Off Topic Airplane Talk

[mansr]

9 hours ago, Ralf11 said:

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

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

 

9 hours ago, Ralf11 said:

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)

Any transformer will block common-mode noise. I thought a grounded centre tap was mainly a safety feature since at 60 V, touching either side is harmless. Touching both at the same time is still dangerous, but you're much less likely to do that.

[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...

[mansr]

2 minutes ago, AudioDoctor said:

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

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.

[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.

[mansr]

40 minutes ago, AudioDoctor said:

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.

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?

[fas42]

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

[Ralf11]

1 hour ago, AudioDoctor said:

 

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.

 

https://en.wikipedia.org/wiki/Counter-rotating_propellers

 

I actually stole the idea from the car industry.  Anyway, this PS is indeed a big fancy way to reduce some types of line noise.

 

 

I am waiting for CC to tell us if he really does have a lot of line noise, or if he's in an R1 area or what...

[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.

[4est]

13 hours 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?

It doesn't take much to make an engine run backwards even if there wasn't a gear box.

[mansr]

5 minutes ago, 4est said:

It doesn't take much to make an engine run backwards even if there wasn't a gear box.

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

[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?

 

 

[mansr]

19 minutes ago, AudioDoctor said:

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?

 

 

Now we're getting into fluid dynamics, which is not my area at all. That said, I can imagine that the gas flow through those turbines behaves differently depending on whether they turn in the same or opposite directions.

 

Around here, the most commonly seen turboprop planes are the Dash-8 and ATR-72, both using PW100 engines. I can only assume that the various advantages and disadvantages work out in favour of identical engines/props on this class of plane.

[Ralf11]

you'd just re-design the turbines as well as the props

[mansr]

48 minutes ago, Ralf11 said:

you'd just re-design the turbines as well as the props

[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.