Aerodynamics and aeroelasticity

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Here's where Soren already corrected himself, Bill:

Soren said:
Oh btw, Gene notes that the full elliptical lift distribution occuring in turns wasn't a design feature from the start, which I said it was, so on that point I was wrong and I gladly admit it.
Click to expand...


plus from the original email post:

 

I thought it perfectly sensible to ask those questions when you said a.) you knew perfectly well about Aeroelasticity and had lectured others on this forum, and b.) that WWII state of the art was not 'witchcraft'. I found both statements amusing and decided to give you the forum.

No spin.

No 'diversions'

Just the debate.

So, the thread is not about 'dictating' anything - but if YOU want to demonstrate your vast knowledge, here is the forum. What say you?
 
Here we go:


FACTS:

Erwin Leykauf, LW ace, 33 victories:
The Bf 109s also had leading edge slats. When the 109 was flown, advertently or inadvertently, too slow, the slats shot forward out of the wing, sometimes with a loud bang which could be heard above the noise of the engine. Many times the slats coming out frightenened young pilots when they flew the Bf 109 for the first time in combat. One often flew near the stalling speed in combat, not only when flying straight and level but especially when turning and climbing. Sometimes the slats would suddenly fly out with a bang as if one had been hit, especially when one had throttled back to bank steeply. Indeed many fresh young pilots thought they were pulling very tight turns even when the slats were still closed against the wing. For us, the more experienced pilots, real manoeuvring only started when the slats were out. For this reason it is possible to find pilots from that period (1940) who will tell you that the Spitfire turned better than the Bf 109. That is not true. I myself had many dogfights with Spitfires and I could always out-turn them.
One had to enter the turn correctly, then open up the engine. It was a matter of feel. When one noticed the speed becoming critical - the aircraft vibrated - one had to ease up a bit, then pull back again, so that in plan the best turn would have looked like an egg or a horizontal ellipse rather than a circle. In this way one could out-turn the Spitfire - and I shot down six of them doing it.

Walter Wolfrum, German fighter ace. 137 victories:
"Unexperienced pilots hesitated to turn tight, bacause the plane shook violently when the slats deployed. I realised, though, that because of the slats the plane's stalling characteristics were much better than in comparable Allied planes that I got to fly. Even though you may doubt it, I knew the Bf109 could manouver better in turnfight than LaGG, Yak or even Spitfire."

Herbert Kaiser, German fighter ace. 68 victories:
Personally, I met RAF over Dunkirk. [During this] battle not a single Spitfire or Hurricane turned tighter than my plane. I found that the Bf 109 E was faster, possessed a higher rate of climb, but was somewhat less manouverable than the RAF fighters. Nevertheless, during the campaign, no Spitfire or Hurricane ever turned inside my plane, and after the war the RAF admitted the loss of 450 Hurricanes and Spitfires during the Battle of France." In the desert there were only a few Spitfires, and we were afraid of those because of their reputation from the Battle of Britain. But after we shot a couple of them down, our confusion was gone."

Major Kozhemyako, Soviet fighter ace:
Me109 was exceptional in turning combat. If there is a fighter plane built for turning combat , it has to be Messer! Speedy, maneuverable,(especially in vertical) and extremely dynamic. I can`t tell about all other things, but taking under consideration what i said above, Messerschmitt was ideal for dogfight.

Mark Hanna, Modern 109 pilot:
I like it as an aeroplane, and with familiarity I think it will give most of the allied fighters I have flown a hard time, particularly in a close, hard turning, slow speed dog-fight. It will definitely out-maneuver a P-51 in this type of flight.

Skip Holm, Modern 109 pilot:
"Most of my flights have been in formation with P-51s and the Me-109 is more maneuverable than the P-51 in most conditions"

Skip Holm interview:
View: https://www.youtube.com/watch?v=TFl8X4y9-94

And here Rall explaining himself that he never went past slat deployment:

Günther Rall, LW ace:
The plane had these wing slats and you mentioned they pop open uneven?
"Two meter slots on fore wings. The reason was to increase the lift during low speed take off and landing. To reduce the length of runway you need. In the air, if you make rough turns, just by gravity, the outer slot might get out. You can correct it immediately by release of stick, you know? Only little bit, psssssssht, its in, then its gone. You have to know that. And if you know it, you prevent it


