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Can´t get the static margin right

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Can´t get the static margin right

Old 08-24-2005, 09:08 AM
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juanes1969
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Default Can´t get the static margin right

I´m building a plane and wanted to make sure the formulas are right on it. The problem I have is that the fuse is already built and I found the formulas later. In this forum there are some links to formulas and if I want the CG to be at 30% MAC the my static margin comes out at 48.7%!!! The specs on the plane are as follows:
Cord = 23.5"
Sweep is 0
Wingspan= 140" the fuse cuts the wing in half and is installed with aluminum tubes so
Half span= 70"
Fuse width at wing= 10"
Distance between Leading Edges= 75.5"
Wing area (without fuse included)= 3290 Sq. In
Tail cord= 14"
No sweep
Tail half span 24.5"
Tali span 49"
tail Area= 686 Sq. In.

In the formulas the CG comes at 7.05" back from leading edge, the wing aerodinamic center at 5.88" back from LE at root, my neutral point at 18.49" back from LE and the distance between CG and Neutral point at 11.44 or 48.7% static margin.

It is a big plane as you can see, the wing is semi-symetrical andwill be pulled by a Mac Minarelli 125 twin gaser. It´s sort of a giant second step aircraft meant to have very stable flight... but now that I found the formulas I´m thinking that I should not fly it until i have some assurance that it will not crash because of bad proportions.
BTW the estimated weight will be around 38 to 42 pounds.
Any help in getting that static margin right and hints on how to do it are VERY welcome
John

I found the formulas here http://www.rcuniverse.com/forum/m_2867994/tm.htm


OHH and the aspect ratio of this wing is 6
Old 08-24-2005, 10:34 AM
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Default RE: Can´t get the static margin right

Without running your numbers myself, I believe that you will find Static Margin of 48.7% to be more than enough for a stable flight. Have you done a search for Static Margin here on RCU, If not do so. I remember reading a thread talking about the statc margin.
Old 08-24-2005, 10:51 AM
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juanes1969
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Default RE: Can´t get the static margin right

Thanks danny03, I have found some on the static margin, but everything says that a good starting point is 5 to 15%, and it says on the sites these formulas are, that too much of static margin reduces you elevator authority. so I don´t know what to do[]
John
Old 08-24-2005, 10:59 AM
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Default RE: Can´t get the static margin right

You are right about elevator authority, and right now the only way that I can think of lowering the Static Margin is 1) move cg further back, 2) reduce tail moment by either moving elevator up farther on the fuselage or remove a section of the fuselage between the elevator and wings, 3) Relocate wings further back and shorten your nose. with the exception of option 1 every thing else seems to require some modifications of the fuselage. wish I could help more, maybe someone else will give some different advice.
Old 08-24-2005, 11:08 AM
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juanes1969
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Default RE: Can´t get the static margin right

Thanks danny, I´ll try with moving the CG as you said, if it doesn´t work, then I´ll have to make mayor surgery to the fuse to get it right. If I cut the fuse about 15 inches I´ll get a better Static margin, not in the "recomended" %, but a little better. I hope it works, thanks for everything
John
Old 08-24-2005, 11:44 AM
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Default RE: Can´t get the static margin right

your welcome and good luck, please keep me inform to how it goes...
Old 08-24-2005, 12:34 PM
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juanes1969
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Default RE: Can´t get the static margin right

Will do, it will fly in about a month, I´ll take pictures before and after and post them here so you can see how it turned out
Thanks
John
Old 08-24-2005, 01:20 PM
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WS
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Default RE: Can´t get the static margin right

Your AC and CG calcs are correct, but DO NOT trust that neutral point. It is based only on areas and distances. There are several factors that will move it forward.

1. The stab is operating at a lower AoA than the wing due to the latter's downwash
2. The stab has a lower aspect ratio, further reducing the lift slope of that surface
3. Propeller thrust will cause de-stabilizing pitching moment if the prop is ahead of the CG.
4. Propwash will further reduce the stabilizers AoA making it less effective.

Any calculation of the neutral point should be made for a flight condition offering the least stability (high AoA and high power)

For example, at 15 deg AoA

Wing CL = 1.2 (corrected for AR = 6)
Downwash = 3.6 deg
Stab AoA = 11.4 deg
Stab CL = 0.8

Already, the stab is operating at 67% efficiency, moving your calculated NP 3.5" forward. I would need to know the distance from the propeller to the wing's AC, the prop diameter and some power estimate to factor in that stuff.

