Vmo indicated airspeed decrease with altitude
02-21-2010, 08:45 PM
#11
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Joined APC: Dec 2007
Position: retired
Posts: 992
Originally Posted by UAL T38 Phlyer
Grade: A+
Here's an extreme example:
I used to do functional check flights on F-4 Phantoms. I've been at FL500, indicating 230 knots, and the Mach was 1.10.
Here's an extreme example:
I used to do functional check flights on F-4 Phantoms. I've been at FL500, indicating 230 knots, and the Mach was 1.10.
02-22-2010, 01:43 AM
#12
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Joined APC: Mar 2006
Position: Pilot & A & P
Posts: 60
Thanks UAL T38 Phlyer. It took me a bit, but I think I finally understand the relationship between IAS, TAS and Mach.
As for FL500 in a BE1900, if it did make it up that high, the flight envelope would be rather small between overspeed and stall.
Hypotheticallly, if I where to make it up that high the 110 inch diameter prop setting would have to be at the max 1700 RPM.
Circumference equals Diameter Multiplied by Pi = 110 X 3.141 592= 345.575 in
345.575 in divided by 12 inches = 28.7979 feet in Circumference
28.7979 ft X 1700RPM = = 48,956.457 fpm
48,956.457 fpm divided by 6,076 feet = 8.0573497 knots per minute
8.0573497 kts per minute X 60 minutes = 483.44 knots (speed of propeller blade tips)
Assuming standard temperature lapse rate at 50,000 feet the outside air temperature would be -85 C which results in a mach 1 speed of 534 knots.
This means my propeller blade tips would be mach .904... Localized airflow around the prop blade most likely above Mach 1.... That can't be good either.
(For comparision at SL the prop blade tips would be at Mach .73 and at FL250 Mach .81)
I think I would rather be in your F4 Phatom.
Again thanks to all that replied. Have a nice day.
As for FL500 in a BE1900, if it did make it up that high, the flight envelope would be rather small between overspeed and stall.
Hypotheticallly, if I where to make it up that high the 110 inch diameter prop setting would have to be at the max 1700 RPM.
Circumference equals Diameter Multiplied by Pi = 110 X 3.141 592= 345.575 in
345.575 in divided by 12 inches = 28.7979 feet in Circumference
28.7979 ft X 1700RPM = = 48,956.457 fpm
48,956.457 fpm divided by 6,076 feet = 8.0573497 knots per minute
8.0573497 kts per minute X 60 minutes = 483.44 knots (speed of propeller blade tips)
Assuming standard temperature lapse rate at 50,000 feet the outside air temperature would be -85 C which results in a mach 1 speed of 534 knots.
This means my propeller blade tips would be mach .904... Localized airflow around the prop blade most likely above Mach 1....
That can't be good either.(For comparision at SL the prop blade tips would be at Mach .73 and at FL250 Mach .81)
I think I would rather be in your F4 Phatom.
Again thanks to all that replied. Have a nice day.
Last edited by r1830; 02-22-2010 at 03:35 AM.
02-22-2010, 05:39 AM
#13
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Joined APC: Mar 2008
Position: Curator at Static Display
Posts: 5,681
Yes & No
Originally Posted by Dougdrvr
OT question: Did you have to be in burner to do that?
The profile called for climbing to FL400, and accelerating in burner until the intakes started to "program." Mach-2 class fighters have variable geometry intakes, which start to close-off at high speeds (the air that gets inside the intake then expands, slows down, and is therefore subsonic by the time it gets to the engine). Compressors don't like supersonic air--they tend to compressor-stall.
Incidentally, this was a slow process: it took about 120 miles, and you would gain about 3 knots a second. The air is pretty thin at FL400, so thrust---even in burner---is low.
The big "splitter-plate" (vertical board of the intake, next to the fuselage) would start to creep out at about 500 kts indicated. The Mach was usually between 1.7 and 1.8; depending on OAT.
Still in burner, you would climb to FL480, and then take them out of burner (one at a time) to make sure:
1. The engine(s) didn't compressor-stall, cough, and flameout (going in or out of burner sets up huge pressure-transients in the engine...perfect set-up for a compressor-stall)
2. Moments later, when re-initiating burner, that it wouldn't cough as above.
During the "zoom" to FL480, it was easy to overshoot and end up higher (FL520 was my highest; a guy in my squadron had been to FL630, and one backseater had been to FL820).
