INNflyer
Topic Author
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What Is The Max Speed In Ground Proximity?

Fri Sep 11, 2009 8:57 pm

When an aircraft takes off, it is obviously trying to build up speed and altitude, while it needs to decrease both when preparing to land. If an aircraft (commercial airliner, not military jet) would fly in 1,000 feet altitude with full engine power (as if it is traveling in 33,000 feet altitude), which would be the maximum speed ? Which component is more important- air flow resistance or physical stability of the aircraft?
 
Soku39
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RE: What Is The Max Speed In Ground Proximity?

Fri Sep 11, 2009 9:24 pm

You are opening a can of worms, but here is my attempt at an answer (it may be incorrect, someone correct me if I am wrong)

The max speed any aircraft can fly is where the power available curve and its drag curve cross.

see power required curve (the power required for an aircraft to maintain level flight, and hold up its weight)

http://www.auf.asn.au/groundschool/umodule1b.html

now on this graph look for parasite drag-this is drag that increases with airspeed, where as induced drag (drag as a result of lift) decreases with airspeed.

http://en.wikipedia.org/wiki/File:Drag.jpg


Now what you must do is overlay the power required curve on top of the parasite drag curve, and where they cross (on the right side, ie your high airspeed, whereas the left side of the curve is lower airspeed) is the answer to how fast the airplane can fly.

The thing to remember is that the curves can shift up and down as well as left and right given different temperatures, pressures (think altitude) and other atmospheric conditions. In other words the answer will never be the same in day to day use because the atmosphere rarely presents the exact same conditions day to day, thus the reason calculations are done using 29.92 and 15 degrees C, then adjusted accordingly before flight using the present conditions.
The Ohio Player
 
Soku39
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Joined: Fri Nov 03, 2000 7:16 am

RE: What Is The Max Speed In Ground Proximity?

Fri Sep 11, 2009 9:27 pm

Also proximity to the ground does not matter unless you enter what is called "ground effect" and 1000' AGL is too high for this to matter.

Read about ground effect here:

http://en.wikipedia.org/wiki/Ground_effect_in_aircraft

This is a very complex discussion that I attempted to simplify, so if there is something you don't understand don't be afraid to ask for clarification or further explanation.

[Edited 2009-09-11 14:34:04]
The Ohio Player
 
T prop
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RE: What Is The Max Speed In Ground Proximity?

Fri Sep 11, 2009 9:52 pm



Quoting INNflyer (Thread starter):
If an aircraft (commercial airliner, not military jet) would fly in 1,000 feet altitude with full engine power (as if it is traveling in 33,000 feet altitude), which would be the maximum speed ?

Max thrust @ 1000 ft? Who knows, the airplane would probably come apart... Max speed is just before that happens.
 
tdscanuck
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RE: What Is The Max Speed In Ground Proximity?

Sat Sep 12, 2009 12:51 am



Quoting INNflyer (Thread starter):
If an aircraft (commercial airliner, not military jet) would fly in 1,000 feet altitude with full engine power (as if it is traveling in 33,000 feet altitude), which would be the maximum speed ?

Somewhere above Vd. At that altitude, you're structurally limited. There's more than enough power to go right through Vd and eventually fail something. You'd still be accelerating when you broke up, so the "maximum speed" would be hard to pin down.

Quoting INNflyer (Thread starter):
Which component is more important- air flow resistance or physical stability of the aircraft?

At low altitude, "air flow resistance"...you've got structural limits due to aerodynamic load. At high altitude, you pass through the Mach limits long before you hit structural limits.

Tom.
 
71Zulu
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RE: What Is The Max Speed In Ground Proximity?

Sat Sep 12, 2009 12:58 am



Quoting INNflyer (Thread starter):
If an aircraft (commercial airliner, not military jet) would fly in 1,000 feet altitude with full engine power (as if it is traveling in 33,000 feet altitude), which would be the maximum speed ?

The airliner would easily exceed the max speed allowed (barber pole on the air speed indicator).

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There was a post on here a while back about an air show high speed low pass in a DC9 and I think it was traveling at 335 knots close to the ground and the pilot actually to push forward slightly to hold it down, as the the ground effect was causing it to climb.
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Fly2HMO
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RE: What Is The Max Speed In Ground Proximity?

Sun Sep 13, 2009 3:19 am



Quoting 71Zulu (Reply 5):
as the the ground effect was causing it to climb.

That's true of any airplane in ground effect.
 
FredT
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RE: What Is The Max Speed In Ground Proximity?

Sun Sep 13, 2009 12:45 pm

If you are above your trim speed, you will require stick forward to continue flying level. Ground effect, no ground effect - no difference.

The trim speed will be likely be somewhat lower in ground effect though.
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