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#21
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"Jon Meyer" wrote in message ... | Ok, | | Firstly VNE is very much limited by TAS not IAS. If | you understand the maths behind how an ASI works then | you will know that the ASI indicates a TAS based on | an assumed air density equivalent to sea level. TAS | is the speed that affects flutter, IAS is simply a | gash estimate of TAS which is reasonable at low altitudes. | Hence the rules of thumb for safe flight at altitude. | | Secondly, ground speed at altitude is much higher than | TAS due to lower air density and can be greatly affected | by very high velocity winds at altitude. This is how | high average speeds can be achieved without exceeding | TAS VNE. Try looking at your GPS groundspeed when at | high altitude, it should be much higher than your IAS | even when the wind effect is removed. I am confused ! How can the ground speed exceed TAS when measured in still air ? TAS means TRUE Air Speed. i.e. the actual speed of the aircraft through the particular parcel of air around the aircraft. If this parcel of air is stationary with respect to the ground, the ground speed of the aircraft must equal the TAS. Cheers, John G. |
#22
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Arnold Pieper wrote:
To make a long story short : -Look at the ASI on any turboprop or even some turbocharged aircraft, and you will see that they fly at TAS much faster than the VNE painted on their ASI. Example : Twin Commander, VNE=255Kt (it's on it's ASI), Cruise TAS = 280Kt to 300Kt. The same holds true for all airplanes that fly up to the 20s and higher, which aviation has been doing since WWII. Look at the manual of some gliders, that do have the limitation on IAS with Altitude. You will see that the table of IAS is not correcting the VNE as a TAS value. Nowhere in aviation VNE is considered a TAS value. Quoting from the manual for my Schleicher ASH 26 E: "4.5.8 High altitude flight Flutter tests were carried out at about 2000 m msl. As the ASI under-reads at increasing altitude, but since flutter limits for light aircraft are determined by the true air speed, the following limitations apply to high altitude flights:" [table of IAS at Vne for altitudes to 13000m follows] -- ----- Replace "SPAM" with "charter" to email me directly Eric Greenwell Washington State USA |
#23
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Jon,
I don't like to come accross as arrogant, I'm sorry if I do, but you seem to be confusing the issue. IAS is what you see in your ASI. TAS is the speed at which the aircraft is is moving trough the air (nothing to do with groundspeed). To obtain you TAS at any time during flight, you look at what your ASI is indicating, then apply a correction based on altitude and oustide air temperature. You can do that with an E-6B type computer. If you don't have an OAT reading, you're out of luck, but the well-known 2% per 1000ft will give you an approximation. Groundspeed, on the other hand is the speed at which you move in relation to the ground. The Groundspeed is the result of your TAS plus (or minus) the wind component. When I say most Turboprops and Turbocharged airplanes fly at faster TAS than their IAS VNEs, that's exactly what I mean : At altitudes of 20k plus, their TAS will be higher than their indicated VNE, regardless of what the Groundspeed is. The Twin commander will be reading Groundspeeds of 350Kt when eastbound (tailwind) and it may read as little as 230Kt westbound. But in both cases, with the same power setting, it will be doing 290Kt TAS, which is more than the 255Kt VNE. Meanwhile, their ASI will be showing 180Kt to 190Kt, depending on altitude and powersetting, well below the VNE of 255Kt. Same thing holds true for all these airplanes. Other examples : TBM-700, cruises at 290Kt TAS (not groundspeed), while the VNE is 265Kt. King Air 350, cruises at 300Kt TAS, VNE is 265Kt. "Jon Meyer" wrote in message ... Ok, Firstly VNE is very much limited by TAS not IAS. If you understand the maths behind how an ASI works then you will know that the ASI indicates a TAS based on an assumed air density equivalent to sea level. TAS is the speed that affects flutter, IAS is simply a gash estimate of TAS which is reasonable at low altitudes. Hence the rules of thumb for safe flight at altitude. Secondly, ground speed at altitude is much higher than TAS due to lower air density and can be greatly affected by very high velocity winds at altitude. This is how high average speeds can be achieved without exceeding TAS VNE. Try looking at your GPS groundspeed when at high altitude, it should be much higher than your IAS even when the wind effect is removed. |
#24
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Thank you Simon,
