Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: boomerlu on March 08, 2010, 01:15:37 PM
-
Hey guys, I was wondering... why do some of our twin engine planes have co-rotating props instead of counter-rotating ones? Is there any disadvantage to having counter-rotating props that would offset the advantage of having no net torque?
Thanks.
-
I'm sure someone else will have more to say on this, but here's what I know. Basically, if you have counter-rotating propellers, you need two different engines, which increases cost and complexity. However with both engines rotating the same direction, you only need one type of engine.
-
The main disadvantage to counter rotating eng. is the fact that 2 engines must be kept "on hand" for replacement.
With a twin that has noncounter rotating engines,any engine would do as a replacement,OTH the counter rotating twin would need "handed" engines and this could be a logistics nightmare.
:salute
-
Why 2 engines? :headscratch: Aren't most of the props geared down? I'd imagine it would just be the gearing part that would be different.
-
CC all - it's a logistical thing not a combat performance one. :salute
-
It's not just gearboxing, the P-38s (if I recall right) had different crankshats among other things. They were convertable, but it was not just a simple swap of parts. It took some work to rebuild them to spin the other way.
When you consider that many twin engine designs flew and fought with co-rotating props during the war, the one-sided torque was not the deciding factor -- as much as the typically diminishing returns on performance as compared to single engine fighters.
Co-rotating the props meant less work, less reliability, not really much benefit gained for making them counter-rotating. So basically the P-38 was the only (don't kill me for saying this if you think otherwise) "successful" counter-rotating design of the war.
There are also downfalls to the counter-rotating props... you could not reduce one engine, then reduce the other without first bringing that first engine back up to the level of the other. At least on the P-38, that is. So each design is not perfect.
Plus, all throughout WW2 engine design, teething problems, and failures during testing of new planes, were the common thread. Poor construction, lack of understanding, or bad manufacturing processes were rampant. Just getting the thing in the air with both engines running meant (usually) relying on an engine you already trusted.
-
The main reason for counter-rotation of props is to make the airplane more manageable with an engine inop (it's a p-factor thing), however in the case of the P-38 they turn the props "out" (done in search of a cure to the compressibility thing) so in effect have made both engines the critical engine.
-
The main reason for counter-rotation of props is to make the airplane more manageable with an engine inop
you might want to rethink that one ... :D
-
Easy to answer....get in a JU-88, remove Auto-Take-off option, full flaps, let loose of the stick and hit full throttle... :x Really Fun! Right?...Counter rotating engines nearly eliminates torque problems.
"Another method of counteracting the torque effect is the use of contra-rotating propellers, or propellers that rotate in opposite directions. As piston engines became increasingly powerful in the 1940s, some single-engine aircraft required contra-rotating propellers to remain controllable. The Super-marine Seafire and Maachi M72 are examples of aircraft with such powerful engines that they needed twin contra-rotating props on the same shaft to cancel the torque effect. Contra-rotating props also offer advantages in eliminating other assymetrical effects and were found to give aircraft much better handling characteristics. In addition, the torque effect can be completely eliminated on multi-engined aircraft in a similar manner. Most twin-engined or four-engined piston aircraft use propellers that rotate in opposite directions to negate the torque effect. By rotating the right engine clockwise and the left engine counterclockwise, for example, the torques will cancel without any corrective action being necessary."
http://www.aerospaceweb.org/question/dynamics/q0015a.shtml
Obviously the propeller which turns opposite the crank shaft also needs about 100+lbs of gears box to change rotation. The piston engine's don't normally rotate opposite, it's the gear box that reverses rotation. :salute
-
Contra-rotating (single engine, 2 props) turned out not to be so useful. The weight was significant (and on the front of the plane, throwing CoG and balance off) and the added gain wasn't not very large. Later model spitfires tested contra-props but found little performance gain over the single-prop versions. Definitely not enough to be worth the headaches of maint/upkeep on them! :D
But, that's all a side-point. Back to the regularly scheduled program!
-
you might want to rethink that one ... :D
Why?
-
Why?
Because it doesn't matter for a two engined plane when only one of them is running :)
-
Allisons were built in left and right configurations, as were the Detroit Diesel 2 cycle motors. Not sure about the Allisons, but a DDA could be made left or right without much trouble really. I'm really not aware of any other major manufactuer that did this, in any significant numbers anyway.
