Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Motherland on December 02, 2007, 09:20:46 PM
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(http://www.virtualpilots.fi/feature/articles/109myths/kuvat/SLATTIENVAIKUTUS.JPG)
I have no clue what it means, but it seems to suggest that the 109 should have a good angle of attack due to its wing slats. I honestly cant remember if it does in game (my hands are to ingrained with how far to push it to notice now) but my memory seems to suggest that it does not.
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Those slats seem to allow it to turn MUCH tighter than another plane without slats would, given the same wing-loading and similar power-load/drag as the 109.
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Originally posted by Motherland I have no clue what it means, but it seems to suggest that the 109 should have a good angle of attack due to its wing slats.[/B]
A better way to describe it would be to say "the 109 should have a better angle of attack. Slats are just another type of high lift device, like flaps, that are used to create more lift for a wing. Someone else can explain the basis of their use on 109's better than I.
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Originally posted by Stoney74
"the 109 should have a better angle of attack.
Thats what I meant. Now I cant edit it though...
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The effect of the slats are modeled in AH.
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Originally posted by SgtPappy
Those slats seem to allow it to turn MUCH tighter than another plane without slats would, given the same wing-loading and similar power-load/drag as the 109.
that chart only shows the increase of the critical Angle of Attack of a wing with slats compaired to a wing without slats.
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Well, the Bf 109 slats cover only the tip part of the wing, the rest of the wing stalls at same AoA regardless if the slats are in or out. In other words the slats are there to prevent tip stall.
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As Karnak said- as far as I know AH models 109 slats in both categories that they should be modelled: (a) aircraft high-lift device animation, (b) aerodynamically in the flight model.
(A) Aircraft high lift device animation:
Just like other control surfaces (flaps, ailerons, etc.), slats are moving parts and should be represented in animation when they are used. The interesting thing about slats are that they automatically activate. As lift increases with angle of attack the leading-edge suction as the result of the changing pressure distribution of the wing becomes large enough to suck the slat open. When the leading-edge suction reduces the slats automatically are forced closed.
I haven't flown a 109 recently in AH but the last time I flew one I recall the slats automatically deploying whenever higher angles of attack were reached. Just listen for them to automatically pop-open or shut in AH depending on the aoa of the aircraft. They are definitely there on the Me-262.
(B) Slat aerodynamics in the AH FM
As stoney said slats are another form of an high lift device such as the variety of flaps. Like flaps, slats can improve turn performance by increasing the lift limit and decreasing the flyable airspeed of the aircraft but do so at a lower parasite drag penalty compared to flaps.
The effect of slats is represented in increasing the maximum lift coefficient, Clmax of an airplane (albeit gripen makes an interesting point about the location of the 109 slats and their effect on aircraft Clmax). Regardless, as such any effect of slats are embedded in the Clmax figure of the 109 which is readily obtained with actual flight tests reports of the 109 in relationship to stall speeds. Knowing HTC they probably have some pretty credible 109 flight test data that they base the FM from.
Slat Physics
A small point: the picture depicts the misconception that slats "energize" the boundary layer by introducing higher velocity airflow onto the main wing element. Actually this doesn't happen at all. Instead the slats actually reduce the velocity around the leading edge of the main wing element which reduces the leading edge pressure peak allowing the boundary layer to stay attached longer & delaying "stall".
Tango, XO
412th FS Braunco Mustangs
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Found some other interesting things, from the same webpage-
In reference to the spitfire's ellptical wings-
The elliptical planform has very small theoretical advantage, but only theoretical, and only valid if the planform is truly elliptical. Spitfire's planform is only approximating elliptical, and what is left has been sold out by the aerodynamic twist it's wing has. It has effect on just one of several factors of wing efficiency, causing a whopping 0.05 improvement in comparison to a trapezoidal planform used in for example Bf 109, that is, IF Spit's wing were truely elliptical...
You also have to take into account the fact that the profile thicknes ratio of Spit's wing is VERY thin, both in maximum and in average. This in turn leads to the small coefficient of lift. This pretty much takes away the advantage of the large wing area.
BTW, ever wondered where did all the elliptical wings go?
If they are so magically efficient, why nobody uses them anymore?
Answer is simple, later aerodynamic research has proven that most of the benefits of elliptical wing were a fallacy created by insufficient or faulty research methods. They simply were not worth the trouble.
Even the developements of Spitfire, Spiteful and Seafang gave up on the elliptic planform and went to normal trapezoid form. Wonder why?
Only thing special in it is the elliptic planform, that dropped of favour just after it, when it was found out that the theoretical benefits of elliptic planform were actually only theoretical, and practical applications did not yield benefits that would justify the almost astronomical manufacturing difficulties and costs.
In Spitfire's case the benefits of elliptic planform (even lift distribution along the span) are nullified by the 2 degree twist (washout) that was needed for at least partially taming the nasty and violent stall behaviour of such wing. In short, the wing twist negated the effect of the elliptical wing. Although the wing was physically elliptical, its lift was not.
Besides, wing aspect ratio has larger effect on the lift/drag characteristics than the Oswald efficiency factor (where the theoretical difference between Spit's and Bf 109's wing is only of magnitude of 0.05), and Bf 109's wing has higher aspect ratio than Spit's...
Spit's wing uses the exactly same NACA 2300 root profile as Bf 109's wing, but with only 13 % thickness ratio, while Bf 109 has 14.2 % thickness ratio. Lower thickness ratio translates to lower Cl max. Bf 109 uses the same NACA 2300 with thickness ratio of 11%, but Spit's wing profile gradually changes along the span to NACA 2200 (more symmetric profile with smaller Cl max) with thickness ratio of only 9 %.
