Aerodynamics and aeroelasticity

[drgondog]

Soren you have explained to us that I don't know what I am talking about when I say the study of Aeroelasticity was only an art during WWII.

Here is the place to debate it

You also have said you know a great deal about thiis subject and explained it many times to other members of the this forum.

Here is is the place to demonstrate this.

First, I would suggest that you explain the two or three characteristics of an airframe that are of most importance to an airframe structures guy that the Science of Aeroelasticity is close to yielding analytically today and approximately when in time the art started transitioning to a 'science'?

To refresh your memory, here are questions for you to start the ball rolling:

Explain
1. Aeroelasticity - How would you model an airframe system to yield analytical results for a.) natural frequency of the system and components with at least one degree of freedom?

2. How or why you believe that the designers of the Fw 190 (or ANY Airframe Designer - ANY nation) understood aeroelasticity and used that knowledge in their designs

3. Give an example of an airframe Model' period, which was developed during WWII -or even afterward.. and explain how those analytical model results, from any approach in preliminary airframe design used during WWII, were close to actual results under loads.

We will keep it cordial. Maybe I know stuff about this that you don't - maybe you know more. We can learn?

[DerAdlerIstGelandet]

I for one am looking foward to this discussion.

[Glider]

Me too, I was taught the basics but nothing compared to this level of detail

[buzzard]

Me too

...let's get ready to ruuuuuuuumble!

I'll be back as soon as I can find my Kevlar jock.

[drgondog]

Guys - I am going to wait one day to see if Soren answere the questions about aeroelasticity before we 'dabble' - and I do mean dabble.

I know the theory, I know structures and I know a lot about aero - but I called this an arcane art in WWII for a reason.

[renrich]

I am looking forward to this. I hope I can figure out what youall are talking about.

[Arsenal VG-33]

I've read that high speed aircraft like the SR-71 and the civilian Concorde would "stretch" while in flight. I've never understood exactly how this phenomenon occured, or how their designs took this into account. Is this related to the question? If it is, I'm interested.

[buzzard]

I'm not familiar with the Concorde 'stretch' but I presume it had the same cause as with the SR-71...heat expansion.

The SR-71 had several design features to accomodate this problem. One was the longitudal corrugations of the wing surfaces. The hi-flash-point fuel (JP-7?) was also used as a heat sink. The a/c also leaked fuel while on the ground due to the designed loose fit of some of the structure.

JL

[Glider]

Originally Posted by buzzard

I'm not familiar with the Concorde 'stretch' but I presume it had the same cause as with the SR-71...heat expansion.

The SR-71 had several design features to accomodate this problem. One was the longitudal corrugations of the wing surfaces. The hi-flash-point fuel (JP-7?) was also used as a heat sink. The a/c also leaked fuel while on the ground due to the designed loose fit of some of the structure.

JL

Heat was the cause of the Concorde 'stretching' this impacted the C of G and was compensated for by fuel transfer in three trim tanks. Center of lift was impacted by her speed and the tanks were used to compensate for this.

One interesting difference between the SR71 and Concorde. The SR71 was painted black as that was the best way of getting rid of excess heat, whereas Concorde used special white paint.

Same problem, totally different solution.

[pbfoot]

I believe all aircraft stretch after a while . I'm aware of a C47 and a F101 that stretched over the years

[drgondog]

Originally Posted by buzzard

I'm not familiar with the Concorde 'stretch' but I presume it had the same cause as with the SR-71...heat expansion.

The SR-71 had several design features to accomodate this problem. One was the longitudal corrugations of the wing surfaces. The hi-flash-point fuel (JP-7?) was also used as a heat sink. The a/c also leaked fuel while on the ground due to the designed loose fit of some of the structure.

JL

You are right about thermal expansion, and with the very high heat range experienced by this a/c, the allowable stresses are reduced significantly for the structural materials. IIRC at Mach 3., 60,000 feet the surface temp is around 500 degrees F and the stagnation temp much higher (i.e leading edge of the wing.

