Theoretical questions about a twin prop fighter

[totoro]

How is lift on the wing influenced by proximity of the propeller? More precisely, what happens when there are two propellers, close to each other? What happens if they both rotate inwards? What happens if they both rotate outwards? How does the distance between the propellers in both cases influence the lift over the wings? How does the fuselage between the props (or lack of it in the example of twin mustang) influence the interaction of air pushed by the props with the air over the wings?

Yes, I am aware this may be too broad of a topic but I am looking for just rule of thumb guesstimations.

 

[Broncazonk]

Going all the way back to the P-38, the first prototype had inwardly rotating propellers which deleted a significant amount of lift and caused stability problems. The pre-production machines used outwardly rotating propellers and the outward rotation fixed the problem.

Warren M. Bodie, in his book, The Lockheed P-38 Lightning: The Definitive Story Of Lockheed's P-38 Fighter, states, "Engine rotation was changed so that the propellers rotated outboard (at the top), thereby eliminating or at least reducing the downwash onto the wing centersection/fuselage juncture. There was, by then, no doubt that the disturbed airflow, trapped between the two booms, was having an adverse effect on the horizontal stabilizer. No problem was encountered in reversing propeller rotation direction; they merely had to interchange the left and right engines."

From the P-82 article on Wikipedia:

"The XP-82 was to be powered by two Packard-built Rolls-Royce V-1650 Merlin engines. Initially, the left engine was a V-1650-23 with a gear reduction box to allow the left propeller to turn opposite to the right propeller, which was driven by the more conventional V-1650-25. In this arrangement both propellers would turn upward as they approached the center wing, which in theory would have allowed better single-engine control. This proved not to be the case when the aircraft refused to become airborne during its first flight attempt. After a month of work North American engineers finally discovered that rotating the propellers to meet in the center on their upward turn created sufficient drag to cancel out all lift from the center wing section, one quarter of the aircraft's total wing surface area. The engines and propellers were then exchanged, with their rotation meeting on the downward turn, and the problem was fully solved."

Bronc

 

[Sundog]

What happens is the air behind the propeller swirls in the same direction as its rotation. This changes the effective angle of attack behind the propeller on the wing. Where the propeller is on its downward swing, the air will be moving downward as well, reducing the local AOA, whereas, on the side with upswing, the local AOA will be increased.

Also, the wing is "washed" in higher speed airflow where the propellers are located. This can greatly aid lift and control power at low speeds. However, at high speeds, it can make that area of the wing create higher drag as well.

The idea with contra-rotating propellers is that the second propeller, turning the opposite direction of the first, will remove the swirl. That swirling is actually lost energy, so you are making the propeller wash more efficient. Of course, it requires more weight, power, and complexity to do so. However, one of the other advantages of the contra-rotating propeller is to offset the torque of each other. That makes maneuvering easier and it means the pilot can probably add full power at low speeds without any problems. Whereas a single propeller powered aircraft would want to torque over if full power was applied at low speeds.

Now if you are talking two separate propellers counter rotating, once again, the primary goal there is to have the torque effects of the propellers cancel each other.

 

[famvburg]

Technically, the P-38 uses counter - rotating props. A Shackleton uses contra. ISTR that the non-turbo-charged P-38s that went to England used props that turned the same direction. I may be wrong but I'm thinking that. If so, I wonder how that affected performance.

 

[Tony Williams]

Bronc, I am puzzled by your two references because they seeming to be saying different things:

Warren M. Bodie, in his book, The Lockheed P-38 Lightning: The Definitive Story Of Lockheed's P-38 Fighter, states, "Engine rotation was changed so that the propellers rotated outboard (at the top), thereby eliminating or at least reducing the downwash onto the wing centersection/fuselage juncture. There was, by then, no doubt that the disturbed airflow, trapped between the two booms, was having an adverse effect on the horizontal stabilizer. No problem was encountered in reversing propeller rotation direction; they merely had to interchange the left and right engines."

From the P-82 article on Wikipedia:

"The XP-82 was to be powered by two Packard-built Rolls-Royce V-1650 Merlin engines. Initially, the left engine was a V-1650-23 with a gear reduction box to allow the left propeller to turn opposite to the right propeller, which was driven by the more conventional V-1650-25. In this arrangement both propellers would turn upward as they approached the center wing, which in theory would have allowed better single-engine control. This proved not to be the case when the aircraft refused to become airborne during its first flight attempt. After a month of work North American engineers finally discovered that rotating the propellers to meet in the center on their upward turn created sufficient drag to cancel out all lift from the center wing section, one quarter of the aircraft's total wing surface area. The engines and propellers were then exchanged, with their rotation meeting on the downward turn, and the problem was fully solved."

So unless there is something wrong with my reading comprehension, the first quote says that having the engines rotate outboard at the top was the solution, the second quote says this was the problem.....
???