Props Question

[Tomliner]

Reading some of posts concerning props on various types set me wondering about the reasons for using contra rotating props on aircraft such as the Shackleton,Gannet etc.I suppose one reason must be to reduce the torque effect but why two props per engine instead of single ones rotating in opposite directions on multi engine types.I'll bet Garry knows. EricT.

[Garry Russell]

I don't know the whole reason but it has been said on some types it was done to increase the prop area without increasing the diameter which would perhaps not fit on the airframe.

Certainly on a single the torque would be cancelled out

Bet there's a lot more to it though

[DaveB]

Ooo Eric. Fortunately, I'm about to work so my answer is short

The contra prop setup on the Gannet is different to say, the Shackleton so shouldn't be confused as being the same thing. Anyway.. single props waste a lot of airflow through the props. By using a second prop going in the opposite direction, it takes advantage of the waste air so you get significantly more poke. However, they're complex and heavy systems which is why they probably don't get applied much. There are benefits but these are mostly outweighed by the complexity/weight issue. Hopefully, someone else will pop in with a full explanation while I'm toiling downstairs

ATB

DaveB

[Tomliner]

Thanks for the replies so far.Well explained Dave.I would imagine that propeller design and engineering is quite complex.EricT

[Garry Russell]

I was watching a Mega Builder programme yesterday and they were testing the the sealing of the glass used in the Worlds biggest tower in Dubai.

To test it they needed to blow lots of wind and low and behold they wheeled up a a Shack unit on a trolley......still with it's countra prop, spinner and cowling, just as it appeared on the wing .

Garry

[SkippyBing]

Interestingly the second prop has it's blades as a different pitch to the front ones. Basically it's receiving air that's been accelerated by the front disc so to get the same angle of attack, thrust etc. it has to be at a different angle. I'm not sure if it applies to aircraft, due to the different fluid densities, but I know in marine applications the second disc is slightly smaller in diameter to avoid the tip vortices from the front disc.

In the Gannet it's actually two engines and two props, it makes a nice compact installation for a carrier aircraft, plus you don't get torque effects etc. In the Shack/Bear etc. it's two props per engine which reduces the disc diameter needed to absorb all the power the engine can produce. It has the knock on effect of meaning all the power trains are identical rather than having handed engines or gearboxes to reduce the torque and prop wash problems, which makes production and logistics simpler. Cost wise it may be cheaper but it would probably depended on the production run.