Bf-109 K-4
Weight: 3,364 kg
Wing area: 16.15 m^2
Win span: 9.92 m
Wing AR: 6.09
Wing Clmax: 1.70
Cd0 : 0.023

Engine power 1,975 HP.
_______________________
Lift loading = 122.5 kg/m^2
Power loading = 1.7 kg/HP


P-51D Mustang
Weight: 4,585 kg
Wing area: 21.64 m^2
Wing span: 11.21 m
Wing AR: 5.8
Wing Clmax: 1.47

Engine power: 1,830 HP
_______________________
Lift loading = 144.1 kg/m^2
Power loading = 2.5 kg/HP

And then we have Gene's (Crumpp) charts supporting the figures above.

And as for the Bf-109 vs the Fw-190:

Hans Werner Lerche, LW test pilot:
"The La 5FN is best suited to low altitude combat by virtue of its engine performance. Its top speed at ground level is slightly below that of the 190 and 109 (using MW 50). The 109 with MW 50 is superior over the whole height band in top speed and climb rate. Acceleration is comparable. Aileron effectiveness is better than the 109. Turning times at ground level are better than the 190 and worse than the 109."

And there's the two LW comparative test flights I posted elswhere on this forum, both times the 109 easily outturning the 190. And then we have the British comparative trials where a heavy 190 Jabo turns with a Mustang Mk.III.

So time for you to try and refute all this Bill and support your own claims, and seeing that this claim of yours is VERY old I see it only as just that you start out! I'll answer your questions afterwards.

Looking forward to your reply...
 
Soren - gently, this is about Aerodynamics and Aeroelasticity - not manuevering dynamics or anecdotal comments. This is my thread and it bagan with your bold claims.

Bring your claimed expertise in both these fields and hold court.

Answer the three questions. You are engaing in one of your personal favorite maneuvers, evasion and escape... come, meet me head on. Subject

Aerodynamics and the theory of flight, aeroelasticity and the theory of elastic deformation and resonance and fatigue in the design of airframes.

Answer the three questions and we can begin
 
I knew you would chicken out.

And as for this being your thread and me not being on topic, well do you see now how irritating that is Bill ? You did this in countless threads.
 
Soren
This thread is about aerodynamics and Aeroelasticity not maneovering dynamics.
The quotes you gave clearly have nothing to do with aero dynamics.

If I could suggest that both of you start again and ignore what has gone on before. Both of you post two questions and both of ou undertake to reply to those questions then those of us who want to learn can do so.
 
Certainly a good idea Glider, one I've wanted to be followed for a long time now. Bill however seems unable to ignore the past and is stuck.
 
Okay, well on the topic I'll again pose the question on elliptical wings:

How do varying elliptical planforms effect the lift distribution, ie the Spit's wing has the ellipse stretched toward the leading edge, compared to a pure elliptical wing as seen on the He 70, or He 112, or the straight LE with elliptical trailing edge of the P-47/P-35/P-43 (and the Re.2000 series fighters) or He 280. Or elliptcal with clipped tips like the CW spitfire, Tempest, or P-47N.

Or adding rounded wingtips to a trapezoidal planform. (ie Bf 109F)
 
The tips added to the 109's wings were actually semi elliptical and where added to improve the efficiency of the wing. I have a chart somewhere on this which I'll try to dig out for you KK.

But despite all the ongoing debate of elliptical lift distribution the benefit is actually extremely small, and aerodynamicists quickly found out about this and henceforth the elliptical wing was scrapped; Put simply, too much effort for barely noticable gains.
 
Cheer up Soren, at least you both can flame away on each other without sidetracking an interesting thread
 
Soren said:
I knew you would chicken out.

And as for this being your thread and me not being on topic, well do you see now how irritating that is Bill ? You did this in countless threads.
Click to expand...

Go set up another thread and we'll dicuss Gene's math.

Then we will work the math at three different altitudes using the manufacturer's charts for Hp versus altitude at Military and WEP for those altitudes and we will look at drag (I noticed you used best case 109K-4 performance against a P-51D, which by the way has far less drag and i noticed you didn't show it above)) and we will drop 1000 pounds and compare P-51B against K as well as against 109G5, 6, 10 and 14's also with dramatically reduced ata, and you will bring out reports discussing the complete Cl profile for ships.

