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Phoenix Models Sukhoi SU-31 CG and engine Q's

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Phoenix Models Sukhoi SU-31 CG and engine Q's

Old 12-03-2005, 06:52 PM
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Default Phoenix Models Sukhoi SU-31 CG and engine Q's

Does anyone know were the center of gravity is for this plane?

Has anyone put a 60 size 2 stroke engine in this puppy?


I acquired this plane from a friend up at my local field after he crashed it. I had to do some major repairs to the nose, canopy, fuse, landing gear and elevators. I recovered a lot of the plane and hand cut new lettering and added more stripes. What do you think...









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Old 12-04-2005, 11:28 AM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

The manual states 110mm back from LE at fuselage. Mine flew best a little nose heavy. The OS 46 FX pulled it around well a 60 would be fun.
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Old 12-04-2005, 12:11 PM
  #3  
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

I used an old Fox Eagle 60 (from the 70's) on mine and it flew just great; not 3D, but nice smooth aerobatics. I didn't add any weight anywhere and it balanced as specified, I don't remeber the exact point, but if you are 1/4 to 1/3 of the mean chord you can never go wrong!

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Old 12-04-2005, 12:35 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

While rebuilding the plane I found out there was a 1 1/2 oz. lead weight hidden in the nose of the plane. I believe this is original. The friend I got the plane from was surprised the weight was in there. He actually had 1 oz of weight in the tail.
Why would they put so much weight in the plane!? They should have left it out and let the builder decide where to put wieght... if any...
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Old 12-04-2005, 04:12 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

I balanced mine at 102mm. It still does pretty decent flatspins. Not sure about the nose weight. Some have them..some don't...mine doesn't.
That' s a fine repair job you did...looks very sharp.
The big thing to remember with the phoenix suk is that it's not a 3D plane. Keep elevator throws as recommended. I think I have around a half inch up and a half inch down on the elevator. This plane will snap if you get too much throw on the elevator. As for rudder and ailerons..well, those are maxed out.
It lands surprisingly well, and does really nice scale tumbles. All in all, a great sport plane.
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Old 12-04-2005, 05:28 PM
  #6  
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

Looks really nice. The bars on the underside are going to come in real handy. The stock color scheme looks the same top or bottom. I did a sunburst on mine 1st thing and it really shows up in the air. Every time it gets far out there and I see those black sunrays I grin.

Mine is still new enough that I'm still flight trimming. First flights showed it was stable, maybe too stable and elevator stick position from rightside up to upside down suggested the too stable came from nose heavy. The book said the CG range in 100mm to 120mm which is about 4" to 4.6". I ran the dimensions to see what my old CL stunters flew best at and came up with about 4.5" as the start point for this layout. I moved some stuff around inside and got about 4-3/4" and flew it. It's still steady and inverted takes lots less elevator.

That lead in the nose from the mfg really frosted me off when I discovered it. No excuse for it other than the mfg wanted to use heavy wood. If the thing wasn't such a good flyer it surely would be the last Phoenix for me. BTW, the book also says there isn't any down or up thrust, but their motor mount directions show the mount located for right and UP THRUST! and mine had as much down thrust as right thrust, maybe more. Bet a lot of people wound up with prop shafts way off center in the cowl. Guess that's why the hog out the cowling way more than scale.

Mine is a good flyer nonetheless.
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Old 12-05-2005, 07:08 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

My instructions said to use no up or down thrust and 2 degrees right thrust. My firewall was perpendicular to the fuselage axis in both planes. To get any thrust adjustment would require shimming the motor on the mount or the mount on the firewall. Shimming the motor on the mount with the engine at a 45 deg angle would result in unwanted vertical thrust adjustment. The mount would have to be shimmed on the firewall to get pure right thrust. Just moving the mount around on the firewall wouldn't have any effect on thrust (OK - a minor effect if it was off axis).

I mounted my engine and mount at zero-zero and the Sukhoi flew fine.
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Old 12-05-2005, 08:20 PM
  #8  
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

Excellent job on the "re-kit". I especially like the drop shadow effect. How did you do the canopy?

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Old 12-05-2005, 08:30 PM
  #9  
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

GWILLIE,
My instructions had a cute little picture that shows the model as having 0 up/down thrust and 2 degrees of right thrust. It actually didn't say anything anywhere about tuning thrust into the model or not tuning thrust into it, either way. And most people would expect that a model that comes with a firewall type engine mount is going to have the suggested thrusts built into the firewall if the model is prebuilt and advertised as an ARF.

The instruction booklet also had a paragraph about installing the engine mounts and has a picture to illustrate. The picture shows the mount being centered on the pre-drilled access hole in the firewall. It shows the hole offcenter to the right and down. I happened to take a digital picture of the firewall of my airplane to use as sort of a blueprint. I then printed it out at the exact size and did my locating with it. Before drilling, I plugged in the wing and did incidence measurements of the wing and stab and worked out the C/L of the fuselage. They were close to zero zero zero, so I use the zero fuselage line and measured the firewall's thrusts. I have a very accurate incidence measuring gauge. My model's firewall has a measured 3.5 degrees downthrust and a little over 2 degrees right thrust.

