% MAC balance point for a plank

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Author Topic: % MAC balance point for a plank  (Read 1038 times)

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Offline cliffhanger wrote % MAC balance point for a plank on May 17, 2011, 10:18:25 AM
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.


Reply #1
Offline skirmish wrote Re: % MAC balance point for a plank on May 17, 2011, 10:23:14 AM
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%.


Reply #2
Online paul w wrote Re: % MAC balance point for a plank on May 17, 2011, 10:30:17 AM

Goodwind Slope Soaring     blogtastic hill side adventures

Reply #3
Online satinet wrote Re: % MAC balance point for a plank on May 17, 2011, 11:33:40 AM
Last Edit: May 17, 2011, 11:35:07 AM by satinet
1/4 oz is the new black (and yellow).

« Last Edit: May 17, 2011, 11:35:07 AM by satinet »

Reply #4
Online paul w wrote Re: % MAC balance point for a plank on May 17, 2011, 11:47:14 AM
Last Edit: May 17, 2011, 11:50:01 AM by paul w
Usefull stuff, but I only have 10g and 5g weights ... :embarassed:

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

« Last Edit: May 17, 2011, 11:50:01 AM by paul w »
Goodwind Slope Soaring     blogtastic hill side adventures

Reply #5
Offline cliffhanger wrote Re: % MAC balance point for a plank on May 17, 2011, 13:31:34 PM
Thanks for the info chaps :af

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


Reply #6
Offline n-tropic wrote Re: % MAC balance point for a plank on May 17, 2011, 13:36:52 PM
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


Reply #7
Offline skirmish wrote Re: % MAC balance point for a plank on May 17, 2011, 14:15:03 PM
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!


Reply #8
Online paul w wrote Re: % MAC balance point for a plank on May 17, 2011, 14:45:49 PM
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.


Goodwind Slope Soaring     blogtastic hill side adventures

Reply #9
Offline the.Timinator wrote Re: % MAC balance point for a plank on May 17, 2011, 17:16:49 PM
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  :af

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

Ah HAH! Thats how you do it!

Reply #10
Offline Woodstock wrote Re: % MAC balance point for a plank on May 17, 2011, 17:18:08 PM
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...

Chris van Schoor

Reply #11
Offline skirmish wrote Re: % MAC balance point for a plank on May 17, 2011, 17:39:15 PM
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!  ;D


Reply #12
Offline Cactus wrote Re: % MAC balance point for a plank on May 17, 2011, 18:46:53 PM
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.

I know you believe you understand what you think i said, but i am not sure you realise that what you think you heard is not what i meant.

Reply #13
Offline the.Timinator wrote Re: % MAC balance point for a plank on May 17, 2011, 20:52:17 PM
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....

Ah HAH! Thats how you do it!

Reply #14
Offline Cactus wrote Re: % MAC balance point for a plank on May 17, 2011, 22:11:51 PM
never had a problem with it, i tend to get it about right by eye alone most times on the slope.

I know you believe you understand what you think i said, but i am not sure you realise that what you think you heard is not what i meant.

Reply #15
Offline enginetorque wrote Re: % MAC balance point for a plank on May 17, 2011, 22:56:55 PM
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  :af


Reply #16
Offline yellowblue wrote Re: % MAC balance point for a plank on May 17, 2011, 23:11:37 PM
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


Reply #17
Offline Rimmer wrote Re: % MAC balance point for a plank on May 18, 2011, 07:13:22 AM
just for us not in the know !   What dose MAC mean?  o and while i'm at it what dose ARFT mean?

Mmmmm Ascot!!

Reply #18
Offline mr ed wrote % MAC balance point for a plank on May 18, 2011, 07:46:23 AM
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.

"I learned a lot from my second marriage... I learned they won't sell you a hand gun if you're crying..."

Reply #19
Offline Woodstock wrote Re: % MAC balance point for a plank on May 18, 2011, 08:34:32 AM
Mean Aerodynamic Chord.
ARFT means toward the back of an upper crust model.

On the other hand ARTF means Almost Ready To Fly.
:af ;D!

Chris van Schoor

Reply #20
Offline enginetorque wrote Re: % MAC balance point for a plank on May 18, 2011, 13:12:35 PM
Then there's ARTC - almost ready to crash

FUBAR

SNAFU

etc  :study:


Reply #21
Offline Zim wrote Re: % MAC balance point for a plank on May 18, 2011, 22:07:48 PM
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!!

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