CG at 21%?

[Joel K. Scholz]

I am almost to the test fly stage on a 1/7 th scale RE2005. Reading thru the book I found the actual plane was balanced at 21%. I normally balance my planes at 25 to 30%. The plane has a semi eliptical wing section with an eppler 197 airfoil.Might only get one chance on this one. Any suggestions?

[Ollie]

For pitch stability the CG must be ahead of the aerodynamic center (neutral point) of the whole plane. Where there is no horizontal tail or tail moment arm as in a pure delta, the aerodynamic center of the whole plane is at or very near the aerodynamic center of the wing. The aerodynamic center of any wing is very near 25% of the mean aerodynamic chord (MAC) of the wing. The MAC is the chord that passes through the centroid of the wing planform. In the case of a triangular planform, the MAC is 1/3 of the way from the root to the tip. The aerodynamic center is 25% of the MAC back from the leading edge of the MAC. The CG, then, should be at less than 25% of the MAC for pitch stability. Typical locations for the CG run from about 15 to 20% of the MAC depending on how much pitch stability is desired. How much stability the aircraft should have depends on the purpose of the model and the skill of the pilot.

The location of the CG relative to the neutral point (NP) determines the static stability. When the CG is at the NP the plane is neutrally stable and the plane goes where it is pointed without diverging or correcting itself. When the CG is aft of the NP the plane diverges and tucks under. When the CG is ahead of the NP, the plane returns to a trimmed flight speed and attitude after a disturbance. How strongly the plane returns to its trimmed flight condition depends on how far the CG is ahead of the NP (static margin).

The NP and CG can be calculated. However, the calculations have small uncertanties due to unknown flow conditions over the prop, fuselage, landing gear, etc. Some airfoils, at low reynolds numbers, have AC locations that shift a little with angle of attack. These variations and uncertanties make the final CG adjustments by flight testing highly desirable. Calculations are good enough to arrive at a safe starting CG which can then be refined by adjusting the CG between test flights.

As the CG is adjusted it will affect rudder and elevator control response. A forward CG results is less control response and an aft CG results in greater control response. It is often necessary to adjust control throws as a result of adjusting CG in order to maintain the desired control response.

[Gary Warner]

I am almost to the test fly stage on a 1/7 th scale RE2005. Reading thru the book I found the actual plane was balanced at 21%. I normally balance my planes at 25 to 30%. The plane has a semi eliptical wing section with an eppler 197 airfoil.Might only get one chance on this one. Any suggestions?

Hi Joel,

Balance it at 25% MAC. The full scales have a number of reasons for forward CG, most notably the changing loading (fuel, payload, pilot being behind the CG, etc.). I found a pix of the plane on the web and it looks like the MAC might be a bit forward of the normal constant cord layout, or not since I haven't done the math on the plan form.

25% should be ok for this plane. Get someone with talented fingers to do the first flights, as you have done before.

Good luck with the model.

Can I assume you'll be at DEAF this year?

Gary
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(Link to a site about this plane) (http://www.aldini.it/re2005/)