Determining the CG
by Sal Calvagna [email protected]
Welcome back to the "RC Giants" column. Well, they're finally here, the dog days of summer. I hope your winter modeling projects have been successfully completed and you're in the midst of a great flying season. Plan to attend a local flying event. You'll see old friends and great models, and more than likely there will be freshly barbecued burgers available!
Recent changes in the AMA rules regarding the size of Giant Scale models means that we'll see even larger models being constructed.
One of the last and most important steps in constructing a model is balancing the CG. For smaller models, this can be as simple as lifting the model by the wing at the specified CG point using the tips of your fingers. It is easy with an 8-pound model; however, try this with a model weighing 40 pounds or more — it ain't gonna work!
Balancing large models with three scales
One way to determine the CG on larger models using three scales is to block the airplane up level. Assuming we're talking about a taildragger such as the Edge, you will have to block up the scale under the tailwheel.
Carefully measure the distance parallel to the airplane's longitudinal centerline from the spot where the mains contact the scale to the spot where the tailwheel contacts the scale. Call this distance D1.
Take the total weight on the mains and call this W1. Take the weight on the tailwheel and call this W2. In the following equation, the distance to the CG (DCG) is the distance from a line, perpendicular to the longitudinal centerline, and passing through the center of the mains contact spot, aft to the CG spot.
CG formula and example
DCG = D1 × W2 / (W1 + W2)
Example: D1 = 90 inches, W1 = 30 pounds, W2 = 5 pounds DCG = 90 inches × 5 pounds / 35 pounds DCG = 12.86 inches back from a line through the mains along the fuselage center
The principle of the formula is that at the CG, the sum of the fore and aft moments will be in equilibrium. We know the forces (weights) and the distance between them (D1) and have to solve for the spot where the moments are in balance (moment = force × distance). You can also use this formula to tell how much weight you need to move to get the CG to move a given distance.
I trust that this will work just fine, but I was never good with math, and after reviewing the calculations, my head hurts. Or you can use a device such as the EZ Balancer II from Southwest Systems and forgo all of these steps. Here's what I found on the Southwest Systems' website:
"The EZ Balancer II is designed for models from 8 pounds up to all legal AMA and IMAA [International Miniature Aircraft Association] limits. Finally! You can balance the CG of all your model aircraft by yourself. No more trying to balance them on your fingertips. No more excessive flexing of the wings trying to lift your model at the wingtips. Built-in stops on the cradles limit the travel so you can balance your model 'hands free' without fear of it tipping over or sliding off the balancer. No more 'guessing' where to put your battery packs or weights. You can move them back and forth until you achieve perfect balance. It will accommodate up to 18-inch-wide fuselages."
Additionally, Southwest Systems offers "Super Size" and "Monster Size" kits that extend the height of the balancer to accommodate even larger models. All of the parts are cut out from aircraft-grade aluminum using high-pressure water cutting. The parts are deburred and prepared for surface anodizing, which keeps them looking good for a long time. Lastly, all of the Southwest Systems kits are 100% made in America. For more information about the EZ Balancer II, please visit the Southwest Systems website, listed in "Sources."
The Fieseler Storch
Sal Iasilli from East Norwich, New York, recently completed an impressive 1/4-scale Vogelsang Aeroscale Fieseler Fi 156 Storch. The model spans nearly 140 inches, weighs 32 pounds, and is powered by a DA-50 gas engine. The kit was designed by Paolo Severin of Italy and imported by Vogelsang.
The fuselage is welded stainless-steel tubing. The rest is built-up light plywood and balsa. The finish is Solartex covering and Rust-Oleum satin white paint.
"After the maiden flight and multiple control adjustments and trim changes," Sal reported, "the second flight turned out to be spectacular! Takeoff was exactly like the full-scale Fieseler — almost 45° with half throttle and liftoff in less than 6 feet!
"After flying a few minutes, I felt very comfortable with the control response. As for the landing, I have never had a model fly this slowly. It was almost like watching a movie in slow motion, and that's without the flaps and just a slight headwind.
"I transported the model on the roof of my Prius. I had some pretty strange stares driving along the LIE [Long Island Expressway] at 70 mph in the HOV [High Occupancy Vehicle] lane.
"Not wanting to offend anyone, I covered the swastika on the rudder. I also didn't want anyone throwing rocks at me. The cockpit detail is still not finished but knowing that the plane flies this great, I can go full bore with the scale detail."
Nice work, Sal, and congratulations on your first flight. The Fieseler Storch Fi 156 was designed in 1935 as a requirement for a STOL (Short Takeoff and Landing) aircraft. It had a 46-foot wingspan and full-length fixed slats and fowler-type flaps that increased the wing area by 18%. One unusual feature was that the ailerons also drooped when the flaps were extended past 20°. This gave the aircraft excellent slow-speed performance.
The demand was so high for this multirole aircraft that the French firm of Morane-Saulnier, in occupied France, was retooled to build the Storch. The aircraft was made famous by its use to rescue the deposed and imprisoned Italian dictator Benito Mussolini. It was also involved in the last dogfight on the Western Front during World War II. It was forced to land by another liaison/spotter type aircraft, the Piper L-4 Grasshopper, after the pilot and observer in the Grasshopper opened fire with their .45 caliber pistols, forcing the Storch to land. The Fieseler was a large liaison aircraft with a 46-foot wingspan, compared to the L-4 at approximately 35 feet. The Fi 156's engine had nearly four times the horsepower of the L-4, being rated at 240 horsepower versus 65 horsepower for the Piper. Nearly 2,900 Fieseler Storchs were produced during the war.
That's all for this month. Fly safely, enjoy the rest of the summer, see you in the fall.
Sources
- EZ Balancer by Southwest Systems — [email protected] — www.ezbalancer.com
- IMAA — www.fly-imaa.org
Transcribed from original scans by AI. Minor OCR errors may remain.