-
Contra-rotating (single engine, 2 props) turned out not to be so useful. The weight was significant (and on the front of the plane, throwing CoG and balance off) and the added gain wasn't not very large. Later model spitfires tested contra-props but found little performance gain over the single-prop versions. Definitely not enough to be worth the headaches of maint/upkeep on them! :D
But, that's all a side-point. Back to the regularly scheduled program!
Yes, a major prohibiting factor against the Sikorsky helicopters with contra-rotating blades, size, weight & complexity of Transmissions (Already huge in a chopper anyway), and thus the expense, but MAN, can they fly!!
-
Because it doesn't matter for a two engined plane when only one of them is running :)
But it does matter. With both props turning the same direction (clockwise from cockpit) having only the right engine will cause more control problems than having only the left engine due to p-factor. It's the down moving blade being farther from the center of the airplane that is the issue...or at least that's what they teach when you get the multi rating. <G>
-
For most planes counter rotating props gave no real advantage and just added mechanical complexity. Canceling torque is important almost only at low speeds and the planes were not supposed to fly slow, especially not in combat and even less in high alt combat that is more "energy" style - Do not take AH as an historical example. P-38 was somewhat "old" plane in its concept and design. The Germans for the 110 were more focused on energy fighters and wanted speed much more than slow turning ability, so did De-Havilland that built the Mosquito to fly as fast as possible as cheap as possible. It is not like P-38s did not enjoy the added stability, ease of landing/takeoffs and the rare occasions where this helped in combat - it did make it a "better" plane, but in the grand scheme of things, reliability, logistics and costs often make bigger difference than performance in a small part of the envelope the pilots were repeatedly told not to get into.
-
The De Havilland Hornet twin engined fighter did have counter rotating props. IIRC Rolls Royce built a special low frontal area version of the Merlin with a removable reduction gearbox on the front. So the engine could be adapted from left to right rotation by just changing the gearbox and propellor.
Beautiful fighter, shame it was just too late for WW2.
-
The P-38 design uses the relative ease of rotation direction change inherent in the V1710 Allison to good effect - the counter-rotating blades have often been stated as being a minor performance multiplier by some(probably the same that insist that the Merlin should have been shoehorned into the -38), but when the Brits bought the Lightning, they insisted upon same-directional allisons installed. The performance was not exceptional, and Lockheed referred to these examples as "castrated" lightnings(if my memory serves).
This lends credence to the concept that counter rotational props, at least in the P-38 installation, had very real and substantial performance implications.
-
The De Havilland Hornet twin engined fighter did have counter rotating props. IIRC Rolls Royce built a special low frontal area version of the Merlin with a removable reduction gearbox on the front. So the engine could be adapted from left to right rotation by just changing the gearbox and propellor.
Beautiful fighter, shame it was just too late for WW2.
Yes indeed Greebo,it is a beautiful plane,IIRC DH built 1, count them,Mossie with counterrotating props.As has been mentioned,the preformance gains didnt outweigh the cost,logistics,etc.,same could be said about the dosal filet.If you look at the Hornet you'll see a dosal filet,DH knew the mossie could benefit from it's addition but the cost and disruption of assembly lines just weren't worth the trouble,instead Mossie pilots were told to be gentle on the rudder.
:salute
-
Its too bad the Mossi didnt get a fair shake w the counter rotating props. Who knows how much better it would have been?
-
The advantage of counter-rotating propellors is balancing out the effects of torque and p-factor and eliminating the problem of the critical engine.
The P-38 though was kind of special, in that it had 'reverse' counter-rotating propellors with the tops of the propellors rotating outwards. This is what helped make the P-38 such a stable gun platform.
ack-ack
-
the Allison is not that hard to "reverse" cam shaft,cam shaft drives, supercharger impellers and starter motors, not crankshaft is the only major components that needs to be changed, if i remember right!
i had one out of the left wing of a p-38 in an old pulling truck once.
-
The specific reason the Mosquito did not have counter rotating propellers was because the RAF did not want to have the logistics complication of dealing with handed engines.
-
The specific reason the Mosquito did not have counter rotating propellers was because the RAF did not want to have the logistics complication of dealing with handed engines.
I had read that somewhere. I was just stating the fact that it was a shame teh Mossi didnt get the chance to try out teh counter rotating props. I wonder just how much faster it would have gone and just how much more capable it would have been.
