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User currently offlineWardialer From United States of America, joined Sep 2001, 1201 posts, RR: 0
Posted (14 years 7 months 5 days 21 hours ago) and read 1626 times:

Is the speedometer in a car the same as groundspeed in an aircraft?

6 replies: All unread, jump to last
User currently offlineOPNLguy From , joined Dec 1969, posts, RR:
Reply 1, posted (14 years 7 months 5 days 21 hours ago) and read 1585 times:

>>Is the speedometer in a car the same as groundspeed in an aircraft?

If you're asking if the speed measured by the auto speedometer is the same thing as groundspeed in an aircraft, yep, conceptually they're the same.

Autos are pretty straightforward, but the two main factors that influence aircraft groundspeed (GS) are true airspeed (TAS) and wind component (WC). Take a TAS and subtract a headwind (HW) component, and you'll have a lower GS than you would if there was no wind, or if you had a tailwind (TW) helping you along.

User currently offlineJwenting From Netherlands, joined Apr 2001, 10213 posts, RR: 17
Reply 2, posted (14 years 7 months 5 days 6 hours ago) and read 1536 times:

Adding to that, TAS is IAS compensated for airpressure (IAS decreases as pressure decreases, TAS remains constant).

I wish I were flying
User currently offlineEssentialPowr From United States of America, joined Sep 2000, 1820 posts, RR: 1
Reply 3, posted (14 years 7 months 5 days 6 hours ago) and read 1528 times:


Calibrated - Indicated corrected for installation errors
Equivalent - Calibrated corrected for compressability
True - Calibrated corrected for Temp (not pressure)

User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 8
Reply 4, posted (14 years 7 months 5 days 5 hours ago) and read 1528 times:

Hi Wardialer.

Just to expand on what OPNLguy said. If you have a true airspeed (TAS) through the air of 300 mph, and the wind is calm, then your speed over the ground (Groundspeed) will also be 300 mph. However, if you're flying with a TAS of 300 mph into a headwind of 50 mph, then your Groundspeed (GS) will only be 250 mph. (thus making your flight take longer). Likewise, if you're flying with a TAS of 300 mph, and you have a tailwind of 50 mph, your GS will be 350 mph. (thus making your flight shorter).

The shorter your flight...the less fuel you burn, plus you arrive sooner! That's why pilots LOVE tailwinds  Big thumbs up and HATE headwinds.  Pissed

Chris  Smile

"Just a minute while I re-invent myself"
User currently offlineRalgha From United States of America, joined Nov 1999, 1614 posts, RR: 5
Reply 5, posted (14 years 7 months 5 days 5 hours ago) and read 1516 times:

In answer to the original question...........YES.  Big thumbs up

09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
User currently offlineTAA_Airbus From Australia, joined Nov 1999, 726 posts, RR: 0
Reply 6, posted (14 years 7 months 5 days 1 hour ago) and read 1499 times:

Well GS is actually ETAS+Wind Component.

(Effective true airspeed.)

A wind vector relative to an aircraft has 2 components, HW/TW and XW. To fly straight along your track, naturally, you have to correct into wind. Therefore, reducing the forward velocity of the aircraft, the new forward velocity is what we call ETAS. Then when you add your HW or TW, you get your ground speed, basically the same as what everyone has already said and rarely is there much difference. But its there and it phas to be considered.

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