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Twin or single? Pros and cons

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Twin or single? Pros and cons

Old 06-24-2014, 03:28 PM
  #26  
David Searles
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Originally Posted by Harley Condra View Post
Don't forget about the AMA turbine rules...3. For Turbojets and Turbofans single engine static thrustshall not exceed 45 pounds; multiple engine static thrustshall not exceed 50 pounds combined.
I am positive that more than just a few jet fliers in the USA pay no attention to this rule, but shame on all of us if the unthinkable happens by someone who does violate this or any other safety rule.



.
Harley,

I would presume(could be wrong) that this jet should qualify for LTMA-1 (55 lbs fueled RTF)status at least, wherein max thrust is 75 lbs. This would give greater flexibility as well as the safety of using up to two 160 size turbines, full thrust. In a single engine event, since the thrust lines are so close, the added thrust could be a life saver.

I agree with Ali and others, that these larger, super light airframes actually perform better with a little added weight. At WJM weights they seem to float a bit too much for my taste.

David S
Old 06-24-2014, 03:40 PM
  #27  
Prop_Washer2
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Originally Posted by sysiek View Post
I have twine bvm rafale and changing my skymaster mig 29 to twine ,two vt-80s and my next project will be the big 120" long F-18 with two turbines setup ,the biggest advantage is the fast respound and grate power no power losing on the y pipe and the sound it's much dipper and powerful when using two turbines ,twine is way to go the two smaller turbines have much faster respond than one big.
Damn I hope that you build and fly better than you spell....just sayin'...
Old 06-24-2014, 03:42 PM
  #28  
sysiek
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No with Nick aviation design su-27 .
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Old 06-24-2014, 05:09 PM
  #29  
sysiek
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Chicago public school .I'm doing just fine thanks.
Old 06-24-2014, 05:15 PM
  #30  
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JC,

Go with the twin set up. The turbines are so close to each other in your plane that the loss of a turbine is not going to put it in a tail spin. For what you are going to pay for a P140RX, you can buy the new KingTech K210 for the same price and have a turbine that doesn't have flame out issue's. The K210 is in the same size housing as the K-140 and probably weighs the same. I would also use a good gyro as long as you are making that kind of investment.


Ghostrider 1 out!!
Old 06-24-2014, 09:13 PM
  #31  
David Gladwin
 
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There's some flawed thinking in this thread.

A gyro fitted to a model as a yaw damper will not control asymmetric thrust just as yaw damper on a fullsize jet won't either, (they can need a LOT of rudder, dependent on speed. The Canberra for example was uncontrollable below about 140 knots if an engine failed on take off, ie full power ) The gyro has nowhere near enough control when set to the gain required for normal flight.

The last thing you need on a twin is excess power. At the speeds we operate models at the rudder simply does not have enough power to overcome the yawing moment and it's rudder NOT aileron which is the primary control. Unfortunately loss of an engine on a model twin is first seen as roll, secondary affect of yaw induced by thrust loss, and when countered by aileron, a spin often ensues.

Of course engine out handling depends on the model configuration. I am currently completing refurbishment of a Fibre Classics Mig 29 for twin JetCat 140RXI power.This model has HUGE fins, which will reduce the yaw angle with an engine out, and near center-line thrust and has been proven (Wolfgang Khlur flew his in the JWM in Austria with only one of its p80s running for much of the flight) to be very flyable with an engine out.

That said, if mine loses an engine, thrust will be kept to a minimum on the live engine, full opposite rudder will be applied (the rudders are small) and the speed kept high for maximum controllability, until lined up to land and the live engine can be reduced to idle.

The further out the thrust from the center line the greater the yawing moment and the more difficult the handling becomes.

But twins sound really good !!

