An idiot's guide to building a Barnstormer

[Nigel Day]

Thanks Danny.

Erm I don't remember. I think it was something to do with increased flight duration.

It's the Turnigy 3548/4 - 1100 KV one and the ESC was a Plush 60A. A 16 x 6 prop is recommended but I seem to remember seeing somewhere that you liked 12 x 8 or 9?

The battery is big so it'll need to go between F3 and F4. There's plenty of room for the ESC between F2 and 3. That might mean fixing the servos underneath the battery (tray). I'm probably going to fit hatches across the top but I'll need to do a bit of playing with the position of the servos and rails to see if I can get the battery out past them easily. I'll report back here with my cunning plan when I've conceived one.

Edited By Biggles' Elder Brother - Moderator on 28/03/2014 14:51:31

[Biggles' Elder Brother - Moderator]

That's coming along very nicely Nigel - looking good. We all resort to the "kitchen spray" from time to time for one reason or another!

BEB

(PS Slapped wrist! No naughty words "bleeped" out with asterisks allowed!)

[Danny Fenton]

The reason I say this Nigel is you have quoted Pat as using a 900 kv motor on 4 cells, so if you up the cell count you would need to lower the kv. Decrease the cell count you up the kv, It sounded like in your reply you had figured that out?

Yet you are fitting a whopping great 5000 5S probably weighing nearly 650 grams and you have increased rather than decrease your kv. A 5S is around 18.5 volts spinning a 1100 kv motor you will be looking at potentially 20,000 rpm which will end in tears and possibly injury!

On a 5S pack you are looking at motors from 500 -700 kv typically.

I think you need to go back to what was said originally copy somebody elses set up, its easier in the initial stages.

Can you check your setup before you fire it up?

Cheers

Danny

[Chris Bott - Moderator]

Danny's spot on Nigel.

I have just, this afternoon, tested a 600KV (almost half of yours) motor on a 5S. LiPo.

Using a 13'6 prop I got 787 Watts. This was at about 42 Amps.

I see an 1100Kv motor as a motor for a 3S LiPo.

[cymaz]

Keep up the good work Nigel.

[Nigel Day]

Whoops, sorry BEB, forgot myself there.

And there's an identical smiley coming up shortly.

You can see know why I entitled this blog as I did!

I really should have worked through the calculations (and guidelines) for the electric setup - or just stuck to the suggestions kindly provided. Ah well, I'll put the 5000 lipo in suspended animation for the right time and get myself a smaller one. Looking on the bright side, I might then be able to get one a bit more compact.

So a big thanks to Danny and Chris for looking over my shoulder and stopping me from doing something stupid. I had figured I could always throttle well back if it was overpowered but there's always the possibility that I might forget, get carried away or simply switch on/connect up with a lot of throttle set.

Before I go away and attempt to answer my own question, If I don't want to do myself a mischief with too much power but I'd like to extend the flying time as long as possible, what kind of lipo would be best?

Thanks once more.

 

Edited By Nigel Day on 28/03/2014 15:44:56

[Nigel Day]

Oops, and I meant a 12x6 prop (not 16 x 6) in my earlier post. Does that make a significant difference?

Edited By Nigel Day on 28/03/2014 15:47:45

[kc]

Buying another motor with a lower Kv might be cheaper than sidelining the 5S Lipo. So keep the 5S if it's a large capacity. What milliamps is it?

( I think Danny mistook my comments reminding about Kv for a reply from Nigel. My point was it's necessary to use the complete combination recommended - volts ,Kv, ESC size and prop size otherwise you are on your own!)

[Danny Fenton]

Hi Nigel, you were quite right in your assumption that the bigger mAh pack would give better duration. So a 5000mAh will give twice the duration of a 2500mAh you have to make a judgement call on weight versus duration.

I am sorry I haven't asked but assuming this is a 60" model and will end up around 6lbs? So using the rough guide of 100 watt to the lb, you will need roughly 600 watts. Now you need to look at a motor that will give that amount of power for a given prop size, and the current stay within the design parameters of the motor.

If we take the Turnigy 4240 640kv motor it will produce around 630 watts on a 14 x 7 prop when fed with your 5 cell pack. It will draw around 35A this will last you 8 and a half minutes at full throttle. That may not sound a lot you may think, but actually i would be very surprised if you got less than twenty minutes. The reason is we don't fly at full throttle, often only 1/3 to half is needed. Plus the propellor will unload in flight and mean you use even less.

Personally I would look at around 3000mAh 5S for your model, and a motor like the 4240 or 4258 at around 650 - 700 kV.

I am using a similar setup in my BiStormer 2300mAh a 600kv motor and a 14 x 7 prop. The cell count on mine is 6S because an A123 has a lower voltage per cell than a lipo. A 6 cell A123 is roughly equivalent to a 5 cell lipo.

Or copy somebody elses set up

Hope that helps?

