2H4 From United States, joined Oct 2004, 7047 posts, RR: 54 Posted (3 months 1 week 5 days ago) and read 3413 times:
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Flightglobal ran an article today about the A400M rollout. The photos show engines 1 and 2 (and presumably, 3 and 4) in a counter-rotating configuration:
I understand the benefit to counter-rotating props, but I've never seen two engines that turn in opposing directions on the same wing. Then again, this configuration could be more common than I think, and the shape of the A400M's propellers simply makes their direction of rotation evident.
So what is the reasoning for this? What benefits does this configuration offer that a "traditional" counter-rotating configuration (ie: 1 and 2 opposing 3 and 4) does not?
This counter-rotation characteristic is known as Down-Between-Engines (DBE) and the A400M will be the first aircraft ever to use such a configuration. The advantages of DBE have far-reaching effects both aerodynamically and structurally. Firstly, airflow over the wings is symmetrical, improving lift characteristics and the lateral stability of the aircraft. Secondly, DBE allows for an optimum wing design by eliminating most of the effects of torque and prop-wash on each wing, concentrating the airflow over the most efficient portion of the wing located between the engines. DBE also reduces the “critical engine” effect of severe yaw in the event of an outboard engine failure. The result allows a 17% reduction in the area of the vertical tail surface.
Further aerodynamic advantages inherent in DBE have been found to give a 4% increase in lift from the wing at slow speed, which enables, for the same total lift, a simpler, lighter flap system to be employed. As a consequence of the lessening of the aerodynamic forces applied to the flaps, the surface area of the horizontal tail-plane can also be reduced by 8%.
We can't solve problems by using the same kind of thinking we used when we created them.
Flipdewaf From United Kingdom, joined Jul 2006, 537 posts, RR: 0 Reply 3, posted (3 months 1 week 4 days 23 hours ago) and read 3377 times:
I believe, though im not 100% on this, that it helps with engine out performance and can reduce the control surfaces and helps the TE high lift devices because of where the air is "blown". I hope that describes it ok.
2H4 From United States, joined Oct 2004, 7047 posts, RR: 54 Reply 4, posted (3 months 1 week 4 days 23 hours ago) and read 3379 times:
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Quoting Cdekoe (Reply 2): Interesting indeed. And Google to the rescue...
Bloody hell. It never even occurred to me to check the A400M website! Thanks!
Quoting Airbus: This counter-rotation characteristic is known as Down-Between-Engines (DBE) and the A400M will be the first aircraft ever to use such a configuration.
Hmm..."ever" is such a strong word, I feel compelled to find an existing example of this configuration just to prove them wrong....
Overall, very interesting. I wonder if other airframe manufacturers will start adopting this configuration.
Dougloid From France, joined Jul 2005, 7275 posts, RR: 49 Reply 6, posted (3 months 1 week 4 days 22 hours ago) and read 3350 times:
It does make certification of the engines a more interesting proposition, because you now have two different types. It could also make for spares provisioning problems.
If you believe in coincidence, you haven't looked close enough-Joe Leaphorn
GST From United Kingdom, joined Jun 2008, 185 posts, RR: 0 Reply 7, posted (3 months 1 week 4 days 20 hours ago) and read 3291 times:
Quoting Dougloid (Reply 6): It does make certification of the engines a more interesting proposition, because you now have two different types. It could also make for spares provisioning problems.
I dont see why. Give me a single example of a military aircraft operator that uses only a single type of engine, and also has heavy lifting aircraft requirement. Most military forces use dozens of aircraft types from trainers to fast jets, to heavy lifters, many using different engine types. What is a single additional type between friends? Most of the maintenance procedures and spare parts will be the same also.
Gliding is to power flying as seduction is to rape.
KELPkid From United States, joined Nov 2005, 3417 posts, RR: 3 Reply 8, posted (3 months 1 week 4 days 20 hours ago) and read 3289 times:
Quoting Dougloid (Reply 6):
It does make certification of the engines a more interesting proposition, because you now have two different types. It could also make for spares provisioning problems.
Obviously, with 8 bladed props, overall cost of ownership wasn't part of the design equation...
