The 35-degree wing sweep in the Tu-95 Bear

CxxTxx

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I saw this unanswered question on another forum, and I've always wondered the same thing so I decided to post it here.

"Why was the Tu-95 Bear designed with 35-degree swept wings? The Bear is fast, but not that fast, that is, it cannot achieve the near supersonic speed that would warrant that degree of sweep. Modern airliners are capable of speeds that exceed the Tu-95, but their wing sweep is much less. Is there something about the NK-12 turboprops that require that sweep, or is the design merely an artifact of when the aircraft was designed (in the early 1950’s?) If the design is merely an artifact from 1950’s Soviet aerodynamic research, why were the 35-degree wings retained in the Tu-142 series of aircraft? Also, are the contra-rotating propellers (the tips of which exceed the speed of sound) still the best way to go given the wide cord six and eight blade props are a proven technology and widely available?"

Good questions. Does anyone have insight into these?
 

TomS

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The Bear is a bit faster than Mach 0.8 (officially Mach 0.82, IIRC), very close to the large jet aircraft of the era. The B-47, B-52, and Boeing 707 were swept at almost exactly the same angle, and were only a few percent faster. The rationale for the swept wing is the same in both cases -- it reduces transonic drag. Modern airliners still tend to have sweep angles bwtween 30 and 35 degrees and speeds around Mach 0.85 -- not very far at all from the Bear.

Clearly large contraprops aren't a preferred solution anymore or we'd see other aircraft with them. The closest we see are proposed open-rotor engines, but they aren't gaining ground yet.
 

Machdiamond

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The wing sweep main purpose is to move the wing box structure forward of the bomb bay, which occupies the full depth of the fuselage and needs to be near the aircraft center of gravity.
For the contra-rotating propellers, it depends on the design requirements. It may still make sense today in some cases. It is interesting to see that the A400M does not have those while the An-70 did. I would say that it is always better to avoid complexity if you can.
 

malipa

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Furthermore a fairly large part of the wing is subjected to the wake of the engines, which accelerate the air as well. The angle actually decreases after the part of the engines ;P
 

flateric

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35 degrees was a 'magic angle' recommended by TsAGI
 

Steve Pace

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Its sweptback flying surfaces sure do add to its overall beauty though. -SP
 

AeroFranz

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Part of the reason is also airfoil technology. Today's airliners employ airfoils that delay compressibility effects without needing as much sweep. For a given cruise Mach number, an A350 needs less sweep than say, a 707. I don't know exact numbers, but it could be as much as 5-10 degrees.
 

Sundog

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Machdiamond said:
For the contra-rotating propellers, it depends on the design requirements. It may still make sense today in some cases. It is interesting to see that the A400M does not have those while the An-70 did. I would say that it is always better to avoid complexity if you can.


However, don't forget the A400M propellers rotate opposite each other on each wing. The inside propellers, 2 & 3, rotate top away from center line (the fuselage) and the outer two, 1 & 4, rotate top away from center line (the fuselage). Which counter acts the torque and keeps the airflow over both wings symmetric about the center line as well. Even though you don't get the benefit of taking the swirling angular momentum out of the propeller slip stream, you sure save a lot of weight without the complex gear boxes. Although I'm sure you personally knew most of that, I just wasn't sure if you knew about the opposite rotation of the propellers on the A400M.
 

CxxTxx

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Sundog said:
However, don't forget the A400M propellers rotate opposite each other on each wing. The inside propellers, 2 & 3, rotate top away from center line (the fuselage) and the outer two, 1 & 4, rotate top away from center line (the fuselage). Which counter acts the torque and keeps the airflow over both wings symmetric about the center line as well. Even though you don't get the benefit of taking the swirling angular momentum out of the propeller slip stream, you sure save a lot of weight without the complex gear boxes. Although I'm sure you personally knew most of that, I just wasn't sure if you knew about the opposite rotation of the propellers on the A400M.

I didn't know any of it....

Steve Pace said:
Its sweptback flying surfaces sure do add to its overall beauty though. -SP

Agreed, the Bear is a sublime work of art.
 

Machdiamond

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Yes Sundog that is counter-rotating propellers as opposed to contra-rotating.
Like the Piaggio Avanti, Piper Navajo, Seneca and Seminole, etc.
 

CxxTxx

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Russian Bears Saved From Extinction

http://www.strategypage.com/dls/articles/Russian-Bears-Saved-From-Extinction-5-14-2015.asp
 

Basil

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CxxTxx,

during cruise speed the propeller tips of the NK-12 remain well below the speed of sound despite the large propeller diameters; only during high speed dashes the addition of forward and rotational speed makes the tips reaching low supersonic regions.

For the A400M engines a contra-rotating propeller gearbox was designed by a German engineering bureau (which traces its roots back to the propeller gear design of the Jumo 022) but finally a single rotation solution was chosen. Arithmetically the contra-rotation offered a slightly higher propulsive efficiency at high speeds.
 

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