Aluminum didn't cut it so both Titanium and stainless steel were introduced to solve the 'stress problems' (lightweight, higher stress/wt and higher yield points)

Two more complications for something like an SR-71

At the surface (outer skin) to inner caps (stiffeners) to webs (connecting caps) to internal and shielded from High Temp surface - there is a temperature gradient going from high to low.. this temp characteristic has two effects - One introduce thermal stresses within the materials, and 2.) lowers the rigidity of the structure introducing Creep... simply 'stretch'

All of these associated 'strecthes or deformations' have a tendency to alter both the airfoil and the dihedral of the wing.

The top of the wing for example in high G manuever is at lower temp than bottom of wing (Higher CL, lower pressure/higher velocity on top surface) resulting in introducing 'more' dihedral' to wing and possibly negative camber to airfoil - all altering handling characteristics.

Creep under these conditions and under repeated loads will introduce factors similar to fatigue

I think I have this right - it has been a long time

[syscom3]

Aeroelasticity - Wikipedia, the free encyclopedia

[drgondog]

Originally Posted by syscom3

Aeroelasticity - Wikipedia, the free encyclopedia

Syscom - Good post - it actually pretty much got basic definitions and avenues of investigation right, at least nothing in my knowledge base over rides it. I note with amusement that they aren't delving into analysis techniques.

I'm also not entirely certain of the date, as described, because certain aspects of aeroelasticity were investigated earlier when it was possible to isloate the system under analysis from the overall airframe.

Von Karman, for example figured out alternating vortices on bridge suspension cables were the complex frequency inputs derived from a cross strong wind - leading to resonance, Departure and failure of a bridge (in WA state? 1930's??)

Flutter on a P-38 elevator, as symmetric, would reduce to analysis with the math in hand in WWII, ditto the 109 elevator when they went to pure canteliver on the 109F.. trying to figure out the failure mode - which I believe was resonance/departure.

I know when i was fooling around with it in late 60's/early 70's there was a long way to go - and frequently impossible to do in Preliminary Design.

Thanks for the post - everyone can take a look - including Soren if he needs it.

[Soren]

Christ

Bill when will you sieze with the pissing matches?? Don't you think I know what this is all about ?? Could you answer every single of those questions above Bill ?? No. Also when did I ever become an a/c designer Bill ? Have I ever claimed to be one ?? All you want is a fight, you have no intentions of keeping this cordial.

It's also very convenient that you avoided all other of my questions, and the reason is clear: You can't support your claims, your bold claims that the P-51 & 109 are close in terms of turn performance and that the field of aeroelasticity was considered witchcraft by aerodynamicists during WW2 being perfect examples.

Now I can tell you what aeroelasticity is and what its effects are on an a/c (Although wiki covers allot of it), I can also tell you that it was in no way witchcraft during WW2 which you claimed it was and that even the Soviets had Scientists specializing in this field, namely M.V. Keldysh, in the early 1930's. That having been said we get better at each field within science as time goes by, and ofcourse aeroelasticity is better understood today, and also A LOT easier to guard against because of the ability construct and test an airframe in sophisticated computer simulations before ever deciding to actually build it. During WW2 the methods were crude by comparison and the most reliable results were achieved by conducting test flights. One method used was carefully examining the wing profile under heavy loads while at the same time establishing the maximum load factor of the wing itself.

Finally I asked you to wait until Crumpp came on the scene, why did you ignore this Bill??

Anyway following your next reply I'll consider wether it is at all worth participating in this thread..

All in all I consider myself friendly not to just ignore this thread..

[drgondog]

Originally Posted by Soren

Christ

Bill when will you sieze with the pissing matches?? Don't you think I know what this is all about ?? Could you answer every single of those questions above Bill ?? No. Also when did I ever become an a/c designer Bill ? Have I ever claimed to be one ?? All you want is a fight, you have no intentions of keeping this cordial.

I intend to keep it cordial

Specify the quaestion you want an answer to - but first demonstrate that you can answer the questions - even if it is "this is the approach, theses are the complications, and this is why, or why not, atha approach can be relied upon?

If you know anything about the subject you can put at least the APPROACH out in front of us - I don't expect detail knowledge from a practitioner but I do expect one from someone knowledgable about the topic

It's also very convenient that you avoided all other of my questions, and the reason is clear: You can't support your claims, your bold claims that the P-51 & 109 are close in terms of turn performance and that the field of aeroelasticity was considered witchcraft by aerodynamicists during WW2 being perfect examples.

