Center of Gravity Basics – The secret to a plane that flies well
It occurs all too usually, sadly: Someone with a model new airplane exhibits up on the area and has every part working completely. The radio checks out, and all of the controls transfer within the appropriate instructions; the engine runs reliably and gives loads of energy. The mannequin taxis out and runs down the sector and turns into airborne. Right away, it begins to rock its wings; the nostril factors up sharply; and after a number of horrifying seconds, it hits the bottom—arduous! The pilot did every part proper besides that he forgot to test the mannequin’s steadiness.
Better than utilizing your fingers, a exact balancing machine, such because the Great Planes CG Machine, will provide help to get the steadiness level appropriate.
To efficiently test-fly a brand new mannequin be completely sure that the mannequin is balanced on the appropriate middle of gravity (CG). Nothing can spoil your day greater than making an attempt to kind out a tail-heavy mannequin after it has left the bottom. It is a good guess that improper CG location is answerable for extra damaged airplanes than engine and radio points mixed. Let’s take a look at some strategies for figuring out the CG.
Balancing Act
While balancing a monoplane, use the situation indicated in your mannequin’s directions or plans measured again from the forefront. If you don’t have this info, it isn’t arduous to determine it out your self.
For most fashions, the steadiness level falls between 25 and 30 % of the imply aerodynamic chord (MAC); 27 to twenty-eight % appears to be the typical. With a continuing chord wing, like on a Piper Cub, the MAC line is the wing’s chord line, or the gap from the forefront to the trailing edge, and it may be measured anyplace alongside the wing’s span (Figure 1). With a tapered or swept wing, it’s important to decide the exact location of the MAC line (Figures 2 and three). Using the wing’s high view, the chord size of the tip rib is added to both aspect of the foundation rib, and the foundation chord size is added to both aspect of the tip rib. The ends of those extension strains are then related with projected strains forming an X. Where these two strains intersect is the situation for the MAC line. The MAC line is parallel to the foundation chord. The MAC line is then divided by 4 to get the 25 % MAC location and by three to seek out the 33 % MAC location. These two factors present the wing’s CG vary. You can then prolong measure strains from the 2 MAC areas, that are 90 levels to the foundation chord, and you may then steadiness your mannequin anyplace alongside these strains.
Figure 4 exhibits Andy Lennon’s variation for locating the MAC line of a tapered wing panel the place the MAC location is on the intersection of a single diagonal projected line and a line connecting the midpoints of the foundation and tip rib chords. The end result for the 2 methods is extraordinarily shut and nicely inside the total CG vary.
Biplanes
When it involves biplanes, the method may be very related. Figure 5 exhibits a aspect view of a biplane with two straight fixed chord wings. The MAC is estimated because the horizontal distance from the highest wing’s forefront to the underside wing’s trailing edge. You then divide the MAC line to seek out the CG steadiness vary. There are different methods to discover a biplane’s CG location, however that is the only. If, nevertheless, your biplane has a swept high wing and a straight backside wing, like with a Pitts Special, you utilize the identical method as for a tapered wing proven in Figure 6. The distance from the highest wing’s root forefront to the underside wing’s root trailing edge turns into the foundation chord line, and the gap from the highest tip’s forefront to the underside tip’s trailing edge turns into the tip chord line.
After figuring out the place of the MAC and discovering the 25 % and 33 % MAC areas, add nostril weight or shift the onboard tools round in order that the mannequin’s steadiness level falls someplace inside the CG vary. Use a sturdy balancing stand that’s sized in response to your mannequin, then make it possible for your ballast weight is correctly secured in order that it may well’t transfer or fall out of the airplane. I like to begin in the course of the vary as a result of the one approach to really determine whether or not the mannequin is barely nose- or tail-heavy is to test-fly it.
The Dive Test
An wonderful approach to test your mannequin’s CG is to make use of the dive check methodology (Figure 7). We realized this system from Keith Shaw, an knowledgeable electric-airplane designer and a MAN contributor, and though it might sound backward, the check works very nicely. According to Keith, “An incorrect middle of gravity, normally too nose-heavy, additionally will increase undesirable drag. A nose-heavy aircraft has to hold up-elevator trim to take care of longitudinal stability. This signifies that the stabilizer is ‘lifting’ downward, and that creates some induced trim drag. It additionally signifies that the wing should now develop much more carry to take care of degree flight, producing much more drag!
“Fly your plane at half throttle, and adjust the elevator trim until it can maintain hands off level flight. Check this by making several passes without changing the throttle setting. You should be flying at an altitude of 100 to 150 feet. When the plane is nearing center stage, gently push it into a 30-degree dive, and hold it until the air speed has increased noticeably. At this point, take your hand off the stick and observe what happens. If the plane pulls up sharply, it’s very nose-heavy. If it continues in the dive or pulls up slightly, its CG is just right. If it tries to tuck under, it’s tail-heavy. This happens because the increase in speed amplifies the trim corrections. If the model was carrying some up-trim to correct a nose-heavy condition, the increased dive speed makes the model pull up and vice versa. “A bunch of extra advantages comes with having a correct CG location. The amount of elevator throw necessary for any maneuver will decrease, and that will mean less control drag. There will be virtually no need for down thrust, which is an inept attempt to ‘fix’ the tendency of an over stabilized aircraft to nose up as power is increased.”
Let’s Go Flying
With the mannequin’s CG is within the secure steadiness vary, you’ll be able to take off and kind issues out on the primary flight. Fly the mannequin at about 1/2 to three/4 throttle, and set the trims for straight and degree flight. After touchdown and checking the trim-lever areas, alter the clevises in order that the radio’s trim levers could be set again to impartial. Make one other flight, retrim, and see how the mannequin performs. If it flies usually—not climbing or diving with minor energy adjustments—it is a good CG indicator and you’re very shut. If there’s nonetheless quite a lot of up- or down-trim, then the CG nonetheless must be adjusted. Plenty of up-trim tells you that the CG is simply too far ahead (nose-heavy); quite a lot of down trim signifies an aft CG (tail-heavy).
Bottom Line
The finest insurance coverage coverage for any mannequin airplane is to have it balanced as shut as attainable to its appropriate CG earlier than flying. Feeling what the mannequin tells you when test-flown will provide help to fi ne-tune its steadiness. Tail heaviness will increase maneuverability, whereas nostril heaviness will increase stability. A nose-heavy airplane, nevertheless, tends to land at a quicker velocity, whereas extraordinarily tail-heavy airplanes can turn out to be uncontrollable. Some fundamental calculations earlier than that first flight will assist maintain your mannequin entire. Get the steadiness proper.
Editor’s notice: Please notice that these methods are good for shut estimations and can get your mannequin balanced within the ballpark nicely inside the correct CG vary in order that it’s secure to test-fly. We’re not considering uncommon configurations or lifting tail surfaces.
Text & Illustrations by the Model Airplane News Crew