Timz From United States, joined Sep 1999, 5350 posts, RR: 6 Posted (8 years 5 months 6 days 21 hours ago) and read 781 times:
Not contra-props (two props rotating in opposite directions around one axis)...
How many aircraft (piston or turboprop) can you think of that had two or four wing-mounted engines with the engine(s) on one wing rotating the opposite direction from the other wing? I was surprised how few come to mind-- the P-38 and its descendents the P-49 and P-58, the DeHavilland Hornet, and some Piper Navajos. There must be others, but how many?
Stallspeed From , joined Dec 1969, posts, RR: Reply 1, posted (8 years 5 months 6 days 20 hours ago) and read 706 times:
Seminoles, Senecas, and Dutchesses all have counter rotating props. This is common with multi-engine trainers. Counter rotating props reduce the Minimum controlable airspeed in an single engine situation(Vmcse).
My understanding is that although the P-38 had counter rotating props, the props rotated to the outside which made both engines "critical". Perhaps someone could tell me why it was designed like this. My guess is that it was a performance thing.
DesertJets From United States, joined Feb 2000, 6882 posts, RR: 28 Reply 3, posted (8 years 5 months 6 days 18 hours ago) and read 673 times:
Mechanically how different is a left turning piston engine vs. a conventional one? In order for the prop to turn the other way, without a complicated transmission, the engine has to turn the other way, right? So does it just have a different crank and firing order to allow for that?
Stop drop and roll will not save you in hell. --- seen on a church marque in rural Virginia
Prebennorholm From Denmark, joined Mar 2000, 5111 posts, RR: 55 Reply 4, posted (8 years 5 months 6 days 17 hours ago) and read 668 times:
Yep DesertJets, they simply produce a mirrored engine. First of all the camshaft, oil pump, ignition systems - and starter! - are changed, but there may be several more minor adjustments.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs, Preben Norholm
L-188 From United States, joined Jul 1999, 28550 posts, RR: 72 Reply 5, posted (8 years 5 months 6 days 17 hours ago) and read 665 times:
The first airplane with conter-rotating props was the Wright Flyer. Although I don't htink you are counting that since both props where powered off one engine.
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LZ-TLT From Germany, joined Apr 2001, 431 posts, RR: 0 Reply 7, posted (8 years 5 months 5 days 13 hours ago) and read 626 times:
Well, to "mirror" an engine, you need to do following:
Crankshaft - not obligatory, you may live well with the right-rotating one
Oil Pump - obligatory change(Mirror)
Cyliner Block - not obligatory, as long there are no problems with external equippment placement or oil lines
Cylinder head(s) - obligatory
Camshaft - obligatory
Ignition - not obligatory and not always needed
The main problem is the cylinder head and the steering times. You either need a "mirrored" cyliner head or some type of transmission which reverses the camshaft's direction of rotation. If the engine has some unconventional steering or multiple camshafts/tappetes(ie radial engines), you need to mirror the whole valve steering(and the cyliner heads as well). As long there are problems with the oil lines in the cyliner head, you'll be forced to mirror the block as well.
Also, don't underestimate the "external" parts of the aggregate like generator(s), exhaust pipes and so on. IMHO, this is the worst bottleneck which could rule "mirroring" an engine out.
Timz From United States, joined Sep 1999, 5350 posts, RR: 6 Reply 8, posted (8 years 5 months 5 days 12 hours ago) and read 613 times:
Don't forget: the fact that the props rotate in opposite directions doesn't prove that the engines do. I think Bill Gunston said the Allison V-1710 could switch its prop rotation just by adding an extra gear to the transmission.
Shark From Trinidad And Tobago, joined Jun 2005, 0 posts, RR: 0 Reply 9, posted (8 years 5 months 5 days 7 hours ago) and read 607 times:
The North American OV-10 Bronco uses conter-rotating props. The differance is the gearboxes. Both engines are the same. They just use gearboxes that a setup to rotate in the oposite direction.
Twotterwrench From , joined Dec 1969, posts, RR: Reply 10, posted (8 years 5 months 5 days 7 hours ago) and read 600 times:
The TIO-540 and LTIO-540 used on the Piper Navajo are really quite simple. The "reversing" is accomplished in the AGB by adding a spur gear. Some accessories are different, but these are mostly external, such as the dry air pump, the engine driven fuel pump and the starter. Internally, they are almost identical. The cylinders are completely interchangeable. The mags can be used on either engine by reseting the index point for the internal timing. No need to overcomplicate this, folks. The engine doesn't care which way it turns, what matters is which way the prop turns. I can't explain to you all the engineering, but I have worked on Navajos for years and can tell you that the vast majority of components internal and external are interchangable.. including crank and cam.
