Jim Gibson just successfully maidened his scale model of the SGS 1-23 with several tows to altitude and uneventful landings. Jim modeled his Schweizer kit after N91887 "Tinny Hawk", the D model flown by Paul MacCready at the 1953 Nationals to first place victory in Green and White color scheme. More images may be seen at http://www.rcgroups.com/forums/showpost.php?p=11743248&postcount=159 along with Jim's documentation of MacCready's historic airframe shown below with its current owners.
Jim's maiden was not without incident and his story shared here points to the need to not only document one's incidence and decalage but also the control throws and rates, the C of G location and trim conditions before first flight and every flight.
Jim asked for help in sorting out the condition that caused his initial ground loop (and minor damage) and I went to Herbert Eberbach for his take, not knowing the full details initially . I bring this up here to remind us all that its easy to get out of shape on tow when starting out and equally imperative to know the whole story to analyze the conditions that may have caused a mishap to understand and derive an appropriate solution.
Here is our correspondence:
Subject: 1-23 ground loop
5/28/09
Tom,
I aerotowed my 1-23 last weekend for its first flight and I had problems on takeoff. The 1-23 did not want to get airborne and I experienced a ground loop before about the same time my brain hit the release. Fortunately, damage was minor and I'm going to give it another go next weekend.
I think my decalage is off. I have the C.G. at roughly 76mm from the leading edge and I think approximately +1.5 degreees of decalage all based on Herbert's posts. Should the decalage be plus or minus relative to the wing?
Any input would be appreciated. I'll let you know how I do next weekend.
Jim
Subject: FW: 1-23 ground loop
5/28/09
Jim,
I've forwarded this message to Herbert as well so you can converse directly with him. He is the "resident" expert with the most flights on the 1/5 scale 1-23 so I would contact him for assistance.
In the meantime, your decalage measured as a positive angle is the difference between the positive incidence of the wing and the relative negative incidence of the horizontal stab. As shown on the plans the decalage is measured at 3 degrees and the balance point at 74mm.
What exactly did you experience after the plane came up to speed?
Tom
Subject: RE: FW: 1-23 ground loop
5/30/09
Hi Jim,
“I am glad to give you my two cents of advice,
First of all, I am not sure what you mean by "plus" degrees of decalage?! In any case, the elevator has to be in an "up-elevator" position with reference to the wing! C.G. at 74 mm with 3° decalage or C.G. at 76 mm with 1.5° are in "the same ballpark" and will not make much difference for the first flights.
Second, you experienced a ground loop while in the take-off run. This might be due to the fact that the 1-23 did not want to get off the ground due to the decalage, but there might be another reason - or maybe both work together in the wrong direction. Where did you position the tow hook? Apparently you have followed my build log and there I presented "my" hook position at the bottom of the front former. I do this always on purpose following bad experience some 30+ years ago.
This position yields a "natural line" from the sailplane`s C.G. slanted down to the hook of the tug. Second, and this is important if the hook is positioned higher, it may occur (particularly on a "rough" grass runway) that the pull from the tug is pulling the sailplane`s nose down somewhat. The plane is then "balancing" solely on its wheel without enough speed for the controls to be effective. The result is likely that it "falls" over to the side and/or nose which ends in a ground loop or even in flipping over on its back. (This exactly what happened to me when I arrived in a new club (after moving) with a new airplane as the experienced "aero-tower".
Hope it helps and happy landings, Jim. The 1-23 is a very nice and stable flyer. Feel free to ask more questions.
Herbert
Subject: RE: 1-23 ground loop
5/31/09
Herbert,
Our club had another aerotow this past weekend which gave me a chance to give the 1-23 another go. This time---success!! We had a beautiful day and I was able to get in about a half dozen tows.
Your comments were pretty much right on the money with the cause for my ground loop. My 1-23 has 3 degrees of decalage(up elevator position relative to wing) with a CG pretty close to 76 mm from the leading edge. I also discovered that when I had my ground loop I had maybe 2mm of down trim already in the elevator and my radio was not programmed for any aileron differential when in launch mode(rudder not coupled to ailerons). Also, I did miss your original build post regarding tow hook position and my release is right at the end of the nose. Sure enough, the 1-23 wants to hug the ground while the tug is still on the ground which I wasn’t expecting previously. All of these items, when combined, caused the 1-23 to hug the ground on rollout. I corrected my elevator trim and differential prior to this past weekend. I apply just a touch of up elevator when it reaches flying speed and hold it until the tug is well clear of the ground. Now I know why it’s a good idea to move the tow hook position down a bit. Oh well, another lesson learned.
With the CG at 76mm I’m still carrying about 8mm(1/2 out on elevator) of up elevator for level flight. A couple of dive tests confirmed the CG could still go back a little further. How far back do you think I could go before I reach the center of lift(neutral position) for the wing? Otherwise I’ll increase the decalage so I can get the elevator closer to a neutral position for level flight.
For your information, I’ve attached a couple of photos of my 1-23(D) with a little history of the full size that the color scheme is based on.
Jim
Great work guys in sorting it out and congratulations to Jim for his excellent build and successful maiden flight!
A reminder too, the TMRC 1-23 in 1:5 and new 1:4 scale come with parts and drawings to build all variants of the 1-23 from the "Standard" though the D, E, G, and H and H-15 variants. This provides a fleet of historic full scale ships and color schemes to model yours after! I suggest the shorter winged "Standard" and D models for slope flyers who will appreciate the faster roll rates but still want to fly a fifth or quarter scale ship at the aerotow field with the club.
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