Exaclty like I said in a previous message : The table below confirms it, It is NOT simply the VNE corrected as TAS. Go ahead and check this table if you will : 100Kt IAS @ 26000ft is LEGAL, ALLOWED and SAFE as per the manual. That's because table shows 111Kt IAS as being the limit at this altitude. Now calculate your TAS for this condition (100Kt IAS @ 26000ft), with various range of temperatures. Here's what you will get as TAS for the various temperatures : 163Kt @ -45 Celsius (way below Standard), 167Kt @ -35 Celsius (aprox. std temperature for this altitude) 170Kt @ -25 Celsius (above Standard) All of these TAS are higher than the original Low-altitude IAS VNE of 146Kt, painted on the ASI. Which confirms what I said : the VNE is NOT simply to be corrected as a TAS figure. This limitation in IAS with Altitudes are "NEW" values for VNE, due to flutter or whatever the reason with altitude. But the resulting TAS at these conditions will be higher than the original VNE. "Simon Kahn" wrote in message ... From DG website DG1000 flight manual: http://www.dg-flugzeugbau.de/index-e.html ========== Warning: At higher altitudes the true airspeed is higher than the indicated airspeed, so VNE is reduced with altitude according to the table below, see also section 4.5.5. Altitude in [m] 0-3000 4000 5000 6000 7000 8000 VNE indicated km/h 270 256 243 230 217 205 Altitude in [ft] 0-10000 13000 16000 20000 23000 26000 VNE indicated kts. 146 138 131 124 117 111 ------- Simon. 19:54 09 December 2003, Arnold Pieper wrote: This seems to me to be some arbitrary way of staying conservative. Look at the manual of some gliders, that do have the limitation on IAS with Altitude. You will see that the table of IAS is not correcting the VNE as a TAS value. Nowhere in aviation VNE is considered a TAS value. |
#25
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Arnold Pieper wrote:
Thank you Simon, Exaclty like I said in a previous message : The table below confirms it, It is NOT simply the VNE corrected as TAS. Go ahead and check this table if you will : 100Kt IAS @ 26000ft is LEGAL, ALLOWED and SAFE as per the manual. That's because table shows 111Kt IAS as being the limit at this altitude. Now calculate your TAS for this condition (100Kt IAS @ 26000ft), with various range of temperatures. Here's what you will get as TAS for the various temperatures : 163Kt @ -45 Celsius (way below Standard), 167Kt @ -35 Celsius (aprox. std temperature for this altitude) 170Kt @ -25 Celsius (above Standard) All of these TAS are higher than the original Low-altitude IAS VNE of 146Kt, painted on the ASI. Which confirms what I said : the VNE is NOT simply to be corrected as a TAS figure. This limitation in IAS with Altitudes are "NEW" values for VNE, due to flutter or whatever the reason with altitude. But the resulting TAS at these conditions will be higher than the original VNE. Perhaps you missed this post: Quoting from the manual for my Schleicher ASH 26 E: "4.5.8 High altitude flight Flutter tests were carried out at about 2000 m msl. As the ASI under-reads at increasing altitude, but since flutter limits for light aircraft are determined by the true air speed, the following limitations apply to high altitude flights: [table of IAS at Vne for altitudes to 13000m follows] If above airspeed limits given as IAS are regarded the true air speed above 3000 m altitude will remain constant at 300 km/h = 162 knots." So for this glider, the Vne in TAS does increase to 10,000', then remains at a constant 162 knots TAS. They are quite explicit about the Vne depending on TAS. -- ----- change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#26
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Eric,
Thanks, I see how explicit they are about it. DG chose to not get into specifics, because the story is a bit more complex than that. The VNE of your glider is 146Kt (IAS), but above 10000' you should start observing other IAS limits, that translate into 162Kt TAS due to flutter considerations. Therefore, simply using 146Kt as a TAS value is not correct, which is what was being suggested earlier, and is what I had trouble with. Nowhere in Aeronautic literature is VNE defined as a TAS value, it HAS to be presented to the pilot as Indicated. In high-performance aircraft where flutter is less of a consideration, there is no such table for high altitude, VNE is always VNE, until MMo becomes a factor. "Eric Greenwell" wrote in message ... Arnold Pieper wrote: Thank you Simon, Exaclty like I said in a previous message : The table below confirms it, It is NOT simply the VNE corrected as TAS. Go ahead and check this table if you will : 100Kt IAS @ 26000ft is LEGAL, ALLOWED and SAFE as per the manual. That's because table shows 111Kt IAS as being the limit at this altitude. Now calculate your TAS for this condition (100Kt IAS @ 26000ft), with various range of temperatures. Here's what you will get as TAS for the various temperatures : 163Kt @ -45 Celsius (way below Standard), 167Kt @ -35 Celsius (aprox. std temperature for this altitude) 170Kt @ -25 Celsius (above Standard) All of these TAS are higher than the original Low-altitude IAS VNE of 146Kt, painted on the ASI. Which confirms what I said : the VNE is NOT simply to be corrected as a TAS figure. This limitation in IAS with Altitudes are "NEW" values for VNE, due to flutter or whatever the reason with altitude. But the resulting TAS at these conditions will be higher than the original VNE. Perhaps you missed this post: Quoting from the manual for my Schleicher ASH 26 E: "4.5.8 High altitude flight Flutter tests were carried out at about 2000 m msl. As the ASI under-reads at increasing altitude, but since flutter limits for light aircraft are determined by the true air speed, the following limitations apply to high altitude flights: [table of IAS at Vne for altitudes to 13000m follows] If above airspeed limits given as IAS are regarded the true air speed above 3000 m altitude will remain constant at 300 km/h = 162 knots." So for this glider, the Vne in TAS does increase to 10,000', then remains at a constant 162 knots TAS. They are quite explicit about the Vne depending on TAS. -- ----- change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#27