-
and you could always get it completely wrong with counter rotating props. luckily they were able to just swap engine and carry on
"The XP-82 was to be powered by two Packard-built Rolls-Royce V-1650 Merlin engines. Initially, the left engine was a V-1650-23 with a gear reduction box to allow the left propeller to turn opposite to the right propeller, which was driven by the more conventional V-1650-25. In this arrangement both propellers would turn upward as they approached the center wing, which in theory would have allowed better single-engine control. This proved not to be the case when the aircraft refused to become airborne during its first flight attempt. After a month of work North American engineers finally discovered that rotating the propellers to meet in the center on their upward turn created sufficient drag to cancel out all lift from the center wing section, one quarter of the aircraft's total wing surface area."
-
I had read that somewhere. I was just stating the fact that it was a shame teh Mossi didnt get the chance to try out teh counter rotating props. I wonder just how much faster it would have gone and just how much more capable it would have been.
It wouldn't have been any faster, just a bit easier on the pilots.
-
i often wonder why they just didn't just flip the engines to counter rotate the props for the advantages they give an airframe.
col is the p-factor effect different when the torque is not on the center line of the airframe?
-
i often wonder why they just didn't just flip the engines to counter rotate the props for the advantages they give an airframe.
col is the p-factor effect different when the torque is not on the center line of the airframe?
Flipping an engine, will change that particular engines Center of Gravity and would most likely have a negative effect on the craft.
-
But it does matter. With both props turning the same direction (clockwise from cockpit) having only the right engine will cause more control problems than having only the left engine due to p-factor. It's the down moving blade being farther from the center of the airplane that is the issue...or at least that's what they teach when you get the multi rating. <G>
I agree. I have seen this with B-26's with only Rt engine very difficult to stay airborn. :salute
-
Flipping an engine, will change that particular engines Center of Gravity and would most likely have a negative effect on the craft.
Flip the engine over and it still spins the same way. Roll a plane until it's inverted and that prop still turns the same direction!
:lol
To get it spinning the other way you've have to spin it around to a pusher. THAT's a setup (left push, right pull) I don't even wanna contemplate!
-
Flip the engine over and it still spins the same way. Roll a plane until it's inverted and that prop still turns the same direction!
:lol
To get it spinning the other way you've have to spin it around to a pusher. THAT's a setup (left push, right pull) I don't even wanna contemplate!
No. Flip an identical engine over and the COG of the engine changes. Do it with ANY engine, or get your hands dirty for a change and put one on an engine stand. The weights are different from top to bottom on a horizontal 1/2 way point, it's actually quite simple Krusty. I am not discussing Radial Engines either. I'm talking Inline Engines.
Keep worrying about fictional propeller setups and rotations that I have never once mentioned though.
-
I'm not talking about CoG... I'm not talking about being able to run inverted, oil starvation or any of that... The comment was made to "flip the engine over" so it would have counter-rotating props... No matter what way you flipped it, it's still spinning the same way. The blades still have the same "handed-ness"!
It's all a moot point because you roll a plane upside down and the blades don't magically spin backwards!
-
I'm not talking about CoG... I'm not talking about being able to run inverted, oil starvation or any of that... The comment was made to "flip the engine over" so it would have counter-rotating props... No matter what way you flipped it, it's still spinning the same way. The blades still have the same "handed-ness"!
I think he meant to turn it around and make the shaft come out of the back end. This is not so simple modification.
Also, having counter rotating props means a different kind of prop for each engine - those are not reversible either.
-
I'm not talking about CoG... I'm not talking about being able to run inverted, oil starvation or any of that... The comment was made to "flip the engine over" so it would have counter-rotating props... No matter what way you flipped it, it's still spinning the same way. The blades still have the same "handed-ness"!
It's all a moot point because you roll a plane upside down and the blades don't magically spin backwards!
....and you're assuming what you say is what I meant. You're wrong and missed the point. Again, I never GAF about the props. Inverting one engine, as someone else mentioned, would cause disturbance in a design. Because you are disrupting the COG.
You are going off on some tangent that has absolutely no bearing.
-
flipping engine over is not what i meant, mash understood right away, krusty did not.
how much work reversing the prop fittings/gears, flipping the exhaust, adjusting the water cooling if there is one depends on the engine but none of that would necessarily require a total redesign of the entire engine. i do not know but i could imagine an engine that was designed so one could have the prop mounted on either side after all the shaft/s runs all the way along the row of pistons. i would be surprised if this had never been done, in fact i would be very curious how allison managed to offer the rotation option on their engines.
as far as how much the C of G would effect the airframe, i do not know that either but after looking at many designs both radial and inline i find that the entire engine is often forward of the wing and i am not convinced that the C of G differences in the engine would be terribly significant to the overall balance of the aircraft and certainly would be correctable with a bit of work.
would it be easier than an entire factory retool? probably. would a gearing solution be easier still? probably.
gentlemen this is just a hypothetical conversation, no need to get nasty with each other about it.
its just fun with our imaginations is all ...
