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Building Tips and technical articles. => Building techniques => Topic started by: Bootlegger on May 29, 2010, 11:42:47 AM
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I have seen several models with rudder's at the end of the stab. How do you assure that they aren't off set the wrong way?
Am I correct in thinking that they should be straight ahead, and where do you get your measurements to assure this?
Also does this help keep air on the stab to make it and the elevators more effective?
Thanks, Gil H^^
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There's got to be a bazillion ways to insure they're straight. Assuming a straight hinge line I'd do it by making sure they were square to the hinge line (or offset by my desired amount), then making sure the stab is square on the fuse by the usual triangular measurements (you do make sure of this, don't you?). If for some reason there isn't a straight hinge line, I'd draw a reference line and make sure they were square to that and proceed as above.
Speaking as an engineer with a long-time interest in aerodynamics, I can authoritatively quote numerous folks who actually know this stuff and say that yes, absolutely, it does make a difference. Just like end plates on a wing, or winglets, they will slightly increase the effectiveness of the stab/elevator as a lifting system, and done right they will reduce the induced drag.
Speaking as an engineer with a long time to develop cynicism about nearly everything, I can also opine that whether they make enough difference to matter is a matter of conjecture, and whether the difference is positive is a matter of getting the details right. While the induced drag at high elevator deflection will almost certainly be lowered, the interference drag from the extra surface intersections will be higher. Further, when you have movable surfaces working against end plates like this, the necessary gap width to gain any advantage can be incredibly small; make it too big and you don't get much improvement in induced drag, you lose your extra lift, and the interference drag goes way up.
History tells all, and history tells that a lot of planes have been successful without them, and a lot of planes have been successful with them. I think whether you use them really falls to aesthetics and personal preference.
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Same as with a regular rudder, its your choice if you want to use offset or not. For no offset all you have to do is make tips of the stab (where the rudders will mount) square to the stab's TE, and then during alignment make sure stab/elev hinge line is parralled to flap hinge line; and both of those perpendicular to fuse. I am building an Olympic right now. The plans show both rudders are airfoil shaped and in addition have 3° offset.
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Done right twin rudders can be awfully nice looking. I've already staked out a position as a doubter for any stunning aerodynamic benefits, but that doesn't keep them from looking darn nice on the right plane.
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Same as with a regular rudder, its your choice if you want to use offset or not. For no offset all you have to do is make tips of the stab (where the rudders will mount) square to the stab's TE, and then during alignment make sure stab/elev hinge line is parralled to flap hinge line; and both of those perpendicular to fuse. I am building an Olympic right now. The plans show both rudders are airfoil shaped and in addition have 3° offset.
Don't forget they also tilt in to the top. I think the plans show a jig for that. H^^
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Don't forget they also tilt in to the top. I think the plans show a jig for that. H^^
Just checked the plans, from a Brodak kit. I dont see anything about tilt in at the top.
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Looks like a good place for me to jump in. Pics are of my almost finished Olympic. The plans give the option of vertical stabilizers with zero degree offset (most popular set-up) or 3 degrees. Don't know which is most authentic but I think the airfoil in the stabilizers will provide all the side lift one will need. Doc is right, they are supposed to be tilted, don't remember the degree but it patterns are shown for a guide so both will have the same degree of tilt. The pic shows the tilt. The overhead shows the airfoil sanded into the fin. FWIW: I pinned mine onto the horiz stab using small dowels, OK, you got me, they're really round toothpicks. Another great design with outboard vertical fins is Lou Wolgast's "Fury" which is kitted by RSM. 8)
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I need to make a correction here. The Brodak Olympic plans do show that the tops of the rudders tilt in at 8°. The rudder tilt is also mentioned in the instruction book.
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This may be the place to put this question, so here it comes.
Is there any advantage to having the little rudderlets on the end of the stab like the Geiseke bear, or Randy's Evolution?
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In theory they act as end plates and alter the vortice pattern at the tips making the elevator more effective thus allowing a slightly smaller surface to be as effective as a larger surface.This is my understanding of them anyway. Oh, and put Randy's "Olympia" on the list of stunters with small tip plates on the ends of the horiz stabilzer also. 8)
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I need to make a correction here. The Brodak Olympic plans do show that the tops of the rudders tilt in at 8°. The rudder tilt is also mentioned in the instruction book.
Glad you caught that. Was ready to run out to shop in my skivvies and open the kit I have and check. Yes I do have another kit courtesy of another modeler at VSC.
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As another Olympic builder, I can say the fins tilt in at the top by design. Why? Gialdini says in his 1963 Olympic article that it looks better that way! Plus, artists and architects will tell you the fins would probably appear to tilt away from each other if actually vertical; the world's greatest example of this is the Parthenon, where all the columns tilt slightly towards the building center and would meet at a point far in the sky if extended. No straight lines anywhere in that building. Very apropos to the "Olympic." Anyway, I used a jig and a disc sander to machine the stab ends to the exact angles needed. I believe mine are not offset at all, but Gialdini's build article calls for 3 degrees offset.