maybe they've got some mechanical properties tables listed.


Very nice find, thank you for digging. Attached
a) the raw wood table from the book, page 73 in the pdf, with tree names added in English
Following there a few explanations, since wood is a living material and directionally different. Not read, yet.
b) the aviation plywood tables from the book, page 76 in the pdf.

What is extremely interesting is that the properties of wood and plywood @ 0° agree with Holtzauge's ball park of G=600-750 N/mm². However, by proper application of the wood, i.e. proper use of stress resistant direction, this goes up by a factor of about 5-6, ending up at around 4000 N/mm².
With Aluminium having a bit more than 3 times the density of the plywood, the wooden structure could have about half the torsional resistance for the same weight as an aluminium one.

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#18 Holtzauge

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Posted 27 March 2017 - 17:28

My Russian sucks but I’m assuming the upper table (11.10) reads something like “aviation veneer first quality” and the table below (11.11) second grade?

 

Looking in the upper table then the G modulus as you say works out to a sursprisingly high number: around 4220 MPa. Seeing that G modulus for the unlaminated birch only goes as high as 640 MPa (As given in the other table) I was up till now thinking that you can’t get above that since that is if you go with the grain and ypically for wood the G modulus perpendicular to the grain is only about 1/10 of that. So to be begin with, I could not make any sense of this but then I started thinking about composites and the way the matrix material stabilizes the composite. I mean carbon fibre is not very stiff in itself but once stabilized by the epoxy you get a very high modulus. Maybe there is something similar going on here?: The birch in itself only gives 640 MPa but when bonded with phenolic resin this rises to an impressive 4220 MPa?

 

OTOH I now see that for the wing midsection, it looks like the Russian fighters had about half the wing skin thickness I was assuming originally (6 mm) with the Yak-1 at 3.5-3.7 mm and the LaGG-3 at 2.8-2.9 AFAIK. So to be fair, since the Me-109E had a wing skin thickness of 0.5 to 1.15 and I was taking the mid section there, it only seems fair to do the comparison with same assumption for the Russian fighters:

 

So a revised calculation of how an aluminium and wooden wing of the same construction would compare using the torsional resistance G*t as a base:

 

(27000*0.8)/(4220*3.2)=1.6

 

Or, using the density method like you did:

 

(27000/2.7)/(4220/0.68)=1.6

 

So this is a dramatic drop from the first estimate of a factor of more than 5 and taken together with the fact that the Russian fighters as you say had a hefty rear spar as well may well indicate that the resistance to torsion does not differ that much at all. However, this of course assumes that the revised G modulus figure of 4220 Mpa holds which I’m still having problems digesting seeing that unlaminated birch has a modulus of 640 MPa when loaded parallel to the grain and that a modern birch plywood only manages a G of 620 Mpa, i.e. only about a 1/7 of what the WW2 Russian plywood managed…..

 

@JG13_opcode: Big thanks for digging up and posting the Russian aircraft construction handbook! :good:


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#19 JG13_opcode

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Posted 28 March 2017 - 02:42

@JG13_opcode: Big thanks for digging up and posting the Russian aircraft construction handbook!

 

My pleasure.

 

Regarding JtD's figure of ~4 GPa for deltawood:

 

The material in question is a wood-fiber plastic using bakelite as the matrix, right?  Without knowing which of a nearly limitless variety of bakelite formulations were used, I did a cursory google search for its mechanical properties[1][2][3] and took the mean of the three shear moduli that I came across while simmering my pasta sauce with baked ricotta, and it came out to about 4.7 GPa or 4700 N/mm**2.  That's just for the bakelite alone.

 

I don't pretend to be a composites specialist but using the mixtures rule we can get a decent estimate of the shear modulus based on assumed volume fraction:  

 

Attached File  composites.png   11.19KB   0 downloads

 

I plotted this assuming 4700 MPa for the bakelite matrix and 650 MPa for the wood fibre.  I think 80% fibre by volume is probably a representative estimate.  That gives us G = ~ 2 GPa.  Depending on the Bakelite composition and void coefficient I could maybe see shear moduli in the 4 GPa range.

 

[1] - http://www.campuspla...mbH/79/a701c8a5

[2] - http://www.matweb.co...da0efa0a&ckck=1

[3] - http://www.matweb.co...5d1c20685810483


Edited by JG13_opcode, 28 March 2017 - 03:37.

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#20 Kai_Lae

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Posted 28 March 2017 - 03:35

In general, based on the above information, what does this mean theoretically with regards to La-5/LaGG roll rates?


