a question for those who fly in the flight levels
- Thread starter deafsound
- Start date
TMetzinger
Final Approach
At those altitudes, you're typically measuring airspeed in mach numbers and traditional IAS in knots changes very little as you climb through the 20s.
If I cruise a 737 (the only airplane/sim I've ever flown above 25000), I'll probably be at .76 - .78 Mach, and airspeed will be somewhere in the 260s (I think - I'll check next sim ride). As I descend, the indicated airspeed will increase, and usually I start watching IAS instead of mach when .67M = 280 KIAS.
There's a couple of really good books on flying the transports. Flying Jets by Pendleton, and Fly the Wing by someone else. You'd be welcome to borrow either/both if you are interested.
If I cruise a 737 (the only airplane/sim I've ever flown above 25000), I'll probably be at .76 - .78 Mach, and airspeed will be somewhere in the 260s (I think - I'll check next sim ride). As I descend, the indicated airspeed will increase, and usually I start watching IAS instead of mach when .67M = 280 KIAS.
There's a couple of really good books on flying the transports. Flying Jets by Pendleton, and Fly the Wing by someone else. You'd be welcome to borrow either/both if you are interested.
deafsound
Line Up and Wait
Thanks for the response. I'm not really interested in flying the transports..I was just conversing with a pilot friend, and he seemed to think that the IAS would be outrageously low....like 100kts, which to me seems insane. I mean, if you were getting 100kts IAS at FL360, wouldn't the plane be really close to stalling? I figured the IAS would be somewhere around 230ish, give or take.
TMetzinger
Final Approach
It's definitely up in the mid 200s, but I'm not sure where stall speed would be... Most planes flying that high have stall systems defined by angle of attack, not IAS, so the shaker/pusher/etc is really independent of IAS.
tonycondon
Gastons CRO (Chief Dinner Reservation Officer)
ive had a 421 up to FL250, i cant really remember what the IAS's were, i dont think they were too far from normal. we didnt spend much time up there. glider pilots have to deal with the issues of the "coffin corner" quite often when they are flying in mountain wave in the Sierras and the Andes, and elsewhere. flying without an engine at 25 or 30000 would definitely be cool. they get constrained by lower redlines at altitude due to higher TAS and higher stall speed of course due to thinner air. sounds like fun!
jesse
Touchdown! Greaser!
As you climb the air gets thinner. This thinner air means there is less for the wings to grab onto and less air to ram into the pitot tube. The drag of the air also decreases. This decrease in drag is what makes you so efficient.
Now let's get back to indicated airspeed. Your airplane is going to stall at the same indicated airspeed for the given weight. The airplane will stall at different true airspeeds. True airspeed really does not mean much. If you did not have any winds aloft to consider--true airspeed equals your ground speed. Indicated airspeed does not equal your ground speed.
100 knots? I doubt it.. Some quick numbers
FL350, 400 KTAS, -50C == 220 KIAS
FL400, 400 KTAS, -60C == 200 KIAS
FL450, 400 KTAS, -70C == 180 KIAS
FL500, 400 KTAS, -80C == 160 KIAS
That's all basically pretty linear looking which surprises me. So my math could be wrong but it's probably about right. The higher you climb and the closer you will get to stall speed. Can you say Pinnacle Flight 3701?
It's interesting how many general aviation pilots really do not understand true airspeed. They also don't understand how to best take advantage of their piston engine. This brings up an entirely different subject albeit still interesting.
As you climb in a piston engine your maximum power decreases. The drag on the airplane decreases as well. The nice thing is since the drag has decreased you no longer need as much power to develop the same true airspeed. This equates to lower fuel burns as well. The sweet spot for a normal aspired piston engine is generally between 7,000 to 10,000 ft. You can achieve almost the same KTAS as you can at sea level while being WAY more efficient which means less engine stress and a lot of fuel saved.
Last edited:
poadeleted20
Deleted
- Joined
- Apr 8, 2005
- Messages
- 31,250
We usually cruised the F-111 at 480 KTAS up at altitude, which usually meant the FL250-350 range, depending on weight. That would put IAS's in the 320-270 range -- and anything less than 270 meant we'd fall out of the sky in a clean configuration even with the wings forward at 26 degrees (didn't go to 16 degrees until in the landing configuration). BTW, we could go a LOT higher, but it took burner to do it, and that burns gas big-time.
FWIW, a U-2 up in the mid-60's has a roughly 5-knot window between stall and mach buffet. Remember that stall speeds are IAS, and don't change with altitude, but mach buffet is closely related to TAS.
