*Effect of Weight*. Weight is an all important consideration and is most
difficult to estimate unless one has accurate data on existing machines
of the same type. The total weight in flying order depends upon the
useful load to be carried, and upon the weight of the power plant. The
weight of the latter varies both with the useful load and with the
speed, climb, and duration of flight. The type of aeroplane determines
the relative head resistance which again reflects back to the weight of
the power plant.
The only reason for the existence of an aeroplane is to carry a certain
useful load for a given distance, and this useful load is the basis of
our weight calculations. The basic useful load consists of the
passengers and cargo, although in some specifications the live load may
be construed as including the weight of the fuel, oil and instruments,
and in the case of military aeroplanes, the weight of the armament,
armor, ammunition, wireless and cameras. For comparison, the elements
constituting the live load should always be specified.
For a given horsepower, speed and climb, it is obvious that the dead or
structural weight should be at a minimum for a maximum live load
capacity. The dead load carried in present aeroplanes will be
undoubtedly reduced in the future by the adoption of lighter and
stronger materials, better methods of bracing, and by reductions in the
weight of the power plant. Just as the automobile industry developed
light and powerful materials of construction, so will the aeroplane
designer develop more suitable materials for the aircraft. While the
present power plant has been refined to a remarkable extent when
compared with the older types, it is still far from the lowest possible
limit. At present the complete power unit—the motor, radiator,
propeller, water, etc.—will weigh from 2 to 5 pounds per horsepower.
With a given aeroplane, the performance is determined by the total
weight and power. The duration and flight range can be increased by
increasing the fuel weight at the expense of the passenger or cargo
weight. The power available for climbing is the excess of the total
power of the motor over the power required for horizontal flight. Since
the power for horizontal flight depends principally upon the weight, it
is at once evident that the weight is a regulating factor in the
climbing speed. In fact the climbing speed may be almost directly
determined from the weight carried per horsepower at normal flight
speed. A fast climbing scout may weigh from 8 to 12 pounds per
horsepower, while the large low climbing machine will weigh from 16 to
20 pounds per horsepower, the respective climbing speeds being
approximately 1,200 and 350 feet per minute.
*Fuel Efficiency and Weight*. The efficiency of the motor, or its fuel
consumption for a given output, has a very marked effect upon the total
weight of the aeroplane. Under certain conditions a very light motor
with a high fuel consumption will often contribute more to the total
weight than a heavier but more economical motor. In short flights, up to
3 hours, the very light rotating cylinder motor with its high fuel
consumption probably gives the least total weight, but for longer
flights the more efficient and heavier water-cooled type is preferable.
For flights of over three hours the fuel weight is a considerable
percentage of the total weight. The proper motor for any machine must be
selected by computing the weight of the fuel and oil required for a
given duration and then adding this to the total weight of the engine
and its cooling system.
*Distribution of Weight*. Practically the only way to predict the weight
of a proposed machine is to compare it with a similar existing type.
After the ratio of the useful load to the total load has been
determined, the useful load of the proposed machine can be divided by
the ratio factor to obtain the total weight. It should be noted in this
regard that if the proposed machine is much larger than the nearest
existing example, a liberal allowance must be made to compensate for the
increase in the proportional weight of the structural members. There
have been many mathematical formulas advanced for predicting the weight,
but these are very inaccurate in the majority of cases.
As a rough estimate, based on a number of successful machines, the
weight of the actual aeroplane structure without power-plant, live load,
fuel, oil, or tanks, is very nearly 32 per cent (0.32) of the total
weight. The remaining 68 per cent is divided up among the power-plant,
fuel and live load. Thus, the aeroplane structure proper of a machine
weighing 2000 pounds total will be 2000 x 0.32 = 640 pounds. Taking the
weight of the power plant, tanks and piping at 28 per cent, the total
dead load of the bare machine without fuel or oil will be 60 per cent of
the total. With a training aeroplane built for a 6-hour flight, the fuel
and oil will approximate 16 per cent, so that the total percentage
possible for the crew and cargo will be 24 per cent. With a given live
load, the total load can now be calculated by dividing the live load by
its percentage. Using the above value, for example, the total weight in
order of flight with a live load of 720 pounds becomes: W = 720/0.24 =
3000 pounds.
