reply to post by scott3x
scott, applause for this post. Fortunately, I took a break after reading a post or two above, where I thought you needed some gentle
corrective nudging, but it looks like you're learning quite well on your own.
Really, for non-pilots (and laymen) you aren't going to find as much info from the Internet as people like me who've actually flown the airplanes
will be able to tell you.
Of course, since what I say about how to operate the airplanes isn't always available ON the internet for you to check, and since no one wants
lessons from me (
) then I'll stick to checking and clarifying what you're already learning.
I doubt it makes much of a difference, but according to this page, the VMo for the 757 and the 767 are actually slightly different; the 767
is the one with the 360 knots VMo; the 757 has it at 350 knots. 767 that has the VMo at 360 knots.
Note #1: You are following the style of "turbo" here...in all of my Flight Manuals and training materials, it is either "Vmo" or "VMO". Picky,
but....
Note #2: I was teasing about being picky, because you have a sharp eye, and caught something that "turbo" obviously never noticed.
Originally posted by turbofan
Look at this another way...it might help to understand:
IF...IF... a 767 could fly at 510 knots at sea level (without breaking apart,
or going out of control), it has enough power available to travel over
Mach 1 (the speed of sound) at 35,000 feet.
Ermmmm...once again, in the Pentagon thread about AA 77, "turbo" goes off the rails here....but, this is his mantra, and it's wrong. He is
forgetting about the effects of using gravity AND thrust to accelerate beyond a speed that would be achievable in level flight with thrust only.
---------------------------------------------
I'm not sure how fast a 757 could travel at 35,000 feet, knots wise. However, I think it would probably be good to introduce the following
link:
www.seattle911visibilityproject.org...
Since MMO (or, if you prefer, "Mmo") is M.88, then simply solve for M.88 at FL350, standard temperature. You will then find the TAS that is
equivalent. Hang on, I'll do it..... OK, I'm back:
When you reach M.88 at FL350, in standard temp for that altiude (-54.3C) your TAS is 507Kt, and the CAS is 303Kt. (CAS is what the airpeed indicator
should read, +/- a few knots. It's important, here, to also point out that the 'barber pole' that I mentioned many posts back, while it is
normally at the 350Kt or 360Kt position (depending on airplane) will move, as altitude increases into the Mach regime, to reflect MMO,
instead
of VMO...because MMO is more limiting, from a CAS standpoint, at thigher altitudes. Oops, surprise lesson....sorry.
Quoting from it:
Here is how you figure out how fast a 767 can go at 700 ft (basically sea level)
We see that the "speed limit" is much lower at sea level than at 30k. 403 mph at sea level, and 581 mph at 30000 ft. But now the rubber to the road
question is, how much beyond the "speed limit" can a Boeing 757 or 767 airplane be taken? To exceed Vmo/Mmo is not catastrophic. Boeing notes higher
speeds can be authorized. To quote the Boeing Flight Ops review: "At speed in excess of Vmo/Mmo ... normal airplane handling characteristics are not
assured." What they are saying is that an airplane can be taken somewhat beyond Vmo/Mmo by a skilled pilot. We would expect a safety factor of at
least 10%, probably more like 20% or 30%, before structural damage may result.
How much beyond the "speed limit" can a Boeing 757 or 767 airplane be taken? Pilots For 9/11 Truth asked that question over a year ago, and still do
not have an answer. It is fairly well established that Vmo is about 400 mph at sea level, but no solid numbers for Vne have been documented. While
this question is still in the hopper, the assumption by Pilots For 9/11 Truth has been that all of the alleged manuvers of the airplanes on 9/11 were
within the scope of a skilled pilot and the Boeing aircraft.
The "flight 175 Impossible speed" Thrust-Drag Argument has been mentioned on the internet.
The argument is basically this: thrust must increase 134-fold to maintain the same 542 mph it had at 35,000' altitude down at 1,000' altitude. I
believe this is defintely wrong. First, the quantity given for air density in this "thrust/drag argument " is incorrect: The air density quoted is
for 35000 meters, or 114829 ft, not 35000 ft. At 114829 ft, not only would you not be able to breathe, but you would also explode, so it looks like
someone got meters and feet mixed up in thier calculations.
The 757 or 767 has enough engine thrust to go about as fast at sea level as at 30000 ft. Here is why I believe that: Thrust increases by about the
needed amount to compensate for the difference in air density between the higher and lower elevation.
At lower altitudes and denser air structural damage may occur due to vibration or flutter, while at higher altitudes and thiner air, structural damage
due to shock waves may occur. Thats what the "speed Limits" are all about. The key question is, by how much can a 767 (or 757) exceed Vmo without
incuring structural damage , and at this point, we just do not know.
Originally posted by turbofan
IF...IF... the engines were able to move the aircraft through more dense
air at 510 knots, it will be able to move the plane at an equivalent speed
[EAS] of over Mach 1 at higher altitudes in less dense air (less aero drag).
But then there's the issue of increased thrust at lower altitudes; I'm guessing it has to do with the increased air
density.
Good guess, critical reasoning...BUT there is still that pesky issue of drag.
~Caution: Lesson approaching!~ We are interested, here, in two types of drag: Parasitic (sometimes called 'form') drag, and Induced drag.
Parasitic is obvious, and is a direct result of forcing a body (airplane) through a fluid (air). It is an inverse curve relationship to speed.
Induced drag is related to the creation of lift. THEREFORE it varies by speed AND angle of attack.
~lesson ending~
Originally posted by turbofan
At this equivalent speed, the aircraft is experiencing the same forces from
air pressure as it would traveling at 510 knots at sea level.
Does this make sense now?
It does, but after taking a look at the formula for equivalent air speed, I admit I'll have to take your word that 510 knots at sea level =
Mach 1 + at 35,000 feet. What I'd like to know is, does weedwacker agree?
scott, you found that page from Wiki with the EAS formula...it's easier to use the one I linked before, from
www.aerospaceweb.org...
Again, as I mentioned about, "turbo" just can't let go of one idea, and he's wrong.
As you can no doubt calculate by now, and realize, Mach 1 at sea level is well above 510Kt. (Yes, I'm using that one now, because it's worst case
scenario...from the three that hit buildings).
Originally posted by turbofan
If you agree that the aircraft can move at 510 knots at sea level in control
and 'safe structural' flight, then you must agree that the same plane can
break the sound barrier at 35,000 feet.
Ask yourself if this is possible.
Sigh. Again, "turbo" is making an illogical assumption, and comparison. He is, still, completely misunderstanding the concepts of Indicated versus
True airpeeds, and the Mach number relationships of air temperature and speed.
Honestly, I don't know. Hopefully you'll respond to that link from seattle911; I note that he also says 30,000 feet instead of 35,000
feet; the author is using PFT as his back up though; since you haven't been banned from PFT, you can ask them directly if he's characterizing what
they said right.
Hopefully he will. We are all hoping he will.
[edit on 30 September 2009 by weedwhacker]
