nuts & bolts
technical counselor Balancing Act Measuring and calculating your airplane’s center of gravity Jack Dueck
E
very homebuilt aircraft is unique. And every homebuilt requires its own weight and balance considerations and calculations. Whether you are completing a quick-build kit from a well-known company or your own design, you are the manufacturer and are responsible for the weight and balance calculations and records required by regulators. In Figure 1, the center of gravity (CG) is defined as the location at which the aircraft’s weight can be assumed to act. It is usually expressed as a location from the leading edge of the wing, relative to the wing chord, which is marked in the figure as C. On conventional aircraft, this will usually range from 20 percent of C for a forward CG limit to 30 percent of C for an aft CG limit. In straight-and-level flight, the weight component is counteracted by the airplane’s lift. If the combined lifting forces act as a force in front of the center of gravity, the aircraft will have a tendency to lift its nose into a climbing attitude.
How Has a Tech Counselor Helped You? Sharing information is the ultimate goal of this new department in EAA Sport Aviation. Whether you’re a new or experienced builder or a Tech Counselor, on these pages we want to know how the team of a TC and builder met a particular challenge. You can either submit a fully written story or just the “who, what, when, where, and why,” and we’ll have a staffer contact you for the rest of the details. Please don’t forget pictures, because they often tell a story better than words. E-mail your TC success stories to
[email protected], with TC as the subject. 102
NOVEMBER 2005
If behind the CG, the aircraft will tend to nose down. Additional forces are then provided by the elevators to tailor the lift in the desired direction. If the elevator authority is insufficient to maintain the desired horizontal flight profile, the aircraft will become uncontrollable. In the above discussion, we have ignored the horizontal force imposed by engine thrust, and the opposite horizontal drag the aircraft produces. But the underlying point for center of gravity calculations is made. Kit manufacturers ascribe great importance to their CG range. The builder must weigh the completed aircraft, determine its CG location under all conditions, and compare the results to the kit manufacturer’s recommended center of gravity range. The center of gravity will vary with each aircraft. If you are designing your own airplane, start out by using the following weight estimates for major components of conventional aircraft: The engine weighs 10 percent to 20 percent of the airplane’s gross weight, the wings and fuselage 10 percent to 12 percent, the landing gear 4.5 percent to 5.5 percent, the empennage 1.5 percent to 2.5 percent, and additional equipment 3 percent to 5 percent. Together, these structural elements compose 50 percent to 55 percent of the airplane’s maximum weight. These design weight parameters will approximate the correct weight distribution for conventional aircraft. (Note: If you are contemplating an airplane that has a pusher prop, canard, or other non-conventional configuration, engineering evaluation
and analysis are required.) CG CALCULATIONS If you are building a kit produced by a known manufacturer, you will be provided with the important data and samples necessary for CG calculations. If you do not have access to this information, you need to develop your own data and criteria. The following steps will get you started. Weigh the aircraft. It should have its usual equipment in place, be positioned in level flight attitude, and be empty of fuel, baggage, and people. It should have its usual level of oil in the engine. Place appropriate scales under each wheel, ensure that the aircraft gear is in its relaxed flight condition (no horizontal loads imposed by lateral tire friction), and record the weights. Using the above configuration, measure the horizontal distances with a plumb bob, marking these locations on the floor: 1) Nose of the propeller spinner 2) Gear locations 3) Front seat occupant location 4) Rear seat occupant location 5) Baggage location 6) Fuel location Note for items 3-6, you will need to estimate where the respective weight could be expected to act. With the data recorded, you can proceed to your CG calculations. CG calculations are based on two parameters: the first is the weight of the component, (aircraft, pilot, fuel, etc.), and the second is the distance this weight acts from some arbitrarily chosen datum, usually at or ahead of the tip of the propeller spinner or at the wing’s leading edge. By using a datum in front of the aircraft, all numbers in the calculations are positive. If the datum is chosen with distances ahead and behind this line, as in the wing’s leading edge, the numbers ahead of the datum become negative, and the numbers behind the datum are positive. The next step is to calculate the CG of the empty aircraft (without usable fuel). Then final calculations are made for the flight-specific conditions by adding weights for the pilot, passengers, fuel, and baggage to the empty aircraft CG conditions. To calculate the CG of the empty aircraft, enter the values recorded into
The all new Sportsman 2+2: Much more than one airplane!
Fly it as a taildragger (TD) now...
Convert it to a trike (FT) in one hour.
Install floats or skis in a few hours. Fold wings in 5 minutes to save thousands every year on hangar costs! All of this plus unmatched: ■ ■ ■ ■ ■ ■
Performance: Takeoff and land in 400 feet; Cruise at 140 kts. Utility: 900 – 950 lb useful load accomodates the kids, large golf bags, mountain bikes, snow skis, or 300 lbs of your gear! Safety: Rugged steel safety cage provides superior occupant protection as compared to an aluminum structure. Ease of Assembly: Pre-drilled, matched-hole parts that fit perfectly, and world-class manuals combine to make assembly vastly easier than other kits. Confidence: Designed and supported by a team with over 25 years of experience in bringing the highest quality, best performing aircraft to market. Community: Join over 3000 pilots who enjoy building/flying our aircraft.
