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How Does Flight Envelope Relate To Performance?  
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Posted (12 years 1 month 3 days 22 hours ago) and read 4701 times:

This will be a difficult question with specific regards to performance limits and their structures. I've spent two weeks coming up with this, and it happens to be a core factor in my thinking; approach with caution.  Wink/being sarcastic



But I guess I want to specifically focus on wing loads during cruise, particularly the affect of lift as speed increases. Throughout the flight envelope, there are three limits: slowest, highest and the fastest they could go. I guess a typical supersonic (I assume an airliner), for example, will have the following (which was based more on fighters so it is probably way off, I donno):



question: Are the maximum number of permissible g's related to the cruise as opposed to the minimum speed or stall at a particular altitude assuming a standard flight profile?

I know there are two different g-loads, the effect from an upward pitch and the effect from accelerating straight and pitching down to prevent a cruise climb. The latter is what my question is in regards to. Since lift force is proportional to both velocity squared and the density of air then it is preferred to rise into lower densities of air as the airplane goes faster to maintain a permissible load. It has to rise into the air or the faster it goes in a straight path low in the atmosphere will render the wing to bend up higher.


I am going to use that picture as if it was Concorde's envelope. Now I don't know what her limits are (whether absolute or operational), just cruise conditions, but I can calculate estimates. For example:

@ T/O: Concorde weighs 186000kg, has wing area (w/o flap) = 358 m2, has T/O speed = 112m/s, and we’ll assume standard atmosphere and temperature. Using lift equation its coefficient of lift it about 0.69. Using an equation for air density as a function of altitude based on tables, we can graph the horizontal takeoff speed (or stall) with respect to altitude. According to the function, Concorde stalls at 380m/s at cruise altitude.

Since the aircraft cruises as 605m/s, does the reverse calculation of lift equation with Concorde’s cruise showing that it experiences a 2.53g load make sense?

In other words, the wings are bending at 253% during cruise; tell me if that makes any sense, cuz it doesn’t to me.

Technically, the stall speed is where the plane cannot maintain lift at all, before which it cannot maintain a straight level of flight. Beyond that point, the speed increase will generate more lift, unless it is compensated by going either upwards into lower air densities or slowed down, those wings will continue to bend upwards until failure.

I know the way that delta wings are built, the can be stronger than regular airplane wings, so can Concorde’s wings take a +250% bend? It sounds kind of high considering that the 777’s wings failed at 154%.

I know I am making a lot of assumptions in order to make this easy to understand (for me), but based on those assumptions, what do you guys think? Even if you haven't a clue, take a shot. Someone will correct you even if they don't know what I am talking about.  Smile/happy/getting dizzy


The meaning of life is curiosity; we were put on this planet to explore opportunities.
8 replies: All unread, jump to last
 
User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 1, posted (12 years 1 month 3 days 21 hours ago) and read 4680 times:

Dear Lehpron -
xxx
It looks like I will have to go to my dusty "Aerodynamics for Naval Aviators" to answer your many questions, a manual - quite easy to get at GPO, and quite affordable if you want it... -
xxx
I do not have an aswer for you at the snap of my fingers...
And in Latin, the saying is "sapiens cum libro" - ("knowledgeable with a book") which applies to old me...
xxx
For the 747 as an example, we cruise with buffet speed protection of 1.4 G loads... and our altitude capability, and cruise Mach are based on that figure, which protects us to about 40 degrees bank... Theory of the old "Coffin Corner"... I think about it, but as to hitting the slide rule for it, pilots don't...
xxx
I do not know anyone flying Concorde, would be nice to get their scope of knowledge to throw among us here... For the guys who fly Concorde, their ground school is not much longer than 747 training, and I know they merely use tables in manuals to decide what buffet margins are applicable...
xxx
This more of aerodynamic design engineering subject than... a humble pilot subject... but I will look at the discussion that will be here - darn attractive subject, indeed...
 Smile/happy/getting dizzy
(s) Skipper


User currently offlineBellerophon From United Kingdom, joined May 2002, 585 posts, RR: 59
Reply 2, posted (12 years 1 month 3 days 17 hours ago) and read 4573 times:

Lehpron

Last time we discussed CD, I see this time the topic is CL and load factors! Some quick thoughts.

The manoeuvre load factors for Concorde are +2.5g to −1g, limits in common with other transport aircraft, and like them, she never comes anywhere near them. To suggest that she experiences loads of 2.53g in the cruise is ludicrous.

The maximum take of weight of Concorde is 185,070 kg.

I don’t know the exact wing area but your figure of 358 m2 seems reasonable.

If by T/O speed you mean the aircraft speed at the instant the wheels first lose contact with the runway, then 112 m/s is an accurate figure in still air.

I’m puzzled why you call that T/O speed a “stall speed”, because it isn’t.

A little background here. A slender delta wing doesn’t stall in the conventional sense, it keeps on producing lift up to amazing angles of attack, but at these high angles of attack, the wing also produces massive amounts of drag. It is this high drag that becomes the limiting factor, because it requires more and more thrust to counter the increasing drag as the aircraft decelerates, if the aircraft is to maintain level flight.

