INTERACTION WITH OTHER SPECIALTIES
It cannot be mentioned too often that there are many disciplines involved
in designing, developing, and manufacturing engines. How well one discipline
understands the problems and technology of other disciplines plays an
important role in determining costs as well as the elapsed time between the
initiation of design and the delivery of an approved product.
An aerodynamicist's lack of appreciation of stress and material technology
may require him to design and redesign a blade a number of times
before the aerodynamic and endurance requirements are satisfied. If manufacturing
technology is ignored, an unnecessarily expensive process may
have to be devised to make and inspect the blade. It is especially note
worthy that a part which can be thoroughly inspected is often preferred to
a potentially superior part which does not lend itself to inspection. Frequently,
the potentially superior part turns out to be inferior because it
does not conform to design requirements. Simple design changes that
improve the ability to reproduce a product, perhaps with a slight penalty in
latent performance, should always be in the thoughts of an aerodynamic
designer. This emphasizes the requirement for the designer to understand
the real needs of the customer.
As mentioned in Sec. 1.2, the purpose of an airplane is to render a service
that enough people want and can afford. The costs to the eventual
customer include his share of the expenses associated with the initial design
and development, manufacturing, maintenance, and availability. (Availability
signifies, among other connotations, whether nine or ten engines
must be purchased to be sure that at least eight are available when needed.)
By understanding the total picture, an honest evaluation can be made
about whether the added cost of a supposedly more efficient part is worth
its latent contribution to reduced fuel consumption, reduced weight, or
increased thrust. This knowledge, of course, cannot be acquired instantly.
The successful aerodynamic designer will always be alert to opportunities
that will improve his understanding of these interrelationships.
Besides adopting this long-range philosophy, there are many areas where
a good understanding of peripheral technology should be sought almost
immediately. A few subjects and problem areas have been selected to
illustrate the need for the aerodynamicist to be involved in many activities.
The important subject of controls is not covered here since some attention
was directed to them earlier.
Dimensional Integrity
Notice was taken in previous sections of some effects of the dimensional
changes that accompany temperature variations throughout an engine. The
existence of variations in temperature and dimensional changes along the
length of the engine during steady-state operation are readily appreciated.
The effects of transient operation on changing these variations of temperature
and dimensions with time should also be recognized.
Circumferential variations in temperature near an outer casing can cause
bulges in the casing, with a resulting local increase in clearance. Simple
calculations can indicate how easily noticeable increases in such clearances
are achieved.
It is practically impossible to keep uniform clearance between stationary
and rotating parts at all times. There is, however, always some operating
condition and one or more areas of an engine where the clearance is a
minimum. The circumstances depend upon the temperatures and vibrations.
Airplane maneuvers are also involved. The requirement of mechanical
integrity sets the magnitude of the minimum clearances. The clearances
at other operating conditions are then automatically defined.
The relative growth of the rotating and stationary parts depends to some
extent on the aerothermodynamic design. Heat-transfer rates and blade
shapes are involved.
It cannot be mentioned too often that there are many disciplines involved
in designing, developing, and manufacturing engines. How well one discipline
understands the problems and technology of other disciplines plays an
important role in determining costs as well as the elapsed time between the
initiation of design and the delivery of an approved product.
An aerodynamicist's lack of appreciation of stress and material technology
may require him to design and redesign a blade a number of times
before the aerodynamic and endurance requirements are satisfied. If manufacturing
technology is ignored, an unnecessarily expensive process may
have to be devised to make and inspect the blade. It is especially note
worthy that a part which can be thoroughly inspected is often preferred to
a potentially superior part which does not lend itself to inspection. Frequently,
the potentially superior part turns out to be inferior because it
does not conform to design requirements. Simple design changes that
improve the ability to reproduce a product, perhaps with a slight penalty in
latent performance, should always be in the thoughts of an aerodynamic
designer. This emphasizes the requirement for the designer to understand
the real needs of the customer.
As mentioned in Sec. 1.2, the purpose of an airplane is to render a service
that enough people want and can afford. The costs to the eventual
customer include his share of the expenses associated with the initial design
and development, manufacturing, maintenance, and availability. (Availability
signifies, among other connotations, whether nine or ten engines
must be purchased to be sure that at least eight are available when needed.)
By understanding the total picture, an honest evaluation can be made
about whether the added cost of a supposedly more efficient part is worth
its latent contribution to reduced fuel consumption, reduced weight, or
increased thrust. This knowledge, of course, cannot be acquired instantly.
The successful aerodynamic designer will always be alert to opportunities
that will improve his understanding of these interrelationships.
Besides adopting this long-range philosophy, there are many areas where
a good understanding of peripheral technology should be sought almost
immediately. A few subjects and problem areas have been selected to
illustrate the need for the aerodynamicist to be involved in many activities.
The important subject of controls is not covered here since some attention
was directed to them earlier.
Dimensional Integrity
Notice was taken in previous sections of some effects of the dimensional
changes that accompany temperature variations throughout an engine. The
existence of variations in temperature and dimensional changes along the
length of the engine during steady-state operation are readily appreciated.
The effects of transient operation on changing these variations of temperature
and dimensions with time should also be recognized.
Circumferential variations in temperature near an outer casing can cause
bulges in the casing, with a resulting local increase in clearance. Simple
calculations can indicate how easily noticeable increases in such clearances
are achieved.
It is practically impossible to keep uniform clearance between stationary
and rotating parts at all times. There is, however, always some operating
condition and one or more areas of an engine where the clearance is a
minimum. The circumstances depend upon the temperatures and vibrations.
Airplane maneuvers are also involved. The requirement of mechanical
integrity sets the magnitude of the minimum clearances. The clearances
at other operating conditions are then automatically defined.
The relative growth of the rotating and stationary parts depends to some
extent on the aerothermodynamic design. Heat-transfer rates and blade
shapes are involved.
No comments:
Post a Comment