Then I would like to to comment on both charts from Gene (not you) relative to the Fw 190 and ask yourself

At which point in the nice smooth high G versus TAS does the BOTTOM DROP OUT as we have been discussing for the last week?

Take it to another thread.

Answer the questions I posed you on this thread.

Thank you
 

First and most important. All wing section data is maximum, or best, that can be expected because it represents a two dimensional wing - in other words one stretching to infinity with no downwash distribution and no tip vortex.

If you presented a lift distribution it would be constant from wing tip to wing tip. You would visualize a rectangle. This could never exist in real life as there would be a discontinuity at the tip because the lifting line vortices distributed along the span have to 'go somewhwere' and they do.. and it is manifested as two tip vortexes that 'theoretically in a perfect non viscous air) would extend all the way back to original point of lift.

All three dimensional wings will exhibit the above characteristics regarding tip vortex.

All three dimensional wings with constant or tapering chords (either rectangular, trapezoidal, delta or elliptical) will have maximum Lift (Circulation in wing theory) in the center of the wing (when looking at Lift Distribution versus semi span plots), and then all will exhibit 'elliptical like' Lift distribution and curve 'downward' from Maximum at Center to zero at the tip.

A TRUE elliptical wing from a Planview, the spanwise downwash from center to tip due to the distibuted Circulation is a Constant and will exhibit the least induced drag in comparison with all the other planform types with same aspect ratio..

Note: in the last thread where you posed this discussion I pointed out that ALL real world (and specifically the Mustang, Fw 190 and Spitfire) exhibited 'ellipital like' distributions when Soren seemed obsessed with a.) Spitfire didn't have an elliptical lift distribution but the Fw 190 designers did that just to 'improve high G turn capability'..

I pointed out that while all exhibited the 'elliptical like' Lift distribution, the Spit was better than the Fw 190 and the P-51.

In a trapezoidal wing for example, varying the tip/chord ratio 'higher' to say .4 will mitigate the differences in efficiency (for subsonic - actually for supersonic the delta is essentially more efficient).

But real world says you do a lot of things to a wing to a.) perform best in as wide a spectrum as possible, and b.) be safe to fly in a wide range.

Wing twist is primarily to enhance the low speed landing safety by lowering or redcucing the chord angle of attack relative to freestream velocity as you move outboard from fuselage. When you do that you are trying to reduce the effective CL going outboard. When you are successful you still get the high lift in the inboard section of your wing (where you want it) but when it reaches CL max, the inboard section will stall first (by design) and the local angle of attack outboard keeps that section from reaching CLmax at the same time..

So you get buffeting over elevators feeding that to your stick, but you still have lift over the ailerons and a chance to correct to a lower angle of attack (push stick slightly forward) to regain control while not losing roll control.

(which the 190 did because of the unanticipated structural deformation in the tip area causing it to stall at the same time - or earlier as the inboard.)

Now we're back.

The rounded tip does slightly diffuse the tip vortex (in theory) from the last point of true airfoil to the tip. In many cases it will increase the wing efficiency slightly in a similar way to increasing the aspect ratio (slightly) - but more costly.

The Trapezoidal wing planform is simpler that an elliptical wing planform - and it is easier to design placing the main spars (for both structural efficiency and weight) for trapezoidal wings than elliptical wings like a Spit and a Jug.

I'll check to see if I answered your questions

Net Elliptical Wings are more efficient with respect to induced drag given same airfoil, same twist, same aspect ratio..

ALL have 'elliptical like' Lift distribution with max at center, zero at tip.
 

Soren, you choose to ignore facts and opinions that are at variance with your own. That could be one reason to ignore this thread.

You could choose to ignore because you may realize you may be out of your depth. We haven't fully tested mine so my 'expertise' may be subject to question also. This thread is one way to focus on that without getting personal.

You could choose to ignore the thread, but 'peek' every once in awhile because it is possible I have a body of knowledge that you don't and you want to learn but are too proud to ask... particularly after all the bi-directional insults (both of us) - but very specifically because you have completely dismissed my knowledge in these fields in a very contemptuous manner and it would be embarrasing to ask.

I hope you stick around - maybe both of us can learn
 
I am going to do this in multiple parts because it is necessarily complicated and will bore the crap out of Marcel.

We will start at the point the (Fw 190, P-51 etc) preliminary design teams had settled on the airfoils and wing design criteria, sized the tail (Vertical stabilzer, rudder, Horizontal stabilizer and elevator, trim tabs, etc) based on stability and control requirements for all the flight profiles, fuselage, fuel, engine, landing gear, length of take off and landing length req's, etc., etc.