+S+
t
-
You say "flip" something, it means turn it upside down. You say "switch" something then it might apply to what you're discussing.
You flip cars over. You flip planes on their backs. You flip pancakes. You flip burgers.
You don't use that term to mean "turn a prop the other direction" -- it's not what the word means.
Sorry, but you weren't making sense and I had to reply.
P.S. I think Karaya's getting stressed. To help de-stress, I'm going to clarify something... You may be under the impression I'm targetting you because I quoted you. In fact the original comment was from thorsim: "wonder why they just didn't just flip the engines to counter rotate the props" and I quoted you because yours was the direct reply to him. YOU took it to mean turning the engine upside down (same as I did), but failed to grasp the point that he was talking about flipping it upside down in order to reverse the direction of the blades' spinning.
So, once again:
"wonder why they just didn't just flip the engines to counter rotate the props " <-- couldn't let that go without SOME comment.
-
You say "flip" something, it means turn it upside down. You say "switch" something then it might apply to what you're discussing.
You flip cars over. You flip planes on their backs. You flip pancakes. You flip burgers.
You don't use that term to mean "turn a prop the other direction" -- it's not what the word means.
Sorry, but you weren't making sense and I had to reply.
P.S. I think Karaya's getting stressed. To help de-stress, I'm going to clarify something... You may be under the impression I'm targetting you because I quoted you. In fact the original comment was from thorsim: "wonder why they just didn't just flip the engines to counter rotate the props" and I quoted you because yours was the direct reply to him. YOU took it to mean turning the engine upside down (same as I did), but failed to grasp the point that he was talking about flipping it upside down in order to reverse the direction of the blades' spinning.
So, once again:
"wonder why they just didn't just flip the engines to counter rotate the props " <-- couldn't let that go without SOME comment.
ADOBE agrees with me in the ability to flip along both the vertical and horizontal axis ...
;)
probably because ...
flip (flp)
v. flipped, flip·ping, flips
v.tr.
1. To throw or toss with a light brisk motion: flipped me the ball; flipped his hair out of his eyes.
2. To toss in the air, imparting a spin: flip a coin.
3.
a. To turn over or around, especially with a light quick motion: flip over a card; flipped the tape to play the other side.
b. To turn through; leaf: flipped the pages of the report.
4. To strike quickly or lightly; flick.
5. To move or act on with a quick motion: flip a switch; flipped open her briefcase.
v.intr.
1. To turn over: The canoe flipped over in the rapids.
2. To turn a somersault, especially in the air.
3. To move in twists and turns: fish flipping about in the net.
4. To move quickly and lightly; snap: The lid flipped open.
5. To leaf; browse: flipped through the catalogue.
6. Slang
a. To go crazy. Often used with out.
b. To react strongly and especially enthusiastically: She flipped over the new car.
n.
1. The act of flipping, especially:
a. A flick or tap.
b. A short, quick movement: a flip of the wrist.
c. A somersault.
2. Informal A reversal; a flipflop.
3. A mixed drink made with any of various alcoholic beverages and often including beaten eggs.
adj. flip·per, flip·pest Informal
Marked by casual disrespect; impertinent: a flip answer to a serious question.
Idiom:
flip (one's) lid Slang
1. To react strongly, as with anger or enthusiasm.
2. To go crazy.
-
You say "flip" something, it means turn it upside down. You say "switch" something then it might apply to what you're discussing.
You flip cars over. You flip planes on their backs. You flip pancakes. You flip burgers.
You don't use that term to mean "turn a prop the other direction" -- it's not what the word means.
Sorry, but you weren't making sense and I had to reply.
P.S. I think Karaya's getting stressed. To help de-stress, I'm going to clarify something... You may be under the impression I'm targetting you because I quoted you. In fact the original comment was from thorsim: "wonder why they just didn't just flip the engines to counter rotate the props" and I quoted you because yours was the direct reply to him. YOU took it to mean turning the engine upside down (same as I did), but failed to grasp the point that he was talking about flipping it upside down in order to reverse the direction of the blades' spinning.
So, once again:
"wonder why they just didn't just flip the engines to counter rotate the props " <-- couldn't let that go without SOME comment.
Well then sound off when you're doing the quoting. :devil Thorsim is incorrect on flipping an engine upside down, the crankshaft will still spin the same way.