All the above leaves the lower wingloading as the only even theoretical advantage for Spit's wing, but even that is somewhat negated by wingprofile that has less Cl max and Cl in general.
- Pentti Kurkinen, enthusiast
Drop tanks:
The droptank system in every Messerschmitt worked the same way. Fuel to the engine was always drawn from the main tank. The droptank replenished the main tank. This was done with an automatic float controlled device that opened the flow from droptank if the fuel level in main tank dropped. There was no pump driving the fuel from the droptank, it was kept pressurized by bleeding compressed air from the engine supercharger into the droptank.
The plumbing was routed from the droptank to the right upper forward edge of the cabin, and from there along the cabin edge to rear, into the fuel tank. There was a piece of perspex tube at the right side of pilot, from where he could see the fuel flowing. When the tube became filled with air (easy to see from the colour) it was time to release the droptank.
A nice system. If you had to jettison the droptank, you always knew that your main tank was full. And it also did not heed any preliminary actions like turning a feed selection valve or somesuch, just tug the release cord...
"109's controls locked up in high speed."
- Another very mythical subject. Before answering one must be asked: "What model are you talking about?"
- There was large differences between various types in the high speed controls. Each newer version handled better in high speeds, the best being the 109 K series which had flettner tabs for enhanced aileron control - at least in theory, as it is debated whether many Me 109 K-4s actually had those flettners enabled. 109 G series were much better on this regard compared to 109 E, which yet again wasn't such a dog as many claim. 109 test pilots, Russians included, have said that the 109 had pretty good roll at higher speeds - again not as good as the 190s, P-51 or P-47 - but it maintained a good lateral control ability. Recovering from extremerely fast 750-900 km/h vertical dives was the problem - not level flight or even normal combat flying.
- Spitfire and a 109 had equal roll rates at up to 400 mph speeds. Not even the favourite warhorse of the Americans, P-51, exactly shined with its roll rate at high speeds. P-51 pilots have actually said that flying P-51 at high speeds was like driving a truck.
- An often quoted British report made of a Me 109 E talks about the "short stick travel", "due to the cramped cockpit a pilot could only apply about 40 pounds side force on the stick" and "at 400 mph with 40 pounds side force and only one fifth aileron displaced, it required 4 seconds to get into a 45 degree roll or bank. That immediately classifies the airplane as being unmaneuverable and unacceptable as a fighter."
- The report claims that The 109-E needed 37lb stick force for a 1/5 aileron deflection at 400mph. Coincidentally, the Spitfire 1 required 57 lb stick force from the pilot for similar deflection at similar speed. This is a 54% higher stickforce for the Spitfire pilot.
- The British test is taken as gospel by many, while it is just one test, made by the enemy, using a worn out and battle damaged airframe. German flight tests report pilots using aileron forces of over 45 lbs and 109's stick was designed for elevator stick forces of up to or over 85kg, over 180 lbs. Finnish Bf 109 G-2 test revealed that at 450 km/h the stick could be still fully taken to the limit with ~10 kg force (20 pounds). Aileron roll without rudder could be performed to both direction from 400-450 km/h in 4-5 s. This is better than the Spitfire with fabric ailerons, about the same as Spitfire with metal ailerons and slightly below clipped wing Spitfire. So it was more matter of the pilot and the test procedures, than maneuverability of the Bf 109. Several details of that test are suspicious and German chief test pilot Heinrich Beauvais disagreed with it and with Eric Brown. Beauvais tried to get into contact after the war with Eric Brown to discuss the matters, but Brown refused to discuss with him. This being the case, it seems that Brown wasn't willing to listen a pilot who'd flown more on the 109 than he ever had, and was more interested on believing his negative findings of the 109 than being proven wrong by a real expert.
http://www.virtualpilots.fi/feature/articles/109myths/
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The slats do give a slight increase in angle of attack in the game. Very noticeable if you can maneuver it so one slat is out and the other is not.
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Originally posted by Motherland
Found some other interesting things, from the same webpage-
In reference to the spitfire's ellptical wings-
So what's his point here? The effort to produce the elliptical wing was done so because of faulty NACA research done before the war. Everyone in the aerodynamic field thought the same thing as the Supermarine engineers did. Heck, the idea for the wing design came from a German aircraft. Look at all of the horizontal and vertical tail surfaces on a lot of the early WWII aircraft--they have elliptical planforms for the same reason. Taper ratio is the key, as a .45 taper ratio from root to tip actually achieves almost ideal spanwise lift distribution. I don't know what the Spit or 109 had for taper ratios. The elliptical wing actually has a reference planform from which you can derive the taper and make a comparison. Regardless, revisionist aerodynamics doesn't help his argument. Only a wind tunnel and some fairly well controlled test flying can properly demonstrate some of his arguments. Don't know if wind tunnel data exists, and the existing test flight data is surely suspect, at least in comparison, or at least, hotly debated as to its precision.
But, he starts getting carried away in the second paragraph. Airfoil thicknesses can't be judged better or worse out of the context of the rest of the wing. A very light aircraft can be a highly maneuverable plane, even with a low Clmax merely because of the wing loading. Interestingly enough, a majority of WWII fighter aircraft used the same 23000 series NACA airfoil, Axis and Allied. They even used practically the same thickness distributions. So, there must be more to the difference between aircraft performance aside from airfoil Clmax. He alludes to the wingloading as being the only remaining factor between the turn performance of the aircraft. Well, if that's the case, it must have been a huge factor, as the Spit out turned the 109--that's not debatable by anyone, regardless of theoretical aerodynamics or actual test data.
