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  1. #1
    Avistarpilot's Avatar
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    First Time Kit Builder: Project GP Super Decathalon

    Well as the title states I'm in the process of building my first kit plane. Figured since it's my first, and I'll probably have a bunch of questions I would just start a build thread...I mean who doesn't like those right? Unfortunately the model is discontinued but looks like everything is here. I got the kit from a co-worker years ago for next to nothing and decided to give it a go.

    I made myself a little work station devoted to this bird. Has been working out pretty well. I've got the wings mostly done. Need to install the wing tips and aileron control linkage yet, as well as sand it all down smooth. Tail feathers are also done minus some standing. My plan is to tinker on her over the winter and hopefully have it perfect by spring, ready for its maiden. As of now I'm thinking maybe a .46 or .51/55 2 stroke for power. I've heard these are a bit heavy so may stick a .55ax in there. That's later on down the road.

    Anyway, here are some piccs. Hopefully someone that's built one of these will chime in, as well as all you veteran builders out there to give me some insight on this thing!

    Workbench:

    Wing build-up

    Wing almost complete, starting fuse build up

    Tail feathers


  2. #2
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    RE: First Time Kit Builder: Project GP Super Decathalon

    Avistar,

    Welcome to building model airplanes. I am impress with your progress. You are definitely in the right track. A way to keep the bird light is to be easy on the glue, specially CA and epoxy. Another trick is to replace some heavy wood with light soft wood. But, that increases the cost and it might not be under your consideration on your first build. Another way is to increase or add lightening holes in different parts of the fuselage. How many and how large? It depends on the structure of the airplane.

    Keep up the good work!!! We are all here to help.

    Pedro
    Spitfire Brotherhood #96, Ultra Sport Brotherhood #20

  3. #3
    Avistarpilot's Avatar
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    RE: First Time Kit Builder: Project GP Super Decathalon

    Thanks! As this is my first kit build I think I am going to stick to the plan. If it ends up like a brick...well I'll deal with it. Sorry for the "crappy" cell phone pics, my digital camera broke, with a little persuasion once I knew it was done, so my cell will have to work for now. Honestly I think my 5mp cell cam is almost better than my old digital.

    Anyway, thanks for the comment. Hope to hear from all of ya!

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    RE: First Time Kit Builder: Project GP Super Decathalon

    I built one of those years ago. Get somebody with some experience to help you if you are not a very good flyer. They were known to snap when slowed down too much while landing. If I remember correctly, Great Planes issued a technical advisory that recommended building some washout into the wing. I built mine without any dihedral to help with the snapping. Engine was an O.S. 70 Surpass turning a 14X4 or 5 wood prop. Great flying aerobatic model and the snapping problem (certainly mine) was inexperience. Good luck with yours. Edit. It may be to your advantage to give Great Planes a call. Somebody there may be able to help you with that advisory..

  5. #5
    Avistarpilot's Avatar
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    RE: First Time Kit Builder: Project GP Super Decathalon

    Ya I've heard these have a tenancy to snap upon landing, also heard they can tip stall upon take off if giving too much elevator or taking off before the wing is ready to fly. I plan to fly the crap out of my avistar again and planning on getting another ARF before this decathalon is done. I consider myself to be coming into an intermediate category flyer in my opinion.. I'm re-joining my local club also, so if the time comes and I don't yet feel comfortable I may see if there would be a member who would want to buddy box me on it.

    How do you stay away from the snap on landing? Keep your airspeed up and "fly" it onto the runway?

  6. #6
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    RE: First Time Kit Builder: Project GP Super Decathalon


    ORIGINAL: Avistarpilot
    ...

    How do you stay away from the snap on landing? Keep your airspeed up and ''fly'' it onto the runway?
    Yes, you don't cut throttle while you are in approaching. You have two to four clicks on the throttle stick to keep some speed. It really depends on your link adjustment. Then you fly until the wheels touch ground or you are less than 6 inches from ground. A good tip is to practice very low speed at high altitud and watch how the plane stalls. You will get the right feeling of how many stick clicks you need to keep the plane flying at a reasonable slow speed for landing.


