Co-rotating props

[Dick Whittingham]

Can anyone define co-rotating propellers for me? It refers to twins, so it is either what I would know as handed propellers, lh on one side and rh on the other. or it means the exact opposite.

Thanks,

Dick W

[Tinstaafl]

That's a bit of a confusing term, isn't it? 'Co' normally means 'in agreement/associated etc. Alternatively it could be an abbreviation so that gives:


Co-rotating: different engine/prop combinations rotating in the same direction
Co(unter)-rotating: different engine/prop combination rotating in opposite directions
Co(ntra)-rotating: same engine, props rotating in opposite directions.

In what context was the term used?

[willbav8r]

Counter rotating props as in light piston twins (PA44 180)?
Referring to the critical engine aspect? Or a contra rotating prop (one engine, two props) like on the Shackleton / TU95 Bear?

[Dick Whittingham]

It is with reference to the different asymetric effects with twins with engines that go the same way and those that don't. We understand the effects, the problem for us is to determine what the JAA mean by the term when it stands alone in a question.

Dick W

[sycamore]

With respect to co-rotating props( ie props that rotate in the same direction- either clockwise/ or anti), the "critical" engine is the one that ; if it fails, you will have the worst handling effects, and the highest Vmca and Vmcg, worst single-engined climb performance.

This is because the "live" engines` downgoing-blades are farthest from the a/c centre-line, and at shallow climb angles this gives a centre -of -thrust, outboard of the prop centre-line, requiring more rudder/ aileron to counter, giving more drag, etc,etc. For most American twins/4`s , the critical engine will be No1,( from the cockpit blades rotate c/w).

A/c which have counter-rotating props have either; no critical engine,or both are critical, depending on whether both engines rotate inboard(at the top of the arc) or out board. Where you have counter-rotating props , they will mostly rotate inboard; only the P-38 is different.

[Dick Whittingham]

OK. Sycamore says that co-rotating props all go in the same direction. Do we all agree with that?

I have to say that the feeling I get from feedback is that the JAA think otherwise. Anyone know of a definitive reference?

Incidentally, the asymetric blade effect occurs when the flightpath and prop axes are not in line, usually at low speed and therefore high alpha, as well as with tail draggers on the early takeoff roll. Climbing doesn't come into it.

Dick W

[Tinstaafl]

Yes. I read co-rotating to be the same as same direction engines/props. Counter rotating is the term I know for opposite rotating. However, it wouldn't surprise me if JAA had decided to use 'co' as it's own abbreviation for 'counter'... :suspect:

[Onan the Clumsy]

Think about it, if they used swash plates, there'd be no assymetrical thrust and no p factor, well none caused by the descending blade at least.

[Specnut727]

OH S**T !!! Now we are in trouble !

Next you'll be suggesting that we turn the whole transmission so the shaft is vertical and the prop(s) are at the top. Then think about the complexities of counteracting the torque...........etc, etc,

[ft]

Jane's Aerospace Dictionary does not mention co-rotating propellers, which makes it doubtful whether it is a good term to use. It does however mention co-rotating wheels, as "wheels on live axle and thus constrained to rotate together".

In my mind, co-rotating would in fact mean a single spool arrangement. I e, if one rotates so does the other, same direction and speed. I do not believe that is what the JAA means though, but I think it is established that they'll be turning the same way.

Cheers,
Fred

[sycamore]

DW,remember, climb can be positive or negative with the same attitude in some twins.
Is there not a JAA glossary of definitions somewhere--- hmmm, suppose not as they wouldn`t be able to agree.. Suggest a call to the CAA and ask for further advice--I really am sure they would be able to sort it out, despite what people say about them!:ok: