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Flight Trimming the CG

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Flight Trimming the CG

Old 05-15-2011, 07:17 PM
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RC_Fanatic
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Default Flight Trimming the CG

I know that for a pattern plane with a symmetrical wing, you want the CG such that only a slight forward pressure on the elevator is required to maintain level flight when inverted. What about a plane with a semi-symmetrical airfoil or one with a flat-bottom airfoil? Is it the same?

Old 05-15-2011, 08:53 PM
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Default RE: Flight Trimming the CG

In essence, yes. But with cambered airfoils, because of their poor inverted performance, the best method is the 'dive' test. Simply, enter a 30 to 40 deg dive from trimmed 'S&L', allow the speed to build, then release the elevator. The required reaction from the model is a gentle pull out to S&L. Quick pull out = balance too far forward, steepening dive = balance too far aft.
Evan, WB #12.
Old 05-16-2011, 04:03 AM
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rmh
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Default RE: Flight Trimming the CG

FWIW- I find that the trim and how speed affects it -is the same on either shape airfoil
Why?
It's all about built in angle of attack
The 2412 or Clark Y, etc flies ok -but it is a draggy setup- It is a low speed design-always was
Build a flat foamie - with big ailerons and set it up so that you can rig ailerons to trail slightly up or down -
see what happens
Speed will affect trim on either setup
Old 05-16-2011, 04:29 AM
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Default RE: Flight Trimming the CG

http://www.polecataero.com/handlaunchu/cg-location

Curtis
Old 05-16-2011, 05:56 AM
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Default RE: Flight Trimming the CG

Thanks, guys. I was rather imprecise and sloppy in my question. I was not implying the same airframe with three different airfoils, but rather 1) a pattern plane with a symmetrical airfoil, 2) a sport plane with a semi-symmetrical and 3) an aileron trainer with a flat-bottom airfoil. I see that the dive test would work for all to determine the "neutral" CG point but the CG might be moved forward various amounts for stability. Is this correct?
Old 05-16-2011, 06:22 AM
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rmh
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Default RE: Flight Trimming the CG

ORIGINAL: RC_Fanatic

Thanks, guys. I was rather imprecise and sloppy in my question. I was not implying the same airframe with three different airfoils, but rather 1) a pattern plane with a symmetrical airfoil, 2) a sport plane with a semi-symmetrical and 3) an aileron trainer with a flat-bottom airfoil. I see that the dive test would work for all to determine the ''neutral'' CG point but the CG might be moved forward various amounts for stability. Is this correct?
Well - no-
three different cats
basically it has to do with speed /power differing
The trainer:
designed to fly at a low speed and to then climb (recover)as speed increases
The sport:
fly a little faster and still recover (climb) as speed increases
The pattern :
Ideally is fairly neutal and so has cg placement and various thrust/drag angles setup so that speed changes offer little pitch difference.
IF- you setup a Seniorita (basic trainer) to be neutral- you won't like it and the character designed into it will be gone.
Let's get airfoils sorted out
the first shapes used were draggy things designed to work at low speeds ( these were undercambered_
the next step were thicker shapes -usually flat bottomed and were stronger because they were thicker - they also were less sensitive to angles of attack
The streamline shape (sym)used in current fighter craft and on pattern planes flies at very low AOA and may use movable surfaces to work better at low speeds
Now - the opposing angles designed into the trainer -will fight you getting a "nuetral" dive - if you keep sliding the weights back-to move cg back- the plane will become weird and very sensitive and speed sensitive
ALL cg/power settings are compromises- No exceptions
You have to come to grips with
type of flying desired
speed range desired
Also maueverability desired
which brings up wing loading requirements for speed range
(more compromises.) After all this is digested - you may want to look up formulas and graphs which are simply models representing how these things interact.
I don't know any math so I don't use em.
Old 05-16-2011, 07:17 AM
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Default RE: Flight Trimming the CG

You can also set trim at Transmitter.There is an adjustnent for all servos?
Old 05-16-2011, 07:25 AM
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Default RE: Flight Trimming the CG

Thanks for the clarification, rmh. I do have a copy of von Mise's "Theory of Flight" but it is tough slogging! Somewhere I have a copy of Eric Lister's sailplane design guide which, as I recall, has a good explanation of stability but it does not get into the effects of power.

After a few years of intermediate low-wing trainers, I'm getting into pattern and war birds so these questions interest me.
Old 05-16-2011, 09:41 AM
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Default RE: Flight Trimming the CG

In all cases the balance location determines pitch (speed) stability. And that is all it does. The other determinimg factor in its location is the amount of angular difference between the the wing and tailplane. Gliders and trainers tend to have their 'trimmed speed' built in by having some positive difference between the two, that is the wing is at a couple or so degrees more than the tail. The dive test is then used to achieve the the most rearward balance that is still pitch stable at that trim. This setup can be used with scale models, and warbirds, just to make them easier to fly, in that they too can be set up to have a natural 'cruise' speed and will then respond to changing speeds in a predictable way. Aerobatic models, for want of a term to group them all in, tend to have no angular difference between the flying surfaces, so now the speed range, without the model changing pitch attitude, is designed to be much wider. Modellers like them to display the same control responses either way up, so although the dive test works for models that spend most of their life upright, it needs to be modified for those that don't care which way up they are. So the methods for determining the balance is different. However, the overall position of the 'best' balance is individual, you, the pilot, will decide which is the 'nicest' place to fly it.
Evan, WB #12.
Old 05-29-2011, 12:11 PM
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Default RE: Flight Trimming the CG

Thanks, Evan. That helps clear up my confusion. I'm just learning that my new pattern plane (Focus Sport) has very different characteristics than my pylon racer (WM T-34) or my "war bird" (BH 60-size T-28).
Old 05-29-2011, 02:44 PM
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Default RE: Flight Trimming the CG

ORIGINAL: RC_Fanatic

........... but the CG might be moved forward various amounts for stability. Is this correct?
Correct.
By playing with the location of the center of mass or CG, we modify the distance that separates that center form the center of lift and drag.

During flight, the plane rotates in any of the three axes only around the CG.

The bigger that separation (CG more forward):
1) There is more lever for the aerodynamic forces to keep or restore the alignment of the center line of the plane with the direction or trajectory of the CG.
2) There is more load on the horizontal tail (and also on the wing), either during upside up or inverted flight. Hence, more elevator surface or deflection is needed for flaring.
3) During loops, the force on the CG increases much as the G's, multiplying the demand on the tail and elevator. Loops of tight radius at high speed are not achievable.

The smaller that separation (CG more aft):
1) There is less lever for the aerodynamic forces to keep or restore the alignment of the center line of the plane with the direction or trajectory of the CG.
2) There is less load on the elevator (and also on the wing), either during upside up or inverted flight. Hence, less elevator surface or deflection is needed for flaring and authority of the elevator during flight is higher (to the point of becoming excessive when the CG is too close to the CL).
3) During loops, the force on the CG imposes less demand on the tail and elevator. Loops of tight radius at high speed are possible.

The airfoil type has nothing to do with those things.
One type lifts more than another for the same variation of AOA, that is all.
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Old 06-02-2011, 04:12 PM
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rmh
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Default RE: Flight Trimming the CG

ORIGINAL: RC_Fanatic

Thanks for the clarification, rmh. I do have a copy of von Mise's ''Theory of Flight'' but it is tough slogging! Somewhere I have a copy of Eric Lister's sailplane design guide which, as I recall, has a good explanation of stability but it does not get into the effects of power.

After a few years of intermediate low-wing trainers, I'm getting into pattern and war birds so these questions interest me.
I am presently flying gliders as well as smaller sport models but have run the gamut of pattern- IMAC Quickie etc..
Trimming is a science to some and a religion to others
There are no secrets but there are some basic guidelines as others here have mentioned
A few other points :
1 Changing the speed range of the model changes the BEST trim setup
for example get setup as good as you can for a good easy to fly setup - then- with no other changes - either try flying everything faster or slower
You will find pitch settings are different - roll is different and ability to hold vertical lines different.

2 Changing the thrust setup (relative prop size ) has a more pronounced affect
for example
The typical glow setup (1.4 cu in four cycle ) which pulled a 14" prop had good response but some power spool up was always required.
A electric setup will pull more blade (and disc) and power response is instant- so no leading (early power application) required .
Some may say no -but try it - you will see.
It is simply because electric setups have max torque available at zero rpm.
The type aerobatic model YOU may like is a personal thing but it is possible to fly any sequence -very well -with scale models of good full scale aerobatic planes
Some people will insist you need to severely modify things to get pure yaw/pitch/roll
There are plenty of varients to choose from so the choice is yours
Just bear in mind -ALL of them are compromises
Nothing beats a lot of experience with many types of models -big -little fast or slow
each type lends more insight into what trim setup fits you the best.
And it is a hell of a lot more interesting and fun
and remember

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