Spacewalker half scale 168" electric

[Chris Lane]

The trouble with weighing methods is that there are few accurate weighing machines - bathroom scales are a disaster (ask the wife)! 1.5% accuracy is all you can hope for. Go for the suspension method! the model only needs to be just clear of the floor. I put my models on the floor, place the slings on wings and fuselage, and then pull up each wire in turn and make fast to cleats on the garage wall. Then I lower the bob weight until its tip is just above the surface of the model. A quick dab with a felt tip pen and job done. No errors, no weighing machines, you can see where the C of G is. . . . .

[Phil Clark]

Phil,
I like the moment method, probably really accurate if done carefully.

Yes...very

I no longer use bathroom scales though as I found these not accurate enough on lighter large models (30lb bracket) I have now got a set of digital scales accurate to 50g, but capable of holding 75kg's. When the time comes for a final inspection, I'll bring them with me and we can double check what you have.

Phil

[Phil Clark]

Phil,

Now the only thing I found tricky, which might of had an effect on the sums is that AA, BB and SS are not straight forward and I had to decide where to take the measurements. Do you include the elevator? If so, how do you handle the curve? At the tip of the stab the chord is zero (because it is curved), so I took the tip chord just a little way in. The tail is this shape..

F101.jpg

Any ideas? or just go for 28% which would be 191mm.....about the same as using 14% in the programme.

Cheers, Andy

I'd turn you 'curved' stab profile into straight lines.......make the tip straight, but reduce the span. What area you loose by reducing the span a little you will gain back by turing the curved tip into a tip with sharp corners. Should be pretty simple to make a plan view drawing of the actual shape (on graph paper) and do some chopping about so you gain back what area you loose by altering the overal shape slighty (& yes, include the elevators)

Phil

[Phil Clark]

The trouble with weighing methods is that there are few accurate weighing machines - bathroom scales are a disaster (ask the wife)! 1.5% accuracy is all you can hope for. Go for the suspension method! the model only needs to be just clear of the floor. I put my models on the floor, place the slings on wings and fuselage, and then pull up each wire in turn and make fast to cleats on the garage wall. Then I lower the bob weight until its tip is just above the surface of the model. A quick dab with a felt tip pen and job done. No errors, no weighing machines, you can see where the C of G is. . . . .

The 'hanging' method does work, but personally, I don't like the idea of hanging 80-100lb models (especially working alone). The other benefit of the 'weights & moments' method as I showed is it can also be used to determine exactly how much weight is required & where to put it to get the CG to the desired position......far better than 'trial and error' as far as I'm concerned.

My scales......

http://www.digital-scales-company.com/index.php?main_page=product_info&cPath=97&products_id=428

And accuracy of +/- 50g is perfectly good enough when we're considering 20+kg models (0.25% accuracy).......and all for only £ 55.00. £ 55 seems like a tiny amount to spend when it can prevent the possible demise of a multi thousand pound model. I have checked several items weighting up to 1kg on my small 'resin & paint' scales in the worshop and weighed them again on the 75kg scales.......the the weights match, so I'm happy they do as they say they do.

Phil

[Andy Boylett]

Maybe I will hang it and weigh it as well then

Actually joking asside it is probably easier to hang it my garage as there is not really enough floor space to fit the outer wings. Once hanging we can fit the outer wing panels.
If we get some nice weather we could of course go outside

Thanks for your help guys.

[Mike Booth]

I guarantee your CG will be 25% of the root chord measured from the leading edge plus 30mm.
Is that close to 28% ?

[Chris Lane]

Phil,

I had a look at your recommended digital scales site and I must say the shipping/parcel scales look pretty good. I won't be buying though because they talk at you and I have had enough of Nagging Noras in machines!

Chris

[Phil Clark]

The fact they talk to you is very useful. I generally have to CG check large models outside as my shop is only single garage size.......if it happens to be sunny, reading the LCD display can be tricky, so the fact it tells you the weight is great.

Phil

[Andy Boylett]

I guarantee your CG will be 25% of the root chord measured from the leading edge plus 30mm.
Is that close to 28% ?

Hi Mike, chord is 684mm so your way brings it in at 29.3%.

Cheers, Andy

[Andy Boylett]

The fact they talk to you is very useful. I generally have to CG check large models outside as my shop is only single garage size.......if it happens to be sunny, reading the LCD display can be tricky, so the fact it tells you the weight is great.

