Topic: % MAC balance point for a plank

I''m having a go at my first homebrew plank.
Without getting to in depth with the theory, what would be a starting point for the %mac balance point?

For conventional models I start at around 28% but I've got a feeling that for a plank it will be a fair bit less than this.

The planform is just a straightforward taper with slight rear sweep.

This may be of some use.

http://fwcg.3dzone.dk/

Regarding the actual CG position though I think it rather depends on the section you are using.
Generally though on most of my own racing planks I tend to put the CG at around 22% and the ballast tube is a tad behind that at 23%.

great post here for setting up planks

http://www.rcgroups.com/forums/showpost.php?p=9800191&postcount=55

1/4 oz is the new black (and yellow).

Usefull stuff, but I only have 10g and 5g weights ...

so long as you're on 2.4 you'll be ok with either 

Thanks for the info chaps

The RCG article says start at 18% MAC - isn't  that a bit far forward even for starters?

I work on the basis that it will be somewhere around 19% which has proved to be a pretty good starting point for the section(s) I use but as has been said, it can vary with section.

Steve

In the end it's very much down to how you like to fly. I know some people who fly their models with a far back cg and very little elevon movement. Others prefer a more conservative set up with one that's further forward but has more elevon throws. i think it's often down to the model in question and also the wing section that it uses.
In the end, there's no right and wrong apart from what suits you is obviously also right for you!

You're trying to tie down 3 variables, the cg, the reflex and the elevator throws. All of these are far more critical when you have no tail out the back.

18% is given as a starting point because (with a bit of excess reflex) you'll get the model to fly (all be it badly) even if any of the other factors are out. From this first flight you can then eek back the cg and dial down the elevator throws in gradual increments until you get to the sweet spot.

Make sure all your linkages have ZERO slop
Don't confuse a bad launch for anything else
Have plenty of aileron throw, but tiny amounts of elevator
Don't use expo until you have the cg and throws nailed.

You're trying to tie down 3 variables, the cg, the reflex and the elevator throws. All of these are far more critical when you have no tail out the back.

18% is given as a starting point because (with a bit of excess reflex) you'll get the model to fly (all be it badly) even if any of the other factors are out. From this first flight you can then eek back the cg and dial down the elevator throws in gradual increments until you get to the sweet spot.

Make sure all your linkages have ZERO slop
Don't confuse a bad launch for anything else
Have plenty of aileron throw, but tiny amounts of elevator
Don't use expo until you have the cg and throws nailed.

Wot he said 

Mostly.

As Skirmish also implied, 18% is conservative but relatively safe. I go for 21% for maidens, and I also build my ballast tubes in to centre on 23%, so ballasted up I am probably back to nearer 22% too.

People have commented planks are critical to CG, and on a typical 60" plank with MAC of around 190mm the difference between 18% and 22% is, obviously 7.6mm.

This might seem fairly generous, but the important thing is that while most planks will fly within this range there will be a "sweet spot" where the performance is much better.

Pauls approach is the way to go.

Tim

You're trying to tie down 3 variables, the cg, the reflex and the elevator throws. All of these are far more critical when you have no tail out the back.

18% is given as a starting point because (with a bit of excess reflex) you'll get the model to fly (all be it badly) even if any of the other factors are out. From this first flight you can then eek back the cg and dial down the elevator throws in gradual increments until you get to the sweet spot.

Make sure all your linkages have ZERO slop
Don't confuse a bad launch for anything else
Have plenty of aileron throw, but tiny amounts of elevator
Don't use expo until you have the cg and throws nailed.

Sounds way easier to get a plane with a tail...

Sounds way easier to get a plane with a tail...

Ah, but the OP is designing/making his own model so if he had a tailplane he would still have the problem of getting the incidences right. You don't have that problem with a plank! 

rule of thumb, about 8-10% of chord further forwards than you would of put it if you had a tailplane.
5-8% for swept wings.

rule of thumb, about 8-10% of chord further forwards than you would of put it if you had a tailplane.
5-8% for swept wings.

Really?

On a conventional layout I would be looking at 33% to start with.

So you are suggesting 23 to 25% for a plank, and 25 to 28% for a chevron?

To me, that sounds like you are cruisin' for a bruisin' man....

I find it much easier to lob off and correct a dive if necessary, than lob off and try to regian control while ducking as it gets whipped back over my head.

Tell me, do you often get smacked in the face by your own model on inital launch?

ERRRRRrrr  you aren't Lec in real life, are you?

JMTC

Tim

Who has built a LOT of planks....

never had a problem with it, i tend to get it about right by eye alone most times on the slope.

Rule of thumb for an unknown model - in other words if I was designing a straight (no sweep either way - no taper - nothing - just a constant chord) flying wing then the starting point for a completely safe test flight is 15% from the L E!

Yes it's 'safe' and then some, but far enough back to be able to fly - from there you simply move it back one test flight at a time until you like the way it flies.

This is good for any wing with any section 

It depends a lot on the foil, and how you like the setup.
Some foils are happy with starting at 22% (PW51), others need to be nearer to 18%. As has been mentioned...it depends if you like to fly a tad nose heavy an with more elevon movements or are happier with a more reasward CoG and less movement.....On a new toy I usually aim for 18-20%, they usually will fly, but will need lots of trial flights to get the sweet spot that suits you.



He who is always playing with new formats of wings (planks)...an not always forward swept

just for us not in the know !   What dose MAC mean?  o and while i'm at it what dose ARFT mean?

just for us not in the know !   What dose MAC mean?  o and while i'm at it what dose ARFT mean?

Mean Aerodynamic Chord.
ARFT means toward the back of an upper crust model.

On the other hand ARTF means Almost Ready To Fly.

Mean Aerodynamic Chord.
ARFT means toward the back of an upper crust model.

On the other hand ARTF means Almost Ready To Fly.

!

Then there's ARTC - almost ready to crash

FUBAR

SNAFU

etc 

I think if you are familiar with planks you can afford to start 20% or more as long as you understand plank elevator throws.

Splinter we started at 22 and it was an angel on the first flight

Splinter DSing on the Skirrid

Z, who loves to post that vid!!