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Old Aug 09, 2010, 10:17 AM
Build ALL the models O_O!
WLJayne's Avatar
United Kingdom, England, Nottingham
Joined Oct 2009
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My first scratch build, EP trainer build/advice thread.

Hi folks,

EDIT: The post below is a bit redundant now, I'm building a 3' Cub from the plan in Kelvin Shacklock's book "Aircraft Workshop: How to build models that fly." The book is quite simply excellent.

Yep, I'm finally biting the bullet. Whether it flies or not I'm going to build a plane from scratch. I don't fully understand 90% of the principles, but in my experience there's no better way to learn than to just dive in. I've read around alot, especially the articles at Airfield Models.

So, the mission, should I choose to accept it: build an EP trainer using the gear from my now retired GWS Pico Toger moth. I'm feeling ready to advance to a secondary tariner, but my girlfriend had a flying lesson yesterday and loved every minute so it would be good to build her a trainer to learn on. The death of the moth was not her fault I must add (she was darned good after 20 mins, touch and go circuits? Yes please. Stall recovery? Didn't even break a sweat.), I shorted the battery somehow while packing up and the burn made me drop the model which kamikazed on the ground. I never liked it much anyway, so I thought "stuff it I'll build a better plane."

Next, the specification - or at leat as much of a spec as I can understand.

Purpose
High wing trainer.

Style/Looks
Very loosely based on a Cessna Skywagon.

Construction
Built up balsa, truss fus and constant chord single piece wing. Probably tissue covering.

Powerplant
GWS IPS DX2BB-AXC turning a GWS 10X4.7 prop.

Flight Time
About what the moth managed, 30 mins give or take.

Stability
Self stabilising.

Vertical Performance
Good lift, vertical climbs not needed.

Control response
Smooth and slow, "floaty."

Stall Characteristics
Forgiving, minimized chances of tip stalls.

Control System
Futaba Skysport 4 35mhz. GWS Pico Rx 4ch. 9g servos. Ripmax 7amp LiPo ESC.

Control Surfaces
Full 4ch, Throttle, Rudder, Elevator and Ailerons.

Battery
7.4v 2s LiPo 800Mah.

Landing Gear
Tail dragger, 42mm foam wheels.

Breakdown
Wing removable for transport.

Now for some numbers (calculated here)

Weight RTF: 256gms/ 9oz. (This is a total wild guess, the hardware comes to 128g/ 4.5oz, perhaps someone can make a better guess about the airframe.)
Minimum Airspeed: 5mph
Maximum Airspeed: 16mph
Rate of climb: Slow but steady.

Wing Loading: 17g/sq.dm (5.538 oz/sq.ft)
Wing Area: 15sq.dm (234sq.in)
Aspect Ratio: 6.66:1
Taper Ratio: 0
Wing span: 100cm
Root Chord : 150mm (5.90 in)
Tip Chord "
Dihedral 3
Airfoil (root): 16% Modified flat bottom
Airfoil (tip): " "

I really hope that makes sense to the experienced builders here. I can more or less picture how I want the model to fly and I hope those numbers are on the mark. Please feel free to suggest modifications to them. If no one can find any serious flaws in that spec then I can start the drawings . I'm looking forward to seeing your responses and to getting started.

Will.
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Old Aug 09, 2010, 01:26 PM
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Materials ? Wood or foam ?

BTW 30 minutes power from an 800mAh battery means you're assuming 1.6A average or no more than about 12 Watts...seems a bit low for a 9 oz plane.

Steve
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Old Aug 09, 2010, 01:44 PM
Build ALL the models O_O!
WLJayne's Avatar
United Kingdom, England, Nottingham
Joined Oct 2009
837 Posts
Thanks for the point about construction, I've added it to the spec. It'll be built up balsa with tissue or film covering . I think I've over estimated the weight, to be honest it's difficult to give an estimate with such little experience. I basically took the weight of the hardware and doubled it. It may well be that I can build it lots lighter than that. I should weigh the moth and set that as my target perhaps.
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Old Aug 11, 2010, 08:15 AM
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Since you already own the motor, ESC, and battery, I think you should just build the model and see what you end up for weight. I do think that 16% chord thickness will give you too much drag at your slow speeds and would recommend ~12%.

