max air speed.

[chris.dever]

Flying my MS flight sim B738 CHC=>BNE. Documented Max cruise air speed 320k. Take off and climb all normal. As I approach FL350 I notice that the max air speed decreases. I get OVERSPEED. Moving back to 280k fixes the problem. As I decend from FL350 into BNE, I notice that the Max airspeed (visable on the left hand side of the display) increases. Logic tells me that at FL350 - I should be able to go faster.

My question is - is this a flight sim 'glitch' or is there a reason why at altitude the max speed reduces ?

(yea - I know I'm just a sim jockey :sad: )

PS... I did manage to find BNE!

[BerksFlyer]

It's because it shows Indicated Air Speed (IAS), which is derived from Calibrated Air Speed (CAS) rather than true air speed. So the thinner the air (higher altitude means thinner air) the slower your IAS. IAS isn't that useful if you're thinking of speed as you would if driving a car (ie. ground speed).

That's it in laymen's terms, I'm sure someone else will explain it a lot better.

[SNS3Guppy]

Chris,

Good observation. As you climb, True Airspeed (TAS) increases if indicated airspeed is held constant. By the same token, if one flies true airspeed (which we don't), IAS decreases.

Around 27,000', we stop flying by indicated airspeed and start flying by mach number. This isn't a hard altitude, but generally during the climb there will come a point when we're no longer airspeed limited, but mach limited. The mach number will become the important reference. It's a little easier to visualize on an analog dial airspeed indicator, but there are two upper airspeed limits. One is your max indicated airspeed, and one is max mach. One will be depicted as a red radial line, or other marking, on the airspeed indicator. For airplanes using mach, the mach is a floating pointer generally marked with red stripes, and referred to as the "barber pole."

As one climbs higher and higher, the "barber pole" begins to move down toward a lower speed. The effects of compressibility increase as one climbs higher, for a given airspeed, and the effects of compressibility, or mach effects, become the chief concern. When one first takes off, the airspeed at which the mach limits would occur is much higher than the maximum indicated airspeed, and we pay no attention to the barber pole or mach number. However, as we climb, the airspeed at which the mach limit occurs (easier to think of as the speed of sound decreasing as the air gets thinner and thinner) drops. At first we may have a maximum indicated airspeed of 350 knots, for example, but the mach limiting speed would occur at 400 knots. As we climb, there will come a point as the speed of sound (which is our reference for mach numbers) decreases, we will note that the mach limiting speed is the same as the limiting airspeed...say both now at 350 knots. As we climb higher, we will see the limiting mach number decrease further, and perhaps be 220 knots indicated...that's where the "barber" pole sits. The limiting max airspeed is still 350...but that's of no concern to us because now we're mach limited.

Have you ever gone underwater in a swimming pool and listened to sounds traveling in the water? Sounds are transmitted very well, and move quickly through the water; you can hear the slightest noises from the other side of the pool. The water is more dense. It appears to amplify everything. You try to run, you move slowly. You can feel the water holding you back.

Flying down low is a little like running in the water. You can go to a higher indicated airspeed, but you're fighting thick air. The speed of sound is higher in that thick air. Climb a little, and your indicated airspeed remains the same, but you're really going much faster; true airspeed increases. Somewhat like if the water could magically be thinned to a lesser consistency. As the air thins, the speed of sound decreases; sound doesn't move as fast or as far through the air. Put a block of wood near your ear and tap the other side with a spoon; you hear the sound just fine. Put a pillow by your ear and tap the other side, and you don't hear much at all; the sound doesn't travel through the less dense pillow very well, nor does it travel through less dense air very well. We use the speed of sound as a handy reference number for our mach effects because mach effects are somewhat tied to the same air properties that affect the way sound moves through the atmosphere: density.