-
Flip the engine over and it still spins the same way. Roll a plane until it's inverted and that prop still turns the same direction!
:lol
To get it spinning the other way you've have to spin it around to a pusher. THAT's a setup (left push, right pull) I don't even wanna contemplate!
I think he meant flipping it end to end. If an engine was built with a fly-wheel on both ends, it would be reversible. :banana:
-
I think he meant flipping it end to end. If an engine was built with a fly-wheel on both ends, it would be reversible. :banana:
:huh
-
I think he meant flipping it end to end. If an engine was built with a fly-wheel on both ends, it would be reversible. :banana:
exactly ...
or if you "moved" the flywheel mechanism to the other side of the engine, or if the shaft protruded through the block on both sides so either rotation was an option ...
remember those little electric motors where the shaft passed through on both sides that you could "flip" to produce the drive direction you wanted
;)
mash way to follow krusty's fail even after i posted the definition of the word flip ...
maybe you posted before you caught up with the thread
-
mash way to follow krusty's fail even after i posted the definition of the word flip ...
maybe you posted before you caught up with the thread
Funny, but in the definition given, except for the word "around", all the examples given seem to pertain to "over" or "up and down".
Semantics.
wrongway
-
Funny, but in the definition given, except for the word "around", all the examples given seem to pertain to "over" or "up and down".
Semantics.
wrongway
what do you do with a switch ...
-
what do you do with a switch ...
Turn it on and off.
-
Turn it on and off.
:D
-
Ok, my two cents worth. In the P38, the counter-rotating props would cancel out the counter torque from the two engines.
I remember reading in a book, a long time ago (and no, I don't still have the book), that both engines were the same, but the left hand engine had a gear system which reversed the rotation of the left hand prop.
As for the direction of the props, when I was younger, I always thought they rotated that way so that if you crashed, and the props broke off, they would roll away from the plane, not toward the cockpit. :D
Turns out it was actually because it made for better airflow. Go figure.
-
Ok, my two cents worth. In the P38, the counter-rotating props would cancel out the counter torque from the two engines.
I remember reading in a book, a long time ago (and no, I don't still have the book), that both engines were the same, but the left hand engine had a gear system which reversed the rotation of the left hand prop.
As for the direction of the props, when I was younger, I always thought they rotated that way so that if you crashed, and the props broke off, they would roll away from the plane, not toward the cockpit. :D
Turns out it was actually because it made for better airflow. Go figure.
in the 38 the motors did not rotate in the same direction!
-
I can't find the web page that explained why the P-38's props turned as they are. All I remember is the P-38 props were tried with the propeller tops rotating both towards and both away from the pilot's point of view. With the propellers tops rotating towards the pilot's cockpit (left prop=clockwise, right prop counter-clockwise) allowed a better torque advantage should one engine fail. The reference article said the main decision to make the propellers tops rotating away from the pilot's cockpit (as they were all built; left prop=counter-clockwise, right prop=clockwise) was to prevent rain water from flying off the props and onto the pilot's windshield, thereby, possibly blinding the pilot's frontal view. I can't remember if the article said the water splashing (left prop=clockwise, right prop counter-clockwise) was a problem on the ground, in the air, or both.
A couple of interesting quotes:
http://bbs.hitechcreations.com/wiki/index.php/P-38_Lightning (http://bbs.hitechcreations.com/wiki/index.php/P-38_Lightning) :
Another issue with the P-38 arose from its unique design feature of outwardly rotating counter-rotating propellers. Losing one of two engines in any twin engine non-centerline thrust aircraft on takeoff creates sudden drag, yawing the nose toward the dead engine and rolling the wingtip down on the side of the dead engine. Normal training in flying twin-engine aircraft when losing an engine on takeoff would be to push the remaining engine to full throttle; if a pilot did that in the P-38, regardless of which engine had failed, the resulting engine torque and p-factor force produced a sudden uncontrollable yawing roll and the aircraft would flip over and slam into the ground. Eventually, procedures were taught to allow a pilot to deal with the situation by reducing power on the running engine, feathering the prop on the dead engine, and then increasing power gradually until the aircraft was in stable flight. Single-engine takeoffs were possible, though not with a maximum combat load.
http://www.answers.com/topic/counter-rotating-propellers (http://www.answers.com/topic/counter-rotating-propellers)
Counter-rotating propellers, are found on twin-, and multi-engine, propeller-driven aircraft and have propellers that spin in opposite directions.
