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I was talking to someone who got to fly in the U2. He mentioned that the pilot said that a descent from 70k' would take two hours with the engine working, or three hours without.

I have every reason to believe this source, but I can't see why it would be true that the plane can descend more quickly with an operational engine. Any usage of the engine, unless it's providing reverse thrust, would add energy to the system, and any added energy will increase flight time.

Anyone have any insight?

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    $\begingroup$ Just spitballin', but one guess would be a pitch attitude limitation that prevents you from diving beyond a certain nose-down angle, so that at the maximum dive angle you can fly at, you can't achieve the max dive speed, and get a higher descent rate, without some thrust on. $\endgroup$
    – John K
    Jun 24, 2022 at 4:34
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    $\begingroup$ @JohnK that has a certain ludicrous sense to it. Although I wonder why there'd be a shallow max nose-down angle, considering the steep angle of initial climb-out. $\endgroup$
    – Kenn Sebesta
    Jun 24, 2022 at 4:52
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    $\begingroup$ This article seems to contradict the basic premises of this question. I.e to descend one pulls throttle to idle and you’re at pattern altitude in 40 minutes. barryschiff.com/high_flight.htm $\endgroup$
    – Jim
    Jun 24, 2022 at 6:00
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    $\begingroup$ That doesn't make any sense to me at all, $\endgroup$
    – GdD
    Jun 24, 2022 at 9:25
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    $\begingroup$ Would love to know why people would downvote the lived experience of a man who got to fly to >70k'. If you don't believe he's remembering correctly (extremely unlikely, but anything's possible), then say so. If you have specific knowledge of the U2 which contradicts what he was told by his pilot, please share that as well. Otherwise, as a glider pilot I am really curious why the pilot of one of the world's highest-performance gliders would report needing more time to come down without the engine than with. $\endgroup$
    – Kenn Sebesta
    Jun 25, 2022 at 5:39

3 Answers 3

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From this the engine was used to extend "everything" to create drag and was only kept at idle power. 23 minutes was the typical descent time, not two hours, which went up to an hour if engine power couldn't get drag creating protuberances extended.

For descent almost everything possible on the aircraft is extended. The throttle to idle, lowered landing gear, raised spoilers and flaps in the gust-up configuration and extended fuselage-mounted speed brakes. Once stable on descent, the rate is close to 3,000 fpm. A speed of Mach 0.715 is used to 53,000 ft., when a speed of 160 kt. is established. In the case of either an engine or electrical failure, with the aircraft descending clean, it could easily take longer than an hour to descend from altitude.

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From the flight manual (1967 edition) we learn that

A glide from 70,000 feet to sea level will take about 73 minutes and from 35.000 feet to sea level will take about 48 minutes.

and, in section III (emergency procedures):

If it becomes necessary to reach lower altitudes in the minimum time, the descent can be made faster by remaining in a banked spiral and increasing the speed to maximum allowable IAS limits. The bank angle and stick forces should be varied as necessary to avoid exceedig G limits and to stay out of heavy high speed buffet. The only disadvantage is an increase in general roughness. Shutting the engine down will increase the rate of descent but may result in windshield and canopy frosting.

Section VI warns that

The descent in the high altiude area from 75,000 feet down to 70,000 feet is slow. The engine power, even on minimum flow, is still considerable at this altitude and the drag items do not produce much drag at indicated airspeeds at 90 to 100 knots.

Now I wonder which exact type of U-2 your friend has experience in since those points in the flight manual all contradict his statement.

If even the landing gear should be lowered in order to add some drag, it is obvious that the U-2 needs all the help it can get to descend quickly. There is a hydraulic accumulator and a battery of 35 Ah, so even a power loss will still allow to deflect the drag items. However, without sustained hydraulic pressure, the speed brakes will be slowly pushed in by the airstream, so a power-off descent, while subtracting the still considerable idle thrust, will also cut speedbrake drag.

