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Jul 20, 2020, 11:11 AM
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Build Log

A-10 T-bolt 2x65mm Oldskool Build

Hello all,

usually I hang out more in the thermal and FAI high performance sections. But being in a wood building frenzy at the moment, I decided to pick up this project and try to finish it within one year from now.

The topic is (yet another) A-10. There are some fantastic build threads on here, and the type is widely available in ARF form as well. I just thought I would share my approach, as maybe some of my steps could be interesting to the wider public.

In my day job I spend lots of time behind the screen, including CAD and CAM work. And I find it truly impressive how these tools have made their way into the hobby, and I admire the work done by many scratchbuilders to come up with super nice and detailed designs which turn into amazing models.

Usually when I build, I do it for some recreation to be off the screen and desk.

Therefore, these are the 'oldskool' rules of this build project:

- Minimum use of computer. Printing airfoils is OK, making a mass and balance sheet is OK and online research is OK and that is it.

- No CAD, no CAM. No milling. No lasering. No 3d Printing. All built from hand with some nice tools like (electric) saw and knife. And sanding.

- Primary material is to be wood. Foam for some form pieces is OK. Minimum use of composites, carbon, glass etc.

A couple of years back I picked up two cheap Haoye 65mm 6 blade fans at the local hobby shop when it was still around. I quickly decided they would need to go into an A-10 as I always wanted to fly the Warthog. I started to build a nacelle assembly in order to test the fans, and purchased two turnigy motors to spin them. But disappointment followed: The motors had obviously too low KV of 2750/V, not enough for my standard 3s batteries. But worse, the fans made a terrible noise starting already at low throttle. They seemed badly out of balance. To make matters worse, the fan centerpiece is very long and puts the fan disc far to the front of the motor, exacerbating any out of balance problem. At the time this seemed a show stopper and the pieces were soon stashed away, attention quickly moving to other projects.

Until recently, when during the lockdown I decided to finish all open projects or clear them out. And this A-10 is too tempting to let go. So I decided to approach the project as follows:

Make the fans work. Shorten the centerpieces and get higher KV motors. If this does not work, get better rotors like the WeMoTech Mini cut down to 65mm to fit the housing.

In parallel:
Design the airframe and start cutting.

So stay tuned for some upcoming posts on the general design of the plane, and powerplant progress!

Blue skies,
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Jul 20, 2020, 11:12 AM
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Jul 20, 2020, 11:13 AM
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Step 1 - General Concept and Design

Step 1 - General Concept and Design

My scratchbuilt planes are in categories of standard batteries I defined for myself years ago. These categories roughly double in capacity for each step. Further assuming a power to weight of about 400W/kg, which gives reasonable performance for all but wild 3d flights, and ca 200sec of full throttle time, the battery weight should be ca 20% of the all up weight, resulting in the following categories:

Battery - AUW (400W/kg)
3s850 - 425g
3s1800 - 750g
3s3300 - 1375g
6s3300 - 2500g
12s3300 - 5000g

As next step, the motor size (ca 10% of the AUW) and motor KV and propeller/fan are chosen.

The 65mm Haoye match in size to a scale 1/16 A-10 Thunderbolt (CF34 fan diameter ca 1070mm). Such a model comes in at 1100mm span and about 1000mm length. This model would reasonably fit into the 3s3300 category.

Trying to achieve a weight of 1300g and slightly cheating on the span and wing chord for a total FAI area of 0.253m2, this would result in a wing loading of just above 5kg/m2, sounds OK for a slow jet as the A-10.

The required power would be in the order of 250W per fan, fairly low for a 65mm size blower. Motors finally should be in the 75g range (2x 75g is 11% of 1300g AUW).

With these considerations, the general specifications of the model are as follows:

Fairchild A-10 Thunderbolt
Scale 1/16
Span: 1170mm (slightly increased vs scale span)
Length: 1000mm
Total area: 0.253m2
All up weight: 1300g
Wing loading: 5.13kg/m2
Battery: 3s3300mAh
2x 65mm EDF

Next up will be some more detailed mass considerations, as well as starting the drawing process.

Blue skies,
Jul 22, 2020, 03:30 AM
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Step 2 - Design Part 2

Step 2 - Design Part 2

Let's look in detail into the mass sheet next.

Out of the overall mass budget, the following distribution follows:

Wing: 200g
Fuselage: 250g
Nacelle Assembly with fans: 210g (exists already)
Tail: 50g
Motors (2pcs): 150g
ESC (2pcs): 50g
3s3300: 275g
Wheel and struts: 25g
Electronics: 60g
Wiring and misc: 30g
AUW: total 1300g

From previous projects, a 1.5m Pilatus Porter wing was built at 200g and a recent 0.8m Acro wing at 80g, so the 200g for this 1.17m wing look reasonable as it also includes the landing gear mounts.

For the fuselage on the other hand, building a 1m long fuse at just 250g will prove a challenge.

Construction will be classic rib and spar for the wings. D-box sheeted with balsa, rear section open and foil covered. No flaps.

The fuselage is a box of ply formers and balsa skin, with ply reinforcements for wing attachment and battery compartment.

For the landing gear, a particular design is chosen. As the mass budget above does not reasonably allow for retracts, there will simply be a fixed landing gear with the main wheels in the retracted position as found on the original. Then a tailwheel is added underneath the tail.

To start off, a scale drawing at 1/5 size of the final model is created. For this a 3side view is transferred to the drawing using pencil and calculator. On this drawing all components, formers, ribs etc are located.

