Econoboy From United Kingdom, joined Mar 2004, 157 posts, RR: 0 Posted (5 years 1 week 5 days 9 hours ago) and read 5977 times:
I believe that I am right in stating that a contra-rotating prop is more efficient than a single rotation prop with the same number of blades. This is because the rear set of blades is able to exploit the vortices shed by the forward set to generate more thrust than they would on their own (or something like that).
This being the case, why are there so few contra-props in service? Does the complexity and weight of the gearbox required negate any performance advantage, or has blade design moved on to the extent that there is no longer any benefit from contra-rotation?
Avt007 From Canada, joined Jul 2000, 2064 posts, RR: 7 Reply 1, posted (5 years 1 week 5 days 9 hours ago) and read 5942 times:
My impression is that a contra-rotating prop is only needed if a single prop can't handle the power output of the engine. The only place you'll see them is on very high powered engines. Given the extra costs, design and maintenance, and the weight penalty, they don't make sense otherwise. Engines such as the Allison 501 (Convair 580, Electra, P-3) and the PW150 (Dash 8-400) handle 4000 shp ok with a single prop.
SlamClick From United States, joined Nov 2003, 9996 posts, RR: 79 Reply 2, posted (5 years 1 week 5 days 8 hours ago) and read 5931 times:
I'ts been my informal observation that fixed-wing types are pathologically afraid of gearboxes. This ignores the fact that they have functioned flawlessly in helicopters for millions of hours.
I know of at least one very large ship that uses a counterrotating screw. It is a single-engine, single screw ship that has a freewheeling propeller mounted co-axial to, and just aft of the main propeller. It might be slightly smaller in diameter and it has fewer blades. (9 and 7 IIRC) Being uneducated in this field I can only guess that its function is to straighten out the wake of the thrusting prop which ought to produce some additional thrust.
[Edited 2004-11-02 22:00:59]
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QantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 50 Reply 3, posted (5 years 1 week 5 days 7 hours ago) and read 5897 times:
As far as I know, contra-rotating propeller designs really came into existence for p-factor issues. Quite simply, having two different propellers rotating in the opposite direction to one another pretty much eliminates any possible torque effect on the aircraft. As engines became more and more powerful, contra-rotating prop layouts became more and more popular, as planes often couldn't handle the amount of p-factor-induced roll. Now, however, contra-rotating designs are becoming relatively obsolete.
Fly2HMO From United States, joined Jan 2004, 3824 posts, RR: 4 Reply 4, posted (5 years 1 week 5 days 1 hour ago) and read 5827 times:
I have to second QuantasA332's comment. On a C172, it's a pain in the ass to always have the rudder to the right (not to mention the extra drag) during a climb. In a PA-44 with counter-rotating props, torque and p-factor are cancelled...
... that is if one engine doesn't commit suicide
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Sovietjet From Bulgaria, joined Mar 2003, 1876 posts, RR: 17 Reply 5, posted (5 years 1 week 5 days 1 hour ago) and read 5820 times:
Well if we are going to study contra rotating props...what better examples than the NK-12 engines powering the An-22, Tu-95, and Tu-114? Also the Kamov bureau has many helicopters with contra rotating props.
FredT From United Kingdom, joined Feb 2002, 2101 posts, RR: 30 Reply 6, posted (5 years 1 week 4 days 14 hours ago) and read 5764 times:
Getting enough power into the air with a propeller of a manageable size is the reason. And yes, there are beneficial interaction effects between the two propellers which improves the efficiency.
There aren't many of them as most aircraft with such a large need for power are better off with turbojet/turbofan engines anyway. The complexity of the system is another issue. I wouldn’t say that complex gearboxes are scary, they are old technology... but to remain reliable, they need maintenance and maintenance means operating costs.
Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
Spacepope From Vatican City State (Holy See), joined Dec 1999, 1928 posts, RR: 1 Reply 7, posted (5 years 1 week 4 days 13 hours ago) and read 5754 times:
Also look at single engined airframes that use single and contra props. In many cases the torque causes some tricky handling on takeoff. I've seen Sea Furys using full rudder and a little brake on one side to keep the nose pointed straight down the runway. Some Griffon engined Spitfires went with contraprops, and of course the Wyvvern and Gannet were hugely powered single engine types (if you count the double mamba as one engine) that used contraprops.