Cheers

Danny

[Nigel Day]

Thanks again Danny. Yes, I think it's predicted to be about 6lbs - hopefully a bit less.

Hi kc. It's a 5000.

[kc]

5000 mah is what Danny calculated to last for 8.5 minutes full throttle / 20 minute flight.
I suppose the other thing to ask is what Danny & Chris would advise for the Chipmunk( if you build an electric one ) You dont want to stock another size of Lipo if you can avoid it as you need several for a days flying. ( unless you are rich.....)
So a setup which would cover 2 or more models would be nice. Of course consider whether the extra weight could be useful to achieve correct CG and whether there would be space for a 5000 mah in the Chipmunk.

[john stones 1 - Moderator]

I think that's a very good idea KC

the builds looking good Nigel

[Danny Fenton]

Hi KC it is a very difficult thing in my opinion to calculate as there are a great many variables at play here. The drag of the aircraft, the weight, the type of flying, and the speed of flight. Not to mention the way the pilot flies. They all have a bearing on the duration you will get. I have known the same set up in different hands produce nearly twice the duration. For the Barnstormer I wouldn't fit a 5000, it is too high a percentage of the overall weight. I have watched dozens and dozens of you tube videos of people flying models and they rarely last more than 10 minutes. So I would aim to only lug around the sky the amount of battery you need. After all it does affect the performance of the model. My 60" Bistormer will have a 2300 A123 pack and i am pretty sure i will get 10 minutes from that, which is fine for me.

The Chipmunk is a slightly harder model to judge because of the weight being unknown. Two builders building the same model will rarely come out the same weight. This is where the skill comes in. Brian Taylor is famous for building light. I feel i am pretty good at building light, but I cannot get within half a pound of his weights. Nor can most people. Not many people ensure that the wood they are using, especially for tail ends is less than 6lbs per cubic foot. all of this even the dabs of glue adds up.

Chris and I are pretty sure we have a setup that will work, but we are still doing some testing. The only given we have at the moment is that the prop size should be 13.5" as that's scale.

Anyway this is more for the Chip thread than clutter up Nigels build

Cheers

Danny

[Nigel Day]

Thanks John, kc, Danny.

[Nigel Day]

Slow progress at the moment but the top, rear stringers are on and the u/c doublers are in.

Next step is to pull in the nose and fit a reduced size F2 about 3cms forward of where it is on the plan. Then I'll add the battery etc shelves between F2 & F4 and add the bottom sheeting & hatch.

F2 is sloping slightly forward so the motor will have a natural down angle. Should I also add a degree or two side angle too or isn't that necessary?

[Nigel Day]

So, nose pulled in a little and F2 added.

Then I added the battery shelves, with the forward one removable so I could get at the ESC below.

No the best bit of ply drilling ever but I'll use better ply and drill next time.

Then the bottom sheeting and u/c plate were added.

And there's space for a hatch to get at the rear-driving servos. Only there's not really enough space between the servos and F4 for me to fit the pushrods and snakes comfortably.....

Danny will appreciate that this was a bit of a slice into the water hazard.

Still, I've plenty of room behind F4 to fit the servos so I fitted a couple of braces and then the cross members to sit the servos on. The first hatch will get glued in and I'll need to create another for the area behind F4 that sits flush with the longerons. This is where someone says "you didn't wanna do thaa-at".....

Now I need to create a fixing point for the Rx just in front of F4 and above the battery shelf.

Next it's fitting the fixing points for the snakes down the fuse and soldering the bullet connectors etc for the ESC.

I thought I'd try Phil W's deadman plug idea too. I just hope my soldering iron is good enough or it's off to the shops I'll be going.

[Biggles' Elder Brother - Moderator]

Hi Nigel,

nice progress, its certainly coming along.

One tip for drilling ply - always clamp a piece of scrap wood behind the ply then drill from the ply side right the way through into the scrap wood. That why you avoid splitting off the outer ply on the far side and get much neater hole!

BEB

[Nigel Day]

Thanks BEB, I should have thought of that.

[Danny Fenton]

Keep going looks great, you are getting there

Cheers

Danny

[Nigel Day]

Slowly. Thanks Danny.

[Nigel Day]

Right. Too much slow and not enough steady here so I'll try and buck my ideas up. Trouble is, I'm spending quite a while at this stage working out the best way (from my limited view) of doing something. I'll have a think, a bit of a play/measure up, and then wander off for a cuppa. Eventually i'll arrive at my preferred solution but it's taking time.

The servos seem to fit nicely. They're cushioned from the bearers and the former so they shouldn't suffer vibration.

I've also fitted the braces for the snakes and the snakes themselves. One good thing about where I've fitted the servos - I can get both snakes from one 1 metre length.

