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Flymach2
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Hi...
Not quite sure what you're asking. Do you want to know what P-factor is? Or how it effects Vmc?
Simply put, P-factor, also known as asymmetrical thrust, is a result of the descending propeller blade being at a higher angle of attack in a climb configuration, subsequently increasing the thrust on that side of the prop arc.
For the engine rotation, depends on what type of twin we are talking about. Some have counter-rotating props while others rotate in the same direction.
For instance, a Seminole or Duchess have props that are counter- rotating and as a result, don't really have a "critical engine".
However, U.S. conventional type aircraft will have engines that rotate clockwise when viewed from the cockpit. So, placing a greater thrust-line on the descending part of the prop arc, you can see where,(in relation to the longitudinal axis of the airplane), the lines of thrust are the greatest.
So, as a result, the left engine,(when viewed from the cockpit), would be the critical engine because if this engine failed, it would have the most adverse effect on aircraft directional control.
There are also British conventional aircraft where both engines rotate counter-clockwise when viewed from the cockpit.
Coffin corner, or Q-corner, is an area that is of most concern for subsonic and transonic category aircraft.
As altitude increases, the speed of sound decreases. Unforunately, the true speed at which an aircraft will stall will increase with the decrease in density. The region where these two areas nearly meet is known as the "coffin corner".
The problem arises when that aircraft is approaching the speed of sound and may be experiencing what is known as Mach buffet.
Mach buffet feels like a stall buffet, so the reaction would be to lower the nose to recover from a stall. If it is Mach buffet, not a stall buffet, pitching down the aircraft will increase it's speed and exceed the design limits and possibly involve some type of structural failure.
Most of these aircraft are designed with safeguards to prevent this. Such as shaker sticks or clackers to warn when approaching the aircraft's Mmo.
Hope some of this helps.....Good luck
Not quite sure what you're asking. Do you want to know what P-factor is? Or how it effects Vmc?
Simply put, P-factor, also known as asymmetrical thrust, is a result of the descending propeller blade being at a higher angle of attack in a climb configuration, subsequently increasing the thrust on that side of the prop arc.
For the engine rotation, depends on what type of twin we are talking about. Some have counter-rotating props while others rotate in the same direction.
For instance, a Seminole or Duchess have props that are counter- rotating and as a result, don't really have a "critical engine".
However, U.S. conventional type aircraft will have engines that rotate clockwise when viewed from the cockpit. So, placing a greater thrust-line on the descending part of the prop arc, you can see where,(in relation to the longitudinal axis of the airplane), the lines of thrust are the greatest.
So, as a result, the left engine,(when viewed from the cockpit), would be the critical engine because if this engine failed, it would have the most adverse effect on aircraft directional control.
There are also British conventional aircraft where both engines rotate counter-clockwise when viewed from the cockpit.
Coffin corner, or Q-corner, is an area that is of most concern for subsonic and transonic category aircraft.
As altitude increases, the speed of sound decreases. Unforunately, the true speed at which an aircraft will stall will increase with the decrease in density. The region where these two areas nearly meet is known as the "coffin corner".
The problem arises when that aircraft is approaching the speed of sound and may be experiencing what is known as Mach buffet.
Mach buffet feels like a stall buffet, so the reaction would be to lower the nose to recover from a stall. If it is Mach buffet, not a stall buffet, pitching down the aircraft will increase it's speed and exceed the design limits and possibly involve some type of structural failure.
Most of these aircraft are designed with safeguards to prevent this. Such as shaker sticks or clackers to warn when approaching the aircraft's Mmo.
Hope some of this helps.....Good luck
Last edited:
Singlecoil
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Timebuilder
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jimbo
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Hey there,
Thanks for the reponses. So in theory with the left engine inop the right engine will most adversly effect flight characteristics because the lines of thrust are farther away from the fuselage.
Thus left engine being the critical engine. This brings up 1 other question. There were criteria making up VMC...I believe there were 4 factors...I.E. CG location, etc etc. Can anyone elaborate?
