What was the goal of dual props?

[Robert Porter]

I noticed both during and after WW2 there were several aircraft that had 2 props on the same shaft/engine. I can't help but think they would do more to harm than improve airflow. One Soviet example had the props rotating in opposite directions.

Did these odd prop configurations actually improve performance?

 

[Capt. Vick]

Torque negation and aerodynamic efficiency to name two...

 

[pbehn]

I noticed both during and after WW2 there were several aircraft that had 2 props on the same shaft/engine. I can't help but think they would do more to harm than improve airflow. One Soviet example had the props rotating in opposite directions.

Did these odd prop configurations actually improve performance?

They reduced torque reaction. On the seafire it was impossible to put a bigger prop on the plane so a contra rotating prop was the next best thing. It weighed more, it was problematic at times under very high G load. To my knowledge they all had the two props going in opposite directions. To me one of the most interesting variants was the Fairey Gannet which had two engines driving the two parts of one contra prop. This gave the power for take off and landing while allowed a longer loiter time for its job in Anti Submarine ECM plane.


Here is a video to make your eyes hurt, remember it is the strobe effect though.

View: https://www.youtube.com/watch?v=0n31Ku-y7jA

 

[gumbyk]

Propellers are kind of self-defeating for high-power engines. As you produce more power, you need a bigger prop to absorb that power (either more blades or longer blades), problem then is that a bigger prop produces more torque, longer blades mean the prop has to turn slower, as the tips exceed Mach 1, and there may not physically be enough room for the mechanisms to control the pitch of 6 blades in one hub.
The torque produced could easily roll the aircraft over at low airspeeds and high power, such as a go-around.

The solution to all of these was contra-rotating props, with essentially two three-bladed propellers turning in opposite directions.

 

[pbehn]

The solution to all of these was contra-rotating props, with essentially two three-bladed propellers turning in opposite directions.

I think I read somewhere that even with a contra prop the tips are supersonic on the Tu 95 but the engines are 11,800 shp each.

 

[gumbyk]

I think I read somewhere that even with a contra prop the tips are supersonic on the Tu 95 but the engines are 11,800 shp each.

yeah, they would have been pushing the envelope, even of contra-rotating props.

 

[Robert Porter]

Thanks guys I wondered about it and the articles I could find were about as clear as mud.

 

[Shortround6]

One reason the articles were as clear as mud is that it took a few years (ok more than a few) to actually get the things to work performance wise. Getting better handling/eliminating torque reaction was easier. But a few Experimental US planes (XP-72 and Corsairs) using the R-4360 engine actually went slower using contra-rotating propellers. The NACA had been fooling around with contra props since the 30s (and some of the early experiments seem bizarre to modern eyes. Like flow straighteners between the two actual props.)

 

[Crimea_River]

I knew about the torque negation but have always wondered about whether the rear prop got the same lift or was the air between the props disturbed enough to affect the rear one. Would they have run at the same pitch?

 

[swampyankee]

I knew about the torque negation but have always wondered about whether the rear prop got the same lift or was the air between the props disturbed enough to affect the rear one. Would they have run at the same pitch?

I worked with propfans when I was at HSD. To be sure, I'll have to dig up the CR I wrote, but, yes the rear prop blades are at a somewhat greater pitch, but not as much as you may expect because of swirl. It also depends on how the gearbox is set up, in that it can be set so both props get the same torque, like the wheels ona car with a normal differential, or the same rpm. In the former, blade pitch is controlled to match rpm, while in the latter pitch is controlled to match power.

 

[mikewint]

First off there is a large drag penalty associated with any local flow going supersonic due to the energy needed to generate the shockwaves in the flow. Additionally trans-sonic flow (Mach 0.8-1.2 roughly) creates a lot of instability in the overall aerodynamics. The shockwaves, which are actually huge changes in pressure over a very small distance, change the overall pressure distribution on the surface which can mean you aren't nearly as aerodynamically efficient or effective. In trans-sonic flow the locations and strength of these shock waves is dynamically shifting. On a propeller this can cause oscillations which obviously load up all of the associated structure in ways it wasn't designed for.

