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Thread: Question about Airspeed

  1. #1

    Default Question about Airspeed

    In the aircraft handbook on the 737 in FS2002, it says that the cruising speed is over 400 knots. But when I fly it, I can't go faster than about 345 knots. Why is this? The 747 has a similar error as well, though I'm not sure about the 777. Please help me if you know how to solve this? :-hmmm

    lasindi :-wave

  2. #2
    KBUR Guest

    Default RE: Question about Airspeed

    I believe they're talking about GROUNDSPEED, the speed your aircraft is actually moving across the earth, rather than airspeed. You can monitor your current ground speed on the GPS provided with FS and a few other places.

    Kyle
    KBUR

  3. #3

    Default RE: Question about Airspeed

    [font size="1" color="#FF0000"]LAST EDITED ON Jan-05-02 AT 02:28PM (EDT)[/font][p]Actually they're not talking about groundspeed, it's the true airspeed (TAS) that they're referencing. Up at say, FL330 the indicated airspeed is maybe 250 kts indicated at .74 mach (I'm generalizing the numbers here), but the TAS with be up around 440-450 kts. 345 kts indicated is actually just over redline or barber pole at lower altitudes; watch the barber pole speed drop on the a/s indicatore as you climb... but the TAS is still way up there. There's no problem with your planes or computer setup, it's normal.

    John
    B737 FO


  4. Default RE: Question about Airspeed

    Or go to the Help/Simulator Help/Glossary
    Look up indicated airspeed IAS and
    true ground speed TAS
    They're defined without going into
    a long scientific explaination of
    air density altitude and temperature.

    Here's what you can try to give you an idea
    of the effects of altitude. As John and Kyle pointed
    out, look for TAS with your GPS displayed.

    Fly the C182 up to 5000'.
    As you climb, watch the IAS compared to the GPS speed.
    You can see the effects of altitude on ground speed.

    Now for temperature.
    Keep flying at 5000'
    Go to local weather and enter 75 degrees as a temperature setting.
    Save the setting by pushing OK.
    Check out your IAS. To confirm the setting your
    OAT (outside temperature)
    should read something close to what you set in.
    You'll be lucky to hit 120kts IAS.
    Now go back and set the temperature to 0 (zero degrees F).
    Save that by hitting OK again. Notice how how much
    more speed you can get out of the C182.
    Happy flying.
    :-wave
    Fred
    http://www.flight-sim.net

  5. #5

    Default RE: Question about Airspeed

    You believe the claims for top speed in an airplane about as much as you believe the sticker on a new car that tells you gas mileage.
    Assuming Microsoft tweaked the models to reflect real life vs what the ad writers put down I would expect the airspeed to be lower. Remember, the wamer the air the less dense so you can go faster.
    If you look at the brochure for a brand new Cessna Skyhawk or Skylane you will see some impressive figures. Cessna used to test for airspeed on a brand new plane, with antennas removed, openings taped over and the CG moved as far back as legally allowed. This would wring out a few more knots. If you believe book figures for performance you are gonna get a rude awakening some day.

  6. #6
    Join Date
    Mar 2005
    Location
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    Default RE: Question about Airspeed

    [font size="1" color="#FF0000"]LAST EDITED ON Jan-06-02 AT 03:20AM (EDT)[/font][p] lasindi,

    (1) Start FS2002.
    (2) Select or Create any flight of your choice.
    (3) Select AIRCRAFT from Tool Bar at top of screen. (press alt/enter to display tool bar).
    (4) Select Realism setting.
    (5) Select True Airspeed, not indicated Airspeed.

    This will give You What your looking for! Not trying to offend any one, but it sounds like all 4 pilots who responded to your question, may also want to try this?
    Let me know if this helped.

    Randy Burton

  7. #7

    Default RE: Question about Airspeed

    > (5) Select True Airspeed,
    >not indicated Airspeed.
    >

    Selecting the airspeed indicator to reflect true airspeed in FS is possible, and maybe many fly it that way, but it's not adviseable or realistic. In real life there's no way we'd want to fly with TAS on the a/s guage. For the wing, speed is life, and that speed is indicated (or calibrated, but that's too much for my brain). All the a/s indicator knows is what air pressure is flowing thru the pitot tube and over the wings; the wings don't care about density altitude, temperature, things like that, it just knows that at a certain angle of attack and weight it needs x amount of breeze blowing over it to produce lift. Indicated a/s will show it.