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#21 JtD

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Posted 28 March 2017 - 07:03

The main point in the 4000 MPa figure is the angle at which the wood is subjected to the stresses. Keep in mind that at a 45° angle between stress and fibre wood is weakest. Meaning that if you just take a push-pull rod, you'd probably go with fibre direction along the rod, parallel to push-pull. This in turn means torsion will get to work at 45° against the fibre, making the rod very weak against torsion. A torsion rod with 45° fibre direction would be factor 6 stronger against torsion, but factor 4 weaker against push-pull. So, ideally, in combined load scenario, you take half the material parallel to deal with push-pull, and half the material at 45° to deal with torsion, which is stronger than a homogeneous 22.5°. In addition, it will also allow you to use 0° in the centre, and 45° on the outer shell, making the rod even more resistant.

This is pretty much what the Soviets did on their aircraft, just way more complex and sophisticated.

Angular dependencies are being discussed in the book on pages 74/75 in more detail than I have found in other publications. However, I did find a test with a result of about G=2000MPa for simple wood at 45° elsewhere. They also showed a huge dependency on the angle, in that case factors up to 10 between best and worst angle.
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#22 Holtzauge

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Posted 28 March 2017 - 17:38

I’ve been thinking about the 4000 MPa figure in the Russian table and I think I may have an explanation: I think it becomes clearer to visualize if one leaves the 2d Bredt-Batho view and instead looks at the wing as an idealized rectangular box structure, i.e. more of a 3d structure. Now if you apply a torsional moment T, this will induce a shear flow q in the structure. In this case the structure is made up of the upper and lower wing skins working together with the forward and aft spar shear webs to carry the torsional load. If you then for the purpose of inspection “cut” out the upper skin you will consequently have a panel with a shear flow q around the circumference. This shear force will try to deform the rectangular panel into a rhomb. However, since we have two plys each in the 45 degree direction, these serve to triangulate the panel surface. The third ply I’m assuming is in the spanwize direction, i.e. in the direction from root to tip.

 

Now the plys in the 45 degree direction are oriented so they are loaded almost in tension and compression in which case it would be more appropriate to look at the E not G modulus. In addition, the best way to orient the spar shear web is also in the 45 degree direction so essentially the wood fibres would form closed rectangular sections at plus minus 45 degrees relative the span direction.

 

So, using this analogy to calculate a composite G modulus for the panel: Russian birch, G modulus 650 MPa parallel to grain. E modulus circa 9800 MPa. Assume a three ply build (See attachment in OP) with a plus and minus 45 and a 0 deg ply. Since the shear force is not acting as a pure tension and compression but angled 45 degrees assume the contribution to G to be E/sqrt(2) from the 45 degree panels:

 

Estimate of composite G modulus for three ply plywood: (2*9800/sqrt(2)+650)/3=4836 Mpa

 

This is why I think the Russian data JG13_opcode found specifies a 45 degree grain orientation (in table by JtD above) and since this is an anisotropic and not like aluminium isotropic material that the E modulus comes into play and could go some ways explain the +4000 MPa figure in the table. At least it did for me. :)


In general, based on the above information, what does this mean theoretically with regards to La-5/LaGG roll rates?

 

Nothing so far I would say since it's still a bit unclear how much more a wooden wing would flex relative an aluminium one.


Edited by Holtzauge, 28 March 2017 - 17:50.

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#23 AndyJWest

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Posted 28 March 2017 - 18:44

One other factor that may need taking into account if when discussing wing torsional stiffness and its effects regarding aileron reversal is the wing planform. The Soviet fighters have highly-tapered wings, and it seems to me that this may mitigate the effect at least slightly, since aerodynamic forces are concentrated closer to the root than with a less-tapered wing. For a given amount of 'twist', the effects on roll rate are going to be less.


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#24 Dakpilot

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Posted 28 March 2017 - 19:38

De Havilland Hornet was very high performance post war fighter with 4000hp and noted for its very high roll rate and record breaking performance, amongst other good flight qualities.

