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Nutball

Foam board quick build

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Nigel R05/01/2019 15:55:41
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Well this promises to be the shortest build thread ever.

I built a nutball from foamboard. This is the stuff hobbycraft sell, four sheets for a tenner.
Nigel R05/01/2019 15:56:24
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20190105_103938.jpg

Nigel R05/01/2019 15:59:46
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It is 23 inches diameter. Weight 400g exactly.

Two hobbyking 9g servos. Lemon rx. 2822 motor 1450kv on a 7x4 prop.

Nigel R05/01/2019 16:48:54
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2440 forum posts
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Today was calm. Ideal for a test flight.

Pleased to report the rumours are true. It flies superbly.

My six year old did circuits with it for three flights. I chucked it around on a fourth. Great fun. Hovers brilliantly and turns as tight as you like.

I did some laps around a big bramble bush and nearly caught it by hand when landing.

Great stuff.

Took three hours to make it. About a hour of foam and the rest was motor mount and gear install

Dave Hess05/01/2019 16:57:48
301 forum posts
18 photos

There are instructions and free plans here if anybody else wants to build one:

**LINK**

loads of other free plans for nice-flying foamboard planes here:

https://forum.flitetest.com/index.php?threads/sp0nz-plans-index.17136/

Edited By Dave Hess on 05/01/2019 17:02:50

David Mellor05/01/2019 17:13:33
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1254 forum posts
611 photos

Nutballs are huge fun to fly......

The original design is by 'goldguy' over on RCGroups, here:-

**LINK**

You can gauge its popularity by the size of the thread - it is one of the biggest on RCGroups. The original design (on page 2 of the RCGroups) is the one to go for.

David Mellor05/01/2019 17:21:14
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1254 forum posts
611 photos

One strange characteristic they have is that fly at quite high alpha, yet won't stall.

Here's another one I made earlier.....

Nigel R05/01/2019 18:44:05
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2440 forum posts
401 photos
It was one of your postings on increasing the strength of Depron that first led me to the design, dave m.

Dave h useful link thank you.

Next foamboard I'm thinking of doing a dekan delta which is a halfway between pizza box and regular delta.

Edited By Nigel R on 05/01/2019 18:44:33

Percy Verance05/01/2019 20:03:21
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7511 forum posts
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This model kind of reminds me of my old 70's/early 80's Mick Reeves Disco flying saucer, from 1 inch thick polystyrene. It was a completely flat 24 inch diameter disc with no tip dihedral. Nor did it have an undercarriage. Boy did I have some fun with that way back when........OS .30 power.

Used a sliding tray servo mixer for elevon control. No transmitter mixing back then..........

 

 

Edited By Percy Verance on 05/01/2019 20:20:14

Piers Bowlan06/01/2019 04:54:43
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1676 forum posts
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This is an old video from nine years ago of Capricorn, the flying 'W'. It has been on my build list ever since (I know, I know, - another one!). These low aspect ratio aircraft do seem to fly well, I suppose very light weight and loads of power has it's benefits.

There have been several full sized examples of these low aspect ratio aircraft like the Arup S2 and S3, the Vought V173 flying flapjack, Eshelman Flying Flounder and more recently Milt Hatfields 'Little Bird'.

Edited By Piers Bowlan on 06/01/2019 05:20:21

Piers Bowlan06/01/2019 05:38:27
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In case anyone is interested, the plans for the Capricorn is in the second post here.

Percy Verance06/01/2019 09:12:50
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I'd fully agree re: low aspect ratio craft Piers. My Disco had some quite remarkable handling chractristics. It was completely unstallable for one.....and a fairly rapid roll rate was another.

Dave Hess06/01/2019 11:13:36
301 forum posts
18 photos

You can buy Flitetest waterproof paper covered foamboard in the UK for £2.50 per sheet. It's an excellent material for building planes like the Nutball - and anything else for that matter. Here's my Hots 40 coming on nicely. A plane like this took me only a few hours to make.pre finish.jpg

Nigel R06/01/2019 14:55:23
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2440 forum posts
401 photos
Nice work on the Hots Dave. Foam board is ideal for angular shapes. Is it a proper airfoil?
David Mellor06/01/2019 16:12:03
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1254 forum posts
611 photos
Posted by Percy Verance on 06/01/2019 09:12:50:

I'd fully agree re: low aspect ratio craft Piers. My Disco had some quite remarkable handling chractristics. It was completely unstallable for one.....and a fairly rapid roll rate was another.

If anyone is interested in the variations in round, disc-shaped or other flat LAR (Low Aspect Ratio) there is a review thread here of ...... quite a few designs.

**LINK**

Dave

Dave Hess06/01/2019 16:38:28
301 forum posts
18 photos
Posted by Nigel R on 06/01/2019 14:55:23:
Nice work on the Hots Dave. Foam board is ideal for angular shapes. Is it a proper airfoil?

