max air speed.
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So...
In otherwords, when you stick you arm out of the window and the forces get higher, you could (theoretically, and if the car could fly) increase your altitude and the forces would reduce (assuming constant speed, not IAS). Therefore as you went higher, you could go faster and keep your arm out of the window as the air pressure reduced. (read no bits falling off). Eventually new contraints would begin to take place as you got faster and higher, these would be related to compressability (super sonic air related) rather than air getting out of the way?? Here then, your arm would still get the bad end of the deal because of the pressure effects of air becoming super sonic?
Surley though the air is still passing around and over you at 320kts (as indictaed at the barber pole, not super sonic) even though the mach reading is .99999 etc
Sorry, still a bit confused.
Surley though the air is still passing around and over you at 320kts (as indictaed at the barber pole, not super sonic) even though the mach reading is .99999 etc
Sorry, still a bit confused.
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assuming constant speed, not IAS
which sort of speed are you keeping constant, here ? If EAS, then the loads would stay the same, if IAS (or CAS - near enough) they would reduce a tad, TAS and groundspeed are not really relevant ...
you could go faster and keep your arm out of the window as the air pressure reduced
that all depends on what you are doing with your EAS
super sonic air related
not quite ... compressibility effects will be measurable well before you get to sonic velocities.
It's no different to most aircraft limitations ... the pilot sees the limiting case (of a number of various limits) either in the POH/AFM or on his/her gauges. With the barberpole, the Vmo limit is limiting (ie Vmo<Mmo, measured as IAS) at lower levels while Mmo becomes limiting (ie Mmo<Vmo, measured as IAS) at higher levels.
the air is still passing around and over you at 320kts ... even though the mach reading is .99999
the speeds are related by the various equations (you probably don't want to go there). Typically, the Mmo might be mid- to high-0.8s (doesn't really matter .. it is a Type-specific thing). You would not see an Mmo near sonic velocity as the general idea is to operate reasonably subsonic (say 0.9) or supersonic (say 1.2). Sustained operation in the transonic region is not a useful idea as the drag problems don't offer any advantage.
which sort of speed are you keeping constant, here ? If EAS, then the loads would stay the same, if IAS (or CAS - near enough) they would reduce a tad, TAS and groundspeed are not really relevant ...
you could go faster and keep your arm out of the window as the air pressure reduced
that all depends on what you are doing with your EAS
super sonic air related
not quite ... compressibility effects will be measurable well before you get to sonic velocities.
It's no different to most aircraft limitations ... the pilot sees the limiting case (of a number of various limits) either in the POH/AFM or on his/her gauges. With the barberpole, the Vmo limit is limiting (ie Vmo<Mmo, measured as IAS) at lower levels while Mmo becomes limiting (ie Mmo<Vmo, measured as IAS) at higher levels.
the air is still passing around and over you at 320kts ... even though the mach reading is .99999
the speeds are related by the various equations (you probably don't want to go there). Typically, the Mmo might be mid- to high-0.8s (doesn't really matter .. it is a Type-specific thing). You would not see an Mmo near sonic velocity as the general idea is to operate reasonably subsonic (say 0.9) or supersonic (say 1.2). Sustained operation in the transonic region is not a useful idea as the drag problems don't offer any advantage.
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john -
Thanks for all the help with this. Forgive me if this is frustration, but -
Lets say the barberpole on a turboprop stays at 260knots until about 15-20,000 feet and then starts decreasing by about 2% for every 1000 feet. Why is there this sudden reduction in speed at this altitude?
Thanks for all the help with this. Forgive me if this is frustration, but -
Lets say the barberpole on a turboprop stays at 260knots until about 15-20,000 feet and then starts decreasing by about 2% for every 1000 feet. Why is there this sudden reduction in speed at this altitude?
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26.5,
Just throwing in a formula in here. Maybe it might help.
IMN = TAS/LSS
IMN = Indicated Mach No.
TAS = True Airspeed
LSS = Local Speed of Sound
LSS is a function of temperature. The lower the temperature (higher altitudes), the lower the LSS. We know that if we climb at constant IAS, the TAS will start increasing. Using the formula above, we can see that to maintain a constant IMN, TAS has to decrease as we climb higher (because LSS decreases with altitude). As a result, IAS decreases as we climb at constant IMN. If you imagine this IMN to be your barber pole (Mmo) and it is being communicated to you on your airspeed indicator (which displays IAS), you will find the barber pole decreasing to lower IAS as you climb. And as a result, you will have to reduce your IAS as you climb (at constant IMN) because the barber pole gets lower along the way.