After the re-light, we needed to get down to about 15000 for some other checks, so you went to idle (again, one at a time!), and glided down. That was where I saw the 230 kts / just above the Mach. During this glide, the landing-gear warning system would be flashing, because it thought "He's below 240 knots, and the gear isn't down!"
Never mind that we were still 8 miles above the ground and still supersonic.
02-22-2010, 01:40 PM
#14
Gets Weekends Off
Joined APC: Dec 2007
Position: retired
Posts: 992
Originally Posted by UAL T38 Phlyer
Doug:
The profile called for climbing to FL400, and accelerating in burner until the intakes started to "program." Mach-2 class fighters have variable geometry intakes, which start to close-off at high speeds (the air that gets inside the intake then expands, slows down, and is therefore subsonic by the time it gets to the engine). Compressors don't like supersonic air--they tend to compressor-stall.
Incidentally, this was a slow process: it took about 120 miles, and you would gain about 3 knots a second. The air is pretty thin at FL400, so thrust---even in burner---is low.
The big "splitter-plate" (vertical board of the intake, next to the fuselage) would start to creep out at about 500 kts indicated. The Mach was usually between 1.7 and 1.8; depending on OAT.
Still in burner, you would climb to FL480, and then take them out of burner (one at a time) to make sure:
1. The engine(s) didn't compressor-stall, cough, and flameout (going in or out of burner sets up huge pressure-transients in the engine...perfect set-up for a compressor-stall)
2. Moments later, when re-initiating burner, that it wouldn't cough as above.
During the "zoom" to FL480, it was easy to overshoot and end up higher (FL520 was my highest; a guy in my squadron had been to FL630, and one backseater had been to FL820).
After the re-light, we needed to get down to about 15000 for some other checks, so you went to idle (again, one at a time!), and glided down. That was where I saw the 230 kts / just above the Mach. During this glide, the landing-gear warning system would be flashing, because it thought "He's below 240 knots, and the gear isn't down!"
Never mind that we were still 8 miles above the ground and still supersonic.
The profile called for climbing to FL400, and accelerating in burner until the intakes started to "program." Mach-2 class fighters have variable geometry intakes, which start to close-off at high speeds (the air that gets inside the intake then expands, slows down, and is therefore subsonic by the time it gets to the engine). Compressors don't like supersonic air--they tend to compressor-stall.
Incidentally, this was a slow process: it took about 120 miles, and you would gain about 3 knots a second. The air is pretty thin at FL400, so thrust---even in burner---is low.
The big "splitter-plate" (vertical board of the intake, next to the fuselage) would start to creep out at about 500 kts indicated. The Mach was usually between 1.7 and 1.8; depending on OAT.
Still in burner, you would climb to FL480, and then take them out of burner (one at a time) to make sure:
1. The engine(s) didn't compressor-stall, cough, and flameout (going in or out of burner sets up huge pressure-transients in the engine...perfect set-up for a compressor-stall)
2. Moments later, when re-initiating burner, that it wouldn't cough as above.
During the "zoom" to FL480, it was easy to overshoot and end up higher (FL520 was my highest; a guy in my squadron had been to FL630, and one backseater had been to FL820).
After the re-light, we needed to get down to about 15000 for some other checks, so you went to idle (again, one at a time!), and glided down. That was where I saw the 230 kts / just above the Mach. During this glide, the landing-gear warning system would be flashing, because it thought "He's below 240 knots, and the gear isn't down!"
Never mind that we were still 8 miles above the ground and still supersonic.
What is so special about the F-22 that allows it to stay above Mach 1 without AB, the so-called "super cruise"? Thanks......
02-22-2010, 04:21 PM
#15
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Joined APC: Mar 2008
Position: Curator at Static Display
Posts: 5,681
Engines & Drag
Correct: the jet was usually clean (no stores), although we always had the inboard pylons...pretty draggy. Start with 12,000 lbs internal, and finish the sortie with about 1500. (45-55 minutes).
The F-22 answer is: relatively clean airframe (drag-wise), and humongous engines.
F-4 was moderately draggy, and the engines (big for their day) didn't have the thrust-to-weight-to-DRAG ratio the F-22 does. The F-22 has enough thrust in Mil to do what I could not in burner.
The F-22 answer is: relatively clean airframe (drag-wise), and humongous engines.
F-4 was moderately draggy, and the engines (big for their day) didn't have the thrust-to-weight-to-DRAG ratio the F-22 does. The F-22 has enough thrust in Mil to do what I could not in burner.
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