It was mainly constructed of wood including main spars, it served successfully from 1945-1955 well as having a navalised carrier version

 

http://www.baesystem...avilland-hornet

 

http://users.skynet....dh103/dh103.htm

 

https://en.wikipedia...avilland_Hornet

 

It just seems to me that wood/composite as a construction medium still had serious relevance in 1945

 

I have no idea how accurate the figures posted in many of the above posts are, and do not suggest anything wrong with the science, simply that there seems to be a vast amount of supposition/assumptions taking place  :) to prove that wooden construction wings were much more inferior, simply adding to the already overused "Stalinwood" rhetoric. trotted out by many who do not have much experience or deep knowledge of aircraft design (not that I do)

 

Just a thought from a more general perspective, thinking of the 'bigger picture'

 

carry on...  ;)

 

Cheers Dakpilot

 

 


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#25 Holtzauge

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Posted 28 March 2017 - 20:05

One other factor that may need taking into account if when discussing wing torsional stiffness and its effects regarding aileron reversal is the wing planform. The Soviet fighters have highly-tapered wings, and it seems to me that this may mitigate the effect at least slightly, since aerodynamic forces are concentrated closer to the root than with a less-tapered wing. For a given amount of 'twist', the effects on roll rate are going to be less.

 

A very good point. The taper certainly helps and in addition, as JtD already pointed out earlier, the Russian fighters had a two spar construction which also helped in that the aileron forces need not only be carried by the torsion box formed by the wing but also carried in bending in the rear spar. So, yes both these effects would have a positive impact and taken together with the new estimate of a shear modulus in excess of 4000 MPa as opposed to the initial estimate of 675 MPa then things are certainly looking up for wooden wings right now!


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#26 JG13_opcode

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Posted 28 March 2017 - 20:17

there seems to be a vast amount of supposition/assumptions taking place   to prove that wooden construction wings were much more inferior, simply adding to the already overused "Stalinwood" rhetoric. trotted out by many who do not have much experience or deep knowledge of aircraft design

 

There's definitely assumptions being made, but I think the opposite of what you describe is happening here.  Facts and knowledge are examining the "common knowledge", and nobody is dogmatically following their preconceived notions. 

 

Assumptions are part of life, especially in science.


Edited by JG13_opcode, 28 March 2017 - 20:18.

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#27 MiloMorai

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Posted 31 March 2017 - 15:52

De Havilland Hornet was very high performance post war fighter with 4000hp and noted for its very high roll rate and record breaking performance, amongst other good flight qualities.

It was mainly constructed of wood including main spars, it served successfully from 1945-1955 well as having a navalised carrier version

 

The wing of the Hornet is a one-piece cantilever structure consisting of two spars with compressed plywood webs, extruded light-alloy bottom booms and spruce top booms.

A stressed plywood double top skin is reinforced by wooden stringers placed spanwise, and the bottom skin is of Alclad reinforced by extruded duralumin stringers extending from the outer engine-rib to the tip.

 

dh-103-37c.jpg

 

http://users.skynet....wing design.htm


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#28 AndyJWest

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Posted 31 March 2017 - 16:18

Wood in compression, aluminium alloy in tension, when the wing is under greatest design load. Evidently the Hornet designers considered that the benefits entailed overrode the added complexity of mixed construction. 


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#29 Dakpilot

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Posted 31 March 2017 - 17:31

The wing of the Hornet is a one-piece cantilever structure consisting of two spars with compressed plywood webs, extruded light-alloy bottom booms and spruce top booms.

A stressed plywood double top skin is reinforced by wooden stringers placed spanwise, and the bottom skin is of Alclad reinforced by extruded duralumin stringers extending from the outer engine-rib to the tip.

 

dh-103-37c.jpg

 

http://users.skynet....wing design.htm

 

I understand there was mixed media, the links in my post give the full details of construction, my only point is that wood was still relevant (main spars are wood..laminated birch plywood) in a very high performance carrier born aircraft of late 40's design and was in use until 55

 

Cheers Dakpilot


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#30 Holtzauge

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Posted 31 March 2017 - 20:03

The wing of the Hornet is a one-piece cantilever structure consisting of two spars with compressed plywood webs, extruded light-alloy bottom booms and spruce top booms.

A stressed plywood double top skin is reinforced by wooden stringers placed spanwise, and the bottom skin is of Alclad reinforced by extruded duralumin stringers extending from the outer engine-rib to the tip.

 

dh-103-37c.jpg

 

http://users.skynet....wing design.htm

 

Thanks for the link and interesting that they used aluminium and wood together like that. I guess it makes sense to use wood on the compression side since it is more resistant to buckling (for a given absolute strength). OTOH mixing materials like that must have been a challenge from a design standpoint due to the different maintenance, environmental and thermal expansion properties of wood and aluminium. Is there any indication of the Hornet's aileron reversal speed?

 

As a sidenote, I think I found the reason the modern Finnish bírch plywood shear strength is so much worse than the WW2 Russian: It seems the Finnish plywood is made up of sheets oriented at 0 and 90 deg where as the Russian plywood is at oriented at +45 and -45 degrees. So the Finnish value of 640 MPa shear strength  is for a panel which is loaded in shear either at 0 or 90 deg relative the grain where as the Russian data at +4000 MPa is for a panel where the shear load is oriented +45 -45 degrees relative the grain.


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#31 Holtzauge

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Posted 01 April 2017 - 09:58

In the OP I said that the Spitfire and Me-109 had an aileron reversal speed in the order of 500-600 mph and since this turned out to be controversial when it came to the Me-109 maybe it would be a good idea to clarify what I base this on:

 

The data comes from a German WW2 report on flight trials on a Me-109F (See attachment 1)

 

On page 7 and 11 in the report, it is stated that the highest Mach number reached in the flight trials was 0.62. From this information you can conclude that the speed on the x-axis of the attached graph (attachment 3) is TAS since M=0.62 gives TAS circa 735 Km/h at 3 Km altitude.

 

So reading off the TAS aileron reversal speed of circa 980 Km/h TAS at 3Km altitude (Attachment 2) we can calculate this to an IAS speed of 234.5 m/s or 844 Km/h or 525 mph at 3 Km.

 

So it turns out that the Me-109 is quite a poor performer in this aspect.

 

PS: The reason I include watermarks and don’t include more than necessary is that I have been asked by my source not to spread the complete report.

 

Copy righted material removed.


Edited by Bearcat, 02 April 2017 - 13:17.

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#32 VO101Kurfurst

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Posted 02 April 2017 - 11:43

"Aileron reversal.... 850 mph"

 

I'd rather trust the work of actual engineers over speudo engineering and selective quoting of documents on forums. ;)

 

"Trying to follow this thread but it is really beyond me. In particular I had no idea that the Me 109 could do Mach 1.1, let alone use ailerons at that speed - Chuck Yeager obviously a typical Yank braggart."

 

Obviously the 109 itself could not safely reach such high speeds, its safe dive speeds being in the order of 0.8-0.85 Mach, much like other WW2 fighters; the point of the this discussion/speculation is the aileron reversal speeds - these, since they are usually above safe dive speeds were not established empirically (through testing) but were calculated it from available test's raw data.

 

Aileron deflection creates torsional forces on the wings, twisting them (how much is dependent on the wing's design, some designs are worse, some are better) will make the wing behave like a huge aileron working in the opposite way and contributing to reduced roll rate at higher speeds, and after a certain point, even reversing the roll's direction. Aileron reversal speed is where the aileron's deflection is powerful enough to make the wing twist so much that the plane actually starts to roll in the opposite direction. However this point is rather theoretical as I have seen no WW2 design so far that would have its reversal point so low that it actually occurs below the safe dive speeds, moreover its a result of multiple factors rather than just one - powerful aileron forces may induce it earlier in a wing structure less able to resists twisting loads.

 

Edit - added some additional aileron reversal speeds for some other types.

Attached Files


Edited by VO101Kurfurst, 02 April 2017 - 12:57.

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#33 JG19_Leaf

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Posted 02 April 2017 - 11:58

I'm Kurfuerst on this one.. these calculations all seem very "back of the envelope" to me.


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#34 JtD

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Posted 02 April 2017 - 13:16

Why would aileron reversal occur at 850 mph if dynamic pressure at supersonic speeds is lower than it is in the transonic region? How would you use a 2D calculation to solve a non-linear problem?

Personally, I'd consider DLF calculations more reliable than British figures based on a 2D calculation based on a Bf109E test. They are probably both off in one way or another. I agree it doesn't really matter, as they both give reversal speeds larger than permitted diving speeds.

It's interesting to see a good even if not phenomenal high speed roll for the 109, 70°/s at 550TAS is not that bad.

Thanks for sharing the info to both of you.

Edited by JtD, 02 April 2017 - 13:17.

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#35 Holtzauge

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Posted 02 April 2017 - 14:00

First of all, the British RAE measurement the americans use in their theoretical calculation  is for the wing at a certain wing station (aileron midspan), so does not include the effects of cut-outs such as for landing gear etc. which explains the high value. In addition it’s for a different wing, an Me-109E not a Me-109F wing. So when it comes to determining the aileron reversal speed for a Me-109F, I would be more inclined to base that on a flight trial on a Me-109F rather than a theoretical calculation based of an idealized Me-109E wing.

