View Full Version : Counter-rotating props...

17th Jun 2003, 17:25
...on twin recips. Are they still around?

Thought spares logistics got rid of those way back when, but ran into someone who claims otherwise. Are they still around in more or less modern designs? And if so, in what aircraft?


17th Jun 2003, 20:06
How modern do you mean? BE76 Duchess, PA44 Seminole, PA31 Navajo with the CR (is that the 225 HP model?) are still used in quite a few numbers. None of them could be thought of as really modern designs though.

17th Jun 2003, 20:46
That's new enough to prove me wrong. :)

Now I'm trying to find a bit of detail on the technical side. How do they go about having counter-rotating props without creating a spares nightmare? How much do the left and right fuel-to-noise converters differ?


18th Jun 2003, 03:36
ft - where I am from (Australia) this is an issue with regards to spares and logistics etc. but not an overwhelming one.:)

18th Jun 2003, 05:09
now that you mention it, I am interested in knowing what are the advantages/dissadvantages of counter rotating props? (I know they cause much less yaw) but what else?

18th Jun 2003, 08:34
Not quite true to say they cause much less yaw, presuming you're comparing asymmetric conditions. It's not always easy to show much of a difference.

The 'problem' is that the downgoing blade produces more thrust than the upgoing blade at higher airframe AoA (to distinguish from the prop disk's AoA). The increased thrust of the downgoing blade offsets the thrust line of the whole prop disk a bit towards the downgoing blade, instead of the thrust line being in line with the crankshaft.

If both engines rotate the same way then the engine with the downgoing blade closest to the wingtip will have its thrust line a little bit further from the a/c longitudinal axis. If one engine is stopped then the rudder will be needed to counter the yaw caused by the remaining engine. The further offset the thrust line of the running engine then the more rudder will be needed to counter yaw if the other engine is stopped. Another way of saying it is that for the same amount of rudder input more speed will be needed to produce enough force from the rudder to counter the asymmetric yaw. Rudder input is limited (there's only so much it can move) so as speed is reduced there will come a point where you can't use more rudder to stop the yaw - Minimum Control Speed, Vmc - hence one engine is more critical than the other ie more speed will be needed at full rudder deflection if one engine fails than if the other engine fails. The failed engine that leaves you with the worst effect is the 'Critical Engine'. In other words, the engine that when it fails leaves the engine with the most offset thrust line still running.

Counter rotating installations are way of eliminating this effect. Instead of having the downgoing blade outboard on one of the engines, reverse the rotation direction so that the downgoing blade is nearer the fuselage. A mirror image of the (now formerly) critical engine.

Nothing's free. The extra cost of all of this is that many parts now can't be swapped between the two engines. You now have to keep individual spares for each engine/prop. Economies of scale of manufacturing are reduced a bit (producing two different engines instead of twice as many that are exactly the same) etc. Most times the benefit isn't worth the cost & hassle. Keep the speed above the worst case Vmc & you're covered in both cases.

Interestingly, I seem to recall at least one a/c type opted for the reverse case. The P38 Lighting had both engines with a downgoing blade outboard instead of inboard. This means you're in the 'worst case' scenario re Vmc if either engine fails. It was done to solve another problem (oil cooling? Tailplane vibration? Not sure) at the 'expense' of having a slightly higher Vmc than would be possible with 'conventional' counter rotating props.

Tim Zukas
20th Jun 2003, 09:53
I read somewhere (a Gunston book?) that the P-38's engines turned the same way-- the gearing made the props counter-rotate. Zat sound right?

Col. Walter E. Kurtz
20th Jun 2003, 11:59
Tinny - that explains a USAAF pilots' little 'ditty' of the time 'Don't give me a P38 - the props they counter-rotate'.

I always wondered why it was looked upon that way - knowing the benefits of no CE.

Thanks for that!

PS How are those sheep looking in the northern summer sun??!!!

23rd Jun 2003, 06:56
The new AIRBUS A 400M heavy airlifter will have counter rotation dual props to cancel the torque generated by the 11,000 shaft horsepower per side turbines, also to cancel the prop tip vorticies - nothing new here - the Rusky'ss been doing this for years on the bear.. Concerning small twins - it is done to cancel low speed additive "P"-factor - IE t improve low speed directional stability - I think ..