ParrotHead From , joined Dec 1969, posts, RR: Reply 11, posted (8 years 5 months 5 days ago) and read 590 times:
What makes props rotating to the outside "critical"? Can somebody briefly explain the various configurations of prop rotations. i.e. both rotatating cw, both ccw, and then both rotating to the inside and both rotating to the outside -----and the various idiosyncrasies associated with each setup? Thanks in advance!
Twotterwrench From , joined Dec 1969, posts, RR: Reply 12, posted (8 years 5 months 4 days 10 hours ago) and read 567 times:
The tendency for a twin with a failed engine is for it to roll into the dead engine. For example, if the right engine failed, the aircraft would tend to roll to the right due to P factor of that enigne. If you make the engines rotate so that the "power" stroke of the propellor, which is the down stroke of the blade, is made close to the fuselage, the the dead engine roll tendency is greatly reduced as the centrifical force is concentrated much closer to the centerline of the air craft. Did I explain that right??? anyone else wanna try???
Stallspeed From , joined Dec 1969, posts, RR: Reply 13, posted (8 years 5 months 3 days 14 hours ago) and read 552 times:
Great Job Twotterwrench! Give that boy a star To continue on what Twotterwrench said: The center of thrust does not come from the center of the propeller (P-factor already explained) So if both props turn in the same direction then the center of pressure of one of the props will be further from the longitudinal axis of the airplane. It has a longer arm, therefore more leverage to cause the airplane to yaw. The critical engine is the one that is oposite that engine with the longest arm. If you loose the critical engine the airplane will be more diffficult to control than if you lose the non critical engine.
To point out how serious this problem can be, there are certain situations (high power, low airspeed) that it would be safer to make a 270 degree turn into the good engine than it would be to turn 90 degrees into the dead engine.
I apologize that my question was poorly asked, and that it may have caused confusion. Perhaps someone could still verify if, and explain why the P-38's props both descend to the outside instead of to the inside, causing a higher Vmcse.
Jetguy From , joined Dec 1969, posts, RR: Reply 14, posted (8 years 5 months 3 days 13 hours ago) and read 550 times:
This may be picking nits, but as a CFIME I used to teach my students that counter-rotating did NOT make one engine "critical" and therefore implied that the other engine wasn't. It simply made both engines equally critical. All theories aside, piston powered, twin engine aircraft have two engines because they need two engines and under certain, limited conditions they can manage to remain airborne and controllable on one engine. The graveyards are packed with pilots (and their passengers) who either didn't understand or didn't believe that fact.
Twotterwrench From , joined Dec 1969, posts, RR: Reply 15, posted (8 years 5 months 3 days 8 hours ago) and read 543 times:
I think maybe a better way to say it Jet, is that it makes both engines "less" critical, although it is true that most light twins come with the guarantee that upon losing an engine, the remaining engine will fly you to the scene of the crash. I have seen a few real "stick and rudder" guys bring a plane around that lost one engine on take off, but not many.
JETPILOT From United States, joined May 1999, 3020 posts, RR: 42 Reply 16, posted (8 years 5 months 3 days 8 hours ago) and read 544 times:
Planes wit hcounter rotating props do not have a critical engine by defenition.
The FAA defines the critical engine as the engine who's loss most adversly affects the performance and handling characteristics of the airplane.
The reason one engine has a higher performance lass than the other on a non counter rotating engined aircraft is because the right side of the propeler disk produces more thrust than the left side.
The right side of the prop disk on the left engine is closer to the thrust centerline and has a lower thrust moment due to the reduced arm.
The loss of the left engine (the critical engine) leaves a majority of the available thrust, which has a higher thust moment, left to provide power.
The higher thrust moment of the right engine requires more rudder to counteract that thrust moment. Hence a hiher penalty in performance.
There you have it.
Keep the plane below blue line and you will live to tell your story.
Jetguy From , joined Dec 1969, posts, RR: Reply 18, posted (8 years 5 months 3 days 7 hours ago) and read 547 times:
Jetpilot...