    Spitfire Brotherhood #96, Ultra Sport Brotherhood #20

  7. #7
    Avistarpilot's Avatar
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    RE: First Time Kit Builder: Project GP Super Decathalon


    Yes, you don't cut throttle while you are in approaching. You have two to four clicks on the throttle stick to keep some speed. It really depends on your link adjustment. Then you fly until the wheels touch ground or you are less than 6 inches from ground. A good tip is to practice very low speed at high altitud and watch how the plane stalls. You will get the right feeling of how many stick clicks you need to keep the plane flying at a reasonable slow speed for landing.
    That's kinda what I thought. A while back after letting my plan sit for a few years it didn't want to idle for anything so I would normally come in hot at about 1/8-1/4 throttle to keep it running in case I needed to do a GA. Now that I have it tuned and running right I still find myself coming in hot until I know it's going to settle in. Guess its habit now lol. I will say the avistar floats a while with a lot of airspeed/ground effect. Observing the slow flight characteristics at a safe altitude is a good idea.

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    RE: First Time Kit Builder: Project GP Super Decathalon

    Mine sounds just like iron bottoms, I used the OS 70fs too. It did come in just a bit hot but I never had a stall or snap problem. I did have a problem when I flew it through a tree though. It was in a snapping,spinning roll at the time. That is the plane I learned never to take my eyes off the plane when flying!!! That was an outstanding plane and the old 70fs was a perfect match for the plane. Another one that GP discontinued! That was pretty rude of them. They used to have some good kits not too many years ago with some pretty good prices.
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    RE: First Time Kit Builder: Project GP Super Decathalon

    They will stall on take off If you are not going fast enough. Easy to avoid that. Power is what you want. Nail that OS 70 with a 14" low pitch prop and the one I built would be flying almost before you could react. One of the guys I flew with had a .60 2 cycle in his. With that Surpass, I don't remember adding any weight to balance it. It is indeed a fine choice. If you fly from a grass field a fast landing will rip the landing gear off if the field is not kept in good shape. And to quote Gray Beard, Don't take your eyes off of it.

  10. #10
    Avistarpilot's Avatar
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    RE: First Time Kit Builder: Project GP Super Decathalon

    Thanks for the replies gents. Originally I was thinking 2 stroke but if both have ran a 70 four stroke with success I may throw one on. I'm used to 2 strokers and was thinking of putting a 55 AX up front. How are the different manufacturers of 4 strokes? Any to shy away from? I'd prefer a surpass O.S but if something else comes available at a better price I may jump on that too.

  11. #11
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    RE: First Time Kit Builder: Project GP Super Decathalon

    Totally what they said... don't let that high-wing fool you. This plane has a perfectly symetrical airfoil, and is short coupled. Nothing trainer like about it.. it doesn't float through the air at all.. and it can ground loop very easily.. with all that said.. it is one of the funnest classic aerobats that I've ever flown. I built mine about 20yrs ago.. and its still with me.. Have fun.
    H

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    RE: First Time Kit Builder: Project GP Super Decathalon

    The four stroke did help out getting the correct CG but any good two stroke .60 will fly the plane. I remember that I had just bought that engine at a club auction for $100.00. I knew the owner and the history of the engine.
    What four stroke to buy will open a can of worms. I think Magnum and ASP are total crap but others will swear by them. I also think OS has priced themselves out of the market. Today I pretty much use nothing but YS and OS four strokes and the smallest I have is the .91. I really like the quality of YS and OS with Saito coming in a far third place.
    With this being said I would no longer bother buying a new glow engine. We have too many good little gas engines on the market today. IF, a Capital IF, I was going to go with a glow engine in my Decathlon today I would just buy a new SK .91 two stroke from Kangke. Other then one, I have had good luck with the SK engines. They break in a bit different then other brands and there instructions that come with the engine were written for another engine a long time ago and are not correct!! They are still a very good choice and only $100.00 for a .91 with a muffler.
    If I couldn't afford or didn't want to spend the money on an OS four stroke then I would just buy the SK, low RPM and high torque engine.
    YS is a great engine but not an engine I would tell everyone to go out and buy. I think they are idiot proof but I have been proven wrong!!
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  13. #13
    Avistarpilot's Avatar
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    RE: First Time Kit Builder: Project GP Super Decathalon

    Man there's some good info coming in! A .91 in a 40 Decathalon though?...whoa.

    I can definitely see how this bird can become a handful, and I am in no way expecting to perform like a high wing trainer. With a fully sym airfoil, and its short fuse I can only imagine the carnage...err I mean Fun! It will take some getting used to but, like anything, once you get to know her characteristics there's no doubt it will be a blast to fly. I love the full scale decathalons and Taylorcrafts. Can't wait to get this one at home in the air!

