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Thread: Planned descents of 12-13,000/minute by four engine a/c?

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    Default Planned descents of 12-13,000/minute by four engine a/c?

    I have been reviewing the flight plans for Bomber Command operations to Oberhausen (November 1-2, 1944) and to Dusseldorf (November 2-3,1944). The flight plan for the Tholthorpe 62-Base Summary of Operations, November 1/2, 1944 states: "Cruise at 19/20,000 ft. 160 IAS: 20,500/21,000 ft. 155 IAS to just short of target. After bombing, turn wide left and increase speed to 240 IAS, letting down at 12/13,000 ft. per min. to 10/11,000 ft."

    The flight plan for the Tholthorpe 62-Base Summary of Operations, November 2/3, 1944 states: "After bombing...Let down at 240 IAS losing height at 12/13,000 ft. per min. to reach 5028 N 0600 E at 7/8,000 ft.

    Is this a transcription error? Just how realistic are descents of this magnitude? I wonder if it should be 1,200-1,300 ft. per min? What say the pilots on this forum? My sources are photos of the archived documents.

    Jim

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    Hi Jim
    That must be an error, mustn't it?
    12,000 ft per min is 200 ft per second!!
    Ian

    Edit: Actually, it might be ok!?
    I don't know what angle a heavy bomber could dive at, but if a 45 deg dive was possible, a descent rate of 200 ft/sec would be a linear rate of about 280 ft/sec. Divide by 3 = 93 yds /sec and x2 (roughly, give or take) would be about 180-ish mph. Well within a Lanc or Hali's speed range.
    So maybe not outside the bounds of possibility even with a faster speed in a gentler dive angle!!
    Last edited by ianh; 23rd February 2020 at 21:32.

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    Ian: Thanks for the math, but not certain of the angle of descent. I am still a little suspicious. Wish my dad was around to corroborate this. :-(

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    Hi Jim
    Yes, I suspect a 45 deg dive would be excessive, but with the airspeed and the vertical rate of descent given, it'll be possible to work out mathematically what angle of descent that represents.
    I only tried 45 deg because it was easy to work with for starters.
    We're not talking Stukas here!
    Ian : )

    Edit: It works out at (near enough) a 35 deg dive, using the vertical rate of descent per min and the 'distance flown' per minute at 240 mph to give two sides of a triangle, then seeing what (dive) angle (arcsine formula) gives that proportion of one over the other.

    Presumably a 35 deg dive for about 40 seconds would've been possible for a fast but controlled descent without pulling the wings off when levelling-out again?
    Last edited by ianh; 24th February 2020 at 13:38.

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    Perhaps a slightly different perspective may help


    "20,500/21,000 ft. 155 IAS
    240 IAS letting down at 12/13,000 ft. per min. to 10/11,000 ft.
    ...
    Let down at 240 IAS losing height at 12/13,000 ft. per min. "
    Source stated as "photos of the archived documents" from Flight Plans detailed (but without NAA refs).

    If direct from images of NAA original docs, transcription error seems highly unlikely, as orders in two docs are explicit and consistent about both speed and descent rate.

    In other words, taken at face value:
    At 21,000 ft 155 IAS increase IAS to 240 and lose 10,000ft or 11,000ft altitude at 12 to 13,000 ft/min

    That is: at 4 miles/min speed, lose 2 miles altitude at 2+ miles/min descent rate.

    The max speed of a Lancaster was 270-280mph at altitude.
    "Maximum permitted speed" rather more than that.
    Plainly neither ordered nor required.

    4 miles/min speed, lose 2 miles altitude at 2+ miles/min descent rate
    implies descend expeditiously for 1 minute to return height.

    This seems to be well within a/c specs and far from improbable.

    Having disposed of the majority of my reference collection and with no pilot quals (let alone 4-engine experience in war-time night bomber ops or orders), open to comment and or correction.

    Don Clark
    www.211squadron.org
    Last edited by Don Clark; 24th February 2020 at 00:01.
    Toujours propos

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    Thanks Ian and Don. I have gone through the trigonometry and at a 30 degree angle of descent, an 11,000 feet vertical difference in height/minute could be accommodated at 250 mph. Very approximately. The distance covered through the air would be 22,000' in 60 seconds which matches 250 mph. Distance covered over the ground would be (square route of 3)*11,000' or 19,000' or 3.6 miles. So it seems reasonable. Thanks for bearing with me on this one.

    This is the simple case of the 30:60:90 degree isosceles triangle. http://www.themathpage.com/aTrig/30-60-90-triangle.htm

    Jim
    Last edited by JDCAVE; 24th February 2020 at 01:18.

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    Hello All,
    OK, I’m not a qualified Pilot, and I don’t have Lancaster/Halifax type on the FSX.
    But I went from 20000ft to 10000ft in 1min 9secs, and covered 6.23 nm over the ground. TAS when 10000ft was reached was 257kts (I had ‘switched out’ various ‘failures’ on the a/c I used!).
    What was the purpose of this fairly extreme height change?
    I can only presume that the German flak batteries had 20000ft set on all the rounds in the 88mm ‘ready-use’ lockers. If the stream suddenly dropped to 10000ft it would take some time to manually change the burst height on each round to 10000ft? By which time, the ‘targets’ – at 250kts – would have gone? Also, if the predictors were set for a stream speed of 150kt then an actual airspeed 100kts faster would throw them out as well?
    Would like to know the real reason!
    HTH
    Peter Davies
    Last edited by Resmoroh; 24th February 2020 at 12:35. Reason: QSD
    Meteorology is a science; good meteorology is an art!
    We might not know - but we might know who does!

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    Thanks Peter. Im pretty sure the changes in heights and speeds had to do with tactics, although I had been thinking about the night fighters rather than flak. Note that most of 6-Group were still using MPH at this stage of the war and 419 continued with this practice for the rest of the war. Evidently there was a shortage of ASI units calibrated in MPH. Going forward Im going to have to be mindful of understanding exactly what is meant with flight plans, ensuring that I understand the units that are being used. For now, they mention air speeds as 190 IAS, so the units are uncertain.

    Jim

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    JD

    IAS = Indicated Air Speed
    So in this case it will be 190 mph indicated air speed which simply means that 190 will be displayed on the Air Speed Indicators in the aircraft.

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    My guess is that is a typo. Dropping at a rate of 12,000 feet per minute in an nonpressurized aircraft, there would few eardrums left.
    Richard

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