Spacewalker half scale 168" electric

[Andy Boylett]

Hi Nick,
I wondered if anyone would notice

We were going to put my roof box on but since there was no room for Sam in the car he put the wings in his own car

However, just measured wing box and its a no go

[Nick Reeves 3055]

well i certainly haven't regretted getting a van!! it wouldn't fit the biggest of the lma models but i can get 9 club sport models in it! the spacewalker would easily go in it too

[Andy Boylett]

A van would be nice Nick we have a caravan with an opening rear window - and we nearly went to our club meeting with it last night.

Anyway, it was worth going to the meeting, we won some trophies and got the "best in show".....

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That is Sam, who will be the pilot of this...

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[Andy Boylett]

I have completed all the control horns now. All made from 2mm GRP board..

Rudder horns and slots, ready for fitting..

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Rudder horns epoxied in place..

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Elevator horns cut ready for fitting..

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And fitted..

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[Andy Boylett]

At my recent club event some people where saying my plane was very tail heavy. I pointed out that the main wheels are centered just in front of the wing leading edge and that the flight batteries were not in (which sit in the nose) and that there was no cowl or prop. However, some people still thought it was tail heavy. The onyl way to sollve this was to measure the cog. So I have buitt a simple cog balance. This uses the axel off my small truck and a purpose built support to fit the plane, such that the axel is directly under the wing main spar...

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The balance seems to work really well because as well as being sat on a pair of wheels, the platform is also attached via the wheel bearings, so very little friction overall.

Here is the plane sat on its balance....

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The plane as it currently stands was balanced with just half of the flight batteries fitted in the front of the plane. So this leaves me with the other half of the batteries (6Kg) to fit anywhere I need to for final balancing after covering.

[Phil Clark]

Have you calculated the exact CG position required Andy, or is 'over the main spar' a guestimate at the moment?

With the wings attached (assuming they weren't when you did the check seing as they aren't shown in the photo), most of their weight will be behind the CG, so this will effect your current CG position remember. Most of your covering, dope & paint is the same......so I'd bank on needing the bulk of the remining 6kg's up front.

Phil

[ian redshaw]

It seems from looking at the pics, that there is a lot of scope for rocking and perhaps inaccuracy with that method for checking CofG. Since in the wing structure there is a big square socket at the CofG, could you not make a fitting to suspend the model from this point? There would be far less rocking possible from this point. If thats not possible, make two ply ribs that fit over the wing tubes before the panels are slid on. This second method would make the chosen CofG point adjustable if you feel the ideal position not on the rectangular spar socket. Love the scale U/C nicely engineered (the ali wheeled one, not the red wheeled one!!!) . Whats the estimated all up weight ?

Just a thought, Ian.

[Andy Boylett]

Thanks guys.

I like the idea of hanging it, easy to do from my garage as it has a built in lifting beam . I also have 2 spare ply ribs already, so well on the way.

I have not done any calcs re balance point and as yet have no idea how to do so. The trial was very rough just to see where the land lies! I did the balance with the wings off, although I have measured the 'out of balance' they add and it about 500g at the leading edge (each).

Can someone tell me where/how to decide on balance point?

Thanks for the comments re the undercarriage Ian, it took ages to make and it was my first go at silver soldering as well.

I'm currently fixing the caravan so that both Sam and myself can fit in the car when we go to Bradford next weekend - and be able to fit the planes in. I know the place where the meeting is as my Mother in-law lives nearby - there is a nice big carpark so room for the caravan (plane trailer) .

Cheers, Andy

[Chris Lane]

Hi Andy,

The recommendation for an unswept and untapered wing such as this one is that the Centre of Gravity (the balance point) should be between 25 and 28% of the wing chord. During flight testing the C of G is moved back if required to increase the pitch (elevator) sensitivity.

As you can hang the model from the roof, my favourite way is to hang it from three wires such as the control-line guys "laystrate"; two to wing slings and the other to a rear fuselage sling. The important point is that all the wires must come from a single point at the top. Now a fourth wire (also from the top single point) with a plumb bob on its end is set up and it will point to the C of G. As yours is a low-wing model it may be helpful to hang the model inverted so that the bob point directly to a position on the wing. The joy of this method is it clearly indicates any lateral off set in the C of G and which should be compensated for. Also if you hang the model other than level by hoiking up the tail you can get the height of the C of G as well.

I have a steel plate with a small hole in it screwed to a joist which I use as the central point to thread the suspension wires through when I am setting up a new model.

Chris Lane

[Phil Clark]

Andy

I'll send you details on CG calculations and another method of determining where the CG is using a simple 'weights & moments' method late on. (The 'hanging' method does work, but with big heavy models, I find it a bit of a pain).