As JT mentioned, at lower altitudes Vmo is the limiting speed and at a 'certain altitude' upwards, Mmo is the limiting one. That is why we climb at a constant IAS until that 'certain altitude' after which we climb at constant IMN.
Now if you imagine that 'certain altitude' as 15,000' to 20,000' in the example you gave, you can see why the barber pole starts decreasing. That is a pretty low Mmo I think. But then again it's a prop. Hope I have made the picture clearer for you.
9V.
Just throwing in a formula in here. Maybe it might help.
IMN = TAS/LSS
IMN = Indicated Mach No.
TAS = True Airspeed
LSS = Local Speed of Sound
LSS is a function of temperature. The lower the temperature (higher altitudes), the lower the LSS. We know that if we climb at constant IAS, the TAS will start increasing. Using the formula above, we can see that to maintain a constant IMN, TAS has to decrease as we climb higher (because LSS decreases with altitude). As a result, IAS decreases as we climb at constant IMN. If you imagine this IMN to be your barber pole (Mmo) and it is being communicated to you on your airspeed indicator (which displays IAS), you will find the barber pole decreasing to lower IAS as you climb. And as a result, you will have to reduce your IAS as you climb (at constant IMN) because the barber pole gets lower along the way.
As JT mentioned, at lower altitudes Vmo is the limiting speed and at a 'certain altitude' upwards, Mmo is the limiting one. That is why we climb at a constant IAS until that 'certain altitude' after which we climb at constant IMN.
Now if you imagine that 'certain altitude' as 15,000' to 20,000' in the example you gave, you can see why the barber pole starts decreasing. That is a pretty low Mmo I think. But then again it's a prop. Hope I have made the picture clearer for you.
9V.
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260 knots CAS (more or less the same as IAS) is just under M0.4 at sea level.
260knots CAS at 18,000ft, say, is about M0.55
225 knots CAS at 25,000ft is also about M0.55
So if you had two sets of limits - one that said "thou shalt not exceed 260 knots CAS", another that said "thou shalt not exceed M0.55", then up to 18,000ft the first one would apply, and would 'mask' the M0.55 limit, but above 18,000ft you'd hit the M0.55 limit first. So as you get higher than that, you have to keep cutting back your IAS (CAS) to respect the Mach number limit.
As to why there's a M0.55 limit (in this case) - a common reason for a Mach number limit on turboprops is to protect against sonic tip speeds; if you do the vector addition of the tip speed at max rpm with the forward speed you'll get a pretty high subsonic speed. (There's been discussion elsewhere on the specific limits for specific turboprops...)
260knots CAS at 18,000ft, say, is about M0.55
225 knots CAS at 25,000ft is also about M0.55
So if you had two sets of limits - one that said "thou shalt not exceed 260 knots CAS", another that said "thou shalt not exceed M0.55", then up to 18,000ft the first one would apply, and would 'mask' the M0.55 limit, but above 18,000ft you'd hit the M0.55 limit first. So as you get higher than that, you have to keep cutting back your IAS (CAS) to respect the Mach number limit.
As to why there's a M0.55 limit (in this case) - a common reason for a Mach number limit on turboprops is to protect against sonic tip speeds; if you do the vector addition of the tip speed at max rpm with the forward speed you'll get a pretty high subsonic speed. (There's been discussion elsewhere on the specific limits for specific turboprops...)
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Golly, haven't we professionals made this complicated!
chris......
Pilots fly IAS (Indicated Air Speed). This is not the True Air Speed (TAS) which you will see displayed somewhere - TAS varies with height (air density) for a constant IAS (higher altitude = faster TAS).
Mach number, also displayed somewhere (e.g M0.76) depends on TAS, not IAS. As you climb at 320 kt. IAS your TAS increases, and so does your Mach number.
Aircraft are limited by IAS and Mach number, to avoid a) falling apart, and b) unwanted aerodynamic events leading to falling apart. The manufacturer helps us to stay within limits by displaying the lower of the two.