Again section VI:

The airplane has conventional speed brakes which are moderately effective. Their primary use is as a drag producing device for descent, approach and landing. […] The speed brakes are fully variable and can be set at any desired position. In some cases they will creep closed from an intemediate position after a period of time. They cannot creep closed from the fully extended position if the switch is left in the extended position.

The only reason for a slower descent with power off is the closing of the speed brakes from loss of hydraulic pressure. This will become more effective at lower altitudes when Mach limits allow to fly at an IAS of 220 knots (gust control to faired) rsp. 240 knots (gust control to gust). Still, the manual advises to shut the engine down in order to increase the sink rate. Windmilling should still create some hydraulic pressure, but the manual does not go into details here.

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    $\begingroup$ Not sure that the U-2 has powered flight controls. This article suggests it does not: "The controls provide feather light control response at operational altitude. However, at lower altitudes, the higher air density and lack of a power assisted control system makes the aircraft very difficult to fly. Control inputs must be extreme to achieve the desired response..." $\endgroup$
    – Ralph J ♦
    Jun 25, 2022 at 15:35
  • $\begingroup$ This makes a lot of sense, and I'm starting to see how a pilot could hand-wavingly refer to "two hours" vs. "three hours". I feel like an engine-out scenario, where there is loss of control of speed brakes, could absolutely tip the scales in favor of a slower descent. And an extra +1 for digging up the U2 flight manual. $\endgroup$
    – Kenn Sebesta
    Jun 26, 2022 at 14:26
  • $\begingroup$ More Ah on the battery might help, perhaps with a twirling device in front to charge it even more, no? $\endgroup$
    – Robert DiGiovanni
    Jun 27, 2022 at 20:16
  • $\begingroup$ @RobertDiGiovanni As I understand it, loss of hydraulic pressure is more important because you lose control of the speed brakes. Electricity is needed for the instruments and radio but not flight-essential stuff. And there is a big twirling device inside the plane: The shut-down engine will idle from dynamic pressure with RPM between single digits and 20% of full thrust, depending on airspeed and density. $\endgroup$
    – Peter Kämpf
    Jun 27, 2022 at 20:23
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    $\begingroup$ @Cloud The is a nitrogen bottle to keep the canopy seal pressurized in case of engine shutdown. So in regular operations an engine shutdown will keep the cabin pressurized long enough to descend. All bets are off, of course, when a damage opens a gaping hole in the cockpit walls. Then only the space suit of the pilot will keep him alive. $\endgroup$
    – Peter Kämpf
    Jul 1, 2022 at 19:34
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"descent from 70k would take 2 hours with the engine working, and 3 hours without"

As with many learning aides, or acronyms, this seems so crazy one has to remember it.

The foundation for the "myth" can be found in here, in a reference provided by @quietflyer, paragraph 4, section "on final approach". Clean, at 109 knots Vbg, the U2 has a glide ratio of 28:1. "Normal" descent is 37 Nm for every 10000 feet, roughly 2/3 clean glide ratio.

For those proposing use of thrust to descend faster (sort of a diving version of Vy), while theoretically correct, generally runs up against Vne in reality. Rate of descent is determined not only by speed, but also by angle of descent.

where speed is limiting, greatest angle of descent gives the greatest rate of descent.

So, as we all do with flaps, spoilers etc., increasing drag increases angle of descent.

Apparently, in this case, the increase in drag is greater than the increase in thrust by leaving the engine on (at idle to operate the drag systems). The angle of descent is lower clean with the engine off, because net drag (Dc) < (Dd-T) "dirty"/engine at idle.

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  • $\begingroup$ Many aircraft have a significantly higher allowable airspeed clean, compared with flaps down. Here, descending at a higher airspeed may produce a higher rate of descent, but gravity will provide all the "thrust" that is needed. $\endgroup$
    – Robert DiGiovanni
    Jun 27, 2022 at 11:48

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