Jet spin

[forthbridge]

Following on from helo rotors...... this got me thinking about jets. Take say a B737. Assuming the engines both turn clockwise (or not) is there any reason a jet engine needs to spin one way or can they be built to spin any way?

Second, from a flying POV, would it make any difference to have engines spinning opposite to each other? Thrust should be equal of course, and I can't think of a good reason to do so, but just one of those daft questions

[SkippyBing]

Jets can spin either way, there's no real problem with the jet wash rotating as there are stator vanes alternating with the turbine blades which stop the build up of rotation in the flow. If it didn't Harriers would have major snags as all the thrust would come out of one side!

In some aircraft i.e. the F-22 Raptor the engines are handed so one spins one way and one the other. As I understand it this is to balance the gyroscopic force of the rotating mass in each engine so at low speed and high angles of attack nothing undesirable happens.

[forthbridge]

Thanks Skip. I wonder if such a solution would be feasible on other aircraft or if it's just down to the airframe/engine fit and application? Seems almost like a modern application of the citra rotating prop.

[SkippyBing]

I'd say for 99% of aircraft it's irrelevant, the gyroscopic forces aren't usually great enough to be noticeable so there'd be no need to go to the extra expense of having two models of the same engine along with the extra hassle in terms of the logistics chain. I'm fairly sure the F-22 is unique in this respect as I don't think even the Typhoon bothers.

[forthbridge]

Gotcha

It makes you think as to the need for the system on the Raptor, or if it was just added either 'in case' or for the hell of it as opposed to being totally necessary.

[SkippyBing]

I do wonder about the Raptor case myself. At the time it and the B-2 were being developed it was a case of no expense spared, I think the only bit of the B-2 that isn't EMP proof is the ABS system!

[Chris Trott]

The reasoning on the F-22 was that when they were doing the ATF flyoff with the YF-22, they had a couple of very nasty spins during upset testing (i.e. where they intentionally fly the airplane past the limits of flyability with the flight control laws that prevent you from doing it turned off or way down). A lot of this is because of the large amount of thrust produced (almost 50% more than the Typhoon) and the fact that the thrust vectoring allows for sustained angles of attack impossible in other aircraft. When they changed the rotation of the engines, they eliminated most of that problem. Without it, I doubt the "hover" maneuver they've been showing off with the aircraft would be possible.

As for engine rotation - the RB.211 spins "backwards" to most others as does the Trent. I guess RR just couldn't get over the fact that they were the only ones still building engines that rotated counter-clockwise.

[SkippyBing]

Thanks for that Chris, having seen the F-22 rehearse for RIAT last year I can see it certainly enters areas of the envelope most other types can't get near so it's not too surprising they had some unique challenges.

[Jetstreamsky]

Actually the RR Pegasus engine for the Harrier is quite unusual, it has the LP and HP spools rotating in opposite directions to greatly reduce the gyroscopic effects, especially important during hover.

[SkippyBing]

I had read that somewhere in a book about the development of the Harrier, however the one time I saw a 'working' cutaway at Farnbrough everything seemed to be spinning the same way so I wasn't 100% sure. I can imagine the precession effect could be interesting in the hover!