Got a request from an avid aviation fan with a doozy of a question. Can anyone help him? I think he is getting confused on the yellow speed tape portion. The actual stall speed is near the bottom. You have to pass through Vls and alpha prot...
Heck, as an airline pilot I am paid to stay out of the red tape and the yellow tape (see below). If I do enter this envelope then I get really apprehensive, it sends mesaages to MCC (Maintenance Control Centre) and I have to answer for everything.
So all you Airbus experts out there....bring it on for this guy!
NOTE: PFD should read as PRIMARY Flight Display...late night for me. :))) Oopsie
I've got a question about the Coffin Corner. I read a post on your blog and have heard different explanations from different sources, I am trying to get it straight in my head. The intent of this question is not to prove you right or wrong, I'm just confused.
In your post you describe it as the altitude where Vs increases and your Vne decreases ( i know the terms in the airbus are a bit different) to create a small margin for error.
Your indicated stall speed would stay the same with an increase in altitude would it not? The True stall speed would increase, but all things being equal indicated stall speed stays the same. On the PFD pic of Coffin Corner on your blog, it shows your Vs at around 250 KIAS, but if the previous fact is true that indicated stays the same with altitude, then that means that your Vs in clean configuration is 250 KIAS at Sea Level. Is that true? That speed seems high to me (I dont fly a jet so I could be very wrong or missing a huge piece of the puzzle)
Where I get confused is I have also read it is the altitude where the low speed buffet and high speed buffet meet.
The other explanation I have heard is that low speed buffet is an angle of attack where the air over the wing is accelerated above the magic speed where you get a shockwave which separates the laminar flow with a decrease in lift and increase in drag (very similar to a stall). And the high speed buffet is where your actual speed is that magical speed where the shockwave separates the flow causing you to lose lift.
Can you enlighten me?
You had me at E=mc^2. I've stopped trying to understand and started to appreciate physics.
Duane. Physics is the science that answers "why." That's why I liked it. :)))
Does PFD mean Primary Flight Display rather than Personal?
wiki is your mate ;)
Supplementary question: I accept that the aim is to keep the "speed bug" out of the red and yellow area's, but above FL250, or thereabouts, you fly to the Mach number, which itself varies with altitude, static air pressure and whether or not you are heading in the right direction in a 150kt jetstream?
As to the "250kt limit" at sea level, is your question asker confusing that with the 250kt limit below the 10k transition level - Canada/USA? I've flown a JP5A at 300ft agl (mean!!) & 300kt ias; that was fun. Also saw a RAE BAC 1-11 do a circuit join from the dead-side at about 750ft & 350kt! That was at a secret RAF Scotland multi-role airfield - interesting to watch!!
...from a "still awaiting news" Foggy on an overcast and damp south coast of England...
Apologies... I forgot to ask!
At what FL would you usually change from IAS to MACH? ...and would it be the same level coming down as going up?
Cheers / Foggy
Christopher. You are absolutely correct. I added a note stating that it should be PRIMARY.
Afteer reading your last blog that why I,m doing a arts Degree in History,lol. Anyways does your aircraft has a head up display like does on the military aircraft?
One more thing this is to getjets nice shot exspecally the one how a aircraft flies. Now I understand,lol. Take carer and happy flying, Chris.
well as a philosophy major in a past life i'm all too happy to delve into "the meaning of things" (coffin corner included).
"Your indicated stall speed would stay the same with an increase in altitude would it not? ... it shows your Vs at around 250 KIAS, but if the previous fact is true that indicated stays the same with altitude, then that means that your Vs in clean configuration is 250 KIAS at Sea Level. Is that true?"
good observation. the issue here is that indicated stall speed does NOT actually stay the same with altitude (even though this is drilled into you in flight school). the rule that "indicated stall speed stays the same with altitude" is an attempt to correlate the critical angle of attack (for a given weight) with something tangible for the pilot (the airspeed indicator). this is fine until you start climbing into the rarefied air of the flight levels where decreased air density/viscosity means it's time to trade simple "rules of thumb" for a slide ruler to explain things.
it would be more accurate to say that stall is more consistent with EAS(not IAS). as air density decreases (ie with an increase in altitude), compressibility has two important effects: there is a divergence in EAS and IAS (IAS increases faster than EAS so the EAS Vs correlates with an increasing IAS value) and a reduction in the wing's CLmax (due to a diminishing Reynolds number). to further complicate things, the EAS stall speed also begins to increase because of the changing relationship between EAS and Mach number. at higher and higher altitudes, the EAS stall speed occurs at an ever increasing Mach# and as Mach# increases, the pressure distribution around the wing changes which has the effect of reducing its ability to produce lift. all these factors contribute to good reasons to keep the airplane going fast (although there is another set of good reasons to keep the airplane from going too fast).
shockwave formation is only associated with the high speed limit (Mcrit).
(see you in 2wks capt doug!)
oops, should have proof read before posting...
"...correlate the critical angle of attack (for a given weight)"
i should correct myself here- the critical angle of attack of an airfoil is always the same (err...for the most part) but corresponds to a relatively consistent IAS stall speed(which varies with weight).
