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Old Feb 05, 2013, 01:33 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
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Craftsmanship!

I'm specially interested in all your motor / prop / battery measurements. What piece of kit do you use for that Trevor? Those are very big numbers compared to what I'm used to.

Still a masterclass!

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Old Feb 06, 2013, 02:24 AM
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Trevorh's Avatar
Hampshire, U.K.
Joined Jun 2008
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Nick, the instrumentation is very simple - just a rev counter and clamp meter to measure the current. I tend to be a bit lazy and not measure the on-load battery voltage, but as you can see by the difference between my two 4s packs, it can make quite a difference.

The only 'advanced' measurement I do is to check the internal cell resistance of my LiPos from time to time. For this I use an iCharger 106B+. There is a thread about the iChargers and another about the relevance of internal resistance measurements as a way of seeing through all the marketing fog of C ratings.

The Aquabird is going along to tonight's club meeting for it's in-build scrutineering. I think I'll leave those part-built sponsons off though or it might start shedding pins all over the skittle alley which could get us into trouble!
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Old Feb 07, 2013, 03:45 AM
Slip the surly bonds...
Sopwith Mike's Avatar
Christchurch,England
Joined Aug 2004
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Trevor brought his nearly-finished Aquabird to our Club meeting last night. I assure you the workmanship is astonishingly good, and consistent over the model, so it looks as if it has been made by one of these 3-D printers from a solid block of balsa. Very nice.

My only quibble is that with a cockpit that small, the plane would be twice the size of an Avro Vulcan....I had assumed that it would be fighter-size...

Not that it's a scale model of course!
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Old Feb 07, 2013, 05:14 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
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Maybe like a Shoestring, the guy just has his head poking out?

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Old Feb 07, 2013, 09:50 AM
...design-build-fly-publish...
eye4wings's Avatar
Ware, herts. U.K.
Joined Sep 2008
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I think we should just try and imagine a very thin bloke laying on his back warming his feet on the ESC. What you think is a cockpit canopy is really the visor of his helmet!
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Old Feb 10, 2013, 05:43 AM
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Hampshire, U.K.
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I have to admit that the whole question of the scale of the cockpit hadn't crossed my mind! Happily the cockpit canopy will be tinted so I don't have to decide how much of the poor pilot's anatomy is visible. To be honest, I was looking to make the minimum modification necessary to turn this from a bird into a plane so that I don't have to tackle the feathered colour scheme!

Meanwhile, the sponsons are basically built. The wing is now complete apart from a good filling and sanding session, and there's no point in doing that until I'm ready to cover it. The uncovered wing, with aileron servos fitted, now weighs in at 18oz, and the whole airframe as photographed (i.e. without nose or hatch) is 30oz. The motor, battery, prop, spinner, Rx, and ESC account for another 30oz so, if the remaining woodwork and the covering amount to 12oz, the AUW will still be around 4lb 8oz.

A last balance check shows that, with the nose length as per plan, the model currently balances nearly an inch ahead of the marked cg with the battery fully forwards. Hopefully this is sufficient to accommodate the rearward cg shift which always seems to occur during covering.
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Old Feb 10, 2013, 05:52 AM
Slip the surly bonds...
Sopwith Mike's Avatar
Christchurch,England
Joined Aug 2004
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...and with the weather we are having, you will be able to fly it off water in the street!

The bird decorations seem almost irresistible noiw that it is fully assembled. I'm sure Tim could come up with an easy-to-apply scheme, in particular, claws on the sponsons
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Old Feb 10, 2013, 07:28 AM
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Trevorh's Avatar
Hampshire, U.K.
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Motor mounting.

Having decided how far forward to mount the motor, I'm now forced to make my mind up on motor access arrangements. I have on occasions managed to get away without a detachable cowl, fitting the motor through the nose ring. However, in this case it looks a bit tight.

The motor has a cross mount measuring 68mm across, with mounting holes on a 58mm diameter circle. The motor drum is just under 50mm diameter and my spinner backplate is 63mm.

If the nose ring is opened up to 57mm then it would be possible to thread the motor through the front and to get a ball driver onto the 4mm hex fitting bolts. However, this leaves a very thin cowl ring of just 3mm and the mounting bolts would be very difficult to get into position, probably needing to be tack glued to the cross mount before inserting the motor.

A radical alternative, which I have never tried, is to open out the central hole in the firewall and insert the motor from behind. The fixing bolts would then also go in from behind, through the cross mount, through the firewall and into captive nuts mounted on the front of the firewall. This of course demands a ginormous hole in the firewall (50mm diameter, plus a cutout for the wires to come through). The firewall is though about 80mm wide so I can't see any reason why this couldn't work.

Has anybody tried this before? If I can't pluck up the courage to give it a try, then it is back to the good old detachable cowl - or buy a bigger spinner and return to plan A!
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Old Feb 10, 2013, 07:33 AM
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Hampshire, U.K.
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Quote:
Originally Posted by Sopwith Mike View Post
The bird decorations seem almost irresistible noiw that it is fully assembled. I'm sure Tim could come up with an easy-to-apply scheme, in particular, claws on the sponsons
Look here Mike, if you're going to get picky about the scale of the pilot, you can't expect me do do a model of an aquatic bird with a propeller for a beak! Anyway, it would definitely need webbed feet.