I'm not a particularly light builder so I'll limit my comment about total weight to my belief that 9oz is optimistic for a wing of that size, a fuselage that has volume and a landing gear. I would guess that a total fly-away weight of 12oz (for a balsa structure with covering) is more realistic. That weight would put the wing loading around 7.4 oz/sq ft - a bit more realistic number for a powered trainer since performance gliders have wing loadings around 5 oz/sq ft.

Alan

Please consider my comments as a challenge to see how light you can build the plane rather than possible discouraging comments. I think it is more important for you to build according to your beliefs and learn from that rather than modifying your beliefs too early.
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Old Aug 11, 2010, 09:00 AM
Build ALL the models O_O!
WLJayne's Avatar
United Kingdom, England, Nottingham
Joined Oct 2009
837 Posts
Hi Alan,

Thanks for the comments , don't worry I'm totally new to the numbers game so I know they may be very different in the end.

In fact I have had something of a change of heart. I bought Kelvin Shacklock's book, and having marvelled at the models within I've decided to try to follow it. Sure, many things will be slightly out of date and I'll work around those but the fundamentals seem very sound. The first "real" project (I'm going to skip the chuck glider bit I think I could handle that!) is a 3ft span balsa Piper Cub. My girlfriend saw the Cub logo and insisted I build it (not that I needed much persuasion.)

So, I will be following Kelvin's plan, but i'm sure I will need help and feedback along the way. Not least because this plan is for rubber power and the conversion to EP may have many pitfalls. I went to my local and bought all the materials I need, except tissue which I forgot. I've sanded of my work bench and laid a nice piece of MDF over it which I will cover with cork tiles. Once that's all done I'll be ready to rock !!

Will.
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Old Aug 11, 2010, 10:30 AM
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Did a little digging and found that it can be done. Check out Pat Tritle's 40 inch J-3 Cub - it might give you some ideals:

Pat Tritle’s 40 inch J-3 build thread
http://www.rcgroups.com/forums/showthread.php?t=1020826

Short kit with plans (ships overseas)
http://www.patscustom-models.com/40%20inch%20J3.htm

Good luck,
Alan
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Old Aug 11, 2010, 10:43 AM
Build ALL the models O_O!
WLJayne's Avatar
United Kingdom, England, Nottingham
Joined Oct 2009
837 Posts
Very nice! I doubt mine will be so elegantly built but it's a good standard to shoot for . I'll prep the plans and clear out the workshop tonight, then I'll start marking and cutting tomorrow.

Will.
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Old Aug 12, 2010, 05:01 AM
Build ALL the models O_O!
WLJayne's Avatar
United Kingdom, England, Nottingham
Joined Oct 2009
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Ok, yesterday was productive. My workshop is (nearly) pristine, the workbench and cork board is set up and the plans are all printed. Tonight I'll stitch the plan tiles together and transfer the cutting templates to the balsa. This is getting exciting!

Will.
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Old Aug 12, 2010, 07:03 AM
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Will,
I looked up the Cub in Kalvin's book last night. It reminds me of the first plane I built (rubber band powered) and the one that got me hooked in this hobby. The outline of individual parts in the parts layout sheets make the build almost like a kit.

Did you use a copy shop to enlarge the plans? I ask because if you did (and the print is made of graphite from a 'Xerox' machine) you might want to try transferring the parts layout sheet directly to the balsa sheet using a hot clothes iron (as he shows in another part of the book). I did not have much luck with the technique and ended up tack-gluing the layout sheet to the balsa and cutting through the sheet.

