mach number and weight
Thread Starter
Join Date: Jul 2008
Location: New Delhi
Posts: 43
Likes: 0
Received 0 Likes
on
0 Posts
mach number and weight
At an aircraft weight of 70,000lb your aerodynamic ceiling in 1g level flight will be:
A) FL440
B) FL390
C) FL320
D) FL420
ans is fl 420 ....why and how...i dont get it
A) FL440
B) FL390
C) FL320
D) FL420
ans is fl 420 ....why and how...i dont get it
Thread Starter
Join Date: Jul 2008
Location: New Delhi
Posts: 43
Likes: 0
Received 0 Likes
on
0 Posts
well the source is a JAA question bank
ITs a part of the JAA curriculum
..its even on ths website
Principles of Flight Theory Test Question Bank - 081-02-02 Normal shockwaves - Question 986
..its even on ths website
Principles of Flight Theory Test Question Bank - 081-02-02 Normal shockwaves - Question 986
The link takes me to an isolated example question with no context. It looks to me as if it's possible it's one of a sequence of several about the same aircraft and without the other information that has been provided earlier in the "paper" it's impossible to answer.
Anyhow at a second glance it looks like flite was right
take a closer look at the fine print at the bottom of the linked page:
I think you need to download the full package to get all the information needed.
Anyhow at a second glance it looks like flite was right
take a closer look at the fine print at the bottom of the linked page:
If this question refers to a chart or figure (as some do), please accept our apologies - figures are not available online
Last edited by wiggy; 14th Sep 2011 at 21:59.
The real question includes a Manoeuvre Capability (Buffet Boundary) Diagram.
The diagram provided in the question includes a worked example which makes it all self-explanatory.
Get the diagram and you will be able to answer the question.
The diagram provided in the question includes a worked example which makes it all self-explanatory.
Get the diagram and you will be able to answer the question.
Join Date: May 2000
Location: Seattle
Posts: 3,196
Likes: 0
Received 0 Likes
on
0 Posts
Simple version is that for max range you fly a constant angle of attack in cruise. As weight goes down, airspeed/Mach goes down to provide less lift at that angle of attack.
Join Date: May 2003
Posts: 409
Likes: 0
Received 0 Likes
on
0 Posts
Intruder, you say "for max range you fly a constant angle of attack in cruise".
Could you re think that one? Did you perhaps mean max endurance? (And even then, I'm not sure that is true given compressibility). From what I remember, the angle of attack for max range decreases as weight decreases.
Of course....I could be wrong
Could you re think that one? Did you perhaps mean max endurance? (And even then, I'm not sure that is true given compressibility). From what I remember, the angle of attack for max range decreases as weight decreases.
Of course....I could be wrong
Intruder, you say "for max range you fly a constant angle of attack in cruise".
Could you re think that one? Did you perhaps mean max endurance? (And even then, I'm not sure that is true given compressibility). From what I remember, the angle of attack for max range decreases as weight decreases.
Could you re think that one? Did you perhaps mean max endurance? (And even then, I'm not sure that is true given compressibility). From what I remember, the angle of attack for max range decreases as weight decreases.
Have a look at page 178 of "The Mechanics of Flight", by A C Kermode.
HN39, interesting, but the question is how the speed/AoA varies with change in weight.
I think the "max L/D" term on your chart should be "L/D". There is only one point of "max L/D", which looks to be about 0.5 Mach, at 5.5° AoA. Not knowing what the altitude is for that chart, I would have thought that that AoA is far too high to be best range speed ie best L/D.
I think the "max L/D" term on your chart should be "L/D". There is only one point of "max L/D", which looks to be about 0.5 Mach, at 5.5° AoA. Not knowing what the altitude is for that chart, I would have thought that that AoA is far too high to be best range speed ie best L/D.
Last edited by Capn Bloggs; 15th May 2012 at 11:00. Reason: added "is for that chart"
Join Date: May 2003
Posts: 409
Likes: 0
Received 0 Likes
on
0 Posts
Hazelnuts, I too would be very interested in further info on this graph. What altitude would that be for? I can't think it is in the 30's, as a speed of circa mach .5 for max L/D seems a bit slow for a typically airliner at that altitude.
