mach number and weight
Originally Posted by HN39
You should read more carefully. I wrote:
Quote:
The point at M=0.5 and AoA=4.5 corresponds to (...) mass/delta=392 tonnes
Quote:
The point at M=0.5 and AoA=4.5 corresponds to (...) mass/delta=392 tonnes
Could you address my comment about the drag change from 0.2 to 0.8?
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Capn Bloggs;
The chart is not for a particular altitude. It covers all weights and altitudes.
"delta" stands for the ambient pressure at altitude expressed as a ratio to sealevel standard pressure, i.e delta=Pamb/Psls.
P.S.
If you plotted a family of curves on the graph, each curve showing for a particular weight and altitude the variation of L/D versus Mach, then each curve would have a maximum L/D at a particular Mach. The L/D curve shown on my chart connects all these maxima, i.e. all curves for any weight or altitude would stay below the L/D curve on the chart.
The chart is not for a particular altitude. It covers all weights and altitudes.
"delta" stands for the ambient pressure at altitude expressed as a ratio to sealevel standard pressure, i.e delta=Pamb/Psls.
P.S.
The more I look at it, the less that chart makes sense.
Last edited by HazelNuts39; 15th May 2012 at 16:06. Reason: P.S.
Hawk37,
Point taken. Will not use L/D in jet discussion any more! 
Nevertheless, the answer to Capt NK Airbus' question in post 8:
is related, is it not? As the weight reduces, the lift required reduces and so the drag for any particular speed also reduces (given we're on the front side of the drag curve) and we would normally accelerate. Therefore, we can reduce the thrust to match that reduced drag. The issue is whether the speed value for max range reduces as weight reduces. Airbus says it does, in that earlier reference I gave.
Where does the curve go as weight reduces? That is the crux of the issue (with thanks to DP Davies
) because that will determine where the tangent line is, speedwise.
Do you have a reference for the AoA not being constant at the Max Range Speed for various weights? I was always under the impression that Max Range speed at any weight was at the same AoA.
Found a Boeing document that talks about the subject:
Aero 12 - Angle of Attack
What I don't get is that "It can be seen that the optimal long-range cruise Mach number does not vary significantly as gross weight (hence, lift and AOA) changes." seems to be counter to those Airbus figures which show a max-range speed change of .67 to .73 over a 40t weight change. I would have thought that that was significant (although a 40 tonne burn is almost more than my aeroplane weighs!).
Originally Posted by Hawk37
It is dangerous to use the terms "max range" and "best L/D" in the same sentence. A jet at an IAS for max range is not at the speed for best L/D.
Nevertheless, the answer to Capt NK Airbus' question in post 8:
Why does Indicated Mach No. reduce if weight reduces, even if Flight Level is the same. I tried hard to find out the reason. No good.
Where does the curve go as weight reduces? That is the crux of the issue (with thanks to DP Davies
) because that will determine where the tangent line is, speedwise.Do you have a reference for the AoA not being constant at the Max Range Speed for various weights? I was always under the impression that Max Range speed at any weight was at the same AoA.
Found a Boeing document that talks about the subject:
Aero 12 - Angle of Attack
What I don't get is that "It can be seen that the optimal long-range cruise Mach number does not vary significantly as gross weight (hence, lift and AOA) changes." seems to be counter to those Airbus figures which show a max-range speed change of .67 to .73 over a 40t weight change. I would have thought that that was significant (although a 40 tonne burn is almost more than my aeroplane weighs!).
Last edited by Capn Bloggs; 15th May 2012 at 15:22.
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Bloggs, I agree with what you have just posted.
You ask "Do you have a reference for the AoA not being constant at the Max Range Speed for various weights?"
No I don't, probably because no one has made an technical argument that it should be, for a jet. However, figure 7 from the boeing doc you linked to appears to suggest that the heavier situation is indeed at a higher aoa, for max range, although one has to extrapolate a bit. Additionally, the boeing doc makes no reference at all to a jet's max range speed being at constant aoa. Now, having said that, for a piston aircraft that may be true, though I'd have to brush up on my aerodynamics before I made that assertion.
You say you don't get that "optimal long-range cruise Mach number does not vary significantly as gross weight (hence, lift and AOA) changes"
I have to agree with you on that. I consider it significant, everyone has their opinion on what constitutes "significant", I guess.
You ask "Do you have a reference for the AoA not being constant at the Max Range Speed for various weights?"
No I don't, probably because no one has made an technical argument that it should be, for a jet. However, figure 7 from the boeing doc you linked to appears to suggest that the heavier situation is indeed at a higher aoa, for max range, although one has to extrapolate a bit. Additionally, the boeing doc makes no reference at all to a jet's max range speed being at constant aoa. Now, having said that, for a piston aircraft that may be true, though I'd have to brush up on my aerodynamics before I made that assertion.
You say you don't get that "optimal long-range cruise Mach number does not vary significantly as gross weight (hence, lift and AOA) changes"
I have to agree with you on that. I consider it significant, everyone has their opinion on what constitutes "significant", I guess.
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I see on your question in question, that this is a teaser question ( I clicked on the Link) there is also a quick solution to the answer (which I make to be between the last option and the second to last multi-choice thang. I have got a horrible feeling that the . . .establishment in question has a nice juicy formula for calculating the FL under the given conditions of weight . . . I mean, do look at what all the other guys have said/suggested - and, while I am here thank you guy(s) for your input on my little bit of . . . input before, also.
Referring to the formula - for example, there was a formula for the A330 which I have since forgotten - hopefully an A330 Captain will chime in but it was an approximate referring to a specific type something like for optimum level take 256 (hell, it could have been 567, I forgot . . !) . so take 256-GW=Opt FL. So, therefore; 256-230=26
Using the other figure of 567 gives another flight level. 560 - 230 = 330, yup, that looks like it.
So, I think maybe they (the advert) are just trying to get you hooked into joining the course.
Referring to the formula - for example, there was a formula for the A330 which I have since forgotten - hopefully an A330 Captain will chime in but it was an approximate referring to a specific type something like for optimum level take 256 (hell, it could have been 567, I forgot . . !) . so take 256-GW=Opt FL. So, therefore; 256-230=26
Using the other figure of 567 gives another flight level. 560 - 230 = 330, yup, that looks like it.
So, I think maybe they (the advert) are just trying to get you hooked into joining the course.
Last edited by Natstrackalpha; 16th May 2012 at 16:13.