For your indicated airspeed, remember that the source for that instrumentation are small tubes, pitot tubes, that sample the air pressure rammed into them as the airplane moves forward through the atmosphere. The faster the airplane goes, the more pressure rammed into the pickup, or pitot tubes, and consequently the higher the indicated airspeed. Imagine if the air were twice as thin (which it is at roughly 18,000')...you'd have to fly much faster to produce the same pressure in the tubes. Which is why to get the same indicated airspeed at high altitudes, the true airspeed is much faster. You're flying faster and faster, but still indicating the same speed. Finally you fly high enough and the air is thin enough that you simply can't pack enough air into those tubes to produce the same indicated airspeeds. You're going fast, but the air is just too thin to register as well.

At this point, because the air is thin, it reacts a little differently to the wing. It's easier to compress or pack together in some areas, such as in front of the wing or nose, and there it forms "shock waves." These shock waves in turn affect the airflow about the wing and the way lift is develop, as well as control features of the airplane. These are mach effects, and the designers of the airplane have determined exactly how far one can go before running into adverse effects. This is the purpose of the barber pole; to keep you away from those effects. It's for this reason that you're mach limited at altitude, rather than airspeed limited.

Indicated airspeed is still valueable, even though we don't use it as our go-fast reference. That is, even though we're watching our mach number for our fast limit, we still closely watch the indicated airspeed for our lower "buffet boundary," or stall. We may be able to get away with a stall at 100 knots at low altitude where the air is thicker and we can make the same lift with a smaller angle between the wing and the wind (angle of attack)...but at altitude, we have to fly faster and perhaps use a larger angle to do the same thing...our stall speed goes up. We may have a stall speed of 160 or 170 knots now. Suppose we're limited to 210 knots due to mach at cruise, and our stall is 170 knots. This means we have a 40 knot window, between 170 and 210 indicated, in which we can operate. The higher we go, the smaller this window gets. Eventually it can get very small if the airplane has enough power to get up that high. That little window is sometimes referred to as "coffin corner," or the place where slowing down any more will cause a low speed stall, and speeding up any more encounters mach effects.

That may be a little more than you wanted, but it's also why as you climb higher and higher the airplane appears to be restricted to slower and slower speeds, and why if you look at an indication of your groundspeed (how fast you're really moving, in relation to the earth) in a no-wind situation...you're really going faster and fast. Clear as mud?

[john_tullamarine]

.. haven't flown the -800 but it sounds like you are missing the significance of Vmo/Mmo limits.

If you are referring to the barberpole indications, the typical picture is either a constant IAS (or a modest increase) from SL to crossover (at lower levels the aircraft is Vmo-limited) above which the Mmo limit takes over and IAS has to reduce with increasing Hp to maintain a constant Mach ...

... and conversely on the descent .. the aircraft can increase IAS steadily (maintaining constant Mmo) until crossover and thereafter follow the Vmo limit for the rest of the descent

If I have missed your point .. please feel free to ignore the post ...

[PK-KAR]

Guppy...good post.If I have any questions about Microsoft Flight Sim, I know there is an expert on here that I can go to.
Ssg v8.0, I thought U're the expert on Microsoft Flight Sim, especially on the aspects of dangers of flex t/o, and your trademark specialty, aborting past V1.... :D

Guppy, you need to get a life and not folow everyone around that doesn't agree with you.
I seem to recall Ssg v7.0 actually following Guppy around. By the way Ssg, after 7 bans, it seems that you're still sticking to your real life specialty, the inability to spell correctly... :D What's "dead reckonking" anyways? :D

[chris.dever]

Thanks Guppy. The answer was much more comprehensive than I expected from a Professional Pilot forum. I now understand. :D

[Ex Cargo Clown]

Chris, easiest way to make it real life and to keep the dreaded "overspeed" coming up is to transition from SPD to MACH in the Fl 200s. Best bet is around Fl 245 !!!!

Oh and Vice-versa !