 

JtD: I think it does matter though what figure you use since if it’s 525 or 850 mph has a very large impact on how much aileron authority you have left at Vne. With an aileron reversal speed of 525 mph there is very little aileron authority left since most of your effort is spent twisting the wing, not rolling.


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#36 Holtzauge

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Posted 02 April 2017 - 16:08

Just as an example: Here is a comparison on what it means in practice if the aileron reversal speed is 525 or 850 mph: Both are estimated values but IRL it would make a huge difference for the pilot what value the wing was actually capable of. As an example in the figure below, a pilot exerting a certain force to deflect the ailerons, will at 400 mph be rewarded either with a roll rate reduced 22% or 56 % from the theoretically possible maximum depending on how stiff the wing is in torsion. Note, the percentages are just for comparison of course, not absolutes since the actual IRL percentages would depend on the actual stiff wing derivate and the shape of the underlying curves but it gives the idea why the aileron reversal speed is not only a theoretical value but has practical implications as well.

 

Attached File  Aileronreversalexample.gif   39.19KB   0 downloads


Edited by Holtzauge, 02 April 2017 - 16:17.

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#37 Venturi

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Posted 06 April 2017 - 03:16

Why would aileron reversal occur at 850 mph if dynamic pressure at supersonic speeds is lower than it is in the transonic region? How would you use a 2D calculation to solve a non-linear problem?

Personally, I'd consider DLF calculations more reliable than British figures based on a 2D calculation based on a Bf109E test. They are probably both off in one way or another. I agree it doesn't really matter, as they both give reversal speeds larger than permitted diving speeds.

It's interesting to see a good even if not phenomenal high speed roll for the 109, 70°/s at 550TAS is not that bad.

Thanks for sharing the info to both of you.


Yes, aerodynamically ok. but what were the stick forces?...
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#38 Dave

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Posted 17 April 2017 - 02:01

Dunno about the Wulf but the 109's aileron reversal speed was in the order of 800 mph, as demostrated by the relevant historical reports.

Also I suggest you should take a close look at its wing sheet thickness when you have a chance, since assuming 0,8 mm thickness for a box spar design is quite an amusing statement.


800mph is supersonic.

Edited by Dave, 17 April 2017 - 02:02.

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#39 Dave

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Posted 17 April 2017 - 02:30

[quote name="Holtzauge" post
If we factor these we can see that the aluminium wing is more than 5 times better at resisting torsional deflection than the wooden one.
[/quote]
No, what you can see is that, for the same thickness, a sheet of aluminium has 5 times the torsional stiffness of a sheet of plywood. There is more to a wing than its skin. And there is more to roll rate than wing material. A better argument would be that being heavier for the same design load, the wooden wings would add more mass further from the CG which should increase inertia, reducing the rates of rotational acceleration and deceleration for the same wing section, area, plan, and aileron geometry. Lots of variables in there and it really affects acceleration (ie snappiness) more than angular velocity limits.
I think the only way you're going to find out one way or another is to perform an FEA of forces (and therefore stresses and deformation) integrated over the entire airframe. The software will take you a while to write and you'll likely need to buy yourself some large scale compute cluster time - or maybe start learning CUDA and build a GPU-based home super computer.
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#40 Holtzauge

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Posted 17 April 2017 - 14:17

No, what you can see is that, for the same thickness, a sheet of aluminium has 5 times the torsional stiffness of a sheet of plywood. There is more to a wing than its skin. And there is more to roll rate than wing material. A better argument would be that being heavier for the same design load, the wooden wings would add more mass further from the CG which should increase inertia, reducing the rates of rotational acceleration and deceleration for the same wing section, area, plan, and aileron geometry. Lots of variables in there and it really affects acceleration (ie snappiness) more than angular velocity limits.

 
No, I don’t agree about this being a better argument because you are muddying the waters by mixing two different effects here: So far we have been discussing aileron reversal speed and not the effects of inertia on roll acceleration. Roll acceleration and the effects of inertia are of course also interesting but aileron reversal speed is a separate phenomena as outlined here.

 

I think the only way you're going to find out one way or another is to perform an FEA of forces (and therefore stresses and deformation) integrated over the entire airframe. The software will take you a while to write and you'll likely need to buy yourself some large scale compute cluster time - or maybe start learning CUDA and build a GPU-based home super computer.


Thanks for the lecture but as an aeronautical engineer already having done the type of structural analysis you refer to above on aircraft structures I still think that doing ballpark estimates and having the kind of constructive dialogue we had so far in this thread can be helpful as well.


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