Rulebooks and regulations are made from paper...and they do a poor job at preventing metal from contacting rock-Ernest G
2H4 From United States, joined Oct 2004, 7047 posts, RR: 54 Reply 9, posted (3 months 1 week 4 days 20 hours ago) and read 3277 times:
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Quoting GST (Reply 7): Most military forces use dozens of aircraft types from trainers to fast jets, to heavy lifters, many using different engine types.
That doesn't change the fact that an aircraft with four common engines will require the operator to inventory fewer spare parts than an aircraft with dissimilar engine components.
Will this make the aircraft uneconomical to operate? Not necessarily, nor does Dougloid seem to be arguing that. But while this sourcing/stocking challenge may not ruin the overall economics of the aircraft, it could absolutely be directly and solely responsible for canceled flights and failed missions.
KELPkid From United States, joined Nov 2005, 3417 posts, RR: 3 Reply 10, posted (3 months 1 week 4 days 19 hours ago) and read 3254 times:
So, it would seem that this design configuration would give the aircraft two critical engines, one on each side (not that critical engine considerations seem to be that big a deal on a large transport...the Vy3e speed is probably a good speed above the stall margin in this bird).
Rulebooks and regulations are made from paper...and they do a poor job at preventing metal from contacting rock-Ernest G
That's because you aren't thinking clearly. You now have to certify two different types of engines. Engines that turn backwards to each other are different even if they share common parts.
Quoting GST (Reply 7): Give me a single example of a military aircraft operator that uses only a single type of engine, and also has heavy lifting aircraft requirement.
what the hell is that supposed to mean?
Quoting GST (Reply 7): What is a single additional type between friends?
If you're willing to pay for it that's ok-but it introduces an additional layer of complication and spare parts stocking and "not commonality" that is unnecessary.
Look at the stackup. Europrop now gets to develop 2 different types of engines, at no increase in profitability, because they aren't going to sell more-they'll sell half as many of each type. The people who make the props are faced with the same problems. And you haven't even started trying to figure out what the changed airflow will do to the engine operating parameters. it woul;d be just plain foolish to assume without knowing that they'll act the same.
Quoting GST (Reply 7): Most of the maintenance procedures and spare parts will be the same also.
And you know this because..........?
Quoting CTR (Reply 11): Could the four engines be common except for a reversing gear in two of the gearboxes?
That's not nearly as simple as it sounds. If you want to maintain the same prop speed, you're going to have to shoehorn those gears in there somewhere. This is not a freakin' bicycle, people.
If you believe in coincidence, you haven't looked close enough-Joe Leaphorn
Tdscanuck From Canada, joined Jan 2006, 3467 posts, RR: 28 Reply 13, posted (3 months 1 week 4 days 13 hours ago) and read 3021 times:
Quoting 2H4 (Thread starter): I understand the benefit to counter-rotating props, but I've never seen two engines that turn in opposing directions on the same wing.
I think the engines go the same directly and they just reverse at the gearbox.
Quoting CTR (Reply 11): Could the four engines be common except for a reversing gear in two of the gearboxes?
Yes.
Quoting Dougloid (Reply 12): That's because you aren't thinking clearly. You now have to certify two different types of engines. Engines that turn backwards to each other are different even if they share common parts.
It wouldn't be that bad. If the only difference was the rotation direction and everything was just mirrored, qualification by similarity would be pretty straightforward.
Quoting Dougloid (Reply 12): That's not nearly as simple as it sounds. If you want to maintain the same prop speed, you're going to have to shoehorn those gears in there somewhere. This is not a freakin' bicycle, people.
Yes, but a counterrotating gearbox is a lot less work and cost than two types of engine.
CTR From United States, joined Jul 2005, 229 posts, RR: 0 Reply 14, posted (3 months 1 week 4 days 12 hours ago) and read 2985 times:
Quoting Dougloid (Reply 12): That's not nearly as simple as it sounds. If you want to maintain the same prop speed, you're going to have to shoehorn those gears in there somewhere. This is not a freakin' bicycle, people.
Wow. The fact that the V-22 does it must be a miracle.
