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May 05, 2009, 03:37 PM
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Build Log

F-84G Thunderjet 70mm scratchbuilt


Hej guys!

Thought I'd share a few pics of my first ducted fan project in the works now!
After long consideration I decided to go for the Thunderjet. It's a plane you dont see around often, and it offers a lot of nice advantages constructionwise..and of course also some challenges! We'll see when time comes..;-)

Some specs:
Scale: 1/13
wingspan: 850mm
length: 900mm
MTOW: 950g (well lets hope that !)

Drivetrain: WeMoTec Mini 70mm with Mega 16/15/2 on 3s
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May 05, 2009, 03:49 PM
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Some construction features: Wings will be styrofoam cores sheeted with balsa skins. Fuselage is a stressed-skin type with plywood frames, balsa stringers and balsa skin. Stabs are simple balsa.

For the wing airfoil I chose the MH-43 blown up to 10% rel thickness.
May 05, 2009, 05:18 PM
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trondb's Avatar
Where´s the pics ? We need to see you know .Intersting project
May 06, 2009, 04:40 PM
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Ok so here we go:

As usual I bought about twice as much wood as I would need eventually..
First to the fuse. It's built in two halves. First the frames are cut from plywood, then fixed on the plan and connected via the stringers.

Note that the center fuse frame has some extensions which will match with pockets in the wings. necessary to give some bending support to those wings because they cannot go through the fuse due to the duct.
May 06, 2009, 05:00 PM
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Stringer by stringer the fuselage took its shape. Stringers are 4x4mm balsa and their main job is to keep the skin from buckling.
The stringers gave me a bit a hard time on the frontmost frame. Because of the strong curvature of the stringers, they tended to not stick to the frame. Problem was quickly solved with a lot of patience..

2.5mm balsa skin was applied in stripes of approx. 10mm width. In the past I found it to work best if two or three adjacent stripes are applied together. Then a next set of three applied at some distance, and finally the gaps filled with some trimmed stripe segments.

Result is a light yet strong fuselage half in any desired shape. And its quick and big fun to build
May 06, 2009, 06:54 PM
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Larry Dudeck's Avatar
Interesting method of sheeting the fuse

What is the proposed wing area?

Drivetrain: WeMoTec Mini 70mm with Mega 16/15/2 on 3s
Can't go wrong with this choice
May 07, 2009, 12:09 AM
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Ed Waldrep's Avatar
Quote:
Originally Posted by Larry Dudeck
Interesting method of sheeting the fuse
That's the way I intended the the T-33 to be built Strip planking is laborious but it gives the model that natural, gradual flowing curve, at least in the longways direction. Getting the planks smoothed out is another story (starring mr. sanding block and sanding sponge).

This one looks great!
May 07, 2009, 12:36 PM
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Will's Avatar
I've done two F84E Thunderjets. They fly very nicely, very benign stall characteristics and a nice axial roll. They are very similar in size to yours here are the details:

F84E Thunderjet (one with yellow and black chevrons on fin):
Span: 42 ¾" (1086mm) with tip tanks
Length: 40 ¾" (1035mm)
Motor: Kontronik Fun400-36
Fan: WeMoTec Mini Fan 480
Battery: 11 x GP2200 NiMH
Radio: JR 35Mhz PCM
Weight: 55.7oz (1578g)


F84E Thunderjet (one with strips on fin):
Span: 42 ¾" (1086mm) with tip tanks
Length: 40 ¾" (1035mm)
Motor: Medusa MR-028-56-2800 with heatsink
Fan: Velocity-rc WM400
Battery: 6 x A123 (Li-FePO4)
Radio: JR 35Mhz PCM
Weight: 59oz (1671g)

Both feature GRP fuselages and tip tanks, balsa skinned foam wings on one and built up balsa wings on the other.
Good luck with yours.

Will
May 07, 2009, 02:02 PM
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Will, thats two wonderful thunderjets! They really look scale, and its nice to know that plane has positive flying characteristics..
Seeing you all up weight makes me a bit doubt on my target weight. Trying to save every gram.. the air will tell the truth!

@Ed, agree on that one, to have a smooth circumferential surface takes some sanding, always in fear that the skin might go too thin

@Larry, wing area as measured by FAI rules (ie including projected wing inside fuse plus horizontal stab area) is 0.168m2, resulting in a wing loading of 5.5kg/m2. If the weight goes up to 1.1kg then im still at 6.5kg/m2.

Any ideas/experience how much thrust loss results from such a long duct?

andy
Last edited by Dotcom; May 20, 2009 at 05:50 PM.
May 07, 2009, 03:35 PM
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trondb's Avatar
Will , your Thunderjets looks fanatstic Really scale looking
Did you make your own GRP fuse/mouldings. I would really get one !

Trond
May 07, 2009, 06:42 PM
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Larry Dudeck's Avatar
Dotcom,

I built a T33 from Ed Waldrep's plans which used your proposed power system. My T33 developed 465 watts using a 3Sx4400 TP LiPo. I measured the static thrust at 22.4 oz using a digital scale.

Some time ago I came across a 2nd order polynomic expression used to estimate the static thrust of the MF480 as a function of power. The expression is

Thrust(gms)=(-.0006*Watt^2)+(2.099*watts)-1.2284. I wish I could take credit for this but I can't. No idea who the author is. The expression works very well for EDF's that are exernally mounted like on my TBJ (Avatar) or an A10

Using the above expression at 465 watts and converting to oz of thrust, I calculate 29 oz. of thrust while measuring 22.4 oz. So the duct losses are (1-(22.4/29)) or 23%. But the T33 has bifurcated inlets so the losses might be a bit higher than a single, straight in/out.
May 09, 2009, 05:08 PM
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The duct on the Thunderjet is 900mm long. It has a basic diameter of 69mm. Towards the intake the duct narrows down conically to 65 mm. Behind the fan the duct continues with 69mm and narrows down to 62mm at the nozzle. The fan is 650mm from the intake due to cg considerations.

I am also expecting duct losses around 25% for this chimney. Plus there will be some imperfections in the duct surface.

Sorry I cant serve proper duct pictures because I was to eager to get the fuse halves closed to get those pics done

For the duct I used heavy black paper (approx 120g/m2). The paper was cut into proper pieces which then were glued into the fuse halves before they were closed. In the end I had one fuse half with the complete duct in it and the other half was more or less matching. Was quite a tricky method, not so sure wheter I can recommend it..

greets andy
Last edited by Dotcom; May 09, 2009 at 05:14 PM.
May 09, 2009, 05:25 PM
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Here you see the soon-will-be intake before the ducts were glued in. Skin surface will look much better after sanding and filling
May 09, 2009, 05:57 PM
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Larry Dudeck's Avatar
"...For the duct I used heavy black paper (approx 120g/m2)..."

Ed Waldrep and I are much more sophisticated in our choice of duct material. We use the heavy paper used to make "Krispy Kreme" donut boxes
May 09, 2009, 06:06 PM
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:-)))))))


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