Step up to the Sportsman 2 + 2 today! New Glastar LLC (www.newglastar.com) 360.435.8533x232 EAA Sport Aviation
103
technical counselor the chart below: WHEEL
WEIGHT
X
MOMENT ARM
=
PRODUCT
Left Right Nose/Tail Total
_________ _________ _________ ________
X X X
____________ ____________ ____________
= = =
____________ ____________ ____________ ____________
Total product/Total weight = ___________ CG location aft of datum
Sample 1: Nose Wheel Aircraft (datum 70 inches in front of wing leading edge): WHEEL
WEIGHT
Left wheel 448 pounds Right wheel 428 pounds Nose wheel 269 pounds Total 1,145 pounds CG
X
MOMENT ARM
=
PRODUCT
X X X
91.9 inches 91.9 inches 34.5 inches
= = =
41171.2 in-lbs 39333.2 in-lbs 9,280.5 in-lbs 89,784.9 in-lbs
89,784.9/1,145=78.41 inches aft of datum
Sample 2: Tail Wheel Aircraft (datum is wing leading edge): WHEEL WEIGHT X MOMENT ARM Left wheel 430 pounds X 3 inches Right wheel 436 pounds X 3 inches Tail wheel 54 pounds X 183 inches = Total 920 pounds CG 12,480/920 = 13.57 inches aft of datum
= = =
PRODUCT 1,290 in-lbs 1,308 in-lbs 9,882 in-lbs 12,480 in-lbs
Weight And Balance 101 Builders new or uncomfortable with weight and balance issues should take a look at a new DVD from the HomebuiltHELP, Weight & Balance 101 for Homebuilts. The video presents the importance of weight and balance for safe aircraft flight, and then progresses through each step with detailed explanations of how to weigh your homebuilt aircraft, determine the center of gravity, and understand and fill out weight and balance worksheets for the inspection process. The 90-minute video is divided into nine chapters to make it easier for more experienced builders to find the information they need. Weight & Balance 101 for Homebuilts is available for $33 online at www. homebuilthelp.com.
With the center of gravity of the empty aircraft or below your aircraft’s maximum gross weight in hand, you can then undertake the familiar exercise and that your CG location is within the CG of entering the data for the specific flight case. As an range as discussed above. example, assume the nose wheel aircraft in Sample Calculate a number of loading scenarios at 1 with a pilot weighing 200 pounds and a passenger light weights and maximum weight in a variety weighing 120 pounds. In addition assume fuel of 36 of loading configurations to see if you can stay gallons and additional baggage of 50 pounds. within the CG limits. Also consider that if your Along with the known weights you will need to use ITEM WEIGHT X MOMENT ARM = PRODUCT the moment arms Aircraft 1,145 pounds X 78.4 inches = 89,768.0 in-lbs associated with each of Pilot/passenger 320 pounds X 92.7 inches = 29,664.0 in-lbs the additional weights listed. Remember the Fuel (36 X 6) 216 pounds X 76.8 inches = 16,588.8 in-lbs distances you measured Baggage 50 pounds X 122.0 inches = 6,100.0 in-lbs with the plumb bob? Total 1,731 pounds 142,120.8 in-lbs This is where they’re put to use. Use a CG Location: 142,120.8/1,731 = 82.10 inches aft of datum moment arm of 92.7 inches for the pilot and passenger, 76.8 inches for the fuel, and 122.0 inches for the baggage. Enter these values fuel tanks are located ahead of the CG under into the weight and balance chart and complete the full fuel conditions, your CG will shift aft as fuel calculations: is burned. You could have a condition where Check to make sure that your total weight is at you start your flight within CG limits, but end 104
NOVEMBER 2005
up with an airplane that is out of balance after a few hours’ flight. Experiment with several different situations. You will quickly gain an understanding of your aircraft’s sensitivity to various loading conditions. Record your CG calculations and keep this information along with the other required documentation in your aircraft as well as in your aircraft files. One additional aspect of weight and balance calculations needs to be addressed. During the service life of an aircraft, adding, removing, or changing equipment affects the weight and balance calculations. Regulations require you to recalculate the aircraft’s empty weight and balance with each equipment change unless it is negligible. These calculations will include both the weight of the component, its moment arm, and the subsequent product of these two multipliers. The totals of these weights and moment arms are added to those of the empty weight of the aircraft just as you would another load or item in your initial CG calculations. Include the list of these items with your flight documents and keep a copy in your records or files. When your aircraft is complete and the final weight and balance is calculated, your first flight tests should be conducted at less than gross weight, with your CG at or near the 25 percent chord position. Add and secure ballast to achieve this condition, and recheck your calculations. Proceed slowly in both forward and aft CG directions, following a preplanned flight-test program as you develop your flighttest envelope. An aircraft loaded with a forward CG will be sensitive to control inputs and unstable. An aircraft with an aft CG condition will react sloppily to control inputs, and be generally unresponsive to smaller control inputs. If the airplane is loaded aft of its allowable center of gravity envelope, the airplane could assume an uncontrollable nose-up attitude on liftoff, followed by a steep climb to stall, then an uncontrolled descent and impact ground.
Answer: Yes you do!* * Question: Do I need one of these? Find out why at:
www.ellison-tbi.com
Carburetors You Can Bank On! Ellison Fluid Systems Inc. • 350 Airport Way • Renton, WA 98055 • 425-271-3220
KIAS RPM MP OP OT FP
Fuel Flow - Altiltude 4 CHTs & 4 EGTs (Graphic & Numeric) Comprehensive Pilot’s Checklist Optional OAT & Density Altitude
Large-Character LCD Display Back-Lit, Sunlight-Readable Screen Clearly Labeled Information Color-Coded LED Bargraphs
$1,499 + PROBES Dimmable for Night Flight Optional Audible or Visual Alarms For Standard & Alternate Engines Lightweight Radio-Rack Width
I-K Technologies 818-302-0606
www.i-ktechnologies.com
EAA Sport Aviation
105
