I am interested in finding energy consumption (% kN) of unretracted landing gear (on a jet fighter) per hour. Although the shape and size tell us how much drag is being produced I need a general rule of thumb.
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$\begingroup$ FMI, what is “kn”? $\endgroup$– Michael HallJan 4, 2021 at 17:13
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$\begingroup$ @Michael Hall, Kilo Newton $\endgroup$– ThomasJan 4, 2021 at 17:34
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2 Answers
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While it’s not a true air to air fighter, following are some comparison numbers for the EA-6B Prowler that might help provide a useful ratio for a tactical "fighter type" aircraft. (Information comes from the NATOPS pocket checklist, under the constant altitude “bingo” fuel divert tables.)
Theses figures are for a flaps-up aircraft, fully loaded with 5 external stores, and both engines operating. (There are multiple configurations and distances covered in the tables).
Fuel required to go 100 Nautical Miles:
| Altitude | Gear Up | Gear Down |
|---|---|---|
| Sea Level | 3970 lbs | 5220 lbs |
| 10,000 ft | 3580 lbs | 4590 lbs |
| 20,000 ft | 3270 lbs | 4160 lbs |
- NOTE: The gear down figures are worst case and presume a hydraulic failure. In the event of a hydraulic failure the gear will be blown down pneumatically and the forward main landing gear doors, (that normally re-close to cover the empty wheel well) will remain open. This imposes some additional drag penalty over and above normal flight with the landing gear down.
You should be able to extrapolate this into something useful, but if you would like more data points please let me know.
answered Jan 4, 2021 at 1:56
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2$\begingroup$ Fuel has fixed amount of energy per pound, so just divide the two numbers for matching cases. Result is about 30% more power with gear. $\endgroup$ Jan 4, 2021 at 17:12
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$\begingroup$ @Jan Hudec, 30% more power means if a plane is moving with 500 KN of thrust with retracted landing gear , it will use ~650 KN thrust? $\endgroup$– ThomasJan 5, 2021 at 4:00
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2$\begingroup$ @Thomas as rough estimate, yes. The fuel required table probably assumes you fly slower with the gear extended, which complicates things quite a bit. $\endgroup$ Jan 5, 2021 at 6:48
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1$\begingroup$ I remember Terry telling us once he had to fly a 747 with gear down and used twice the fuel. $\endgroup$– PerlDuckJan 5, 2021 at 16:15
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1$\begingroup$ @Thomas, it will definitely take more than comments—and someone with better idea about behaviour of nozzles. $\endgroup$ Jan 8, 2021 at 21:50
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It is doubtful whether you will find any data beyond "a lot". This is the reason for retracting the gear whenever possible. A while ago an Airbus A320 was unable to retract the gear. The pilots didn't find specific data in their manuals. The additional fuel use resulted in the plane never flying again.
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2$\begingroup$ Welcome to Aviation.SE! While I agree with your post, it would be better suited as a comment. Also, HF3378 was an A310, not A320. $\endgroup$ Jan 3, 2021 at 22:29
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1$\begingroup$ Anecdotally, a glider with and without retractable undercarriage (typically just one wheel, and only partly exposed) changes the glide ratio from 70:1 to 100:1 $\endgroup$– FrogJan 3, 2021 at 22:46
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$\begingroup$ @Bianfable didn't get your meaning of suited as a comment $\endgroup$– ThomasJan 5, 2021 at 4:01
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2$\begingroup$ @Thomas, he means that it isn't an answer to the question. Which is isn't. It is a comment. $\endgroup$ Jan 5, 2021 at 4:50