[FE Hoppy]

Post of the day goes to Guppy.:ok:

That's what this forum is all about.


and a special commendation to ssg who is getting banned faster than i can read his posts!:ugh:

[Turbavykas]

As far as I remember from ATPL theory(I am not a pilot) speed of sound decreases because of temperature. It's like -50c up there. So you cannot fly the same IAS because you will reach the speed of sound. And you cannot fly conventional aircraft faster than the speed of sound...
I am not sure about the density... The density is the amount of molecules and the temperature is the energy:bigger temperature faster movement of molecules. So when you reach the spead of sound you reach the speed of air molecules and molecules cannot move out of your way... So you hit something like concrete wall.
(I think in hard materials sound moves quite different so it's difficult to compare gas and liquid ect...)
from wikipedia:
In fact, assuming an ideal gas (http://en.wikipedia.org/wiki/Ideal_gas), the speed of sound c depends on temperature only, not on the pressure or density (since these change in lockstep for a given temperature and cancel out). Air is almost an ideal gas.

[boardpig]

I'd been mulling this very question over for a time as I didn't fully understand the whole overspeed at high alt thing.
Am I correct then in saying that the red overspeed dial is basically marking the point where undesirable aerodynamic effects begin? I understand why this limit comes down as you climb but now I see its because the air is thinner (doh!). I used to look at it and think, I can fly at 400kts sub 10000 so why am I limited to 320kts at 35000? Obviously I am "going" much faster with respects to ground speed but used to think the overspeed limit was to do with aircraft mass and ground speed (ie structural limit) not air density related.
You learn something new every day eh?

BP

[john_tullamarine]

Not quite .. it is a case that any envelope is the result of overlaying a number of requirements to define the limiting regions of those requirements.

That is, at low level, there are compressible limitations .. but you don't get to see them as the EAS limits inherent in Vmo cut in first. Similarly, the EAS limits still apply at altitude .. but the Mmo problems cut in before Vmo.

[SNS3Guppy]

Obviously I am "going" much faster with respects to ground speed but used to think the overspeed limit was to do with aircraft mass and ground speed (ie structural limit) not air density related.


Ground speed, of course, is entirely irrelevant to the question at hand...except perhaps as a framework reference in still air. It doesn't present any aerodynamic limitation.

[boardpig]

So what begins to happen then as you push through the red dial (apart from the alarms), I mean in an aerodynamic sense?

BP.

[Wizofoz]

Depending on the aircraft and altitude, things like Mach Buffet, mach Tuck or, eventually, bits falling off the aircraft!!

[OutOfRunWay]

Often one of the first indications will the air going supersonic over the cockpit roof, creating a local shock and breaking laminar flow. This can be clearly heard as a pretty loud rumble and felt as a shaking of the aircraft.
If you get local shocks in the vincinity of control surfaces, youre in for a very nasty surprise: this means controls become ineffective, or even that the control effects are reversed!

This does not happen easily on airliners, though, and you would have to be a complete idiot to deliberately push one that far.

Airliners have gone supersonic after failures and survived though, notably a 727 which went into a spin and then a vertical dive, and I believe the China Airlines 747SP which took a spin-dive over the pacific. Both lost bits and pieces, but both managed to land after the gears got extentded by g-forces and slowed the aircraft. Wet pants all 'round.

regards, OORW

[twentysixpointfive]

Now I've got a question!
Why are there two different speed limits? Specifically, Vmo and Mmo. I understand Mmo exists to prevent the effects of compressibility, but why is there a Vmo speed and why does it predominate at lower altitudes?

[john_tullamarine]

Vmo relates to pressure loads on the structure. While nothing is going to happen a modest margin above Vmo, there comes a point where bits can break off .. eg wing LE D-cell skins.

If I recall correctly, the RAAF P3 accident which ended in the lagoon was an undesired consequence of a pass at (an unintentionally) too high an EAS which saw some tin cease to maintain company with the parent structure.

Similarly (and the memory is a bit scratchy here) I recall the early V-tail Bonanzas had a similar problem at lower than expected EAS due to inadequate structure margins in the tail nose ? Hence a mod to strengthen the region after a number of inflight problems.

Vmo is an EAS limit so you would normally expect to see an AFM limitation in IAS (CAS) either simplified to constant, or increasing modestly with altitude.

