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AMELIA is the product of a partnership between NASA and the California Polytechnic State University. One of its innovations is circulation control. An engine is mounted over each wing, and high pressure air is redirected from the exhaust over the wing and through tiny slots, adding lift.

AMELIA is still in wind tunnel testing, but if it makes it to the prototype stage, it would have a carrying capacity of about 100 passengers. A good commercial comparison might be a low-seating version of the Boeing 737, although it's old. A newer (and perhaps better) comparison is with the Airbus A318. Its cousin, the A319, is a similar aircraft that has been more popular.

Anyway, assuming that a prototype/production version of AMELIA is manufactured, it might be pretty similar in size to these aircraft. They're all twinjets; AMELIA, though, has its engines on top of its wings. In theory, one could modify AMELIA to have more engines. This version would be different, just as the 747 is a completely different craft that the 737, or how the A318 can't compare to the A380. Scaling it up, though, could be possible.

Would the addition of two more engines1 interfere in any way with the airflow from the original engines and the subsequent lift produced? I doubt it, because engines on four-engine aircraft (e.g. the 747 and A380) don't seem to present any interference issues that impact performance. But the setup for AMELIA is pretty unique. Could adding an additional engine on either side interfere with airflow?

Image credit: NASA.

1For spacing considerations, imagine spacing like that on the A380. This gives some basic specifications, while this gives some drawings outright.

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    $\begingroup$ (1) I guess, the engine is mounted above the wing to lower the acoustic signature as heard from the ground. (2) On a different note, are the designers planning to do boundary layer control using high pressure air from the engines? If so, I would guess that it wouldn't principally matter where the engines are: above or below, and how many engines. The engines are far enough from the wing. (finally) These are just guesses, which can be completely wrong. $\endgroup$
    – Nick Alexeev
    Feb 26, 2015 at 0:33
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    $\begingroup$ (3) There is a fairly successful Soviet aircraft Antonov An-72, which was design for a similar niche: STOL transport. It also has got engines above wings. The engines produce Coanda effect, which generates lift and shorten the takeoff. I wonder if Coanda effect is involved in AMELIA too? $\endgroup$
    – Nick Alexeev
    Feb 26, 2015 at 0:42
  • $\begingroup$ @NickAlexeev (1) seems accurate; I've seen that rationale given for other aircraft using similar-ish designs. I don't know the answer to (2), but boundary layer control could be an advantage. I wouldn't be surprised if a variant of that makes its way into advanced designs. (3) I had heard of the Coanda effect, though only in connection to the development of the Avrocar. I don't know how much of an influence that could have. $\endgroup$
    – HDE 226868
    Feb 26, 2015 at 0:55
  • $\begingroup$ Is this SLO or Pomona's design? As a Pomona Alumni, this makes a huge difference in my opinion of if the design will be an improvement. $\endgroup$
    – Mark
    Jul 1, 2015 at 19:22

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No, but it would take a slight modification and I don't think they'll do it.

First, you have to understand why AMELIA is injecting high pressure air over the top of the wing. In aerodynamics, lift is produced as a result of "circulation", which means that the air above the wing travels further in the same amount of time than the air on the bottom of the wing. The high pressure air from the engine, in this case, aids the air on the top of the wing because NASA has put a slot on the top of the wing (probably about 3/4 of the way back) to give more energy to the air flowing over the top of the wing. Since it has more energy, they can force it to take an even longer path (remember the air on top takes a longer path than the air below) without stalling. Stalling is when the air above the wing doesn't have enough energy to travel fast enough to reach the back of the wing at the same time as the air traveling beneath the wing, and so just gives up and separates from the wing.

To answer your question, the addition of a second engine doesn't preclude this in any way, however, it would mean that you have more exhaust air available. I predict (although I don't work for NASA) that the engineers would make the wings wider and have each engine provide high pressure air to its own section of the wing. This would give the same benefit, only scaled up.

Note however, that I think it is quite unlikely that four engines will ever be used in this configuration. Modern two-engined craft are as resilient against engine failures as old four-engined craft and are now allowed to fly almost every commercial route (See ETOPS). Combined with the fact that two-engined planes can be made more efficient, I don't think that AMELIA will ever have four engines.

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Disrupt: no. Effect somehow: yes.

New engines would over-power the aircraft, so you would have to choose smaller main (or inner) engines.

With more engines, engine maintenance tasks would take longer, and reliability figures (mean time to between failures, etc) would decrease to some degree. It would also aeroelastically effect the wing (which must be considered).

If this is a proof-of-concept demonstration (on paper), there's no need to increase the complexity of the aircraft by adding more engines. If someone needs to flight-prove this concept, then adding more engines can be postponed till there's a major benefit from having the extra engines.

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