catfishjohn: You really don't need an engine that big. Decathlons are not met for speed; they are not racers. Like a Cub, they are a short-coupled high-wing taildragger that stears with the rudder and flies on the wing rather than the prop. I have one that's similar, only it's a discontinued Lanier Citabria with slightly higher wing loading than the Phoenix Decathlon. The .46 AX will pull it around just fine, and it also has a very long glide ratio considering that it's not a sailplane.

NorfolkSouthern

Thanks for the reply NorfolkSouthern. I just had that K&B 61 sitting around and just put new bearings in it.
I was just wanting to put it back in service. If you say a .46 engine would be a better match I have
a Evolution .455 2s I acquired from a H9 trainer mustang a friend had. Its in great shape and it runs great.
Do you think this would be a match? I'm not really a speed freak and I like slow and relaxing flying with
lots of throttle control. Sometimes I open her up briefly and do some crazy stuff too. So if the .61 is installed I can always pull that throttle back. The Evolution might be a better match though. Thanks for your thoughts.

Your Evolution should be fine for the Phoenix. The K&B would probably go good with the Goldberg Decathlon 60.

NorfolkSouthern

The plane weighs 7lbs and recommends a MINIMUM of a .46. I would have liked a .61 in mine.

I'll say! I've seen you fly so slow that a 20 minute flight takes over an hour!

I would say to wiegh both engines and see what the wieght penalty is for the larger one. If there isn't one or it's next to non-existent, then go with the .61 and carry that ace in your back pocket for the entire flight (should you ever need to use it). If you want, I can bring a small scale home from work to wiegh them both. Besides, I still need to get you that prop for the cub.

Andy

Here's what I have on the Tower Hobbie's site:

Specs:

And here's what I have on mine:

Wingspan: 64"
Wing Area: 625 sq in
Fuselage length: 44 1/2"
Weight: 6lb approximate
Engine: .40 to .50 two-stroke
.48 to .70 four-stroke
Wing Loading: 22.11 oz/sq ft

I can just about set mine up on a hi-start and catch thermals, if that's to my liking. But it is a piston powered plane, afterall. Engine choice depends on what the owner wants to do. Fly on the prop like a Venus II, or fly on the wing like a Piper Cub. Although I would prefer the Cub, others would rather have bigger control surfaces and increased throws so they can 3D hover. And then there are some who like the challenge of a war bird's added weight and speed. It's all good. Mind you, the .61 K&B engine is going to make it a little heavier, and that will also increase the stall speed. You will want to adjust your flying technique accordingly.

NorfolkSouthern

Absolutely right. This is making me think back to the full scale I flew a couple of weeks ago; that Cessna sure as heck didn't have unlimited vertical performance! When I got the Decathlon all I heard was 'aerobatic trainer', but I had no idea what a scale plane flew like and was expecting something quite different. A .46 will fly this plane, and probably fly it very scale. If that's what you're looking for, this plane is a BEAUTIFUL scale flier. I'd probably still go with the .61 though

Well, after doing my homework I decided to go with the Evolution .45 because its a all around better engine. The K&B .61 I have is very old and was made betreen 1980 and 1990. It is rated at only 1.3 hp and weighs 17.00 oz. The Evolution weighs around 17 oz with the muffler and the weighted flywheel removed. I have to assume it being a modern engine it has a lot more hp than 1.3 hp. I will save the K&B for a later project. I wish Evolution listed hp ratings. Thanks everyone for your input and insight. This plane is going to fly AWESOME!

Cafish John

The Evolution is going to fit in the cowl better than the K&B. As for horsepower rating, I'm not real sure. I do know that my OS 46 AX and 11 x 6 prop had no problem pulling a .46 sized Cessna through all the maneuvers. I'm assuming the Evolution will do the same on the Decathlon. It just won't do the unlimited verticals or go as fast as something like a 55 AX.

