2-29 Figure 2-5. Empty Weight CG
ITEMS WEIGHT (LBS)
ARM (INCHES) MOMENT (IN-LBS) Left Wheel 101 60 6060 Right Wheel 99 60 5940 Nose/Tailwheel 42 180 7560 TOTALS 242 80.8 19560 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑀𝑀𝑇𝑇𝑀𝑀𝑀𝑀𝑀𝑀𝑇𝑇 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑊𝑊𝑀𝑀𝑊𝑊 𝑊𝑊𝑊𝑊𝑇𝑇 = 𝑀𝑀𝑀𝑀𝐸𝐸𝑇𝑇𝐸𝐸 𝑊𝑊𝑀𝑀𝑊𝑊 𝑊𝑊𝑊𝑊𝑇𝑇 𝐶𝐶𝑊𝑊 OR 19560 242 = 80.83
2.14 Determining Empty Weight CG.
2.14.1 S ample Aircraft. The sample airplane for determining empty weight is a single seat aircraft, with a design empty weight of 242 pounds and a gross weight limit of 500 pounds. The datum line is located at the nose of the aircraft and the CG range is between 69 to 74 inches from the datum.
2.14.2 To Work a CG Problem. Figure the empty weight CG first. On a piece of paper, draw four blocks. Title each block from left to right as shown in Figure 2-5.
2.14.2.1 U nder the block titled “Items,” vertically list “left wheel,” “right wheel,” “nose/tailwheel,” and “TOTALS.”
2.14.2.2 Place a calibrated scale under each wheel and record the weight on each gear, in pounds, in the weight block beside the appropriate wheel.
2.14.2.3 Measure the distance in inches from the datum line, or imaginary point identified by the manufacturer (e.g., nose of the aircraft), to the centerline (C/L) of the three wheels. Record the distance of each wheel and place it in the “arm” block beside the appropriate wheel. See Figure 2-5.
2.14.2.4 Multiply the inches (arm) by the weight on each wheel to get the moment (inch-pounds) for each wheel. Add the weight on the three gears and the three moments in inch-pounds and divide the total weight into the total moment.
The sum is the “empty weight center of gravity” in inches. In the sample case, the empty weight CG is 80.83.
Note: All calculations should be carried out to two decimal places.
2-30 Figure 2-6. Takeoff CG
ITEMS WEIGHT (LBS) ARM (INCHES) MOMENT (IN-LBS) A/C 242 80.8 19554 Pilot 170 65 11050 Fuel 30 67 2010 TOTALS 442 73.8 32614 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑀𝑀𝑇𝑇𝑀𝑀𝑀𝑀𝑀𝑀𝑇𝑇 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑊𝑊 𝑀𝑀 𝑊𝑊𝑊𝑊𝑊𝑊𝑇𝑇 = 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝐶𝐶 𝑊𝑊 OR 32614 442 = 73.8
2.15 Determining Takeoff Weight CG.
1. Since the aircraft’s empty weight and empty weight CG are fixed numbers, the only way an aircraft’s CG can be changed is by adding weight in other locations.
2. For example, in Figure 2-6, the aircraft’s empty weight has been written in the appropriate blocks. The pilot weighs 170 pounds and fuel—5 gallons—weighs 30 pounds.
3. Again, all measurements are made from the datum to the centerline of the object that has been added. Weight multiplied by inches from the datum equals moment. Add the weights and moments to find the takeoff CG for that particular flight.
4. Loaded in this configuration, the aircraft has an aft CG, but is still within the CG flight envelope and is safe to fly. The pilot must consider the effect an aft CG will have on the flight characteristics of the aircraft.
2-31 Figure 2-7. Additional Equipment Added
ITEMS WEIGHT (LBS) ARM (INCHES) MOMENT (IN-LBS) A/C 242 80.8 19554 Pilot 170 65 11050 Fuel 30 67 2010 S/B 15 75 1125 Strobe 1.5 179 268.5 Fuel 1.5 55 82.5 TOTALS 460 74.1 34090 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑀𝑀𝑇𝑇𝑀𝑀𝑀𝑀𝑀𝑀𝑇𝑇 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑊𝑊 𝑀𝑀 𝑊𝑊𝑊𝑊𝑊𝑊𝑇𝑇 = 𝑇𝑇𝐴𝐴𝐴𝐴𝑇𝑇 𝐴𝐴 𝑇𝑇𝐴𝐴 𝐴𝐴 𝑇𝑇𝐴𝐴 𝑇𝑇 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑊𝑊𝑇𝑇 𝐴𝐴 𝑊𝑊ℎ 𝐴𝐴 𝐶𝐶 𝑊𝑊 OR 34090 460 = 74.1