Phil

When I proposed that these scales could be used in our bathroom (when not being used for models) my wife said that at that accuracy you would be able to tell if you had had a good ****. and that this may skew the measurements being taken

[Mike Booth]

I guarantee your CG will be 25% of the root chord measured from the leading edge plus 30mm.
Is that close to 28% ?

Hi Mike, chord is 684mm so your way brings it in at 29.3%.

Cheers, Andy

Good Andy, 1.3% out, i can live with that even on a Spitfire, she will fly safely.

[Andy Boylett]

Thanks Mike. Are you at Bradford next Sunday?

[Andy Boylett]

I am now attempting to connect up the main power systems that the plane will be 'live' when displayed at Bradford. One of the issue with large electrics is the number of the batteries that have to be plugged in. To help cope with this I manufacture a small unit into which all the batteries plug.

Here is a unit for 12 batteries...

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This is made by soldering 25mm legnths of 3mm steel bar into 4mm bullet connectors. These are then soldered into small brass bace-plates that are screwed to a base board and laid out in an appropriate formation. The one shown connects 12 packs of 3 cell LiPos in a 12 cell, 3 in parralell configuration.

Here is the complete connector along side an EMCOTEC power switch that is all soldered up and ready to fit..

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The EMCOTEC device is used to switch the power on and off, thus making the plane safe when needed. The switch is 'solid state' and can be operated from a spare transmitter channel as well as with a special magnetic 'off switch'. I think that these type of switches should be compulsary on 12 cell electrics because of the danger from both the "silent start prop" and the electrical energy......a home plug socket can deliver 240v at 13amps (3Kw)......the electrics in my plane can deliever nearly 600amps at 51v or over 30Kw......10 times higher than in your house!!

[Phil Clark]

Are you planning any form of stress relief on that connection board Andy?......I appreciate the steel wire is 3mm into brass plate, but having used those red connectors myself, I know they are fairly tight and with constant plugging & unplugging, there is bound to be a bit of flexing going on given the length of the wire & connector off the brass plate which could cause one or more of your soldered joints to fail over time.....

Phil

[Andy Boylett]

Hi Phil,
Already though about that. You can't quite see on the last photo but if you look closely it has:
- had the plywood plate sanded down to 1/3 of its original thickness.
- had a 1/64 ply piece added to stop the brass screws that show through contacting anything
- had thick epoxy, full of fibres, added all over the barss plates and around the bottom of the steel pins.
- had the outside edge trimmed back to just clear all the bits.

I have also used these before (without epoxy, but with solder carefuly done so all round joint made) and had no issues.

I have zoomed in on the above pic so you can see the mods....

IMG_2839 small.JPG (18.41 KiB) Viewed 12830 times

Regards, Andy

[Andy Boylett]

I am continuing to try and get this model all powered up for Sunday, but nice weather means we have to keep going flying

I have now completed the holder for half of the flight batteries. This half is twelve 5000mah 3 cell LiPo's, weighing 6kg and I have fitted them directly under the motor inside the cowl. I do this on a lot of my models because the cowl is not structural and it is therefore very straightforward to put a large access door in the side of it......It is very difficult to put a similar access door in the woodwork near front firewall for access into where a petrol tank would normally go because you do not want to compromise the structure.

Here is the battery compartment, under the motor...

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There are 8 batteries in there and space for another 4. They are held in place by a wooden bar that velcrows in place..

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here is the complete load...

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[Andy Boylett]

After Phil's questions re the possibillity of solder joints fatiuging I have taken a close up photo of the multi pack battery connector. This shows how it is reinforced with epoxy and fibres..

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[Andy Boylett]

Yab a dab a doo, all the power system is now complete and after plucking up courage to press the on button I have had it running. Here it is all armed and ready to go..

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The EMCOTEC power system is amazing. I have installed both the plug in switch (in the fus side above) and a receiver controlled switch. Either of these switches fully disconnects all the power packs and makes it all safe. When the system is armed there are green led's as a warning (you can see the one in the side of the fus). I have installed the warning led for the receiver controlled switch in the dash board.

The other very nice thing is that this system eliminates any 'spark' that you get when connecting high voltage DC power systems. Here is the EMCOTEC box..

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[Andy Boylett]

Satelite receivers

One of the last things I have left to do before starting covering is to finish off the wiring to the servos and the remote receivers. I am using two AR9000 receivers and each one has 2 remote receivers. Anyone got any comments or suggestions on placing of receivers in a large model? I am thinking of puting one in the end of each wing, one in the fus front and one in the fus rear - so as to give maximum variation.

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[Phil Clark]

Spread them about as far as is possible and make sure they are are orientated with the aerials differently, some fore aft, some left right, some up down........