To conclude re-design of Xigris C1, the following details at different CG has been compared.

1. CG @ 20% MAC
2. CG @ 25% MAC
3. CG @ 20% MAC and NO wing Incidence
4. CG @ 25% MAC and NO wing Incidence

NP on CMAC = 0.4956

1.1 CG @ 20% MAC

Tail Volume (TVC) =0,700
Static Margin = 29,7%
Wing Aero Incidence = 0,9deg
Lift coeff at Current V =0,13
Fuselage Deck Angle = 0,68deg
Horiz Stab Incidence =0,01deg
Short Period Pitch Damping =0,83

2.1 CG @ 25% MAC

Tail Volume (TVC) =0,700
Static Margin = 25,0%
Wing Aero Incidence = 0,9deg
Lift coeff at Current V =0,12
Fuselage Deck Angle = 0,66deg
Horiz Stab Incidence =0,1deg
Short Period Pitch Damping =0,89


3.1 CG @ 20% and wing @ ZERO deg
No other differance than Fuselage Deck Angle @ 1,57 deg nose up.
4.1CG @ 25% and wing @ ZERO deg
No other differance than Fuselage Deck Angle @ 1,55 deg nose up.


As one can see, @ 20% MAC the Static Margin increased to close to 30%. This requires a larger deflection (13). At CG @25% this figure is (11).

What to do now?
1. Re-use the wing
2. Build new stab to TVC 0,7
3. Locate CG @ 25% with possibility to reduce this 1-3% (Shuffle batteries etc)
4. Set downthrust to -0,5deg.

WAIT FOR SPRING!!!!!

In meen time watch for Santa!


Regards

Having looked at this model -on line - I see nothing extraordinary about it
That is - looks like it should trim out fine as long as power is correct -and power makes a BIG difference.
On models such as these - I never used a bubble degree referrence for a simple reason
Too much error in these instruments as a matter of general findings. This is due to minute errors in LE and TE
Rather - I used a flat referrence plate (perfectly flat table)
Then I supported the model to level fore aft and side to side
Once I established this and a referrence line thru a zero thrust line , I had referrence for any adjusting which may be needed .
One degree is 1/4 " in one foot -which means - any error on a measuring level can make a sizeable error.
Measurements always went from table UP to given points on the model-using a vertical, adjustable pointer which could be slid around to compare points anywhere
Further -when all is completed -I typically had zero -zero zero OR a measurable wing angle incidence of + 1/16", Next I setup for 25%-27%% MAAC and flew the thing
This is not a secret procedure - it is very common as it gives a starting point.
The more relative angles you put in -the more compromises you will produce
Some flyers use little or no engine thrust deviance.

At cruise speed ( desired level "hands off") level flight speed -Typically THIS setup produced a slightly
nose heavy " setup
again very common. and should be expected
At this point , common procedure is to shift CG to no more than 30% and note how the model changes by doing only cg changes
The amount of exponential in the radio and throw of surfaces can totally confuse this part of setup
Ideally upright (hands off) to inverted flight will result in a predictable slight down pressure on the elevator
These are the most important stages of a setup -in my book
The amount of thrust line change will depend on POWER and your habits
There is no magic setup which will yield hands off in any attitude IF speed remains the same .
Many guys I have designed for and flown with, do the setups in a similiar manner - No secret trim formula.

I am glad to hear that someone else uses the flat table method for a reference for setups...

If you ever worked in a machine shop - you saw the advantages of this procedure.
You also learned the folly of cascading measurements .
I designed industrial machinery at one point in my life- and found out how easily errors in measuring can occur
Especially when curved and straight lines were involved.

I too use the 'flat table' set up method.

I have the most accurate digital incidence meter I have ever seen !
I don't use it.

I can set up with a pinpoint height gauge and then when I double check it with the
digital incidence meter I find that I am "dead on".

Having proven this over and over again I just use the height gauge now. Takes almost no time.

Like race cars I have found that most people have their own preference in aerobatic A/C set ups
and that it is NOT a 'one size fits all' phenomenon.

It still takes me about fifty actual flights to get it where I really like it.
After that I find virtually no difference for the rest of the year, in some cases multiple years.