    Back to the build, I found out the hard way not to rush bending balsa sheeting. Was putting the lwr fwd sheeting on the bottom fuse last night. Had it wetted and was slowly bending it over the radius and snap! Must not have been wet enough. Easy fix, rotated it 180 degrees to start with a fresh balsa edge and made sure it was good and soaked. Turned out pretty well 2nd go round.

    Lwr fuse sheeting, aft fuse bulkheads, and mlg ply mounting base installed.

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    RE: First Time Kit Builder: Project GP Super Decathalon

    I have not owned or flown this plane, but for whatever it may be worth, I have a theory about the tip stalling on this plane. I think it's the small size of the stab. The stab stalls and you lose control over the angle of incidence of the wing, and it snaps. The solution is to build a larger stab.

    Here's why I think this: The wing is very conventional and it is not tapered. It is not super thin and it does not have a sharp leading edge. It is NOT a wing that should have a tip stalling problem. But the stab is quite small compared to the stabs on other RC planes. I think that's the problem. I think it's also the reason people seemed to keep having problems long after the symptoms were well known. People tried washout, or a more forward CG, but no one tried a bigger stab.

    Of course, the stab is scale size, and the full size plane flies just fine. But the models operate with much lower Reynolds numbers and the stab on a 1/6 Citabria is a pretty small aerodynamic surface.

    Just one guy's opinion. But this problem was discussed a lot without anyone coming up with something that worked. I almost bought one just to try it.

    Jim

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    RE: First Time Kit Builder: Project GP Super Decathalon

    You're right on the money, Buzzard Bait. It's close coupled and the stab is rather small. I checked the plans of the kit I have and the ratios (stab area to wing area, ECT) could stand a little adjustment. Adding two inches or so to the fuselage and a little more stab area would probably calm it down. But then it would turn into something like a Cub. In this case, I think this kit is exactly what Great Planes wanted. This thread has got me thinking about building another, I still have that 70 surpass also.

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    RE: First Time Kit Builder: Project GP Super Decathalon

    I'm sorry, I really didn't mean to suggest you install a .91 but I shudder when I see the prices of a good 70 four stroke. I do have a small 60 stunt plane with the SK .91 in it but I do hold the stick down except when going straight up. I also tend to use what I have on hand and the smallest four stroke is a .91 and so is my smallest two stroke I have these days. Back when I used the .70fs they were under powered by todays standards. You will get a ton of people saying to give the Magnum a try and if you get a good one they are a fine engine, if not you will be swearing at it. for the price they can be worth the risk and power wise they are on par with the old OS line. The old 70 gaqve that plane some zip.
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    RE: First Time Kit Builder: Project GP Super Decathalon

    I once read a post about this plane from a guy from Great Planes who said they learned a lesson not to put a kit on the market without plenty of flight testing. I believe he said it was the first scale kit they ever marketed. It sounded like they were kind of embarrassed by the plane.

    Obviously some people have mastered it and like it. I wouldn't advocate lengthening the fuselage, but if it were me, I'd go with about 20 to 30% more stab area, given all the complaints about it. That means increasing linear dimensions by 10 to 15%.

    Anyway, it's looking good. Best of luck with it!

    Jim

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    Avistarpilot's Avatar
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    RE: First Time Kit Builder: Project GP Super Decathalon

    If Great Planes is embarrassed about it that makes me feel real warm and fuzzy inside! BUT, I know it flies, and I will deal with its quirks when I stumble apon them. I've already got the tail made up so I think I'm just going to go per the plan and hope for the best. Obviously there are pilots that love to fly this bird as well, just have to fly it different. I can tell you, however, I will probably be a nervous wreck when I go to maiden her.

    Back to the subject, I've seen a few "lightly used" .70 four strokes on flea bay ranging anywhere from 100-200 depending on condition. Would it be worth buying a used engine? If you can trust the history of some of these it may not be terrible. Some have said to have a couple of flights on them? There is also a magnum .70 brand new for 150...if it runs like crap I guess I'd be out 150 bucks...not much for this hobby lol.(hope I didn't start an engine debate)

    Another question I had was about the tail gear. I keep reading about how almost everyone replaces the stock tail gear on many kits and ARF's. Is this one worth a darn or should I go ahead and plan on replacing it too?

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    RE: First Time Kit Builder: Project GP Super Decathalon

    That Magnum is WAY overpriced. You can get a brand new one for less: http://www.hobbypartz.com/4strokeengines.html

    ASP is the same.