When you build the wing, I suggest you add turbulator strips (those 1/16th sq strips running the length of the wing in front) as in http://www.rcgroups.com/forums/attac...mentid=2412741 I found the film covering sagged quite a bit between ribs in this area. You can cut slots in the ribs with two hacksaw blades held together when the ribs a stacked with the main 'spar' for sanding as shown in his book.

Alan
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Old Aug 12, 2010, 07:36 AM
Build ALL the models O_O!
WLJayne's Avatar
United Kingdom, England, Nottingham
Joined Oct 2009
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Hi Alan,

I scanned the pages and then enlarged them in photoshop and matched them precisely to a ruler so they are bang on 200%. I printed the cutting templates on my ALPS printer, which uses a wax based ink which means it should transfer perfectly to the wood under heat (or at least that's my theory.) I was thinking of adding that extra strip, for a little extra rigidity and to help the tissue as you suggested. Thanks for the link .

The book is very well laid out, it's not perfect but anyone with a brain can fill in the blanks. I like all the models in there and I would be very proud indeed to build and fly all of them. I especially like the Spirit of St. Louis and the Sea Fury.

Will.
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Old Aug 14, 2010, 08:17 AM
Build ALL the models O_O!
WLJayne's Avatar
United Kingdom, England, Nottingham
Joined Oct 2009
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Ok today's job is to attatch battens to the underside of my build board, my workbench isn't flat enough. I'll get a propper metal one when I have a propper workshop, when my ship comes in etc and so on. Then I'll start cutting.

A question just occured to me, can you thin down dope with cellulose thinners and spray it onto tissue with a spray gun? That would give a nice even coat and it would be great if I could.

On with the batten making...
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Old Aug 14, 2010, 03:15 PM
Build ALL the models O_O!
WLJayne's Avatar
United Kingdom, England, Nottingham
Joined Oct 2009
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Well, mission one was sucessful. However the parts transfer thing didnt work at all. Kelvin neglected to mention in the book that the ironing techniqe doesn't work with an inkjet printer . I'll have to go to the printers in town on monday and get them to do it on their big laser printer. I was tired of stitching tiles together anyway.

Bummer, I was hoping to be cutting happily tonight. Oh well .
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Old Aug 16, 2010, 08:22 AM
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Quote:
Originally Posted by WLJayne View Post
A question just occurred to me, can you thin down dope with cellulose thinners and spray it onto tissue with a spray gun? That would give a nice even coat and it would be great if I could.
Unfortunately, dope is not all the same. Take a look at this thread http://www.rcgroups.com/forums/showthread.php?t=533922 I would suggest you buy dope thinner from the same vendor as you buy the dope.

And yes, you can spray dope with an airbrush if you thin it out: something like 1 part dope to 2 (or 3) parts thinner. Before airbrushing, you will need to brush thinned dope (1 part dope: 2 parts thinner) over the tissue until the pores are sealed. And before covering, you will need to apply dope to the edges of the piece you want to cover before doping the tissue on the piece.

Alan
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Old Aug 16, 2010, 08:26 AM
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Quote:
Originally Posted by WLJayne View Post
I'll have to go to the printers in town on Monday and get them to do it on their big laser printer. I was tired of stitching tiles together anyway.
I hope you don't have any problems with copyrights. I did not notice any copyright waiver statements on the parts sheets.
If they won't copy it, let me suggest that you use use tacky spray on the backside of the parts sheets, stick the sheets to the balsa and cut through the pattern lines. In addition, the heat/dry transfer method did not leave enough graphite on the balsa to be very visible.

Alan
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Old Aug 16, 2010, 08:51 AM
Build ALL the models O_O!
WLJayne's Avatar
United Kingdom, England, Nottingham
Joined Oct 2009
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Hi Al,

Copyright shouldn't be a problem as I'm not selling the plans. The author expects you to photocopy and enlarge the plans anyway, so I don't see that harm in getting a pro to do it . They came out very nicely, and I can get started now.

Thanks for the tip about the dope .
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