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GE (General Electric) flies Boeing 747s equipped with new engines that are being tested. Why can't they equip the test airplane with all 4 of the new engines? Wouldn't the thrust be imbalanced by replacing only 1 engine? As you can see, it may have 1 big engine and 3 smaller ones:

747 with GE-90 engine fitted for testing

Other times, smaller engines are being tested. Would the smaller engine provide enough thrust for a large, heavy airplane like the 747?

747 with CFM LEAP engine fitted for testing

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    $\begingroup$ "on an airplane this size it wouldn't have much thrust because of the weight of the 747." I'm not sure this is what you meant to write. Thrust is not related to aircraft weight. I think you mean to ask if the lower thrust would cause performance problems (compared to the default engines). $\endgroup$ – egid Sep 4 '15 at 0:07
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    $\begingroup$ Would the smaller engine provide enough thrust for a large, heavy airplane like the 747? It flies and GE were happy to fly it. That's probably all the answer you need to that one. $\endgroup$ – Simon Sep 4 '15 at 5:07
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    $\begingroup$ Also note that these 747's are testbed aircraft: likely empty, and almost certainly running fairly low fuel loads. They aren't taking 14 hour passenger flights, so they aren't at maximum take off weight and don't need as much power as an in-service aircraft. $\endgroup$ – Jon Story Sep 4 '15 at 12:34
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    $\begingroup$ It's perfectly legal to operate a 747 on only 3 engines for purposes of a ferry flight. The restrictions that apply are that it must be carrying no revenue payload and have aboard only the required crew. I would imagine the same rules apply for test flights. Thus there would be no problem operating with a smaller than normal engine thrust-wise. They would, of course, have reduced performance for a given weight. $\endgroup$ – Terry Sep 4 '15 at 21:27
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They only change one, because that one is a new engine that is being tested.

If you are testing a new engine you change 1 at first so if it fails you have 3 others. If you change all 4 and there is a systematic defect in the design, now you have 0 good engines instead of 3.


You will also see that the test engine isn't always one suitable for a 747. This is because they may be testing a new design or a new alternative fuel and it is safer to test an unknown with 3 good engines (e.g. a 747) rather than with just 1 good engine (any two-engine airplane). This increases the safety margin if the test engine fails.


Running a different engine with a different thrust rating requires some thought about asymmetric thrust. Doing this on a 4-engine airplane simplifies this greatly over a 2-engine airplane. Ways to accommodate different thrust from the odd engine:

  • De-rate the new engine so it only produces thrust equivalent to the normal engine
  • Run the new engine at reduced thrust
  • If the new engine is inboard, reduce the outboard engine thrust to balance the total yawing moment from that wing with the other side.
  • Don't worry about it and trim the airplane to compensate.

The other advantage to doing this on a 747 is that a 4th engine is technically not really needed as a 747 can takeoff, fly and land with only 3 operating engines.

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    $\begingroup$ Any idea how GE addresses the asymmetrical thrust? That's a good part of Ethan's question, although I imagine they just throttle back #3 to match #2. $\endgroup$ – egid Sep 3 '15 at 23:59
  • $\begingroup$ @egid Yes. That's what I think they do. $\endgroup$ – curious_cat Sep 4 '15 at 8:41
  • $\begingroup$ @egid derating would work, as would compensating loadout. Of course the 747 can handle a certain amount of asymmetric thrust without such measures quite well. $\endgroup$ – jwenting Sep 4 '15 at 8:53
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    $\begingroup$ Possibly also worth mentioning that these testbed aircraft run fairly light, so there's no real need for 4 engines at all: as such, two engines running on partial power isn't much of an issue anyway and the "partner" engine can be safely throttled back $\endgroup$ – Jon Story Sep 4 '15 at 12:33
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    $\begingroup$ @egid one way that they are doing that is having it as the inner engine, where too much/little thrust will have less lateral impact than if it was on the outside. Think of the lever arm that the engine thrust is affecting, outside engines have a longer arm than inner ones. $\endgroup$ – enderland Sep 4 '15 at 12:45
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They are not testing that the engine works, that's already been proven with hundreds of hours of regulated and rigorous ground running. They are testing the "on wing" systems and behaviours in various flight conditions.

The on wing systems will also have been tested in ground runs before the first flight.

Therefore, only one engine is needed.

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Although you already have a correct answer and it is accepted too but i would add another point to the mix. They are testing a new engine which uses a new technology and is uncertified/unproven. Why should they make 4 of that version and then find out a number of things to fix and go back to all 4 of them first before moving on to produce more models? That costs a lot of money and time.

One is enough.

Wouldn't the thrust be imbalanced?

Even on our R/C Aircrafts we can have different engine RPMs for different engines (on a multi engine model), let alone a Jumbo Jet! They can all operate at different turbine speeds and different thrust levels to balance as and when needed.

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