flight vehicle loads(1)

8/13/2019 Flight Vehicle Loads(1)

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Flight Vehicle Imposed Loads

All loading conditions imposed on a structure must be known before the

flight vehicle structure can be designed. The load conditions are those

encountered both in flight and on the ground.

Since it is impossible to investigate every loading condition which a flight

vehicle may encounter in its service lifetime, it is normal practice to select

only the critical loading conditions for design.

The critical loading conditions are usually determined from past experience

and are definitely specified by the certification or procuring agency.

Limit loads are associate with normal operation of the flight vehicle.

Ultimate loads are typically defined as 1.5 x Limit Loads.


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Inertia Loads

The maximum loads on any part of the flight vehicle structure occurswhen the vehicle is being accelerated.

The loads produced by landing impact, maneuvering, gusts, boost and

staging operations, launches, and dockings are always greater than

the loads occurring when all the forces on the vehicle are in Equilibrium.

In many of the loading conditions, a flight vehicle may be considered

as being in pure translation or in pure rotation.

Translation:


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Inertia Loads(Continued)

Rotation: Rigid mass rotation about point Owith a constant angular

velocity . x-axis chosen through center of gravity C.


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acceleration . x-axis chosen through center of gravity C.


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acceleration , angular velocity , and linear acceleration aox-axis chosen through center of gravity C.


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Load Factors for Translational Acceleration

When the vehicle has only translational acceleration, every part ofthe vehicle is acted on by parallel inertia forces which are

proportional to the weight of the part. Load factor is a convenient

way to express the magnitude of inertial forces.


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Load Factors for Translational Acceleration(Continued)

Flight vehicles frequently have horizontal acceleration as well as

vertical acceleration.


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When the airplane horizontal thrust line is not horizontal, it is usually

convenient to obtain the components of the forces parallel and

perpendicular to the airplane thrust line

Perpendicular to the thrust line

Parallel to the thrust line


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The landing load factors are defined by dividing the landing loads

by the airplane weight.

In general, the load factor nin any axis iis given by:


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Load Factors During Maneuvers

A common airplane maneuver is a motion along a curved path (pullup from a steady flight or pull out from a dive)

If the velocity of the airplane

along the path is constant,

then at= 0 ad Ft= 0.


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Load Factors During Launch

Lumped Mass Model Discrete Mass Model


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Example Problem 1

When landing on a carrier, a 10,000 lb airplane is given a deceleration of 3gby means of an arresting hook as shown in the figure below.

(a) Find the tension in the cable, the wheel reaction, and the distance e from the

center of gravity to the line of action of the cable.

(b) Find the tension in the fuselage at vertical section AA and BB if the portion of

the airplane forward section weighs 3000 lb and the portion aft of section BB

weighs 1000 lb.(c) Find the landing run if the speed is 80 ft/sec


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Example Problem 2

A cargo plane shown below is making a level landing with the nose wheel

slightly off the round. The vertical ground reaction is 300,000 lb. The pitching

moment of inertia of the plane is 40,000,000 lb sec2 in.

(a) Find vertical acceleration at the aircraft c.g.

(b) Find vertical acceleration at the pilot c.g

300,000


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Example Problem 3

An airplane weighs 14,000 lb. It is flying horizontally at a velocity of 500 mph

(733 ft/sec) when the pilot pulls up into a curved path with a radius of curvature

of 2500 ft. Assume engine thrust and drag are equal and colinear with each other.

(a) Find airplane acceleration

(b) Find wing lift (L) and tail (T) forces

(c) Find airplane load factor


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Example Problem 4

A spacecraft payload weighs 600 lbs. It experiences load factors nz= 5g and nx= 0.4gduring the launch sequence. Calculate the reactions forces and moments at

the launch vehicle interface

Launch Vehicle

Interface

flight vehicle loads(1)

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