Everything here is from the references shown below, with a few comments in an alternate color. The purest way to measure an engine's thrust is to mount it on a stand that measures force — think of a bathroom scale turned on its side — add some fuel and ignition and read the scale. They do that with the engine in the design and test phases, but that is hardly practical when the thing is mounted to an airplane. You can also stick a pressure probe in the inlet to measure what the engineer typically calls P 2 , and another in the aft end of the engine to measure P 8.
The basic definition is that you can measure an engine's thrust by dividing the pressure at the tail pipe pt 8 in the drawing by the pressure and the inlet pt 2 to come up with an Engine Pressure Ratio suitable for viewing in the cockpit as an instantaneous representation of thrust. Does the pilot really need EPR to fly the airplane? No, but some indication of engine thrust is key to flying the airplane precisely in what professional aviators call the Control and Performance technique.
In many airplanes engine speed is measured directly in terms of revolutions per minute, RPM, and that is present to the pilot as a percentage of its maximum rated RPM, such as with the T While the RPM was rarely a good measure of differences in thrust — a 2 percent change near idle was hardly noticeable; a 2 percent change nearer the maximum produced a big change — it was good enough.
The T also used RPM but needed additional indications to reflect the performance of its after burning engine. But pilots primarily relied on RPM:. Yes, during descents at idle thrust settings and high airspeeds. Answer D is correct. Nov 8, - PM. Windshear threats are impossible to anticipate, and pilots must be especially vigilant to act upon a windshear warning.
Is this statement true or false? Nov 1, - PM. Which of the following hazards increase the risk of a runway overrun? Oct 25, - PM. To convert some of this energy in the hot gases to rotational mechanical energy, turbines at the rear of the engine are present, attached to the inner and outer coaxial shafts. These turbines are responsible for driving the low pressure and high pressure compressor turbines, which keeps the engine cycle alive.
The remaining air that entered the compressor only helps keep the engine cycle running. At this take off thrust, the inner coaxial shaft that drives the fan, and the Low Pressure compressors is spinning at revolutions per minute rpm. The outer coaxial shaft which drives the high pressure compressors is running at 10, rpm. With these values, we can get down to explaining the figures that a pilot sees in his cockpit engine displays. The same definition holds good even today. The maximum go-around EPR on the thrust ratings charts is 1.
Below the EGT, is displayed the N1, or the fan speed, or the speed of the inner coaxial shaft. On the sides of the semicircular N1 gauges are the N2 readings of the corresponding engine. The N2 is the speed of the outer coaxial shaft, or the shaft that drives the high pressure compressor.
GE uses the N1 as the thrust reference parameter. With the chosen thrust reference parameter, thrust is controlled to target the thrust at takeoff, and climbs. Cruise is a speed based reference, while descent is usually at idle thrust.
The advantage with EPR is that it is a pressure ratio, and is indicative of engine thrust. According to Boeing, EPR is more directly related to, and a much better indicator of thrust than the compressor speeds N1 and N2 , and therefore is more advantageous in terms of accuracy to utilize EPR to control engine operation.
Because changes in thrust setting take time to respond, and the filtering of noise from sensors delays response time, there is a negative impact on stability. Besides, EPR is dependent on the prevailing local atmospheric conditions as pressure is affected by temperature and aircraft altitude. With a parameter such as fan speed, or N1, the response is much better and the measurement of speed a lot more accurate than the measurement of pressure difference, which allows for excellent stability in control.
N1 is simply the fan rotational speed, which is independent of the prevailing local atmospheric conditions. EPR is the measure of a quantity that relates to the performance of the engine. N1 relates to a parameter which is responsible for the performance of the engine. As such, N1 does not take into account the other variables which may affect thrust, such as engine performance degradation after several years. If, for example, 50,lbs of thrust demanded a hypothetical EPR of 1.
In this way, N1 is not a reliable parameter for thrust setting over very long periods of time, while it is the presence of an N1 indication that enables crew to recognize performance degradation.
Consider an engine suffering a bird strike. Blades will get damaged, due to which the pressures developed across the engine will suffer. This way, the crew can ascertain that the engine has been damaged. A display with N values only will not be able to convey as much information to the pilot.
A clear diagram from a Boeing manual colored and edited for clarity showing th EPR pressure sensors in pink. Blue arrows are the bypass air, while red indicate the hot exhaust from the engine core. But EPR relies on two pitot probes: one that is ahead of the fan, and the other that is aft of the fan. The pressure sensed by these two pitot probes results in the indicated EPR.
However, they are faced with the same operational issues as other pitot probes: they are susceptible to foreign objects, such as insects and ice, clogging the opening. This can lead to faulty EPR readings, which are not indicative of the actual thrust generated by the engine.
The crash of Air Florida Flight 90 into the Potomac river is a result of the EPR probes having been clogged due to improper de-icing practices. When the crew realized that they were low on thrust, the throttle levers were advanced to full thrust position a little too late to salvage the situation, taking away the lives of 70 persons. This makes the N readings very reliable. The N readings do not fluctuate as a result of atmospheric variations, unlike the EPR.
For this reason, when penetrating a turbulent region in flight, N1 values are used as reference, even if EPR readings are available.
In normal mode, engine thrust setting is made through control of the EPR. The required EPR is set by controlling the fuel flow to the engines. In this mode, the N1 power setting is determined as a function of the thrust lever angle, altitude, and engine inlet total temperature. N readings, if not the primary readings in certain engine-aircraft combinations, are definitely a lot more reliable due to their independence from other measured parameters.
This EPR control depends upon two probes providing minimum 4 data, in contrast to the N sensor which needs only rotational speed data from a single sensor. In case of an inability to control thrust using EPR, the thrust control system always falls back upon a more reliable N1. In fact, when setting thrust, crews always cross check the developed EPR with the engine N1. In effect, this increases crew workload. While N readings are not true indicators of thrust, an undamaged engine over a substantially long period of time will hold a good correlation between generated thrust and N readings in known atmospheric conditions.
EPR may provide the crew with better awareness about the engine performance, but the EPR itself is nowhere as reliable as the robust, independent and highly reliable N readings. Jbizzle said:. September 5, at am. Archie said:. September 27, at am. This is exactly what I was looking for- a great article. Very well explained, good examples and arguments. Also, those probes are prone to damage and icing, which will throw off the reading completely. On Air Florida Flight 90, the probes iced up because the engine anti-ice system was left off despite taking off in a snowstorm.
Although the readings displayed the takeoff EPR of 2. Had the pilots of Air Florida Flight 90 cross-referenced their EPR to N1, they would have seen the discrepancy and prevented the crash. N1 is advantageous in the stability of the measurement, since it only relies on the shaft speed of the fan and convert that value into a percentage.
But that is as far as the advantages go.
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