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"Arnold Pieper" wrote in message om... | Eric, | | Thanks, I see how explicit they are about it. | DG chose to not get into specifics, because the story is a bit more complex | than that. | | The VNE of your glider is 146Kt (IAS), but above 10000' you should start | observing other IAS limits, that translate into 162Kt TAS due to flutter | considerations. | | Therefore, simply using 146Kt as a TAS value is not correct, which is what | was being suggested earlier, and is what I had trouble with. | | Nowhere in Aeronautic literature is VNE defined as a TAS value, it HAS to be | presented to the pilot as Indicated. | | In high-performance aircraft where flutter is less of a consideration, there | is no such table for high altitude, VNE is always VNE, until MMo becomes a | factor. I think the statement that you should use TAS as the limit for Vne rather than IAS, comes from earlier times, when less was known about instabilities such as flutter. I have heard it said that something between IAS and TAS is what affects flutter, but the percentage will change from one aircraft to another, so it is safe to use TAS as a Vne limit. No one said it was an exact statement, but it is known to be *safe*. TAS is calculated by power pilots as a matter of course for flight planning, so the conversion is well known and easily available on the various flight computers ("prayer wheels" and such) It is sometimes mandated by local Authorities that a table based on manufacturer's data or TAS is provided. The Gliding Federation of Australia has such a stipulation, tied to the installation of oxygen in a glider. Part of the installation is a placard to be fixed in the cockpit which shows the table of Indicated Vne versus altitude, derived either from the manufacturer's table or calculated using TAS conversion for a standard atmosphere (pressure and temperature with height) Whether either table is "right" is immaterial, as long as the table gives the pilot *safe* information. Cheers, John G. |
#28
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Sorry,
Misprint, The Groundspeed at altitude will be equal to TAS and therefore much higher than the IAS (not TAS as I earlier stated). That is why your groundspeed will be higher than IAS. This was simply meant in answer to the confusion of how someone could achieve an average speed of 250+kph at 20000ft nwithout exceeding VNE at altitude. I am not confused about the issues concerning flutter. Flutter is dependant on TAS. IAS is an arbitrary value based on sea level air density. I still believe that the reason you are confused is that the VNE of the 'high performance' aircraft you describe is specified as IAS at cruising altitude - not at sea level. Therefore your calculation of TAS being higher than VNE is flawed because you have taken the wrong air density as your datum. At 20:12 09 December 2003, Jon Meyer wrote: Ok, Firstly VNE is very much limited by TAS not IAS. If you understand the maths behind how an ASI works then you will know that the ASI indicates a TAS based on an assumed air density equivalent to sea level. TAS is the speed that affects flutter, IAS is simply a gash estimate of TAS which is reasonable at low altitudes. Hence the rules of thumb for safe flight at altitude. Secondly, ground speed at altitude is much higher than TAS due to lower air density and can be greatly affected by very high velocity winds at altitude. This is how high average speeds can be achieved without exceeding TAS VNE. Try looking at your GPS groundspeed when at high altitude, it should be much higher than your IAS even when the wind effect is removed. |
#29
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Jon Meyer wrote:
Secondly, ground speed at altitude is much higher than TAS due to lower air density ^^^^^ ^^^ ^^^^^^^ Are you joking ? -- Denis Private replies: remove "moncourrielest" from my e-mail address Pour me répondre utiliser l'adresse courriel figurant après moncourrielest" dans mon adresse courriel... |
#30
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Jon Meyer wrote:
Sorry, I am not confused about the issues concerning flutter. Flutter is dependant on TAS. IAS is an arbitrary value based on sea level air density. I still believe that the reason you are confused is that the VNE of the 'high performance' aircraft you describe is specified as IAS at cruising altitude - not at sea level. Therefore your calculation of TAS being higher than VNE is flawed because you have taken the wrong air density as your datum. So Vne of power planes is a figure which describes Vne at the highest cruising altitude? So this means that actual Vne at a lower altitude may be faster? Hmmm...this seems to make sense for power planes... Unfortunately gliders don't have an altitude limited by power. So this is much stickier. I noticed the PW-5 initially had a 15,000 foot altitude limitation (1998?) and there were some vigorous complaints. Then apparently the limitation was removed (don't know exactly why). Perhaps test pilots determined the "flutter" was caused by gaps in the elevator connection and not actual "flutter." But what this seems to suggest is that a manual which does not specifically address Vne and altitude should be viewed with skepticism. A grob 102 at 49,000 feet pushed to Vne makes you a test pilot... Excellent discussion! Fantastic fountain of (sometimes varied) opinions and advice. It's also nice to see the thoughts converging, and I can visualize a lot of readers shuffling through arcane texts asking "is that really true?" Thanks for your posts... |
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