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#21
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Akaflieg Karlsruhe AK-X
Thank you so much Andreas. This project and Mu-31 are projects that seem like they have potential to develop new technology.
On Monday, December 11, 2017 at 12:41:31 PM UTC-8, Andreas Maurer wrote: On Mon, 11 Dec 2017 14:03:36 +0000 (UTC), Kiwi User wrote: Yes, I was aware that this is a complex set of three control surfaces per wing, but I'm a little confused about their function, but I think that, listing from root to tip they were elevator,aileron,drag rudder. Is this a fair description. Close. One needs to know that the AK-X works in a completely different way than any other flying wing ever designed. Despite its similar appearance, it is not even similar to the SB-13 aerodynamics-wise. 1. The AK-X is a flapped wing. At low speeds, all (!!) control surfaces move downwards, at high speeds all move upwards. Just like an ordinary glider. The rudders are in the winglets. 2. Pitch control is done by the inner flaps which work the same way as a canard. Pitch up: Control deflection down, and vice versa. Perfect solution concerning lift-distribution. 3. Compare the wing sweep of the AK-X to other flying wings: It is much greater. This shows good promise to get rid of the pitch axis oscillations experienced by other flying wings and the CG sensitivity that has plagued all flying wing designs so far. However, it needs an extremely stiff wing , which has just become possible in the last few years after the latest progress in carbon fibre stiffness. (Fun fact: The wing is so stiff that the structural test did not result in the wing spar breaking but in a torsional fracture of the wing shell!) Comparison to previous flying wing designs: All previous flying wings had one huge basic fault: In order to pitch up (or to fly slow), you had to deflect the controls up, therefore reducing airfoil camber and thus lift coefficient - basically exactly the opposite of what you'd like to have aerodynamically. The wing of the AK-X works exactly like that of any flapped glider: Low-speed flight: All flaps deflected "down" High-speed flight: All flaps deflected "up" The idea behind this aredoynamic design is, frankly spoken, a touch of genius. It's the first ever flying wing design ever that in theory will be able to compete with a conventional design in all areas of the flight envelope up to very high speeds. Plus, there are a couple of other benefits: The wing uses conventional airfoils whose aerodynamic qualities can be predicted well today. The flapped wing creates the same lift coefficient as the wing of a conventional design, allowing high aspect ratio and wing loading. Behind the cockpit there's a 40 liter water tank (directly at the center of gravity) and no other structural parts - pretty simple to replace this tank with an angine and some serious battery capacity. To me, the only remaining question is the influence of the wing sweep on spanwise flow - but as I heard the guys are pretty optimistic so far (they've got a 1:2 model flying with very good results). Cheers Andreas |
#22
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Akaflieg Karlsruhe AK-X
On Mon, 11 Dec 2017 21:41:25 +0100, Andreas Maurer wrote:
To me, the only remaining question is the influence of the wing sweep on spanwise flow - but as I heard the guys are pretty optimistic so far (they've got a 1:2 model flying with very good results). If a description I read many years ago of what makes a Hoerner tip work and why its beneficial is true, then the spanwise flow shouldn't be a problem. I know that a lot of tip shapes were described as Hoerner tips, but the one I'm talking has: - a minimum LE sweep of 10 degrees on the outermost wing panel - a straight edge to the tip raked outward toward the TE at at least 30 degrees and should meet the TE at an acute angle, i.e. not rounded off - the upper surface curves down to meet the lower surface at an acute angle The idea was that the LE sweep promoted spanwise flow toward the tip, which was encouraged to oppose the tip vortex rotation as it slid over the convex tip profile. The pointed at the end of the TE anchors the tip vortex while the roll-down of top surface flowing spanwise out along the panel and down over the tip shape will tend to move the tip vortex outward. I used this tip design for many years on competition free flight F1A gliders. It worked for me. It was notable that, while models with conventionally rounded tips needed a lot of tip washout to prevent tip stalling, my design worked best with unwarped [flat] tip panels. Directional stability was good too. Minimal fin area is beneficial to F1A performance and thermal centering, the optimum being just big enough to kill dutch rolling tendencies. On my design the fin had to be reduced to a surprisingly small size before the first signs of dutch roll appeared. Benefits of solid balsa fins: you keep chopping bits off until the dutch roll appears and then stick the last bit back on. -- Martin | martin at Gregorie | gregorie | dot org |
#23
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Akaflieg Karlsruhe AK-X
At 20:41 11 December 2017, Andreas Maurer wrote:
Comparison to previous flying wing designs: All previous flying wings had one huge basic fault: In order to pitch up (or to fly slow), you had to deflect the controls up, therefore reducing airfoil camber and thus lift coefficient - basically exactly the opposite of what you'd like to have aerodynamically. The wing of the AK-X works exactly like that of any flapped glider: Low-speed flight: All flaps deflected "down" High-speed flight: All flaps deflected "up" The idea behind this aredoynamic design is, frankly spoken, a touch of genius. It's the first ever flying wing design ever that in theory will be able to compete with a conventional design in all areas of the flight envelope up to very high speeds. Plus, there are a couple of other benefits: The wing uses conventional airfoils whose aerodynamic qualities can be predicted well today. The flapped wing creates the same lift coefficient as the wing of a conventional design, allowing high aspect ratio and wing loading. Behind the cockpit there's a 40 liter water tank (directly at the center of gravity) and no other structural parts - pretty simple to replace this tank with an angine and some serious battery capacity. To me, the only remaining question is the influence of the wing sweep on spanwise flow - but as I heard the guys are pretty optimistic so far (they've got a 1:2 model flying with very good results). Cheers Andreas Andreas, Thank you very much for the insights. If my father were still alive today, he would be wanting to follow the progress very closely. The aerodynamic genius of using the wing sweep to emulate canard characteristics as opposed to using the sweep for high speed flight is extremely intriguing. Being able to use current modern airfoils while reducing drag considerably, should result in very noticeable performance increases. Also, the advent of newer construction materials and methods which enable the builders to achieve the stiffness that is required to overcome other previous wing's aeroelastic issues cannot be overstated. Please post any progress updates here whenever you may hear of them in the future.... RO |
#24
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Akaflieg Karlsruhe AK-X
At 14:03 11 December 2017, Kiwi User wrote:
BTW, have you see this article about the Ho S.IVb: https://scalesoaring.co.uk/VINTAGE/D...n/Horten%20IV/ Horten_IVb.html Lots of photos, good plan showing the three control surfaces per wing and a useful write-up about construction, flying characteristics and where the airframes went. A bigger plan is downloadable from he https://scalesoaring.co.uk/VINTAGE/D...n/Horten%20IV/ Horten_IV_model.html As far as I know, the replica has not flown. Thanks for the scalesoaring links, I had not seen them before. RO |
#25
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Akaflieg Karlsruhe AK-X
On Mon, 11 Dec 2017 15:48:44 -0800 (PST), "Jonathan St. Cloud"
wrote: This project and Mu-31 are projects that seem like they have potential to develop new technology. Absolutely. Since you mentioned the Mu-31: Also an extremly interesting project, much more consequent wing-root design than the JS-3, and since it's otherwise identical to the ASW-27 it should be easy to compare the benefits of the new design. |
#26
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Akaflieg Karlsruhe AK-X
On Tue, 12 Dec 2017 01:52:24 +0000 (UTC), Kiwi User
wrote: Hi Martin, I'm rather worried about spanwise flow originating at the wing root (similar to the SB-13) - but let's wait and see. The guys (and gals) know their stuff. The idea was that the LE sweep promoted spanwise flow toward the tip, which was encouraged to oppose the tip vortex rotation as it slid over the convex tip profile. The pointed at the end of the TE anchors the tip vortex while the roll-down of top surface flowing spanwise out along the panel and down over the tip shape will tend to move the tip vortex outward. I used this tip design for many years on competition free flight F1A gliders. It worked for me. It was notable that, while models with conventionally rounded tips needed a lot of tip washout to prevent tip stalling, my design worked best with unwarped [flat] tip panels. Directional stability was good too. Minimal fin area is beneficial to F1A performance and thermal centering, the optimum being just big enough to kill dutch rolling tendencies. On my design the fin had to be reduced to a surprisingly small size before the first signs of dutch roll appeared. Benefits of solid balsa fins: you keep chopping bits off until the dutch roll appears and then stick the last bit back on. |
#27
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Akaflieg Karlsruhe AK-X
On Tue, 12 Dec 2017 03:30:19 +0000, Michael Opitz
wrote: Thank you very much for the insights. If my father were still alive today, he would be wanting to follow the progress very closely. The aerodynamic genius of using the wing sweep to emulate canard characteristics as opposed to using the sweep for high speed flight is extremely intriguing. Being able to use current modern airfoils while reducing drag considerably, should result in very noticeable performance increases. Also, the advent of newer construction materials and methods which enable the builders to achieve the stiffness that is required to overcome other previous wing's aeroelastic issues cannot be overstated. Please post any progress updates here whenever you may hear of them in the future.... I promise. I happen to be at the Akaflieg Karlsruhe workshop two times per year so with a little luck I can provide you with some updates if the Akaflieg guys allow that. But since we are talking: Has your father ever talked about (or even flown) the Horten VI (the 24m glider)? I was always amazed that such a thing could be built in the pre-carbon fibre aera. There are a couple of reports about the IV, but I couldn't find any halfways detailed source about the VI. And, second question: Do you know if the Horton guys aver considered winglets instead of the drag ailerons? Cheers Andreas |
#28
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Akaflieg Karlsruhe AK-X