This does not explain why there were contraprops on the Shackelton, Bear, etc, but perhaps this is because there is more than one reason to use them.
MD11Engineer From Germany, joined Oct 2003, 10349 posts, RR: 67 Reply 8, posted (5 years 1 week 4 days 11 hours ago) and read 5725 times:
Also I think that counterrotating props produce a lot more noise through the siren effect (the rear prop blades choping through the airstream produced by the front ones). Ever heard an AN-22 on take off?
Econoboy From United Kingdom, joined Mar 2004, 157 posts, RR: 0 Reply 9, posted (5 years 1 week 4 days 10 hours ago) and read 5701 times:
Some intersting answers here - thanks! Spacepope and Fly2HMO mention the benefit of eliminating torque effects when taking off. On a slight tangent, does this explain why the Lockheed P38 was changed so that the left and right hand engines rotated in opposite directions? What about other high power twin engine planes like the Gumman Tigercat and the DeHavilland Mosquito?
Spacepope From Vatican City State (Holy See), joined Dec 1999, 1928 posts, RR: 1 Reply 10, posted (5 years 1 week 4 days 10 hours ago) and read 5700 times:
For the P-38, they introduced "Handed" props because of the torque as well. Early P-38s were delivered to the RAF with both props turning the same direction, and these airframes were rejected. When both props turn the same way, the plane turns one way pretty well, but not the other direction, as it is fighting against the prop torque. Handed props also eliminate the problem in twins of a "critical engine" so that the plane will behave pretty much the same with either engine out.
You also may notice on some WWII fighters that the engine is actually canted a few degrees from straight. I think there were some Italian fighters and maybe a french one like that. Basic idea was to turn the engine to help neutralize torque as well.
Pilotpip From United States, joined Sep 2003, 2610 posts, RR: 13 Reply 11, posted (5 years 1 week 3 days 23 hours ago) and read 5629 times:
Spacepope,
That the offset of the engine doesn't just apply to warbirds with large engines. Even today's piston singles incorporate a couple degrees of offset to help with torque. If you look at a Bonanza from directly in front it's clear that the engine isn't straight.
QantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 50 Reply 12, posted (5 years 1 week 3 days 22 hours ago) and read 5627 times:
While offsetting the engine to counter p-factor is still seen on some designs, a more common anti torque-effect measure is canting the vertical stabilizer instead. Or, in the case of the PC-12 and some other aircraft, weather radar or another such device is mounted in a pod on the wing to generate enough drag to reduce/eliminate discernible p-factor.
Phollingsworth From United Kingdom, joined Mar 2004, 825 posts, RR: 9 Reply 13, posted (5 years 1 week 3 days 15 hours ago) and read 5589 times:
contra-rotating, coaxial shaft systems have a couple of advantages over single prop systems.
1. It removes the swirl from the prop-wash, this increases the percentage of the work performed on the flow that can be used for thrust.
2. It allows a greater effective pressure rise across the entire prop disk. This is similar to a multi-stage compressor. Since the pressure rise lasts only as long as the flow takes to get through the disk it is relatively immeasurable. However, the flow speed is measurable. This produces greater thrust from a smaller diameter prop.
3. It allows the prop to turn at a speed closer to that of the gas turbine, decreasing gearbox size and weight
It has a couple of detriments.
1. For slower speed vehicles it is actually less efficient from a propulsive standpoint, i.e., Mach losses are less dominant and the power required for a given thrust is higher.
2. The counter rotating shafts are a bearing nightmare. The systems, in practice, cost a lot more to maintain. I also believe that the thrust bearings are even more problematic than the shaft isolation bearings.
As for using tip vortices, that is usually something that is avoided as the cyclic loads introduce interesting fatigue problems. If you want to eliminate the vortices problem put a duct around the prop.
Timz From United States, joined Sep 1999, 5355 posts, RR: 6 Reply 14, posted (5 years 1 week 3 days 14 hours ago) and read 5584 times:
"...does this explain why the Lockheed P38 was changed so that the left and right hand engines rotated in opposite directions? What about other high power twin engine planes like the Gumman Tigercat and the DeHavilland Mosquito?"
The P-38 and the DH Hornet had handed props-- very few (no?) other twin-eng fighters/bombers did.