    With TAS shown you could end up surprising yourself with an unexpected stall while approaching to land. Try landing at Telluride, CO (KTEX)for example, 9100 ft fld elevation in a 182. At normal approach speed might be 70 kts or so indicated, but at that high altitude the TAS will be much higher. So if you come in trying to hold 70 kts on the guage while it shows TAS, it's gonna stall and drop out from under you. Not a good idea.

    John


  8. Default RE: Question about Airspeed

    flyinggriffin,
    Are you sure you are correct with this? I'm not saying your wrong, I could be confused as well.
    I see it this way. The aircraft is powered and pulled along
    through the air with the propeller. Thus, the denser the air
    the better its ability to do so. I realize cold dense air
    inhibits the plane's ability to fly through the air is
    a factor. The fact the propeller can get a better bite in dense air seems to outweigh the fact that denser air offers more resistance to fly through it.

    Please, your inputs mean a lot, and I'm not saying you're wrong. I'd like to see a facts sheet one this sometime. I could be a bit confused as well.

    > Assuming Microsoft tweaked
    >the models to reflect real
    >life vs what the ad
    >writers put down I would
    >expect the airspeed to be
    >lower. Remember, the wamer the
    >air the less dense so
    >you can go faster.

    This is what I'm eluding too, BTW.
    :-wave
    Fred
    http://www.flight-sim.net

  9. #9
    dettling2 Guest

    Default RE: Question about Airspeed

    The answer is both simple, and complicated, lasandi. Let me try to explain it to you. First, you have to understand the distinction between indicated and true airspeed.

    As any aircraft climbs, the air's density decreases. The pitot tube is the mechanism that is used to measure deflection of a pressure diaphram, and transfer that into a reading on your airspeed indicator. The airspeed indicater is calibrated for standard atmosphere and pressure — 59 degrees F and 29.92 inches of mercury. The pitot will have a lesser deflection, as altitude increases. The result is a lower than actual reading of airspeed, called "indicated" airspeed.

    True airspeed is the actual speed through the air — in other words, it is the indicated airspeed corrected for altitude. The conversion factor is about 1.75 % increase in indicated airspeed for each 1000 feet above sea level. Most aircraft have an airspeed indicator that has a small window in the lower left quadrant. There, the needle will point at the indicated airspeed and at the corrected value of true airspeed simultaneously. You can set Microsoft Flight Simulator to read either true or indicated airspeed.

    Now to answer your original question. Jet transports carry massive amounts of fuel, and that fuel is typically stored in large tanks at or near the wing root. If you look carefully at most transports, you can see the very thick wings they have, especially at the the root. Well, thick wings equals low maximum allowable indicated airspeeds. In contrast, supersonic fighters have thin wings, for high allowable indicated airspeeds.

    So, you must climb jet transports up high, to 25,000 to 35,000 feet, to find an altitude where the air is very thin. This gives you the atmospheric conditions that will permit a high true airspeed and optimum fuel efficiency, combined with an indicated airspeed that is at or near the barber pole. So, using your 345 knot maximum indicated airspeed, and our 1.75 % per 1000 feet of altitude, the approximate true airspeed at 30,000 feet would be 345 plus (30 x 1.75 %). If you do the math, it is 345 plus 52.5 percent of 345. That equals 345knots + 181.125 knots, or 526.125 knots.

    But there is one more very important consideration to make. At high speeds and altitudes, you are dealing with the compression of air in front of the airplane as it approaches the speed of sound, commonly called Mach 1. The 737 has a maximum allowable Mach number of, I think, .72 Mach. So, you have to throttle back somewhat in cruise, so as not to exceed that mach number— regardless of true airspeed. So the limiting factor at altitude is Mach number, and neither true nor indicated airspeed. If you exceed this "crritical mach", you will get the same sort of overspeed warnings as you would if you exceed 345 knots near the ground. Believe me, you don't want to overspeed a transport. You could induce flutter, and literally start the airplane disintegrating. NASTY!!!!!

    I hope this explains it to you. If it doesn"t, I'd recommend you get a good general book on flight training. Then you can read up on this topic at your leisure.

    Happy landings! -dettling2

  10. #10
    Join Date
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    Default RE: Question about Airspeed

    MMO for 737 is .82. Normal cruise speed is .74, 273 KIAS at 37,000'.

    -E

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