Mcrit, mmo and mach trimmer confusion
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Mcrit, mmo and mach trimmer confusion
Hi all,
I know that this thread has been discussed already but i couldnt find a proper amswer to my question and i was wondering if someone could shed some light on it.
-most of the jet aircrafts cruise speeds are above mcrit so that means there is a range of speed between mcrit and mmo at which the aircraft flies. How can this good performance wise if above mcrit you will have a drag increase due to shockwaves?in another post i read that at 0.05 M above Mcrit the drag increase isnt much but still why would you want to fly at speeds that are going to increase your drag?
-i know what a mach trimmers is there for but what i didnt get is how does it actually work? Does it activate at a preset mach number even tough mcrit is not a fixed value but it changes according to outside conditions and aircraft configuration?
I hope my questions are clear enough and someone will be able to enlighten me
Thanks in advance
I know that this thread has been discussed already but i couldnt find a proper amswer to my question and i was wondering if someone could shed some light on it.
-most of the jet aircrafts cruise speeds are above mcrit so that means there is a range of speed between mcrit and mmo at which the aircraft flies. How can this good performance wise if above mcrit you will have a drag increase due to shockwaves?in another post i read that at 0.05 M above Mcrit the drag increase isnt much but still why would you want to fly at speeds that are going to increase your drag?
-i know what a mach trimmers is there for but what i didnt get is how does it actually work? Does it activate at a preset mach number even tough mcrit is not a fixed value but it changes according to outside conditions and aircraft configuration?
I hope my questions are clear enough and someone will be able to enlighten me
Thanks in advance
You don't fly many aeroplanes at Vmd or Mmd, and for the same reason you don't necessarily cruise below Mcrit.
Min drag is about best endurance speed. Aeroplanes don't usually fly around that, as the real objective is to get to the destination with the minimum fuel burn. That is best done at the higher best range speed.
Real world economics however push the cruise speed up further, as there's more to the cost of running an aeroplane that fuel burn, and the airline will want to maximise the utilisation of the aeroplane (and arrive on time!). So, the aeroplane will be flown at higher speeds again. Hence flight above Mcrit.
As you approach transonic speeds, there's a pitch trim change. A Mach trimmer links into the pitch control circuit and applies what in effect is a pull back on the stick of the right amount at the right Mach number(s) so that the pilot doesn't have to. There's nothing particularly complicated about it to be honest, and it's really just there as a safety improver / workload reducer. The internal mechanism is really just mechanical gearing (or electrical / electronic simulation of the same).
And the plural of "aircraft" is "aircraft" not "aircrafts". A common mistake in non native English speakers.
Min drag is about best endurance speed. Aeroplanes don't usually fly around that, as the real objective is to get to the destination with the minimum fuel burn. That is best done at the higher best range speed.
Real world economics however push the cruise speed up further, as there's more to the cost of running an aeroplane that fuel burn, and the airline will want to maximise the utilisation of the aeroplane (and arrive on time!). So, the aeroplane will be flown at higher speeds again. Hence flight above Mcrit.
As you approach transonic speeds, there's a pitch trim change. A Mach trimmer links into the pitch control circuit and applies what in effect is a pull back on the stick of the right amount at the right Mach number(s) so that the pilot doesn't have to. There's nothing particularly complicated about it to be honest, and it's really just there as a safety improver / workload reducer. The internal mechanism is really just mechanical gearing (or electrical / electronic simulation of the same).
And the plural of "aircraft" is "aircraft" not "aircrafts". A common mistake in non native English speakers.
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Mcrit is the sped at which a small part of the airflow will reach M1.0. Usually at in indicated Mach between 0.6 and 0.7 ish.
As Mach number increases at some point the aircraft will become uncontrollable because of high Mach effects. That speed becomes MMO.
Google drag divergence Mach number.
As Mach number increases at some point the aircraft will become uncontrollable because of high Mach effects. That speed becomes MMO.
Google drag divergence Mach number.
