non-coutner-rotating jet engines
 

Hello,

I'm sorry if this has been covered in another thread (I have looked but with no success), and also if this is not the most appropriate forum (the subject matter here seems rather more lofty than my question).

Is the overriding reason for jets using non-counter-rotating engines the cost involved in setting up two different assembly lines?

I'm not downplaying this as a factor, as I appreciate this extra cost would be huge. I have not found any information directly dealing with this issue in any detail, although a poster on another aviation forum hinted that the torque and p-factor experienced by an aircraft with jet engines is not as detrimental to its performance as that of a twin-prop?

Any help would be greatly appreciated

You appear to have answered your own question. Massive cost. Negligible benefit.

Not so much that, as there is no POINT to making counter-rotating jet engines.

It is a fact that there have in fact been a few attempts at making counter-rotating propellor engines, but that is because P factor means that a prop aircraft with both engines rotating in the same direction has a reduced performance in the single engine case on one engine in comparison to the other.

Still, the economic factors involved in having two different engines means even this has been a minority proposition.

This is NOT the case with jet engines.

While there is a small amount of "swirl" in the eflux of jet engines, it does not make any significant change to the aircrafts single engine performance, no matter which engine fails.

Jets don't have a "critical" engine, so there is no point in having counter-rotating engines.

My half arsed guess goes something like this:

The issues associated with counter-rotating engines are more applicable to propeller driven aircraft

Asymmetric blade effect (uneven thrust at high angles of attack) does not apply to jets (as much) as the inlet ducting prevents air approaching anywhere but front on; and

Torque effect doesn't apply to jets because torque has to be applied to something fixed to the airframe (cylinder heads for piston or perhaps a gearbox for turboprops) for the aircraft to 'feel' it. Jets have neither as the torque provided by the turbine is comes from hot gas expanding into a free airstream rather than against a cylinder wall.

Though most "left-handed" piston engine cranks actually rotate in the conventional direction, but have a gearbox that produces prop rotation in the opposite sense.

The 'swirl' affect on a fan on a jet engine is 'reacted' by fan exit guide vanes in both the fan and the core. So while there is a torque in one direction from the fan, the exit guide vanes provide a counter torque in the opposite direction that is nearly equal to the fan torque. The final stages of the turbine work similarly. Hence the net torque from the engine that ends up being applied to the airframe is minimal (plus minimizing the exit swirl helps efficiency).


Some propeller engines do more or less the same thing by using counter rotating props.

Thanks for the answers, really useful and well explained information here.

Actually most CR piston engines simply have a reverse ground camshaft, and possibly accessory gears. This doesn't apply to all, but most direct drive types

I think that you're mistaken about this. The only twin engine airplanes I know of which have counter rotating props have direct drive engines.

Common airplanes with counter-rotating props include: The Beech Duchess, Piper Seminole, Seneca, certain models of the Navajo, including the Chieftain, and the Cessna Crusader. all with direct drive engines ie: Prop is bolted directly onto the crankshaft.

I think you'd be pretty hard pressed to name a common GA aircraft with geared engines and counter rotating props.

'Jets don't have a "critical" engine, so there is no point in having counter-rotating engines. '


Agree with their being no need for 'counter rotating engines' on jets but they do have a critical engine it's just that it's not always the same.


It's the outboard engine on the upwind wing.

That still doesn't make it a 'critical engine'. Just makes it aerodynamically critical; nothing to do with the engine though.

You're right, of course, must engage brain before opening mouth. :\

Undaunted, I'll stick my neck out farther and assert that it's the right-hand engine that has a left-hand rotating crankshaft on a C/R twin.

Though if we're were talking turboprops, all bets are off, at least in respect of the A400M. It has both left-and right-hand props on each wing, geared obviously, and in this case it's the gearboxes that are handed.

I was about to say - 100% of the C/R piston twins I have flown are direct drive. Geared prop piston twins are not real common.
Trivia fact - the P-38 C/R engines are on the "wrong" sides. The plane flies better that way.

The GeNX fitted on the 747-8 is counter rotating.

And the Trent 900 on the A380, and presumably the A350 engine too, is counter-counter-rotating.

So which wing has the left-hand rotating engines and which the right-hand ones? :O

and

interesting, so there *are* counter rotating jet engines. Why is that? Seems like if torque and "p-factor" are negligible to nonexistent, there wouldn't be much advantage to building different engines for different sides.

Oh dear. Next time I'll have to put two smileys on my post. :ugh:

A sq,

The three concentric shafts on the engines swap direction from inside to outside - but all the same way on all of the engines.

It avoids the need for a set of stator vanes between intermediate and high-pressure components, at the expense of scarier bearings.

The RB199 compressors spin in opposite directions to counteract gyroscopic effect.

TurboUnion

Right, I'm aware that there's counter rotating spools in the same engine, I thought we were talking about having left and right engines rotate in opposite directions.

Quote:
Agree with their being no need for 'counter rotating engines' on jets but they do have a critical engine it's just that it's not always the same.

It's the outboard engine on the upwind wing.
That still doesn't make it a 'critical engine'. Just makes it aerodynamically critical; nothing to do with the engine though.
tmpffisch is offline Report Post Reply


It certainly is, anyway the effect of a critical engine is most certainly aerodynamic no matter what the type or installation.

Oh dear oh dear, the engines don't turn different ways on different wings, the engines have counter rotating shafts as in the LP turns one way and the HP turns the other. This is the first counter rotating Hi Bypass ratio turbine engines I have come across on modern aircraft. :rolleyes:

Technically speaking it is contra rotating rather than counter rotating, which is what I thought the originator was alluding too, perhaps not, so I'll leave it to the more technically adept people who know more! :ugh:

Aaaaah! Halcyon days brought back in earnest!

The sweet sound of 4 x Rolls-Royce Griffons at full chat prior to 15+ hours wheels-off to wheels-on.

Not quite a counter-rotating jet, but a magnificent sound now only heard on the DVDs and videos. :{

Just put GE's on one side and RR's on the other.
Problem solved.

"Critical Engine" is a term used in certification. Given that the VMCG/A data is for zero crosswind (these days) there is no critical engine on a jet.

There is however an engine who's failure will require more control that the other in a cross wind. This is due to the fin not the engine.

The affect is still the same FEH, that's the point I was trying to make.


Best wishes.

P1127 and Kestrel/Harrier had contra-rotating spools to counter gyroscopic effects. The test pilots insisted on this after bending the Bell X-14 during training flights. I recollect Bedford/Merewether? Ah, yes, Merewether X-14 with Vipers. Subsequently J-85s. Okay, Mr Farley?

Actually the OP wants to know about non-coutner-rotating engines.....

Henry

As you say the Pegasus low pressure and high pressure spools have always rotated in opposite directios to minimize any gyroscopic moments in the hover. Given the simple non autostabilised reaction control system used on the P1127 to control attitude in the hover if gyroscopic couples had existed the thing would have been impossible (rudder would have made it pitch and pitch would have made it yaw for example). Bedford, Merewether and Robin Balmer (the controls and aero man) are all dead so I don't know how much the US experience counted. My feel is that it only confirmed what they already realised and so insisted on with Bristol before the first Pegasus was designed.

The engine men were naturally reluctant to have contra rotating spools because this hugely increased the inter spool bearing speeds to numbers outside their ken.

The A350 engine you refer to is the R-R Trent XWB: Trent XWB - Rolls-Royce

How about "CACA" on the Airbus A400M.
The change in prop rotation is made in the gearbox.