If he's merely trying to get some credibility for the 109 as a successful WWII fighter--I don't think anyone with a knowledge of WWII aviation would state otherwise. There exists much myth, legend, and folklore surrounding WWII aircraft. Emotions, mis-read or faulty test data, nationalistic tendencies--all of these have played a part in skewing our perception of these aircraft some 60 years after the fact. If you consider quality in the same light that most manufacturers do today, it is a matter of "suitability to purpose". When examined in that light, you could judge the value of each aircraft with respect to its intended purpose, and how well the design performed with respect to that purpose. Its success or failure on the battlefield is a entirely different matter, as there are many other factors involved.
I'm a P-47 fan boi, but I don't think it was a panacea aircraft. It did what it was designed to do very well. It also did some things it was not designed for very well. The same could be said for the 109 and the Spit. Not having a plane designed to excell at 30K feet is a lack of foresight, not poor design.
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Well, the problem with this kind of sources is that these are just as or more selective than the sources they try to downplay. In other words, the outcome is creation of new myths instead busting the old myths.
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Don't think it's been said...
The main reason the elliptical wing was stuck on a Spitfire was not really its theoretical aerodynamic efficiency (may have been a factor in choosing, but it was not the main factor). The main factor was the need for a very thin wing that could cram what was needed inside. I.e. MG's, gear, rad/oil coolers etc.
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Originally posted by Stoney74
If he's merely trying to get some credibility for the 109 as a successful WWII fighter--I don't think anyone with a knowledge of WWII aviation would state otherwise.
I dont think that this was the purpose of what he wrote- most of the stuff on the webpage is from many different sources- when this was written its possible that he had nothing about the 109 in mind- notice that the 109 was never mentioned in the excerpt- and he was just trying to debunk the 'uber elliptical wing' myth. And that it was put into the webpage for the obvious reason that the Spitfire was a big competitor to the 109.
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The Spitfire's wing made it harder to build and gave it a distinctive image. The eliptical shape was not the reason for its success in combat, but it is likely that it played a substantial part in the Spitfire's media success.
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Just like Karnak said ... there was nothing very special about the elliptical wing. It was good because it had a good amount of wing area for the size of the tiny Spitfire, which gave it its turning. It was also very thin which was essentially more aerodynamic than a thicker, non-laminar flow wing.
It's just shaped like that for the sake of stuffing everything in, not because they thought it was magical.
The Tempest was fitted with a similar wing for the same reason. Those 20 mm cannon had to go somewhere and since they wanted it recessed in a thin wing, they got the same shape; albeit the wing was much larger so the cannon didn't jut out as much as on the Spit. They joked around saying that the only reason the wing was chosen was because everyone else was obsessed with the Spit's wing.
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Originally posted by Motherland
notice that the 109 was never mentioned in the excerpt- and he was just trying to debunk the 'uber elliptical wing' myth.
Uh, end of the first paragraph you posted, the author began comparing the Spitfire wing with the 109. Planform, thicknesses, aspect ratio...all were compared between the 109 and Spit.
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Originally posted by Stoney74
Uh, end of the first paragraph you posted, the author began comparing the Spitfire wing with the 109. Planform, thicknesses, aspect ratio...all were compared between the 109 and Spit.
Woops. Missed that part when I was skimming back over it to check myself.:o
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Originally posted by SgtPappy
...there was nothing very special about the elliptical wing. It was good because it had a good amount of wing area for the size of the tiny Spitfire, which gave it its turning. It was also very thin which was essentially more aerodynamic than a thicker, non-laminar flow wing.
It's just shaped like that for the sake of stuffing everything in, not because they thought it was magical.
Wrong. After NACA's research was published, major aircraft manufacturers around the world began using it. NACA stated that an elliptical wing would realize almost ideal spanwise lift distribution and reduce induced drag, with respect to the planform, to a minimum. As a document, the research was the first published of its kind. It was considered a breakthrough from a technology standpoint, as it was the first publication of such a monumental amount of research. It was widely embraced as truth for a number of years--some still continue to repeat it almost verbatim. That some of it was flawed was only considered after the War, and the debate over its legitimacy continues today. The amount of specialized manufacturing was the main reason most manufacturers did not pursue elliptical wings, not because they didn't think the theory worked. Even today, with the use of double tapered wings on the Lancair and Columbia production planes, designers are still chasing the theory of elliptical wings and ideal spanwise lift distribution. There are differing opinions on its worth.
Further, wing thickness has nothing to do with respect to it being considered "laminar" or "non-laminar" (or properly termed, "turbulent"). You can have an aircraft with a wing that's 20% thick with respect to chord that uses a "laminar" airfoil. The 2200 series airfoil used by the Spitfire is actually considered a "turbulent" airfoil. The difference in the classification is a matter of how much laminar flow is achieved over the wing. All airfoils create areas of both turbulent and laminar flow. The turbulent airfoils only create laminar flow over a small percentage of the chord. The laminar flow airfoils create much larger lengths of turbulent flow, and delay its formation to a point further back on the chord. After a certain point, due to the geometry of the wing, the air becomes turbulent on either airfoil type. The amount of laminar flow that is achieved is the basis for determining wheter a certain airfoil is "turbulent" or "laminar". Perhaps a better description for them would be "high drag" and "low drag" airfoils, even though this could be confusing in certain circumstances. The P-51 was the first WWII aircraft that I can find that took advantage of a "laminar" airfoil. All of the rest, including the Typhoon/Tempest, 190, Spit, 109, etc., etc. used "turbulent" airfoils.
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Let me see if I've got this straight...
In a normal airfoil turbulence (turbulence is basically disturbed air, right?) is generated behind the wing. In a laminar flow wing, the turbulence is minimized (or is it completely eliminated?) From what I know about the P-51's performance, laminar flow wings are very low drag, but they do not produce as much lift as a traditional airfoil at low speeds, but seem to at higher speeds? Or is the Pony's highspeed maneuverability due solely to it's highspeed flaps and control surfaces?