If you look at the picture below you'll see my picture and where I drew center lines to locate the center of the firewall. I figured to have the engine's prop shaft wind up squarely in front of the center of the fuselage so as to be centered in the cowling. The cowling would be lined up with the fuselage so the fuselage sides were excellent reference lines. To build your plane so that the prop is centered in a cowl like that, if your plane has right thrust in the firewall, you move the motor mount off the the right (when viewed from the front) based on the right thrust angle and if it has upthrust you move it down etc. If you look at the picture of the Phoenix firewall and hole, it's obvious they positioned the hole for right thrust and up thrust.

Problem is, they show the model as having no up/down thrust yet have one picture showing the engine mount install based on up thrust and the actual model has the "locating" hole positioned for up thrust. And the model has downthrust. Anybody following the booklet for my model is going to have an offcenter prop.

And your booklet and model are bigtime different than mine. Wonder how many different runs they made of this model? Good thing your instructions matched your plane. Mine flies great with right and down, too. So wonder what else differs between our two?
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Old 12-05-2005, 11:08 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

Pretty sure mine has right and down built into the firewall.
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Old 12-06-2005, 01:41 PM
  #11  
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

Really nice rebuild job! Darock, I think we have the same instruction manual and my Sukhoi had the hole in the firewall positioned in the same place. I didn't like the supplied mounts, they seemed too flexible. I used a Dave Brown mount. This meant new holes. Also, my throttle needed a different location for the pushrod to exit. In addition, as you could guess, the hole for the fuel tank was in the wrong location for the mount. I cut a light ply disc for the tank hole smeared epoxy over snug fittong dowels, and plugged the engine mount and throttle holes. When the epoxy was set, I mounted everything to my preference, installed the throttle to my needs, and made a new hole for the fuel lines to exit.

As I mentioned, my fuselage had no down or right thrust built in. I'm watching for another one of these planes. Swap meet season is coming up and maybe I can score one. I'd kind of like to strip the factory covering, resand everything a tad to get the leading and training edges of the tail group and ailerons more "aerodynamic", then cover and trim it to my taste.

It is a shame things like this engine mounting are confusing. Not a problem for someone with a building background, but that leaves out an awful lot of new fliers. I personally think these planes are the best bargain I've ever seen. I takes so little work to get them flying and they are built as nice as most guys could ever build. And to top it off, these planes fly just great!
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Old 12-18-2005, 12:07 PM
  #12  
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

You got to love the Phoenix Models company, they have no website! I'm tryin to find the manual online with no luck. Does anyone know where I can get one? I would like to take a look at it. I need to figure out where to mount my engine and how far from the fire wall.


Mine does have the down and right thrust built in built in...
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Old 12-18-2005, 05:51 PM
  #13  
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

gumhead,
The manual isn't much help. There is only a single paragraph about Installing the Engine Mounts and it basically says nothing everyone wouldn't know already. There is a small picture showing the motor mount centered around the hole. That's NOT where it ought to be if you want the prop to be centered in the cowl. The picture also shows the mount arranged with the engine pointing down at 225 degrees. That puts the muffler for the 4stroke they picture elsewhere down in the hole at the bottom of the firewall. They say to drill four 5mm holes for the engine mount, and to make "sure to space the engine mounts to fit the engine you are using." That is pretty much all they say in the paragraph.

In the Installing the Throttle Pushrod Housing section that follows the Installing the Engine Mounts section, they say that "The distance from the firewall to the front of the engine thrust washer should 4-3/4" (121mm)" (that's EXACTLY how they printed it)

Since you fit the cowling later in the assembly, the distance is not critical for that. Matter of fact, the cowl has a lot of leeway front to back. I chose to install mine probably 1/2" back from where their cowl seems to be in their pictures. Their pictures show the front of their cowl probably 1/4" behind the back of the prop drive washer. Their pictures also show a cowl that has a lot more cowl up front. Their front part extends farther into the hole and makes a lot narrower hole than mine.

If you use that hole as a centering point for your motor mount, you also could have problems with the tank being too high. I mounted my motor at 270degrees for a couple of reasons. Turned out the tank wouldn't be too high that way. And I used a Pitts muffler and I wanted the tubes to stick straight out the bottom. As I mentioned in other posts about this mount installation, the hole is correctly located for the right thrust so the left-to-right location of that hole is useful for lining up the mount. But the hole is located for upthrust, and the firewall has downthrust, so I moved the mount location straight up the amount the hole is missplaced downward.

Hope this provides what you need to know and the other stuff helps.
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Old 12-18-2005, 06:50 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

I used the hole as a centering point for the crankshaft of the engine. I had no problem getting the engine centered in the cowl. Mine flys great with the Saito .82. Oh and dont ever try to fly it without the cowl. It changes the trim at low speed dramatically.

