WEIGHING FORM MAKE ... Rotary MODEL. SERIAL .. 0242 N . 411 DATUM LOCATION Leading edge of wing at root Aircraft weighed with full oil.
1. Main weighing point is located (- " forward) (+68" aft) of datum.
2. Tail or nose weighing point is located (-26" forward) (+ " aft)of datum.
Weighing Point Reading Scale - Tare Weight = Net X Arm = Moment
3. Left Main Wheel 622.00 -5.00 817.00 68" 41,956.00 Right Main Wheel 618.00 -4.00 614.00 68" 41,752.00
5. Sub-Total 1,240.00 - 9.00 1,231.00 68" 83,708.00
6. Tail or Nose Wheel 155.00 -3.00 152.00 - 26" -3,952.00
7. Total as Weighed 1,395.00 - 12.00 1,383.00 57.67 79,756.00 Space for listing of Items when Aircraft is not Weighed Empty.
Net Item Weight Arm Moment
8. Oil - 8 gallons @ 7.5 p/p/g -60.00 - 30.00 1,800.00 Aircraft Empty Weight & c.g. 1,323.00 61.64" 81,556,00 Maximum Allowable Gross Weight 1,773 pounds Useful Load 450 pounds Computed by: Frank A. Adams A & P Number:_1366968 FIGURE 3-10. Sample weighing form.
Item Net Arm Moment weight (1bs.) (in.) (lb.-in.)
then divide the sum of the moments by the total weights involved: Acft. total as 1,383.0 57.67 79,756.0 weighed -ess oil, & gallons -60.0 -30.00 1,800.0
c.g. = Total moment 79,756.0 gallon @ 7.5 Ibs. per Total weight 1,383 Aeft. empty 1,323.0 81,556.0 = 57.67 in. weight and moment Consequently, the c.g., as weighed, is 57.67 in. Again using the formula:
from the datum. Total moment 81,550.0 Since the aircraft was weighed with the oil c.g. = Total weight 1,323 tank full, it is necessary to remove the oil to = 61.64 in.
obtain the empty weight and empty weight c.g. The EWCG is located 61.64 in. aft of the datum.
Datum +23' + 6" - 56" + 9.0° + 18.70
196.0" Tail wheel weight = 50 Ibs.
Tail wheel arm = 198.0" Empty weight (without oil) = 950 Ibs.
Datum location = leading edge of wing.
Engine = 100 hp.
CG range = (+9.0") to (+18.7").
Fuel capacity = 40 gal @ +23".
Number of seats - 3 (one @ + 6" and two @ +34").
Maximum gross weight = 1775 ibs.
Oil capacity = 8 qts @ -41".
Maximum baggage =50 Ibs @ +56".
Note: (This airplane can be flown from either the front or rear seat.)
FreURE 3-11. Schematic diagram for forward weight and balance check.
The maximum allowable gross weight as shown in the Aircraft Specifications is 1,733 pounds. By subtracting the aircraft empty weight from this figure, the useful load is determined to be 450 pounds.
WEIGHT AND BALANCE EXTREME CONDITIONS The weight and balance extreme conditions represent the maximum forward and rearward
c.g. position for the aircraft.
An aircraft has certain fixed points, fore and aft, beyond which the c.g. should not be permitted at any time during flight. A check should be made to ensure that the c.g. will not shift out of limits when crew, passengers, cargo, and expendable weights are added or removed. If the limits are exceeded and the aircraft is flown in this condition, it may lead to insufficient stability, with resulting difficulty in controlling the aircraft.
Adverse loading checks are a deliberate attempt to load an aircraft in a manner that will create the most critical balance condition and still remain within the design c.g. limits of the aircraft.
It should be noted that when the EWCG falls within the EWCG range, it is unnecessary to perform a forward or rearward weight and balance check. In other words, it is impossible to load the aircraft to exceed the c.g. limits, provided standard loading and seating arrangements are used.
