Wing structures complete, she fills up a good portion of the garage, started glassing tonight I plan to have the wings in primer by the end of this weekend

That's looking nice Ken.
I bet you're feeling quite excited now.

It's odd how out of scale the wings look in their naked colour. By the end of the weekend though they'll look a lot smaller in their grey.

Are you planning to give her her first flight in primer ?

Amazing Ken how you did such nice work so quickly. I am so fn slow by comparison. And the outlines look perfect to me. Can't wait to see the colors. Oh, excuse me, "colours"...! What's it weighing these days?
Rob

this is gonna be an awesome and dare I say unique model to bring to the field

All I can say is SUPERB!!! I can't really say the plane is beautiful, but the craftmanship and build quality certainly is.
I will be looking forward to the power tests and finished plane. A truly unique choice. There won't be 10 more at the field either! hehe.
I'm usually looking to model something different too. I have a Buffalo because I've never seen another. I chose a Hawker Hurricane because everyone else has a Spitfire. I would love to build a Dash 7, but unable to find any suitable plans and no computer draftingskills.
Your builds are a true inspiration to me, and others. Doug B

Thanks everyone, I do know what you mean about the wings looking a little disproportioned, they are scale, the full scale was a gun platform so designed to fly slow and stable so this has some bearing I would imagine, I also think the camera wide angle lens is distorting a little.

I only realised last week that the plane is actually very close to being able to fly, all I have left to do is wire up the wing control servos. I think I will maiden in primer but will wait for the weather to improve probably towards the end of March but will see

Rob I will weigh her when the wings are painted, would estimate from the last check that she will be around 7.6kg

I guess if its time to think about this plane flying I had better start looking at CG issues.

Using a calculator suggested CG is 143mm, it is currently around 165mm so until the cockpit and pilot is installed I am going to need some weight up front, dissapointing but considering the shape of the plane not too bad really.

Wing loading is 31.75oz/sq.ft with a stall speed of 28 mph again perhaps a little dissapointing any opinions, I have taken a guess at all up weight of 8.5kg for now

Obtaining the CG without lead was always going to be an uphill battle with the 129's short nose. A cubic wing loading of around 10 isn't bad though. It's a fighter! It should fly fast. The huge flaps should slow it down enough for reasonable landing speeds.

Agreed! If the stall speed was that high with full flaps for landing THEN I'd maybe start worrying!

Since getting into giant scale I have seen how deceptive speed is anyway. 28 mph for a model that size would look very different from 28 mph for a model half that size.

I think 'don't worry about it' would be the right advice at the moment.

Edit: Actually I don't use these fancy calculators so don't fully understand all that's going on on that first chart, but if it is saying that your CofG will need to be at 28.6% root chord, I am a little surprised the figure isn't lower. I would have guessed with that forward sweep you'd be looking at something more like 20%.

Originally Posted by eye4wings

Edit: Actually I don't use these fancy calculators so don't fully understand all that's going on on that first chart, but if it is saying that your CofG will need to be at 28.6% root chord, I am a little surprised the figure isn't lower. I would have guessed with that forward sweep you'd be looking at something more like 20%.

The simplest way to explain it is that that calculator takes the size and distance of the stabilizer from the wing into account and calculates static margin. The contribution of the stabilizer allows you to move the CG back farther than you might if using %MAC.

I've found that particular calculator tends to recommend a CG that is more tail heavy than I like. I would maiden at 25% MAC or less regardless of what the calculator says just to be safe.

I agree with Pat.

J

Thanks everyone, so with a MAC of 368 the best starting point for the CG is around 92mm? that is 25% of 368

Originally Posted by pmullen503

The contribution of the stabilizer allows you to move the CG back farther than you might if using %MAC.

Really? I would find that statement a little worrying as the tailplane is normally used to counteract the weight of the nose to hold the wing at the desired angle of attack and is therefore operating in 'negative' lift condition - its force being applied in the downward direction. The problem with a rearward CofG is that once the weight moves aft of the wing's centre of lift (which can move fore and aft at different airspeeds by a margin dependant on the airfoil section used) the tailplane is asked to supply positive lift as is usual for a canard. At the positive-negative changeover point the angle of attack can change quite suddenly to produce the porpoising effect we have all seen and try to avoid as the pilot tries desperately to counteract the continually changing trim.
I would have thought therefore that the tailplane should not be taken into account - and so my agreement with the concensus that CoG should be about 25% (or maybe less) is with this very much in mind.

Of course the corrollary is that a too far forward CofG would require the tailplane to supply a downward force that could over-tax it's capability. This is what caused the general 'rule' that a model's tailplane area needed to be increased by 10% over the scale area - which is quite unnecessary if the CofG is positioned correctly.

In fact the amount of dead weight required to get the CofG to a desirable position puts more load on the wing - and this has been known to encourage me to risk a slightly more rearward CofG for the first taxi runs.

I try NOT to take off on the first run but do a few fast taxi runs without trying to make the model rotate and climb away. What I do in this process is watch for air to appear under the wheels when the model has reached flying speed. I then chop the throttle and settle the model back on the ground. This process will tell me what elevator trim the model will fly with (plus a couple of ticks of down in the climb-out) before I have to cope with everything all at once.
In these runs aileron trim may also be adjusted if both wheels don't lift off together.
Then when I know trims are in the right area I can make my take-off with reasonable confidence.

Originally Posted by eye4wings

Really? I would find that statement a little worrying ..............

I was inferring the effect of the underlying equations in the calculator. When using that calculator; a more rearward CG is suggested.

Originally Posted by heli_madken

Thanks everyone, so with a MAC of 368 the best starting point for the CG is around 92mm? that is 25% of 368

Yes, that's what I would do. Even if it's very nose heavy, you'll still get it down in one piece.

If you set the CG at the calculator's recommendation of 143mm you would be at 39% MAC!

I designed a shelf for weight up at the front of the nose so its easy to get lead in there, it took 1.08Kg (2lb 6oz) to achieve the 90mm CG

So with the estimated finished weight now at 9.5kg wing loading has increaseed to 35.4 oz/sq.ft with pretty much the same stall speed as before 29.5mph

Any opinions?