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Old 12-25-2005, 06:48 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

I just got one of these from my wife for christmas. All i have left to do is mount the engine. What would be the consequences of
mounting the engine in the center without the proper thrust??
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Old 12-25-2005, 10:56 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

drbenz,
Where you mount it on the firewall, up down left right, won't do a thing to the thrust directions. The firewall for this model is built with right and down thrust and you're going to get that no matter where on the firewall you screw on the motor mount. And some people have reported that their models don't have what others have reported for thrust directions of their firewalls. And the manual shows thrusts that don't match up to the models people seem to be getting nowadays. So it's a big confusing discussion.

If you put the wing on the airplane and measure it's incidence, it'll probably be parallel to the centerline for your fuselage from back to front. If you then look at the angle the firewall makes to the aircraft centerline, you'll see the up/down thrust if there is any. If there is none, the firewall will be absolutely perpendicular to the CL. My model LOOKED like it had downthrust and measured to have 3-4degrees.

If you stand over the fuselage and look straight down, the right/left thrust will show up about the same way. My model LOOKED like it had right thrust and definitely did. About the same as the downthrust.

Where you center the motor mount on the firewall to screw it on the firewall only really determines where the nose of the engine winds up within the cowl opening. When you've got a right pointing firewall that also points down, the center of the motor mount would usually not be in the center of the firewall. It would be above and to the right of the center if you were looking from the front.

Truth is, with my model, Phoenix whacked out so much of the cowl for the opening that whether or not the prop is centered is almost impossible to tell.

I wouldn't worry at all about centering the prop compared to looking inside the fuselage just behind the firewall for that lead slug Phoenix chose to hide in there. Pull that lead and use it if you wind up needing it.
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Old 04-18-2006, 02:41 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

Well I finally remaidened the sukhoi yesterday. It was really windy, 15-20 mh winds... But what the heck! The plane flew wonderful! All it needed was a click of aileron. The roll rate is awesome! I ended up putting Super tiger G-51 it the plane. It has plenty of vertical power! I found that it needed a little speed when landing... I cant wait for some better (less windy) weather to really put it threw the paces. Im also gona try to squeeze 2.5" wheels in the wheelpants, she stinks at taxiing with the 2.25" wheels.



Here's a couple more pics, sorry I didnt get any pics of her in the air. I cant take picture and fly at the sametime, Im not that good YET

Plenty of room for the G-51...







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Old 04-19-2006, 07:42 AM
  #18  
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

Nice looking rebuild. You're a pretty good craftsman to reclaim a wreck and make it look brandnew.

I did notice something in the pictures that might be worth your consideration. To show it clearer, I took your last picture and straightened the perspective and added a couple of lines. The lines are to each side of what would be a centerline drawn through the center of the vertical stabilizer, the cockpit area, and the front of the fuselage up to where the cowl starts (the cowl hides the actual centerline from there forward).

If you look at where the spinner and engine are in the picture, you'll see what I'm illustrating.

If you happen to have some trimming problems with the airplane once you fly it in calmer winds, you might consider the thrust line of that engine. It looks to have maybe a couple of degrees left thrust, maybe more. I couldn't see what the up/down thrust was.
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Old 04-19-2006, 08:14 AM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

ORIGINAL: darock

It looks to have maybe a couple of degrees left thrust, maybe more. I couldn't see what the up/down thrust was.
There is actually a couple of degrees right thrust. The engine is mounted off center on the firewall (trying to center it in the cowl).
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Old 04-19-2006, 08:34 AM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

Heres a better picture...

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Old 06-01-2006, 08:28 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

can anyone tell me what brand of covering Phoenix uses on this model. i'm trying to fix a small hole in the wing.
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Old 06-01-2006, 08:40 PM
  #22  
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

Hangar 9 Ultracote.
Cub yellow... http://www.horizonhobby.com/Products...ProdID=HANU884
and I believe the red was Flame Red...
http://www.horizonhobby.com/Products...ProdID=HANU883

Not positive on the red tho. It was a while ago and it doesnt have the sticker any more. But it is Ultracote.
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Old 06-02-2006, 02:30 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

hello iam just starting to build this plane putting in os61 my plane has the weight already in the noise sould i pull it or let it be thanks steve
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Old 06-02-2006, 04:15 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

I used an OS46AX with pitts muffler when I built mine. I did not know about the weight at the time, so it remained. I wound up moving the battery to the rear to move the CG back. Had I known about the weight, I would have removed it.

I'd suggest that you take the sucker out. It's dead weight no matter what since it's not even as close to the firewall as possible. It would have been much more effective had they stuck it to the firewall.

How much does your engine weigh? How much does the 46AX weigh?
Don't you think that adding a heavier engine probably would negate the value of that lead?

I just used TOWER's online catalogue to look up the weights of the two engines.
Your engine is roughly 6 ounces heavier than the one I used. If I were you, I'd pull the weight.
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Old 06-02-2006, 04:18 PM
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

OS 60 is gona be way nose heavy I would definitely take out the weight. My plane has a ST 51, I have the battery in the tail and it still needed tail weight. The OS60 is 5 to 8 oz heaver. The plane has way more then enough power with the 51...
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