You and I have debated this forever, I have matched your 'related anecdotal comments' with many encounter reports, we have cussed and discussed the RAE reports to death, I have cited Rall's comments about the Mustang versus the 109 when he was running the training program for Luftwaffe leaders - you rejected his comments as 'flawed' because he was afraid to push it to its limit, I have asked for the flight test results from Rechlin that you say disprove the tests at RAE in March 1944.. but you don't have those...

I say 'turn with' you say 'hopelessly inferior' - combat results early and late are heavily skewed to the Mustang. Pilot quality is essential and a known variable.

You dive deep in aero to prove your thesis based on wing CL, I show you aero is more complicated than that - when you get confused you solicit Gene.. So - that is where we are after six months. Please step out and ask one person to substantiate what you can't - namely that the Me 109 clearly outperforms the Mustang in three different flight altitudes at three different speeds to compare stalling, aileron control, turn radius for each and the representative G's in the turn.

So far ONE test has been dissected, but in that test the Mustang III was described as 'clearly superior' - show another report by Luftwaffe professionals discounting this in controlled conditions and we'll talk - end of talk on this subject until you do?

Now I can tell you what aeroelasticity is and what its effects are on an a/c (Although wiki covers allot of it), I can also tell you that it was in no way witchcraft during WW2 which you claimed it was and that even the Soviets had Scientists specializing in this field, namely M.V. Keldysh, in the early 1930's.

Bring to the table ONE example of an analytical approach to modelling an airframe mathmatically and we can accept it as you thesis that it wasn't an art in WWII. By the way 'art versus science' is the phrase. 'Witchcraft' is your way of misquoting what I say.

That having been said we get better at each field within science as time goes by, and ofcourse aeroelasticity is better understood today, and also A LOT easier to guard against because of the ability construct and test an airframe in sophisticated computer simulations before ever deciding to actually build it.

Duh. Which is precisely the point I have been leading you too.. ditto aero simulation for the same reasons. Ditto why P-38 Flutter was discovered after crashes, not during design, or Me 109 horizontal stabilizers (according to Nowarra's research) failed on the early 109F's because they didn't correctly model the cantilever tail or its natural frequency, and the Comet kept putting metal and bone in the ground - for similar reasons - but leading to fatigue versus resonance/departure.

During WW2 the methods were crude by comparison and the most reliable results were achieved by conducting test flights. One method used was carefully examining the wing profile under heavy loads while at the same time establishing the maximum load factor of the wing itself.

That is NOT what you said to start this debate - and does NOT represent the theory of Aeroelasticity - only test to failure.

Vibration and thermal stress testing to failure under repeating loads is not all that old - maybe late 1960's

Finally I asked you to wait until Crumpp came on the scene, why did you ignore this Bill??

Primarily, because I respect Gene's knowledge about these subjects but not yours. If Gene wishes to correct me, we'll make sure he has my thinking right versus his own and we CAN debate civilly, out of mutual respect.

Anyway following your next reply I'll consider wether it is at all worth participating in this thread..

All in all I consider myself friendly not to just ignore this thread..

Soren, here is a fundamental problem. You do not know much about either fundamenta Aerodynamics (the underlying physics of fluid mechanics) or airframe structures and the disciplines of modelling an airframe along different approaches depending on which problems you are trying to solve.

If you would stick to what you know a lot about, we wouldn't get into these 'pissing contests'

Virtually every equation you have used, including CLmax, are mathmatical approximations based on different boundary conditions and the model you are trying to solve.

The classic airfoils in the tables are always 2D, 'infinite' wing, constant cross section and no twist. Then get ready to modify the crap out of it based on twist, based on tip/chord ratios, based on tip geometry, based on wing plan from, based on 'cleanness' of surface (nacelles, bomb racks, dirt, wavy surfaces)..

But you swoop in on someone who may know less than you about boundary conditions and start pontificating.

Fine - sit out. I can debate and I can learn - what have you got to teach me?