Thanks!
Multi challenged
Thanks for the reponses. So in theory with the left engine inop the right engine will most adversly effect flight characteristics because the lines of thrust are farther away from the fuselage.
Thus left engine being the critical engine. This brings up 1 other question. There were criteria making up VMC...I believe there were 4 factors...I.E. CG location, etc etc. Can anyone elaborate?
Thanks!
Multi challenged
Flymach2
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Hi..
Yes, that is true...left engine is critical in U.S. Conventional..
Just a few more criteria than the few you mentioned..
§ 23.149 Minimum control speed.
(a) VMC is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.
(b) VMC for takeoff must not exceed 1.2 VS1, where VS1 is determined at the maximum takeoff weight. VMC must be determined with the most unfavorable weight and center of gravity position and with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with -
(1) Maximum available takeoff power initially on each engine;
(2) The airplane trimmed for takeoff;
(3) Flaps in the takeoff position(s);
(4) Landing gear retracted; and
(5) All propeller controls in the recommended takeoff position throughout.
(c) For all airplanes except reciprocating engine powered airplanes of 6,000 pounds or less maximum weight, the conditions of paragraph (a) of this section must also be met for the landing configuration with -
(1) Maximum available takeoff power initially on each engine;
(2) The airplane trimmed for an approach, with all engines operating, at VREF, at an approach gradient equal to the steepest used in the landing distance demonstration of § 23.75;
(3) Flaps in the landing position;
(4) Landing gear extended; and
(5) All propeller controls in the position recommended for approach with all engines operating.
(d) A minimum speed to intentionally render the critical engine inoperative must be established and designated as the safe, intentional, one engine inoperative speed, VSSE.
(e) At VMC, the rudder pedal force required to maintain control must not exceed 150 pounds and it must not be necessary to reduce power of the operative engine(s). During the maneuver, the airplane must not assume any dangerous attitude and it must be possible to prevent a heading change of more than 20 degrees.
(f) At the option of the applicant, to comply with the requirements of § 23.51(c)(1), VMCG may be determined. VMCG is the minimum control speed on the ground, and is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and using the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued. In the determination of VMCG, assuming that the path of the airplane accelerating with all engines operating is along the centerline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centerline is completed may not deviate more than 30 feet laterally from the centerline at any point. VMCG must be established with -
(1) The airplane in each takeoff configuration or, at the option of the applicant, in the most critical takeoff configuration;
(2) Maximum available takeoff power on the operating engines;
(3) The most unfavorable center of gravity;
(4) The airplane trimmed for takeoff; and
(5) The most unfavorable weight in the range of takeoff weights.
Good luck...
Yes, that is true...left engine is critical in U.S. Conventional..
Just a few more criteria than the few you mentioned..
§ 23.149 Minimum control speed.
(a) VMC is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.
(b) VMC for takeoff must not exceed 1.2 VS1, where VS1 is determined at the maximum takeoff weight. VMC must be determined with the most unfavorable weight and center of gravity position and with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with -
(1) Maximum available takeoff power initially on each engine;
(2) The airplane trimmed for takeoff;
(3) Flaps in the takeoff position(s);
(4) Landing gear retracted; and
(5) All propeller controls in the recommended takeoff position throughout.
(c) For all airplanes except reciprocating engine powered airplanes of 6,000 pounds or less maximum weight, the conditions of paragraph (a) of this section must also be met for the landing configuration with -
(1) Maximum available takeoff power initially on each engine;
(2) The airplane trimmed for an approach, with all engines operating, at VREF, at an approach gradient equal to the steepest used in the landing distance demonstration of § 23.75;
(3) Flaps in the landing position;
(4) Landing gear extended; and
(5) All propeller controls in the position recommended for approach with all engines operating.
(d) A minimum speed to intentionally render the critical engine inoperative must be established and designated as the safe, intentional, one engine inoperative speed, VSSE.