Col. Walter E. Kurtz
23rd Jun 2003, 09:23
Used 2 fly boeing - I think that configuration is known as 'Contra rotating' propellers.

24th Jun 2003, 16:31
The A400M will not be fitted with co-axial contraprops. It will have an advanced propeller system with a 17.5 ft diameter 8-bladed propeller with composite blades, control of which will be integrated into the EuroProp International TP400-D6 high-speed turboprops' FADEC to maintain the engine and propeller system at a constant, optimum efficiency speed. All propellers will rotate in the same direction.

Since the aeroplane will fly using Airbus' FBW magic, a single engine failure will probably be barely noticeable to the crew - no shaking legs on the rudder bar or other asymmetric control problems, just point it in the required direction with the side-stick and let the FBW and FADEC do the hard stuff.......

GearUp CheerUp
25th Jun 2003, 06:35
Further to the question as to which modern aircraft have counter rotating props, next time you see a BAe Jetstream start up or shut down have a look at which way the props turn.

25th Jun 2003, 07:42
Tin, the P38 prototype had inwards rotating props,but this was changed on pre-production a/c to outwards rotation due to tailplane buffet. The UK development a/c had no turbo-chargers as the US didn`t want us or the Germans to know the technology, so those a/c also had right -hand rotation early Allisons fitted, known colloquially at Lockheed as " the castrated P-38"; and what did we do for them ? jet-engine,turbo-prop, follow-up tailplane-- Honour amongst friends?

25th Jun 2003, 19:27
and you're suggesting I'd find what?

Being a bit short on Jetstreams starting up in my immediate surroundings (very much the wrong kind of operator now, did help'em get started on a daily basis on the ramp once upon a time though), here's a snippet found on a BAe Jetstream from the NTSB:

"[...]equipped with two Dowty model R333/4-82-F/12 single-
acting, full-feathering, constant speed, 4-blade propellers, which rotate counter-clockwise as
seen from the cockpit."

Does not sound very counter-rotating to me. Thinking of another version? Or only about the CCW part? Is there a CW version of the same engine?

Still unable to find much info on if there is a way of coping with it, apart from the hassle of having entire mirrored engines available. Not that I have found time to do any in-depth research of the issue.


25th Jun 2003, 20:14
Interesting thread, as a "non-techy" I immediately thought you were referring to counter-rotation as provided on the Fairey Gannet, an aircraft with which I was extremely taken in my "yoof"! :ok: First seen at RAF Nicosia about 1956, wondered how they could go different ways off the same shaft, my knowledge of mechanics being rather rudimentary!

Eventually found that power was provided by twin Armstrong Siddeley Mamba turbines and separate gearing and shafts.

The US came up with something similar in the Douglas Skyshark which, unlike the Gannet, only had the one Allison engine.

25th Jun 2003, 21:29
So many interesting gadgets in aviation. And you're rarely allowed to take them apart! It's very frustrating. :D


26th Jun 2003, 08:16
It's not the pulling apart that's difficult to arrange. It's getting it back together before anyone notices that's awkward.. :8

Tim Zukas
29th Jun 2003, 08:32
Someone in this discussion
said the OV-10 had handed props but the engines turned the same way. Not sure whether the other guy was saying the same about the Navajo. Anybody know for sure if these or any other aircraft have handed props and unhanded engines?

29th Jun 2003, 22:15
When I saw this thread title, I presumed that the discussion was about those a/c with twin props, counter-rotating from the same engine/s, as in Shackleton. It is interesting to see that the thread has equally covered 'handed' rotation on single prop engines and the problems caused by loss of one engine.

On another side ... I am glad to see that the Americans have been witholding technology from us from P38 days and before. It would be sad to think that they had only just started treating us like dirt. But then, if we have a Presi Minister that allows us to be so treated - we get what we deserve.

30th Jun 2003, 07:13
The a/c examples I mentioned have handed engines. They're all direct drive ie no gearing.