The aerodynamicists have done all their calculations and are working on deriving wind tunnel results, in parallel, and will be working constantly with the airframe design guys to discuss changes as issues are discovered relative to stall, fixed and free static margins relative to unanticipated aerodynamic effects or cg changes, etc, discovered over time.

But they feed to loads to the Airframe team and the detail structural analysis begins with Free Body diagrams of the airframe under external loads such as engine torque, thrust, lift, lift distribution, drag, landing G's,etc. Very important in this beginning are also asymmetrical loads due to rolling and diving turns.

A first place to start might be looking at all the load combination on the tail - first to look at the Vertical (dive pull out) , Horizontal (Rudder loads in roll) and Moments about the longitudinal axis of the airframe (largely due to the asymmetrical loads- both rudder in roll as well as torque from prop/engine applied to fuselage)

BTW - this is HUGE in a typical helicopter tail boom and rudder design due to the rotor being required to counter the huge torque of the main rotor systems.

They look at the external loads on the wing because they are interested in a.) the wing not failing, and b.) the transmitted loads to the fuselage, and c.) what compromises they make have to make in wing structure due to placement of landing gear, flaps and ailerons, armament, fuel, etc.

They TRY to anticipate structure to resist 'too much' deflection and torsion due to aileron and flap loads, particularly in turns, to maintain the anticipated aerodynamic efficiency to achieve their design targets.

They look at the powerplant and how the loads tranmitted by that big ass prop (thrust and torque) must be carried to the airfarme - and usually anticipate that both hp and torque will increase over time and the structure must be a.) capable without redesign or b.) redesign of key components are not difficult or long lead time. Example if the primary structur is a forging instead of tubes - you are hosed as that woul be a long lead time item to change.

More later - we'll get to 'aeroelasticity as it existed' in WWII in next post or the one after it
 
And it was all done with slide rules.

In my experience in the aerospace industry, talking to the engineers , they told me something about design.

Some of the smarter engineers know intuitively whats going to happen, and the lesser engineers end up proving them right.
 

That I like
 

At no point in this thread so far (atleast until this point) has Bill been anything but cordial. He is asking you questions, you are skirting around them.

This has the making of a good thread, that a lot of people could learn from. Just answer the fricken questions man!
Frankly Soren he is only talking to you in the way you talk to other people.

Seriously Soren, he has not flamed once in this thread.
 
I know virtually nothing about this topic, but a question keeps floating into my head and was wondering if someone would want to answer it. My rudimentary understanding (if you could call it that) of the flutter phenomena is the wing bending or vibrating because of the forces caused by the airflow past the wings. Yes? Now, if there is more airflow further from the wing root, than there is closer to it, isnt there going to be a greater effect because (for want of a better expression) there is a greater leverage for the forces at work to exert on the wing and airframe structure?

If that is correct, wouldnt an elliptical wing form be inherently less likley to suffer than a trapazoidal wing form, where inherntly a greater percentage of the wing are is further away from the wing root.

Or is this completely wrong......
 

Flutter is usually experienced primarily with control surfaces and usually because they have a rotational degree of freedom (i.e. pivotal as in elevator or aileron.)

The manifestations of 'flutter' is an oscillating deflection normal to the freestream. It is caused by one of two primary reasons.

1.) it is immersed in turbulent or unsteady flow (say near inboard wing stall or ,compressibility, or indicial gusting) and that turbulent flow is shedding vortices on the elevator in a way to approach resonant frequency
2.) it is resonating with an input frequency of some other type input - say the beat frequency of the engine.. and the structural frequency of the elevator/aileron is close to the input frequency

If the wing, for example starts 'resonating' you are in deep and serious trouble as resonance by definition amplifies the deflections - usually in the case of an airframe - to failure.
 
Next phase - you have all the loads and work on individual primary assemblies, for example the tail. With the assumed worst case rudder loads normal to the rudder and the axix of symmetry for the airframe, and the assumed worst case loads on the elevator you design each sub component within the "lines drawings" established during Preliminary Design. You have two dominant boundary conditions - stress and weight.