Let me explain that I am an avid simmer and enjoy reading this and several other equally excellent blogs relating to aviation - I can in no way be called an expert so please be gentle if my understanding is incorrect:
As the pressure/temp falls with altitude you have to fly at a higher mach number to achieve an IAS of 250 kts or whatever speed is your stall speed in a given configuration (basically because there is less air to support the a/c), and so the higher you fly the mach number at which you will stall increases, ie moves closer to the V never exceed speed of the aircraft thereby closing you into a smaller and smaller "corner" as you increase altitude.
Apologies if this is way off the mark or missed the point of the question but I'd appreciate any feedback on whether I am on the right track.
As I said, great blog and love the interactivity - keep up the excellent work!
Dave from the UK (EGHI)
Foggy. Great link.
I too am a little confused about the 250 knot query so that's why I put the question out en masse.
There are lots of smart people out there including you.
I still have my fingers crossed for your "awaiting news."
Bas. The PFD is a great display. I've been staring at it for 15 years. Many are first intimidated by it but once you figure it out and toss out half the nonsense the instructors want to teach on it, it's a beauty. :)))
Foggy. I think you asked this before. I keep meaning to check. The airplane does it by itself. First you are climbing on IAS in knots and then the MACH number appears on the "PERF" (performance) page.
It's around FL180 to the low twenties...I think. I'll try to make a mental note on Saturday when I fly to SFO.
It's about the same in descent but it varies. Next time ATC tells me to "transition to 290 knots" I'll see if I can catch it when it does.
Won't your flight sim do this for you? Curious.
Chris. LOL. No, none of our mainline fleet has "heads up." The RJ fleet does with JAZZ. This stemmed from an incident we had in Fredericton years ago.
Yes, Getjet's pic made it simple to explain the theory of flight. :)))
Hey Captain Doug,
and all this time...I thought a "HIGH SPEED BUFFET" was a short lunch break...oh 30 minutes or so
a "LOW SPEED BUFFET" was at least an hour for lunch.....
are you at least smiling?????
I'VE BEEN BLINDED BY SCIENCE!!!!
then there's the 'no frills' school of thought......."my picture post that you were kind enough to put here...."HOW AIRPLANES FLY"....cartoons that are drawn with the left hands of right handed people.....
Hows that watch band working for ya.....lol
is it a sign dial too....
@Chris Gardner...I am glad you like the pics......
looked gloomy in St. John's today....
good evening to all my friends and former friens...LOL here at Captain Doug's web-home....
@getjets--I definitely like your explanation of how planes fly...sheesh, to think Captain Doug couldn't explain it simply like that...
...all this Newton, Bernoulli baloney...
Brian. Well done for a philosophy major!!! I guess it's on the same line as a meteorologist discovering plate-tectonics.
You know, I almost understood most of what you said. So it didn't take an aerospace engineer to explain things. :)))
So we are talking EAS (equivalent air speed). I might have known that 20 years ago.
I'll stick to keeping everything in the middle of Vls and V max (Mmo). LOL
"See you in two weeks." What's up?
Dave from the UK.
I like your answer. I don't think you are off the mark at all.
Welcome to the blog!
Captain Doug in Canada
Jack. I think you did great in trying to openly decipher things. Yes, the coffin corner eventually will meet as depicted in "Foggy's bottom" Wiki "Q factor" link.
When they meet, your airplane turns into a huge boulder.
Thanks for joining in one of my "higher intellectual" posts. But I'm sticking to weather as my forte. LOL
Captain Doug the "non" aerodynamics guy. :)))
Well I am an aero guy, though I mainly deal with the inside of engines rather than the outside. I think Brian basically has it correct. Your questioner said:
"Your indicated stall speed would stay the same with an increase in altitude would it not? The True stall speed would increase, but all things being equal indicated stall speed stays the same."
The key point is that the indicated speed means something different when you travel at higher Mach numbers, this is due to the changing nature of fluid flows at higher speeds. The indicated air speed is read from a pitot static system which gives you the dynamic pressure at low speed and is directly related to lift but at higher speeds the relationship is much more complex.
Therefore the indicated air speed for stall at high altitude (and high speed) is not the same as the indicated air speed for stall at low altitude.
"The other explanation I have heard is that low speed buffet is an angle of attack where the air over the wing is accelerated above the magic speed where you get a shockwave which separates the laminar flow with a decrease in lift and increase in drag (very similar to a stall). And the high speed buffet is where your actual speed is that magical speed where the shockwave separates the flow causing you to lose lift."
This may be true: low speed buffet being caused by supersonic flow over the top of the wing - I guess the argument would be that in order to maintain lift you increase the angle of attack which then causes the wing to stall due to a shockwave formation rather than the conventional boundary layer separation. However that would be very dependant on the geometry of the wing and requires you to change the angle of attack as well as the speed of the wing.
I hope that helps!
Maybe Mr. Yeager could cut right to the heart of the matter...regarding that technical question.
[only genuine url, from what I can see]
As a test pilot, he was paid to venture into the yellow and red areas [on purpose!], so that question may not be so theoretical to him.
Better still, why not ask "editor" if Yeager would agree to an interview on your blog? [But type the editor's email address from scratch [don't click it, since the site seems to get hacked from time to time]
From Yeager's point of view, it might be a first. In the world of aviation, you two may have different specialties, but you are equals...
I like your comment..."he was paid to venture into the yellow and red areas." I'm paid to stay out of them. lol
"Safety, comfort, and schedule" is the kool-aid I drink. LOL
Thanks for putting me up as an equal with Chuck Yeager. What a promotion!
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