Eventually a speed is reached which requires the maximum available thrust from the engines to counter the drag and maintain level flight. This speed is referred to as the Zero Rate of Climb Speed, VZRC, and is obviously dependent on the number of engines operating. Fewer engines means less available thrust, less drag can be tolerated, which in turn means a higher VZRC.

I don’t know what the VZRC is on take-off with all four engines operating at full re-heated power, other than to say that it is so low that it is not a factor. The three engine VZRC is not calculated because it would be around V1, (roughly 190 kts or 98 m/s) and the aircraft still on the runway.

The two engine VZRC is calculated, the first one that really matters, and at max weight VZRC2e would be around 260 kts or 133m/s.

Once airborne, the minimum speed Concorde is allowed to fly at is called VLA (Lowest Authorised) and at maximum take-off weight that would be around 215 kts or 110 m/s.

Concorde does not cruise at a given altitude, nor does she accelerate in level flight. She cruise climbs, at full cruise power, maintaining Mach 2.0. If a level off is necessary, then an immediate thrust reduction is necessary to avoid accelerating.

In short, as some of your assumptions are invalid, I regret to say I think that trying to calculate some sort of high altitude limit from Concorde’s take off speed, by treating it as a stall speed, is rather futile.

If it helps, Concorde’s limits at altitudes above 41,000 ft are:

MMO: M 2.04
MNO: M 2.00
VMO: 530 kts IAS
TMO: +127°C
ALT: 60,000 ft
VLA: 300 kts IAS

Glad to see your continuing interest though. Someone has got to design the next one!


B747skipper

For the guys who fly Concorde, their ground school is not much longer than 747 training

Let me assure you that your statement is both amusing and wrong. Big grin

and I know they merely use tables in manuals to decide what buffet margins are applicable

No, for the reasons discussed above, buffet doesn’t come into it, and there are no tables. If you want to know what margins are available above the stick shaker, then just look at the incidence meter.

Regards

Bellerophon



User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 3, posted (12 years 1 month 3 days 16 hours ago) and read 4553 times:

Dear Bellerophon -
xxx
The only Concorde I have is knowledge acquired on board AF one time CDG to JFK, was in 1988 - was invited to visit flight deck when they found out I was PanAm 747 pilot urgently needed in JFK - and that I spoke French...
xxx
We talked airplane, obviously, these gentlemen did not make it as Concorde flying was much different than any other airplane, yes, the cruise climb was, but makes sense, the supersonic track structure... that is what they briefed me about, fuel tanks transfer being used for trim... and the flight engineer gave me the performance manual to read on my lap to see their tables... was not different from what any other pilot does... their training, classroom training was a week extra due to high altitude and supersonic flying... so that extra time seems to be within the norms... oh yes, they went to an altitude chamber - which is not required by airlines in USA... while it is for military pilots...
xxx
My training took 38 days for the 747, from first day in classroom until the end of my line training... typical of 727 or MD-80 training...
My attitude in life and aviation is this - I am what I am, a 747 pilot, I do not consider C-152 student pilots sub-humans, and I am extremely sociable and helpful to the many young junior pilots who want to be future B-787 captain. maybe my boy will be one if he does not get to be superstar soccer player.
xxx
And I am glad to retire in 5 years or so to a Brazil beach and forget it all with a Brahma chopp in hand... Right now, I still tangle with friends in the A.net which think planes going "supersonic" due to tailwind... jeeeeeezz...
xxx
 Smile/happy/getting dizzy
(s) Skipper


User currently offlineBellerophon From United Kingdom, joined May 2002, 585 posts, RR: 59
Reply 4, posted (12 years 1 month 3 days 5 hours ago) and read 4426 times:

B747skipper

My training took 38 days for the 747, from first day in classroom until the end of my line training

A bit quicker than I did my B747 course, but not by a lot.

The point I was trying to make was that the planned length of the BA Concorde course, from start to finish, is 180 days, hardly not much longer than B747 training.

The ground school alone takes up around 56 days in the classroom, and the simulator about the same again. Then comes the base flying, route training and final check flights.

I believe the nature and length of the course are among the main reason most pilots decide not to bid for Concorde.

As for charts and manuals, yes, lots of them, just none relating to buffet boundaries.

Anyone who has flown the B747-100/200 using triple INS only, can work well with a Flight Engineer, is used to analogue instruments, knows the NAT track system, can use HF, and is familiar with JFK and the Canarsie approach onto 13L, will find the Concorde operation very familiar.

Sounds like you might have been well qualified!

We did have some US crew do the course at Filton and qualify on Concorde, although I think they were all from Braniff.

By the way, Many Congratulations to your son on his first solo, an impressive achievement at such an age.