I have a concern though so would appreciate some guidance. The distance from the servo arm/head to the former/brace where the end of the snake sits is only around 7-8 cm. With a travel of about a cm left to right as the servo head/arm rotate, that's quite a lot of lateral movement. How will the snake and wire cope with this as it moves back and forward and left to right? I've thought about leaving the snake unattached in the middle section (between the first and second braces) to allow it to flex sideways but from what I've read so far, snake flexing doesn't seem to be a good idea.

Will it work OK as is or should I move the brace back a bit - and if so, how far?

The snakes exit the rear section via the 'thingies' fixed into the side sheets - remembering to keep them well apart so as not to get the elevator and rudder controls snagging each-other.

The front end has been strengthened as per plan for the bottom block and as others have suggested for the engine bulkhead.

Whilst fitting the snakes and generally fiddling about, I managed to break one of the cross braces at the back and one of the bottom longerons. Still, that gave me an opportunity to try my splicing and repairing skills.

Doing that and smoothing it down afterwards reminded me that I've not rubbed down any of the model so far. That partly explains why it looks like a dog's breakfast close up. I'm about to add the side stringers/longerons and when I've done that I'll do a spot of planing and sanding.

I've not used a razor plane before so it might end up looking more like a pencil than a Barnstormer!

Edited By Nigel Day on 18/04/2014 10:00:22

[Pete B]

I know they are called 'snakes', Nigel - but the straighter you can make the control run to minimise the friction, the better. It can be a pain and take a while but it really is worth the extra effort to get it right.

I'd re-drill the holes through the former a little higher, so that the snake emerges in the same horizontal plane as the rotation of the servo arm. Laterally, they should emerge at the mid-point of the arc described by the servo arm hole you choose to use. This will eliminate mechanical twisting and reduce strain on the servo arm and linkage.

I wouldn't expect you'd need the full throw of the outer hole anyway. Full throw will produce a lot of control surface movement and, rather than reducing the EPA on the radio, you might find using the 2nd hole in from the outside will give you more than adequate throw.

Depending on the length of the connector you use, I wouldn't think you need more outer emerging from the former than you already have. I would anchor the outer at at least one other point midway along the fuselage, too. My standard method is to wrap at turn or so of masking tape around the outer, give it a quick roughen and then use epoxy to fix it to the former.

Before you lock everything in place, have a dry run and satisfy yourself that everything is moving with the least binding possible.

With a razor plane, adjust the blade so that just a very thin shaving appears - any more and it can dig in and take out far more than you want......

It's coming on.... just go easy with the sanding!

Pete

[Nigel Day]

Thanks Pete.

I've tried the inner movement and it feels as though there's hardly any resistance. Whether that'll still be the case when it's linked from end to end though....

There is a second former/brace 2/3 of the way to the start of the tail section that might not be obvious from the photo(s). I had planned to connect it all up when the fin and tailplane were in place before gluing anything in place.

I'll take your advice and reposition the snake 'heads' though and line them up with the middle of the inner arms. The rest of the arms will be coming off btw.

Is this where the idea for the film 'snakes on a plane' came from?

Thanks for the advice re the razor plane - and the sanding.

Edited By Nigel Day on 18/04/2014 16:08:36

[Nigel Day]

This is a warts and all, beginner goes a-modelling blog so here goes with the next set of gory details.....

My BS's nose is a little wide. Checking with the plan, it's only a few mm too wide so I'm sure it won't matter. I suspect that I didn't pull the nose in enough. The sides seemed quite firm so I was a little wary of putting too much strain on the bottom block and the leading former.

It's not as fat as it looks in these two photos - honest!

There's scope for me to shave a bit more off all round but I don't want to make the nose too thin where it joins the former. I guess I could glue some additional wood on the inside around the join so I could go in a bit more?

The side stingers (longerons) have been added and the cheeks planed. I don't remember who said in this forum that planing was great fun, but they're right!

I decided to make the whole of the top removable from the nose to the cockpit, and in three sections.

The first two have been formed and largely shaved. Just a little tidying up and smoothing to do I think.

Both have been carved from glued blocks. In the interests of openness and honesty, here's the belly of the beasts too.

I still need to do a little work where they join. I haven't decided quite how to do that yet but I might actually join them with the front fairing. They currently slot over F2, hiding it, so I'd retain that feature.

The last of the three sections will be done with a sheet bottom and sheeting over ribs - just so's I can get a feel for a bit of that ready for later projects. Somewhere in there will be some sort of cockpit front. It'll probably be (very) basic this time but I still might do it as Danny & Phil have done.

My wife is concerned that I don't have a pilot in mind. I did try telling her that it's radio-controlled but she didn't buy that. I have suspicion that I may receive a last minute but of ballast in the shape of Snoopy, Dick dastardly or similar. I'll let you know the outcome.

For those that have spotted the lack of cabanes on the top, fear not, they've been added since the last photo was taken. The decking section will fit over and round them.

Until the next time......

[jolliffee]

Hi Nigel. Looking good,have you thought about a colour scheme yet?