Forward Weight and Balance Check To make this check, the following information is needed:
1. The weight, arm, and moment of the empty aircraft.
2. The maximum weights, arms, and moments of the items of useful load that are located ahead of the forward c.g. limit.
3. The minimum weights, arms, and moments of the items of useful load that are located aft of the forward c.g. limit.
The example shown in figure 3-11 presents one method of conducting an extreme-condition check.
This method makes it easy to visualize exactly where the weights of various loading arrangements are distributed and how they affect c.g. location.
Using the data given in figure 3-11, determine if the airplane can be loaded to cause the c.g. to go beyond its limits.
The minus moment of -615.0 is subtracted from the sum of the plus moments:
13,855.0
- 615.0 13,240.0 THIRD STEP: Find the most forward c.g. position by dividing the total moments by the total weight.
13,240.0 - = 11.17 in.
1,185 Since the total moment is plus, the answer must be plus. Therefore, the forward extreme position of the c.g. is located at 11.17 aft of the datum.
The forward c.g. limit for this example airplane is +9.0 in. aft of the datum; therefore, it is easy to see that it can be safely flown with this loading arrangement.
FIRST STEP: Load the airplane as follows:
Oil -8 qts. a -41 in.= (15.0 Ibs.) (-11 in.).
Pilot - 170 1bs. @ +6 in. = (170.0 Ibs.) (+6 in.).
Fuel, minimum-50 Ibs. @ + 23 in.= (50.0 Ibs.)
(+ 23 in.).
No passengers.
No baggage.
Fill any fuel tanks which are ahead of the forward limit. If the fuel tanks are to the rear of the forward limit, use the minimum required amount of fuel.
Rearward Weight and Balance Check To establish that neither the maximum weight nor the rearward c.g. limit is exceeded, the following information is needed:
1. The weight, arm, and moment of the empty aircraft.
2. The maximum weights, arms, and moments of the items of useful load that are located aft of the rearward c.g. limit.
3. The minimum weights, arms, and moments of the items of useful load that are located ahead of the rearward c.g. limit.
SECOND STEP: Total all weights and moments. The most rearward c.g. position is found by Item Weight (Ibs.) (in.) Arm Moments (Ib.-in.) case the airplane is loaded so that it will be tail- repeating the three steps that were followed in making the most forward c.g. check, except in this Acft. EW 950.0 + 12.3 + 11,685.0 heavy- Oil 15.0 -41.0 615.0 Pilot 170.0 + 6.0. + 1,020.0 FIRST STEP: Load the airplane in a manner that Fuel (min.) 50.0 + 23.0 + 1,150.0 will make it most tail-heavy.
Total 1,185.0 13,240.0 Oil-8 qts. @ -41 in.= (15.0 lbs.) (-41 in.).
The above figures require careful consideration.
Notice that each weight is multiplied by its arm to obtain its moment. All the weights are added to obtain 1,185 Ibs., the total weight. However, when adding the moments, all the plus moments are added:
11,685.0 1,020.0 1,150.0 Pilot - 170 1bs. @ + 6 in. = (170.0 1bs.) (+6 in.).
Fuel (max.) -40 gals. @ + 23 in. = (240.0 1bs.)
(+ 23 in.).
Passengers-two @ 170 Ibs. each-340 Ibs. @ + in. in. = 1bs.) 1bs.) (+34 in.).
Baggage (max.) -50 Ibs. @ +56 in. = (50 Ibs.)
(+ 56 in.).
Fill any fuel tanks which are aft of the rear limit. If the fuel tanks are forward of the rear 13,855.0 limit, use the minimum required amount of fuel.
SECOND STEP: Total all weights and moments movable ballast under all loading conditions for as shown here: which such ballast is necessary.
Weight Arm Moments Item (Ibs.) (in.) (lb.-in.)
Controlling c.g. Position With Ballast Oil Acft. EW 950.0 15.0 + 12.3 -41.0 + 11,685.0 615.0 c.g. exceeds the forward c.g. limit under certain Figure 3-12 shows an example aircraft whose Pilot 170.0 + 6.0 + 1,020.0 loading conditions. The forward weight and bal- Fuel (max.) 240.0 +23.0 + 5,520.0 ance check proves that with only the pilot and Passengers (two) 340.0 +34.0 + 11,560.0 minimum fuel aboard, the forward c.g. is exceeded.
