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May 05, 2015, 06:04 PM
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I slid the wings onto the tube to do a rough check of the balance. Surprised to see it is still slightly tail heavy in this configuration. The servos and wings struts mount behind the cg and will shift the balance even further back once they are installed. I had hoped the 290g motor and 540g battery would eliminate tail heavy condition. But apparently not completely.
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May 05, 2015, 06:43 PM
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eihoward's Avatar
Hey Dave, Great Looking Planes you have there...
May 05, 2015, 06:45 PM
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Quote:
Originally Posted by dbc
I got the cowl mounted, always a tough job for me. It turned out nice, with one exception.

The fuse is now essentially finished. With the battery, it weighs 5 lbs. 12 oz. and balances 80mm back from the LE of the wing. Neither of these things really tells very much w/o the wing.

I'll start on the wing preparation tomorrow.

Dave
Great news, I plan on a 14 x 10 for my Rimfire .55 on 6S... I think that would be plenty of power.
May 05, 2015, 07:04 PM
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Quote:
Originally Posted by eihoward
Great news, I plan on a 14 x 10 for my Rimfire .55 on 6S... I think that would be plenty of power.
That should have plenty of power at half throttle! And the weight of the 6s should help with the balance, too.
May 06, 2015, 02:23 AM
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I fiddled around with one strut the whole evening yesterday, trying to find a good mounting solution. The problem is that, contrary to yours, this old version is molded of fibreglas with something inside that looks like wood from the outside. This wooden part, however, does not extend into the endpart, were the mounting screws need to go through, so this part is hollow and makes some cracking noises as soon as you tighten the mounting screw. It is also not flat, so the screw will touch the strut at an angle and further weakens the mounting area. I have now poured some gorilla glue into the strut in order to stabilize the complete structure. I have just tested on one strut and checked it this morning. The strut is far more rigid now and I will do this to the other one as well, on both ends. As you can see, this takes quite a lot of time and the maiden flight on the weekend is definitely postponed for at least a week.
May 06, 2015, 02:24 AM
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Dave, what is the big one on the piano? This one sure looks really nice!
May 06, 2015, 06:22 AM
dbc
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dbc
Thread OP
It sounds like there were some problems with the design of that older wing strut. I think my new version will work okay, but it doesn't look very finished with the exposed metal and screws.

The balance issue is beginning to concern me. Even with my heavy motor and battery, it's going to require a significant amount of nose weight to achieve the recommended CG of 65-70mm back.

The Great Planes 40 Cessna is a very close match in size and weight to the Phoenix plane:

http://www3.towerhobbies.com/cgi-bin...&I=LXUW98&P=ML

It's recommended CG point is 5 to 10 mm further back than the Phoenix plane, close to where I expect mine to balance without added weight. I'm going to take some measurements and calculate the theoretical CG for our plane. It could be that the Phoenix recommendation is further forward than necessary.

You might check out the online assembly manual for the GP plane. The wing struts and main gear both have separate fairings around the ends of them that look correct. And they are available, along with the wing struts, as inexpensive replacement parts. These might possibly fit on our plane, and even the wings struts might work.

The large plane on the piano is the Seagull 72" Sport Cruiser. Thanks for the compliment. It is indeed a nice looking model.

Dave
May 06, 2015, 07:20 AM
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Don't worry about the CG, on most manuals it is shown too far forward. I would take the one third / two third rule and go from there. Or use any of the free programs available and calculate the correct CG.

I have worked yesterday quite a long time on the struts and have tried a few things already, so I will go with them anyway. Today, I had a few more ideas, involving a piece of brass tube, some rubber foam, fuel tubing and epoxy. If that all works out, the struts will be easily removable and will not break or fail. Tomorrow I will know more...

The landing gear fairings... they are history! At least for now, I will leave them off. Maybe, some day, I will think about them again.
May 06, 2015, 08:53 AM
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Quote:
Originally Posted by eihoward
Great news, I plan on a 14 x 10 for my Rimfire .55 on 6S... I think that would be plenty of power.
Wow, my calculator says around 54A, with a forecasted motor temperatur of 180F. Getting quite warm...

And the thrust should be more than 5000g, with a semi scale plane of around 3500g. Are you planning 3D flying with it??? I would go for a 13x8 prop instead, the numbers are looking far less dangerous.
May 06, 2015, 09:39 AM
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Quote:
Originally Posted by Aussie64
Wow, my calculator says around 54A, with a forecasted motor temperatur of 180F. Getting quite warm...

And the thrust should be more than 5000g, with a semi scale plane of around 3500g. Are you planning 3D flying with it??? I would go for a 13x8 prop instead, the numbers are looking far less dangerous.
Recommended Prop sizes as per TowerHobbies site: 13x10E - 15x8E. for the Rimfire .55

I like it to be fast...
May 06, 2015, 10:33 AM
dbc
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dbc
Thread OP
I'm very skeptical of Tower recommendations. Frequently they are wrong - sometimes way wrong.

I always use a wattmeter, tachometer, thermometer and then a calculator to zero in on an appropriate prop. And then do some experimenting with actual flight and a radar gun to fine tune it.

The Scorpion motor I've installed is way more powerful than needed too - but the weight is needed. The 4 other Phoenix planes I've built have had CG recommendations that were pretty accurate. A couple of different methods of calculating CG indicate the range of 60-75mm for this one. The "one third two thirds" rule for CG really only applies to planform, rectangular wings. The angular tapered wing on the Cessna requires computing the MAC (Mean Aerodyanimic Chord = 8.847" in this case) for a meaningful estimate.

Looks like my AUW is going to be right at 120oz. At the top of spec range.
Last edited by dbc; May 06, 2015 at 12:44 PM.
May 06, 2015, 11:18 AM
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dbc
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There are small light ply mounting pads keyed into the formers behind where the manual is calling for wing strut mounting on the fuse. But I have no idea what's behind the indicated wing locations. I think you have already drilled the wings, Kalli. What did you find?
Last edited by dbc; May 06, 2015 at 12:31 PM.
May 06, 2015, 12:45 PM
Registered User
There is just some thicker balsa. If you take a wood screw to mount the struts, you can use them just once. I am currently gluing the 3mm inlets in, in the struts are already a small 3mm brass tube on each side, which will be used as support for a normal 3mm screw. After the epoxy has dried out, I will do the same on the fuselage. Will be a lot easier as you can work from inside out. When this section is ready, I will post some pics.
May 06, 2015, 01:17 PM
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dbc
dbc
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I'm not planning on routinely removing the wings. I often times never take the wings off an assembled plane because I don't transport them. Do you think the wing material will be the equal of the light ply blocks in the fuse? If so, that will probably suffice for the one installation (and maybe a very occasional removal) I plan to do. What do you think?

Dave
May 06, 2015, 01:40 PM
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Hard to say. I just drilled a hole in there and the drill was going through pretty easy. I think it is not more than 3 or 4 mm. You might glue a thin plywood plate on the area and paint it white. A screw would hold much better then.


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