I note the Y axis, L/D is proportional to 1/D since L is constant for level cruise. this graph then is the inverse of the thrust required curve we typically see.
However, it includes the compressibility effects, is this why it is such a different shape?
Hawk
I note the Y axis, L/D is proportional to 1/D since L is constant for level cruise. this graph then is the inverse of the thrust required curve we typically see.
However, it includes the compressibility effects, is this why it is such a different shape?
Hawk
Join Date: May 2003
Posts: 409
Likes: 0
Received 0 Likes
on
0 Posts
Capn Bloggs says "best L/D (max range) occurs only at one AoA."
We should be careful here, and distinguish between jets and non jets. I don't believe best L/D is max range for a jet. Similarly, for a jet, max range is not at a constant aoa, as weight changes. The thread makes no specific distinction between jets and non jets until Hazelnuts refers to "large jet transport"
We should be careful here, and distinguish between jets and non jets. I don't believe best L/D is max range for a jet. Similarly, for a jet, max range is not at a constant aoa, as weight changes. The thread makes no specific distinction between jets and non jets until Hazelnuts refers to "large jet transport"
Join Date: Jul 2009
Location: France - mostly
Age: 84
Posts: 1,682
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Capn Bloggs
I think the "max L/D" term on your chart should be "L/D".
(*)For this airplane: mass/delta=392 tonnes
Last edited by HazelNuts39; 15th May 2012 at 12:07. Reason: 392t
Join Date: Jan 2012
Location: Not far from the edge of the Milky Way Galaxy in the Orion Arm.
Posts: 510
Likes: 0
Received 0 Likes
on
0 Posts
Why does what? WAT !
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.
N
N
Have I done that right? Is that the answer to your question? the objective being to go as fast as possible . . .as high as possible . . using as little fuel as possible . . .
Power plus Attitude = ?
Last edited by Natstrackalpha; 15th May 2012 at 11:25.
Originally Posted by hawk37
Similarly, for a jet, max range is not at a constant aoa, as weight changes.
Originally Posted by Natstrackalpha
If you are just flying along, as time goes on the weight of the aircraft gets less due to fuel burn. If left to it`s own devices the aircraft either increases altitude or gets faster and faster.
Originally Posted by Hazelnuts39
The point at M=0.5 and AoA=4.5 corresponds to a certain weight/delta, where delta is the ambient pressure ratio. Consequently the altitude varies with weight, such that ambient pressure varies proportionally to weight. (For this airplane: mass/delta=392 tonnes).
Join Date: Jul 2009
Location: France - mostly
Age: 84
Posts: 1,682
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Capn Bloggs
If that chart is for a 392t aeroplane
The point at M=0.5 and AoA=4.5 corresponds to (...) mass/delta=392 tonnes
Join Date: May 2003
Posts: 409
Likes: 0
Received 0 Likes
on
0 Posts
Capn Bloggs; "the IAS for max range changes (reduces) as the weight reduces to maintain the best L/D"
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.
Generally, for a jet, flight at best L/D gives max endurance (usually close to published holding speeds) and is at or near a constant aoa.
If we want best range however, we need to fly at the speed where the ratio of TAS/FuelFlow is the greatest. This is faster than the speed for best L/D, and is not at a constant aoa.
Best L/D speed is the lowest point in the Thrust Required vs TAS graph.
Best range speed can be determined by drawing a line from the origin to the point where it tangents the Thrust Required vs TAS graph. Some assumptions of course, eg constant TSFC Thrust specific fuel consumption (lbs of thrust, per pound of fuel, per hour)
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.
Generally, for a jet, flight at best L/D gives max endurance (usually close to published holding speeds) and is at or near a constant aoa.
If we want best range however, we need to fly at the speed where the ratio of TAS/FuelFlow is the greatest. This is faster than the speed for best L/D, and is not at a constant aoa.
Best L/D speed is the lowest point in the Thrust Required vs TAS graph.
Best range speed can be determined by drawing a line from the origin to the point where it tangents the Thrust Required vs TAS graph. Some assumptions of course, eg constant TSFC Thrust specific fuel consumption (lbs of thrust, per pound of fuel, per hour)