In government specifications the total weight of the pilot and passenger
are taken at 330 pounds, or 165 pounds per man. Gasoline and oil are for
a 4-hour flight. A safer average figure will be 170 pounds per man, and
a fuel allowance of 6 hours. The floats of a seaplane or flying boat
bring the percentage of the dead load much higher than with the land
type of chassis.
The following table will give an idea as to the weight distribution
expressed both in pounds, and as a percentage of the total weight. It
covers a wide range of types, varying from the training types Curtiss
JN-4B and the Standard H-3, to the Handley-Page Giant bomber and the
Nieuport speed scout. The average values found by Hunsaker for a number
of machines weighing in the neighborhood of 2500 pounds is given in the
fourth column. Under each heading are the actual weights and the
percentages of the total weight for each item. Items marked (*) include
both gasoline and oil. Mark (C) is the power plant complete, and (@)
includes radiator.
*Weight Per Horsepower*. As already explained, the weight carried per
horsepower varies with the type of machine. When the total weight is
determined for any aeroplane, the power requirements can be calculated
by dividing the total weight by the weight per horsepower ratio. A fair
value for a training or exhibition machine is from 18 to 20 pounds per
horsepower, while for a very high speed machine, such as a chaser, the
weight will be taken at 10 pounds per horsepower. For two-seater
fighters 16 to 18 pounds is fair practice. For a comparison of the
horsepower-weight ratios used on different well-known machines see
tables in Chapter II. Thus, if our total weight is found to be 2400
pounds as determined from the above table, and if this is a training
machine, the horsepower will be: 2400/20 = 120 horsepower. Using the
same total weight, but powered for two-seater fighter conditions, the
power will be increased to 2400/16= 150 horsepower. As a scout the power
will be increased still further to 2400/10=240 horsepower.
As a problem in solving the weight and horsepower from the data, we will
assume that we are to design a two-seater fighter with a total useful
load of 1200 pounds. This load consists of the following items:
Personnel (2) = 330 pounds; gas and oil = 500 pounds; guns and
ammunition = 370 pounds. The nearest example that we have to this live
load is that of the Standard H-3, which carries 744 pounds and in which
the percentage of live load is 28.1 per cent. As our machine will be
somewhat larger, we will not be far from the truth if we take the
percentage as 0.27 instead of 0.281. The total weight, in flying order,
will now be 1200/0.27 = 4440 pounds. At 16 pounds per horsepower the
motor will be: 4440/16=277 horsepower.
An empirical formula for a high-speed scout was set forth in "Aviation
and Aeronautical Engineering" by D. W. Douglas. This is based on the
horsepower unit. A unit wing loading of 8.45 pounds per square foot, and
a low speed of 55 miles per hour was assumed. The wing section chosen
was the U.S.A.-1. In the formula, H = horsepower:
- Power plant weight = 3 H.
- Chassis weight = 0.7 H.
- Tail weight = 0.25 H.
- Fuel for 2.25 hours = 1.4 H.
- Military load = 250 pounds.
- Tanks and piping = 0.42 H.
- Fuselage weight = 1.84 H.
- Wing weight = 1 lb. sq. ft.
- Propeller = 2.8/H.
- (Total) = (7.61 H + 2.5/H + 250)/7.45 = Weight of aeroplane fully
loaded in the order of flight.