Mmo is a sloping line from high EAS (at low level) to low EAS (at high level). The two lines cross somewhere on the way up .. typically in the high 20s. At lower levels, you run into the EAS limit first .. at higher levels, the compressibility limit.

[twentysixpointfive]

john - thanks for the response!
On a typical twin turbine like a king air/metro/1900, if you see a barberpole and it decreases in value as you climb, why is that the case? Is this a Vmo or an Mmo limit in this case? If its Vmo, and caused by pressure forces on the aircraft, shouldn't it go DOWN with altitude since there is less air impinging on the a/c structure?

Aerodynamics 101...thanks! :ok:

[Dream Land]

Every aircraft has two basic Speed limits, Vmo and Mmo.

(1) Vmo is a structural limit, defined as an Equivalent Airspeed (EAS) which is a direct measurement of the dynamic pressure of the air stream. Aircraft do not have EAS indicators (they should have), and, instead, we refer to CAS indication, which aircraft do have. For a given EAS, CAS increases with increasing Pressure Height, thus, Willy Miller, from the information you've given us, the EAS limit for your aircraft is 284 KEAS.

(2) Mmo is an aerodynamic limit. It is a Mach Number beyond which control problems, and severe loss of thrust from propellers occurs. It has NOTHING to do with the aerodynamic load on your aircraft, but may contribute to undesirable propeller vibration and flexing. High speed propeller aircraft usually do not have Mach Meters, they should have, and the Mmo limit is usually reflected as a decreasing Vmo with increasing Pressure Height.

Mmo is determined at a Mach Number somewhat beyond Mcrit (NOT at Mcrit), where the control problems and/or loss of propeller thrust due to Mcrit exceedance is acceptable. Thus, it is related to Mcrit.

Every component of the aircraft has it's own Mcrit, wings, propellers etc., and Mcrit is usually found at the fastest moving part of the aircraft. For the propeller aircraft, the fastest moving part is the propeller tips, their speed being the VECTOR SUM of their own speed due to rotation and the aircraft forward speed (TAS).

From the information you've given -
I grabbed this snippet from PPRuNe some time ago, I believe from Old Smokey, but not sure and apologize to the author in advance.

Cheers, D.L.

[john_tullamarine]

26.5 ....

if you see a barberpole and it decreases in value as you climb

what you will see is one of

(a) IAS constant during the climb from ground level. This reflects a simplification of the real limit. During the climb to crossover height, the Mmo limit starts somewhat higher (so it is "hidden" by the Vmo limit) and progressively reduces. At crossover, the limits reverse and Mmo becomes less than Vmo .. so the barberpole starts reducing at constant Mach.

(b) IAS slowly increasing during the initial climb to crossover. This reflects a constant EAS .. the rest as for (a)

If its Vmo, and caused by pressure forces on the aircraft, shouldn't it go DOWN with altitude since there is less air impinging on the a/c structure?

.. the problem with flying at too high an EAS is that the aerodynamic loads imposed on the aircraft's surfaces (and underlying structure) by the airflow can become too high and things can break. A bit like driving in your car with an arm out the window .. the loads on your hand/arm increase significantly as your roadspeed (EAS/IAS for an aircraft) increases .. then again if you fly Tigers you will understand exactly what I am talking about here .. One of the concerns is the pressures exerted on skins as the air runs past ... eventually the skin either gets pushed into the structure voids or, more likely, gets pulled off the attaching rivets ..

Your concern about altitude is tied up with a constant EAS resulting in an increasing TAS ... it does get a tad confusing until you are able to put all the different speeds in their right places ...

Dream Land

The barberpole is a combined Mmo/Vmo gauge so whichever limit is limiting is reflected in whatever the needle is doing.

[boardpig]

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.

[john_tullamarine]

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.

[twentysixpointfive]

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? :cool:

[NinerVictor]

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.

[Mad (Flt) Scientist]

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...)

[TyroPicard]

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

[john_tullamarine]

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".

[chris.dever]

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 !

[twentysixpointfive]

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!

[Check Airman]

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!

[Captain_djaffar]

nice instructions guppy:ok:

[john_tullamarine]

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