Quite right Colonel, the "uncovered" CG needs to be somewhat forward of the finished CG due to the extra covering weight at longer moment aft.

I don't know for sure where the Ajax CG should be, but as it has a relatively large and cambered tailplane I would expect that it would be happy with it further aft than your typical radio model. The Witch, similar layout, flies very happily with the CG at 60 percent which is just in front of the free-flight position, my instinct with the Ajax would be to aim for 40 - 45% rather than the 30-35% which would be the recommendation for a "normal" sport radio model. This should be completely safe, but the old truism about tail heavy models only flying once still applies, so if in doubt err on the forward side!

With a lifting tail 40% should be a plenty safe starting point.

Originally Posted by sparks59

Colonel, I usually check the centre of gravity without exploding any grey cells, by popping the measurements into the boxes at the following page link http://adamone.rchomepage.com/cg_calc.htm
sparks

Originally Posted by Sundancer

...my instinct with the Ajax would be to aim for 40 - 45% rather than the 30-35% which would be the recommendation for a "normal" sport radio model.

Originally Posted by rabidrue1

With a lifting tail 40% should be a plenty safe starting point.

Well, our Albert had stuck the mainspar bang on 40% of chord, so that's what I'll use. The disconcerting thing is that the CG calculator comes up with between 62-72% depending upon the Static Margin I pick.....

I know that I'm preaching to the converted here, but I just want to say what fantastic service I have just received from Mike Woodhouse at Free Flight Supplies http://www.freeflightsupplies.co.uk/ !!!!

Placed an order at lunchtime on Friday, and the gear arrived at my desk at 11am today (Monday). Even if it had been ordered Monday and delivered Thursday I would have said it was good, but over a weekend that is exemplary. So don't forget, that's

http://www.freeflightsupplies.co.uk/

I have now received my ebay steel screws and neodymium magnets, plus nylon screws and covering materials from Mike at http://www.freeflightsupplies.co.uk/ (have I mentioned that before?).

This means I now have the wherewithal to nail the motor on the nose, then make an approximation as to the positioning of the Rx & the Lipo, and the sticky metal to hold the two hatches on (only 1 magnet per hatch methinks otherwise I'll be a-ripping the fuz bottom off trying to open the hatches).

Then (or perhaps even during) I can start having a go at covering the old gel's modesty. I've just tried to get a 250ml pot of the Ronseal Diamond varnish from B&Q, but although Googling shows they have it in stock, it's not actually a line they stock.......work that one out!! But I'm not going to spend £25 on a 750ml pot for a model that's already tipped over the £50 mark (due to having to buy everything as I had no 'stock'!!!). I'll carry on Googling but if I fail, I'll be trying my old bockle of Johnsons' Klear which I've had since Moby Dick was a tadpole....

Originally Posted by Colonel Blink

...I just want to say what fantastic service I have just received from Mike Woodhouse at Free Flight Supplies

Yep...I can do nothing but chime in....!......my experiences with MW are also solely great...in fact, there are few of his kind around today...
In my opinion, well worth to support...!

Not really surprised that the CG calculator says 60% Colonel, with that big a lifting tail. 40% should be ultra safe, if it comes a bit aft of that with your preferred lipo position I wouldn't worry.

Re Free-Flight Supplies and Mike Woodhouse. I can only echo your sentiments, I have been buying tissue and mylar from Mike for - what, it must be 15 years now - and his service has never been less than absolutely excellent, as are his products. He also happens to be one hell of a good modeller, which may explain why he cares about what he supplies.

Is the stuff you might be forced into using for the covering water based? If not I wouldn't be sure how it will turn out. You could try Poly-C (http://www.rcworld.co.uk/polyc.html), this is £10 for 500 ML, still a lot more than you will need for the Ajax, but I get the sense you may be building others? As far as I know this works just the same as the water based urethane varnishes I have tried, they say it has "an adhesive" added, but I don't know from personal experience. Of course, you could always lay new hardwood floors in the house, then you would need to buy litres of the stuff on the household budget and there would be sure to be some left over, that's what I did

PS: I thought it was expensive here in France, but the brand I use, 3V3 is "only" 65 euros for 2500 ML, which at current exchange rates makes it a bargain compared to your £25 for 750 ML!

Originally Posted by Colonel Blink

I have now received ..and the sticky metal to hold the two hatches on (only 1 magnet per hatch methinks otherwise I'll be a-ripping the fuz bottom off trying to open the hatches).

.. old bockle of Johnsons' Klear which I've had since Moby Dick was a tadpole....

A trick I learned on RCG is to use one magnet on the hatch and a flat (ish) head screw wound into a small balsa block in line with the magnet. This allows sufficient strength to 'click' and hold the hatch secure, without pulling the structure apart when opening and closing. Better still it can be adjusted to get the hatch flush at the correct magnet tension.

Good luck with the varnish.

sparks

You can mix other water based substances into these varnishes. Pva, acrylic paint or ink and dilute with water. If thin enough they spray well with an airbrush. I find they bond tissue to mylar extremely well. No need to buy "special" products.