Repeat after me: "It's an aeroplane and it's not scale". If you say it often enough, you might even come to terms with the concept
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Old Feb 10, 2013, 09:03 AM
...design-build-fly-publish...
eye4wings's Avatar
Ware, herts. U.K.
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To rephrase your dilemma Trevor, it's a matter of whether to instal motor and X-plate together from the front (which there is not enough room for) or insert motor and X-plate from the rear. The latter requires removal of spinner and prop, but then given the tightness of the front end the X-plate could only be inserted without the motor attached and the motor screwed in from the rear anyway.
It seems to me that you need rear access either way, but of the two I would favour access mainly from the rear.
(This means the X-plate screwing against the rear of the firewall which gives better security anyway)

But I think there is a better way.
Simply put, move the firewall forward and reverse firewall, motor and mount so that the drive is from the rear of the motor via the clamp-on type of adaptor.

There are several advantages first of which is that it is kinder on the motor's bearing should the propellor or the motor be a little out of balance as there is a similar weight behind the mounting to that in front and a shorter distance from the mount for any imbalance to build to serious vibration.
Another is that although the motor has to be extracted rearwards you do have a choice of taking out motor and X-plate together or, by undoing motor screws from the front, just the motor.
And of course there is always the option of dispensing with X-plate altogether and simply screwing motor through the firewall from the front.

And, yes, this is my favourite method lately and I have done it this way on several models - including the Stirling.
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Old Feb 10, 2013, 01:18 PM
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Steve Merrill's Avatar
USA, IL, St Charles
Joined Nov 2002
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I have mounted motors behind the firewall and it works but you must be sure to use heavy plywood for the firewall. I have tried to get away with using lite ply and got severe vibration up to and including breakage of the firewall. Use 1/4in plywood. And make sure the motor screws are not too long
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Old Feb 11, 2013, 01:43 AM
Slip the surly bonds...
Sopwith Mike's Avatar
Christchurch,England
Joined Aug 2004
2,603 Posts
[QUOTE=Trevorh;24090065]....it would definitely need webbed feet.
QUOTE]

What are you waiting for...?
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Old Feb 11, 2013, 03:01 AM
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Hampshire, U.K.
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Thanks Robin for the reminder about the front mounting option - I must admit that it is so long since I did this that I had forgotten all about it! The only downside I see is that you have to ensure the fuselage has good torsional rigidity all the way to the nose ring.

Steve, the fact that you have actually used the 'through the firewall' mounting method has encouraged me to explore this option a bit further. I share your misgivings about Liteply. I haven't used it for years, having been put off by a battle with some seriously warped sheets many years ago. Nowadays, I prefer to just use balsa and stiffen it where needed by by laminating it with a bit of 1/32in ply.

My bench saw can't really handle 1/4in ply (which is why I have had a 4' x 2' sheet of the stuff laying untouched in the workshop for four years!). My plan is to cut the firewall from 1/8in ply, fit another ring of 1/8in ply around the front face to take the T nuts, and maybe laminate the backside with 1/16 ply for good measure.

The more I think about it, I'm not convinced that the central bit of the firewall was doing much anyway so it's maybe not such a loss having to cut it out. The main worry is that the bolt holes will be very close to the big hole, so reinforcing the edge of the hole to stop the bolts tearing out sounds like a good idea. Of course, the T nut flanges will protrude into the hole so will need to be ground away to let the motor through.

However, as new challenges go, I'd rather tackle this than feathers and webbed feet! Talking of webbed feet, Mike and I are off to the flying field this morning, so we might need some ourselves.
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Old Feb 11, 2013, 05:26 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,696 Posts
Quote:
"It's an aeroplane and it's not scale".
Funny looking aeroplane!

I can't wait to see it flying!

Nick
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Old Feb 11, 2013, 06:21 AM
...design-build-fly-publish...
eye4wings's Avatar
Ware, herts. U.K.
Joined Sep 2008
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Quote:
Originally Posted by Trevorh View Post
The more I think about it, I'm not convinced that the central bit of the firewall was doing much anyway so it's maybe not such a loss having to cut it out. The main worry is that the bolt holes will be very close to the big hole, so reinforcing the edge of the hole to stop the bolts tearing out sounds like a good idea. Of course, the T nut flanges will protrude into the hole so will need to be ground away to let the motor through.
Agreed the mount centre just adds a little stiffness - not critical, and torsional stresses are mainly applied in rapid acceleration, so my view is that someone gentle on the throttle such as yourself would not need to worry about it unduly.

How about front mount with installation from the rear, cut away only as much of the mount plate is required to ensure good containment for your T-nuts and save having to grind some of them off?

Actually I would use wood screws (again from the rear) anyway, but some clearance is still required so the ply doesn't fray at the edges.

I showed my 12-year old grandson some of your posted pictures yesterday - you should have seen his face! I think he was quite impressed.
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