You're definition of critical engine is absolutely correct. I reread my post and I realizied that I messed up what I meant to say. (That's what happens when you're tying to type and talk on the phone at the same time.) However, my point remains that all things considered counter-rotating engines merely "equalize" the aerodynamic factors which determine which engine is "critical". I still stand behind my original post, FAA definitions aside, that piston powered, twin engine aircraft have two engines because they need two engines and under certain, limited conditions they can manage to remain airborne and controllable on one engine. There are other factors which must also be considered. For example, on the old Apaches and Aztecs Piper saw fit to install hydraulic pumps on the left engine only. If that engine happened to be the one that failed right at takeoff, you not only had all of the aerodynamic factors working against you, but you also had the opportunity to use the hand pump to try and get the gear retracted. A very sporty proposition at best. I understand that engine accessories are not part of the FAA's critical engine definition. But in the real world, if you're going to be flying one of those beasts you had better have a full and complete understanding of everything that will be happening when that "once in a career" event takes place. Maybe it's the old "is the cup half full or half empty" question, but when I was instructing in a twing with counter rotating propellers, I always perferred to get my students thinking along the lines that both engines were equally critical, not that there were no critical engines. Oh well, it's all a moot point if you keep the airspeed above blue line any way. After spending the last 15 years flying jets I just have one further comment - propellors are for boats.
Timz From United States, joined Sep 1999, 5350 posts, RR: 6 Reply 19, posted (8 years 5 months 1 day 12 hours ago) and read 500 times:
Okay guys, lemme see if I've got this straight. On the OV-10 the engines turn in the same direction but the props don't, but on the Duchess and the various Pipers the engines counterrotate with the props, right? Can anyone confirm that the P-38 engines turned the same direction? How about the Merlins on the Hornet? And by the way-- what about the P-82? Some had Allisons, some had Merlins; did any or all counterrotate?
Jetguy From , joined Dec 1969, posts, RR: Reply 22, posted (8 years 5 months 20 hours ago) and read 480 times:
Hey Aaron,
I've paid my dues sitting between pairs of both Garrett '731s and Pratt & Whitney PT6s for several thousand hours. Just a humble opinion (yea right) but I'd rather fly behind Garretts any day. Granted, the early Garretts certainly had more than their share of "issues" but, all of the problems were addressed and the engines have matured into extremely reliable and efficient powerplants. In other words, all that is ancient history. Unfortuately, old opinions die very hard. I could go on and compare unscheduled inflight shutdown rates, fuel specifics, etc which also favor the Garrett but I've got to go get ready for a trip. Maybe later.
Twotterwrench From , joined Dec 1969, posts, RR: Reply 23, posted (8 years 5 months 18 hours ago) and read 470 times:
Jet... there is a BBBBBIIIIIIGGGGG difference between a 331 and a 731. I have always said that Garret must have hired some P&W engineers when they built the 731 cuz that's the first engine they ever got right. The 331 makes either a great APU or a boat anchor, but it's got no business driving a prop. Maintenance manhours to keep those pigs running are astronomical in comparison to a PT-6. And any engine that needs no less than 5 computers just to keep it running has something really wrong with it's initial design. Just my humble opinion.
Jetguy From , joined Dec 1969, posts, RR: Reply 24, posted (8 years 5 months 8 hours ago) and read 463 times:
Oops, I said 731 didn't I - of course I meant 331 and there is a very big difference. As a pilot I prefer the Garretts over the Pratts, however, I understand why the mechanics out there wouldn't like them. But, hey that's what they call job security.
L-188 From United States, joined Jul 1999, 28550 posts, RR: 72 Reply 25, posted (8 years 5 months 5 hours ago) and read 453 times:
Trotterwrench.....
Most P-38's did have counter-rotating props...The exception what an early back that where built for the brits. They lacked superchargers too, so they where pretty much universally considered dogs.
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To continue on what Twotterwrench said: The center of thrust does not come from the center of the propeller (P-factor already explained) So if both props turn in the same direction then the center of pressure of one of the props will be further from the longitudinal axis of the airplane. It has a longer arm, therefore more leverage to cause the airplane to yaw. The critical engine is the one that is oposite that engine with the longest arm. If you loose the critical engine the airplane will be more diffficult to control than if you lose the non critical engine. 
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