My friend needs to balance his Ultimate biplane. As you may know, the
Ultimate has swept back staggered wings.
The problem is, the indicated CG on the plan has it just about 1/3
(33%) from the LE under the top wing.
I had thought that for a staggered wings plane the CG has to be at the
average 1/3 of the two wings.
Which is correct, 1/3 under the top wing or the average of 1/3 of both
Why does your friend not believe the plans? If nothing else, it's a great
starting point -- you know, like when all else fails, read the instructions
Cheers -- \_________Lyman Slack________/
\_______Flying Gators R/C___/
\_____AMA 6430 LM____ /
Visit my Web Site at www.LymanSlack.com
Lyaman, you're right about reading the instructions, but only the CG
was drawn in the plans without commentary. If my friend was to go by
the given CG in the plans, he'd have to add 36 oz. or 2 1/4 lbs. of
weight in the nose section. He would like to avoid dead weight.
If he could use the already balanced CG at the average of 1/3 of both
staggered wings, he need not add weight, just some adjustments by
moving some inside components.
He is very nervous about getting this correct on the Ultimate, as he
had crashed two planes recently.
I have a DPM Ultimate and it balances between 50% to 60% of the center
cord on the top wing. The 60% setting is needed for torque rolling
maneuvers. The 50% settiing is quite docile. I think you can use the
same ballpark numbers regardless of your model size as lon as it is
fairly close to scale. Sizewize(sp?) the DPM Ultimate is 60.5" span,
64.5" length, 1230sq in area, and weighs about 10.5lbs. Using a Saito
150.(180 would be better)
Don"t forget guys that the 1/3 back thing is for a clark Y airfoil(flat
bottom)...the ultimate, if scale, has a fully symetrical airfoil which
is not tapered but constant cord swept. If you are gonna err, do it on
the nose heavy side that way it will just fly like a dog but at least
Phil AMA609 (Lotsa biplanes here.)
Actually it's not directly related to the airfoil -- see
http://www.zenithair.com/kit-data/ht-90-4.html by a full-scale aircraft
designer, who doesn't mention airfoils.
It _does_ relate to the size of the tail and the tail moment, so if you
put on an oversized tail or if you put the tail way back then you'll be
able to move the C/G back.
It is _indirectly_ related: Because the Clark Y center of pressure
ranges around 33% of chord instead of the 'natural' center of pressure
at 25% of the chord*, a successful craft with a Clark Y has to have a
bigger stabilizer to overcome the moment; this bigger stabilizer means
that you can move the center of gravity back.
I mostly agree with your recommendation, however -- as long as it's not
so nose heavy that you have to land at 100mph to keep the nose up, when
in doubt you should start with the thing nose heavy. Make that nose
weight easily removable for test flying, however, and play around with
it a bit -- try to sneak up on the right balance (don't overshoot by too
much -- that may re-kit the airplane for you).
* The description I've heard is that every airfoil has three factors
that impact the center of pressure and coefficient of lift in an
un-stalled, subsonic condition: First, inside of every airfoil there's
a symmetrical section trying to get out. The result of this is that
every airfoil acts like there's a lift force at 25% of the chord that
varies with angle of attack. As long as the wing isn't in stall and
isn't too stubby this lift varies in direct proportion to angle of
attack and airspeed squared. Second, because of camber there's an angle
of attack offset, so a cambered airfoil may need a negative angle of
attack to have zero lift. Finally, every airfoil has a torque that
varies only with airspeed squared -- so a symmetrical airfoil wants to
fly straight, while a Clark Y wants to pitch down. You can use this to
advantage in a flying wing: by building an airfoil that wants to pitch
_up_ and putting the center of gravity ahead of the 25% point you'll
have a craft that's stable in pitch even without an elevator.
Thank you for posting so often and contributed so much here. Also for
the link to the full sized airplane designer, who doesn't mention
airfoils. My friend is your basic model builder and has a very
elementary knowledge of geometry. What he wants is to have someone to
tell him "exactly" where the CG must be for his Ultimate.
However, we know that flying model airplanes is not an exact science.
Still, it is very interesting to read all the posts here and I could
use some of the pointers myself.
His plane is nearly finished. I don't like to see him discouraged
from a bad crash if his plane is not properly balanced. As one person
said, the drawing may be in error.
So, what should I tell him about the CG on the plans he purchased? Or
should he go ahead and use the indicated CG on the plan and add the 36
oz. of lead?
So he makes it easy to remove when he's tuning it. There are numerous
articles out there on telling where the center of gravity needs to go
based on flight performance.
The test that the sailplane guys do is to get the thing trimed out at
one speed, then dive. If it tends to pull out of the dive then the C/G
is too far forward; if it tends to tuck under the C/G is too far back.
This gives you neutral stability, which is just right if you're a good
flyer with quick reflexes (trainers do _not_ put the C/G back so far).
For an intermediate pilot -- put it in an intermediate spot.