Baggage (max.) 50.0 + 56.0 + 2,800.0 Total 1,765.0 31,970.0 Most forward c.g. check THIRD STEP: Find the most rearward c.g. posi- Weight Arm Moments tion by dividing the total moments by the total Item (1bs.) (in.) (Ib.-in.)
weight.
Most rearward c.g. when loaded as shown in Acft. EW 1,600.0 + 15.6 + 24,960.0 figure 3-11: Oil 22.5 - 22.0 495.0 31,970.0 = 18.11 in. Fuel (min.) Pilot 115.0 170.0 + 10.0 + 18.0 + 1,700.0 + 2,070.0 1,765 The rearward c.g. limit for this example airplane Total 1,907.5 +28,235.0 is +18.7 in. aft of the datum; therefore, it can be Total moment flown safely with this loading arrangement. Most forward c.g.= Total weight 28,235 INSTALLATION OF BALLAST = 1,907.5 = 14.8 in.
Ballast is used in an aircraft to attain the desired c.g. balance. It is usually located as far aft or as far forward as possible to bring the c.g.
within limits using a minimum amount.of weight.
Ballast that is installed to compensate for the removal or installation of equipment items and that is to remain in the aircraft for long periods is called permanent ballast. It is generally lead bars or plates bolted to the aircraft structure. It may be painted red and placarded: PERMA- NENT BALLAST-DO NOT REMOVE. In most cases, the installation of permanent ballast results in an increase in the aircraft empty weight.
Temporary ballast, or removable ballast, is used to meet certain loading conditions that may vary from time to time. It generally takes the form of lead shot bags, sand bags, or other weight items that are not permanently installed. Temporary ballast should be placarded: BALLAST _ LBS.
REMOVAL REQUIRES WEIGHT AND BAL- ANCE CHECK. The baggage compartment is usually the most convenient location for temporary ballast.
The places for carrying ballast should be properly designed, installed, and plainly marked. The aircraft operation manual must include instructions regarding the proper placement of the re- Without ballast placed somewhere aft to bring the c.g. within the designated limits of +16.5 in.
to +20.0 in., the aircraft is unsafe to fly when loaded with the pilot and minimum fuel. The problem of determining how many pounds of ballast are needed to move the c.g. within the approved limits can be solved by using the following formula:
Ballast weight needed:
(Weight of acft as loaded) (Distance out of limits) Arm from variable weight location to limit affected Inserting in the formula the applicable values:
Weight of the aircraft as loaded = 1907.5 Distance out of limit = +1.7 in.
Arm from variable weight location to the limit affected =53.5 in.
We obtain the following:
(1907.5) (1.7) needed in the baggage 60.6 Ibs., ballast weight
53.5 compartment.
When the mathematical computation ends in a fractional pound, use the next higher whole pound as the actual ballast weight. Consequently, 61.0 pounds must be placed in the baggage compartment to bring the c.g. safely within the c.g. range.
Datum 22" + 10' +34 + 70 CG range
16.5" to 20.0" 162" Tail wheel arm = 166".
Tail wheel weight = 115 Ibs.
CG range = (+16.5") to (+20.0") Fuel capacity = 38 gal @ 18" (19 gal. in each wing Maximum baggage = 100 Ibs @ 70' Maximum gross weight = 2620 Ibs.
A/C EW (without oil) = 1600 Ibs.
Datum = leading edge of wing.
Engine = 230 hp.
Oil capacity = 12 qts @ -22".
Number of seats = 4 (two @ + 10" and two @ +34").
Note: Flight controls in front seat only.
FIGURE 3-12. Example aircraft whose c.g. exceeds the forward c.g. limit.
A final forward weight and balance check should Most Rearward c.g. check.
be made to prove that by adding 61.0 pounds of Weight Arm Moments ballast in the baggage compartment, this aircraft Item (Ibs.) (in.) (lb.-in.)
could be safely flown with minimum fuel aboard.