2.16 Additional Equipment Testing.
2.16.1 During flight testing, a strobe battery and handheld radio are added. The battery/battery box weight is 15 pounds and the location is 75 inches aft of the datum. The strobe assembly weight is 1.5 pounds and is located 179 inches aft of the datum. The radio’s weight is 1.5 pounds and is located 55 inches aft of the datum. See Figure 2-7.
2.16.2 In the sample problem, the previous figures for takeoff weight and moment are still accurate, hence those numbers have been listed in the appropriate blocks.
2.16.3 Add the battery, strobe, and radio data in the appropriate locations and calculate the totals. At 465 pounds, the aircraft is still 35 pounds under its design gross weight limit of 500 pounds but is out of balance because the CG has moved 0.3 inches further aft
(74.1 inches) and exceeds the allowable rear CG limit of 74 inches.
2.16.4 Since the aircraft is out of balance with an aft CG, it is no longer stable in pitch and would be dangerous to fly. In most cases, it is not the amount of weight added to the aircraft that can cause a major safety problem but its location.
2.16.5 To bring this aircraft back into the safe CG range, the battery would have to be moved 4 inches forward or 71 inches from the datum line. Another alternative is to install 8 pounds of ballast in the nose, 20 inches from the datum.
2.16.6 If the sample aircraft exceeded the designer’s gross weight limit, such as with a 300-pound pilot instead of the CG limit, its climb, stall, and performance capability would be poor and the possibility for in-flight structural failure would be high.
2-32 Note: In the sample W&B, positive numbers were chosen by placing the datum line on the nose of the aircraft. Some manufacturers prefer to use a datum located somewhere between the aircraft’s nose and the leading edge of the wing.
2.16.7 This kind of datum will set up a system of positive arms, or items located aft of the datum, and negative arms—items located forward of the datum.
2.16.8 When working a W&B problem with negative and positive moments, subtract the sum of all negative moments from the sum of all positive moments to reach a “total moment” for the aircraft.
SECTION 9. PAPERWORK
2.17 Objective. Have the proper documentation and paperwork to conduct the flight test.
2.17.1 W eight and Balance (W&B) . The W&B for the aircraft should be carefully done. The gross weight and CG range should be determined prior to every flight. An equipment list should be developed and maintained to identify the appliances and articles installed in the aircraft at the time of weighing for certification. It should also be kept current as installed equipment changes from additions or removals.
2.17.2 Airworthiness/Registration/Operating Limitations/Placards/W&B. The aircraft is not considered to be in a condition for safe operation if these documents are not on board.
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2.17.3 C hecklists . In addition to the assembly/airworthiness inspection previously discussed in Chapter 2, Section 7, the builder should prepare the following checklists:
1. Preflight;
2. Before starting;
3. Starting the engine;
4. Before takeoff;
5. Takeoff/cruise;
6. Descent/before landing;
7. After landing;
8. Securing the aircraft; and
9. Emergency procedures.
Note 1: A checklist to cover the above procedures may seem a tedious task, but it may only be the size of a 5x8 card similar to a checklist for a Cessna 150 or a Piper Cherokee.
Note 2: The amateur builder should anticipate several revisions to the checklist.
2.17.4 F light Manual . It is imperative that a flight manual describing the anticipated performance of the aircraft be written by the aircraft builder/kit manufacturer. The manual will be revised several times during the flight test phase until it accurately reports the aircraft’s performance, as well as limitations, normal and emergency procedures, and descriptions of those systems installed. Amateur builders should follow community and/or kit manufacturer recommendations for flight manual formatting and content.
2.17.5 Maintenance Records (Logbooks). Operators of amateur-built aircraft are required to maintain a record of the annual condition inspection in accordance with their operating limitations. The FAA recommends that owners record all maintenance (to include inspections) performed in the aircraft’s logbooks. This will create a history of the aircraft’s maintenance and will be invaluable in spotting trends.