(e) At VMC, the rudder pedal force required to maintain control must not exceed 150 pounds and it must not be necessary to reduce power of the operative engine(s). During the maneuver, the airplane must not assume any dangerous attitude and it must be possible to prevent a heading change of more than 20 degrees.
(f) At the option of the applicant, to comply with the requirements of § 23.51(c)(1), VMCG may be determined. VMCG is the minimum control speed on the ground, and is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and using the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued. In the determination of VMCG, assuming that the path of the airplane accelerating with all engines operating is along the centerline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centerline is completed may not deviate more than 30 feet laterally from the centerline at any point. VMCG must be established with -
(1) The airplane in each takeoff configuration or, at the option of the applicant, in the most critical takeoff configuration;
(2) Maximum available takeoff power on the operating engines;
(3) The most unfavorable center of gravity;
(4) The airplane trimmed for takeoff; and
(5) The most unfavorable weight in the range of takeoff weights.
Good luck...
jetalc
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Part 23 has to do with aircraft certification, and does not elaborate on aerodynamics. The FAA is not necessarily saying that all those factors affect Vmc - this is a common misunderstanding, and too many MEI's make their students memorize this ridiculous list. According to the Flight Training Handbook, Chapter 16, the FAA states only three factors that affect Vmc (whether we agree or not is irrelevant) - CG, Density Altitude and Angle of Bank. That's it. End of story. The configuration that a new aircraft has to be in while determining its PUBLISHED Vmc has very little to do with the factors affecting Vmc...
Anyone else have any references where it actually states that the factors that affect Vmc are anything more than these three? I've been looking for 15+ years, and haven't found any others...
Anyone else have any references where it actually states that the factors that affect Vmc are anything more than these three? I've been looking for 15+ years, and haven't found any others...
IP076
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Those may be the 3 that the FAA seems to think makes the biggest difference....but what about Power? Are you saying VMC is the same at Idle as it is at Max? Or that a student doesn't need to know if he reduces power, Vmc essentially goes away? Why would those factors be included for certification if they didn't affect Vmc?jetalc said:
http://av-info.faa.gov/data/practicalteststandard/faa-s-8081-12b.pdf
On PDF page 114 of that document (Commercial PTS) it states that the applicant must:
2. Configures the airplane at VSSE/VYSE, as appropriate—
a. Landing gear retracted.
b. Flaps set for takeoff.
c. Cowl flaps set for takeoff.
d. Trim set for takeoff.
e. Propellers set for high RPM.
f. Power on critical engine reduced to idle.
g. Power on operating engine set to takeoff or maximum
available power.
In that statement alone I would think one would need to know why each item is configured as stated, unless of course, we just teach robots, which that landings thread is starting to imply......
Also you could imply needing to know more than those 3 from pages 84 and 113 of that PDF file.
I think, and I also think most FAA people would agree, that a commercial multi pilot should have a idea of how items such as Flaps, Gear, Props, and Power affect the controllability and performance of an aircraft while operating single engine.
Bus Driver
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One minor point on the coffin corner / Q-corner answer by Flymach2. While TAS Vs does indeed increase with altitude/temp (DA), IAS Vs remains fairly constant. In a high altitude/ high angle of attack / low temp. situation the buffet encountered is shock wave induced. Not a normal boundary layer separation stall buffet. With an increase in angle of attack, the increase in airflow velocity on the upper wing surface goes Mach Crit. With an identical recovery technique of reducing the angle of attack this would be an academic distinction, except, that this can occur at IAS’s far in excess of Vs for the given weight.