At 00:34 13 December 2017, Andreas Maurer wrote:
On Tue, 12 Dec 2017 03:30:19 +0000, Michael Opitz wrote: Thank you very much for the insights. If my father were still alive today, he would be wanting to follow the progress very closely. The aerodynamic genius of using the wing sweep to emulate canard characteristics as opposed to using the sweep for high speed flight is extremely intriguing. Being able to use current modern airfoils while reducing drag considerably, should result in very noticeable performance increases. Also, the advent of newer construction materials and methods which enable the builders to achieve the stiffness that is required to overcome other previous wing's aeroelastic issues cannot be overstated. Please post any progress updates here whenever you may hear of them in the future.... I promise. I happen to be at the Akaflieg Karlsruhe workshop two times per year so with a little luck I can provide you with some updates if the Akaflieg guys allow that. But since we are talking: Has your father ever talked about (or even flown) the Horten VI (the 24m glider)? I was always amazed that such a thing could be built in the pre-carbon fibre aera. Martin Gregorie provided a pretty good link for a brief Horten history: https://scalesoaring.co.uk/VINTAGE/D.../Horten%20IV/H orten_IVb.html From that article: "As Heinz Scheidhauer was the most experienced and long serving Horten pilot, it is also strange that the test flying of the H IVb should be passed to Strebel at such an early stage. Scheidhauer had flown most of the Horten types and despite some peculiarities, their handling had been generally benign as they were highly stall and spin resistant. Although the H IVa had been liable to flutter, it had proved possible to damp it by simultaneous use of the drag rudders. None the less, he had refused to fly an HIII to explore its handling at extreme rear centre of gravity positions. On 24 May 1944 he made the maiden flight of the ultra high aspect ratio H VI, discovering that not only was it subject to flutter at both high and low speeds but that the entire wing was far too flexible and fragile for even an expert pilot. If Zubert’s log book entry is correct, the pilot of the H IVb on 11 August was almost certainly Scheidhauer and this may also have been its maiden flight – long before the H XIIIa. Unfortunately just ten weeks after test flying the H VI, Scheidhauer discovered that the H IVb suffered from truly appalling handling characteristics and would abruptly drop a wing and attempt to spin at speeds as high as 77km/h, yet above 110 km/h the wings would flutter. Later, also he refused to fly the H XIIIa which Strebel tested from the outset. It seems that Scheidhauer was losing faith in Reimar’s ability as a designer." Scheidhauer was a student of Dad's from the first military glider pilot training class at the DFS in ~1937 where Dad was one of the three original instructors. Dad also later got him a flight in a Me 163, even though he was from the Horten camp. By 1944 when Scheidhauer first flew the Ho VI, Dad was pretty deep into getting operational Me 163 units up and running. His glider logbook shows one or two Ho III entries earlier, but no Ho VI. He never really talked about the Ho VI at all. I doubt it flew much considering the info from the above story. There are a couple of reports about the IV, but I couldn't find any halfways detailed source about the VI. And, second question: Do you know if the Horton guys aver considered winglets instead of the drag ailerons? I don't know if they considered winglets, and I can't remember what the aircraft they designed after the war in Argentina looked like. The person to ask is Peter Selinger. He was also a friend of Scheidhauer's and received all of Scheidhauer's personal logs, etc when he passed away. Peter also wrote a book on the Hortens, but unfortunately, I don't have a copy of it. "Nurflügel", by Peter F. Selinger and Dr. Reimar Horten ISBN-103900310092 ISBN-139783900310097 I do have Peter's contact info in case you want it though. We have been in fairly regular contact with each other since 1985. RO |
#29
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Akaflieg Karlsruhe AK-X
On Wed, 13 Dec 2017 01:27:16 +0100, Andreas Maurer wrote:
Hi Andras, I'm rather worried about spanwise flow originating at the wing root (similar to the SB-13) - but let's wait and see. The guys (and gals) know their stuff. I notice that initial drawings put the wing at the bottom of the pilot's pod, but in the 1:2 model its just below the canopy rim. Was this for wing clearance or aerodynamics? I'm a little surprised, too, at the quite minimal root fairings. Is this what you were referring to when you mentioned spanwise flow at the root? It would be interesting to see flow visualisation round them. Though, as you say, the guys and gals know their stuff, so maybe cleaning up the wing roots is being left for full size detailed design. After all, the 1:4 model had nothing except a couple of sensor probes at its root, so just adding the pod was quite a big step aerodynamically. -- Martin | martin at Gregorie | gregorie | dot org |
#30
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Akaflieg Karlsruhe AK-X
This thread really highlights what a diverse interesting group, peoples the soaring world! I have immensely enjoyed not only the history told in this thread, but the technology these young engineers are exploring. Wish there was a way for non-German speakers to stay updated on the various Akaflieg projects. Thank you for all who shared!
Jon |
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