Hello there,
In the majority of contra-props, both props spin at the same speed (for ease of engineering and maintenance), and have equivalent blade counts - good examples are the Avro Shackleton, and the Tupolev Tu-95 Bear :
And most others - Spitfire Mk. 22, Westland Wyvern, Fairey Gannet, etc. etc.
[Contra-Prop Epitome - the Spitfire Mk. 22 ; Seafire Mk. 47 was very similar]
[Westland Wyvern - First turboprop warplane - I’ve always had a soft spot for it - mostly due to the contra props, but its form is also lovely]
Airflow over the contra-prop, while a bit turbulent at roots and tips, actually energises the flow over the rear set of blades. The blades are, as they enter the trailing edge vortex of the forward advancing blade, actually moving through air that is moving anywhere between 1.25 - 2 x as fast as if they encountered still air by direct attack. Its smoother in flow too.
Here’s a CFD flow analysis of a contra-propeller. The red areas on the rear blades are under high pressure = energy from the front blade set, though flow breaks down at the tips and the roots.
Its not immediately obvious that there is any advantage, but I also found this rather charming example from a Computer Fan manufacturer who has validated the utility of the contra-prop in a new product for electronic cooling :
[Image courtesy of San Ace electronics]
As a result of this, very few contra-props have been designed to take advantage of this on a full-sized aircraft. The only one I have ever seen is, of course, the Antonov AN-70 :
Here seen where the front props have a total of 8 blades, and the rear props six. You’ll also note that the rear props have a shallower pitch angle (angle of attack), and that the blade shape is also designed to take advantage of the higher energy, faster flowing air coming off the front blades - the sweep angle of the blades is greater !
In videos, the propellers appear to be spinning at the same speed - again, this is a sensible thing to do from an engineering perspective.
Another Antonov featuring contra-props is the earlier massive An-22 Antheus :
Nice shot of it with contra-props - and people. As Mr. Trump would say - it’s YUUUUGE !
To date, about the only two aircraft I know of that could realistically vary their second propeller speed where the Macchi-Castoldi MC.72 racing seaplane of the 1930’s
Where each prop was independently driven by a seperate V-12 engine. The other was the Fairey Gannet carrier AEW and ASW platform, where , again, the Bristol-Siddeley Double Mamba turboprop could independently power each propeller :
Here’s a shot of it doing this at Farnborough.
There might be others of course - but the An-70 isn’t one of them as far as I know.
Therefore - and in answer to your question - Not really. If anything, performance is increased for the rear propeller from the energised airflow from the trainling edge of the front propeller.
Lower rotation speed is not needed (or practical), though so far on the Fairey Gannet would permit this theory to be tested, and the Antonov An-70’s engine contra-prop appear to be the only design example taking advantage of it.
Hope this helps, my 2c worth.
L’Envoi
Jim Antrill did note that , for the Shackleton, the rear propellers are a smaller in diameter than the front set by a small %. That’s quite true.
From my research, the blades began life as the same length, and prior to fitment would have their tips shortened followed by balancing. For the An-70, An-22, Bear, Wyvern, MC.72, Gannet and Spitfires/Seafangs, the difference was 0% from the get-go - and none of those types appeared to suffer from performance issues.
As he did note, there is this small difference - and I suspect that this was in a (vain) attempt to keep the rear blade tips completely within the vortex of the front blades, to maximise efficiency of thrust.
Not that a Shackleton is quiet….[ YES ! - love the sound of four Big Griffons in close formation - its spine tinglingly awesome ! ]