Forum: F-35 Lightning II

outlaw162 wrote:

580 knots TRUE airspeed at 39,000 feet is Mach 1.02.

Mach 1.02 is 327 knots calibrated airspeed at 39 grand.

680 KCAS at 39,000 feet is Mach 2 (that's 1176 knots True airspeed).

Mach 1.2 is 580 knots KCAS at 20,000 feet.

Trust me, I'm a doctor.

OL

true or calibrated?
this sounds interesting.

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Quote:

Even when loaded internally with two 2,000lb GBU-31 Joint Direct Attack Munitions and two AIM-120 AMRAAMs, Griffith says the sheer power of the Pratt & Whitney F135 is evident. “The engine has a lot of thrust. It’s been fun to outrun the F-16 (chase aircraft). They can’t keep up. If we go to full military power the F-16 has to go to afterburner to keep up.”

I'm guessing that's a clean F-16 as well? That's impressive.

Salute!

Sheesh, OL, I wonder how many times the newbie has ever flown at 40K or mach 0.01

Roscoe? Where's Roscoe?

For great examples of CAS/IAS/EAS/TAS versus mach, go look at the SR-71 tech order..........

http://www.sr-71.org/blackbird/

One of my buddies that flew the sucker had to remind me that once above a certain altitude, that temperature was the driver of mach. Of course, air density decreased while temp remained someplace around minus 55 deg C. So true airspeed was lots higher than what your pitot tube experienced. Those pesky air molecules got further and further apart per cubic cubit.

Bottomline? We're seeing a troll who has no clue about mach, TAS, CAS, screw-all.

later,

Gums sends ....

munny wrote:

I'm guessing that's a clean F-16 as well? That's impressive.

In most cases the chase planes carry drop tanks 1 or even 2.

Through some snooping around the internet, I found this calculator?

http://www.hochwarth.com/misc/AviationCalculator.html
(Since I don't hold a doctorate in math or engineering, someone want to check this out?)

Seems like a good educational tool.

TEG

http://www.tscm.com/mach-as.pdf

Try this, the second chart down for high Mach numbers. Just go directly left from 650 Cal on the right side to the left side Mach 2 line and you'll intersect the 40K altitude line.

Something about the Hochwarth KCAS calculation isn't kosher. Put in 39,000 feet and Mach 2. They compute a KCAS of 770 for Mach 2 at 39 grand. That seems high.

770 Cal at 39K puts you out to about Mach 2.3 on my references.

The chart from the above link shows a 650 KCAS for Mach 2 at 40,000 feet with a TAS around 1150 (-56 C) which is more like what I remember from the Phantom.

The fastest KCAL I ever saw in a Phantom was 750 and that was headed downhill at 10-15000 feet, not level at 40K.

I'm an old guy and it's been years ago. Maybe global warming has changed the whole KCAS-Mach-KTAS relationship. Maybe knots calibrated are sneaking in the hole in the ozone layer?

Very Happy

OL

(TSCM is a civilian counter measures contractor, I have no idea what these charts were incorporated in, nor do I probably want to know.)

outlaw162 wrote:

Something about the Hochwarth KCAS calculation isn't kosher. Put in 39,000 feet and Mach 2. They compute a KCAS of 770 for Mach 2 at 39 grand. That seems high.
770 Cal at 39K puts you out to about Mach 2.3 on my references.
The chart from the above link shows a 650 KCAS for Mach 2 at 40,000 feet with a TAS around 1150 (-56 C) which is more like what I remember from the Phantom.

Yeah, there's definitely something wrong with the Hochwarth calculator. As you said, Mach 2 at 40k is in fact 652 KCAS and 1147 KTAS.
I played around with it (the Hochwarth thing) a bit....
It looks like true airspeed (TAS) is correct everywhere- all Machs and altitudes.
The calibrated airspeeds (CAS) appear to be good up to Mach 1.0 at all altitudes. But, as you go faster than Mach 1 the CAS numbers drift farther and farther off --- except, interestingly enough, at sea level, where CAS is good everywhere.
Also, interesting is the fact that at Mach 2.0, from 20k-50k altitude, the CAS numbers you get (which are supposedly in KNOTS) agree pretty well with the correct CAS values, IF you expressed them in MPH (makes no sense at all, I know, but they do). Looks like there is a bad switch or logic somewhere in the calculator.

Gums wrote:

Salute!

Sheesh, OL, I wonder how many times the newbie has ever flown at 40K or mach 0.01

Roscoe? Where's Roscoe?

Been on vacation...

Quote:

One of my buddies that flew the sucker had to remind me that once above a certain altitude, that temperature was the driver of mach.

Speed of sound is 100% dependent on temperature, nothing else matters.

That said, as luck would have it I took my "book of all that is Holy and Good" back to work Monday, so I had to dig out my P&W pocket guide...too hard to read accurately but Outlaw has it partially correct. I believe that because the article linked 1.2M and 580 KCAS (emphasis on calibrated) that this occurred at 20,000 feet where these match. Doesn't matter if it's standard day or not if you assume that 20,000 feet was pressure altitude (and in flight test it always is, that is based on an altimeter setting of 29.92 in Hg)).

Flight test airspeed is never measured in true airspeed as it has no relevance, whereas Mach is used a lot (due to compressibility effects) and calibrated is king otherwise (because an airplane will always behave the same measured relative to calibrated regardless of temperature, pressure, etc. because calibrated is essentially a measure of the number of molecules passing over the wing and that is what causes loift, drag, stalls, etc...)

Quote:

Speed of sound is 100% dependent on temperature, nothing else matters.

Only in an ideal gas.

The Earth's atmosphere is a very close approximation.

OL

Close enough that the differences are lost many digits down...

In a nutshell the difference between science & engineering.

Engineers make things work without necessarily having to care what precisely is going on. Scientists know precisely what’s going on, but don’t necessarily care whether anything in particular works.

Scientists provide all the possible reasons why something may not work, and then some engineer makes it work anyway.

Very Happy

OL

Forum: F-35 Lightning II

Good News and Mach 1.2 at Edwards