Settle a bet: Mach speeds

"Jeff Crowell" wrote:
Mike Marron wrote:

Oops!

The above question should read, "...turn around and fly back to
airport "A" now with a 20 mph tailwind at the same TAS."

In other words, if you takeoff from airport "A" and fly 200 miles into
a 20 mph headwind to airport "B" at 100 mph (TAS) and then turn around
and fly back to airport "A" now with a 20 mph tailwind at the same TAS
will you complete the roundtrip flight in the same time as if you had
made the flight in no-wind conditions?

Okay, I'll bite.

Looks like the no-wind solution is 4 hours, the headwind/tailwind
solution is 4.17 hours.

Just in case I win, I prefer Lagavulin 16. I'll skip the seegar.

Al Gerharter was the first to answer correctly, but disqualified
himself by admitting that he's an old hand instructor so it looks
like you won indeed. Congratulations! A large bucket of Lagavulin
16 is on the way...

In other words, if you takeoff from airport "A" and fly 200 miles into
a 20 mph headwind to airport "B" at 100 mph (TAS) and then turn around
and fly back to airport "A" now with a 20 mph tailwind at the same TAS
will you complete the roundtrip flight in the same time as if you had
made the flight in no-wind conditions?

Okay, I'll bite.

Looks like the no-wind solution is 4 hours, the headwind/tailwind
solution is 4.17 hours.


Because you spend more time going against the wind and fighting it, than you
spend with the wind and benefitting from it, so thats why you lose time




Ron
Tanker 65, C-54E (DC-4)
Silver City Tanker Base

(Ron) wrote:

Because you spend more time going against the wind and fighting it, than you
spend with the wind and benefitting from it, so thats why you lose time

Amazing how many folks fail to grasp this concept.

Instead of forever battling headwinds, one of these days I'm going
to get in and burn an entire tank of gas or two while intentionally
flying WITH the wind and see where the tailwinds take me....

"Mike Marron" wrote in message
...
(OXMORON1) wrote:
Walt wrote:

BTW, on the deck 760 mph is close to .1M (STP), so when
your car is showing 76 mph, you're buzzing along at about .1M. (For
interstate travel, 75 mph is 12.5 miles in 10 minutes . . . we just
completed about 6000 miles of driving where I used this sort of mental
T=D/R to predict ETAs thus boggling my better half. Same sort of calcs
can be used ina light plane such as C152 . . . 6gph = 1 gal in 10 min,
90KTS = 1.5 nm/min; 120(tailwind)=2 nm/min. As above, use the
calculator if things look tight. (If they do you've screwed up.)

Glad I am not the only one running continuous ETAS while cruising down
the
Interstate :-) Drift gets to be a bitch though!
Habit! Ingrained by instructors who are really old farts now!

Speaking of winds and such, as an "old fart" flight instructor myself
(it's all downhill after 40, right?) here is a little homemade
question that I find many aviators both military and civil routinely
get wrong:

If you takeoff from airport "A" and fly 200 miles into a 20 mph
headwind to airport "B" at 100 mph (TAS) and then turn around and
fly back to airport "B" now with a 20 mph tailwind at the same TAS
will you complete the roundtrip flight in the same time as if you had
made the flight in no-wind conditions?

Yea or nay?

(Correct answer gets a cigar and large bucket of his favorite
drink....)

No.
Pepsi please.

On Sat, 15 May 2004 21:25:59 -0500, "tscottme"
wrote:

I ask because I've heard NASA folks mentioning that the Shuttle, at some
point in it's descent, is going Mach 25. I claim that means the vehicle
is going 25 times faster than Mach at that particular pressure and temp
where the Shuttle is, while someone else claims NASA means 25 times the
sea level value of Mach, even though the Shuttle is in the very upper
atmosphere at the time.

[I'm sorry to be so late in replying, but life got weird and it took
me a little while to get back to this. MFS]

It's a definition thing. The Orbiter goes Mach 25, by definition,
until the dynamic pressure gets high enough to be sensed, which is at
(surprise!) Mach 25. If you use the real dynamic pressure, which is
zero, the Mach number goes to infinity, so we set it to Mach 25 by
definition to avoid that.

It isn't sea level Mach number for that airspeed. No one uses
"equivalent Mach number". That's because the aerodynamics and
aerothermodynamics are functions of Mach number, so it has to be
local.

This means you're right and someone else is wrong.

Perhaps that someone else is confused by equivalent airspeed, usually
presented by PAO as miles per hour. That is the sea-level airspeed
that the airplane would have to fly at to have the same Mach number at
sea level that it has at altitude.

I don't have my calculator or my standard atmosphere here, so the
numbers I'm about to give you will be only approximate. Flying Mach 3
at 80,000 ft is an indicated airspeed of about 300 knots (300 KIAS).
At sea level, the equivalent airspeed would be about 3,000 fps, which
you'll have to convert to knots (this will be KEAS).

I know to discount what reporters say, but I'm referring to engineers
and their official spokesmen.

You can check this in "Space Shuttle Hypersonic Aerodynamic and
Aerothermodynamic Flight Research and the Comparison to Ground Test
Results", Kenneth W. Iliff and Mary F. Shafer, NASA-TM-4499, June,
1993. If you go to http://www.dfrc.nasa.gov/DTRS/1993/Bib/H-1894.html
there's a link to a pdf version.

One of the early references explains the air data system, including
this Mach number definition.

Mary

--
Mary Shafer Retired aerospace research engineer

On Sun, 06 Jun 2004 20:42:51 -0700, Mary Shafer
wrote:

At sea level, the equivalent airspeed would be about 3,000 fps, which
you'll have to convert to knots (this will be KEAS).

I make 3,000 fps to be 2.758 knots....


all the best -- Dan Ford
email: (put Cubdriver in subject line)

The Warbird's Forum www.warbirdforum.com
The Piper Cub Forum www.pipercubforum.com

"Cub Driver" wrote in message
...
On Sun, 06 Jun 2004 20:42:51 -0700, Mary Shafer
wrote:

At sea level, the equivalent airspeed would be about 3,000 fps, which
you'll have to convert to knots (this will be KEAS).

I make 3,000 fps to be 2.758 knots....

Did I miss something, like a really strange joke or
that you're calculating some (correction?) factor at
3000 fps and want the answer in knots?
Did you really determine 3000 feet per second to
be less than 3 knots? Knots as in nautical miles per hour?
Or perhaps it was a type and you meant 2758 knots?
2578 knots would still be way off, but it would at least
be of the right magnitude;
3000 fps = 2045.4545... mph ~= 1777 1/2 knots.

Got to be the correction factor...

Perhaps my calculator left off some zeros.

On Tue, 8 Jun 2004 03:17:26 -0400, "John Keeney"
wrote:


"Cub Driver" wrote in message
.. .
On Sun, 06 Jun 2004 20:42:51 -0700, Mary Shafer
wrote:

At sea level, the equivalent airspeed would be about 3,000 fps, which
you'll have to convert to knots (this will be KEAS).

I make 3,000 fps to be 2.758 knots....

Did I miss something, like a really strange joke or
that you're calculating some (correction?) factor at
3000 fps and want the answer in knots?
Did you really determine 3000 feet per second to
be less than 3 knots? Knots as in nautical miles per hour?
Or perhaps it was a type and you meant 2758 knots?
2578 knots would still be way off, but it would at least
be of the right magnitude;
3000 fps = 2045.4545... mph ~= 1777 1/2 knots.

Got to be the correction factor...


all the best -- Dan Ford
email: (put Cubdriver in subject line)

The Warbird's Forum www.warbirdforum.com
The Piper Cub Forum www.pipercubforum.com