NorfolkSouthern

Hello Decathlon flyers. Got mine this holiday and what a plane it is. I ended up mounting a 1500mah nicad to the back of the fuse. I added a plywood plate and velcro straps for easy removal of the battery. Engine choice is the O.S. 46 and 11 x 6 wood prop. Added a charge and on/off switch to the side. First flight was crazy. take offs were heavy on wanting to guide left. Allot of right rudder to keep it straight. Not right! I looked at the right thrust and mine was all washed out due to the motor mount bad install. I actually had a little left thrust!. After fixing that problem the Decathlon tracked very nicely down the runway and good take off with no issues. Flew about a half a tank and the engine was dying out. Made a dead stick landing on the first new flight. Plane had a nice glide and brought her home back to the runway with no issues. I ended up covering the right cowl port to increase the air flow instead of having an air dam with all the holes exposed. O.S. I read to have at least a 2:1 or even 3:1 air flow from the front to the exit of the cowl. I also enlarged the hole for the glow starter. Plane flew great on the next flight. Very easy in the air. I set the CG with the battery position at 90mm from the leading edge. No extra weight has been added. Plane is about 6.5lbs without fuel. I also used the foam supplied with the kit to wrap around the tank. I upped the tank size from the kit 8oz to 10oz without issues. I only should have gave a little more room for the throttle wire as its to close to the tank like I would like to have.

Looks nice! Your cowl is cut exactly like mine. My battery is mounted just like yours too. I have a .45 evolution with a 11x7 prop. I maidened mine about a week ago and all went pretty well. It performs good especially for the money it cost. I have only one flight on her because the weather turned bad. The only thing I have to work on is getting it to land right. When it touches down it wants to hop back up. I think we had four or five good landings (hop ups) in one attempt. The touchdowns were not harsh so I was surprised when it came back up. My friend suggested maybe moving a little weight forward and testing again. We will see. It will be two more weeks before I can make it to the field again to mess around with it.

Very good, I added the black stripe as i couldn't get the cowl to line up with the red and white lines. When they paint the cowl they dont allow for the offset of the cowl mounting. If the cowl is lined up at the end of the firewall the paint lines match up. Small thing. I noticed the plane next to your Super D looked like a good flier. I see Sullivan sells lite tires that claim to have a lower bounce rate that may help the landings. I did notice on mine at full throttle it did have a tendance to want to climb up. Mid to low throttle is easy level flying. The glide slope without the engine was easy.

I flew my second and third flights today. I'm still messing around with the CG and engine settings. During the second one I noticed it was still tail heavy and when I landed it did the hopping thing. I moved the CG forward more and it make a big difference in bringing the tail up and handling on the third flight. The hopping at touchdown went away too. I dont know where the CG is now but I will measure it later. My cowl paint job doesnt line up either but it doesnt bother me. I dont notice it as it passes by me over the runway . I really like this plane and I know that when I get it like I want it it will be a favorite to take to the field. Special thanks to Tcrafty for his help in dialing this plane in. Two heads are better than one.

My cowl paint job didn't match up. What I did was get some Great Planes striping trim - I think I got 1/4" red and 1/8" white. You can extend or 'move' the lines wherever you need to, and it came out looking AWESOME. Sold the plane, so sorry I don't have any pics.

Ok, now I remember. I added some pin striping, so that's why I had the white trim tape. For the stock scheme, you just need 1/4" red trim tape. Wrap it so it's even with the red covering at the fuse and around the front of the cowl, either ending at the breather holes or wrap it just under and go all the way around.

I chose to do a 1/8" white stripe evenly spaced above the existing color line. I searched for pics of full scale planes on Google Images and modeled one exactly. Really came out spectacular, and only took a half hour and five bucks.

Edit: What I did was put a 1/4" red stripe 1/4" below the line where the red meets the white, and a 1/8" white stripe 1/8" above that line, so I had a few nicely spaced trim lines.

Well I exchanged my 1500mah battery for the standard 600mah battery from futaba in the same fuse position. That changed my CG from 90 to 80. Thats 20 off from the orginal booklet setting of 100mm. However, at 90mm CG on a straight pass at 3/4 to full throttle this plane just wants to climb up. I've been using down elevator to keep it more level flight. The two batteries are about 1.3oz in weight difference. Will see what happens on the next flight. The bigger battery will keep it also at 80mm CG if i move it forward on the fuse and just behind the servos.

