Holverson Swinger RG – resurrection of a nostalgic classic

Discussion in 'Rocket Boosted Gliders' started by Rktman, Jan 18, 2019.

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  1. Oct 24, 2019 #61

    Rktman

    Rktman

    Rktman

    Eric Noguchi

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    Sounds like that would work great with the upscale I hope to build. No complicated fiddly bits to mess with, just a nice easy to construct solution. I like it!
     
  2. Oct 29, 2019 #62

    Alan15578

    Alan15578

    Alan15578

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    I'm finally continuing with the Groundhog evolution. I re-read Jon Robbins Groundhog 16 article. It seems that he was most concerned with boost drag, and minimizing frontal area, while the real advantage of swing wings is that they have very low load during boost. Of course ideally you always want to boost high and establish a high enough sink rate that you keep thermals from stealing you model, while still winning. In practical terms you want to optimize over all three flight phases, boost, transition, and glide to maximize duration and then work with or around thermals. The Groundhog 16 was flown with a A5-2S, and has a very generous wing area. The large wing area is good because it is generally timed until the timers loose sight of it, and you prefer that timers lose sight of competitors models sooner. Sizing is one of the most important aspects of scaling the Groundhog to different motor classes. Through experience, I have found 1.5 " chord to be best for B motors, 1" for A (A3-2t) and 1/2A motors, 3/4" did not work well for 1/4A Motors, 2" for C motors, and 2.5" for D motors. I have not had occasion to fly E or higher R/G, but The swing wing should be best suited to higher boost speeds.

    The first evolution is to get rid of that heavy wire and piston, and use a simple burn string release. At the same time I moved the elastic out from the pivot to pull forward, much like on the Hot Turkey. If you do not constrain the wing tips, the wings will flutter on boost and often shred. I push a straight pin into the bottom of the fuselage, just ahead of the tail surfaces. Pull the pin out and cut it to a shorter length. Push it back in the pin hole and and use pliers to drive it in until just enough is remaining to catch the burn strings. I use a thin nylon cord of the type commonly used for parachute shroud lines. Cut a long piece of string and fix a a small Avery adhesive pad (the same material commonly used to secure shroud lines to parachutes) to one end of the string. Hook it around the pin and stick it tightly to the underside of the folded back wing at the trailing edge. run the string straight up the bottom of the fuselage, over a shallow notch cut in the front if the fuselage, loop it through the motor pod vent ports twice, forming an X in front of the motor, run it back over the notch and down the fuselage and around the pin in the pin from the opposite side the the other folded wing. Pull the string tight, there will be some elasticity in the nylon chord, and tape it to the other wing just like the first one, and trim off the excess. This has proven to be enough to keep the wings from fluttering on boost, and I have never had deployment failure, aside from catos and human error (forgetting to attach the elastic).

    The second evolution is using an undercambered airfoil. This will give about 15% more duration performance in glide with no boost penalty, due to the swung back wings. The undercambered wing will have more pitch down moment that will require more more trim down force from the stabilizer, or moving the CG aft. At this point it may be helpful to look at some AMA towline gliders. You can find designs with undercambereed lifting stabilizers, and far aft CGs. We could calculate the neutral point and establish aft CG stability limit, or find the condition that minimized trim drag. However, the R/G still has to contend with transition, so I recommend moving the CG just a little aft, but not as far as possible. You can pick an airfoil used by a successful towline glider flying a similar Reynolds Number, or you can pick one out of a catalog, or even design one. I'm not very particular about airfoils, I just use what seems about right.

    The third evolution is just materials. For the wings, I use thicker C grain contest balsa, but I sand most of that material away, but save your lightest wood for other projects. I cover the inner portion of the wing with tissue, and on large models I'll add an second layer of tissue on the inner most portion of the wing. The Spruce Fuselage on the B and smaller sized Groundhogs is fine, but is too heavy when scaled up. For a 2.5" chord model, I used hard balsa laminated with carbon fiber reinforcement. It will not have the impact strength of Spruce, but it will be stiffer and lighter. Covering the stab with tissue is also a good idea.

    The fourth evolution is simply a hodgepodge of improvements such as adding a pop up stab DT, more attention to streamlining the pivots, and making the wings/ pivots repairable by using replacing the pivot axles with nylon screws and bushings, and bit of plywood reinforcement.
     
  3. Oct 29, 2019 #63

    Alan15578

    Alan15578

    Alan15578

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    I neglected the tow hook in the Fourth evolution. I added this to assist trimming.

    There are several issues with the Groundhog. The wing aspect ratio of 24 is extremely high for duration glider. The fuselage length is very long, making the short period mode highly damped, and the phugoid mode lightly damped.. The wings actually flex quite a bit during the phugoid oscilations. If you shortened things up it would have better dynamics, and move closer to the XP-2. Induced drag can be minimized with an elliptic lift distribution, but at an aspect ratio of 24 it is already very low. One could change to an elliptical planform, or twist the wings (washout) which will also eliminate tip stall. I generally try to select wood with a suitable twist or try to build in twist when I cover the wings with tissue. I'd also like to try a tapered wing, say 1.75 at the root and 1.25 at the tip, with the wing thickness tapered as well. Although, this would increase the pivot size and drag, and we might not be able to call it a Groundhog. So, there is still scope for further evolution even in a classic well used design.
     