The worst problem I have seen with the bubble level setups is that the V blocks in each end-follow the curves which may be way off
Many novices sand the leading edges on each panel such that the true cl is high on one panel and low on the other
It's a left handed right handed thing and very tough to see in some cases
Of course if one never has built from scratch - these things are not apparant.
As for all the incedence stuff-and how much is needed, that's another thing as on most really GOOD aerobatic stuff the required force difference from upright to inverted flight may be only a couple of clicks of elevator trim.
After 40 or so years of doing this stuff the really relevant stuff becomes pretty obvious.
Get it really straight -
find a really good compromise in balance for inverted vs upright flight
Then fiddle with the rest of it if you must
you will do best if you stick with a setup which is easy to predict -then fly it -a lot
One winner I knew flew with the elevator control reversed>
Anotoer flies with no expo (how -I don't know )
Others can take any basic setup and whip up on the rest of the bunch simply because they FLY each part of any maneuver and don't expect any hands off flight .
Having a huge reserve of thrust is misunderstood by those who have never flown electric and found the profound differences it can make-This also eliminates the thrust adjustments some feel necessary for UPlines where speed decay is a problem.
A fast trip forward to watch really GOOD indoor aerobatics -shows how important power can be in the equasion .

Interesting to me was the full scale aerobatic guys regarding proper trim.
" Get it about right and then compensate."

It really is simply a matter of how much compensation 'in flight' you are comfortable with.
"comfortable" meaning automatic, IE: un-thought.

My preferred set up for aerobatics is with about 3 to 5 degrees of sink (upright as well as inverted)
That way there is no such thing as 'neutral' !
Tiring but accurate.

You are so right, I used to work on the process machinery used in the manufacture of semiconductors very tight tolerance type of stuff, and know all to well the issue of "cascading measurements".
This is the type of thing can really throw off the performance of an aircraft if you don't get it right, and it all boils down as to how good your reference is.
I would imagine it is even more critical for precision aerobatic aircraft to get the setup dead on to start with...

I forgot to give you the new measurments of the new stab.

H-stab Area 215in^2 = TVC =0,7

Span 1/2 width 13 inch
Root chord 10 inch
Tip 6,5 inch
Elevator fraction 40%

All the rest is up to your self, but do'nt measure anything if you do'nt know what you measure.

Good luck!!!

Regards

Hmmm - hands off either way????
We found for our tastes -years back that simply altering amount of expo for up vs down elevator - allowed us to run MORE FORWARD cg's
The down elevator movement was a bit brighter than the up -around center.
Prior to split expo settings- this was done by altering the geometry of the servo linkages -which was more work but it did help
a bit of down pressure on the stick (very minute) for inverted flight -or straight down lines was accomplished easily
I was never able to eliminate the forces of gravity (unlike some) so I had to make do -

Hi,
New stab/rudder are now built and is 50gr lighter than the previous one. This had the expected result as needed on the CG.

Re-shuffling of battery (Zippy 5000mah 30C) was succesful re. CG.

Pre- re-build CG was @25% and plane was tail heavy. After re-built CG @ 25% and battery moved back towards tail 40mm.

Now, wait for spring!!

Regards

You just made a good argument for either method.

Dag,why you want to run with such a strong stability margin? Running with that much stability margin means that with even a 25% CG location you're going to have a very strong nose up pitching tendency due to speed changes one trimmed for hands offin level flight.

You wrote in your first post that "NP on CMAC = 0.4956". Am I correct in reading this as the Neutral Point is at 49.56% of the MAC? Or what is the C in "CMAC"? Or did you mean the Neutral point is at 49.56% of the root chord? Which, for a swept leading edge such as an Edge design would be about right.. If I interpreted your statement correctly and the NP is located at 49.56% of the root chord then you're going to want to aim at moving your CG back to around 45% or maybe even closer to the NP in order to achieve a better 0-0-0 thrust to wing to tail alignment in flight where you only require a smidge of down when inverted.

Hi BMatthews,

I understand that all measures given may cause some confusion, i shall do my best to clarify this matter.
BMatthews
Dag, why you want to run with such a strong stability margin?
Answ:2.1 CG @ 25% MAC

Tail Volume (TVC) =0,700
Static Margin = 25,0%
Wing Aero Incidence = 0,9deg
Lift coeff at Current V =0,12
Fuselage Deck Angle = 0,66deg
Horiz Stab Incidence =0,1deg
Short Period Pitch Damping =0,89

BMatthews
You wrote in your first post that "NP on CMAC = 0.4956".
Answ: This relates to NP of Cmac at 0,4956% where C is 14,25 inch and hence SM @ 25%.