More details will be added to the drawing, such as formers crossection and landing gear mounts. Then those parts are scaled up to full size in order to be cut.

Blue skies,
Jul 24, 2020, 11:25 AM
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Ed Waldrep's Avatar
You could run two of your 3S packs wired in series to create a 6S pack, but I don't know what those fans would pull. I ran 2800 kv motors in Wemotec MF480 70mm fans and they pulled, for a single motor, about 31 amps on 4S and 38 amps on 5S. On two motors in parallel the amp draw was 85% more (because of voltage drop, these are numbers from 15 years ago, early lipos). Running two motors in parallel on 6S might work, it depends on the pack size/weight you can carry. Finding just the right motor is the key, it depends on what pack configuration you want of course, 3s and higher kv per motor, or 6s with lower kv per motor.
Jul 25, 2020, 02:49 AM
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Hello Ed, thanks for chiming in!
Going 6s could of course be an option. But my 2750kv motors are rated 5s max, and 6s would yield 2750/V x 6 x 3.7V x 80% = 49000rpm which sounds like a very high power setup. Of course this estimate is before any measurements of the Haoye fans. Further, those motors are long and heavy with 113g.

With the goal of 250W per fan, I would aim at 22.5A per motor at 3s or half that current at 6s.

I also have laying around two Track Star 380 with a KV of 3200/V and 86g weight. At 3s this could give something equivalent to your reported result of 31A at 4s with a 2800KV motor. As a further option, the same motor series is also available at 3800/V.

Once I have modified the fan centerpiece I will run a test series with both motors for current, voltage and rpm logging. Thrust and efflux velocity would be nice as well but require a more sophisticated setup.

Aug 06, 2020, 04:04 PM
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A small update.

The nacelle assembly was started in summer 2017 before the project was shelved for 3 years. Plan is now to finish this assembly, and use it for testing of the fans while the rest of the plane is drawn and built.

The fans themselves need to be modified in that the centerpieces are too long, putting the blades too far in front of the motors. They also need to be balanced before running them again.

Apart from that, I am contemplating making a dedicated test stand for EDF to measure also the thrust.

Blue skies,
Aug 17, 2020, 01:12 PM
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Hello all,

I think I found the root cause of the noisy, unbalanced running characteristics of the fans.
The aluminium adapter are total duds. When mounted to the shafts by themselves, I measured them to run out of the shaft axis by as much as 0.5mm. No way to make these useful. I will have to source new adapters. What I found so far is all 6mm outer dia, when the original ones are 5mm, so I will need to re-drill the fans.

Meanwhile the wing plans take shape. Some sheets of office paper were taped together and the wing planform and ribs location transferred in full scale. This is also when all the small parts like wing joiners, gear mounts etc are positioned before drawing them separately for cutting.

Happy amps,
Aug 21, 2020, 09:51 AM
Classic jets rule
AIR SALLY's Avatar
Nice work enjoying your old school and cool build
Sep 07, 2020, 02:36 PM
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Cheers Air Sally ;-)

Just finalized the aerodynamic design. I took a fairly simple approach as follows:

As this A-10 is supposed to be a rather slow and light but agile jet for low-level action, I went for a thick airfoil section in the order of 15%. I then chose two velocities 'slow' and 'fast' at 10m/s and 30m/s respectively. With that and the remaining airplane specification of area and weight, the resulting lift coefficient and Renolds Number are calculated where the airfoil candidates are to be compared.

The full size A-10 has a NACA6716 which is highly cambered (6%) at a very far aft camber maximum (70%) and 16% thickness. Such an airfoils is unusual for model airplanes, at least I never tried it nor heard of it's use. The aft camber results in continuously high lift moment through the angle of attack range while a 'normal' airfoil relaxes the lift moment at higher AoA. Not necessarily a bad thing but I didn't dare.

A comparable but more benign choice could be the S4233 which maintains an aft camber maximum but less overall camber, and I added the standard N2414 to the comparison. The NACA2414 has the advantage of being easier to build as it does not have the concave bottom line. The polars are somewhat comparable (in terms max CL) as I am in no way looking for the last optimum of lift to drag or low overall drag but simply good flight characteristics. With a max CL of 1.3 both would allow a minimum speed of around 8.5m/s.

The NACA2414 has a bit less moment coefficient but to be honest I settled for it mainly for the ease of building.

When mounted at a incidence of zero degrees it yields a lift coefficient corresponding to 20m/2 which sounds OK to fly around at with a neutral stab. With the engines mounted up high and blowing over the stab, the incidence question of the stab will be the biggest unknown of this build anyway.

Next the airfoils can be printed and the wing plans finalized ready to go cutting.

Cheers, Andy
Sep 16, 2020, 11:41 AM
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Dirty Dee's Avatar
Great progress.
Sep 29, 2020, 03:28 PM
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Not much posting recently but some slow progress on the center panel. Straight assembly of all the pieces from the bottom up, and in the end the panel comes out at 82g including the center skin and gear mounts an joiners.

I'm not decided yet on the skin over the rear part of the wing, originally I intended to just cover the center part under the fuselage with balsa and leave the rest open for foil covering. But I reckon I'd like to 'feel' the wing torsional stiffness with the outer panels on before making the call.

Also some details on the joint are still missing, as I will include another thin spruce joiner in the rear section. Then there is the landing gear fairing.

Meanwhile the plans for the outer panels, including the servo mount and aileron details are finalized.

Blue skies,

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