If I got this horribly wrong, please forgive me. I find aerodynamics very intriguing, though I know little about it.
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Though later than the P-51, I seem to recall reading that the Ki-84 also had a "laminar" airfoil.
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Originally posted by Motherland
Let me see if I've got this straight...
Let's start with a simple airfoil shape, one that's flat on the bottom and has a basic teardrop shape on top. I wish I had a picture handy to illustrate, but alas...I went ahead and made this half way through the post...
(http://i125.photobucket.com/albums/p61/stonewall74/airflow.jpg)
Forgive the crude drawing.
As the air hits the leading edge of the wing, it momentarily "stops" and then splits, with some air going under the wing, and other air going over the wing. The air going over the wing accelerates as it travels up and over. Ideally, we want the air to pass over the top of the wing in a smooth, "laminar" fashion. The air closely follows the profile of the top wing surface. But, since air has mass, and doesn't want to be disturbed...if the transition vertically is severe enough, the air can "trip" or "stumble" and become turbulent. It is still attached to the wing--but swirling from being disturbed too much by the profile of the wing. This happens on ALL wings--the difference is how far back along the top of the airfoil length (aka the "chord") the turbulence begins. The turbulence can either be created by the front part of the chord, where it forces the air up, or be created by the middle of the chord, where it transitions from an upward motion to a downward motion, or can be created as the airfoil tapers down to the trailing edge. All these different direction changes can cause turbulent flow, depending on the geometry of the wing.
What NACA tried to do was test whether or not they could force the air to remain laminar as far back along the chord as possible by changing the geometry of the airfoil. The longer they could delay the air becoming turbulent, the less drag the airfoil shape would create, therefore reducing the overall drag of the wing caused by the airfoil geometry. What they discovered was that there was a limit to how far back they could push the turbulent flow. About 55% along the top of the airfoil was as far as they could force the turbulent flow and still have an airfoil that was useful for aircraft. There are other shapes that can maintain laminar flow for their entirety, but they don't create enough lift to be considered for aircraft. They usually possess some other nasty qualities as well that are beyond the scope of this discussion.
Anyway, when discussing the P-51's "low drag" wing, it basically was the first WWII fighter to use some of the airfoil shapes that pushed the turbulent flow further back along the top of the airfoil. The result was a wing that caused much less drag than other wings designed before it. I can't remember the actual percentage, but IIRC (don't have time to look it up), airfoil shapes that only maintain laminar flow for the first 25% or less are considered "turbulent" airfoils. If the airfoil shape can maintain laminar flow for more than 25%, they are considered "laminar" airfoils. Some laminar airfoils maintain the flow longer than others, but all are still considered laminar.
Depending on the camber, thickness, and chord length (wing width), a laminar flow airfoil can create as much or more lift than a turbulent section. It depends on the geometry of the airfoil, wing, Reynolds number--a host of other considerations.
Its late here, and perhaps another can chime in tomorrow while I'm at work. I'll check back tomorrow and try to follow up with this post.
Do a Google Search on airfoils, airfoil design, airfoil selection, etc. There are a bunch of good websites out there that explain this much better than my gross generalization above.
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Stoney, pretty good descriptions above my friend :). I'm guessing you were looking for a diagram similar to the following:
(http://brauncomustangs.org/upload/flow.gif)
As Stoney mentioned all wings have laminar and turbulent flows as depicted above. When an object passes through a fluid (e.g. wing through the air) a boundary layer of flow forms along the surface. This boundary layer has two characteristics: laminar flow (flow steady & layered) and turbulent (flow layers unsteady & mixed). On a wing the boundary layer starts out laminar and then becomes turbulent at some point. Viscous and pressure forces acting on the boundary layer directly impact where it is laminar or turbulent.
Laminar flow airfoils attempt to increase the length of the laminar flow along the chord of the wing. The shape of the laminar wing controls the pressure distribution across it to achieve more laminar flow. Of course another factor that affects laminar flow is surface roughness which has nothing to do with shape. The smoother the surface, the more likely you'll be able to lengthen the laminar flow.
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It's an over-generalization to say that laminar wings cannot produce as much lift as traditional airfoils.
One of the benefits of laminar airfoils are that they produce high Clmax for lower drag. If you skim NACA Report 824 you'll find that NACA 6-series airfoils have max lift coeff. as high or higher than NACA 24- and 44- series airfoils. Disadvantages of laminar airfoils are poor stall characteristics, suceptibility to surface roughness, higher pitching moments, and being very thin at the trailing edge which makes them more difficult to make.
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P-51 High-speed Maneuverability
I think the oft quoted comment about the reason for the P-51's high-speed maneuverability is because of it's combat flaps it can deploy at higher speeds. To be honest I don't know where the idea that the Mustang had high-speed maneuverability comes from. That being said, the 2nd not so obvious reason I believe that is totally overlooked comes from…you guessed it…the laminar wing :).
Most people are aware that compressibility effects drastically affects aircraft controls. What is less obvious is that at high aoa's and moderate airspeeds compressibility effects can arise forming shockwaves on the wing that degrades an airfoil's maximum lift coefficient as well. Thanks to gripen pointing it out to me years ago, there are some NACA flight tests that compare various USAAF aircraft that demonstrate that the Mustang's Clmax is not as drastically impacted by the degradation in lift due to compressibility. Avoiding all the nasty physics, I believe basically the pressure distribution of a laminar flow wing allows it to delay the formation of shockwaves due to compressibility.