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a mach trimmer works by pulling back on the stick.
on the DC9 series, it is called a Mach Trim Compensator. AS you go faster the nose wants to drop, so a nifty gadget pulls back on the stick so you don't have to.
you test it prior to flight. if it fails, you have to fly slower.
there is a pretty nifty little rod that comes up to show you how much p;ull up is being used.
on the DC9 series, it is called a Mach Trim Compensator. AS you go faster the nose wants to drop, so a nifty gadget pulls back on the stick so you don't have to.
you test it prior to flight. if it fails, you have to fly slower.
there is a pretty nifty little rod that comes up to show you how much p;ull up is being used.
When studying transonic flight it is tempting to think that something very dramatic occurs at Mcrit, but this is not the case. Mcrit is simply the lowest mach number at which any part of the airflow over the surfaces of an aircraft reaches the speed of sound. Because this airflow is only just sonic it produces few if any shock waves. And because the sonic airflow is restricted to a very small region of the aircraft surface, any shock waves formed affect only a very minor part of the overall flow. Because of these factors, there is no significant increase in drag, and no significant changes in aircraft performance or handling at Mcrit. So Mcrit is really just of academic interest to pilots.
why would you want to fly at speeds that are going to increase your drag?
In order to achieve maximum endurance we must fly at the speed at which fuel consumption rate is lowest. For jets this is generally accepted to at or about Vmd. In order to achieve maximum range for a given fuel low we must fly at the speed at which we achieve the maximum ratio of TAS : Fuel Flow. In jet aircraft fuel flow is proportional to thrust and in steady flight thrust is equal to drag. So for minimum fuel consumption between two points (and maximum range) we need to fly at the speed at which we achieve the maximum TAS : Drag ratio.
If we look at a graph of drag against TAS for a typical jet aircraft at subsonic speeds, the curve is a shallow bucket shape with the minimum drag value at Vmd. As speed increases beyond Vmd the drag increases, but the rate of increase is initially very small. This means that at speeds slightly greater than Vmd, the rate of increase in drag is less than the rate of increase in TAS. This means that increasing the TAS within this speed range increases the drag slightly, but actually increases the TAS : Drag ratio. But if we continue to increase our TAS, the drag curve becomes steeper, and a speed is eventually reached where the drag is increasing faster than the increasing TAS.
The Drag Divergence Mach Number (Mdd) is a mach number slightly higher than Mcrit, at which the shock waves become sufficiently widespread and intense to create a large increase in drag. This causes a marked decrease in the TAS : Drag ratio. So at speeds between Mcrit and Mdd increasing airspeed will have a beneficial effect on the TAS : Drag ratio.
To find the speed at which we get the best TAS : Drag ratio we draw a tangent from the origin of the graph to touch the drag curve. The speed at which this line touches the drag curve is called Velocity Maximum Cruise Range (Vmrc). Vmrc gives the minimum fuel consumption between two points, which in turn means that it gives the maximum attainable range for any given fuel load.
But as Genghis has said, the fuel used does not represent the full cost of flying from A to B. The full cost includes Crew salaries, aircraft depreciation/lease costs, ATC charges and many others. Some of these costs are time-related and some are not. Flying faster than Vmrc increases some of these costs and decreases others. So the lowest overall cost is achieved by flying at the speed which gives the best compromise between these different costs. This speed is usually slightly faster than Vmrc, which itself is faster than Mcrit.
why would you want to fly at speeds that are going to increase your drag?
In order to achieve maximum endurance we must fly at the speed at which fuel consumption rate is lowest. For jets this is generally accepted to at or about Vmd. In order to achieve maximum range for a given fuel low we must fly at the speed at which we achieve the maximum ratio of TAS : Fuel Flow. In jet aircraft fuel flow is proportional to thrust and in steady flight thrust is equal to drag. So for minimum fuel consumption between two points (and maximum range) we need to fly at the speed at which we achieve the maximum TAS : Drag ratio.
