rotax618
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Everything posted by rotax618
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This was my solution.
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I stand corrected, hope the extra effort gives the design some advantage in weight and thrust. Still seems more complex than bolting the prop onto the flange of the engine and hanging the tail from either a low boom or twin booms.
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Prop around the boom can and has been done BUT for what gain? The engineering complexity in a half ton MTOW sports aircraft makes no sense. The prop drive bearing is only a small part of the solution - Since the prop is driven by a belt the the engine vibration cannot be isolated from the boom by rubber mounts. Propellers are not turbines and have their own modes and vibration frequencies, all of this is being transferred to a thin walled boom tube and amplified by the length of the boom to the tail cluster, this can induce flutter and fatigue.
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Prop rotating around the boom is a complex solution looking for a problem.
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Great that you could get the same angle, will be an easy job from here and the spar will be dead straight and like new.
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Yes I didn't explain myself correctly , all aircraft have to exhibit positive stability. Some military aircraft are so unstable that they can only be flown because of artificial stability in their fly by wire systems. As Facthunter (Nev) pointed out too much dihederal can have the unintended consequence of causing "Dutch Roll", and some designers have limited control surface deflection in an attempt to provide stall and spin proofing - not very good at low speed.
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From the Nurflugel site: NACA paper by Zimmerman on the subject (1929), where he presents wind tunnel force measurements for a series of low aspect ratio wings. Each wing model was composed of a straight and rectangular center section, to which two kinds of wingtips could be attached: with "square" or with semicircular planforms. Zimmerman progressively reduced the span of the center section, until it reached zero. Obviously, in that condition only the wingtips remained, directly attached to each other and forming a circular planform wing in the case of "round" wingtips. Apparently "by accident", he noticed that the circular wing developed a maximum lift much higher than that for other aspect ratios, either with round or square tips. I may not remember the exact numbers (I don't have the paper here right now), but I think CLmaxpeaked from a bit more than1.0 for the other AR's to almost 2.0 for the circular wing, that is, maximum lift almost doubled. We're talking about Clark Y airfoils, under low Reynolds conditions in 1929."
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I agree that you should have control authority especially at low speed, usually large enough control surfaces will give that authority without having to interfere with the stability margin.
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It is much more pleasant and a lot safer to have positive stability so that it is possible to fly hands off for a period without the slightest gust sending the aircraft into a spiral dive, it is a CofA requirement that an aircraft should return to level flight in "X" number of oscillations following a control input. Combat and aerobatic aircraft are an exception. An aircraft that is neutrally stable in any plane is demanding and tiresome fo fly any distance. Aircraft are subject to all sorts of external forces, neutral stability means the pilot must compensate immediately to maintain to counteract these disturbances.
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Low aspect ratio has a lot of advantages for a small sports plane, the planform has been shown to be stall and spin proof, due to their low wing loading have STOL characteristics. The can have a higher power off sink rate which means they can land in smaller clearings. The dont take up much hangar space. They can be built much lighter because of the short span and deep spars, have a low parts count, and can have low drag at cruise, (see performance of UFO on 50HP 503) You will never thermal an LA aircraft, but if you want a small safe STOL aircraft don't disregard them.
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Rtfm, your design has merit, and has been tried sucessfully before. You probably don't need all of the fuselage and elevator. My sketch could have a circular wing like Davd Rowe's UFO, and would have far easier access and visibibility.
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The Flying Flea has been described as a "Super Staggered Tailess Biplane", probably as accurate as calling it a canard considering the interaction between the front and rear wings.
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The 701 and classic Sav have a much rounder leading edge for the slat vent, the leading edge sheet wrapped around that LE far easier than thr VG/XL/S wing section.
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The 701 and Savannah share the same designers, I should have said Nearly All Of The Metal Aircraft require a preformed LE. Just look at the RVs and Monnett designs.
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All of the plans for metal aircraft I have seen require that the leading edge covering sheet be pre-formed before being attached, I'm sure that preforming the fuselage corner sheets will make for easier construction and a better looking result (provided that you dont kink the part).
