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Glide ratio

Old 14th Apr 2014, 03:30
  #21 (permalink)  
 
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When flying the Classic 747, one must adjust idle power descent point depending on TOD weight if planning a min fuel burn descent without ATC interference. The heavier the weight, the longer the glide, and the lower the weight, the shorter the glide. The difference in a no wind situation glide between landing weights of 630,000# and 380,000# could be ~25-30 NM.
Explain please.

All glider pilots know that best no-wind gliding range is achieved at best L/D and the range is indipendent of weight. Counter to intuition, the heaver aircraft will glide the same distance, but it will do so at a higher speed. That's why racing sailplanes load up with water ballast. They can glide the same distance at higher speeds.

It's not immediately obvious to me why a Classic 747, unrestricted by ATC considerations, would obey a different set of aerodynamic laws. Perhaps you're flying the heavy version at best L/D (or more likely at some speed above best L/D) and comparing that to the light version being flown at the same high speed; i.e. even further above the best L/D for the light version. That would indeed cause the light aircraft to glide a shorter distance. Could that be it?
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Old 14th Apr 2014, 03:43
  #22 (permalink)  
 
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Mozella, the reason is they fly a fixed Mach/Speed schedule irrespective of weight, they have no interest in the L/D.
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Old 14th Apr 2014, 05:23
  #23 (permalink)  
 
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So while sinking at the same rate per second, the distance travelled will be greater. The 'apparent' glide Ratio will be greater, while L/D still remained the same
But the sink rate is also proportional to TAS so the glide angle remains the same at 1/(L/D) no matter what the altitude.
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Old 14th Apr 2014, 05:43
  #24 (permalink)  
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As a glider pilot allow me a few remarks : pure Gliding ratios ( or Finesse max as we call it) in modern gliders can reach high levels but the most important other factor is the minimum sink rate and at which speed range it can keep that , That what is interesting, not the gliding ratio per se.
For info the ASH 25 ( which I sometimes fly) with winglets and 26m wingspan has a finesse of 62 with 0,45m/s and will keep a good sink rate through a large speed range. You will never get that sort of perfection in a jetliner because it is build for speed , not gliding.

A modern jet liner (e.g.B767, A330 B787 ) have "official" gliding ratios around or slighly above 20:1 but only at a very specific speed , outside of that exact speed, they will probably start dropping like stones, (i.e close to 10.1) . If I remember correctly the Gimli B767 got something like 12:1 with a Glider pilot at the commands.
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Old 14th Apr 2014, 07:02
  #25 (permalink)  
 
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Mozella, the reason is they fly a fixed Mach/Speed schedule irrespective of weight, they have no interest in the L/D.
"They" used to be "me" before I retired. I'm quite familiar with flying airliners having done so for 33 years. I've also done my fair share of sailplane racing. But fixed mach/speed procedures were not part of the discussion. At least I didn't think so. The post said, "............planning a min fuel burn descent without ATC interference."

It seemed to me that the situation involved stretching the glide using minimum power (since the head office, not to mention the passengers, get all upset if you shut all four down). That would be done the same way in a 747 as it would be in a sailplane; i.e. best glide ratio or best L/D. That's how you go the most distance for a given altitude, ignoring wind.

Of course, airliners have other pressing interests, cost being a big factor. That's why all speeds, climb, cruise, and decent, are faster than optimum from an aerodynamic standpoint. Each time the flight attendants got a raise we flew faster and each time the pilots took a pay cut, we flew slower as directed by the company bean counters.

Cost also explains why we didn't make decents at best L/D. It's simply too slow. But sometimes I flew slower even than that when, for example, I got instructions to hold at a fix for an extended period of time. In that case I immediately slowed to max endurance speed (minimum sink speed for glider pilots) and sauntered toward the holding fix at which time I accelerated a bit to holding speed. Efficient flying isn't hard, but you can't save fuel and/or time by being lazy which explains why some pilots raced toward the holding fix.

In our company, we didn't ever fly a fixed Mach/Speed schedule. Nearly every aspect of selecting cruse, climb, and decent speeds (and the associated power settings) took into consideration such things as the aforementioned cost, aircraft weight, wind, our schedule, leg length, other airlines published schedules, and even maintainance items which wore out based on time in service.

But I got the idea the theoretical 747 example was free of such constraints as well as ATC restrictions, but I could have misunderstood what was being said. That's why I asked.
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Old 14th Apr 2014, 18:31
  #26 (permalink)  
 
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Originally Posted by Owain Glyndwr View Post
But the sink rate is also proportional to TAS so the glide angle remains the same at 1/(L/D) no matter what the altitude.
Ouch.
Serious Blond moment on my side.
Obviously the difference between IAS and TAS does not only apply in the horizontal plane.
Proving the point that with wrong assumptions taken as a given (constant descent rate) one can re-define physics. Not.
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Old 14th Apr 2014, 21:06
  #27 (permalink)  
 
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Mozella

But I got the idea the theoretical 747 example was free of such constraints as well as ATC restrictions, but I could have misunderstood what was being said. That's why I asked.
Not theoretical...real world. No speed brake, power off descent compensating for wind, weight and airport arrival pattern, adding power at ~1,000' AGL, if left alone by ATC, which obviously wasn't always the case.