If we look at a graph of drag against TAS for a typical jet aircraft at subsonic speeds, the curve is a shallow bucket shape with the minimum drag value at Vmd. As speed increases beyond Vmd the drag increases, but the rate of increase is initially very small. This means that at speeds slightly greater than Vmd, the rate of increase in drag is less than the rate of increase in TAS. This means that increasing the TAS within this speed range increases the drag slightly, but actually increases the TAS : Drag ratio. But if we continue to increase our TAS, the drag curve becomes steeper, and a speed is eventually reached where the drag is increasing faster than the increasing TAS.
The Drag Divergence Mach Number (Mdd) is a mach number slightly higher than Mcrit, at which the shock waves become sufficiently widespread and intense to create a large increase in drag. This causes a marked decrease in the TAS : Drag ratio. So at speeds between Mcrit and Mdd increasing airspeed will have a beneficial effect on the TAS : Drag ratio.
To find the speed at which we get the best TAS : Drag ratio we draw a tangent from the origin of the graph to touch the drag curve. The speed at which this line touches the drag curve is called Velocity Maximum Cruise Range (Vmrc). Vmrc gives the minimum fuel consumption between two points, which in turn means that it gives the maximum attainable range for any given fuel load.
But as Genghis has said, the fuel used does not represent the full cost of flying from A to B. The full cost includes Crew salaries, aircraft depreciation/lease costs, ATC charges and many others. Some of these costs are time-related and some are not. Flying faster than Vmrc increases some of these costs and decreases others. So the lowest overall cost is achieved by flying at the speed which gives the best compromise between these different costs. This speed is usually slightly faster than Vmrc, which itself is faster than Mcrit.
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Maybe I didn't explain my self clearly, my question is why is it profitable flying at speeds so high that exceed mcrit increasing drag?
Also about the Mach trimmer I got the reason why is fitted much tuck and all that but in practice how does it actually work?it must be linked to the Asi and activates at a certain speed and it works proportionally to the speed.what is this speed?isnit a fixed value?
About the aircraft/aircrafts thanks for that even my English fiancé has always used aircrafts for the plural so there is always something new to learn
Also about the Mach trimmer I got the reason why is fitted much tuck and all that but in practice how does it actually work?it must be linked to the Asi and activates at a certain speed and it works proportionally to the speed.what is this speed?isnit a fixed value?
About the aircraft/aircrafts thanks for that even my English fiancé has always used aircrafts for the plural so there is always something new to learn
my question is why is it profitable flying at speeds so high that exceed mcrit increasing drag?
As an example let’s assume that we have the following:
Fuel consumption rate at Vmd = 1000 kgs per hour.
TAS at Vmd = 250 knots
Fuel flow at Vmrc = 1200 kgs per hour
TAS at Vmrc = 330 kgs per hour
If we fly 1000 nautical miles form A to B at Vmd we have
Flight time = 1000 Nm / 250 Nm/hr = 4 hours
Fuel consumed = 4 hours at 1000 kgs per hour = 4000 kgs.
If we fly the same 1000 Nm from A to B at Vmrc we have
Flight time = 1000 Nm / 330 NM.hr = 3.03 hours. (To 2 decimal places)
Fuel used = 3.03 hours x 1200 kgs per hour = 3636 kgs
By flying at Vmrc which is 80 knots faster than Vmd, we have used 364 kgs less fuel.
When considering the fuel cost over a given distance the important thing is not the number of kgs of fuel we burn per hour. It is the number of kgs of fuel we burn for each nautical mile. This is why get the maximum range and hence minimum fuel cost if we fly at the speed at which the TAS to Drag ratio is maximum. The fact that this speed is slightly higher than Mcrit is not important, because there is no significant drag rise at Mcrit.
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Thanks a lot keith we actually wrote at the same moment so I didn't read your first post that explain my questions really clearly thanks a lot again u helped a lot.
Thanks genghis for your help even if I don't why you have a bit of an attitude.
Thanks genghis for your help even if I don't why you have a bit of an attitude.
Thanks genghis for your help even if I don't why you have a bit of an attitude.