Place a placard in the cockpit in a conspicuous Acft. EW 1,600.0 + 15.6 24,960.0 place for the pilot, or anyone concerned, to see. Oil 22.5 -22.0 495.0 The placard should read: FOR SOLO FLIGHT Fuel (max.) 228.0 + 18.0 4,104.0 CARRY A MINIMUM OF 61.0 POUNDS IN Pilot 170.0 + 10.0 1,700.0 BAGGAGE COMPARTMENT. Passenger 170.0 + 10.0 1,700.0 Passengers (two) 340.0 + 34.0 11,560.0 Baggage (max.) 100.0 + 10.0 7,000.0 Maximum Load Conditions Total 2,030.5 50,529.0 termine whether the airplane shown in figure 3-12 A rearward weight and balance check will de- Most rearward c.g. - Total weight Total moments can be flown safely when fully loaded without exceeding the aft c.g. limit or its maximum gross = 50,529.0 = 19.21 in.
weight. 2,630.5 68
LOADING GRAPH CO-PILOT GALLONS * :./Gal.)
LOAD WEICHT IN POUNDS ‹ PILOT FUEL PASSENGERS 43 40 REAR 30 Add weight of items to be carried to the Licensed Empty Weight of the airplane. Add moment in thousands of pound - inches of these BAGGAGE items to the total airplane moment L in thousands of pound-inches found on the Center of Gravity Envelope.
40 2 4 10 12 14 16 18 20 22 24 26 28
MOMENT IN THOUSANDS OF POUND-INCHES FIGURE 3-13. Typical loading graph.
The c.g. is well within the c.g. range when fully loaded; however, the maximum allowable gross weight is exceeded by 10.5 pounds. In this case a number of alternatives are available to remedy this overloaded condition without appreciably reducing the aircraft payload or flight range, as follows:
Alternative No. 1-reduce baggage by 10.5 Alternative No. 2-reduce fuel by 10.5 Ibs., or 1.75 gals.
Alternative No. 3-reduce passenger load by one passenger.
Each alternative listed will require a placard stating the loading arrangement by which the gross weight and c.g. will be retained within their designated limits. Compute a new c.g. position for each alternate loading arrangement.
LOADING GRAPHS AND C.G. ENVELOPES The weight and balance computation system, commonly called the loading graph and c.g. envelope system, is an excellent and rapid method for determining the c.g. location for various loading arrangements. This method can be applied to any make and model of aircraft.
Aircraft manufacturers using this method of weight and balance computation prepare graphs similar to those shown in figure 3-13 and 3-14 for each make and model aircraft at the time of original certification. The graphs become a permanent part of the aircraft records. Along with the graphs are the data for the empty weight arm and moment (index number) for that particular make and model aircraft.
The loading graph illustrated in figure 3-13 is used to determine the index number of any item or weight that may be involved in loading the aircraft. To use this graph, find the point on the vertical scale that represents the known weight.
Project a horizontal line to the point where it intersects the proper diagonal weight line (i.e., pilot, copilot, baggage, etc.). From the point of intersection, read straight downward to the horizontal scale to find the moment or index number.
After the moment for each item of weight has been determined, all weights are added and all moments are added. With knowledge of the tota.
weight and moment, project a line from the respective point on the c.g. envelope shown in figure 3-14, and place a point at the intersection of the two lines. If the point is within the diagonal lines, the loading arrangement meets all balance requirements.
The following is an actual weight and balance computation using the graphs in figure 3-13 and
3-14. For this example, assume that the aircraft has an empty weight of 1,386.0 pounds and a moment of 52,772.0 pound-inches. The index 69
2400 AIRPLANE WEIGHT IN POUNDS 2300 2200 CENTER OF GRAVITY ENVELOPE 21 00 2000 1900 1800 1700 1600 Any point falling with- 1500 in the envelope meets all Balance require- 1400 ments.