FWIW, a U-2 up in the mid-60's has a roughly 5-knot window between stall and mach buffet. Remember that stall speeds are IAS, and don't change with altitude, but mach buffet is closely related to TAS.
Let'sgoflying!
Touchdown! Greaser!
Danos
Line Up and Wait
Was that that flameout over Missouri?
tonycondon
Gastons CRO (Chief Dinner Reservation Officer)
yes..
Everskyward
Experimenter
- Joined
- Mar 19, 2005
- Messages
- 33,448
- Display Name
Display name:
Everskyward
Real close! We have to write down this stuff so I looked at what I did today. At FL 390 the IAS was 238 knots and .77 mach. The TAS was about 440 knots.
Jesse,
You're kinda on the right track. It's more correct to say that an aircraft stalls at the same AOA, not airspeed. The 2 are somewhat related. With that, AOA is a pretty arbitrary number for a given aircraft. For example, in the T-6, the airplane stalls at 18 units of AOA. In the '38, it stalls at 1.0 AOA. What I'm driving at is that different aircraft measure AOA in different units, but will stall at its specific stall AOA every time, regardless of altitude, airspeed, etc. To add to the mix, AOA does not equate to pitch or G's. Clear as mud? Hope this helps.
Break break
To answer the original question my answer is that it depends alot on what the winds are doing. We usually tool around at 280 or so in the '38 these days due to its high potential of compressor stalling at high altitudes. I remember one time cruising at .9 mach with a groundspeed readout of 630 knots (pretty good tailwind). Not really sure what the calibrated a/s was (we read calibrated, not indicated), we fly mach when we cruise. We typically set .5 mach plus altitude for max range (example: FL 280 = about .78m max range), or just .9 for when we have the gas.
You're kinda on the right track. It's more correct to say that an aircraft stalls at the same AOA, not airspeed. The 2 are somewhat related. With that, AOA is a pretty arbitrary number for a given aircraft. For example, in the T-6, the airplane stalls at 18 units of AOA. In the '38, it stalls at 1.0 AOA. What I'm driving at is that different aircraft measure AOA in different units, but will stall at its specific stall AOA every time, regardless of altitude, airspeed, etc. To add to the mix, AOA does not equate to pitch or G's. Clear as mud? Hope this helps.
Break break
To answer the original question my answer is that it depends alot on what the winds are doing. We usually tool around at 280 or so in the '38 these days due to its high potential of compressor stalling at high altitudes. I remember one time cruising at .9 mach with a groundspeed readout of 630 knots (pretty good tailwind). Not really sure what the calibrated a/s was (we read calibrated, not indicated), we fly mach when we cruise. We typically set .5 mach plus altitude for max range (example: FL 280 = about .78m max range), or just .9 for when we have the gas.
Last edited:
colomtnflyer
Cleared for Takeoff
- Joined
- Feb 18, 2007
- Messages
- 1,008
- Location
- Guess it depends on what week it is!
- Display Name
Display name:
iAM in AK!
Today, at FL270, -30C, IAS 160 = TAS 249.... (unless I screwed up my cipherin') By the way, GPS showed 277 across the ground, except when we had to kick out the ice vanes... dumped between 15 and 18 kts!
jesse
Touchdown! Greaser!
I'm fully aware of that. I'm saying that Vs0 or Vs1 is indicated and will stay constant for the given weight. True airspeed has no effect on it.
Truth is very little of us have AoA indicators in general aviation and understanding the concept helps--but it's feel and situational awareness that'll save your ass.
tonycondon
Gastons CRO (Chief Dinner Reservation Officer)
pretty much got it. weight = load factor in your example. most precise wording is indicated stall airspeed is the same for a given configuration but we know what you mean
Patch
Pre-takeoff checklist
Today at FL 280, I was at 0.80 mach with a Calibrated Airspeed of 318
At FL 270, I was at 0.77 mach with a calibrated airspeed of 311. OAT was -34 degrees.
Bull - we don't go above 280 due to the fact that we are non-RVSM.
At FL 270, I was at 0.77 mach with a calibrated airspeed of 311. OAT was -34 degrees.
Bull - we don't go above 280 due to the fact that we are non-RVSM.
Earthroamer
Pre-takeoff checklist
I echo Mari's speeds today. Mine are pretty close to hers by a couple of knots. She is usually running about .77 to .80 depending on how warm and fuzzy she wants to be. I am usually at .74 in the slowtation. So my IAS is just a couple of knots slower than hers. Even in the slowtation, which is the slowest jet I have flown...we are not out of the 200's IAS even at FL450. At least, not that I have seen.