Generally, the propellers on both engines of most conventional twin-engined aircraft spin clockwise (as viewed from the rear of the aircraft). Counter-rotating propellers generally spin clockwise on the left engine, and counter-clockwise on the right. The advantage of counter-rotating propellers is to balance out the effects of torque and p-factor, eliminating the problem of the critical engine.
In designing the Lockheed P-38, the decision was made to reverse the counter-rotation and having the "tops" of the propeller arcs rotating outwards, away from each other. Tests on the initial XP-38 prototype demonstrated greater accuracy in gunnery with the unusual configuration. The German World War II Henschel Hs 129 ground attack aircraft, Heinkel He 177 heavy bomber and Messerschmitt Me 323 transport's counter-rotating powerplants used the same rotational "sense" as the production P-38 did.
Drawbacks of counter-rotating propellers come from the fact that, in order to reverse sense of rotation of one propeller, a gearbox needs to be used or the engine or engine installation must be different. This may increase weight (gearbox), or maintenance and spare parts costs for the engines and propellers, as different spare parts need to be produced in lower numbers, compared to a conventional installation.
-
I can't find the web page that explained why the P-38's props turned as they are. All I remember is the P-38 props were tried with the propeller tops rotating both towards and both away from the pilot's point of view. With the propellers tops rotating towards the pilot's cockpit (left prop=clockwise, right prop counter-clockwise) allowed a better torque advantage should one engine fail. The reference article said the main decision to make the propellers tops rotating away from the pilot's cockpit (as they were all built; left prop=counter-clockwise, right prop=clockwise) was to prevent rain water from flying off the props and onto the pilot's windshield, thereby, possibly blinding the pilot's frontal view. I can't remember if the article said the water splashing (left prop=clockwise, right prop counter-clockwise) was a problem on the ground, in the air, or both.
During prototype developements of the P38, the propellers actually spun with the props bottom arc away from eachother; with the top of the prop arc turning inwards. Even though there was still counter torque effects, the nose had the tendency to pitch downwards.
The operational configuration of the P38 was different, and as you correctly stated, the top of the props arcs turned away from eachother; with the bottom of the prop arc turning inwards. While I dont have the sources on hand to back this up, one of the main reasons why they configured the engines like they did is so that the gun platform would be more stable. Another reason for the engine configuration is because the counter-torque would create a nose-up attitude and thus helped improve the P38s turning ability.
-
in the 38 the motors did not rotate in the same direction!
the allison was designed with a "flip-able" ;) err reversible crankshaft ...
>>>
Another feature of the V-1710 design was its ability to turn the output shaft either clockwise or counter-clockwise by assembling the engine with the crankshaft turned end-for-end, by installing an idler gear in the drive train to the supercharger and accessories and by installing a starter turning the proper direction. So, there was no need to re-arrange the ignition wiring and firing order, nor the oil and Glycol circuits to accommodate the direction of rotation.
<<<
http://en.wikipedia.org/wiki/Allison_V-1710
-
The main reason for counter-rotation of props is to make the airplane more manageable with an engine inop (it's a p-factor thing), however in the case of the P-38 they turn the props "out" (done in search of a cure to the compressibility thing) so in effect have made both engines the critical engine.
i was wondering why they rotated the engines out, rather than in......
-
i was wondering why they rotated the engines out, rather than in......
When stating if the p-38 engines are critical or not may be a moot point. The defintion of a critical engine is very arguable for some planes. However, in the p-38 (w/props outward) neither one of the engines could be considered a "critical engine," they may only be considered "equally critical." See link below for reference.
http://en.wikipedia.org/wiki/Talk:Critical_engine (http://en.wikipedia.org/wiki/Talk:Critical_engine)
Another reason for the engine configuration is because the counter-torque would create a nose-up attitude and thus helped improve the P38s turning ability.
From a pure mechanical perspective, I wonder if the nose-up attitude was a direct affect of the wing and/or elevator flexing do to the large engine torque placed on the twin booms. For example, I would believe the under full power an engine on the p-38 could twist the entire engine boom structure and change the aerodynamics of the wing and/or elevator which produced a nose-up or nose-down attitude. Depending on how the elevator was connected to the rear tail boom, the elevator could experience a bowing up or down. This bowing effect could change the lift on the elevator same as if the elevator trim were adjusted by the pilot. Remember, I’m only speculating based on a purely mechanical point-of-view.