40 50 60 70 80 90 100 110
MOMENT IN THOUSANDS OF POUND-INCHES FIGURE 3-14. Center of gravity envelope.
number for the empty weight of the aircraft is ELECTRONIC WEIGHING EQUIPMENT developed by dividing the empty-weight moment by 1,000. This gives an index number of 52.8 for the airplane's empty-weight moment. Load the aircraft to determine whether the c.g. will fall within the diagonal lines of figure 3-14. Arrange item weights and index numbers in an orderly form to facilitate adding.
Moment Electronic weighing equipment greatly simplified the mechanics of weighing large, heavy aircraft. Figure 3-15 shows one type of electronic scales. The complete weighing kit is contained in a portable carrier. Included in the kit are a steel tape, plumb bobs, spirit level, straightedge, hydrometer (for determining the fuel specific gravity) and the load cells. The load cells are actually strain gages that reflect the load imposed upon them by the aircraft in terms of voltage change.
Weight (thousands This change is indicated on a scale that is cali- Item (Ibs.) of Ib.-in.) brated to read in pounds.
One load cell is placed between the jack-pad Acft. EW 1,386.0 52.8 and the jack at each weighing point. Each load Oil 19.0 - 0.4 cell must be balanced or "zeroed" before applying Pilot & copilot 340.0 12.2 any load to the cell. After completing the weighing Rear passengers (two) 340.0 24.1 operation, remove all load from the cells and check Baggage 20.0 1.9 to see if the cell reading is still zero. Any deviation Fuel 245.0 11.8 from zero is referred to as the "zero scale shift" and constitutes the tare when using electronic Total 2,350.0 102.4 weighing scales. The direction of shift is the factor that determines whether the tare is added to or The total airplane weight in pounds is 2,350.0, and the moment is 102.4. Locate this point (2,350 @ 102.4) on the c.g. envelope illustrated in figure 3-14. Since the point falls within the diagonal lines, the loading arrangement meets all weight-and-balance requirements.
subtracted from the scale reading. Always follow the instructions of the manufacture whose scales you are using.
HELICOPTER WEIGHT AND BALANCE The weight and balance principles and procedures that have been described apply generally to 70
FIGURE 3-15. Aircraft electronic weighing kit.
helicopters. Each model helicopter is certificated for a specific maximum gross weight. However, it cannot be operated at this maximum weight under all conditions. Combinations of high altitude, high temperature, and high humidity determine the density altitude at a particular location. This, in turn, critically affects the hovering, takeoff, climb, autorotation, and landing performance of a helicopter. A heavily loaded helicopter has less ability to withstand shocks and additional loads caused by turbulent air. The heavier the load, the less the margin of safety for the supporting structures, such as the main rotor, fuselage, landing gear, etc.
Most helicopters have a much more restricted
c.g. range than do airplanes. In some cases this range is less than 3 inches. The exact location and length of the c.g. range is specified for each helicopter and usually extends a short distance fore and aft of the main rotor mast or the centroid of a dual rotor system. Ideally, the helicopter should have such perfect balance that the fuselage remains horizontal while in a hover, and the only cyclic adjustment required should be that made necessary by the wind. The fuselage acts as a pendulum suspended from the rotor. Any change in the center of gravity changes the angle at which it hangs from this point of support. More recently designed helicopters have loading compartments and fuel tanks located at or near the balance point. If the helicopter is not loaded properly and the c.g. is not very near the balance point, the fuselage does not hang horizontally in a hover. If the c.g. is too far aft, the nose tilts up, and excessive forward cyclic control is required to maintain a stationary hover. Conversely, if the
c.g. is too far forward, the nose tilts down and excessive aft cyclic control is required. In extreme out-of-balance conditions, full fore or aft cyclic control may be insufficient to maintain control.
Similar lateral balance problems may be encountered if external loads are carried.
Upon delivery by the manufacturer, the empty weight, empty weight c.g., and the useful load are noted on the weight and balance data sheet in the helicopter flight manual. If, after delivery, additional fixed equipment is added or removed, or if a major repair or alteration is made, which may affect the empty weight, empty weight c.g., or useful load, the weight and balance data must be revised. All weight and balance changes should be entered in the appropriate aircraft record.