If flying an airplane where the variety of broad landing weights was not possible, then the weight factor for descent planning was/is negligible and unnoticeable.

To be clear, at same Mach/airspeed descent schedule: heavier weight...longer glide. Lighter weight...shorter glide.
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Old 15th Apr 2014, 00:03
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Not theoretical...real world. No speed brake, power off descent compensating for wind, weight and airport arrival pattern, adding power at ~1,000' AGL, if left alone by ATC, which obviously wasn't always the case. .......... snip .........
To be clear, at same Mach/airspeed descent schedule: heavier weight...longer glide. Lighter weight...shorter glide.
OK, that makes sense. I took the post to mean a minimum fuel decent which involves operating at best L/D. That's why it didn't make sense. Now I see the examples were idle decents using the same speeds at widely varying weights; quite different from a minimum fuel decent.

For reasons discussed earlier, such things are never done in the airline business except (perhaps during a fuel emergency) since airliners usually have sufficient fuel and not much extra time or money. However, back when I was one third my age and one half my weight I flew fighters for the U.S. Navy where we used a best L//D decent frequently. We had plenty of time and money but we were always out of fuel.
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Old 15th Apr 2014, 01:31
  #29 (permalink)  
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back when I was one third my age and one half my weight

.. now, that evokes whimsical memories for most of us older folks ... or was it half my age and a third of my weight ? .. whichever ...
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Old 15th Apr 2014, 04:46
  #30 (permalink)  
 
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To be clear, at same Mach/airspeed descent schedule: heavier weight...longer glide. Lighter weight...shorter glide.
I've read that an emergency descent in a light weight 747 is somewhat eye watering with respect to the nose down attitude. True??
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Old 15th Apr 2014, 06:30
  #31 (permalink)  
 
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Brian,
True, and, of course, something like 15 or more degrees nose down appears far steeper than it really is.
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Old 15th Apr 2014, 18:15
  #32 (permalink)  
 
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15:1 at green dot. Does green dot represent best L/D, or close to it?
yes, green dot corresponds to speed with best L/D
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Old 15th Apr 2014, 20:12
  #33 (permalink)  
 
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A reasonable present rule of thumb for heavies is that they glide (nil engines) somewheres around 2nm/1000ft .. which approximates 12:1.
Slight thread drift, but doesn't 'glide' mean no thrust, ie all engines out?

I agree with John_tullamarine I have seen 2nm/1000 feet work to perfection several times (in the (real) simulator ) with both engines out on various series of 737.

Last edited by Hobo; 23rd Apr 2014 at 07:03.
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Old 15th Apr 2014, 22:10
  #34 (permalink)  
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I have seen 2nm/1000 feet work to perfection several times (in the (real) simulator ) on various series of 737.

Indeed .. on the 733 it was an infrequent, but regular, end play to be left very high by ATC, or one chose to be very high due wind related turbulence until entering the circuit (over CNS at 10,000ft was typical with a strong westerly) .. 25/gear/idle/limit speed and somewhat better than 2/1000 was achievable .. however, the mental arithmetic machine was running hot all the way down ... let it get away ever so slightly and there was nothing left in the toolkit to claw it back. Naturally one had briefed the passengers on PA prior ..

SOP proscription prevented our investigating 40 in lieu of 25.

Great satisfaction, though, when it all worked out well .. and, of course, one nonchalantly would give the impression that it was all a routine walk in the park ...

However, I guess this sideline is not what the OP was after ..

Mind you, the Electra at 2/1000 still needed a bit of power to avoid going low .. but that is an another tale ..
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Old 23rd Apr 2014, 06:45
  #35 (permalink)  
 
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John, you forgot the electra had no wings..
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Old 23rd Apr 2014, 13:33
  #36 (permalink)  
 
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Sim demonstrations - dual engine failure at 5000'/250kts/20 DME and 10,000'/250kts/35-40 DME.

Establish L/D speed and glide to the airport.

18:1 would give a range of 15 nm at 5000' and 30 nm at 10,000'.

Trading airspeed and flap extension at the latest possible moment allows you to reach the runway.

2nm/1000' would have you at 10 DME at 5000' or 20 DME at 10,000'. That's more than enough energy and no where near the max capability of the a/c.
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Old 24th Apr 2014, 03:10
  #37 (permalink)  
 
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Doesn't GROUND SPEED play a role in figuring glide ratio?

I get that it plays no role in best L/D speed and such, but if you want to figure out whether you can make the Azores or whatever from 37,000 feet, don't you need to know if you have a tail or headwind component?

Seems to me TAS is irrelevant. IAS tells you how to fly for the best range/glide ratio possible, and GS tells you what that ratio will be (and whether it will be enough!)
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