The long story:
I recently put together an AT-6 Texan model. With about a 9.5" chord
and positive taper the directions had me put the C/G at 1.5" back from
the leading edge. That didn't seem right, so I ran the numbers and came
up with a C/G about 3.5 or 4" back (that taper does make a difference).
With the C/G toward the front of that range it was _way_ too noseheavy
-- it ended up with a 1 ounce block of tungsten screwed onto the
tailwheel mount, which brings the C/G back far enough that it handles
The short story: You can't always trust C/Gs in plans.
refer to Alan's Hobby, Model & RC Web Links
sub section :
"Calculators, Conversion, Center of Gravity, Electric Flight , MAC & Servo
Torque calculation charts."
Balsa Density, Float size, Vacuum forming pictures etc - inc downloads
Conversion & Calculation tables etc
metric - Imperial Converter - down load free zip file.
CG & MAC - Graphic
CG & MAC calculator = PJB's Seriously Aeronautical Stuff
CG Calculator & Basic Design of Flying Wing Models
CG Sling & Balance beam "Vanessa" Zip file - email Alan for a copy.
Convert - free download - convert units from one type to another ****
Design Calc - from Van's Place (site down - email Alan for a copy.)
E/Flight - Drive Calculator V2.1 ***
Entisoft Units -- Measurement Conversion Calculator --
Free Software ex NASA_design jet engine, airfoils, kites, Rockets etc.
Martindale's Calculators On-Line Center - (over 1900 covers almost every
Motor Calc Program - find which motor, battery and prop is right for your EP
Performance Slide Rule Calculator - Servo torque, Horsepower, Stall Speed
R/C Aircraft Proving Grounds - CG, MAC, Servo & Basic Model Calculators
RC Calculators - C.Gadd. [Servo Torque, Wing Loading, Glow to Electric etc.]
RC Performance calculators - Thrust, hp, Airspeed, RPM vs Load. etc
Weight Estimation Spreadsheet - Jim Ryan
Alan's Hobby, Model & RC Web Links
On 2 Apr 2006 14:06:36 -0700, email@example.com wrote:
What brand and 'size' Ultimate is this ? Is it a kit or an ARF ? The
reason I ask if it's a kit is that some people tend to build real
heavy and can make for a tail heavy airplane. Many biplanes need a lot
of nose weight...one that comes to mind for being notorious for this
is the Sig Skybolt.
32 oz is a lot of weight for a .40 size aircraft but not so bad for a
Also , is he putting the recommended engine on the aircraft or does he
possibly have a a lighter one ?
I'm looking at plans for a .40 size Great Planes Ultimate and it shows
the CG 2 3/8" from the LE at the fuse on the Bottom wing which looks
to be about 25-28%. Don't have the aircraft to measure.
If it would help , I have a book on RC Airplane design that shows a
simple way to arrive at the CG. I could send you a sketch of it.
Finding the CG is really not a complicated thing , but with this
method you don't even have to know the wing area.
But , like most everyone else here , I would say follow the
instructions , especially if it's a well known brand of aircraft.
I can't recall ever having an aircraft that didn't balance where they
said it should.
Ken, keep that thought in mind. I won't see my friend until the Toledo
Weak Signals Show (We live in Toledo). I do know he bought the plans
and built his Ultimate from "scratch" and may not have selected the
lightest balsa. The motor is a 150 cc gasolene engine and it could be
on the light side, which may result in a tail heavy plane.
The reason I question the CG is there is a discrepancy between the
amount of stagger of the wings on the plans and the actual built plane
of at least 2 inches. That is, the plans show more stagger and the
plane has less.
I would like to double check the measurements and come back for your
further suggestions. I could be wrong.
<snip>150 cc ?
Ken, you're right. I checked about the engine and it is a 3W 75cc gas
burner. I had misunderstood my friend.
I still have to check the measurements of the stagger when I next see
On 5 Apr 2006 18:05:14 -0700, firstname.lastname@example.org wrote:
Wan , an airplane that requires a 75cc 3W would be in the
neighborhood of 20 lbs or more. The weight your're speaking of to
balance this thing is not uncommon at all. Didn't have any idea you
were talking about a 1/3 scale or thereabouts.
Yes, it's a very large aircraft. But I think I will let my friend work
it out by calling the designer of the plane about the CG. He had a rash
of bad luck in the last couple of weeks, as he had crashed his Twist
twice. The second time it was damaged beyond repair. Then the troubles
he had with his Edge 540. Nearly lost it due to a glitch in the control
system. When he thought he had replaced his warped tail feathers of the
Edge, on his first test flight with the modifications, nearly lost
control but saved it.
Thanks for all your help, but I don't want to suggest anyting to my
friend that may lead to problems that may contribute to his already
deep discouragement. And if he have problems after my suggestions with
I had been looking into your site about your busy involvement in
musical instruments and modeling history et all. You have been and
still are a very active person.
With sincere thanks,
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