GENERAL Fuel is a substance that, when combined with oxygen, will burn and produce heat. Fuels may be classified according to their physical state as solid, gaseous, or liquid.
Solid Fuels Solid fuels are used extensively for externalcombustion engines, such as a steam engine, where the burning takes place under boilers or in furnaces. They include such fuels as wood and coal. Solid fuels are not used in reciprocating engines, where the burning takes place inside the cylinder, because of their slow rate of burning, low heat value, and numerous other disadvantages.
Gaseous Fuels Gaseous fuels are used to some extent for internal-combustion engines, where a large supply of combustible gas is readily available. Natural gas and liquefied petroleum gas are two of the more common types. Gaseous fuels can be disregarded for use in aircraft engines. The large space they occupy limits the supply of fuel that can be carried.
Liquid Fuels Liquid fuels, in many respects, are the ideal fuel for use in internal-combustion engines.
Liquid fuels are classified as either nonvolatile or volatile. The nonvolatile fuels are the heavy oils used in diesel engines. The volatile class includes those fuels that are commonly used with a fuel metering device and are carried into the engine cylinder or combustion chamber in a vaporized or partially vaporized condition. Among these are alcohol, benzol, kerosene, and gasoline.
Aviation fuel is a liquid containing chemical energy that, through combustion, is released as heat energy and then converted to mechanical CHAPTER 4
FUELS AND FUEL SYSTEMS energy by the engine. This mechanical energy is used to produce thrust, which propels the aircraft.
Gasoline and kerosene are the two most widely used aviation fuels:
CHARACTERISTICS AND PROPERTIES OF
AVIATION GASOLINE Aviation gasoline consists almost entirely of hydrocarbons, namely, compounds consisting of hydrogen and carbon. Some impurities in the form of sulphur and dissolved water will be present. The water cannot be avoided, since the gasoline is exposed to moisture in the atmosphere. A small amount of sulphur, always present in crude petroleum, is left in the process of manufacture.
Tetraethyl lead (TEL) is added to the gasoline to improve its performance in the engine. Organic bromides and chlorides are mixed with TEL so that during combustion volatile lead halides will be formed. These then are exhausted with the combustion products. TEL, if added alone, would burn to a solid lead oxide and remain in the engine cylinder. Inhibitors are added to gasoline to suppress the formation of substances that would be left as solids when the gasoline evaporates.
Certain properties of the fuel affect engine performance. These properties are volatility, the manner in which the fuel burns during the combustion process, and the heating value of the fuel.
Also important is the corrosiveness of the gasoline as well as its tendency to form deposits in the engine during use. These latter two factors are important because of their effect on general cleanliness, which has a bearing on the time between engine overhauls.
Volatility Volatility is a measure of the tendency of a liquid substance to vaporize under given conditions. Gasoline is a complex blend of volatile 73
- Gage Bomb Sample container FIGURE 4-1. Vapor pressure test apparatus.
hydrocarbon compounds that have a wide range of boiling points and vapor pressures. It is blended in such a way that a straight chain of boiling points is obtained. This is necessary to obtain the required starting, acceleration, power, and fuel mixture characteristics for the engine.
If the gasoline vaporizes too readily, fuel lines may become filled with vapor and cause decreased fuel flow. If the fuel does not vaporize readily enough, it can result in hard starting, slow warmup, poor acceleration, uneven fuel distribution to cylinders, and excessive crankcase dilution.
The lower grades of automobile fuel are not held within the tolerances required for aviation gasoline and usually contain a considerable amount of cracked gasoline, which may form excessive gum deposits. For these reasons, automobile fuels should not be used in aircraft engines, especially air-cooled engines operating at high cylinder temperatures.
Vapor Lock Vaporization of gasoline in fuel lines results in a reduced supply of gasoline to the engine. In severe cases, it may result in engine stoppage.