As the rudder and elevator are movable surfaces, Flight Controls group, is engaged to help design control horns, tab linkages, etc - which are largely determined by the forces that have to be applied over a moment arm, and the moment arm length or 'swing' are largely determined by the space available.. So you look at the design, the hinge points and control forces required, the amount of deflection permitted by the aero and stability and control guys for rudder and elevator, the spars, torque box, etc to take out bending cause by the aero forces applied to them.. If they (load carrying structure) are built up beams or torque boxes then compression is carried in one cap, tension in the opposite cap, and the shear transfer in the web between the caps - normally riveted.

So visualize an "I" beam with the very top surface being the skin of the elevator, then an "L" or a "T" extruded aluminum Cap section, then a Web made of thin aluminum connected to both the top and bottom "Cap" section, with rivets. It will be sized based on buckling stress threshold in carrying shear from the Compression Cap to the Tension Cap. When Buckling is calculated it usually means going up one more dimension in the sheet metal web - say from .040 to .050.

I am going somewhere with this relative to aeroelastic analysis, but later

Usually a spar built up like this will be stiffened with vertical Caps joining the top Cap, Web and bottom Cap.. this results in a closed solution for shear analysis and enables stiffening the web in some cases without going to a thicker web - or simply a desirable loacation to attach a rib.. or located there (think three D) to attach another web perpendicular to this beam we are building to yet another Spar built up the same way - But Now we are building a Torque Box rather than a simple spar.

So now assume we have built the top and bottom skins, we have our beam or torque box analyzed, we have all the cut outs required to position control linkages, that it successfully meets the 8 G limit load to Yield at its WEAKEST point, and meets 12G ULTIMATE for its weakest point also.

Maybe the elevator is not so large as to require torque boxes and simple tube truss works, but we have used the same approach for the Horizontal and Vertical Stabilizer..

For the moment we have finished the elevator, and similarly the rudder and say for the moment we know how to build the vertical spar of the Vertical stabilizer, and we have finished looking at tail wheels and are at the bulkhead that will be the 'Tail" to "Fuselage" interface.

At that point we know we have to safely and efficiently carry the loads from the tail in all the worst case scenarios up to 12 Ultimate (at least we have done the best we can - and fought with the airframe design guys in the eternal Strong versus Light battle.

I know I can declare victory with the analysis because safety usually trumps Performance in US doctrine - but I do care because I want MY ship to be the best of class and weight is HUGE.

So Now I'm at the interface aft and I know I have to take out Forces normalized to three Planes and Bending in at least two axes - but I'm gonna check with all three.

At this point you will be looking at perhaps 4 main beams of the airframe (two top, one on each side, and two bottom, one on each side... they will transmit the Compression and Tension Loads, a Bulkhead will take some of the Torsion Load, pass it to the shear panels connecting the Four Main beams, and in turn distribute the load successfuly going forward... i.e "the Load path" for the artists in the audience - because this analysis combines art with the know physics and math.

Syscom had it right.

If some one wants me to go into more boring detail about the wing, what to do about cut outs for inspection panels, ammo doors, cowlings, etc I will.

The Net Net.

The airframe must take all the applied external loads from High turns, to dive pullout, to rolling manuevers in dive, to the propeller torgue, account for all the holes and inefficient structure (like a cockpit) or a mid wing versus a low wing design - and it's gotta fly.

The airframe is not a "tube" or a "box" of homogeneous material properties that are simple to calculate in many cases for a.) natural frequency, b.) Stiffness, and c.) harmonic motion to different forces.

No, it is a complex mess of torque boxes made up of stiffeners and sheet metal and rivets, connected to castings and forgings through shear or tension devices, with different properties at different locations or different properties along a constant line span wise of fuselage station wise. It is not even simple to calculate DEFLECTION or TORSION Displacements due to Precise Loads - much less complex aerodynamic loads.

Now, (an edit to this post) I have seen attempts at striffness calculations performed after the fact to attempt to solve a problem - the spitfire wing tip comes to mind when the control reversal issues came to play.. and the equations they used from my perspective were technically correct for that small region as an approximation for the stiffness)

Now, Soren tell me how the 'Aeroelastic' properties, some of which I have named
were not a 'well known science' even AFTER the Korean War... much less during the day of Kurt Tank?

And once you have given me the background I don't have on that subject, tell me how the Germans analyzed theri design for a.) natural frequency, b.) deflections under all the theoretical loads, and c.) changed their designs before the a/c was built?
 
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