2-34 SECTION 10. POWERPLANT TESTS
2.18 Objective. Ensure that the engine has been properly run-in and is safe to operate in all rpm ranges.
2.18.1 T esting . You can conduct an engine pre-oil and cold compression test as follows:
1. Remove the rocker-box covers and one spark plug from each cylinder.
2. Using an external oil pump, or by rotating the propeller in the direction of rotation, pump a substantial supply of oil up from the sump into the rocker arms.
3. When the engine is pre-oiled, run a cold compression test of each cylinder.
4. The results will serve only as an initial benchmark for comparing other compression tests taken after the engine has been run-up to operating temperature.
2.18.2 Ne w/Newly Overhauled Engine Run-In Procedures.
2.18.2.1 Most amateur builders start with a new or newly overhauled engine and proceed to “run it in” on the airframe. This practice is followed because builders usually don’t have either a test cell or a special “club” propeller that is specifically designed to aid in engine cooling during run-in. There are pros and cons to using an airframe to run in an engine, but the best advice has always been to follow the engine manufacturer’s instructions, found either in the manufacturer’s overhaul manuals, service bulletins, or service letters.
Following the manufacturer’s instructions is especially important if the engine has chrome cylinders which require special run-in procedures.
2-35
2.18.2.2 Also, before running-up the engine, be certain that it has the proper grade oil in the sump. Some new and newly overhauled engines are shipped with a special preservative oil to prevent corrosion. Drain this out and service the engine with the correct oil before starting.
2.18.3 U sed Engine Run-In Procedures. Some amateur buil ders install a used engine from a flyable aircraft. The same checks and adjustments as are performed on a new or newly overhauled engine should be conducted on a used engine.
Note: B oth new and used engines require special attention to engine cylinder baffling to ensure cylinder cooling is within the engine manufacturer’s Cylinder Head Temperature (CHT) specifications.
2.18.4 Pr e -Run-In Checks.
2.18.4.1 Before beginning the powerplant tests, inspect the engine and propeller carefully. All fuel and oil line connections should be tight. Check the torque on the engine mount attaching bolts. Be certain that there are no tools, hardware, or rags lying between the cylinders or under the magnetos.
2.18.4.2 Check for the proper amount of oil in the engine and that the dipstick gives an accurate reading of the oil quantity. Be advised that some engines are mounted on an angle in TC’d aircraft. These engines have a special oil dipstick, which corrects for the different angle of oil in the crankcase. The same engine, mounted level in an amateur-built aircraft with the original dipstick, will not show the correct oil quantity.
2.18.5 T est and Support Equipment.
2.18.5.1 A CHT gauge is needed to ensure that all cylinders are receiving the proper flow of cooling air.
2.18.5.2 On the newer aircraft engines, the cylinders are drilled and tapped to accept a bayonet-type CHT thermocouple probes. For older engines, the thermocouple is designed like a spark plug washer and fits under a spark plug. It can be installed in any cylinder, under either the top or bottom spark plug.
2.18.5.3 CHT monitors can have multiple thermocouples which may be connected to a selector switch in the cockpit, which the pilot uses to select a cylinder to monitor. This also is an excellent troubleshooting tool for identifying fouled plugs and bad ignition leads.
2.18.5.4 If there is only one CHT thermocouple, attach it to the rearmost cylinder on the right side of the engine, as viewed from the cockpit, and run up the engine.
Run the same test on the opposite rearmost cylinder to be certain the hottest running cylinder is monitored. You also need calibrated oil pressure and oil temperature gauges to test the accuracy of the engine instruments installed in the aircraft.
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2.18.5.5 Exhaust gas temperature (EGT) thermocouple probes, commonly known as EGTs, are becoming popular in amateur-built experimental aircraft. EGTs help monitor the engine and can be an excellent way of determining which spark plug/plug wire requires attention during a magneto check and provides information on the health of the valves and cylinders. Just like CHTs, there can be one for the entire engine or one for each cylinder exhaust pipe. EGT manufacturer installation instructions and cautions should be closely followed.
Accurate readings for EGTs are greatly affected by placement on the exhaust and the relation to airflow around the engine.
2.18.5.6 The following support equipment is recommended to be carried on board the aircraft (not all-inclusive):
• Fifty feet or more of tiedown rope,
• Tiedown stakes,
• Two chocks for each wheel,
• Fire extinguisher,
• Assorted hand tools,
• Safety-wire,
• Cotter-pins,
• Ear and eye protection,
• Grease pencils,
• Logbooks,
• Clip board,
• Pen and paper,
• A watch to time the tests,
• Rags, and
• Manufacturer’s instructions.