  4. Oct 30, 2019 #64

    Rktman

    Rktman

    Rktman

    Eric Noguchi

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    Much thanks. Lots of great info here applicable to glider design; invaluable when scratch-building original designs and making improvements to older proven ones.
    Since I now have the opportunity to build all those gliders that I always wanted to as a kid but never got the chance to, I'm on a quest to do as many as I possibly can get to. Doesn't matter if they were superior fliers or just average, since I'm doing it just for sport and the fun of it, not for competition.
    It's also been a good learning experience that has allowed me to dabble in some original designs along the way, which is why I find your input valuable.
     
  5. Oct 31, 2019 #65

    Alan15578

    Alan15578

    Alan15578

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    Thanks. There are probably dozens of things about the Groundhog that I have not said, or even totally forgotten. One on them is that the dihedral shown in the Groundhog 16 plans is much greater than needed. It is sad that I have acquired so much skill and experience, and there no one to pay it forward to, at least not locally. I have yet to build a 1/8A, or an E or larger Groundhog, there would be no point.

    I've been waiting for John Bean to release his mark 4 altimeter. What I really want is a similarly sized follow on product, a DT release, or maybe an altimeter with DT function built in. This would make sport flying B/Gs and R/Gs practical.
     
  6. Oct 31, 2019 #66

    Rktman

    Rktman

    Rktman

    Eric Noguchi

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    [QUOTE="Alan15578, post: 1932230, member: It is sad that I have acquired so much skill and experience, and there no one to pay it forward to, at least not locally. I have yet to build a 1/8A, or an E or larger Groundhog, there would be no point.[/QUOTE]

    I'd say you're certainly paying it forward here on the Forum by sharing your knowledge and expertise gained from years of experience. It's invaluable to BARs and those just getting into the sport. I'm a glider fanatic but don't have the benefit of an aeronautics, engineering, or even balsa glider flying/design/construction experience, so any knowledge I gain here or from other online or printed sources is always much valued.

    For myself, the point of building a 1/8A or 24mm Groundhog or other swing wing would just be for the challenge and fun of it, to see if I could get it to work successfully and what I could learn in the process of designing, building, and flying it. (My one attempt at a swing wing, as a clueless newbie 3 years ago, was an eye opener...it unintentionally spun like a top on the way up and refused to fully deploy its wings. I sure learned a lot from that less than stellar attempt). Now that I'm a little bit wiser, I intend to give it another try this winter.
     
  7. Oct 31, 2019 #67

    Ez2cDave

    Ez2cDave

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  8. Oct 31, 2019 #68

    Rktman

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    Eric Noguchi

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    You, sir, are a fount of knowledge! Great info, and just what this forum category could use, especially since there seems to be more beginners here (than on YORF) and we can really benefit from a deep knowledge library like this. Really helps to build it right the first time, and not to have to make the same mistakes that someone else has already made and found the solutions to. Hopefully others will also leave tips and advice gleaned from hands-on experience or their background.
     
  9. Oct 31, 2019 #69

    Ez2cDave

    Ez2cDave

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    You are very welcome . . . I hope that everyone benefits from it !

    Dave F.
     
  10. Nov 2, 2019 #70

    Rktman

    Rktman

    Rktman

    Eric Noguchi

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    Couldn't make this month's launch, looks like my next opportunity is the 16th of this month if the winds drop below double digit.
     
  11. Nov 9, 2019 #71

    Rktman

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    Rktman

    Eric Noguchi

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    Looks like I'm going to have to postpone the Swinger's maiden yet again. With sustained double digit wind speeds and 43mph gusts forecasted, it'll have to wait for next month's Tripoli East launch date. :(
     
  12. Nov 10, 2019 #72

    BABAR

    BABAR

    BABAR

    Builds Rockets for NASA TRF Lifetime Supporter TRF Supporter

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    Good choice!
     
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  13. Dec 1, 2019 #73

    Rktman

    Rktman

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    Eric Noguchi

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    Finally got the chance to launch the Swinger yesterday. It would've been a glorious flight...if it had not gotten hung up on the launch rod.
    With the wings still folded, the top wing got charred pretty severely. Guess metallized mylar tape wasn't a good protective solution. The heat from the ejection charge also caused the piston to swell and jam in place. Well, at least the wings deployed perfectly--though that seemed more anti-climatic comic relief than dramatic.
    I need to take a closer look but everything looks fairly easy to repair. Unjam the piston and use a strip of aluminum tape to protect top wing and she should be ready to fly at the next launch in 2 weeks.
    IMG_3862.JPG
    1st flight damage.JPG
     

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