8
$\begingroup$

I recently saw a picture of a Russian Bear(TU-95) intercepting one of our(US) military aircraft over the Pacific Ocean.

Many years ago, when I flew in reconnaissance aircraft, we were frequently intercepted by Bears which surprisingly flew at 500+ mph airspeed.

Yet no US Military Aircraft uses contra-rotating props(to my knowledge). Why is that?

$\endgroup$
12
$\begingroup$

In short, the US didn't need one.

The main reason for the development of Tu-95 was the rather unique situation existing after WWII, when the Soviet Union needed a heavy bomber to carry the (then very heavy) nuclear weapons. Due to the lack of bases anywhere near contiguous United States, the Soviets were at a significant disadvantage when it came to nuking the US before the ICBMs came into picture (US had bases in Europe and in Pacific). At that time, heavy bombers were the only way to deliver a nuclear payload and it required the aircraft to be fast, have a intercontinental range and be able to carry a reasonable payload.

The first attempt in developing a long range bomber during the last days of WWII by the Soviets didn't work out and as a result, they simply reverse engineered USAAF B-29s which had 'landed' in Russia into the Tu-4 'Bull', which lacked the necessary range. In a way, the B-29 was the forefather of Tu-95. Further development continued on this airframe, culminating in the Tu-80 'Barge', which had the necessary range, but was considered unsuitable as it lacked speed, which would be fatal against jet interceptors.

The Tu-95 was developed at the same time of the B-52 'stratofortress'. Actually the Soviet Air Force preferred the jet powered Myasishchev M-4 and Tupolev had to convince Stalin about the Tu-95's suitability. During the initial design phase, the Tupolev bureau considered two versions- one with four turbojets and another one with turboprops. The turbojet version was discarded as it would've lacked range due to the fuel hungry turbojets (Tupolev turned out to be right about that as the M-4 was sidelined due to lack of range).

The payload and speed requirements required a very powerful engine (the Kuznetsov NK-12 is the most powerful turboshaft engine ever) and the contra-rotating props were selected to overcome the problems associated with the large diameter of the propellers.

Western countries also did a lot work with contra-rotating props, but the advent of jet engines and the complexites associated with the counter-rotating props meant few graduated from the prototype stage. The Huges XF-11, for example had counter rotating props initially; they were changed to conventional ones after Huges crashed in it. The Fairey Gannet and Avro Shackleton were among the few western contra-rotating aircraft to see regular service (both were used in ASw roles, which highlight the increased endurance offered by them). A number of contra-rotating aircraft were tested during the end of WWII, but the advent of jet engines meant that the required speed could be achieved in a far better way.

$\endgroup$
  • 1
    $\begingroup$ Also Soviet engines were less efficient due to lower precision of manufacturing, which exacerbated the problems achieving sufficient range. $\endgroup$ – Jan Hudec Mar 13 '16 at 21:17
  • $\begingroup$ If you consider the military version of the Cessna 337, the O-2, this [cool looking] aircraft has two separate motors that spin props in opposite directions (aka contra-rotating). It is called a coaxial twin. $\endgroup$ – Pugz Mar 14 '16 at 6:54
  • $\begingroup$ @Pugz, I don't think that qualifies as contra-rotating. They are not coaxial, so they are only counter-rotating. $\endgroup$ – Jan Hudec Mar 14 '16 at 9:44
  • $\begingroup$ @aerolias Excellent answer-I always thought it was because they couldn't shut down(feather an engine) in flight; unlike standard turboprops. What I was amazed by is the fact they still use them. $\endgroup$ – user2479 Mar 14 '16 at 10:33
  • $\begingroup$ @JanHudec oh sorry, I meant centerline thrust twin, not coaxial. Oops :-) $\endgroup$ – Pugz Mar 14 '16 at 14:22
3
$\begingroup$

There were a couple attempts (Douglas Mixmaster, Hughes XF-11) at same, but none of them ever made it to serial production.

The Mixmaster, though fast, arrived at the dawn of the jet age and wasn't fast enough to compete with jets. The Air Force transitioned to jets pretty quickly.

The Hughes developed hydraulic problems it's first flight and crashed, nearly killing the pilot (Howard Hughes). Again, fast but not faster than jets.

The Soviets spent a lot more time developing that tech because they had extensive experience with turboprops (they were still flying turboprop airliners long after US / Western carriers had transitioned to jets) and early, Soviet jet engines were very inefficient. Not having quite the budget of the USAF, they needed something fast yet fuel efficient. Contra-rotating props gave them speed while turboprops gave them efficiency.

$\endgroup$
1
$\begingroup$

Contra rotating props are mechanically complex and pose serious maintenance issues when used on piston engines, and tend to be very noisy. The advantages are counterbalancing of torque, and more thrust in a smaller space than a single prop. However, the rear prop will not have the smooth airflow the front prop encounters - it will be cutting into the pulsing airstream from the front prop, so additional stress is imposed.

Several military aircraft in the US tried them in the prototype stage but abandoned them when the drawbacks outweighed the advantages. These include the Northrop XB35, which had gone to single props in the YB35 production version, before the turbojet variant, the XB-49 eliminated propellers. The Hughes XF-11 abandoned contra rotating props in its final version. Another Northrop design with contra rotating props was the XP56, although its contra rotating props were but one of many problems that design had.

The Avro Shackleton, which was successful, was known for high maintenance on it's prop gears.

A notorious example was the XF84H"Thunderscreech", infamous for its very high noise levels... just like the TU-95. The advantages of contra rotating props were not found to outweigh the disadvantages.

Later western military turboprop designs, such as the Lockheed C130, dispensed with the complexity of contra rotating props in favor of better single prop design. Same is true of the Airbus A400 military transport - four single props.

Contra rotating props were part of the Unducted Fan design, which saw some use in the Soviet Union, but didn't prove to be advantageous enough to see commercial use in the US, due partially to high noise that would have had problems with noise restrictions at most western airports. Ironically, some of the prop blade tech developed for the GE CFM36 unducted fan was later used on the huge and highly successful GE90 turbofan.

In the end, the western nations solved the issue with the high bypass turbofan, which is equally efficient, capable of higher speeds, and produces far less noise.

If anything, the TU-95 stands as a testament to the Russian ability to build robust military platforms, and a testament to western tech to solve problems with a different design rather than brute force.

$\endgroup$
0
$\begingroup$

The US Air Force used the OV-10 Bronco during Vietnam. It had twin turboprop engines. The props counterrotated by using an extra gear in one of the engines to make it spin in the opposite direction from the other prop. Although the OV-10 was ostensibly an observation aircraft, it carried some serious firepower, such as rockets and miniguns. It was also capable of carrying a small complement of passengers, for either inserting a special ops group or exfiltrating wounded personnel. I don't know if the Air Force still has any on active service, but they are still flying in other places. It seems to me that I read that they might be used in a similar role as the A-10, when the A-10, which is more expensive to operate would be, ahem, overkill.

$\endgroup$
  • $\begingroup$ That is not what OP means. They refer to two propellers on ome engine shaft, turning in opposite direction. The Bear has four sets of these, 8 propellers in total. $\endgroup$ – Koyovis Nov 9 '17 at 3:05
  • $\begingroup$ Howard Hughes was developing such a system. He crashed and was seriously hurt. He never built another aircraft with this propeller arrangement. It turned out that a single O-ring had failed. I can see the advantage of this kind of arrangement, but I suspect it results in a very heavy and complex propeller system. $\endgroup$ – Howard Miller Nov 10 '17 at 14:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.