Dougloid From France, joined Jul 2005, 7275 posts, RR: 49 Reply 15, posted (3 months 1 week 4 days 11 hours ago) and read 2972 times:
Quoting Tdscanuck (Reply 13): It wouldn't be that bad. If the only difference was the rotation direction and everything was just mirrored, qualification by similarity would be pretty straightforward.
Quoting Dougloid (Reply 12):
That's not nearly as simple as it sounds. If you want to maintain the same prop speed, you're going to have to shoehorn those gears in there somewhere. This is not a freakin' bicycle, people.
Yes, but a counterrotating gearbox is a lot less work and cost than two types of engine.
Tom.
Tom, it's not a question of mirror imaging everything. You folks are all assuming that this is easily accomplished by merely "putting in a counterrotating gear in there.
Listen. I spent a lot of time working on TPE331s and we had two flavors. One was a straight, 2000 rpm clockwise rotation engine with a supr gear train. The other was a counterclockwise, 1591 rpm number that swung a bigger prop.
I can tell you that the reduction gearing was entirely different between two engines with tthe same model number. Here's how they did it and I think that here's how Europrop will do it.
The 2000 rpm model had a spur gear train and a fixed planetary gearset. The 1591 rpm reverse rotation one had to have a floating planetary hearset and an extra bull gear to get the reverse rotation while making sure that all the accessories worked properly.
Think about it. Draw yourself a little picture of a geartrain that will reduce shaft speed to prop speed.
Then think about what you have to do to get the prop to run backwards. Never mind what you've got to do with your accessories, prop governor, whatever, but don't forget it's got to occupy the same physical envelope.
You've got to get one gear that's gonna take all that torque and reverse it in there that's gonna handle 15,000 hp reliably.
Easy? Simple? In an armchair maybe.
[Edited 2008-06-26 20:56:22]
If you believe in coincidence, you haven't looked close enough-Joe Leaphorn
Tdscanuck From Canada, joined Jan 2006, 3467 posts, RR: 28 Reply 16, posted (3 months 1 week 4 days 11 hours ago) and read 2946 times:
Quoting Dougloid (Reply 15):
Tom, it's not a question of mirror imaging everything. You folks are all assuming that this is easily accomplished by merely "putting in a counterrotating gear in there.
That particular portion of the thread was about having the engine run the other direction. It doesn't take any extra gears, you just mirror the whole thing and run it the other way.
Quoting Dougloid (Reply 15): Listen. I spent a lot of time working on TPE331s and we had two flavors. One was a straight, 2000 rpm clockwise rotation engine with a supr gear train. The other was a counterclockwise, 1591 rpm number that swung a bigger prop.
Right, but that's not what was being discussed. The suggestion was two identical engines that spin the same speed (with the same prop), just in opposite directions. It's been done many many times before.
Francoflier From France, joined Oct 2001, 1358 posts, RR: 3 Reply 17, posted (3 months 1 week 4 days 9 hours ago) and read 2925 times:
Quoting Tdscanuck (Reply 16): The suggestion was two identical engines that spin the same speed (with the same prop), just in opposite directions. It's been done many many times before.
Sorry to jump in, but 2 identical engines cannot turn in opposite directions. Their innards would all be backwards, hence different parts, hence different engines.
Unless you meant the different gearbox scenario, in which case it sounds like an easier solution, though not simple, as Dougloid mentioned. An extra set of gear could take care of the problem, even though it would make the gearbox slightly heavier and bigger, and might rob a bit more power from the turbine, all of which can be compensated for, I suppose.
It would make more sense if everything but the gearbox was interchangeable.
Looks like I picked the wrong week to quit posting...
Keesje From Netherlands, joined Apr 2001, 7977 posts, RR: 26 Reply 18, posted (3 months 1 week 4 days 6 hours ago) and read 2844 times:
I remember reading that the engine configuration also has advantages for people jumping from the side doors of the aircraft. It reduces downwash on one side.
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From United States, joined Oct 2004, 7047 posts, 
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(not that critical engine considerations seem to be that big a deal on a large transport...the Vy3e speed is probably a good speed above the stall margin in this bird).
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