Also, mechanically speaking, assuming the p-38 props could produce enough torque to flex the wing and/or elevator and change the aerodynamics. I could see the advantage of counter-rotating props for the p-38. If the p-38 didn’t have counter-rotating props the wings and/or elevator could experience a wicked change in the lift under full engine power, mainly in the wings. If the p-38 had props that spun the same direction the plane may require a stronger and heavier airframe.
-
Any structure that would be deformed by torque would most likely come apart with any inflight loads.
-
Not meaning to be argumentative, but the flex of the P-38 fuselage/tail boom is probably so small, it would be difficult to measure, unless you did it with lasers during flight. It certainly wouldn't change flight characteristics or cause a nose up attitude. It's simply the force of torque in 3D space. If suspended in a vacuum it would do back flips "'till the cows came home". anyone using high speed power tools, wheel grinders etc., has felt it. :salute
-
The main reason to use counter rotating pros is to eliminate the torque. Your over haul costs double or triple on the counter rotating side, due to the fact that the prop on the counter rotating side are different, as are many of the acessories, such as the vacum pump, Magnetos, and the like. Counter rotating props do not have that much effect in cruise, the big difference between the 2 setups, is in take-off and landing. The Piper Seminle has counter rotating props it is a great plane, and easier to land, and take-off than an Apache, or Aztec, and a Beech 18 is a handful either way. Also there is a BIG difference in "engine out performance". In referance to Fliping the engine over, in a push pull configuration there are a few planes that use that set-up. The Donier 335 was one of them, and the Cessna 337 is a more modern one but the most up to date example is the Adams A-300. For this it is called a center line thrust, if you get your multi engine rating in one of these configurations, your certificate states it and you cant leagaly carry passengers in a conventional multi-engine aircraft. For the simple reason that P-factor and other characteristics of multi engine aircraft are either non existant or nominal. One Problem with a Center line thrust aircraft is that since the thrust line of both engines in in the center line, you do not get the adverse yaw when you loose an engine like you do in a conventional twin. It is harder to determine when you loose an engine on take-off. You still have a "Blue Line" in a push pull but not for the same reasons as in a conventional twin. In a conventional twin the "Blue line" is the minimal single engine controlable airspeed at gross weight. Below the Blue line at gross weight if you loose an engine there is a good chance that you will end up in an aluminum ball somewhere near one end of the ends runway.
-
You still have a "Blue Line" in a push pull but not for the same reasons as in a conventional twin. In a conventional twin the "Blue line" is the minimal single engine controlable airspeed at gross weight. Below the Blue line at gross weight if you loose an engine there is a good chance that you will end up in an aluminum ball somewhere near one end of the ends runway.
Blue line is Vyse....best rate of climb, single engine.
Vmc is marked with a red line.
-
Blue line is Vyse....best rate of climb, single engine.
Vmc is marked with a red line.
My Bad your right.
-
I can't find the web page that explained why the P-38's props turned as they are. All I remember is the P-38 props were tried with the propeller tops rotating both towards and both away from the pilot's point of view. With the propellers tops rotating towards the pilot's cockpit (left prop=clockwise, right prop counter-clockwise) allowed a better torque advantage should one engine fail. The reference article said the main decision to make the propellers tops rotating away from the pilot's cockpit (as they were all built; left prop=counter-clockwise, right prop=clockwise) was to prevent rain water from flying off the props and onto the pilot's windshield, thereby, possibly blinding the pilot's frontal view. I can't remember if the article said the water splashing (left prop=clockwise, right prop counter-clockwise) was a problem on the ground, in the air, or both.
A couple of interesting quotes:
http://bbs.hitechcreations.com/wiki/index.php/P-38_Lightning (http://bbs.hitechcreations.com/wiki/index.php/P-38_Lightning) :
http://www.answers.com/topic/counter-rotating-propellers (http://www.answers.com/topic/counter-rotating-propellers)
dunno the truth to this, but here's somethign i just found.......
http://www.examiner.com/x-21467-Cleveland-Private-Aircraft-Examiner~y2009m11d28-A-Modern-Marvel-Counter-Rotating-PropellersThe-P38-which-way-do-they-Rotate
-
If the p-38 had props that spun the same direction the plane may require a stronger and heavier airframe.
It didn't. Model 322's were the "Castrated P-38s" that didn't have counter-rotating propellors, both spun to the right. There wasn't a need to strengthen or otherwise modify the airframe to compensate for the torque.
ack-ack
-
Just more trimming....
-
On a boat with twins, if the props spin inward you get more stern lift and speed. Would this same rotation in a plane cause more lift? Boat props spinning outward give you more stability, but you loose a little performance.
