11. Propulsive Efficiency
In terms of dynamics where we are concerned solely with forces producing motion, a Jet Engine installed in an Aircraft where the REACTION half of the jet engine's physical working cycle comprises an Air Mass in Motion delivered to the Aircraft which, in a conventional engine/Aircraft configuration is then required to act as the ACTION half of the Aircraft's Motion System, is then, no longer, simply, a Jet Engine.
In this form of utilisation, the Jet Engine is required to simultaneously perform two totally different physical functions each involving two entirely different rates of work upon the Aircraft. The first of these has already been described, namely, the purely physical work involved in converting chemical fuel energy into an accelerating Air Mass Force. The second of these is to deliver to the Aircraft this Resultant Air Mass Thrust Force, via, it's engine mounting bolts. This Resultant Jet Engine Thrust (Gross Thrust at all demand conditions) becomes the ACTION half of the Aircraft's Air Mass Motion System while at the same time the engine is required to act as an integral part of the Aircraft's structure.
The use of the terms Gross Thrust in the sense used here is the quantity of thrust delivered to the Aircraft/engine's thrust bolts at all demand conditions. This is expressed as (EmVj) where Em is the Engine Mass in unit of time and (Vj) it's Final Propelling nozzle Velocity. This expression applies at all demand conditions and is in no way dependant upon or effected by the Aircraft's Velocity at that condition. The fact that the value of this Gross Thrust delivered at all demand conditions is reduced in effectiveness to a Net Thrust value in no way alters this truth.
The Thrust delivered to an Aircraft via it's engine's mounting bolts when powered by a Jet Engine is at all time Gross in value, that is, it is the maximum value of the thrust at the disposal of the Aircraft at all demand conditions as is accounted for in calculating the Aircraft's performance by the use Mathematical expression as (-EmVj).
In summary, in the ACTION half of the Aircraft's Motion Systemthe Aircraft/engine is working on the Air Mass through which it is passing and, in the REACTION half of the same Motion System, the reverse is true, .i.e., the Air Mass (atmosphere) through which the Aircraft is flying is, working on the Aircraft.
Consequently, the Propulsive Effectiveness of the ACTION half of the Aircraft's Air Mass Motion System, Air Mass Motion System No.1, will be determined by the difference between its final velocity leaving the Aircraft's propelling nozzle and, the Resultant Aircraft Velocity at that condition. This is regarded as true of all conventionally powered Jet Powered Aircraft flight today.
However, any motion vehicle that has ever been, must possess a Motion System with both ACTION and REACTION halves. In Jet Powered Aircraft this comprises two totally differently acting AIR MASS, MOTION SYSTEMS. It is a fact that the physical, and hence the motion effectiveness of the thrust delivered to the Aircraft at all demand conditions is both determined and governed by, the difference between the Velocity of the delivery of the Thrust force so created relative to the Resultant Velocity attainable by the Aircraft. If the Velocity of the Air Mass force delivered was the same as the that done by the Aircraft upon the Air Mass through which it is flying at its Resultant Velocity, then, its Propulsive Effectiveness could be said to be, 100%. In this specific case, as things are, where the Air Mass is caused to accelerate throughout the Aircraft/engine this can only hold true when Vj the Final Propelling Velocity of the delivered Air Mass is the same as the Resultant Aircraft Velocity at, any demand condition.
The Propulsive Effect of the Aircraft's Air Mass Motion System No.2 (the Atmosphere), caused to act upon the Aircraft in the REACTION half of the Aircraft's Motion System, will equally be determined by the difference between the Air Mass Flow Velocity delivered to the Aircraft and, that acceptable to the ACTION half of the same System, i.e., at the Inlet to the Jet Engine's Compressor. But, the Jet Engine is a Reaction System, wherein an Air Mass is caused to accelerate throughout its length whilst forming an integral part of the Aircraft's structure. Consequently, the Resultant Aircraft Velocity and hence that of the Air Mass delivered to its engines at virtually all demand conditions will invariably exceed that acceptable at the inlet to the Aircraft/engine Compressor. Furthermore, the effect here is to act as a massive motion thrust force acting against the Aircraft's line of flight described and known as, Intake Momentum Drag quantified and accounted for in the Aircraft's performance by use of the expression, (-EmV). This affect dramatically reduces thereby the Aircraft's potential Velocity made possible by the Gross Thrust Force delivered to the Aircraft's thrust bolts in the ACTION half of its Air Mass Motion System to, a lesser Net Thrust value at, all demand conditions. It should be noted that this penalty is incurred in the REACTION half of the Aircraft's, Air Mass Motion System and is a product of the Aircraft's Velocity which in it's turn is the Resultant affect of the ACTION half of the Aircraft's Motion System upon is Motion State at all demand conditions.
This Resultant effect is solely attributable to the particular manner in which Sir Isaac Newton's 3rd Law of Motion is actually utilised by every Jet Powered Aircraft when either in motion or flight.
Because the Air Mass is no longer simply passing through the jet but through the Aircraft (comprising it's airframe plus engine) per se, the thrust delivered at all times is Gross in value. In the free standing Jet Engine static case at Sea Level, the Air Mass caused to accelerate through the Jet Engine is in motion relative to the stationary Earth Mass whereas, in the installed flight state, the Air Mass is in motion relative to the Aircraft, regardless of the Aircraft's speed. Logically, therefore, the final jet velocity in flight will be relative to the Aircraft itself as a motion vehicle, regardless of the fact that it may well be flying through the air at hundreds of miles per hour.
There can be no argument that the finest exponents of this science are the aerodynamicists employed by the Aircraft manufacturers. Not unreasonably therefore, it can be concluded that the resultant products of these manufacturers today represent the optimum of their respective accomplishments in terms of their knowledge and understanding of the dynamics of Jet Powered Flight.
Note a: In terms of Motion Law or dynamics we are not concerned with how the relative Velocities of either, the ACTION half of the Aircraft's Air Mass Motion System caused to accelerate through the Aircraft, or, that of the REACTION half of the same System caused to accelerate around the Aircraft have in fact, been created.
Note b: However, although the above is correct in terms of Motion Law, in terms of the Laws of Physics the power required to be delivered to the Engines thrust bolts must be sufficient to propel the Aircraft (Air Mass Motion System No.1) and also, to support the Aircraft (Air Mass Motion system No.2). At this point, the engine has done what is physically required of it. However, it is the direct continuous involvement of the physical action of the Jet Engine in the conventional configured Aircraft's Air Mass Motion System beyond this point which is the most serious fault and principal obstacle to the future economic survival of the entire World's Jet Powered Aircraft Industry beyond the year 2005.
Note c: It should be understood however, in physics, although inter -related in Action (a) & (b) are both quite separate issues in determining the Motion Efficiency of a Jet Powered Aircraft as a Motion Vehicle that is caused to fly (Sections 16, 17 & 18 by way of explanation).
|
