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Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

latimes.com

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Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#111

Post by willyt »

bamboozled wrote: post #76

That is 1289.085 kilometres per hour! The title should be updated on to include a measurement used by the rest of the world including the scientific community.

If you want to get pedantic then you and the article are both wrong! Navigation, marine and aeronautical, is done in Nautical Miles, called Knots when used to measure speed. 1NM used to be exactly 1 minute of latitude but is now defined as very closely approximate to this as earth is now understood to be not a perfect geoid. There are 60 Nautical Miles to a degree of latitude. This allows distances and courses to be measured and plotted easily on charts which are Mercator projections.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#112

Post by tjohns »

cmurf wrote: post #87
kgermino wrote: post #6

This is probably a stupid question, but how is "ground speed" measured? Is it how quickly the airplane moves past a fixed point at it's altitude, or along the earth's surface? I.e. the plane is effectively moving along the surface of a sphere, but if the plane is 30,000 ft up, then the radius of the plane's sphere is 30,000 ft greater than the radius of the "ground" so traveling at the same angular velocity will requ…

In the old days, you'd take the time between two "fixes". A fix could be a visual reference, or a NAVAID. That'd result in your shadow's speed. Then came DME, distance measuring equipment. I don't know if DME ever took altitude into account or if the error resulting from altitude was just ignored, but analog DME radios would just tell distance at first and then digital versions had a groundspeed feature. At least ins…

DME equipment explicitly does not take altitude into account.

It's colloquially called "slant range" for this reason, and if you care about actual ground distance you have to use trigonometry.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#113

Post by jhayward »

snowwrestler wrote: post #66
btian wrote: post #27

Earlier quoted context omitted.

It's measured by repeatedly taking GPS coordinates divided by time.

GPS measurements are comprised of a 2D surface coordinate system (e.g. lat/long) and an altitude above mean sea level. So, "speed" as measured by GPS is speed along the Earth's surface. EDIT: assuming a constant altitude flight

No, GPS measurements are taken as receiver distance from orbiting satellites whose orbits are calculated in a 3-D+t inertial reference frame (ECI), which is then converted to an earth-centered-earth-fixed (ECEF) coordinate. From there an end user devices can transform to desired Datum reference (a model of the Earth's surface) depending on the application. Everything about GPS is inherently 3D+t.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#114

Post by kevin_b_er »

bamboozled wrote: post #76

That is 1289.085 kilometres per hour! The title should be updated on to include a measurement used by the rest of the world including the scientific community.

The aircraft is rated in knots equivalent air speed. Everywhere but China and Russia tend to call out speeds in knots. If anything, it should be in knots. So, about 696 knots.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#115

Post by photojosh »

bamboozled wrote: post #97
photojosh wrote: post #90

Earlier quoted context omitted.

If you're being pedantic about science-related matters, it's 1290 km/h. Don't give false precision with your conversion; there's no way the original measurement has >3 significant figures. Even better yet, 361 m/s. km/h is a derived unit. ;)

You’re right I did just paste that figure from Google conversions. Using the metric system over the imperial system isn’t being pedantic, it’s just courtesy in 2019

It's from a USA-based newspaper. I hate imperial with a passion, but it is still their backarsed standard. Surely anyone with a scientific bent can do a rough calculation in their head. For me, it was "oh, around 1300 km/h, cool" and moved on...

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#116

Post by ahartmetz »

bartread wrote: post #95
FabHK wrote: post #50

Earlier quoted context omitted.

Wind is "named" after where it comes from. If you fly north, and the wind is a north wind, you fly slower. (This terminology is in line with what GP said.) Then, ground speed = air speed - wind speed. Thus, GP seems to have either the sign wrong, or ground and air speed mixed up (but not both :-)

> Wind is "named" after where it comes from. This is absolutely true, but has always infuriated me because it seems needlessly contrary as compared to the way we speak about anything else travelling in a direction as read from a compass. Does anybody know why it's the case?

Probably because it matters where the air was before. Stuff like: north wind is could, south wind is warm, west wind is humid, etc

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#118

Post by aidenn0 »

astrodust wrote: post #93
aidenn0 wrote: post #83

Earlier quoted context omitted.

When cruising, in level flight by definition, 100% of the engine power is there to combat drag, right? That is if the thrust vector of the engine is 100% level and the airspeed is constant, then 100% of the engine power applied is offset by drag.

Some of that thrust has to be used to generate lift.

If the engines are level, then none of the thrust is generating lift; the wings generate the lift (and with it some corresponding drag).

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#119

Post by markab21 »

theothermkn wrote: post #103
markab21 wrote: post #64

Earlier quoted context omitted.

When I was getting my pilots license I stumbled into a really cool video NASA did showing exactly what happens when you reach a fluttering condition like what you described. This is a link to the NASA recording if anyone is interested. https://www.youtube.com/watch?v=pEOmCkZyXzk https://www.youtube.com/watch?v=OhwLojNerMU (Older with music, kinda funny old-school science video from NASA) Edit: formatting.

In practical terms, flutter due to approaching transonic conditions is probably pretty rare. That kind of flutter is (mostly or most often) due to the interaction of normal shocks with the control surfaces, or coupling with an elastic mode of the wing or tail surface. The flutter in the first video is occurring at very much subsonic speeds, and looks to be either the result of flying a purposely underdesigned tail su…

Interesting.

Do you have any thoughts on composite-based aircraft like a Cirrus and flutter? A few times I've had a Cirrus SR22 into a pretty steep descent with poor controller sequencing for an approach into busy terminal space and had to push it down, but the plane felt solid even at 180-190kts TAS. I backed it off only because I get nervous with any unexpected turbulence which is not uncommon in Florida.

The Piper Saratoga I flew for a bit didn't seem to like the speed as much, that or the toga was a bit more vocal than the Cirrus in what it was feeling with regards to airspeed.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#120

Post by aidenn0 »

ars wrote: post #91
aidenn0 wrote: post #83

Earlier quoted context omitted.

When cruising, in level flight by definition, 100% of the engine power is there to combat drag, right? That is if the thrust vector of the engine is 100% level and the airspeed is constant, then 100% of the engine power applied is offset by drag.

Depends on how you define drag. If you define it as friction due to air against the plane then no, because a lot of the drag is due to sending air downward to keep the airplane up. The distinction is that, that that part of the drag is impossible to reduce, while friction you can work to reduce. That's what the GP meant by "discounting lift".

There are two types of drag, parasitic drag and lift-induced drag, but they are both drag.
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