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As I said the structure as supplied is more than adequate, that doesnt mean that it can't be improved, an aeroplane is a million compromises all flying in formation. The fin/rudder is not the only deficiency in the Savannah design, there is insufficient dihedral, the stability in roll is only a little better than neutral. Increasing the dihedral makes the aircraft far more pleasant to fly, as Mark has observed, the noseleg needs strengthening, particularly on the XL/S as the more upright castor angle (6deg) increases the landing and steering loads. The noseleg attachment on the firewall is prone to cracking (both top and bottom attachments). Having said all that, from my 12 years of ownership I have found the Savannah to be a very practical and safe aircraft, and would build another without hesitation.
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I doubt that the "pillow" look on the sides is a design feature but is an uninteded consequence of ICP needing to send a package of parts that take up the least volume. The same "pillow" look can be seen on the underside of the leading edge between the ribs. The blind rivets are only supposed to be in shear, not tension and sheer, it also applies side load to the fuselarge longeron angles causing them to become non-concentric, lowering their resistance to buckling. Obviously the fuselage is more than adequate, but as a pure structure?
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Reg Brost and I discussed pre-forming the fuse corner skins a while back. It seems the pre-load on the curved corners causes distortion in the flat side, top and bottom sheets. (Look at the bulges between the frames on S Savs). May not apply to your build as you are using thicker sides. Give Reg a ring he may be able to help on the pre-forming method.
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G'day Mark, I have some very old 6061 sheets here which still have the white plastic on them, I made a new overhead switch panel for my Sav the other day. I soaked the piece in mineral turps for an hour and found I could easily peel the white plastic off using a razor blade, I then re-soaked it in turps and easily removed the glue with turps soaked Scotchbrite. Would probably be difficult on large pieces. Danny told me about the vinegar solution but I had no luck, the turps did the trick. I figured that turps must soften the glue of I wouldn't have been able to peel the plastic skin off.
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Looks likme it is only a two seater. Nice looking aircraft though.
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Yes, they have been to the Groppo factory and are aware of the kit contents. They said that like ICP, Groppo have a four seater aircraft. It is only supplied factory built until they sort out the kit production, and like ICP is too heavy at the moment for LSA. It looks remarkably like Roger's plane.
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G'day Mark, the guys in the hangar next to mine bought a Groppo kit, it should arrive next week. Will let you know when I get a look at it.
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My crazy STOL idea ...
rotax618 replied to bexrbetter's topic in Aircraft Building and Design Discussion
It would be extremely dangerous to have anything connected to the rotor system which could cause rotor speed decay. Even with the large kinetic energy stored in the rotor, it can rapidly slow when when it is subjected to negative G situations, so much so that the rotor has been known to strike the tail and propellor. Maintaining rotor speed is essential to autogyro safety. The Groen autogyro was reported to use depleated uranium in the rotor tips to ensure conservation of rotor speed for jump takeoff anf flare on landing. -
My crazy STOL idea ...
rotax618 replied to bexrbetter's topic in Aircraft Building and Design Discussion
I believe that the ideal sport aircraft would be a Gyrocopter with a semi-powered rotor, and short wings. Supplying a small amount of power to the rotor system should accomplish several things. 1. It would compensate for the induced drag of the rotor system, lowering the disc angle and reduce the overall drag, increasing efficiency, overcoming the 1:2, Thrust:AUW requirement in conventional gyroplanes. 2. It would reduce the tendency of the rotors to slow down dangerously in negative G situations, giving a greater safety margin. 3. Supply power for short takeoff and providing a much lower minimum speed (hover in light breezes). 4. Prevent the rotor from slowing down when some of the lift is shared by a conventional wing. Obviously the torque would have to be compensated, I envisiond the thrust engine could be moved laterall, off centre by varying amounts (pusher engine). A tractor engine could be pivoted sideways to offset the thrust by varying amounts. The problem has always been the mechanical complexity, but maybe a hybrid thrust engine and a direct drive electric motor on the rotor would work. Anyway food for thought. -
I ultimately had to build a engine frame to fit the Rotax ring mount to the VG Savannah, but the engine mount bolt hole centres on the original 701 and Classic/VG Savannah are identical, both aircraft were designed by the same people. And one is a knockoff of the other. The XL/S Savannahs seem to suffer more nosewheel cracking-damage etc because of the less caster which increases steering/landing loads, it seems the firewall was made more vertical to increase the windscreen rake and make it look faster. As they say an aeroplane is a million compromises all flying in formation.