  20. #20
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    RE: First Time Kit Builder: Project GP Super Decathalon

    Well haven't posted on here for a bit. Noting huge to report. I'm hoping to get some good progress done this Labor Day weekend. I'll be heading down the hobby shop early tomorrow to pick up my engine, fuel tank, and some other tiddly bits. After some research I decided to go with the Magnum .70 four stroker. I'm to the point of locating the engine on the firewall so I need to get that to continue on to fuel-proofing the fuse and keep on keeping on with the kit.

    As far as whats been done since last post, I have the lower triangle stock and lwr fuse skin done. Glassed the center wing section, and installed the aileron torque rods. I feel that the wing LE back to the aileron hinge well is complete. I finished sanding and blending the LE with the wing skin & ribs. Next up is mounting engine, fuel-proofing the fuse, and continue on with the upper fuse build up. Hoo-rah Cant wait!

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    RE: First Time Kit Builder: Project GP Super Decathalon

    I see I misread...the $150 Magnum is brand new...no problem, good luck.

    I'm sure if you are careful about technique you can avoid the dreaded tip stall and enjoy the plane.

    Jim

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    RE: First Time Kit Builder: Project GP Super Decathalon

    I've got one I still fly and put a 46ax on it at first and it was fun but didn't fly scale so I ended up with a 46la on it now and enjoy it a lot. It's been awhile since I flew it lately but I may get her down to play with next time I'm out. I played with The idea of a 40la but the 46la runs good and is just as light.
    As far as the tip stalling mine does when it's slowed down but I think if you would put some washout in the wing it would greatly improve that issue. I put some in my latest gp extra and she's a kitten compared to others. I'm not sure about the others remarks about the stab, they could be right. I tend to think the washout would be where the problem
    Could arise at.
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    Avistarpilot's Avatar
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    RE: First Time Kit Builder: Project GP Super Decathalon

    Got back from the hobby shop today, was going to break in the engine today but decided to wait. It's going to be a while before I get to fly this thing, I don't want to break it in and then let it sit until spring to fly it. I will use it as mock up and break it in once the weather gets nice again next year. That being said, to the folks that have owned and flown the GP decathalon, how did you mount your engine? I have a .70 4 stroke that I am leaning towards mounting sidways(per the plans) Is it possible to mount this think inverted? Also, do you test fit the cowling at this time also to make sure it lines up with the engine crank? How does one go about getting the crankshaft to get square with the cowl opening before you mount the engine to the firewall?

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    RE: First Time Kit Builder: Project GP Super Decathalon

    Side ways so my NV was straight up from the cowl and I could get a glow driver onto the plug. If you go for inverted I can tell and show you how to make your own remote glow driver so you can't see/notice it like the cluncky store bought untits.
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    RE: First Time Kit Builder: Project GP Super Decathalon


    ORIGINAL: Avistarpilot

    Got back from the hobby shop today, was going to break in the engine today but decided to wait. It's going to be a while before I get to fly this thing, I don't want to break it in and then let it sit until spring to fly it. I will use it as mock up and break it in once the weather gets nice again next year. That being said, to the folks that have owned and flown the GP decathalon, how did you mount your engine? I have a .70 4 stroke that I am leaning towards mounting sidways(per the plans) Is it possible to mount this think inverted? Also, do you test fit the cowling at this time also to make sure it lines up with the engine crank? How does one go about getting the crankshaft to get square with the cowl opening before you mount the engine to the firewall?
    You'll do fine with this plane.. It has its own personality, as is said, some have mastered it.. Although I don't have mine mastered, it flies fine. I installed an ASP .61 2-stroke w/pitts muffler. Also, instead of using the balsa struts.. I fabricated some out of Aerotrend airfoil shaped aluminum tubing. Although the "surfboard stiff" wing doesn't need them at all.. haha..

    The cowling gets mounted after the engine/mount are fitted/installed. Just go by the plans, get the engines' thrust washer/crank in same position as shown on the plan.. take your time here.. Once the engine is in place, the cowling gets positioned.. with engine mounted on airframe, using the thrust-washer, and the front opening in the cowling... those will line up.. with proper spinner clearance(yes.. have the spinner back-plate installed too.. There's a few easy tricks for getting the cowl mounting screws in the correct position too... I won't go to far into that.. just remember that.. when you get to that point.. and I can help you a bit further maybe.... I hope this helps a bit. Have fun.

    I should shoot a current pic of mine. . I've wanted to dig it out and dust it off for months..


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