It's always a good idea to start at 25% MAC and go back from there based on flight tests.
Old 08-25-2005, 08:54 AM
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juanes1969
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Default RE: Can´t get the static margin right

WS, thanks for the help!
The wing aerodynamic center (AC Right?) comes out at 5.88 inches back from the leading edge. and the prop is 26.3 inches to the front from that AC. The engine is an Aviomac 120cc twin and produces about 10HP. The prop is a mejzlik 28x12.
I don´t have the specs for the engine, as the manual doesn´t say the HP rating or static thrust it produces, but I have a ZDZ80 and that engine with a mejzlik 26x10 produces an easy 50 lbs thrust and has 8.5HP, so I´m gessing here that the minarelli will give me somewhere around the 60-65 Lb thrust range. It does give me 6100rpm with the 28x12.(and only has one gallon run on it)
I hope this is the information you need for the calculations and the help.... I´ll be standing by for your numbers
Thanks WS
John
Old 08-25-2005, 09:24 AM
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Default RE: Can´t get the static margin right

If you can wait 'till later, I'll back up the calcs wih my own. I'm at work right now, but have CG calcs in my own on my home PC. I have always gotten reliable results with mine, whether I try to set up like a trainer for first flilghts, a near neutral for more fun. Looks like you have enough info here to give it a shot. I take it the wings and tail are non-tapered...
Old 08-25-2005, 09:32 AM
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juanes1969
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Default RE: Can´t get the static margin right

Hi John, thanks for helping
You are right, the wings are non-tapered and the tail too. I´ll wait, and I´m still in time to make any changes to the aircraft, it is in the bones, so if I have to cut the fuse or anything else, I can do it with no problem. Anyway I´m would not fly it (even if I could right now) without reading all the help I can get here.
Thanks

John
Old 08-25-2005, 10:00 AM
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WS
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Default RE: Can´t get the static margin right

As near as I can figure, at a slow flying speed of 30 MPH, you'll be getting near 55 lbs of thrust. With 15 deg AoA, that's about 5800 oz-in of nose up torque, worth about 0.4 lift coefficient on the tail. I would say your power on neutral point should not exceed about 5" behind the wing aero center (almost 11" back from the LE). If your CG is 7", your margin becomes closer to 17%. You'll really only know for sure by flying it! Interference between the fuse and stab could have a negative effect on stability as well.
Old 08-25-2005, 10:36 AM
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juanes1969
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Default RE: Can´t get the static margin right

WS, I´m a little confused now.... acording to what you said, can I fly the plane as is by just moving the CG? or should I modify something? If the first is correct, it would be great!!!!
Or do you mean I should just fly it and adjust as needed depending on flight results? [8D]
John
Old 08-26-2005, 07:17 AM
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Default RE: Can´t get the static margin right

.
Old 08-26-2005, 07:19 AM
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Default RE: Can´t get the static margin right


My calculations show the neutral point, power on, at 46% MAC. So, for a 10% static margin, you would set the CG at 36% MAC,or 8.5 in back from the leading edge. Although I would recommend 10% margin, a 15% margin would be a CG location 7.3 in back from the LE. Once you are used to the plane, you can move the CG to 5% static margin, at 9.6 in back and then slowly move back to neutral if you want a pure aerobatic setup.

Yes, once you have the shape of the plane analyzed, all you have to do to set stability is move the CG, assuming the anaysis shows it to be a reasonable airplane. These are all reasonable nembers for this plane.

With the tail area being 20% of the wing area, and a relatively long tail moment, the airplane should be well damped in pitch and should be pretty easy to fly with a near neutral CG - that's why I wouldn't recommend more than a 10% static margin. Just test fly with a mild setting on elevator travel /low rate.

PS show us some pics or plans or something. We've earned it.
Old 08-26-2005, 08:22 AM
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juanes1969
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Default RE: Can´t get the static margin right

Thanks John, as soon as I have the tail and wings installed and almost ready to cover I will post pictures for you, as you said you guys earned it!!!! thanks a lot.... I´ll keep you posted
John

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