Yes, much easier than balsa. The wing section is symmetric. I made a test piece that you can see in front of my toolbox back left.

Nigel R07/01/2019 09:39:17
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2440 forum posts
401 photos

Dave M - nice thread. Some good links and "shape" design layouts in there!

The stall characteristics do seem to be the thing that sets the very low aspect ratio craft apart from conventional. I guess it is a side effect of the amount of the wing which operates in the tip vortices.

Nigel R07/01/2019 09:51:41
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2440 forum posts
401 photos

On a side note, if a square has an aspect ratio of 1:1, then a circular wing like this has an effective aspect ratio of 1.3:1.

A little higher than my recent flying carpet effort, which had as aspect ratio of 0.45:1!

The nutball loops much better than the carpet. Other than that it seems somewhat similar in its close-to or post- stall behaviour.

It's also worth noting that the U-channel spine that I used on the underside of this nutball worked very well - I can stash the lipo just in front of the wing in the trough that is formed. And sticking on the motor mount was very easy. Provides something handy to grab when launching, too. I realise this might be a bit of a schoolboy observation to all the foamboard pros out there.

David Mellor07/01/2019 16:23:46
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1254 forum posts
611 photos

 

Posted by Nigel R on 07/01/2019 09:39:17:

Dave M - nice thread. Some good links and "shape" design layouts in there!

The stall characteristics do seem to be the thing that sets the very low aspect ratio craft apart from conventional. I guess it is a side effect of the amount of the wing which operates in the tip vortices.

 

I'm pleased you like the thread, Nigel - it was fun working on it.

I think the evidence - such as there is - supports your thinking about the amount of wing which operates in the tip vortices.

 

I've done a bit of work on visualising the airflow over Nutball wings. I've built some at 48" diameter (and one at 60" diameter), and instrumented them with a video camera on a pentapod (5 legs instead of 3 on a tripod) to observe wool-tuft movement at different speeds and bank angles.

This is the basic arrangement, with a Mobius camera 4 feet above the centre on a 48" diameter Nutball, looking straight down. Using wool-tufts is an old method of monitoring fluid streamlines, particularly on aircraft wings to visualise stall conditions.

Here they are sitting still before take-off.

p1020064.jpg

 

The Nutball wing comprises two outer dihedral panels and an inner main panel.

p1020065.jpg

 

At speed, in level flight, the air flow is essentially all turbulent aligned longitudinally with some span wise flow from the dihedral tip panels.

p1020067.jpg

 

But as the Nutball slows down (and alpha increases), the span-wise airflow tends to increase off the dihedral panel, whilst maintaining longitudinal flow over the main panel. This is one reason the elevator (which is colossal) remains effective at very low speed/high alpha - it has lots of prop wash over it.

p1020066.jpg

 

I have tried to upload some of the clearest the screen shots from the videos that illustrate the sketches, but the "album software" doesn't accept them.

So I've posted a very short clip of mid-to-low speed flight where you can see the airflow over the Nutball wing. Not very clear, but it gives the general idea...

 

 

 

Edited By David Mellor on 07/01/2019 16:29:43

David Mellor07/01/2019 16:50:04
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1254 forum posts
611 photos

 

Incidentally, if you accept the idea that wing tip vortical flow is significant in Nutballs, then altering the planform shape slightly from circular to tear-shaped (a bit like a nan-bread shape) does seem to improve low-speed stability of the Nutball. I think the idea being that decreasing to frontal area of tip and increasing the rearward area of tip moves some of the vortex shedding further aft.

 

The optimum shape to do this seems to be what is termed an Inverse-Zimmerman shape (two half-elipses sharing a common axis).  It approximates to a delta (deltoid) shape and has been much studied for military SUA design. 

e.g. see http://edge.rit.edu/edge/OldEDGE/public/Archives/P0206F/mav_prototype.htm

 

 

One test of how much the dihedral angle affects the vortical flow is to make matched pairs of identical Nutballs - some with zero dihedral (i.e. dead-flat) and some with the standard 20 degree dihedral angle.

 

The differences are breathtaking - the dead flat Nutballs are far faster and more aerobatic that the standard dihedralled-Nutballs.  There is no doubt that increasing the dihedral from zero degrees to 20 degrees increases the drag and stabilises the aircraft in slow high alpha flight.

 

The dead-flat Nutballs not only fly far faster, they also handle high windspeed better too.

 

A lot of folk think that ultra-light foamies can't handle the wind. That just ain't the case at all - here are two short clips showing identical weight/power/wingloading Nutballs. The flat one is handling 28 mph winds, but isn't that keen on going slow in high alpha. The standard (dihedral) Nutball is doing what it does best - low, slow high alpha in low wind speeds.

Edited By David Mellor on 07/01/2019 17:06:33

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