-
On planes that have the propellers close to the tip of the wings, outward turning props will reduce wingtip turbulence and increase the effective lift. I don't know what is the case in the V22. For the boat, this is a little different because you have an air-water boundary that significantly changes the flow, especially from above.
edit:
I just checked and V22 does indeed have outward props. Probably needs it because the wings are so short that it would otherwise loose a lot of lift to turbulence.
http://www.youtube.com/watch?v=4RkA47WPeX0&feature=related
-
True in a boat, the more you get it out of the water, the faster it will go. But it looses stability.
-
Contra-rotating (single engine, 2 props) turned out not to be so useful. The weight was significant (and on the front of the plane, throwing CoG and balance off) and the added gain wasn't not very large. Later model spitfires tested contra-props but found little performance gain over the single-prop versions. Definitely not enough to be worth the headaches of maint/upkeep on them! :D
But, that's all a side-point. Back to the regularly scheduled program!
Tu-95 Bear.....
One of the fastest prop planes(turbo-prop) ever made has contra-rotating props..
(http://upload.wikimedia.org/wikipedia/commons/0/04/Tupolev_Tu-95_Marina.jpg)
There are several Unlimited class racers that have single-shaft contra-rotating props..
Here are just a few...
(http://www.wetwing.com/reno/unlimited/unlimitedfoto/unlimited38.jpg)
(http://www.air-and-space.com/20001008%20Chino/RB-51%2019750618%20l.jpg)
Everything I've read says contrary to what you're saying.. The only reason they really didn't take off was because jet-technology was becoming the mainstream..
I'd be more than obliged to read anything you can provide that says the contrary..
-
great pic of the bear :aok
actually looks quite elegant from that angle, apart from the chin. reminds me of
(http://scottshaw.com/chupacabra/Z%27Dar.jpg)
-
Contra rotating props were a pain to develop in WW2. They often had big vibration issues due to the rear prop blades hitting the turbulence from the front set.
The racing planes get a bigger advantage from the prop than their WW2 forbears would have as they are developing a lot more power from the engine, albeit not for very long. The advantages in terms of drag, handling and the ability to transmit all that power into the air would more than offset the reliablity and weight issues.
The Bear is so fast the prop tips go supersonic and this is what limits the maximum diamater the props can be. Contra props were the only practical way the designers could transmit enough power.
-
The camshaft is what has to be ground in the reverse sequence, to make an engine run in reverse. Crank throws will be in the same positions. All that is neccesary is to reverse the intake and exhaust valve sequence, and spin the engine the other way, Vroom!
Didn't the US and Russia, experiment with crescent shaped prop blades, in an effort to stop the supersonic tip problems?
Like applying the swept wing principal to propellers.. It didn't work.. Not well enough to use anyway..
RC
-
An article I once read about the development of the F8B - Boeing's Navy attack a/c that was too late for WWII and beat out by the AD1 - it was fitted with a contra-rotating setup. One of the down-side issues was that the continual strobo-scopic effect of the blades crossing in the line of sight out front was nauseating to the pilots after an hour or so in flight.
-
great pic of the bear :aok
actually looks quite elegant from that angle, apart from the chin. reminds me of
(http://scottshaw.com/chupacabra/Z%27Dar.jpg)
:lol :lol
Niice...
(http://farm1.static.flickr.com/185/447456492_6ce17a0487.jpg)
-
those crafty germans had all that stuff solved long before that
(http://upload.wikimedia.org/wikipedia/en/0/06/Do335.png)
yea i know not exactly but hey who can resist an opportunity to post a pick of a cool plane like the 335
a variant would probably made a very nifty executive prop job after the war
-
those crafty germans had all that stuff solved long before that
Everyone talks about how good the Germans are at creating things and how superior blah blah blah they are. However they never really fully understand that in the end, Germany was at the tail end of two world wars in a row, now that will be a record to beat for a very long time.
-
The camshaft is what has to be ground in the reverse sequence, to make an engine run in reverse. Crank throws will be in the same positions. All that is neccesary is to reverse the intake and exhaust valve sequence, and spin the engine the other way, Vroom!
Didn't the US and Russia, experiment with crescent shaped prop blades, in an effort to stop the supersonic tip problems?
Like applying the swept wing principal to propellers.. It didn't work.. Not well enough to use anyway..
RC
The schimtar style blades are in use now, the new C-130 has them. So do many of the new civilian airplanes.