I'm not sure how you tune your plane, but CG is not in general tuned by the propensity to climb at full throttle...

Sounds to me like you should try a little more downthrust instead?

I see what your saying, but should elevator trim be adjusted to a point where it deflects all the time forcing a more downward position? I would rather had elevator trim at netural.

Yes the elevator should be neutral. I was refering to the angle of the engine. By adding/increasing downthrust on the engine the plane will climb less at high throttle settings...

The plane should climb with increased throttle, that is wanted and inherent to the design, but the amount can be fine tuned using the thrust angle of the engine.

This is all part of the tuning one should go through with a new plane. To me, CG is tuned at a low throttle setting to get a stable shallow glide with neutral elevator.

Ok on that. That does sound like a good way to setup a new plane. I went ahead and flew this plane with the CG at 80mm behind the leading edge of the wing. It flys pretty stable now with just a little increase in lift when going to full throttle and can fly level with a few clicks of elevator. At low to no throttle it still has a very good glide. Changing the CG from 90 to 80 did make a difference and I like it for now. This test was with hardly any wind blowing today. The plane does have built in some down thrust in the firewall. Its not a really fast areobatic plane with the .46 O.S. No problem doing a loop and basic manuvers. A 55 O.S. would seem a little more needed.

You are both correct about engine thrust vs CG but in this instance I think I know what Marketvending is trying to say. The plane is tailheavy because the CG recommendation in the instructions is all wrong. The instructions say 100 mm and the folks on this thread say try 90 mm. My second flight described earlier was at 90 mm and the result was the tail was always low and the nose high at all speeds especially slow and in turns it would almost knife edge on its own. It was unstable and slow to respond at low speeds. Also the plane while landing would hop up three or four times when touched down gently. If I guessed I would say my Decathlon's CG is near 85mm now and it performs much better now from take-off to landing. I will play around with it more and report where I finally leave the CG. 80mm might be the key.

Thats for sure on the CG postion. Even the manual says to balance this plane upside down. How do you balance a high wing plane upside down on the balance stand? I've got many flights on it now and its stable as can be.

Yeah, when I read about balancing upside down I realized this page might be a universal one for all their model lines. I also figured the CG was lost in translation somewhere too! I hope to soon get mine ironed out so its happy. I cant get to the field again for a couple weeks and that stinks. Hey Marketvending. I just noticed you joined RC Universe one day after me in 2007. Weird!

Well that is wierd on the day after. We've got the best weather right now. Flying at the club is always busy. I used a metal scrap to mount the needle 90 degress vertical. That was a oddball thing on this angle of the engine to the cowl. I did a further adjustment to get it even better at 90 after the picture. Had to make the scrap mounting holes a little bigger for more adjustment.

I just got mine yesterday. I am very impressed. Are you guys using the nylon stopper that came with it? I have probably assembled 30 arf's and I have never seen this before. I've always used brass tubing.

Thanks

Hi all, I am new to RCU and posting on forums. I have been following this thread for a while and I am currently finishing my Phoenix Models Super Decathlon. I am new to building RC planes. I have had several foamies and built a SPAD (Simple Plastic Airplane Design) from internet plans but this is my first ARF. This may sound like a stupid question but I was woundering if anybody had pictures of how they mounted their fuel tanks in this plane. The directions (or lack there of) stated to put a bead of silicone at each end but this seems like a fragile and unsecure way to secure the tank once it has the weight of the fuel in it. Any pics or sugestions would be great. I am using a 10 oz Dubro tank that I already had assembled instead of the stock one that came with it.

Another question I had. I am using a Magnum .52 two stroke and the directions for the engine state not to use bigger than a 12x8 prop. I have a 12x6 and am wondering if this will be enough prop to pull this plane.

Thanks in advance for any advice or help.