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gmcjetpilot
Joined: 04 Nov 2006
Posts: 170
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 Posted: Fri Feb 15, 2008 6:05 am Post subject: Boeing 767 speeds (was External antennas) |
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Mike:
Not sure what your talking about, but I wrote or
should have wrote 460kt (true).
I fly both the 757/767.
I never do more than 250kias below FL100.
We usually climb at about 280-320 kias to FL250.
Than we go to Mach above approx FL270.
Normal econ cruise is about mach 0.82.
Mach 0.82 = about 280 kias, 450 ktas (at) FL350.
Indicated a/s depends on altitude.
Red line, Vmo/Mmo is a moving target.
Max Vmo/Mmo is no longer a Boeing Limit
to memorize, because it moves around and
its always displayed on both the mechanical
airspeed indicator and the speed tape on the
EFIS. Off the top of my head (at) sea level its
approx 350 kias/M.86 range for the B757. The
limit is the lower of the two limits, Vmo/Mmo.
Kts are for lower altitudes and Mach for higher
altitudes above FL270. Of course we don't do
350 kts on the deck, but it would be fun.
During a pilot oral exam there is no Vmo/Mmo
to remember. You don't have to memorize stuff
that is right in front of you. Good thing, less to
memorize. Other speeds are in the checklist.
My point is the antenna does not care what size
plane it is on. Drag is drag.
Think about it, Mike
Cheers George ATP/757/757/CE500
>From: "Mike" <mlas(at)cox.net (mlas(at)cox.net)>
| Quote: | Subject: Re: External antennas on composite plane
>460 kts TAS not indicated. The indicated speed of a
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| Quote: | 757 or 767 is 360kias.
Think about it, Mike
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Be a better friend, newshound, and [quote][b]
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mlas(at)cox.net
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| Posted: Fri Feb 15, 2008 6:57 am Post subject: Boeing 767 speeds (was External antennas) |
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George,
I only pointed it out because I assumed that you were trying to illustrate the speed effect of a higher speed aircraft on an antenna and its relationship to drag/speed. You wrote 460 knots. I was just trying to point out that as it relates to the antenna drag, you use indicated speed not True Airspeed in that speed range (not true for high mach numbers). I then pointed out that the 757 only does 350’ish Knots Indicated.
Mike
The dark side (Airbus)
--
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echristley(at)nc.rr.com
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| Posted: Fri Feb 15, 2008 7:11 am Post subject: Boeing 767 speeds (was External antennas) |
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gmcjetpilot(at)yahoo.com wrote:
| Quote: |
My point is the antenna does not care what size
plane it is on. Drag is drag.
Drag is drag, and planes are planes; but the relative importance of a
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quantity of drag varies from plane to plane.
A Cozy or Quickie can do 150mph to 200mph with 100Hp or less, because
everything imaginable has been done to drop the drag. The drag of the
antennae is no more than it would be on the same 90mph Pietenpol, but it
is a much more significant portion of the entire airplane's drag.
You can't take an antennae and say that it will have a specific speed
penalty. You take the antennae and say how much power will be required
to push it through the air at a specific speed. You have a power budget
set by the engine. Any of that budget spent pushing antennaes through
the air will not be available for pushing the airframe through the air.
It is up to the builder to decide if spending a piece of that limited
budget is worth the effort or degraded performance associated with not
spending it.
A 757 has a rather large power budget (compared to a Quickie). Sticking
another antennae on is unlikely to even register, no matter what speed
the airplane is going. I just isn't worth much effort to try to get it
out of the airstream. Other configurations will differ.
In my case, I know that I tend to be a heavy-handed klutz. No matter
how hard I try, I know I'll break off an antennae every time I was the
plane. My push to clean up my airframe has more to do with what I know
will be a maintenance headache.
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Gilles.Thesee(at)ac-greno
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| Posted: Fri Feb 15, 2008 6:58 pm Post subject: Boeing 767 speeds (was External antennas) |
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Ernest Christley a écrit :
| Quote: | Drag is drag, and planes are planes; but the relative importance of a
quantity of drag varies from plane to plane.
A Cozy or Quickie can do 150mph to 200mph with 100Hp or less, because
everything imaginable has been done to drop the drag. The drag of the
antennae is no more than it would be on the same 90mph Pietenpol, but
it is a much more significant portion of the entire airplane's drag.
You can't take an antennae and say that it will have a specific speed
penalty. You take the antennae and say how much power will be
required to push it through the air at a specific speed. You have a
power budget set by the engine. Any of that budget spent pushing
antennaes through the air will not be available for pushing the
airframe through the air. It is up to the builder to decide if
spending a piece of that limited budget is worth the effort or
degraded performance associated with not spending it.
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Ernest, Matt,
Couldn't agree more with you. I built a low drag airplane (MCR four
seater), and indeed, hiding the antennas made quite a difference. The
present cruise is 130-1135 kt on 100 hp with four on board at low
altitude. We still have some cleaning to perform on the airframe.
I remember having a long discussion with George on this issue some
months ago.
Best regards,
--
Gilles
http://contrails.free.fr
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Gilles.Thesee(at)ac-greno
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echristley(at)nc.rr.com
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gmcjetpilot
Joined: 04 Nov 2006
Posts: 170
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| Posted: Sat Feb 16, 2008 8:48 am Post subject: Boeing 767 speeds (was External antennas) |
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| Quote: | Re: Re: Boeing 767 speeds (was External antennas)
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Matt and Mike, you are both right, thanks for keeping
me honest.
Where I'm are coming from is the KISS principle, antenna
drag is small even on a fast homebuilt, relevant to the
effort and the performance, especially on a metal plane.
Also there's nothing like a good external 1/4 wave dipole.
Fiberglass plane? Have at it. Personally when I "Race", I
take the Com antenna off and connect it to a temp internal
antenna in the cockpit. When I had a VOR antenna, I
took that off as well. So I can be picky about a few drag
clicks, but for daily flying, not so much. 1/4th of a mph
is not going to make much of a difference. Also running
extra coax around, cost weight, money and radio perf.
Cheers, thanks guys, fly safe.
[quote][b]
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