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Redbandit
7th Aug 2011, 16:59
I know counter rotating props help to eliminate a critical engine, so my question is why is it most aircraft don't have counter rotating props? I thought maybe because they may be more complex in design but can't really think why.
Also on a 737-800 do the engines operate in the same direction?
Just a couple of questions that may pop up in my interview.

Avionker
7th Aug 2011, 17:11
Also on a 737-800 do the engines operate in the same direction?

Yes. I'm not aware of any jet aircraft where the engines are 'handed'. On turboprops the engines themselves are not 'handed', but gearboxes are. Not sure about the new geared turbofans, but I see no reason for them to go down the turboprop route.

As for your first question, do you mean a single engine aircraft with contra rotating props, or twins with counter rotating props on opposite engines?

Jig Peter
7th Aug 2011, 17:13
For example, the "handed" (counter rotating) props on the de Havilland Hornet (among others) were to counter swing on take-off. If one engine failed, loss of thrust on one side still caused a swing.
If one jet (fan- or "pure"-) engine fails, there's still a swing (yaw) through loss of thrust: direction of rotation doesn't come into it.

3holelover
7th Aug 2011, 17:16
I think simplicity is the best answer to your first question. Engine manufacturers won't have to produce opposite rotation engines or gearboxes, and those who install them won't find themselves without one that turns in the right direction...
And yessir, all turbofan equipped transport category aircraft will have engines that turn the same way on either wing. (at least all the one's I'm familiar with, but possibly someone will point to an odd exception?) The direction of rotation isn't really a factor on a jet.

Redbandit
7th Aug 2011, 17:31
yeh the aircraft i was refering to was a piper seneca where the props are counter rotating. thanks for the help

barit1
7th Aug 2011, 18:28
"Critical engine" is a relative condition. If the Vmca difference is only a knot or two, it hardly pays to have the expense of two different engines.

And the reason turbofans aren't LH/RH is because of the stator vanes behind the fan rotor. Without them (like in a conventional propeller) the downstream airflow is a vortex about the prop/fan axis. The stator vanes recover that vortex energy, and create a linear airflow field. So it makes no difference if the fan rotor turns LH or RH.

Volume
8th Aug 2011, 09:32
why is it most aircraft don't have counter rotating props? NOISE !!! Ever heard a Tu-95 taking off ? The Noise of four counterrotating props or 16 Blades hitting the wake of the 16 other propeller blades rotating the other way, meaning 256 "bangs" per prop revolution, is just unbelieveable.

awblain
8th Aug 2011, 10:47
There is no steady torque applied to a jet engine by the airflow, due to the stators as noted by barit1 above.

There are still (I believe small) torque effects from the rotating parts, both gyroscopic - from changing the orientation of the engine - and inertial - from changing the engine's rotational speed. Both depend on the angular momentum of the shaft/blades, which scales with the angular speed, and the moment of inertia - an averaged/integrated product of mass and the square of the distance of the mass from the axis.

Most of the mass in a jet is in the shaft, relatively little in the blades, making for a small moment of inertia, whereas a propeller has more mass further off axis. The gyroscopic rotation in the horizontal plane when changing pitch, and the counterrotation effect along the shaft axis when changing the rpm should both be much less significant for a jet than a propeller of the same power.

toolowtoofast
8th Aug 2011, 11:27
NOISE !!! Ever heard a Tu-95 taking off ? The Noise of four counterrotating props or 16 Blades hitting the wake of the 16 other propeller blades rotating the other way, meaning 256 "bangs" per prop revolution, is just unbelieveable.


those are 'contra rotating' - quite different to 'counter rotating'

Mach E Avelli
8th Aug 2011, 11:37
Cost is an issue. With counter-rotation you either need another gearbox or an engine set up to run the other way. Plus there is the extra expense of producing a propeller that is 'handed'. So the spares inventory goes up big-time.
For the average light twin putting out maybe 300 HP at the hub it is hardly worth the effort for a very small gain in handling and performance. Hence it never caught on. It was a sales gimmick.
Big military aircraft probably did need it, but for them cost was no object.

hval
8th Aug 2011, 12:40
Must remember that that there are precession effects in turbofans. Much of the precession effects may be negated by having HP and LP Spools that rotate in opposite directions. Examples of engine that do so are the RR Trent, the GE90 and the Engine Alliance GP7200 ( which is based on the GE90).

EDIT: - Whoops I see awblain has already answered with my point. My apologies.

Dawdler
8th Aug 2011, 12:43
When I saw the title of this thread I immediate thought of the counter rotating props ala Shackleton, where each engine drives two props, one clockwise, the other counter clockwise. I have always wondered about the efficiency of such an arrangement and the gearbox complications (and the power losses) involved.

I assume in a twin engine aircraft where one engine direction differs to the other, it makes a difference which one is turning which way. It certainly has an effect on stability in twin engined boats.

Max Angle
8th Aug 2011, 13:11
Off topic really but what the hell.

"Precious Metal" is a P51 Mustang racer with a Griffon engine driving contra rotating props, sounds and looks very mean, great stuff.

http://www.youtube.com/watch?v=HVWmI1PMU