Ater test flight I expect to lower some of the incidence calculated.
All this figures are a result of re-design of Zigris except for one thing, i wanted to keep main wing as is.

Best regards

But a 25% stability margin is a GROSS amount.  Even trainer designs only use about 10 to 12% stability margin.  Sport and competition  aerobatic models use more like 0 to 5%.

Yes, but the most interesting point about Triangulation Trimming was the recommendation to start with the C/G at 25% MAC and go not far behind that. Incidentally, that results in the static margin being 25% or not much less. Dag and I use the same Plane Geometry spreadsheets, and I tried for the Integral pattern plane and got very similar results (maybe due to similar tail volume). It even basically works in the simulator, but not for Brushfire which I compared it to. Just would have liked to get an explanation (rationale).

My good friend Steve Rojecki designed the Brushfire -as I recall in 1977?
We both lived in Cincinnati at the time - My pattern designs used a little different force setup ( thrust line closer to wing)

UStik, BMatthews

I suspect that SM in most of the small programs have not included for fuselage, hence strange figures. Yes, i use the Plane Geometry spreadsheet and i have used it for a prototype design which have been flying for a couple seasons, and it flies very good with CG at 25% and SM 25%. UStik, try to reduce/increase stab size and watch what happens to figures simular to the ones in my post. If I should have used a very low SM (0-5%) the following would go abselutely crasy; Short Period Pitch Damping =0,89
and any pull outs from say vertical would be very difficuld to perform without overshuting(Fig 12 page 22). Further i suspect that horisontal flying would be difficult due to very very small stab.

Further I think a SM @ 25% is not GROSS for a pattern model, where some people wants to fly at SM @30% or so with its concequences.

Best regards

UStic
Watch your mail box!

Dag

I haven't used the Plane Geometry spreadsheet.  And while I only fly sport models and never built a big pattern ship I know that to achieve a nice neutal handling model I've always ended up with the CG back around 30 to 35%.  And on the flat foamie I've got the balance is back around 40%.  If it were any farther ahead on any of those the models would be nose heavy and need lots of up trim.  But it would seem that Dick, our resident pattern model designing guru, agrees with your numbers.  So all I can do is shake my head in confusion and realise that I still have much to learn.

BMatthews,

Iam 65 and still learning, great fun!!!

Best regards

I believe Dag is expecting purer response from the model upon application of yaw force, with the model in any attitude. His numbers are similar to those I use in my designs, except my TVC is lower.

With CG back to 35% MAC, the model's response will be less pure in yaw. Snap roll exits become a little less predictable and spins could overshoot easier.

Don't want to put words in his mouth but I think Dag is seeking the best compromise for a truly neutrally responding precision aerobatic model. Based on the numbers I see, he will

MTK, if I read your post right the idea is to select a CG that makes all the other stuff work better and then size the tail such that the TVC results in a neutral point very near to 25% to achieve near neutral handling in pitch as well as yaw and the other things.   Is this assumption right? 

BMatthews,

The first part of your question, yes.... choose a CG no greater than 30% MAC, and lower is a little better. I don't go any further fore than 25%MAC, however, on my aerobatic models. A little further fore is tolerated better by trainers.

All of today's pattern models can tolerate CG as far aft as 40% MAC but they won't fly a pure envelope with that far aft a CG.

The second part of the question, no. The neutral point will be aft of the CG and you will need some push on inverted flight, assuming you trim for hands off upright flight.

A lower TVC places more load on the wing as does a fore CG. A lower TVC will reduce pitch stability, allowing smaller elevators and making smaller pitch commands more effective. This will result in a more lively, agile aerobatic model. Lower TVC enhances at least the two bugaboo maneuver categories in aerobatic circles: snaps and spins

Again, great for aerobats but not so great for trainers. Trainers should sport high pitch stability and high TVC.

FWIW- I pick a speed range I like -then setup for GOOD stability
A very stable model will snap extremely well and recover on line
The really aft cg stuff goes off hunting rabbits rather than recovering on line
Also a slightly nose heavy model will drop into th spin (no forcing req'd) and exit better
-Trying for handsoff all around -is to me - futile Now for th little foamies - completely different setups
way aft cgs -and all flying is slow and you can power your way out of anything