Incidentally, the fact that the props are handed doesn't prove that the engines are. One of Gunston's books mentions that the Allison prop rotation could be reversed by the addition or removal of an extra gear, but I've never seen any other mention of that.
FredT From United Kingdom, joined Feb 2002, 2101 posts, RR: 30 Reply 15, posted (5 years 1 week 3 days 14 hours ago) and read 5580 times:
The design philosophies differed prior to and during WWII. In some places, handed engines were the standard, in other places not. Those who did converted eventually, due to supplying spares for both versions of the same engine consuming more resources than it was worth.
Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
Econoboy From United Kingdom, joined Mar 2004, 157 posts, RR: 0 Reply 17, posted (5 years 1 week 3 days 7 hours ago) and read 5520 times:
Handed engines means that they are designed for clockwise or counter-clockwise rotation of the crankshaft. I don't know if the later P38's had one engine of each hand, or if like Timz suggests, the change in rotation direction was achieved simply by using different gear layouts in the reduction gearbox. If the engines had different crankshaft rotations, it means they would not have been interchangeable from left to right - hence a spares issue.
By the way Phollingsworth, thanks for the informed answer.
Pilotpip From United States, joined Sep 2003, 2610 posts, RR: 13 Reply 19, posted (5 years 1 week 3 days 5 hours ago) and read 5487 times:
Econoboy, thanks for the answer. Geared piston engines do seem to be something that most pilots are not fond of. I wonder why this is? Turbine engines have no problem utilizing gear boxes and they reduce an engine turning tens of thousands of RPM down to maybe 2000 RPM at the prop.
L-188 From United States, joined Jul 1999, 28569 posts, RR: 73 Reply 20, posted (5 years 1 week 3 days 1 hour ago) and read 5455 times:
For the P-38, they introduced "Handed" props because of the torque as well
Actually the first airplane with handed props was the Wright Flyer.
Wasn't intentional, just a byproduct of driving both props off the same motor.
Econoboy, typically the second prop on such a counter-rotating system is much less efficient then one in front of it. That is because while the forward prop is operation in clean air, the one behind it is constantly in the was.
As far as single props go. Eventually you will loose efficiency if you put too many blades on it, since at some point the trailing blade will operate in the wash of the leading one.
That is why generally two bladed props are more efficent then three bladed, which are more efficient the four blades....ad nausum.
A more efficient solution is to either widen the blade, as was done with the "Paddle Blades" that where added to the B-17 between the E and F versions of that aircraft or lenghthen the blade.
There is a limit to how much you can lengthen a blade however, because the farther out you go the faster the end of the blade travels, and efficiency really drops off if the ends are supersonic.
Blade design is truly a black art.
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Econoboy From United Kingdom, joined Mar 2004, 157 posts, RR: 0 Reply 21, posted (5 years 1 week 2 days 9 hours ago) and read 5394 times:
Reading your replies, it seems that the contra-rotating prop is pretty complex mechanically, and its main advantage is the elimination of torque reaction when changing throttle settings.
I was puzzled why the griffin Engined Spitfire used a five bladed prop to absorb the power whilst the later Shackleton, which used the same engine, had contra-rotating props (with three blades on each hub). I found, after some poking about on the web, that the contra-rotating griffin arrangement was developed for the carrier version of the Spitfire. The torque reaction from single rotation Prop would turn the plane to the right. If a pilot applied full throttle to go round again after abandoning a landing, he would find himself flying into the flight deck island, so a contra-rotating prop was essential for naval flying. The shackleton inherited the same arrangement of contra rotating prop and engine.
L-188, I agree with you about blade design being a black art!
Spacepope From Vatican City State (Holy See), joined Dec 1999, 1928 posts, RR: 1 Reply 23, posted (5 years 6 days 6 hours ago) and read 5242 times:
Econoboy, that is very true. Later models of Seafires did have contraprops. From observation, it seems that contraprops were used on single engine high horsepower designs, where the cutoff for single prop use is at about 2500 horsepower. I know a few racers out there still use single props on 4000hp or so engines, but those seem to be an exception. Wyvern had 3600hp, Gannet had 2900, however each section of the double mamba drove a seperate prop, so I'm not sure this is a "true" contraprop.
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