David G.
Old 06-25-2014, 02:08 AM
  #32  
Likai
 
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Adjusting both thrust lines go through CG can prevent tail spin in one turbine shutdown event. I took off F-14 with one K-180 without problem.
Old 06-25-2014, 03:11 AM
  #33  
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Excuse me if this is a stupid question but what do you mean by adjusting both thrust lines. Is this where you make sure the tail pipe is straight and turbine is centre of tail pipe?
Old 06-25-2014, 06:16 AM
  #34  
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I bend both tail pipe outward 10 degree on exhaust section, in case of F-14, this will make thrust line go through CG when you look from top. Because the distance between thrust line and CG is zero, so no yaw moment will be generated by one engine shutdown.
Old 06-25-2014, 06:29 AM
  #35  
BlueBus320
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I think he is saying when each thrust line is slightly canted in a favorable direction for single eng opps. Both turbines running cancel the off center thrust & balance the model, but if one fails the operating engine's thrust line is canted to counter act the asymmetrical thrust. Skymaster F-14's single engine performance videos are amazing!
Old 06-25-2014, 06:30 AM
  #36  
Jack Diaz
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I have been flying twins for almost twenty years now (BVM F-4's, BVM Rafales, MIBO A-10). (Btw, the only one equipped with a rudder gyro is the Rafale .... an ancient gyro).

Sometimes a higher position of the throttle stick, or an out of line vertical, have been the only symptom of a dead engine.
A few times I have only noticed it after landing !

In other words, with the exception of some know issues with an airframes out there (that I have heard of), the issue of loss of control and catastrophic situations due to the loss of one engine is in my opinion more theoretical than factual. ..... Or I have been lucky

PS: I am talking about our models ... don't know about full size!
Old 06-25-2014, 06:37 AM
  #37  
Granpooba
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Can only give purpose of twin engines, from the real world. Reason for twin engines ? When the first one fails, the other will take you to the scene of the accident !
Old 06-25-2014, 06:40 AM
  #38  
BlueBus320
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I have a buddy that lost THREE Skymaster A-10's! He attributed each to P-140RX failures. Happily, I was not there to witness any of the crashes, but supposedly that aircraft with the high mounted far spaced turbines has horrible single eng performance.
Old 06-25-2014, 06:46 AM
  #39  
BlueBus320
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Originally Posted by Granpooba View Post
Can only give purpose of twin engines, from the real world. Reason for twin engines ? When the first one fails, the other will take you to the scene of the accident !
Correct me if I'm wrong, but this statement mostly pertains to conventional (both props rotate in the same direction with a "critical engine") multi engine, propeller driven aircraft. With counter rotating props, airliners, & even most conventional twins the extra engine gets you to your car in the parking lot
Old 06-25-2014, 07:27 AM
  #40  
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Like Jack said, engine out situations are not an issue with this kind of aircraft where the engines are so close. I've seen a couple of mibo A-10's crash because of engine out situations. Both planes very flown by inexperienced pilots who just panicked and forgot to fly the aircraft. But, the A-10 will be harder to control in an engine out situation than the Yak because of the engine positions, further apart. No doubt about that. The Me 262 is notorious for crashing when losing an engine as well.

And David is also correct in saying that gyros are not there for engine out correction. That what the rudder stick is for. I use gyros to fix Dutch roll tendencies, to smooth out rough air and to assist on ground for landing and take off roll. A gyro won't save your butt in case of an engine out.

