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Old Mar 13, 2014, 04:44 AM
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Khan's Avatar
France, Aquitaine, Uzein
Joined Sep 2013
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Build Log
500mm wingspan Pilatus PC-21

Hi guys !

So I'm commited to design (decision to build it will come later, after I'm sure the design is good) my own Pilatus PC-21 in a 1/18 scale, which means a 505mm wingspan.



A word about the PC-21: I'm fond of that airplane. A sleek, beautiful goddess of speed, swiss precision and cleverness in engineering. In all my love, I made a LEGO Technic model of it, that Pilatus put in their internal newspaper. And guess what the scale of that LEGO model is... 1/18 !

So, what I'm trying to design:
- Small but cute airplane
- Easy to intermediate handling characteristics (I already fly an E-flite UMX Sbach)
- 5-6 min flight on the 240mAh 2s batteries I already own for my Sbach

How I'm planning to do it:
- Calculate basic wing parameters with the SPP Method (french)
- Calculate basic powerplant parameters with Geeby22's "Moteur_calc" (french)
- Design the airframe on CATIA V5 CAD software. It's not dedicated to RC airplanes, but since we build real airplanes with it, I figure it'll do good.
- Integrate all equipment to the DMU (Digital Mock-Up) and check airframe weight, weight ditribution, CG, etc..

About the design of the airframe :
I plan to have it completely CNC Milled from EPP foam (Any thoughts on that ?). I don't have a CNC mill, and I don't know where I can do that yet, but I'm confident I can find someone through the local RC shop or club.

So let's get started !
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Last edited by Khan; Mar 13, 2014 at 05:45 AM.
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Old Mar 13, 2014, 01:33 PM
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Nice project. I like it to
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Old Mar 14, 2014, 05:19 AM
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OK so here are the first calculation results

Wing area: 3.53 dmē (54.7sq. inch)
Flying weight (preliminary): 125g (0.275lb)

I chose a NACA 2412 wing profile with a 3° mounting angle at the root, washed out to 1.5° at the wingtip.
Cruise speed would be 10.5 m/s (23.5mph) and stall speed would be 7.3 m/3 (16.3mph)
Attached is a Excel sheet where I divided a wing in 10 slices (right part of the sheet) and calculated lift for each of them, at cruise incidence and stall incidence. Lift coefficients were taken from Airfoil Tools with the right Re number for the speed.
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Old Mar 14, 2014, 05:33 AM
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Now it gets (visually at least) appealing

I managed to find a workable 3D CAD file from GrabCAD and imported it successfully into CATIA (1st attachment)

I fiddled with it a little and it started to look good (2nd attachment). However what I need from this file is only the fuselage. I'll make the wing myself with the NACA 2412 profile and the model building plan from Pilatus.
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Old Mar 14, 2014, 06:16 AM
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at cruise incidence and stall incidence. Lift coefficients were taken from Airfoil Tools with the right Re number for the speed.
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Old Mar 17, 2014, 03:52 AM
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OK so now I have a wing (attachment 1)
I'm wondering whether to design flaps for it or not. I'll have to calculate if the added lift offsets the increased weight. 1 or 2 more servos, bigger Rx, a little hardware... not sure !

I also worked a bit on the fuselage (attachment 2). The problem is that the engine cowling and intake part doesn't fit correctly on the fuselage, I have to re-draw it.
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Old Mar 20, 2014, 04:15 AM
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OK now I have a half-fuselage and a wing with improved wingtip and karman.
I decided to not include flaps, for the sake of simplicity.
However I diminished the scale of the fuselage to 1/20 while retaining the same 1/18 wing. This way I can lower the flying weight to 100g and the stall speed to 6.5 m/s (14.5mph)

So now shape work is almost finished, I'll soon move to integrating equipment.
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Old Apr 07, 2014, 04:13 AM
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I have worked a lot on the CAD design, and it's almost finished, it just lacks a few elements like control rods, ESC and minor things like screws and stickers for the rudder (I'm having difficulties importing vectors from Illustrator, some letters don't show).

The prop will be driven by a 1312 3000 kV Brushless and 6A ESC, that will run on 2s 260mAh LiPos
Receiver will be a flight-stabilizer one, as this small plane is meant to fly outdoors.
I still have to choose between the OrangeRx or the LemonRX (cheaper but servo pins don't match with the HK5330 servos I had in mind).
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Old Apr 07, 2014, 05:45 AM
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France, Centre, Romorantin-Lanthenay
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Great! The hatch can be the canopy...
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Old Apr 08, 2014, 07:01 PM
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Great looking plane and an ambitious project to be sure. I love the 5 blade folder. Good luck. I'll be watching this one closely.
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Old Apr 09, 2014, 02:40 PM
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I wonder if doubling the size (1/9 scale) and crunching all the numbers based on that would lead to better/slower stall speed, etc.
At half a meter in wing-span, relatively high aspect ratio (at least from the pics it seems so) and fairly high wing loading, you might be looking at a pretty challenging airplane to fly.
And a meter span wouldn't be all that huge anyway - still fit into a typical car.
Warbirds also benefit greatly (in terms of handling) from scaling up for the same reasons.
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Old Apr 10, 2014, 04:52 AM
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Thanks for that wise comment.
Actually you have synthesized my fears about the design. All along I knew it would be tricky and so I kept on optimizing things in order to facilitate flight.

With the CAD Model almost finished, I now know that the flying weight can be a little under 90g, thus giving a wing loading of 25.5 g/dmē (8,35 oz/sq.in)
And I count on the flight stabilizer to give the plane a "like bigger" feeling, as it is the case with my UMX Sbach.
And IF i can find a stabilizer able to do flaperon (gotta ask LemonRX for that), then it will be all good I think.

I know It would be wiser to choose a larger scale, but let's face it : there already are a few 850-1000mm Pilatuses (Pilati ?) PC-21s kits or plans, so I'd have no design to do (which is what l wanted to do in the first place).
And there's the WAF : small is always cuter than big

Anyway, I can find a little solace knowing that HK/Durafly designed pretty much the same type of plane with their 400mm RareBear and 440mm Critical Mass. So I know planes of this size are manageable.

What I think will be my next course of action will be to make a Phoenix model of it and -roughly- see how it flies.
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Old Yesterday, 10:37 AM
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Aaaand ... finished !

The CAD design is complete now, with stickers, hatch, exhausts, control rods, and the weight is still under 90g, with CG spot on the wing's thrust point.
Now I have to import the 3D model into Phoenix Builder, and for that I have yet to find a way.
I'm able to export the Catia assembly to a .STL file, open that with Blender and convert it to an .OBJ file, but when importing it into Phoenix Builder, nothing is shown...
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