Hello everyone. this is my second year in pattern and I am new to balancing a pattern plane. this is what i have been told after countless tail heavy situations:

(I own a Summit 3)

Aerodynamic theory says for a plane to be stable, the center of gravity
(CG) has to be in front of the center of pressure (CP), the point where
all aerodynamic forces (lift and drag) are centered. The closer the CG
is to the CP, the less stable the airplane. If the CG is at or behind
the CP, the airplane is unstable, that is, it will not have a natural
tendency to fly nose first. The CP is not easily measurable by
conventional means, but if you traced the shadow of the airframe as
viewed from above, it would be roughly the point where the areas in
front and behind the point are equal. The area of the wing usually
dominates this point, and the areas of the fuselage and tail can be
ignored. But take a look at the horizontal stab of the Summit, it is
very large compared to the horizontal stab seen on a typical sport
plane. It has the effect of moving the CP rearward, which allows the CG
to be more rearward than conventional thinking would allow.


Can somebody simplify this for me please. I am used to balancing a sport aircraft where normal theroys take place.

I hate to bust your bubble here...but it just ain't that technical.

That era of planes was simple.....Stick it around the wing tube...Like right on it....Then fly it.

Go to the NSRCA web site www.nsrca.org

There is a trim chart under technical stuff....It has some trimming tests to setup the CG properly. CG is a feel thing.

I like a plane to have a feel....Others seem to measure it by the amount of down elevator needed to hold inverted. I guess a good rule of thumb I use is Pull 45 up line roll it inverted and see how well it holds the line....If it maintains the line without any down elevator needed its tail heavy. It should arc slightly shallowing the 45 deg line.

Another thing is the amount of pitch coupling associated with rudder input. Fly Knife edge and if the plane pitches to the belly its tail heavy and pitching to the nose is usually nose heavy....There are other factors that can cause this so I usually play with it until I find the best feel and best other attributes.

You should find that pattern planes are very tolerant of CG location.....Everybody likes their planes a certain way and not everyone feels the same about CG...In fact most people like their planes too tail heavy for me....The more nose heavy a plane is the better it will hold heading in a snap...But you get to a point where the the plane takes too much control surface throw to achieve the stall in a snap roll.....

Since your just starting out don't worry about the snap rolls. It will not be an issue for you for a while yet.

My best advice is to get with a local pattern flier or go to a contest around that area and get a pattern flier to fly your plane.

I had a Summit but it was a long time ago and I don't know where the CG is noted on the plans....If you have this number then use it. Many people flew those planes it was a very popular and great flying plane for its day.....

Its generation was the last of the 60 sized pattern models and it was a very good combo and very competitive in its era. Most designs in this era had the wing tube on or very near the CG...So use the wing tube as a starting point if you don't know....

The CG will always need to be tweaked to meet your needs....Try the 45 upline with the 1/2 roll....and this will give you a good feel...my personal opinion.


Troy Newman
Team JR

Troy is spot on (as usual) with his assessment that CG is very much subjective and in fact all trimming can only be done by seeing how the plane flies. I personally like my CG quite well back as I like the way this makes the planes roll, a friend of mine in our club who also flies pattern sets his planes up with a CG further forward than I like it, to the extent that I don't really enjoy flying his planes and he doesn't like mine.

I read an article a few years back which gave a formula for waorking out a good starting point for the location of the CG, and I was amazed to find it actually works for me. For others such as Troy I suspect it would place it too far back.

However, I don't usually bother to work it out, I fly it and see what happens! And it's worked ok so far.

If anyone wants a copy of the formula let me know, I won't put it here though as it's not terribly easy to enter into this kind of format.

James

I have the CG marked 8 1/2 inches from the trailing edge of the wing which is where the builder said it would be and it is also where it came out to be when we did the math. we used 25% and 30% of the MAC for starters but for the plane to balance relatively close to level we had to move it back around 50%. i cant do anything to the equipment inside the plane to fix this and it would take too much weight to balance it out. the previous owner actually flew it like this (i have the exact same equipment as he) and he said it is ok. he said normal balance properties do not apply to pattern planes because the large tail section moves the CP rearward. the message he sent me was in the middle of my last message. is this correct????

CG rules still apply to pattern planes...The only difference is they are more tolerant of CG changes.....

If the CG by the designer is where you have that's a great starting point...Ivan was and still is a great flier and has designed some great flying planes.....Trust his advice on CG.....you may like it a little more aft or a little more forward...for taste.....

As for all the talk about Center of Pressure on the wing....I'm a mechanical Engineer and I can tell you your not going to calculate a center of pressure on that wing!

I also seriously doubt you are at 50% of the MAC...maybe 50% of the root cord but not 50% of the Mean Aerodynamic Cord (MAC).


You don't need not worry about the CP on the wing...and you don't really need to worry to much about the percentage of MAC....

The calculations will work for our models so I won't say that they are garbage...but in practical application they mean didily squat!

Making the numbers come are not going to make you fly better...and putting the Planes CG per the numbers is nothing more than a one time exercise that you are going to change right away......Most pattern models are not designed around these principles anyway.....they are designed by trial and error....Not only that but at the reynolds numbers we are flying at the rules are very broad and the results from the proper calculations have a very very broad range...we are talking like 10% of the wing cord difference...The important part is the feel not the numbers....So drop the idea of the numbers they mean squat!

I can tell you from experience run the CG Ivan has on the plans.....We flew tons of these planes back in the early 90's and the CG is on or close to his recommendation.

Just my opinion...If you still want the numbers I'm sure someone can tell you how to calculate the stuff....but be aware the C of P is not the only thing you need to worry about...Reynolds number...What speed is the plane flying at? Air density? Incidence....total weight of the model...and on and on....Not to mention what air foil is on the model?

Do you know the air foil....? You need that before you can even venture into the world of aerodynamics....

Its a lot more practical to just stick it on the CG on the plans and tweak from there.....

Good Luck
Troy Newman

about theory.

seen from a pure theorical point, having the CG as close from the CP as possible make your plane going faster also.
I'll try to explain with my poor english
because the CG is in front of the CP (always on a RC plane) there is a nose down reaction of the plane (imagine that the plane is pull in the air from the CP, he wants to turn around the cg). this pitch down moment has to be compensate by a pitch up moment from the stabiliser. to have a pitch up from the stab, the stab has to produce a negative lift (I mean pull the tail in the direction of the ground). in flight, the summ of all the lift forces must be equal to plane aparent weight (weight*g factor).
as close the CG is from the CP as small the negative lift of the tail is, so the wing may produce less lift and then less drag....

having the cg close to the CP make the plane more unstable, so it has to bee kept in the tolerance. On modern "fly by wires" airplanes, because there is a computer between the stick and the flight controls, the CG may be set BEHIND the CP resulting in a unstable but economical and performant plane.

in real plane we always say "after CG save fuel".

this was my stupid coment

I hope you understood my poor english

Wow this clears things up. I guess ill just fly it the way it is then. Thanks a lot guys.