2.18.6 Safety Precautions. Before the first engine run, ensure the aircraft is tied down, brakes on, and the wheels are chocked. The builder and flight test team should wear ear and eye protection. All flight test participants should be checked out on fire extinguisher use and operation. During engine runs, do not allow anyone to stand beside the engine, or inline or close to the propeller. Making minor adjustments to a running engine, such as idle and mixture settings, is a very dangerous procedure and should be done with great care by experienced individuals.
2-37
2.18.7 T he First Engine Run.
2.18.7.1 The first start of the engine is always a critical operation. The engine should be pre-oiled using the manufacturer’s instructions. For aircraft using other than FAA-approved oil pressure and temperature gauges, the FAA recommends attaching an external calibrated oil temperature and pressure gauge to calibrate the engine instruments. After priming the engine and completing the starting engine checklist items, the first concern is to get an oil pressure reading within the first 20 to 30 seconds after engine start. If there is no oil pressure reading, shut down the engine.
2.18.7.2 There are three common problems that would cause low or fluctuating oil pressure.
1. Air in the oil pressure gauge line. This is easily fixed by loosening the line connection near the oil pressure gauge and squirting oil into the line until full. Another option is to use a pre-oiler to provide the pressure and carefully bleed the air out of the line near the oil gauge by loosening the B-nut connecting the oil line to the gauge.
2. A misadjusted oil pressure relief valve. Cleaning the pressure relief ball, checking for the proper number of washers, correcting spring tension, and re-adjusting the setting could solve the problem.
3. An internal problem within the engine, most likely the oil pump. An engine tear down would be required.
2.18.7.3 With good oil pressure/temperature readings and the engine running smoothly, ensure that the engine oil pressure and temperature gauges in the cockpit match the calibrated oil pressure and temperature gauges attached to the aircraft for the first run. Do not overlook this test. It is critical to determine the accuracy of the cockpit engine gauges not only for the ground engine run-in period, but also for in-flight engine cooling tests.
2.18.7.4 Work through the engine manufacturer’s run-in schedule. Most engine manufacturers recommend a series of engine runs from low rpm to maximum rpm. Each run usually incorporates a 200-rpm increase and lasts no longer than 10 minutes. The secret to a successful engine run is not to let the engine temperatures exceed manufacturer’s limits during engine runs.
Note: Engines with chrome cylinders or chrome rings require different high power run-in programs. Follow the manufacturer’s run-in instructions to ensure the engine will perform satisfactorily over its lifetime.
2.18.8 E ngine Cooldown. After a ground run, the cooling off period takes approximately an hour. This is because a newly overhauled engine needs time for the internal parts, such as rings, cylinders, valves, bearings, and gear faces, to expand and contract several times to obtain a smooth surface that retains its “memory.” This is a lengthy process even when done right, but it is important not to skip any of the recommended runs. You risk
2-38 increasing oil consumption and reducing overall engine performance, reliability, and engine life span—which could be costly in the long-term—if you skip any of the runs.
2.18.9 Record the Engine Run-In Data. During the engine run, monitor the CHTs, oil temperature, and oil pressure. Record the readings and adjustments for future reference.
If the CHTs are rising close to the red line, reduce power and stop the test. Some causes of high CHTs include:
• Using spark plugs with the improper heat range;
• CHT gauges installed on the wrong cylinder;
• Missing or badly designed cylinder head cooling baffles;
• Partially plugged fuel nozzles (applicable to fuel injected engines);
• Fuel lines of improper internal diameter (creates lean mixtures);
• Engine improperly timed either mechanically and/or electrically; and
• The carburetor fuel mixture set excessively lean.
2.18.10 Selecting Adjustments. Narrow down what adjustments need to be made by recording the change and its resulting effects. The record of the change and its effects are then used for making a logical decision for follow-on adjustments as needed to reach the proper engine performance parameters. Two ways of recording the engine performance parameters are:
2.18.10.1 Keeping a detailed paper log of each applicable engine instrument on each run-up.
2.18.10.2 Using an EFIS to record all engine performance indications for all engine runs in their entirety. EFIS for experimental aircraft may consist of either a standalone system or be connected to a multifunction display (MFD).
