A Brushless Bubbles Born - RC Groups
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May 25, 2003, 03:29 AM
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SheldonYoung's Avatar

A Brushless Bubbles Born


It seems like ages ago that I built my first airplane from plans, a Bubbles from the free plan by David Theunissen. Though saddled with a $7 can motor and my horrible building technique the plane tollerated my ineptitude. Eventually it was retired, or rather put out of its misery, and I moved on. Several months ago David published a new version of the fuselage and I figured it was time for another crack at it. What a difference experience and the right equipment makes.

The maiden flight went well and I confirmed unlimited vertical is addictive. The Bubbles is fairly short coupled and surprisingly agile with tight turns and quick rolls. It's not built for precision aerobatics and knife edge is going to take some work but four point rolls look much better than I expected. It lands more gently than any plane I have ever owned.

The maiden flight confirmed I had some modifications to make, including moving the landing gear significantly forward, improving battery cooling and using something more elegant than tape to hold the hatch closed. The modifications made during building consisted of shortening the wing one bay each side, no wing tips and a more forward battery tray so it would balance. I strongly recommend shortening the wing if using less than 10 cells to keep the wing loading from becoming too low.

All up weight: 49 oz.
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May 25, 2003, 03:41 AM
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SheldonYoung's Avatar
I'll try a 13x8 prop next time out with the thought it'll lower current draw at cruise and improve pullout from a hover. The plane is actually designed for 12 cells (on the full length wing) but I find 8 provides plenty of power and duration for what I want it to do.

8 x Sanyo HR-SC 2600 NiMh
Mega 22/20/2
MEC SuperBox 3:1
APC 13x6.5
Castle Creations Phoenix 45 ESC
Berg 5*DSP receiver
4 x Hitec HS-81 servos

A Mega 22/30/3 or 22/30/4 direct drive should work well if you don't want a gearbox and don't care about unlimited vertical.
May 25, 2003, 03:47 AM
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On the fourth flight I moved the center of gravity back and the plane handled noticably better, much crisper.
May 25, 2003, 03:50 AM
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The Bubbles is a nice plane, I'm glad I built it. We invest a lot of time building airplanes so saying you're happy to have built it says a lot.
May 25, 2003, 04:59 AM
Single-task at best...
tim hooper's Avatar
Sheldon,

That's excellent!

How on Earth did you get it down to 49oz? On 8 cells, mine comes in at 58.....

The new fuselage design looks noticeably slimmer than the original, which as you say, was designed to cope with 12 cells. I wonder if knife-edge suffers as result? No matter.

Are the rear mounted servos on the plan now, or is that you own own mod?

After flying mine for a couple of years, I improved its agility a good deal by fitting larger control surfaces all round. Coupling the flaps to the elevator (via the tx) gives 3D-ish performance, with extremely tight loops and bunts. A very worthwhile mod!

https://www.rcgroups.com/forums/show...hlight=bubbles

tim
May 25, 2003, 08:16 AM
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ScottS's Avatar
I think mine is around 53 oz with 10 cells, so the weight seems about right to me. Anyone try Li-poly cells in Bubbles yet? I already get about 10 min flights on my RC2400 - so it almost seems pointless. But I keep thinking about it - a 7800mahr Thunderpower pack would drop the weight nearly 5 oz and should give 30-40 min flights easily.
May 25, 2003, 02:07 PM
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Quote:
Originally posted by tim hooper

How on Earth did you get it down to 49oz? On 8 cells, mine comes in at 58.....

The new fuselage design looks noticeably slimmer than the original, which as you say, was designed to cope with 12 cells. I wonder if knife-edge suffers as result? No matter.

Are the rear mounted servos on the plan now, or is that you own own mod?
To keep the weight low I used nice balsa and thought about everything I was adding to the plane. It's amazing how fast those 5 grams here and there add up. Using nice balsa makes a considerable difference as well, grabbing random balsa from the shelf could double the weight of the bare airframe. One of the reasons the servos are in the back (my modification) is because servo extensions are lighter than pushrods. It takes longer to build being this picky, but the end result is satisfying. I think if I was going to do it again I could probably get it down to 45 oz.

The new fuse is somewhat smaller but mostly it's just much easier to build. There isn't as much torsional rigidity in the front so I'd add a small former to keep everythign square. I need to take a closer look at the behavior in knife edge again next time I'm out.

And as for LiPolys, at the 8 cell and 35A current draw I'm using they would work great. But they certainly aren't necessary to have a blast with the plane.
May 26, 2003, 05:33 AM
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I have a bubbles wing & tail hanging up in the shed. Would they still fit to the new design fuselage? ( the original fuselage did not survive its maiden flight).

I have not rebuilt it due to the construction being a bit time consuming, but that new fuselage looks a bit simpler.

Maybe its time to wipe the dust off it.

Heather
May 26, 2003, 09:06 AM
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SheldonYoung's Avatar
As it happens, the tail and wing are the same as the previous version so you're already half done. The new fuse is much easier to build and takes about 1/5 the effort of the previous version.
May 26, 2003, 03:21 PM
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Thanks Sheldon,

Looks like I need to print out those plans..


Heather
May 26, 2003, 03:33 PM
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SheldonYoung's Avatar
Yes, yes you do.
May 26, 2003, 03:36 PM
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Hi Sheldon
I printed the 95% version a few weeks ago and have almost completed the a/c. I have some fuselage details to complete and it is coming out at about 41+ oz. The span is 48", and I am fitting a Mega 16/15/3, geared 3.1:1 with 11x7 prop and 8 1700CP cells.
I have had 240 watts and 26amps on the wattmeter at full chat.
This is my first aileron plane since I came back into the sport after about 15 years. I will be taking it very easy for a while until I get back into the swing again. A very nice design and easy to build.
I hope it flies well with a newby at the controls.
John
May 26, 2003, 03:39 PM
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It should fly well, just keep the control throws dialed down. I look forward to hearing how it goes.


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