This phenomenon is referred to as vapor locking A measure of a gasoline's tendency to vapor lock is obtained from the Reid vapor pressure test. In this test a sample of the fuel is sealed in a "bomb" equipped with a pressure gage. The apparatus (see figure 4-1) is then immersed in a constanttemperature bath and the indicated pressure is noted. The higher the corrected vapor pressure of the sample under test, the more susceptible it is to vapor locking. Aviation gasolines are limited to a maximum of 7 p.s.i. because of their increased tendency to vapor lock at high altitudes.
Carburetor Icing Carburetor icing is also related to volatility.
When the fuel changes from a liquid to a vapor state, it extracts heat from its surroundings to make this change. The more volatile the fuel, the more rapid the heat extraction will be. As the gasoline leaving the carburetor discharge nozzle vaporizes, it can freeze water vapor contained in the incoming air. The moisture freezes on the walls of the induction system, the venturi throat, and the throttle valves. This type of ice formation restricts the fuel and air passages of the carburetor. It causes loss of power and, if not eliminated, eventual engine stoppage. Extreme icing conditions can make operation of the throttle controls impossible. This icing condition is most severe in the temperature range of 30° to 40° F. outside air temperature.
Aromatic Fuels Some fuels may contain considerable quantities of aromatic hydrocarbons, which are added to increase the rich mixture performance rating of the fuel. Such fuels, known as aromatic fuels, have a strong solvent and swelling action on some types of hose and other rubber parts of the fuel system. For this reason, aromatic-resistant hose and rubber parts have been developed for use with aromatic fuels.
Detonation In an engine that is operating in a normal manner, the flame front traverses the charge at a steady velocity of about 100 feet per second until the charge is consumed. When detonation occurs, the first portion of the charge burns in a normal manner but the last portion burns almost instantaneously, creating an excessive momentary pres- 74
sure unbalance in the combustion chamber. This abnormal type of combustion is called detonation.
This tremendous increase in the speed of burning causes the cylinder head temperature to rise. In severe cases, the increase in burning speed will decrease engine efficiency and may cause structural damage to the cylinder head or piston.
During normal combustion, the expansion of the burning gases presses the head of the piston down firmly and smoothly without excessive shock. The increased pressure of detonation exerted in a short period of time produces a heavy shock load to the walls of the combustion chamber and the piston head. It is this shock to the combustion chamber that is heard as an audible knock in an automobile engine. If other sounds could be filtered out, the knock would be equally audible in an aircraft engine. Generally, it is necessary to depend upon instruments to detect detonation in an aircraft engine.
Surface Ignition Ignition of the fuel/air mixture by hot spots or surfaces in the combustion chamber is called surface ignition. If this occurs before the normal ignition event, the phenomenon is referred to as preignition. When it is prevalent, the result is power loss and engine roughness. Preignition is generally attributed to overheating of such parts as spark plug electrodes, exhaust valves, carbon deposits, etc. Where preignition is present, an engine may continue to operate even though the ignition has been turned off.
Present information indicates that gasoline high in aromatic hydrocarbon content is much more likely to cause surface ignition than fuels with a low content.
Octane and Performance Number Rating Octane and performance numbers designate the antiknock value of the fuel mixture in an engine cylinder. Aircraft engines of high power output have been made possible principally as a result of blending to produce fuels of high octane ratings.
The use of such fuels has permitted increases in compression ratio and manifold pressure, resulting in improved engine power and efficiency. However, even the high-octane fuels will detonate under severe operating conditions and when certain engine controls are improperly operated.
Antiknock qualities of aviation fuel are designated by grades. The higher the grade, the more compression the fuel can stand without detonating.
For fuels that have two numbers, the first number indicates the lean-mixture rating and the second the rich-mixture rating. Thus, grade 100/130 fuel has a lean-mixture rating of 100 and a richmixture rating of 130. Two different scales are used to designate fuel grade. For fuels below grade 100, octane numbers are used to designate grade. The octane number system is based on a comparison of any fuel with mixtures of iso-octane and normal heptane. The octane number of a fuel is the percentage of iso-octane in the mixture that duplicates the knock characteristics of the particular fuel being rated. Thus, grade 91 fuel has the same knock characteristics as a blend of 91 percent iso-octane and 9 percent normal heptane.