2.18.11 After Shut-Down.
2.18.11.1 After each engine run, check for fuel and oil leaks, loose connections, and hot spots on cylinders, which can be indicated by burnt paint. The FAA recommends draining the oil and removing the oil screen/filter within the first 2 hours of running the engine. Check the screen/filter for ferrous metal with a magnet. Wash and inspect the screen/filter for non-ferrous metal like brass, bronze, or aluminum.
2.18.11.2 A very small quantity of metal in the screen is not uncommon in a new or newly overhauled engine. It is part of the painful process of “running-in.” If subsequent oil screen checks—2 hours apart—show the engine is “making metal,” this indicates a problem inside the engine and a tear down inspection is required.
2.18.11.3 The FAA also recommends all fuel sumps, filters, and gascolators should be checked for debris after each engine run. Special attention should be given to
2-39 the fuel system by builders who use composite or fiberglass fuel tanks.
Composite and fiberglass strands can be very fine, making visual detection difficult. Frequent cleaning of the fuel filters and screens early in the flight testing phase will avoid a gradual buildup of loose composite fibers, which would reduce or stop the flow of fuel to the engine.
SECTION 11. ADDITIONAL ENGINE TESTS
2.19 Objective. All of the engine and fuel system tests and inspections should be listed and recorded on a checklist designed for that purpose. That document should note the completion of each of the above tests with information obtained during that test. Record the fuel flow, compression test, magneto timing, idle speed, oil pressure, hot and cold, etc. These checks will determine if the engine supply of fuel is adequate at all angles of attack.
2.19.1 Mi xture and Idle Speed Check . After completing the initial engine “run-in” tests, check the idle speed and mixture settings. To determine if the mixture setting is correct, perform the following:
2.19.1.1 W arm up the engine until all readings are normal.
2.19.1.2 Adjust the engine rpm to the recommended idle rpm.
2.19.1.3 Slowly pull the mixture control back to idle cut-off.
2.19.1.4 Just before the engine quits, the engine rpm should rise about 50 rpm if the mixture is properly adjusted. If the rpm drops off without any increase in rpm,
2-40 the idle mixture is set too lean. If the rpm increases more than 50 rpm, the idle mixture is set too rich.
2.19.1.5 Because most air-cooled aircraft engines need excessively rich fuel air mixtures to provide cooling for full-throttle operation, the condition should be confirmed. This is most easily accomplished by running the engine at full throttle (with the aircraft properly secured), manually leaning the mixture, and observing an increase in EGTs and in some cases an actual rise in rpm. While the amount of EGT rise is variable, the important aspect is that there is a rise of 25 to 50 degrees Fahrenheit (°F) or more. This will help alleviate engine overheating situations during the first few hours of inflight operation.
Note: Some amateur builders, after properly setting the idle mixture/rpm to the manufacturer’s specification, increase the engine idle rpm by 100 rpm for the first 10-plus hours of flight testing. This is to ensure that the engine will not quit when the throttle is pulled back too rapidly, or when power is reduced on the final approach to landing.
Be aware that in some situations, particularly with light wing loading or low drag types of aircraft, adding 100 rpm to the idle speed could cause the landing distance to be increased significantly and also have an unexpected adverse effect on landing behavior.
2.19.2 Mag neto Check.
2.19.2.1 The magneto checks should be smooth and the difference between both magnetos rpm drops should average about 50 rpm. The builder also should perform a “hot mag” check, to prevent unexpected engine self-starts. To perform this check, run up the aircraft until the engine is warm. At idle rpm, turn the magneto switch off, the engine should stop running. If the engine continues to run, one or both of the magnetos is not grounded, or “hot.”
2.19.2.2 The usual causes for a hot magneto are a broken “P” lead coming out of the magneto or a bad magneto switch. THIS IS AN IMMEDIATE THREAT TO
THE PERSONAL SAFETY OF ANYONE NEAR THE AIRPLANE AND SHOULD BE REPAIRED AT ONCE.
2.19.3 C old Cylinder Check.
2.19.3.1 The builder should perform a cold cylinder check if the engine is running rough and an ignition system problem could be contributing. A cold cylinder can be detected easily by switching magnetos and reading modern EGT monitors (if equipped) or by aiming a handheld laser thermometer at each exhaust stack. If you have neither one, perform the following:
1. Run the engine on the bad magneto for about 30 seconds at 1,200 rpm.
Without switching the mag switch back to “both,” shut off the engine.