Tango, XO
412th FS Braunco Mustangs
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Originally posted by Motherland
(http://www.virtualpilots.fi/feature/articles/109myths/kuvat/SLATTIENVAIKUTUS.JPG)
I have no clue what it means, but it seems to suggest that the 109 should have a good angle of attack due to its wing slats. I honestly cant remember if it does in game (my hands are to ingrained with how far to push it to notice now) but my memory seems to suggest that it does not.
the leading edge flaps on the 109 don't actually create the space as pictured here, if i'm correct......but rather, they extens out and down, as the airflow attempts to seperate from the wing(this causes a negative air pressure on the leading edge). as they do this, they add camber, and a litle bit of area to the wing(almost like a fowler flap, but on the leading ege). by adding this camber, they increase the lift of the wing, and delay the onset of the stall.
i haven't flown any of the german fighters in here enough to know if it is modeled that way or not in game.
<>
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Cap:
I pretty sure that 109 slats are, well slats :) - meaning there is a gap. The gap is necessary for vortex formation between the slat airfoil and the main wing airfoil. Otherwise no downwash is created to reduce the spike in pressure peak toward the wing's leading edge at high aoa's.
Take a look at these pics as well:
(http://brauncomustangs.org/upload/109%20slat-1.jpg)
(http://brauncomustangs.org/upload/109%20slat-3.jpg)
(http://brauncomustangs.org/upload/slat6.jpg)
(http://brauncomustangs.org/upload/g10.slat2.jpg)
Pics from the following sources:
http://109lair.hobbyvista.com/techref/systems/control/slats/slats.htm
http://www.rcgroups.com/forums/showthread.php?t=507614
Tango, XO
412th FS Braunco Mustangs
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I would take anything you read from Pentti Kurkinen, or "Penttiku" as I know him from WWIIOL with a large dose of salt. I've never seen a guy spend so much time trying to prove an aircraft (ie the Bf-109) is uber compared to its contemporaries.
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Thats a cheap shot Trukk but I know you enjoy it.
Penttiku was actually providing merely a counterforce to chronic infestation of "allied fanbois" who liked to elevate their views as a fact without actual knowledge of subject.
Of course he could have made his points more politely but then again the apparent ignorance from HTC finally was just too much. But at least he knew his facts and chose to say nothing if he didn't have enough info on subject -which cannot be said of his counterparts. :p
This conversation closely relates to "109's third wing" debate that took place in WW2OL forums. The modification to 109's flight model resulted in flight performance where 109 could not exceed 15 degrees AoA in any condition, where as data clearly states otherwise. Only the drag remained leading to naming them as "slatichutes" since they only slowed you down without any benefit to maneuvering.
When facing a Spitfire you had to make a decision whether to stay fast or you needed to go really slow with slats deployed to match Spitfire's turn If you stayed a bit too fast the Spitfire would out turn you quite quickly. After the flight model revisit you were as good as dead if you went slow making the Spit the Überride the fanbois always wanted it to be.
-C+
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It's my perception of the guy, take it for what it's worth.
I flew the 109 in WWIIOL and never had a problem with the slats. They let you pull more AoA than the Spitfire, but that additional AoA cost you in drag. So at high AoA with the slats deployed you had to get your shot in quickly because you were burning E fast. But (like any aircraft) as long as you didn't play the Spitfire's game the 109 was very capable.
In the Tri-09 days, you could be really sloppy (heck you could out turn a H-75) but once they removed that extra wing you had to fly it smart.
Both sides have their fanboys and Penttiku is one of them.
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Originally posted by Stoney74
Wrong. After NACA's research was published, major aircraft manufacturers around the world began using it. NACA stated that an elliptical wing would realize almost ideal spanwise lift distribution and reduce induced drag, with respect to the planform, to a minimum. As a document, the research was the first published of its kind. It was considered a breakthrough from a technology standpoint, as it was the first publication of such a monumental amount of research. It was widely embraced as truth for a number of years--some still continue to repeat it almost verbatim. That some of it was flawed was only considered after the War, and the debate over its legitimacy continues today. The amount of specialized manufacturing was the main reason most manufacturers did not pursue elliptical wings, not because they didn't think the theory worked. Even today, with the use of double tapered wings on the Lancair and Columbia production planes, designers are still chasing the theory of elliptical wings and ideal spanwise lift distribution. There are differing opinions on its worth.
Further, wing thickness has nothing to do with respect to it being considered "laminar" or "non-laminar" (or properly termed, "turbulent"). You can have an aircraft with a wing that's 20% thick with respect to chord that uses a "laminar" airfoil. The 2200 series airfoil used by the Spitfire is actually considered a "turbulent" airfoil. The difference in the classification is a matter of how much laminar flow is achieved over the wing. All airfoils create areas of both turbulent and laminar flow. The turbulent airfoils only create laminar flow over a small percentage of the chord. The laminar flow airfoils create much larger lengths of turbulent flow, and delay its formation to a point further back on the chord. After a certain point, due to the geometry of the wing, the air becomes turbulent on either airfoil type. The amount of laminar flow that is achieved is the basis for determining wheter a certain airfoil is "turbulent" or "laminar". Perhaps a better description for them would be "high drag" and "low drag" airfoils, even though this could be confusing in certain circumstances. The P-51 was the first WWII aircraft that I can find that took advantage of a "laminar" airfoil. All of the rest, including the Typhoon/Tempest, 190, Spit, 109, etc., etc. used "turbulent" airfoils.
I saw a report on a website supporting your statement on the elliptical wing... this is why Republic used it on many of their experimental post-WWI fighters right? For some reason, I've come to believe that the ellipse was all b.s but I guess I'm wrong.
Also, What i meant about the thin wing was that it was aerodynamically 'cleaner' than other wings of same, but thicker cross sections.