Phil

[Dave Collis 2296]

Andy, you could also make up an aluminum (or ply) rib, to fit between the centre section and the outer panel, slotting over the wing tubes etc, but is larger in height than the wing section. make a hole in the over height portion where the C/balance is to be (my 1/2 scale is 28-30%) and hang the model from a suitable support. Inverted would be preferrable for a low wing plane.

Project is coming along very nicely and looks good.

Cheers,
Dave.

[John Rickett]

All the information you need to calculate the cg is here - a lot of it compiled by our very own Alisdair Sutherland - so you can be assured its correct!
http://adamone.rchomepage.com/cg_calc.htm

[Andy Boylett]

Cheers guys.

I have just used the online tool and at 10% static margin it puts CoG at 220mm form LE, which is about 1/3 of the way through the wing spar from the front.

The range from 5% to 15% is 70mm.

[Andy Boylett]

Andy, you could also make up an aluminum (or ply) rib, to fit between the centre section and the outer panel, slotting over the wing tubes etc, but is larger in height than the wing section. make a hole in the over height portion where the C/balance is to be (my 1/2 scale is 28-30%) and hang the model from a suitable support. Inverted would be preferrable for a low wing plane.

Project is coming along very nicely and looks good.

Cheers,
Dave.

Dave, I already have 2 spare ply ribs so it would be really easy to drill small holes in them for hanging up, either way up.

[Andy Boylett]

Hi Andy,

The recommendation for an unswept and untapered wing such as this one is that the Centre of Gravity (the balance point) should be between 25 and 28% of the wing chord. During flight testing the C of G is moved back if required to increase the pitch (elevator) sensitivity.

As you can hang the model from the roof, my favourite way is to hang it from three wires such as the control-line guys "laystraight"; two to wing slings and the other to a rear fuselage sling. The important point is that all the wires must come from a single point at the top. Now a fourth wire (also from the top single point) with a plumb bob on its end is set up and it will point to the C of G. As yours is a low-wing model it may be helpful to hang the model inverted so that the bob point directly to a position on the wing. The joy of this method is it clearly indicates any lateral off set in the C of G and which should be compensated for. Also if you hang the model other than level by hoiking up the tail you can get the height of the C of G as well.

I have a plate with a small hole in it screwed to a joist which I use as the central point to thread the suspension wires through when I am setting up a new model.

Chris Lane

Chris, using the online calculator puts the cog at 220mm on a chord of 684mm which is 32% ??....but still forward of the centre of the main spar.

[Chris Lane]

I think that 32% is too far aft for a first flight - the risk is an unstable and uncontrollable aircraft. I recommend not more than 28% to start with. The position of the main spar is not a factor.
regards Chris

[Phil Clark]

Agree with Chris 100%....disregard the position of the spar; this has absolutely nothing to do with CG position. 32% is much too far back. I have used a number of these online CG calculators; none have been 100% correct.

I don't know what or why this is, but the larger a model gets, the more forward the CG seems to need to be. I have had 'calculated' positions given as far back as 34% in the past but the model has ended up flying well at 27%. Petty much any model will fly at 25%, this is a very safe staring point, but as you have a constant chord wing I'd go with around the 27% mark.

All a forward CG will do is makes the model less and less sensitive in pitch.....this isn't a bad thing; it just means you need more and more elevator movement to get the nose up (it’ll feel very heavy & dead to fly). If you go too far the other way, you'll have an overly sensitive model in pitch which can be a real pig to fly.

CG position generally relates to 2 things.......stab area and moment length (distance between 25% chord of the stab & 25% chord of the main plane). A small stab generally required a more forward CG that will have a very small 'safe' margin, a large stab will give a more rearward one but also a wider 'safe' margin. A short moment (close couple aircraft like a Bearcat for instance) will normally require a more forward CG than an aircraft with a longer moment (an F3a pattern model for example). The fun comes when we have close coupled models with large stabs (the large stab helps overcome the issues caused by the short moment)...and vise versa......a long moment with a small stab (Spitfire)

Wing planform also plays a part.....heavily tapered wings generally don’t like rearward CG’s unless a generous stab area is present as well (hence why we always see ‘kit’ sports scale Spits etc... with oversized tail surfaces to tame them down a little). A constant chord wing will be a little less critical and more forgiving.

I was once told this by a very experienced modeller....

A model with a forward CG flies badly, a model with a rearward CG flies once.................

Phil

[Phil Clark]

Here is the CG calculation method I use on all 'large' models.......................has served me well over the past 5-6 years. This text is a direct copy from a thread I ran on the Skyraider project on RCScalebuilder......so some of it isn’t all that relevant to Andy's situation.

HOWEVER......following 2 test flights, the model was rebalanced at 27-28% and was MUCH nicer to fly. It flew 'OK' at 33%, but was just 'hard work'.