A propeller is essentially a spinning disc; the tangential velocity depends on the radius outwards from the axis. So, the very center always has a tangential velocity of about zero. If the tip of the prop is super-sonic, then somewhere along the radius you are transitioning between, sub- and super-sonic.

In that setup, the shockwave is just hanging out in the atmosphere between your prop blades. Its location is unstable and can slosh around all over your prop; you don't really have any control over anything. This is opposed to the nicely ordered and well defined shockwaves on super-sonic jets or turbines or rockets. Therefore there is no physics reasoning fundamentally stopping you from running a prop faster than the speed of sound, it is just a bad engineering idea.

Along those lines there was a test bed aircraft, the Republic XF-84H. Which as designed to test the effect of contra-rotating, supersonic, turbine driven, props. They found there were too many other issues for it to work well (top speed Mach 0.83). Namely, you have a prop that's creating a shock wave every time a blade passes. It made for an incredibly loud noise (audible to 25 miles away, giving the aircraft the nickname "Thunderscreech") plus the aircraft was getting hit over and over by the prop pressure waves which disoriented both the aircraft and pilot, in fact, it even gave one guy a seizure.

 

[grunnvms]

The Tu95/TU142/Tu114 family of aircraft are know to be among the loudest aircraft in the air, because the propeller tips are supersonic.

The AVRO Shackleton maritime reconnaissance aircraft / bomber was the grandson of the famous AVRO Lancaster bomber. It had four Rolls Royce Griffon piston engines driving contra rotating propellers.

 

[swampyankee]

Propellers are kind of self-defeating for high-power engines. As you produce more power, you need a bigger prop to absorb that power (either more blades or longer blades), problem then is that a bigger prop produces more torque, longer blades mean the prop has to turn slower, as the tips exceed Mach 1, and there may not physically be enough room for the mechanisms to control the pitch of 6 blades in one hub.
The torque produced could easily roll the aircraft over at low airspeeds and high power, such as a go-around.

The solution to all of these was contra-rotating props, with essentially two three-bladed propellers turning in opposite directions.

Some of the propfan designs had 11 blades in a hub, and production props have eight. The British built five-bladed props, and the Japanese, six. The main -- probably only -- reason to go to contraprops is when there is a diameter restriction. There's also a lot of related nonsense about two or three or one blades being most efficient: also not true.

CRPs do get some efficiency improvement from recovery of swirl; this may be as high as three or four points. They never caught on in the US, probably because the US fighters were physically larger than the Spitfire.

 

[mikewint]

My understanding of the US position was simply that the gains were not worth the mechanical and engineering problems. Essentially the same reason aircraft don't fly by flapping their wings

 

[swampyankee]

My understanding of the US position was simply that the gains were not worth the mechanical and engineering problems. Essentially the same reason aircraft don't fly by flapping their wings

Likely true. The US was also experimenting with contraprops, but the closest they came to service was probably the XB-42, which would have been a great fast bomber in 1943.

 

[mikewint]

I always liked the POGO. Can you imagine trying to land the thing looking over your shoulder

 

[Robert Porter]

That always struck me as a "why" aircraft. It looks cool but have to think the disadvantages, especially for landing, outweigh any advantages for vertical takeoff.

 

[mikewint]

The idea was a sound one. A small fighter that could be loaded onto almost any ship in a sealed container. Since it is a VTOL craft no landing strip required. At need the ship could launch the Pogo for air defense. In effect any reasonably sized ship becomes a mini-aircraft carrier

 

[chuter]

An interesting point on the Shackleton props is the forward blades are longer by a few inches so the rear blade tips didn't get caught up in the tip turbulence from the forward blades. I have no idea if relative tip speeds played any part in that decision. The Do335's rear prop was cut down in diameter to reduce tip speeds because it was in the high speed airflow of the forward prop (and it could still push the plane along 48mph faster than the front prop) but it was independently governed. All present day contra-prop warbirds/racers are flying on Shackleton spares but they've all cut the blades down to the same length.