With the advent of fuels having antiknock characteristics superior to iso-octane, another scale was adopted to designate the grade of fuels above the 100-octane number. This scale represents the performance rating of the fuel-its knock-free power available as compared with that available with pure iso-octane. It is arbitrarily assumed that 100 percent power is obtained from iso-octane alone. An engine that has a knocklimited horsepower of 1,000 with 100-octane fuel will have a knock-limited horsepower of 1.3 times as much (1,300 horsepower) with 130 performance The grade of an aviation gasoline is no indication of its fire hazard. Grade 91/96 gasoline is as easy to ignite as grade 115/145 and explodes with as much force. The grade indicates only the gasoline's performance in the aircraft's engine.
A convenient means of improving the antiknock characteristics of a fuel is to add a knock inhibitor.
Such a fluid must have a minimum of corrosive or other undesirable qualities, and probably the best available inhibitor in general use at present is TEL (tetraethyl lead). The few difficulties encountered because of the corrosion tendencies of ethylized gasoline are insignificant when compared with the results obtained from the high antiknock value of the fuel. For most aviation fuels the addition of more than 6 ml. per gallon is not permitted. Amounts in excess of this have little effect on the antiknock value, but increase cor- There are two distinct types of corrosion caused by the use of ethyl gasoline. The first is caused by the reaction of the lead bromide with hot metallic surfaces, and occurs when the engine is 75
in operation; the second is caused by the condensed products of combustion, chiefly hydrobromic acid, when the engine is not running.
Purity Aviation fuels must be free of impurities that would interfere with the operation of the engine or the units in the fuel and induction system.
Even though all precautions are observed in storing and handling gasoline, it is not uncommon to find a small amount of water and sediment in an aircraft fuel system. A small amount of such contamination is usually retained in the strainers in the fuel system. Generally, this is not considered a source of great danger, provided the strainers are drained and cleaned at frequent intervals. However, the water can present a serious problem because it settles to the bottom of the fuel tank and can then be circulated through the fuel system. A small quantity of water will flow with the gasoline through the carburetor metering jets and will not be especially harmful. An excessive amount of water will displace the fuel passing through the jets and restrict the flow of fuel; it will cause loss of power and can result in engine stoppage.
Under certain conditions of temperature and humidity, condensation of moisture (from the air) occurs on the inner surfaces of the fuel tanks.
Since this condensation occurs on the portion of the tank above the fuel level, it is obvious that the practice of servicing an airplane immediately after flight will do much to minimize this hazard.
Fuel Identification Gasolines containing TEL must be colored to conform with the law. In addition, gasoline may be colored for purposes of identification. For example, grade 100 low lead aviation gasoline is blue, grade 100 is green and grade 80 is red. See figure 4-2.
100/130 gasoline is manufactured (1975) in two grades- high-lead, up to 4.6 milliliters of lead per gallon and low-lead, not over 2.0 milliliters per gallon. The purpose being to eliminate two grades of lower octane fuel (80/87) and 91/96). The high-lead will continue to be colored green whereas the low-lead will be blue.
The low-lead will replace the 80/87 and 91/96 octane fuels as they are phased out. Engine manufacturers have prepared instructions to be followed in making adjustments necessary for changeover to the 100 octane fuel.
A change in color of an aviation gasoline usually indicates contamination with another product or a loss of fuel quality. A color change can also be caused by a chemical reaction that has weakened the lighter dye component. This color change in itself may not affect the quality of the fuel.
A color change can also be caused by the preservative in a new hose. Grade 115/145 gasoline that has been trapped for a short period of time in new hose may appear green. Flushing a small amount of gasoline through the hose usually removes all traces of color change.
AVGAS 115 Military or Special Use Civilian AVGAS 100 Civilian
HIGH LEAD AVGAS 100 Civilian
LOW LEAD
• AVGAS 80 Civilian FIGURE 4-2. Identification of avgas.