Off Topic Airplane Talk

[AudioDoctor]

9 hours ago, mansr said:

Except that they don't do that. If they did, the torques would of course cancel as you say.

 

Of course they do. I don't know if every twin engine plane with props does this, but I know many do.

 

 

19 hours ago, Ralf11 said:

I am trying to think of an analogy for you...

 

none for your old job

 

ok, in a twin engine aircraft using props - you spin them in different directions to reduce torque moments

 

I congratulate myself for that one!

 

 or...

 

some types of noise on the line can be removed by using a center tap to get 2 different ... legs, each 60 V and out of phase with other

 

the noise will then cancel when the component is connected across the 2 legs (and will still get 120 V)

 

Mansr, maybe he doesn't need it.  But if you could take the price of buying them off the profit from your website... just before tax season...

 

 

 

interesting, so it's basically a big fancy way to remove line noise...

[AudioDoctor]

1 hour ago, mansr said:

I don't know every plane either (far from it), but the ones I've flown on had both props spinning the same direction. If you say there are models with counter-rotating props, I believe you.

 

I personally think it is a better idea, but I am also not an aeronautical engineer. It could be that some manufacturers don't do it simply for cost reasons.

[AudioDoctor]

1 hour ago, mansr said:

Manufacturing and keeping inventory of all engine parts in left and right versions would certainly add cost. I suppose the benefit depends on the size of the plane. Also bear in mind that the plane must be able to fly on one engine, so the trim surfaces to balance the forces are always needed. What's an example of a plane with counter-rotating props?

 

The P-38 for sure had them.  I think Diamond Aircraft twins have them as well. Piper aircraft have them.

 

It is much safer to have counter rotating propellers versus identical spinning propellers, when one engine fails. Twins with propellers spinning the same way have a "critical engine" that if fails will cause more controllability problems than if the other engine had failed.

[AudioDoctor]

1 hour ago, fas42 said:

Could be done by having a gearbox - so just one extra subsystem... plenty of planes with counter-rotating props on the same axis.

 

two props on the same axis, turning opposite directions is called contra-rotating propellers.

[AudioDoctor]

2 hours ago, mansr said:

That's true for a piston engine. I was thinking of turboprops.

 

I don't know about all turboprops, but I have flown a King Air. They have props that spin in the same direction and I always wondered why. The propeller is not directly attached to the shaft in a Pratt & Whitney PT6 engine, it is free wheeling. I thought it would be as easy as changing the orientation of the fan blades that the engine pushes air over to make the prop spin the other direction. Am I wrong? Have I not thought about this enough?

 

 

[AudioDoctor]

Grumman tried to get away with NOT doing that and seriously handicapped the F-14 until they put new engines in it...

 

My Uncle flew them and he can talk your ear off for weeks about this one topic.