Ray's Estimate of Concorde's Descent Time

[rayjohn01]

Does anybody have a reasonable estimate of the following, If the Concorde was flying at 6000 ft at velocity of sound ,and it's engines cut out , how long would it take to reach the ground doing the best it could to delay the inevitable ? I've seen answers ranging from <1min to > 7 mins .
Ray.

 

[russ_watters]

According to http://aerodyn.org/HighLift/ld-tables.html site, the Concorde has a glide ratio of 8 (not all that good), meaning for every 8 feet forward it must go down 1 foot. 8 x 6,000 is 9 miles. At 300 mph (guess), that's 1 minute, 48 seconds. Starting at mach 1 will add a some to the distance, an extra mile or two, but with no propulsion, it will quickly drop to its glide speed and not add much to the time. Figure about 2 minutes total.

 

[Janitor]

Since drag forces typically go up as nonlinear function of speed (and I mean more than in direct proportion to speed), the pilot's best tactic would likely be to trade that supersonic speed for altitude by pulling back on the yoke, and then glide back down at a speed not too much above stall speed. My intuition says maybe four minutes.

Anybody else ever see Bob Hoover flying that twin-engine Shrike at an airshow, turning the engines off for fast, quiet, low passes in front of the audience?

 

[rayjohn01]

russ and Jan-

Thanks guys you can see how the guestimates differ, ( the 7 min answer seemed to assume some swap of speed with height based on energy, but it neglected the drag losses.
I visited a 'flying site' which talked of glide ratios etc etc for several hundred posts -- and not one mentioned TIME -- even the 'emergency procedures' omitted time , and yet this seems crucial in an emergency. If I was flying I would sure like to know whether it was 2 or 7 minutes.
I tried a caculation based on drag proportional to speed ( conservative I think) to reduce speed to stall at the same height, and then free fall.
to do this I assumed that free fall in a nose down attitude could reach 1000 ft/sec to define the drag force, I could not get much over 80 seconds total.
Ray

 

[russ_watters]

I visited a 'flying site' which talked of glide ratios etc etc for several hundred posts -- and not one mentioned TIME -- even the 'emergency procedures' omitted time , and yet this seems crucial in an emergency. If I was flying I would sure like to know whether it was 2 or 7 minutes.

In an emergency, distance, not time is the critical factor. A pilot is well-trained and decisive: he doesn't need time to think about his decisions. He already knows what to do.

and then glide back down at a speed not too much above stall speed.

You were good up until there. As you approach stall speed, drag goes up much faster than lift. As you can see from http://www.allstar.fiu.edu/aero/lift_drag.htm graph, the best L/D ratio is at relatively low angle of attack (0 in that case), which means speed well above stall speed (something like 50%). A jet that stalls at 200kts (with no flaps) would glide at around 300. Also, flaps would probably not improve the ratio (maybe just a knotch or two of flaps) - they provide more lift, but a lot more drag.