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Trying to investigate this question, I see that 'Ask the Captain' says:

The weight of the shuttle is calculated like any payload. The 747 produces enough lift to fly and to carry the weight of the shuttle. In this respect it is no different than cargo carried inside the airplane. source

But of course it is different, since cargo is generally carried inside an aircraft.

There was a fairing for the Shuttle's engines for drag reduction, but I'm wondering if the wings of the Shuttle had some aerodynamic contribution to the lifting body, or whether that was all down to the 747's wings.

STS Challenger on 747 source

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    $\begingroup$ Related: aviation.stackexchange.com/questions/35305/… (which gives a partial answer: it costs a lot more drag to get a unit of lift from the orbiters wings than from the 747s wings) $\endgroup$ – Sanchises Feb 4 at 13:40
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    $\begingroup$ The main goal of the fairing was to reduce the turbulence in the wake of the Space Shuttle to maintain the yaw authority of the vertical tail. $\endgroup$ – ROIMaison Feb 4 at 16:11
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    $\begingroup$ If the shuttle does produce lift, then that raises the question of whether the wings of the 747 are in the wash of the shuttle. $\endgroup$ – Acccumulation Feb 4 at 18:38
  • $\begingroup$ I instinctively feel it would handle more like a small monoplane carrying a shipping container, than a biplane! $\endgroup$ – Grimm The Opiner Feb 5 at 12:49
  • $\begingroup$ @ROIMaison That the fairing wasn't entirely successful in this regard is demonstrated by the addition of extra vertical stabilisers (although I don't remember whether those were articulated; I'd guess probably not). $\endgroup$ – anaximander Feb 5 at 13:51
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This podcast with one of the pilots answers just about every question on the shuttle carrier you could have and it's worth a full listen. But to cover the flight dynamics, I would skip to 50:33 minutes, where the pilot states (please note there is no official transcript of the podcast and I typed this as I listened to it, please see the official podcast for the actual verbiage, but this is close):

Markus Voelter (Interviewer): Let's talk a bit about the flying characteristics, did the wings of the shuttle add some extra lift or was it mounted with essentially 0 angle of attack?

Arthur C. “Ace” Beall (Pilot): It was mounted with some angle of attack, you can see that as it's sitting. At the speed we were flying, the carrier itself was about 5 degrees nose high. The shuttle was creating lift, but how that affected it aerodynamically is hard to say, it was always mounted the same way so we had no basis of comparison. It did, however, create a lot of drag and made the aircraft very top heavy so bank angles were limited.

So some lift was generated but at the speeds they were flying, other limitations came into play that were more of a concern. It's also worth noting that when in transport no one was in the space shuttle and the control surfaces were not used in any way.

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    $\begingroup$ Given that the Shuttle had to maintain a high angle of attack while landing (at 200 mph or so), 5 degrees would likely be insufficient to lift the shuttle's weight, so the overall effect would be still that the 747 was carrying the shuttle rather than the shuttle lifting itself. $\endgroup$ – Skyler Feb 4 at 17:47
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    $\begingroup$ @Skyler i added a bit since there may have been a misunderstanding. The carrier its self was 5 degrees nose high and the shuttle had further positive angle of attack due to its mounting. But i agree it provided little lift in the situation. $\endgroup$ – Dave Feb 4 at 17:56
  • $\begingroup$ While no one was in the orbital shuttles during transport, Enterprise was crewed during the "captive - active" portions of the Approach and Landing tests, and of course the ones where it separated and landed on its own. $\endgroup$ – Chris Stratton Feb 5 at 19:06
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Interestingly enough, for the orbiter separation maneuver to work, the Shuttle would need to have a higher lift to weight ratio than the carrier 747. Plenty of people were probably wringing their hands (including me) over the possibility of the orbiter hitting the V stab upon release, but when you see the video, it isn't even close.

This was accomplished with an lighter unloaded orbiter set at a higher angle of attack than the 747. At adequate airspeed, it simply lifted away. The higher AoA tolerance of the delta added to the safety of the manuever. I would imagine they spoiled to lift of the 747 a little too.

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    $\begingroup$ Looking at youtube.com/watch?v=cOmemZl4k1U, it seems the SCA did a bit of a dive to help with separation. $\endgroup$ – Eugene Styer Feb 4 at 16:27
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    $\begingroup$ @EugeneStyer, it had to do that so the orbiter would maintain speed by gliding; otherwise it would immediately start slowing down and hit the tail. Unfortunately the video is not clear enough to see whether the carrier has spoilers deployed (I would expect it does). $\endgroup$ – Jan Hudec Feb 4 at 18:02
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    $\begingroup$ The L/D ratio of the Shuttle is notoriously bad, I'm pretty sure it's lower than that of the 747. So the 747 would have to dive away to separate from the orbiter. $\endgroup$ – Hobbes Feb 4 at 18:20
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    $\begingroup$ @Robert, re "Amazingly", you mean, almost as if they engineered it that way? $\endgroup$ – prl Feb 5 at 3:19
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    $\begingroup$ @RobertDiGiovanni It was. From nasaspaceflight.com/2012/04/… -- the relative AOA for the approach and landing tests was 6 degrees, which was reduced to 3 degrees for ferry flights. $\endgroup$ – Tristan Feb 5 at 15:19
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The space shuttle's wings are small, but they still act like wings. Their lift depends on the actual angle of attack. The space shuttle cannot be neutral for all angles. The engineers had a choice at which precise angle to mount the space shuttle on top of the carrying 747. I would assume that the shuttle is rather close to neutral at cruising speed, to avoid excessive drag. Then it will contribute to the lift when the combination has a larger angle of attack like during start and landing.

Yes, this is a huge biplane, but the "upper wings" are designed for returning from earth orbit and landing, not for contributing much lift to the 747 combination.

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    $\begingroup$ This answer is speculation and assumptions. In fact, the bit about the purpose of the orbiters wings is just wrong. The orbiter's wings were designed to provide cross-track maneuverability so that the shuttle could land after a single orbit polar mission, during which the Earth would have already rotated almost two hours out of phase from it's reentry path. $\endgroup$ – dotancohen Feb 5 at 14:13
  • $\begingroup$ @dotancohen You are welcome to share your knowledge and contribute a better answer. I would also appreciate if you let me know which of my assumptions are unreasonable. To what extent is the single orbit requirement relevant for this question? $\endgroup$ – bogl Feb 5 at 15:39
  • $\begingroup$ I don't have a better answer to give. Bogl, you're not a new user, you know that speculation is frowned up here. As for the single orbit requirement, that is the basis of the orbiter's wing design. Upon reentry from a single polar orbit it would have to maneuver over a thousand kilometers off the reentry course in the atmosphere to line back up with the launch location. The wings and that huge vertical stabilizer provide the control authority to do that. Without that requirement those control surfaces could have been much, much smaller and lighter. $\endgroup$ – dotancohen Feb 5 at 18:08

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