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Originally posted by SgtPappy
I saw a report on a website supporting your statement on the elliptical wing... this is why Republic used it on many of their experimental post-WWI fighters right? For some reason, I've come to believe that the ellipse was all b.s but I guess I'm wrong.
Also, What i meant about the thin wing was that it was aerodynamically 'cleaner' than other wings of same, but thicker cross sections.
Sorry to be so blunt in my last response. But, your absolutely right, and it wasn't just Republic, but they are a good example. It's not so much that the elliptical wing theory is B.S. as it is that NACA used some questionable methods to arrive at that conclusion (source Ribblet GA Airfoils). There are those out there that still believe in the elliptical ideal. Some think it is more easily achieved with planform taper only, and some have begun to question whether aspect ratio is the larger factor in overall wing drag. Peter Garison (Author of the Technicalities column in Flying Magazine) wrote an article a couple of months ago discussing whether or not taper even makes an appreciable difference or whether aspect ratio makes the largest impact. There are as many opinions out there even among aerodynamic folks today. We all see a lot of selectivity when folks quote aerodynamic evidence to support their claims about certain aircraft.
Thin wings are not necessarily aerodynamically "cleaner" than other wings of the same, but thicker section profiles. The airfoil itself may have a lower absolute coefficient of drag, but they usually have narrow laminar buckets and a lower range of useful coefficients of lift (and overall lower Cl). Not to mention that there is a difficult amount of math/wind tunnel use necessary to predict overall wing lift/drag versus the airfoil section lift/drag. All the lift/drag information for each airfoil published by NACA was for 2 dimensional flow. Since wings have 3 dimensional forces acting on them, there's much more involved to derive the actual efficiency of a wing. Perhaps when I get home from work, I'll post some examples illustrating this.
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Elliptical planform and its advantages are not BS, it is just more costly and time consuming to manufacure and the benefits are not always seen worth it.
Elliptical planform itself did not make the qualities of Spitfire but because of the size of the wing every measure had to be taken to minimize drag and making it elliptic was one of them. The wash-out did not have too radical effects on its lift distribution IMO, but it did make it worse but still it was better than those of P51 and FW190 as can be seen in Davis Lednicher's report. That report considers only one state of flight and AoA, the graphs would be different with more AoA so the graph is not a general description of its qualities but a snapshot of one situation -probably at 0 DEG AoA.
The bad feature was indeed the stall characteristics and washout helped the pilot so that the buffeting of the root of the wing at stall warned to pilot to tighten the turn so that the tip would not stall as well. But if that happened the stall was probably quite sudden and I have understood that Spit went easily into flat spin but that has more to do with lift center relation to CoG and their shifting due to centrifugal force more than some negative feature of an elliptic planform.
-C+
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Originally posted by Charge
...as can be seen in Davis Lednicher's report. That report considers only one state of flight and AoA...
Do you have a link or copy of this? I tried to find it on the web with no luck. Furthermore, I can't find a credible source that actually discusses the washout designed into the Spitfire wing.
The bad feature was indeed the stall characteristics and washout helped the pilot so that the buffeting of the root of the wing at stall warned to pilot to tighten the turn so that the tip would not stall as well.
-C+ [/B]
Given the wing geometry used by all WWII fighters, I believe they were all doomed to have poor stall characteristics, regardless of the band aids of washout, slats, etc. Between poor airfoil section stall characteristics, coupled with thickness taper, coupled with planform taper, (none of which could have been foreseen given the knowledge base at the time) and massive amounts of torque from those monster engines all combined to make stick and rudder skills a most valuable commodity.
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Ok, here is an example of why a thinner wing may not be "better" than a thicker wing. I have to admit that this airfoil did not turn out to be as good an example as I thought it would. I'll look for a better one later...
I ran both the NACA 23015 and the NACA 23009 through XFOIL96. XFOIL is an airfoil analysis program. XFOIL is not in total agreement with the NACA airfoil wind tunnel test data shown in NACA Report 824, but the comparison is useful regardless. For my test runs, the Reynolds number was set at 3.0E6 (or 3,000,000) and the Mach at .17 which are the same conditions used by NACA to develop the data for Report 824. These are NOT similar to the conditions that an actual aircraft using this airfoil during WWII would have--I merely used them to set up the same conditions NACA used.
(http://i125.photobucket.com/albums/p61/stonewall74/ClComparison.jpg)
In this image, you'll notice that the thinner airfoil (NACA 23009) a 9% thickness airfoil, stalls earlier than the thicker airfoil (NACA 23015). Also note that beginning at about 9 degrees AoA, the 23015 produces more lift at the same AoA, all the way to the stall.
(http://i125.photobucket.com/albums/p61/stonewall74/CdComparison.jpg)
In this image, you'll notice that the thinner airfoil creates less drag at low AoA, but as the AoA increases, the difference between the two decreases, and by 12 degrees AoA, the thinner wing actually produces more drag at the same AoA than the thicker wing.
I didn't take the time to do a lift/drag curve for both aircraft, but you could see that at certain portions of the flight envelope, the thicker airfoil section can be more "aerodynamic" than the thinner airfoil section.
Another interesting illustration here is the impact of airfoil thickness on tip stalls. Almost all WWII fighter aircraft used thickness taper, where the airfoil thickness at the wing tip was thinner than at the root. The NACA 23012 root, 23009 tip was a popular combination on many WWII aircraft. You can see from this comparison, that as the wing approached the stall condition, the wing tip would stall much earlier than the root. When this happens, the engine torque rolls the aircraft quickly over onto its back, aka a snap roll.
I'll see if I can't follow up on this later...