1) The model has to be jacked up 'level'....by level, with the fuselage datum at '0' degrees. This is easy on the Skyraider as the top of the forward fuz is parallel to the datum, so a spirit level can be used.

2) Hand a 'plumb bob' of somewhere convenient....in this case, the end of the crank shaft stocking through the prop (yellow arrow)......this determines our 'known position.

3) Measure the distance from this 'known position' to the centre (axles) if the main wheels. This is distance 'A'

4) Measure the distance from the 'known position' to the centre (axle) of the tail wheel. This is distance 'B'

5) Using a set of bathroom scales, place them under one main wheel and take a weight reading. When doing this, the other main wheel, and the tail wheel MUST be chocked up the same amount as the thickness of the scales so as to maintain the models 'level'

6) Swap the scales and chock to the other main wheel, and take a 2nd main wheel weight reading. Combine the 2 main wheel weight readings. This is 'Wt1'

7) Using the same method as above (with both main wheels chocked up) take a tail wheel weight reading. This is 'Wt2'

We now have 2 distances (A and B), and 2 weights (Wt1 and Wt2). The 2 weight readings can now be combined to give us the 'total' weight of the model.

The 4 values can then be plugged into the following formula, to determine where the CG is, in relation to the 'known point'.

(A) (Wt1) + (B) (Wt2) = (Wt1 + Wt2) CG

So.....here's the figures

(33.5" x 145.8lb) + (113.75" x 29.2lb) = (175) CG

4884.3 + 3321.5 = 175 x CG

CG = 8205.8
--------
175

CG = 46.89" back from the 'Known point'.

We now know where the CG 'is' (at present).....but we also need to know where it 'should' be................to do this, I've used 3 different CG Calculation formulas, (the one on the UK's RCMF Forum (which I consider a little over complicated), the formula in Gordon Whiteheads 'RC Scale Aircraft' book, that I've used for years, and never had a poorly flying model after using it, plus the one on this website............

http://adamone.rchomepage.com/cg_calc.htm

............which I like as it's simple, straight forward, and the explanation on the page all makes sense, and is factually correct.

Having done this, I've ended up with 3 slightly different figures (though all 3 have ended up within an inch or so of each other, which on a model this size, is pretty good) The calculated position using the 1st 2 formulas is around 38% at the main plane 'average chord' position. This sounds MUCH too far back, but considering the HUGE stabiliser area (23% of the main plane.......BIG when you consider a Spitfire is closer to 10%!!!), and the VERY long moment arm, it becomes a lot more believable, though still sounds and little scary. But....using the 3rd formula, if I add in a 15% 'Static margin' to be 'on the safe side' as well as to cater for the fact that the main gear retracts rearwards, this calculated CG position moves forwards to around 33% at the average chord position.......looks and sounds much safer.

So.....a little nose weight is required, but how much?

1st I had to work out where nose weight could safely be added, and being I know it's not going to be a matter of a few ounces to shift the CG about 1.5" forwards, I'm not happy mounting considerable weight inside the cowl, as once vibration is considered, I don't think the light weight, rolled ply structure of the cowl will take it. So......the only sensible place to mount it is on either side of the engine box, just behind the firewall where there is sufficient structure to bolt it to....this works out to be 12" rearward of our 'known position'

Working back with the original CG formula......we can add in a 3rd set of figures (the 3rd known distance, and the nose weight required) and alter the existing figures to the following (using 8lb's as the estimated nose weight requirement)

(33.5" x 145.8lb) + (113.75" x 29.2lb) + (12" x 8lb) = 183lb x CG

4884.3 + 3321.5 + 96 = 183 x CG

CG = 8301.8
----------
183

CG = 45.36" rearward of our 'Known position'.

Looking back at the original CG position, the addition of 8lb of nose weight has moved the CG forwards just under the 1.5" required to bring it forwards to the 33% position.

[Andy Boylett]

A model with a forward CG flies badly, a model with a rearward CG flies once.................

Phil

Love that

Last year a friend in my local club flew his F15 ducted fan....

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Now, that might look like a really nice flying photo. However, I am not that good a photographer. This was the shot I got as the plane stopped 20 foot off the ground just in front of me. Then it fell vertically and landed in front of me in 3 pieces! Dave who was flying said he was astonished he even got it back to the filed as it was tail heavy he could not even turn without it falling out of the sky.

Great advice, thank you.

[Andy Boylett]

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

It was that link you added above that I used earlier today which came out with 32%.

For that calc my data is:
A=B=684mm
S=0mm
Y=2100mm
D=1679
AA=570
BB=292
SS=282
YY=626
This gave cg at 5% of 254, @10% 220 and @15% 186.

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..

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

Cheers, Andy