Why A Multi-Engine Aircraft?
Have you ever wondered why a manufacturer puts more engines on an airframe? There are many people who think that it's for safety; that a twin is safer than a single. After all, if one engine fails, well, you just keep on flying on the remaining one, right?
No. When one engine on a twin fails, you don't lose half of your excess thrust, you typically lose 80% to 90% of your excess thrust, which means that if you were climbing at 1200 fpm with both engines, if you configure and fly the aircraft perfectly after an engine failure, you will likely see around 200 fpm, which is pretty bad.
Most light twins, when operated anywhere near gross weight, have very marginal single-engine performance, and are very intolerant of pilot error in achieving a positive rate of climb. A non-turbocharged twin will typically have a single-engine service ceiling of around 5000 foot density altitude. So, an engine failure in cruise in summer means you're likely going to descend.
And remember, with two engines, you're twice as likely to have an engine failure.
So why on earth would a manufacturer install two engines instead of one? Apart from specifically-built multi time-builders and trainers, the answer is: for more power.
If a manufacturer can't get 500 hp from one engine for a 5000 lb. aircraft, well, the answer is to put 250 hp on each side. However, there is additional drag created by the drag of the engine nacelle on each side.
It's interesting to contrast the performance of the Twin Comanche, with a total of 320 hp, versus the 260 hp single Comanche.
Unless you need multi time in your logbook, I personally would take the speedy single Comanche over the twin, especially when you add in the extra dollars per hour of the twin.
Newer, serious light business aircraft such as the TBM-700 or PC-12 use a single turbine engine to get the power required with minimal drag. Also, kerosene burning turbines can fly higher than any piston-engined aircraft, may of which are restricted to 25,000 feet. The TBM-700 goes 300 knots TAS at 30,000 feet, which isn't too bad for a single, eh?
There is another problem with putting two engines out in the wings. When both engines are pulling, everything is nice and symmetrical. But when one engine fails, it is no longer providing thrust — the other engine is providing all the thrust, and in doing so, it causes the aircraft to yaw towards the failed, or dead engine.
It's actually worse than that. The dead engine, in addition to no longer providing thrust, is also creating drag, since the propeller is windmilling, and the energy to spin the prop has to come from somewhere.
The drag the windmilling prop creates also causes the aircraft to yaw towards the dead engine.
Instinctively, the pilot will try to stop the yaw by stomping on the rudder pedal on the side of the engine which is still producing thrust.
In fact, this is how a pilot identifies the dead engine; the "dead" foot, which no longer needs to push on the rudder pedal, is on the side of the "dead" engine.
At this point, the pilot "feathers" the dead engine. The prop control is pulled all the way back, beyond the feather detent, and the prop blades rotate to maximum coarse pitch, which minimizes drag. In fact, at this point, the propeller of the dead engine will stop rotating. It's really weird, flying along, looking at the stopped blades of a feathered engine.
Back to opposing the yaw. We all know that as you slow down, flight controls get sloppy — they lose effectiveness. Below a certain speed, the rudder will not have enough authority to oppose the yawing into the dead engine. This results in the aircraft rolling inverted into a spin, and nearly always the deaths of all the occupants, which creates bad press for general aviation.
What is usually recommended in this situation is to reduce the power on the good engine, and to lower the nose to increase airspeed, in order to maintain control. Neither of these is a particularly desirable choice at low altitude, right after takeoff.
As a result, climbing at slow speeds is strongly frowned upon in twins. So strongly, in fact, that this minimum yaw control speed, known as Vmc, is painted as a red line on the airspeed indicator, in addition to Vne. Rotation on takeoff before Vmc is really discouraged
Have you ever wondered why a manufacturer puts more engines on an airframe? There are many people who think that it's for safety; that a twin is safer than a single. After all, if one engine fails, well, you just keep on flying on the remaining one, right?
No. When one engine on a twin fails, you don't lose half of your excess thrust, you typically lose 80% to 90% of your excess thrust, which means that if you were climbing at 1200 fpm with both engines, if you configure and fly the aircraft perfectly after an engine failure, you will likely see around 200 fpm, which is pretty bad.
Most light twins, when operated anywhere near gross weight, have very marginal single-engine performance, and are very intolerant of pilot error in achieving a positive rate of climb. A non-turbocharged twin will typically have a single-engine service ceiling of around 5000 foot density altitude. So, an engine failure in cruise in summer means you're likely going to descend.
And remember, with two engines, you're twice as likely to have an engine failure.
So why on earth would a manufacturer install two engines instead of one? Apart from specifically-built multi time-builders and trainers, the answer is: for more power.
If a manufacturer can't get 500 hp from one engine for a 5000 lb. aircraft, well, the answer is to put 250 hp on each side. However, there is additional drag created by the drag of the engine nacelle on each side.
It's interesting to contrast the performance of the Twin Comanche, with a total of 320 hp, versus the 260 hp single Comanche.
Unless you need multi time in your logbook, I personally would take the speedy single Comanche over the twin, especially when you add in the extra dollars per hour of the twin.
Newer, serious light business aircraft such as the TBM-700 or PC-12 use a single turbine engine to get the power required with minimal drag. Also, kerosene burning turbines can fly higher than any piston-engined aircraft, may of which are restricted to 25,000 feet. The TBM-700 goes 300 knots TAS at 30,000 feet, which isn't too bad for a single, eh?
There is another problem with putting two engines out in the wings. When both engines are pulling, everything is nice and symmetrical. But when one engine fails, it is no longer providing thrust — the other engine is providing all the thrust, and in doing so, it causes the aircraft to yaw towards the failed, or dead engine.
It's actually worse than that. The dead engine, in addition to no longer providing thrust, is also creating drag, since the propeller is windmilling, and the energy to spin the prop has to come from somewhere.
The drag the windmilling prop creates also causes the aircraft to yaw towards the dead engine.
Instinctively, the pilot will try to stop the yaw by stomping on the rudder pedal on the side of the engine which is still producing thrust.
In fact, this is how a pilot identifies the dead engine; the "dead" foot, which no longer needs to push on the rudder pedal, is on the side of the "dead" engine.
At this point, the pilot "feathers" the dead engine. The prop control is pulled all the way back, beyond the feather detent, and the prop blades rotate to maximum coarse pitch, which minimizes drag. In fact, at this point, the propeller of the dead engine will stop rotating. It's really weird, flying along, looking at the stopped blades of a feathered engine.
Back to opposing the yaw. We all know that as you slow down, flight controls get sloppy — they lose effectiveness. Below a certain speed, the rudder will not have enough authority to oppose the yawing into the dead engine. This results in the aircraft rolling inverted into a spin, and nearly always the deaths of all the occupants, which creates bad press for general aviation.
What is usually recommended in this situation is to reduce the power on the good engine, and to lower the nose to increase airspeed, in order to maintain control. Neither of these is a particularly desirable choice at low altitude, right after takeoff.
As a result, climbing at slow speeds is strongly frowned upon in twins. So strongly, in fact, that this minimum yaw control speed, known as Vmc, is painted as a red line on the airspeed indicator, in addition to Vne. Rotation on takeoff before Vmc is really discouraged
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