Counter Rotating Props

[dublinpilot]

Watching a light twin the other day brough back something that I had read before about counter rotating props.

I remember reading about the effect of losing one prop and that one was a "critical prop" to lose, as it caused more yawing due to the position of the downward moving blade.

That much I understand.

Then it mentioned about counter rotating props, which ensure that neither prop is "critical prop".

That I understand too.

However I am sure it mentioned a disadvantage to counter rotating props, which explained why all twins arn't counter rotating....only I can't remember what it was, and I can't find the original text!!!!

Anyone else know the disadvantage of counter rotating props?

[GT]

Immediate thing that springs to mind is cost. Different sets of parts, fewer same parts made, hence more expensive.

GT.

[Clear Air Turbulence]

Possibly something to do with the economics of producing an engine which turns 'in the other direction' and the associated complications of different spare parts etc for each engine.

'...which explained why all twins arn't counter rotating'

There are twins with counter rotating props - Beech Duchess springs to mind.

All the best

CAT

EDIT - Looks like you beat me to it GT

[Tinstaafl]

A counter rotating engine has different parts compared to the normally rotating model. This means that you have to maintain spares for two different engines ie can't keep a particlar spare part that could be used on either engine. This costs more for little or no gain in safety.

Performance & handling limits are based on the worst case failure ie the critical engine. As long as the a/c is operated within its envelope then a failure of the non-critical engine could give you a slight 'bonus' or buffer over & above critical engine performance & handling limits.

[Croqueteer]

Used to fly 4 griffins with contra rotating props. Exciting.

[formationfoto]

Can't see any technical downside of counter rotating props other than as mentioned elsewhere in this post - the cost. Have flown twins with both 'one way' and 'two way' props and would prefer counter rotating for the advantage given in the event of engine failure on the critical engine with 'same way' props. Sadly most of the twins I have flown have been marginal on one engine and therefore I welcome any assistance I can get.

[Tinstaafl]

Not so.

For the same reasons Vmc is different between the critical & non-crit. engine, there will a performance difference due to the additional control inputs needed to maintain a given heading/AoB.

That difference in input will result in different drag effects. At least it's supposed to but always seems to be swamped in the rest of the variables when I do a sequential shutdown & restart.

Probably another one of those things that's more theoretical than explicitly noticeable.