AIRCRAFT WEIGHT ESTIMATION

AIRCRAFT WEIGHT ESTIMATIONTake-Off Gross WeightThe total weight of the aircraft can be divided into crew weight, payload weight, fuel weight and empty weight.

Empty Weight EstimationThe empty weight fraction can be estimated statistically from historical trends.Empty weight fractions vary from 0.3 to 0.7 and diminish with increasing total aircraft weight.If variable swept wing is to be used then, empty weight fraction should be multiplied by 1.04.

Empty Weight Fraction vs W0

Fuel Fraction EstimationThe total amount of fuel used during a mission is based on considering the individual amounts used within each flight phase/segment.If there are N flight segment, then the total amount of fuel used is

If 6% allowance is given for trapped and reserved fuel then,

Where is known as mission

segment weight fraction defined as the ratio of weight of the aircraft at the end of the segment to the weight of the aircraft at the beginning of the segment.

Typical Mission Profiles

For our initial estimation, Mission SegmentWarm up ad Takeoff0.970Climb0.985Landing0.995

Cruise SegmentFrom Brequet range equation, we have

for propeller driven airplane

for jet driven airplane

10Loiter:The loiter phase consists of cruising for a specified amount of time over a small region. Loiter time is usually included to allow for delays prior to landing.

for jet driven airplane

for reciprocating engines

L/D EstimationFor a propeller driven airplane, efficient cruise occurs at (L/D)max and efficient loiter at 0.866(L/D)max.For a jet driven airplane, efficient cruise occurs at 0.866(L/D)max and efficient loiter at (L/D)max.

ExamplesJune10(3b):

W0(guess)We/W0W0(calculated) 100,0000.37387108,730108,700..

Ans: 107840NJune 11(2):Payload= 58600NWeight of crew = 3335NEmpty weight fraction =The fuel fraction in terms of range (R) is Obtain the gross weight of the airplane for two options of range viz. 1500km and 2000km. Comment on the results-how does fuel fraction change? How does gross weight change?

December 2011(4a):A UAV is to be designed with a payload capacity of 50kg. It is powered by a single piston engine having sfc of 0.07 mg/W-s during cruise and 0.09mg/W-s during loiter. After launch by catapult, the aircraft is required to fly a distance of 200 n mile from launch site, loiter for 45 minutes, and then cruise back , where it is recovered by a parachute. Assume that its entire flight is conducted at M=0.3 at sea level under ISA conditions, and the propulsive efficiency is 0.8. If the empty weight fraction is 0.4, and max. L/D of the aircraft is 10; estimate its design gross weight.Given:Weight of payload=50kgSfc during cruise

Sfc during loiterCruise to destination,Return cruise,Loiter time,Max(L/D)=10Propulsive efficiency=0.8Mach no. , M=0.3

We know,(1)

Fuel fraction = 01 32As the UAV is launched by a catapult, no fuel will be used during takeoff and climb and hence fuel fraction during these segments are neglected. Also, the UAV is recovered by a parachute, fuel fraction during landing is also neglected. So the fuel will be used only for three segments namely, cruise out to destination, loiter and return cruise i.e.

Cruise to destination(0-1):We know, for a piston engine, range is given as

For a piston engine, efficient cruise occurs at a velocity where (L/D) is max.

Loiter(1-2):We know,

Since efficient loiter occurs at a velocity where, (L/D) is equal to 0.866(L/D)max and entire flight is conducted at sea level under ISA condition,

T=288K

Since cruise range is same as the cruise to destination range and every other parameters being same, we get

So,

Fuel fraction

Allowing 6% allowance for reserved and trapped fuel,Total fuel fraction,

Substituting in equation (1),

June 12(4a):A twin piston engine aircraft is required to have a payload capacity of 1000kg. The aircraft is required to fly a distance of 500km and back at an altitude of 3km above mean sea level under ISA conditions at a cruise Mach no. of 0.3. The empty weight fraction is 0.4 and max. L/D of the aircraft is 8. If sfc of the engine is 0.07 mg/W-s and propulsive efficiency in cruise is 0.8, estimate its design gross weight.