Regards

Bellerophon


User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 5, posted (12 years 1 month 3 days ago) and read 4402 times:

Dear Bellerophon -
xxx
I am fully aware that USA airline industry has - comparatively "shorter" training courses for aircraft "endorsement" - the course programs are often designed differently... while USA courses are often oversimplified - this with my regrets - courses on the other side of the ocean are overemphasizing subjects which have no practical value - this often because they are just "traditional subjects" - and airlines are conservative, a valued quality.
xxx
The last time I took a pilot conversion training was to fly the 747 as captain for PanAm in 1986 - I had NEVER flown that type aircraft before... I had just been a 707 and 727 captain beforehand...
It consisted of -
80 hours classroom study of aircraft systems - took 2 weeks
non programmed hours of CPT - took 2 days
6 sessions of 2 hours of simulator training - took 6 days
oral exam, simulator test and LOFT program - took 3 days
LOE - line experience with check captain - 25 hours - took 4 days...
xxx
As you can see, from A to Z, allowing for a few days OFF to rest here and there, a complete program, if carefully scheduled - takes 5 weeks to train a 747 captain.
xxx
After the demise of PanAm, I flew a contract for Cargolux... upon hiring, we spent 3 days in classroom in Luxembourg (merely company policies and aircraft differences - CF6), 2 days simulator in Frankfurt, and did a round trip with a check captain from Luxembourg to Montreal... 2 days. The day after, flew my first Cargolux trip to DXB...
xxx
Some other pilots from PanAm elected to go with JAL... as 747 captain, there, it took them - I cannot believe this - 7 months to complete the entire training... and i understand that on the line, JAL requires a "line check" for every route flown... with a check captain... A pilot qualified NRT-SFO route, is not qualified on NRT-LAX, it requires a separate qualification...
xxx
There are extremes in training, USA may be at the end of the scale, but I would say that Japan is at the other end...
 Smile
(s) Skipper



User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 6, posted (12 years 1 month 2 days 1 hour ago) and read 4371 times:

B747skipper, you mentioned a 1.4 G load, does that imply that the your aircraft can experience 141% loading before failure or does one must include gravity and say 241% to failure?


bellerophon: "I’m puzzled why you call that T/O speed a “stall speed”, because it isn’t.....Concorde does not cruise at a given altitude, nor does she accelerate in level flight. She cruise climbs, at full cruise power, maintaining Mach 2.0. If a level off is necessary, then an immediate thrust reduction is necessary to avoid accelerating."

Clearly I know the difference between T/O speed and stall; I just made them the same so that they were easier to deal with. Of course the point at which those wings produce enough force to lift the aircraft at straight level motion occurs at T/O, could that not mean that as long as the wings maintain this flat orientation, it would not be able to lift itself off the ground had it been moving any slower, hence a 'post-stall' effect?

With regards to cruise climb, rather that reducing power, simply pitch down to maintain the same altitude would (or should) cause the wings to bend as if in a high-G turn even though she still accelerates? Granted this is not part of the normal operating procedure, but if done, would a new and probably lower maximum speed be established as such? Furthermore, knowing this, could the absolute velocity maximum be calculated from which without having to deal with either flight testing or CFD's?

Cuz I have neither and I want to know the absolute Vmax of a machine with similar parameters to Concorde, or faster. That's kind of the point here, I am trying so very hard to come up with something that should cruise three times faster, can I come up with an approximation of the speed conditions?



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineBellerophon From United Kingdom, joined May 2002, 585 posts, RR: 59
Reply 7, posted (12 years 1 month 21 hours ago) and read 4345 times:

Lehpron

Clearly I know the difference between T/O speed and stall; I just made them the same so that they were easier to deal with.

By assuming they are the same, you render any calculations that follow of very little value.

Of course the point at which those wings produce enough force to lift the aircraft at straight level motion occurs at T/O, could that not mean that as long as the wings maintain this flat orientation

Concorde's wings produce almost no lift during the "flat orientation/straight level motion" you talk of, unlike most modern subsonic aircraft wings which do.

Only once rotated to the correct attitude does the wing generate significant lift, and then, like any other aircraft, it climbs because of an excess of thrust, not an excess of lift.

simply pitch down to maintain the same altitude would (or should) cause the wings to bend as if in a high-G turn even though she still accelerates?

Why? Broadly speaking, if we maintain the same altitude, the lift equals the weight, and the aircraft will accelerate until the drag equals the thrust, or the aircraft falls apart under the aerodynamic loads (not "g" loads).

B747skipper can expand on his answer for himself if he wishes, but he is talking about a 1.4g buffet margin, that is the margin above the increased stall speed caused by turning flight or encountering turbulence, not a 1.4g margin from wing failure.

I'm sorry to sound so negative, but I believe you need to re-think some of your basic understandings and assumptions.

Regards

Bellerophon







User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 8, posted (12 years 1 month 21 hours ago) and read 4350 times:

Dear Bellerophon -
Slown down your "winged horse" my friend...
Airline pilots have a tendency to be very conservative and fly by the book...
Typical question to fight engineer from me is "can we go any higher"...
(they look in the book) - if NO, I will ask again 45 minutes later...
xxx
We are dealing with supersonic Concorde here - and I forgot all basics...
In a "regular" Mach .84 - FL 350 airplane we dwell in regular buffet margins speeds.
My only concern - ONLY - is to save fuel - negotiate flight levels...
(s) Skipper


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