10 years flying JetCat. Never any real problems. In my book it's the best turbines on the market. So, I'm as confident with the 140RXi choice as I'd be with any other engine.
Old 06-25-2014, 09:01 AM
  #41  
essyou35
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Originally Posted by BlueBus320 View Post
I have a buddy that lost THREE Skymaster A-10's! He attributed each to P-140RX failures. Happily, I was not there to witness any of the crashes, but supposedly that aircraft with the high mounted far spaced turbines has horrible single eng performance.
What kind of failures?
Old 06-25-2014, 09:34 AM
  #42  
BlueBus320
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Originally Posted by essyou35 View Post
What kind of failures?
He said 2 were failures because of a faulty (too small) cable that was supplied with his new P140RX, & the 3rd was an engine failure/shut down, but he said he's not sure why. At the time he told me about the 140 failures I had never had an inflight jetcat failure on any of my turbines (mostly P120's, about 6 over the years) so it sound strange, but I have read other things on this type of failure to corroborate his story. He is a very experienced & talented pilot, & his situation turns me in the other direction from A-10's for now, but I loved my BV Rafale, & would do a yak, F4, 18, or SM F-14 twin as well
Old 06-25-2014, 09:50 AM
  #43  
Granpooba
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Originally Posted by BlueBus320 View Post
Correct me if I'm wrong, but this statement mostly pertains to conventional (both props rotate in the same direction with a "critical engine") multi engine, propeller driven aircraft. With counter rotating props, airliners, & even most conventional twins the extra engine gets you to your car in the parking lot
Quote is an old time joke among real life pilots, as not all twin engine aircraft have counter rotating props. But question for you is, when one engine quits, which is now the critical engine ?
Old 06-25-2014, 10:27 AM
  #44  
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Are you guys really suggesting that a gyro won't help arrest the negative effects of an engine out situation? I don't think that anyone here expects that a gyro will make an a10 with a engine out fly just fine but it will defiantly help.

also weight is absolutely not an issue with this jet and twins running at half throttle are efficient enough that you can get at least the same flight time as a big single producing the same thrust out of a bifuricated pipe, because lets not forget how much extra thrust the single has to produce for the pipe.
Old 06-25-2014, 11:14 AM
  #45  
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Thank you Likai and BlueBus320 for the explenation.
Old 06-25-2014, 12:31 PM
  #46  
ravill
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John,

Even though, you've already made up your mind it seems, but since you're asking for opinions, I'll give you mine.

Stay single bro.

Ha!

Everytime I've seen John, he's got some hot chick with him. And he's a great pilot too.

Because I like low drama flying, I'd stay away from twins. But in "real" life, I love twins.
Old 06-25-2014, 01:12 PM
  #47  
BlueBus320
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Originally Posted by Granpooba View Post
Quote is an old time joke among real life pilots, as not all twin engine aircraft have counter rotating props. But question for you is, when one engine quits, which is now the critical engine ?
lol, can't tell if you are joking, or really asking.. If joking, then your critical engine is the one you still got..lol. Guess the text book answer would be something like; "the critical engine is the engine that if lost would most adversely affect the aircrafts flight performance".haha. I built a few hours doing multi instruction many moons ago, but can still regurgitate a little of that stuff
Old 06-25-2014, 01:54 PM
  #48  
-JC-
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Originally Posted by ravill View Post

Everytime I've seen John, he's got some hot chick with him. And he's a great pilot too.
.
Rav, what can I say... Chicks just love fat, bald guys


I decided on twins for this one because it's a twin in real life. And, after speaking to owners of the Yak who flies it as twin. It can handle the weight easily and has tons of fuselage space for fuel. So it seems like a good candidate
Old 06-25-2014, 02:31 PM
  #49  
Granpooba
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Originally Posted by BlueBus320 View Post
lol, can't tell if you are joking, or really asking.. If joking, then your critical engine is the one you still got..lol. Guess the text book answer would be something like; "the critical engine is the engine that if lost would most adversely affect the aircrafts flight performance".haha. I built a few hours doing multi instruction many moons ago, but can still regurgitate a little of that stuff
Just joking of course ! But out of 13,000 flight hours only about 750 of my hours were in single engine aircraft. All the instructors that I ever knew always enforced the fact that the " Critical Engine " is the one that is still operating. The failed engine is doing nothing for your now, thus trim and fly the aircraft with the critical engine still operating to a safe landing.

But then again, I have quite a bit of time in a Falcon 50. At Flight Safety and other training facilities you train for a ( 2 ) engine failure. When that happens your pucker factor tends to rise. But the aircraft can still be flown very well on one engine. Even had a maintenance fairy flight where we had to cage one engine and fly it to a maintenance facility on the two good remaining engines. Dassault builds one heck of a good airplane.

Sorry about the rambling fellows, guess we kind of got off subject .........

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