Forum: Air Force Life

I'm not old enough to remember the days before INS-generated Great Circle Steering for long flights.

For long-haul, over-water type flights, say 10+ hours or more, was the rotation of the Earth taken into account and if so how was it figured into nav calcs to determine what heading or course to steer? And is there code built into the INS software to take Earth's rotation into account?

And if the answer to both is painfully simple, feel free to rub my nose in it. Smile

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Ever hear of dead-reckoning? Wink

Yo!

New guys can help me, but here goes:

a) INS knows earth is rotating. Older ones would 'level' to local vertical and also slave to true north. Later ones simply 'remembered' how much earth had rotated/ plane moved and thereby reduced gyro bearing 'drag' errors induced by torqueing the platform. The new RLG systems don't have ANY 'stabilized' platform.

b) Planes fly thru the airmass as well as inertial space. So wind was a bigger player than earth rotation. Up/down near the poles, we used a directional gyro and paid no attention to the normal compass - the 'grid' system.

As one flew along, heading had to be constantly changed to fly the planned great circle. Using nav stations and stellar fixes, we could correct the course to agree with what we had planned/wanted.

later

I don't think great circle and dead reckoning are, umm, 'compatible'... Wink

I'm curious. Could an airliner or cargo plane (in theory) fly a straight flightpath from say, JFK to Heathrow? I know they use "Great Circle" to save gas, and cut the trip distance, but how much more gas/time would it take to takeoff, and fly straight over the pond? Confused

Also, when the Blackbird made the record flight from (was it DC or NY) to Farnborough, what flightpath did it take? I'm sure when you're flying Mach 3+ at 80K AGL, that fuel/time, etc becomes less a factor than when you are in a 747. Wink

Well, if you don't use an autopilot to hold a magnetic 'heading', point at your destination AND keep the wings perfectly level as you bomb along, you will fly something close to a great circle.

Think about it........get out your basketball and put your tiny airplane on it and push it to the other side of the ball (read 'world'). Your heading with respect to the 'pole' will change all the way along, but you haven't turned a degree!

Flying a rhumbline, you can only get someplace without changing compass heading if that place is due east/west (same latitude) or due north/south. Otherwise you have to keep changing your heading with respect to the pole. In short, the 'great circle' IS THE STRAIGHT FLIGHTPATH. It only looks like a circle when on a Mercator-type projection.

TRIVIA ITEM 327: The A-7D/E computer used rhumblines within 32 miles ('flat earth' assumption).

tired, brain mushy, need a drink

OH YEAH!...Because when you fly due North/South, you wouldn't fly straight into your projected target (read, airport) because of the rotation of the earth. Think I've got it now. My brain is mushy as well. It's time for a few Silver Bullets. Cool

That's why I love sectionals, they're on a Lambert conical projection, so a straight line drawn on them is the closest to a great circle of any other flat projection.

Habu wrote:

That's why I love sectionals, they're on a Lambert conical projection, so a straight line drawn on them is the closest to a great circle of any other flat projection.

Only if you're flying along the tangent of the projection (or perpendicular to it)

Forum: Air Force Life

Question for the old navigators and star-shooters