*Weight of Wings*. The weight of the wings depends upon the span, very
small machines having wings that weigh only 0.38 pounds per square foot,
while the wings of very large machines may run as high as 1.1 pounds per
square foot. For average size biplanes from 0.75 to 0.80 pounds per
square foot would probably be safe—that is, for areas ranging from 450
to 550 square feet. The weight of the upper wing of the Nieuport is
0.815 pounds per square foot, while the lower wing (short chord) is
0.646 pounds per square foot. The wings of the Standard H-3 trainer will
average 0.77 pounds per square foot, the lower wing and center section
being heavier than the upper wing. The wings of the Curtiss JN-4B will
average 0.75 pounds per square foot. These weights do not include the
interplane wires or struts, nor the fittings. The total weight of the
interplane struts of the JN-4B, the Aviatic, and machines of similar
size will average from 28 to 30 pounds. The ailerons will weigh about 12
pounds each.
*Weight of Motors*. There is a considerable difference in the weight of
air-cooled and water-cooled motors. The water, water piping, radiators
and jackets of the water-cooled motors adds considerably to the weight
of the complete power plant. The mountings are heavier for the
water-cooled motors, and because of the tandem arrangement of the
cylinders, the crankshaft and crankcase weigh more. In taking the bare
weight of the power plant all of the accessories must be included. In
the following table, the "bare engine" includes the carbureter, magneto,
and necessary integral accessories, but does not include the jacket
water, mounting, radiator, oil in base, water piping, nor controls.
Water-cooled motors are marked by (W) and air-cooled by (A). Rotary
air-cooled are (RA), and gallons (G).
WEIGHTS OF AERONAUTICAL MOTORS.
The bare radiator will weigh from 0.48 to 0.56 pounds per horsepower,
the average being safe at 0.52. The water contained in the radiator will
average 0.35 pounds per horsepower. The weights of the piping and the
water contained therein will be computed separately. The circular sheet
metal cowl used over the rotary cylinder air-cooled motor is equal to
twice the square root of the motor weight, according to Barnwell.
Propeller weight varies considerably with the diameter, pitch, etc., but
a safe rule will give the weight as 2.8 √H where H = horsepower. The
tanks will weigh from 0.75 to 1.2 pounds per gallon of contents, or
approximately 1/5 the weight of the contents when completely filled.
*Chassis and Wheel Weight*. The chassis of a two-wheel trainer will
weigh about 90 pounds complete, although there are chassis of training
machines that weigh as much as 140 pounds. The chassis of speed scouts
will be from 22 to 40 pounds complete. Tail skids can be taken at from 6
to 8 pounds.
Tangent wire wheels complete with tires are about as follows: 26 x 4 =
21 pounds; 26 x 5 = 28 pounds; 26 x 3 = 14 pounds. Ackerman spring spoke
wheels are estimated as follows: 20 x 4 = 17.5 pounds; 26 x 3 = 22
pounds; 26 x 4 = 32 pounds; 30 x 4 = 35 pounds; 34 x 4 = 45 pounds.
*Military Loads*. A 20-mile wireless outfit devised by Capt. Culver
weighed 40 pounds with storage batteries, while the 120-mile outfit
weighed 60 pounds with a 180-watt generator. The 140-mile U.S.A.
mule-back wireless of 1912 weighs 45 pounds. The "Blimp" specifications
allow 250 pounds.
The Lewis gun as mounted on the "11" Nieuport weighs 110 pounds,
including mount, gun and ammunition. Lewis gun bare is 26 pounds. The
Davis 6-pounder, Mark IV, weighs 103 pounds with mounting but without
ammunition, while the same make of 3-inch 12-pounder weighs 238 pounds
under the same conditions.
*Controls and Instruments*. The Deperdussin type controls used on the
Curtiss JN-4B weigh 16 pounds per control, while those installed in the
Standard H-3 weigh about 13 pounds. An average of 15 pounds per control
is safe. An instrument board for the aviators’ cock-pit, fully equipped,
weighs from 20 to 24 pounds. The front, or students’ instrument board
will average 10 pounds. Pyrene extinguisher and brackets = 7 pounds;
Speaking tube = 3 pounds; Oil pressure line and gage = 3 pounds; Side
pockets = 3 pounds; Tool kit = 10 pounds.
*Control Surfaces*. The rudder, stabilizer, fin, and elevator can be
made so that the weight will not exceed 0.60 to 0.65 pounds per square
foot.