What you have observed is the Mach limit taking over from the IAS limit, due to a/c altitude.
Suggest you find the TAS and M indications next time and observe what happens as you climb/descend.
TP
chris......
Pilots fly IAS (Indicated Air Speed). This is not the True Air Speed (TAS) which you will see displayed somewhere - TAS varies with height (air density) for a constant IAS (higher altitude = faster TAS).
Mach number, also displayed somewhere (e.g M0.76) depends on TAS, not IAS. As you climb at 320 kt. IAS your TAS increases, and so does your Mach number.
Aircraft are limited by IAS and Mach number, to avoid a) falling apart, and b) unwanted aerodynamic events leading to falling apart. The manufacturer helps us to stay within limits by displaying the lower of the two.
What you have observed is the Mach limit taking over from the IAS limit, due to a/c altitude.
Suggest you find the TAS and M indications next time and observe what happens as you climb/descend.
TP
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MFS gives the story very succinctly (as one would expect) and the numbers are typical for turboprops ...
As a "for instance", if I look up, say, a KingAir 350 POH which happens to be on my desk just now, the relevant limits are
Vmo = 260 KCAS to FL210, then reducing to maintain
Mmo = 0.58 up to the max level (FL350)
The POH even belabours the point by stating that the "red pointer reflects Vmo/Mmo limits".
As a "for instance", if I look up, say, a KingAir 350 POH which happens to be on my desk just now, the relevant limits are
Vmo = 260 KCAS to FL210, then reducing to maintain
Mmo = 0.58 up to the max level (FL350)
The POH even belabours the point by stating that the "red pointer reflects Vmo/Mmo limits".
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Whew.......
I'm just pleased that my question was not so dumb afterall !
I've read every word and thank all for their input..
Now I just have to get thinking about the 1000 other real world things that baffle me on my FlightSim !
I've read every word and thank all for their input..
Now I just have to get thinking about the 1000 other real world things that baffle me on my FlightSim !
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Alright thanks for everyone's posts!
I did not realize that the barberpole decreasing with an increase in altitude was in fact a mach limit on a turboprop, since I figured most of the time it would be no faster than .5-.6M (slow), but I did not consider that it could be because of the props going sonic!
Problem solved! Thanks!
I did not realize that the barberpole decreasing with an increase in altitude was in fact a mach limit on a turboprop, since I figured most of the time it would be no faster than .5-.6M (slow), but I did not consider that it could be because of the props going sonic!
Problem solved! Thanks!
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It's always annoying when simmers come here and post with (what seems to be) the express intention of stirring the pot. This thread has demonstrated what I've always felt. Armchair pilots are welcome here, as long as they identify themselves and not try to impress anybody by building a fake character.
Bravo Zulu chris.dever!
Bravo Zulu chris.dever!
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A restatement of views ..
(a) the site was set up with the intention that it would be a valued resource for professional pilots
(b) for Tech Log, the emphasis is on technical matters
(c) so long as the discussions bear some relationship to (a) and (b) ... and, at times, that can get a bit tenuous ... they can be introduced, and contributions may be made, by folk from anywhere. Pilots have considerable knowledge in their field .. but are not necessarily the sole source of wisdom in the aviation field.
(d) keep in mind that we have a quite significant proportion of Industry folk here who are not professional pilots .. many don't even hold licences ... but their contributions are of great technical value to the professional piloting community
(e) some folk have a dislike of sorts for the flight simmers .. often that is quite inappropriate ... provided that the simmers offer useful contributions, those contributions, and the folk involved, are valued
(a) the site was set up with the intention that it would be a valued resource for professional pilots
(b) for Tech Log, the emphasis is on technical matters
(c) so long as the discussions bear some relationship to (a) and (b) ... and, at times, that can get a bit tenuous ... they can be introduced, and contributions may be made, by folk from anywhere. Pilots have considerable knowledge in their field .. but are not necessarily the sole source of wisdom in the aviation field.
(d) keep in mind that we have a quite significant proportion of Industry folk here who are not professional pilots .. many don't even hold licences ... but their contributions are of great technical value to the professional piloting community
(e) some folk have a dislike of sorts for the flight simmers .. often that is quite inappropriate ... provided that the simmers offer useful contributions, those contributions, and the folk involved, are valued
