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Bone 4 Tuna |
What a way to travel in style! _________________________ An unarmed man can only flee from evil and evil is not overcome by fleeing from it. - Col Jeff Cooper NRA Life Member Long Live the Super Thirty-Eight | |||
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To cancel the angular momentum of the engines. Otherwise, the plane handles bizarrely, related to the phenomenon of "countersteering" in motorcycles. With the third dimension involved it gets even weirder. "Crom is strong! If I die, I have to go before him, and he will ask me, 'What is the riddle of steel?' If I don't know it, he will cast me out of Valhalla and laugh at me." | |||
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אַרְיֵה |
A brief overview re direction of rotation of props. Note that all of my “hands-on” knowledge in this area, as a pilot and an instructor, comes from the civilian world, not the military. With the exception of some Piper products, just about every conventional civil twin-engine airplane has both props turning in the same direction, almost universally clockwise when looking forward. If an engine fails in flight and altitude needs to be maintained or gained (if possible), power is typically increased on the functioning engine. Without going into a lot of detail, normal flight requires a positive angle of attack; that is a slightly nose-high attitude. This means that the propellor blade that is on the descending half-circle of rotation is taking a bigger “bite” than when it is on the ascending half of rotation. This, in turn, moves the center of thrust of the propellor away from the hub and out toward the descending blade. On a conventional clockwise rotating prop, this means that the center of thrust is on the right side, instead of being centered at the hub. Conventional twin, both props turning clockwise, if the right engine fails, the center of thrust from the remaining (left) engine is inboard of the prop hub, but if the left engine fails, the center of thrust from the right prop is outboard of the prop hub. This means that the right engine, in this situation, has a longer lever arm than the left engine. Longer lever arm means greater yaw (turning) force, so it takes more rudder deflection to maintain directional (heading) stability. Because of this, the left engine is the critical engine; failure of the left engine results in more difficulty maintaining directional control than failure of the right engine, Along came Piper, in some of their piston powered twins, with CR (Counter Rotating) props. Left prop turns clockwise, as usual, but right prop turns counterclockwise. This means that the descending blade of the prop is inboard of the hub on both engines, and there is no longer a “critical” engine. The P38 Lightning also had counter-rotating pops, but “wrong” in that the left prop rotated counterclockwise, the right prop rotated clockwise. This means that the descending blades of both props were outboard of the hubs, increasing the lever arm re yaw (turning) force in the event of engine failure. I have no idea why they did it this way; it seems to me that it would have been just as easy to do it the way Piper did. הרחפת שלי מלאה בצלופחים | |||
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Now and Zen |
When demonstrating the Mosquito, Geoffrey De Haviland would perform turns and even rolls at low level into the engine he had shut down and feathered the propellor. ___________________________________________________________________________ "....imitate the action of the Tiger." | |||
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I grew up to the sound of a couple of A26s passing by my house as a kid. I lived in the town that was the home office for Phillips Petroleum and they operated two of them. The local executives referred to them as the "hot rod". It was the fastest thing they had at the time. Tragically they lost both of them to accidents around 1958. If my memory is correct, one was one accidents occurred in Ohio as it was turning final for landing and the other was somewhere down south where the aircraft flew into a tornado. Phillips had been operating aircraft since the twenties with a Boeing 247 and FOR Trimotors and had never had an accident. That same year one of their Lockheed Lodestars was involved with a stall/spin accident here at home. My house was about four miles south east of the airport and you could go out and sit on the front porch and here the A26s as they took off. Their left turn after take off took them right by my house. If I remember right, they had water injection and made a hell of a roar as the initially applied the power. Back in the old days corporate planes were painted in the company colors and carried logos. Now everything is just boring white. "If you think everything's going to be alright, you don't understand the problem!"- Gutpile Charlie "A man's got to know his limitations" - Harry Callahan | |||
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That same plane was parked at the Billings Airport back in 1999. I have color prints somewhere. IIRC, it had not moved in a while. | |||
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P-82 was CR as well. | |||
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| Ammoholic |
Interestingly, many European aircraft have props that turn the other way (counterclockwise). On my initial CFI ride, the FAA examiner screwed with me a bit. He had me do a lesson on P-factor and when I was explaining it and the associated left turning tendency, he interrupted and said that his prior instructor had told him P-factor caused a right turning tendency. Not wanting to offend, I said, "I could be wrong. We'll look at the propellor when we go and it will be easy to tell which side the down going blade will be on." A few minutes later my light bulb came on and I asked if his previous instructor was European. He said, "Yes, German." I answered that many European airplanes have engines that turn the other way. I think he was more trying to fluster me and see how I'd deal with an student whose prior instructor had told them something "wrong", but he was okay with my handling of it. [excellent description of the physics snipped. You must have been instructing for a while and doing a good job of it.]
I have read that putting the engines on the P-38 as they did made the airplane more maneuverable. I don't understand how this would be so with both engines making equal power, but I can see how it could if one where pulled back. Of course, said additional maneuverability would come at a cost of reduced controllability. | |||
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I'm not sure about planes, but we use counter rotating propellors on boats/yachts so they track and steer straight rather than pulling to the right. That being said the counter rotating props on boats turn to the outside of the boat on each side it's mounter.....starboard side turns clockwise (when looking from the stern), port side turns counterclockwise. NOW, one of the race boat pioneers, I believe it was Reggie Fountain experimented with inboard counter rotating props on a raceboat with staggered motors (staggering motors puts the 2 drives closer together). The boat did gain speed as the thrust was more confined, I think it gained 4 mph which is a major feat in the race boat world. BUT, it wouldn't handle hardly at all in docking situations and did not steer well at speed either, so they abandoned the idea. | |||
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