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Charge: very much agree with your comments on the elliptical wing. Unfortunately there's too much mis-information out there on the topic.
Stoney: I have a pdf copy of Dave Lednicer's report that he graciously sent to me. PM me with your email and I'll send it along to you.
Nice work with xfoil :). I'll have to talk to you about how you got it working. When I looked at it a couple of years ago it seemed too much effort to get working on a PC.
Tango, XO
412th FS Braunco Mustangs
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That's a good example, Stoney. Don't worry about being blunt, I'm fine and the world isn't going to worry about being polite to people anyway haha.
Anyway, do you have aircraft examples of those airfoils? Just so I can get a better idea of how they worked.
Charge,
I now know the ellipse wasn't fake at all. As Stoney mentioned, the reports made on the elliptical wing were rather questionable and I, in attempt to understand those reports, found those methods questionable as well.
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Originally posted by SgtPappy
Anyway, do you have aircraft examples of those airfoils? Just so I can get a better idea of how they worked.
You mean like specific airfoils for specific airplanes? Or 3D analyses of wings using specific airfoils?
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"Given the wing geometry used by all WWII fighters, I believe they were all doomed to have poor stall characteristics, regardless of the band aids of washout, slats, etc. Between poor airfoil section stall characteristics, coupled with thickness taper, coupled with planform taper, (none of which could have been foreseen given the knowledge base at the time) and massive amounts of torque from those monster engines all combined to make stick and rudder skills a most valuable commodity."
I once saw a pressure distribution chart for a normal straight leading edge wing and that chart pretty much explained why the arched leading edge of Spitfire is beneficial dragwise. I have often searched that article but never found it again. It could have been one of those NACA articles though...
I'm not sure but I think that the stall problem with the elliptic planform may be due to that arched leading edge causing an even pressure distribution along the wingspan (meaning an even forming of drag) but at the same time causing the lift to be lost all at once if the max permissible AoA is exceeded, so the washout is indeed the only means of making the impeding stall somewhat predictable, and according to what I have read it worked if the stall was approached somewhat carefully. So the tipstalling charcteristics of a normal tapered wing is probably not caused by planform itself but due to fact that the thinner wing section stalls first, but the tapering may delay the stall to a point where the whole wing stalls at the same time.
(I'm talking about planform taper, not profile taper.)
If you compare the commonly used profiles you notice that while the root section profile of 109 (F,G,K) is quite common 14-15% thickness the tip profile is thicker 11% than those of other design's typical 9%. Probably good for stall characteristics but bad for drag unless the planform is kept quite thin at the tip.
This means that accompanied with slats the 109 could probably "ride the stall" for quite long up to significant AoAs (20-25deg?) but only the tips of the wing providing lift in the end as the stalling would reduce the positive wing area as the AoA would increase. I have no idea what is actually the value of that amount of lift in maneuvers and in fact it may well be that 109 ran out of lift before the wing would stall, the remaining effective wing area not able to support the weight of the a/c. (This would suggest that the situation would be different in high speed, but the stalling characteristics would be too...)
Previously I had made a general assumption that the slats would open only at approach of stall but that is not the case as was evident by the flying video of Black 6 where you could see the slats slightly moving in and out even in slow level flight. That didn't make sense to me so I concluded that these matters are not always as straight forward as they may seem.
-C+
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Originally posted by Charge
I'm not sure but I think that the stall problem with the elliptic planform may be due to that arched leading edge causing an even pressure distribution along the wingspan (meaning an even forming of drag) but at the same time causing the lift to be lost all at once if the max permissible AoA is exceeded, so the washout is indeed the only means of making the impeding stall somewhat predictable, and according to what I have read it worked if the stall was approached somewhat carefully. So the tipstalling charcteristics of a normal tapered wing is probably not caused by planform itself but due to fact that the thinner wing section stalls first, but the tapering may delay the stall to a point where the whole wing stalls at the same time.
(I'm talking about planform taper, not profile taper.)-C+
Well, the problem with planform taper is that the Reynolds number at the tip will be much lower than the Reynolds number at the root. So, in addition to a thinner airfoil section (with its reduced Clmax/quicker stall), the reduced chord at the tip has a lower Rn, which also reduces Clmax. There are acceptable amounts of taper, but any taper contributes to a greater potential for tip stall.
I'll do a Reynolds number comparison for the Spit airfoil to demonstrate, and post it.
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Ok, here we go.
(http://i125.photobucket.com/albums/p61/stonewall74/SpitfireWingtipExample.jpg)
This chart shows the same NACA 2209 airfoil used on the Spitfire wing tip. Although the Spit used a 2209.5, XFOIL doesn't like fractional thicknesses, and I figured the difference would be negligible. Regardless, you can see that the same airfoil section thickness stalls earlier at lower Reynolds numbers than at higher Reynolds numbers. This illustrates the effect planform taper has on tip stall. If velocity, density, etc. are held constant, and the wing tip chord is the only variable, the difference shown here is a wingtip of shorter chord (lower Rn) versus a wingtip of longer chord (higher Rn)
So, both airfoil thickness taper and planform taper contribute to a higher potential for tip stall. You begin to get an idea why almost all WWII fighters display such nasty tip stall characteristics. That's not to say that either don't have their uses, but in the ongoing tradeoff that is aircraft design, you could make an argument that many non-combat losses of aircraft may have been avoided by changing the wing geometry that was used by both Axis and Allied manufacturers. For example, perhaps the Corsair would have had more benign slow flight characteristics had there not been any airfoil thickness taper. And, perhaps the Spit would not have needed any washout (which increased drag) had there not been such extreme planform taper.