*General Notes on Weight*. Before starting on the weight estimates of
the machine the reader should carefully examine the tables in Chapter II
which give the weights, and general characteristics of a number of
modern machines.
*Weights and Wing Area*. When the weight of the machine is once
determined, the next step will be to determine the wing area. For speed
scouts or very large heavy duty machines the choice of a wing section
must be very carefully considered. For the speed scout several wings
giving a minimum high speed resistance should be examined, such as the
Eiffel 37 or the U. S. A-1 or U.S.A.-6. For the low-speed aeroplane to
be designed for great lift, a number of sections such as the U.S.A.-4 or
the R.A.F.-3 should be tried for a number of speeds and angles. For
training machines a wing of the "All around" type such as the R.A.F.-6
should be adopted, the structural characteristics in the case of a
trainer having an important bearing on the subject. If W = weight of the
machine in pounds, V = low speed in miles per hour, A = total area in
square feet, and Ky=lift coefficient, then the area becomes A=W/KyV².
Compensation must be made for biplane interference for aspect ratio, and
stagger as previously explained. For an ordinary training machine with
the usual gap/chord ratio, and aspect ratio, the correction factor of
0.85 may be safely employed.
*Example*. We will take the case of an aeroplane carrying a personnel
load of 340 pounds, oil and gasoline 370 pounds, and baggage amounting
to 190 pounds, instruments 100 pounds. Total live load will be 1000
pounds. Taking the live load percentage as 0.30, the total load will be
1000/0.30 = 3333 pounds. If the low speed is 50 miles per hour, and the
maximum Ky of the chosen wing is 0.003 at this speed, the area will be A
= W/KyV² = 3333/0.003 x (50 x 50) = 444 square feet. Since this is a
biplane with a correction factor of 0.85, the corrected area will be:
444/0.85 = 523 square feet. The unit loading, or weight per square foot
will be: 3333/523 = 6.36 pounds. The corrected area includes the
ailerons and the part of the lower wing occupied by the body.
*Empirical Formula for Loading*. After investigating a large number of
practical biplanes, the author has developed an expression for
determining the approximate unit loading. When this is found, the
approximate area can be found by dividing the total weight by the unit
loading. This gives an idea as to the area used in practice.
It was found that the unit loading increased with the velocity at nearly
a uniform rate. This gave an average straight line formula that agreed
very closely with 128 examples. If V = Maximum velocity in miles per
hour, and w = weight per square foot, then the unit loading becomes:
w = 0.065V - 0.25 for the average case. For high speed scouts this gives
a result that is a trifle low, the formula for a fast machine being more
nearly w = 0.65V - 0.15, for speeds over 100 miles per hour.
A two-seat machine of average size weighs 2500 pounds, and has a maximum
speed of 90 miles per hour. Find the approximate unit loading and area.
The loading becomes: w = 0.065V - 0.25 = (0.065 x 90) - 0.25 = 5.6
pounds per square foot. The approximate area will be: 2500/5.60 = 446
square feet.
If the above machine had a speed of 110 miles per hour, the formula
would be changed for the high-speed type machine, and the loading would
become:
w = 0.065V - 0.15 = (0.065 x 110) - 0.15 = 7.00 pounds per square foot.
The required area will be: 2500/7.0 = 372 square feet. When the unit
load is also determined in this way it is a very simple matter to choose
the wing section from Ky = w/V².
*Area From Live Load and Speed*. By a combination of empirical formula
we can approximate the area directly. For the average size machine, w =
0.065V - 0.25. And the total weight W = U/0.32 where U is the useful or
live load. Since A = W/w, then A = U/(0.65V - 0.25) x 0.32 = U/0.021V -
0.08.
Thus if an aeroplane travels at 90 miles per hour and has carried a
useful load of 800 pounds (including gas and oil), the approximate area
is: A = U/0.021.V = 0.08 = 800/(0.021 x 90) - 0.08 = 442 square feet.
This assumes that the useful load is 0.32 of the total load and that the
speed is less than 100 miles per hour.