Like Raf_Bob and his Senator I am still progressing with the Ajax, but the recent sunny spell (4 whole days - count 'em!) and the start of the school holidays has meant that I have been an infrequent and temporary vistor to the loft.....

I've permanently attached the wheels to the u/c - not having any cup washers (and I would be bound to melt my nice lightweight plastic wheels if I tried soldering washers within a country mile of them) nor any collets, I had to find a way of keeping the rolly bits on the sticky out bits. I raided young Pippa Blinkette's craft box while she was in a hypnotic state watching the 47th repeat of Tracy Beaker and came up with a number of tiny beads that threaded on the legs a treat. A dab of Araldite keeps each one in place.

I have also managed to nail the motor up the sharp end; firstly with nylon bolts until I found out that M2 nylon threads have the strength of porridge, and now with good old M2 steel bolts which holds it on a treat. Connected the ESC which actually doesn't want to fit neatly into the compartment in the nose intended for it - but it fits and will stay there. With the motor & ESC in the nose, the u/c plugged in, and the empennage on the back (complete with servos), the fuz is fairly significantly nose heavy. I will need to put the Rx forward of the target CG if I don't want to extend the Rx to ESC lead by an inch or so - that will hopefully mean that the Lipo being behind the CG (easy to do) will balance it. However, there are still servo extensions to add, plus rudder and elevator to add to the equation, so it's all a bit of a movable feast. I suppose all I can do is keep checking and get as much of the model completed before I nail the Lipo in place.

Now then, as the mylar, tissue & Balsaloc have arrived, I have begun to research covering techniques, and I have a couple of questions...
a) Is the mylar effectively used to just 'glaze the openings' of a structure rather than completely cover it? What I mean is, the sheeted area at the front of the fuz would be left unmylarred (if there be such a word); and wing panels would be covered so that as best as possible there is no overlap between the upper surface and the lower surface mylar??
b) I have read that most dope the bare wood before mylarring (another new word!). One of the attractions for me is not to use dope due to my breathing, my diminishing braincells and domestic harmony. Should I apply my 'varnish of choice' to the wood pre mylarring, or just plonk the Balsaloc straight onto the bare wood??

For those who asked about the Johnson's Klear, it is a water based acrylic. I was going to test whether it works before attacking the whole airframe with it...

Hi Blink, Sheeted areas do not need covering in mylar but you will find a difference in shade of the tissue where the mylar ends, so to speak. If the sheeted areas are small (such as infill at the nose or tail) then I cover them with the mylar, but on areas such as wing leading edge sheeting just the open area. Of course, the Ajax will not have large sheeted areas so you could cover the whole thing with mylar. As to treating the woodwork first, it seems best to seal it and I use diluted varnish for this but another way (apart from dope) is very thin Balsaloc. Either way works well but if you use varnish it's best to sand the structure when it's dry. Thus you start with a very smooth framework. Remember when applying mylar to allow an escape route for trapped air, ala solarfilming. A few pinholes is all that's needed as air is pretty thin!
Followed your build with much interest and amusement at your sense of humour.
All the best with the covering. If this your first attempt then I would very respectfully suggest you knock up a trial frame of some sort and use it to practise. Also, on the model, a good place to start is the fuselage under side. BOL.

Echo Bob's sentiments re a trial frame; mylar/tissue is not difficult but there is a technique to be acquired and a bit of practice before committing yourself to tackling your lovely airframe makes sense.

Personally I use the "mylar over open areas only" technique when covering with dope; this way, doping the airframe with 2/3 coats first then rubbing down, brushing thinners through the tissue adheres it nicely to the uncovered wood areas, so it is stuck down firmly when the water shrinkage kicks in. When using varnish, covering overall with mylar and then applying the tissue wet and brushing through is best I find.

The shrinkage of the mylar is such that you don't have to be too concerned about getting it too tight when first applying it, it is more important to get it EVENLY tensioned and to avoid wrinkles in panel corners etc.

Balsaloc, being basically a PVA like substance, does tend to raise the grain on untreated balsa, so if not using dope I would recommend a couple of coats of varnish and a rub down before applying it.

Finally, once again, any amount of advice and coaching is not worth as much as a bit of practical experiment on a test piece.

Good luck!!

Do you really need to use mylar on such a small model as an Ajax? Tissue alone is enough, mylar will warp the wings like a propeller ... I use mylar/tissue on the wings of my Dixielanders, as it makes them nice and strong, but this is a much larger model than the Ajax. Balsaloc on bare wood.

The CG on my FF rubber Ajax is about 1/4" behind the spar.

Good point jj.
My buddy DH covers many of his smaller models with tissue and they come out fine.
sparks

Originally Posted by *jj

Do you really need to use mylar on such a small model as an Ajax? Tissue alone is enough, mylar will warp the wings like a propeller ... I use mylar/tissue on the wings of my Dixielanders, as it makes them nice and strong, but this is a much larger model than the Ajax. Balsaloc on bare wood.

The CG on my FF rubber Ajax is about 1/4" behind the spar.

It adds toughness and waterproofing. Not essential it's true but gives a good finish without any additional dope/varnish so it's not that heavy.
My Senator wing is as flat as a pancake with mylar under lightweight tissue.