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I have been looking for the definition of cruise-climb for gliders that use an electric motor, but so far I have only been able to find a definition based on liquid fuel. I'm asking because that condition appears in an RFP for a self-propelled glider with an electric motor.

"An aeroplane cruising technique resulting in a net increase in altitude as the aeroplane mass decrease"

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    $\begingroup$ Clearly, that RFP isn't using the definition you quoted for that term. (Either that, or the author didn't consider what he's writing about very carefully.) Can you quote a little more of the RFP, or link to it, to provide more context? $\endgroup$
    – Ralph J
    Nov 4, 2023 at 3:24
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    $\begingroup$ I share it google.com/… $\endgroup$
    – andy
    Nov 4, 2023 at 3:47
  • $\begingroup$ andy please note that the answers of Robert and Rob, although being formally correct, do not really apply to the RFP you are analysing. The RFP itself contains all the info that you need and you should only refer to that, see for example the comments of @quietflyer $\endgroup$
    – sophit
    Nov 5, 2023 at 17:20

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"Cruise climb" does have a definition regarding a cruising aircraft rising to a higher altitude as it loses weight from fuel consumption. This relates more towards long range aircraft such as airliners.

A second application of "cruise climb", is one of climbing at an airspeed greater than Vy.

Looking at it in the scope of Vx, Vy, and cruise climb, we see excess thrust and Angle of Attack can be used for a steeper climb, a faster climb, or greater distance while climbing, respectively.

because angle of attack is lower at higher airspeeds, cruise climbing may be a more efficient way of gaining distance while climbing.

Why cruise climb with a glider? It wouldn't matter if it was gas or electric. One needs to go and find another thermal.

Cruise climbing the glider is really an extension of "cleaning up" slats and flaps, minimizing drag, and gaining as much distance as possible for a given amount of energy.

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Reading the RFP one can easily see what is meant with "cruise-climb" (requirement 1.a.ii):

The self-retrieve begins with a self-launch and includes 30 minutes of soaring flight before the glider is unsuccessful in finding atmospheric lift to continue soaring flight and must use motor power to “self-retrieve”. This includes a climb, cruise-climb, then descent during which the aircraft must cover a distance of at least 50 km, ending at an altitude at least 1000 feet above the airport.

So the cruise-climb relates to the need for the glider to cover 50 km and reach an altitude of 1000 feet if it hasn't encounter any good thermal in the previous half an hour, as simple as that.

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  • $\begingroup$ How is "cruise climb" different from the "climb" segment, in the RFP's meaning? $\endgroup$
    – Ralph J
    Nov 5, 2023 at 14:00
  • $\begingroup$ @RalphJ -- see the tables in the RFP -- for the “Design reference Instructional Flight" mission, for the "climb" segment, a climb rate of at least 600 fpm is specified. So that it doesn't forever to get to a reasonable altitude to shut down and start looking for thermals. Also the target shutdown altitude is 2500' above airport altitude. For the "Design reference self-retrieve flight" mission, it is a little more complicated. Two different "climbs" are mentioned, the first being identical to the one mentioned above, and also a "cruise climb", where no minimum vert speed is specified. $\endgroup$ Nov 5, 2023 at 14:16
  • $\begingroup$ (Distance covered in "cruise climb" counts toward a target 50 km range for self-retrieve. A portion of that 50 km self-retrieve range appears to be intended to be accomplished in a power-off glide.) $\endgroup$ Nov 5, 2023 at 14:17
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Constant mass battery electric aircraft have no need to cruise climb.

Cruise climb is a technique where you allow the aircraft to slowly climb during cruise to keep the specific range at its optimal value. You're typically gaining altitude to maintain constant lift coefficient at constant Mach number (or true airspeed, above the tropopause these are the same).

A constant mass aircraft has no need to cruise climb. Just cruise.

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  • $\begingroup$ I woudl rather say that constant mass battery electric acft are unable to cruise climb. They are destined to fly the whole time at constant mass and that is one of their disadvantages. $\endgroup$
    – Jpe61
    Nov 5, 2023 at 15:30
  • $\begingroup$ @Jpe61 Not losing weight in cruise is a small disadvantage, but cruise climb is a maneuver generally not allowed by ATC and seldom taken true advantage of. Long range flights can at best hope to step climb. Battery energy density is enough of a disadvantage that BE aircraft don't really have to complain about being constant mass. $\endgroup$ Nov 6, 2023 at 6:08
  • $\begingroup$ My observation (as a passenger) does not match that. On almost all flights I can think of there certainly has been multiple upwards altitude changes enroute. $\endgroup$
    – Jpe61
    Nov 6, 2023 at 7:06
  • $\begingroup$ Check out FlightAware or similar flight tracking websites. A quick random sample seems to show that transcontinental (USA) flights at night (low traffic time) sometimes show a nice step-climb, typically three segments across the continent. Daytime flights show more erratic altitude changes, likely due to traffic and weather, often descending mid-flight. Shorter flights (half that distance) seem to generally hold constant altitude. I'm sure trans-oceanic (long and lower traffic) get to step climb better -- I have not seen any flight tracks with a true cruise climb. $\endgroup$ Nov 6, 2023 at 16:46
  • $\begingroup$ I'll have inter- and transcontinental samples to report soon 😏 but you are right in that true cruise climb is rare. Intermittent step ups are common in my experience. $\endgroup$
    – Jpe61
    Nov 6, 2023 at 17:25

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