We ran into some operational problems with this in the late 80’s. The holding data that we got from the European manufacturer of our new wide body transports did not include any correction for altitude. It was somewhat disconcerting to our pilots that whenever they entered a holding pattern at high altitude they got the stick shaker, alfa floor/stall warning, srs, etc. While researching this little problem, the good people in the engineering departments of McDonnell Douglas and Boeing ran some computer models for us and determined that this can become a factor as low as .33 Mach. While doing an unrelated test flight, my good friend and partner on this project and I decided to do a little hands on research. Clean, FL310, 1.3 Vs, 45 degrees of bank, pull, OK, normal stall warnings, wings level, recover, no problem. Then someone decided to see what effect spoilers had on the situation. Do Not Ever, Ever, Ever, try this. The result was so fast and so violent i swear it could knock the fillings out of your teeth. So IMHO a low speed Mach buffet is not the same animal as a stall buffet. I’m going to have to disagree with AC 61-107 regarding stall speed in it’s definition of coffin corner. For better explanations try Aerodynamics for Naval Aviators, Fly the Wing by Jack Web, or an old Advisory Circular from the 60’s, Transitioning to Jets by Ken Moon.
We ran into some operational problems with this in the late 80’s. The holding data that we got from the European manufacturer of our new wide body transports did not include any correction for altitude. It was somewhat disconcerting to our pilots that whenever they entered a holding pattern at high altitude they got the stick shaker, alfa floor/stall warning, srs, etc. While researching this little problem, the good people in the engineering departments of McDonnell Douglas and Boeing ran some computer models for us and determined that this can become a factor as low as .33 Mach. While doing an unrelated test flight, my good friend and partner on this project and I decided to do a little hands on research. Clean, FL310, 1.3 Vs, 45 degrees of bank, pull, OK, normal stall warnings, wings level, recover, no problem. Then someone decided to see what effect spoilers had on the situation. Do Not Ever, Ever, Ever, try this. The result was so fast and so violent i swear it could knock the fillings out of your teeth. So IMHO a low speed Mach buffet is not the same animal as a stall buffet. I’m going to have to disagree with AC 61-107 regarding stall speed in it’s definition of coffin corner. For better explanations try Aerodynamics for Naval Aviators, Fly the Wing by Jack Web, or an old Advisory Circular from the 60’s, Transitioning to Jets by Ken Moon.
Lead Sled
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As has been mentioned, pilots use the term "Coffin Corner" to refers to the situation in which an airplane has climbed to such a high altitude that the difference between the low speed stall buffet and the high speed Mach buffet is only a few knots.
Aircraft with the newer technology wings tend to have more margin between the two extremes of the performance envelope and typically don't have much of a problem. I can't speak for any of the new generation large transport category aircraft, but I will use as an example the Astra/G100 series aircraft that I am currently flying. In our case, our low speed buffet at FL390, at normal cruise weights, would be down in the neighborhood of .55 Mach, the high speed buffet is just beyond the "barber pole" or a point or two above .875 Mach. If I were to climb to FL450, the low speed buffet would increase to .56 Mach and there would be no change on the high speed end. With a normal cruise of say .82 or .83 we have no real concerns with the coffin corner. If a pilot tries to take them too high for existing conditions they will either run out of "wing" or out of "power" and simply quit climbing. Conceivably, you could push it and get the speed so low and the AoA so high that you could end up with a real serious problem, but you would have to be brain-dead to allow the situation to get so far out of hand. Many of the "Jurassic Jets" had real issues with the coffin corner phenomenon - like the DC-9.
As far as multiengine propeller driven aircraft...
Engine rotation in twins doesn't really make much difference. For example the MU-2's engines/propellers aren't counter-rotating (one rotating CW, the other CCW) they both rotate in the same direction - either CW or CCW depending on the model. It's not really that big of a deal, in the case of the Garrett engines the "Dash 10's" rotate the direction they do because of an extra gear in the gearbox.
Planes with counter-rotating props do not have a critical engine by definition. The FAA defines the critical engine as the engine who's loss most adversely affects the performance and handling characteristics of the airplane.