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Sgt Pappy/Motherland,
A good general discussion of wing planform, taper, etc. and its effect on wing efficiency located here (http://ocw.mit.edu/NR/rdonlyres/Aeronautics-and-Astronautics/16-01Fall-2005-Spring-2006/380D0491-DF01-4AC6-AFD1-B1DFBEC34F0A/0/spl8b.pdf)
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Originally posted by Stoney74
It's not so much that the elliptical wing theory is B.S. as it is that NACA used some questionable methods to arrive at that conclusion (source Ribblet GA Airfoils).
Originally the elliptical lift distribution was introduce after WWI by a German aerodynamicist L. Prandtl as part of his lifting line theory. The idea was that elliptical planform offers constant downwash along the wingspan and therefore minimizes induced drag. A bit later (1926) Glauert introduced calculation methods to expand the theory to the other planforms. Notable thing is that Prandtl and Glauert dealed only with induced part of the drag (ie only directly lift related) so couple years later (1933) W.B. Oswald at NACA introduced his methods to calculate aircraft performance and he lumped all induced and parasitic drag (due to lift) factors together creating his now famous e factor.
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Originally posted by gripen
...couple years later (1933) W.B. Oswald at NACA introduced his methods to calculate aircraft performance and he lumped all induced and parasitic drag (due to lift) factors together creating his now famous e factor.
In NACA Report 627, NACA made the case for an elliptical planform as the ideal planform. It tested 22 different combinations of aspect ratio, planform taper, thickness taper, etc. and tested their stall characteristics, among other properties.
The following is an excerpt from "GA Airfoils" by Harry Riblet (page 100): "One final comment is needed regarding NACA sample #22, with the elliptical planform. This specimen has the highest Clmax (1.81) of any tested, and is accordingly often cited as proof of the "inherent superiority" of the elliptical planform over uniformly tapered planforms. Note, however, that this specimen enjoys two distinct advantages over the other specimens--it has a unique high-lift section (4412), and in addition it has the same percent thickness at the root and tip, 12%, which is, as we have seen, the optimum thickness in terms of best L/D. In view of this, we wonder if the legendary mystique of the elliptical planform is justified."
The point he's making here, is that from a purely scientific standpoint, the research is flawed. NACA did not take, for example, the same planform, airfoil, planform taper, and thickness taper and then only adjust one of these variables at a time. Instead, they came up with 22 unique combinations and compared them to each other.
Whether or not a properly configured elliptical planform is the best ideal configuration, NACA's research, in retrospect, by itself does not provide a compelling argument for an elliptical planform. The argument is that a uniformly planform-tapered wing can realize elliptical lift distribution just as well as a wing with an elliptical planform.
Too many folks confuse the idea of elliptical lift distribution with elliptical planforms. One can exist independent of the other.
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Originally posted by Stoney74
In NACA Report 627, NACA made the case for an elliptical planform as the ideal planform. It tested 22 different combinations of aspect ratio, planform taper, thickness taper, etc. and tested their stall characteristics, among other properties.
Nope, elliptical planform was introduced as a special case of elliptical lift distribution by Prandtl (1919) and Glauert developed the theory further. Following is a quote from Glauert's work (H. Glauert: "The elements of Aerofoil and Airscrew Theory." University Press Cambridge 1926, p. 143-144):
"The elliptic distribution of circulation or lift across the span of an aerofoil is important, firstly because it leads to minimum possible induced drag for given total lift, and secondly because the load grading curves of most aerofoils of conventional shape do not differ greatly from the elliptic form. The results deduced from hypothesis of elliptic loading are therefore the best which can possibly occur and are also a good first approximation to those actually obtained...
[formulas and discussion for mathematical proof omitted]
Hence the lift coefficient also will have the same value for all sections of the aerofoil. But the circulation... varies elliptically across the span, so also will the chord. Thus the elliptic loading will be obtained from a monoplane aerofoil of elliptic plan form and constant angle of incidence....
Elliptic loading across the span can also be obtained from aerofoils of other plan form by suitable variation of the angle of incidence across the span, but any such twisted aerofoil will give the elliptic loading for one attitude only, since the necessary angle of twist depends on the mean angle of incidence of the aerofoil."
Note that couple pages later Glauert gives also formulas for tapered wings as well.
Originally posted by Stoney74
The point he's making here, is that from a purely scientific standpoint, the research is flawed. NACA did not take, for example, the same planform, airfoil, planform taper, and thickness taper and then only adjust one of these variables at a time. Instead, they came up with 22 unique combinations and compared them to each other.
While Riblet is obviously right in his critics, the elliptical planform was allready invented by Prandtl nearly 20 years earlier.
Note also that the NACA 627 test results contain induced as well as parasitic drag of the wing so they did not exactly measure the effects of planform to induced drag.
BTW we use quite back wards pointed wing tips in the fast RC-gliders to minimize wing tip vortices (idea originally invented by S. Hoerner and tested in the Bf 109 during war). Here (http://www.strat.at/images/raketenwurm_cad_640.jpg) is a drawing showing high performance "Raketenwurm".
Originally posted by Stoney74
Too many folks confuse the idea of elliptical lift distribution with elliptical planforms. One can exist independent of the other.
That was invented by Glauert as continuation to Prandtl's work some 10 years before the NACA 627 as pointed out above.
Notable thing is that similar ideas as Prandtl's theory were presented as early as 1907 by F. Lanchester so overall it's difficult to say who invented what and when.
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Originally posted by gripen
Nope, elliptical planform was introduced as a special case of elliptical lift distribution by Prandtl (1919) and Glauert developed the theory further.
Sorry, I didn't mean to imply that NACA invented the theory of elliptical distribution or the idea of the elliptical planform. I was simply alluding to their windtunnel research of the topic.
Good information about Prandtl and Glauert.