The reason one engine has a higher performance loss than the other on a non-counter rotating engined aircraft is because the right side of the propeller disk produces more thrust than the left side. The right side of the prop disk on the left engine is closer to the thrust centerline and has a lower thrust moment due to the reduced arm. The loss of the left engine (the critical engine) leaves a majority of the available thrust, which has a higher thrust moment, left to provide power. The higher thrust moment of the right engine requires more rudder to counteract that thrust moment. Hence a higher penalty in performance.
That being said, I want my students to understand that counter-rotating do NOT make one engine "critical" and therefore implies that the other engine isn't. It simply makes both engines equally critical. All theories aside, piston powered, twin engine aircraft have two engines because they need two engines and under certain, limited conditions they can manage to remain airborne and controllable on one engine. The graveyards are packed with pilots (and their passengers) who either didn't understand or didn't believe that fact.
Lead Sled
Aircraft with the newer technology wings tend to have more margin between the two extremes of the performance envelope and typically don't have much of a problem. I can't speak for any of the new generation large transport category aircraft, but I will use as an example the Astra/G100 series aircraft that I am currently flying. In our case, our low speed buffet at FL390, at normal cruise weights, would be down in the neighborhood of .55 Mach, the high speed buffet is just beyond the "barber pole" or a point or two above .875 Mach. If I were to climb to FL450, the low speed buffet would increase to .56 Mach and there would be no change on the high speed end. With a normal cruise of say .82 or .83 we have no real concerns with the coffin corner. If a pilot tries to take them too high for existing conditions they will either run out of "wing" or out of "power" and simply quit climbing. Conceivably, you could push it and get the speed so low and the AoA so high that you could end up with a real serious problem, but you would have to be brain-dead to allow the situation to get so far out of hand. Many of the "Jurassic Jets" had real issues with the coffin corner phenomenon - like the DC-9.
As far as multiengine propeller driven aircraft...
Engine rotation in twins doesn't really make much difference. For example the MU-2's engines/propellers aren't counter-rotating (one rotating CW, the other CCW) they both rotate in the same direction - either CW or CCW depending on the model. It's not really that big of a deal, in the case of the Garrett engines the "Dash 10's" rotate the direction they do because of an extra gear in the gearbox.
Planes with counter-rotating props do not have a critical engine by definition. The FAA defines the critical engine as the engine who's loss most adversely affects the performance and handling characteristics of the airplane.
The reason one engine has a higher performance loss than the other on a non-counter rotating engined aircraft is because the right side of the propeller disk produces more thrust than the left side. The right side of the prop disk on the left engine is closer to the thrust centerline and has a lower thrust moment due to the reduced arm. The loss of the left engine (the critical engine) leaves a majority of the available thrust, which has a higher thrust moment, left to provide power. The higher thrust moment of the right engine requires more rudder to counteract that thrust moment. Hence a higher penalty in performance.
That being said, I want my students to understand that counter-rotating do NOT make one engine "critical" and therefore implies that the other engine isn't. It simply makes both engines equally critical. All theories aside, piston powered, twin engine aircraft have two engines because they need two engines and under certain, limited conditions they can manage to remain airborne and controllable on one engine. The graveyards are packed with pilots (and their passengers) who either didn't understand or didn't believe that fact.
Lead Sled
Last edited:
jetalc
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Similar to Part 23, I don't see anything about how those factors affect Vmc - I have my own theories to rationalize that configuration, but I still can't find anywhere where it is stated that these factors affect Vmc. I feel that the FAA, in the interests of safety and FULL education, should re-vamp the way it discusses Vmc, possibly to include statements like "anything that decreases rudder effectiveness will increase Vmc" or "these factors affect Vmc most significantly" instead of apparently limiting the factors to those three.IP076 said:
This isn't flamebait, but I am questioning the way people are answering Vmc questions in interviews - it should be "FAA Approved" and people should stay away from making up their own theories about WHY they stated that configuration in Part 23 and the PTS. Still looking for a concrete answer...thanks for the reply - it's been a while since I've looked in the PTS, and didn't realize they had amended them to include the Part 23 configuration stuff.
jetalc
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