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Vmca and Critical Engine

 
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K.Haroon
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PostPosted: Tue Feb 28, 2006 6:03 am    Post subject: Vmca and critical engine Reply with quote

Single Engine Minimum Control Airspeed (Vmca)

Vmca: is the minimum speed at which directional control can be maintained with the critical engine inoperative. Published Vmca is a calibrated airspeed and is determined by the aircraft manufacturer.

Factors (stipulated in FAR part 23) which the manufacturer uses to determine the published Vmca speed for a particular airplane are outlined below:

1 - Standard Day (Temperature 15 degrees C and Pressure 29.92" at Sea Level)

2 - Maximum sea level takeoff weight

3 - The most rearward (legal) center of gravity

4 - The critical engine "Failed" and the propeller on the inoperative engine

(a) Windmilling

(b) Feathered, if the aircraft is equipped with an automatic feathering
device

5 - Takeoff or maximum available power on the operating engine

6 - Landing gear up

7 - Flaps in the takeoff position

8 - No more than, but not necessarily, five degrees of bank into the good
engine

With regard to the eight items; you should notice that except for the bank, gross weight, and standard day conditions, all the remaining items are the worst case (they increase Vmca) or are related to the takeoff scenario. The following is an explanation of all eight factors and how they affect Vmca.

Banking into the good engine and maximum weight both reduce Vmca. Banking into the good engine about three to five degrees lowers Vmca by vectoring lift to counter yaw (effectively increasing the horizontal component of lift) and also by reducing sideslip. Reducing the sideslip as mentioned previously, yields greater rudder effectiveness, making possible better control of yaw at slower airspeeds.

Maximum weight also decreases Vmca. As you may recall from basic aerodynamics, the lift an aircraft generates must equal the weight. Thus, a lightly loaded aircraft will generate less total lift than a heavier loaded aircraft. Also recall that the total lift is the sum of the vertical and horizontal lift components. Since lightly loaded aircraft generate less lift, there is less horizontal lift (when 3-5 degrees of bank is applied) to control the yaw. The lightly loaded aircraft would therefore require more airflow over the rudder to control the yaw which necessitates a higher Vmca speed.


Vmca for some multi-engine aircraft is determined at a weight less than maximum takeoff weight because at maximum takeoff weight the aircraft's stall speed would be higher than Vmca and then the aircraft would stall before loss of directional control could be attained. Remember an increase in aircraft weight increases the stall speed.

Maximum power produces the greatest yaw and roll toward the dead engine. In fact, anytime such as denser air, lower altitudes, and lower temperatures that increases engine performance will increase Vmca.Reducing power completely on the good engine would eliminate Vmca altogether. The throttle is often considered the pilots "Vmca lever".

A rearward or aft C.G. reduces the lever arm between the C.G. and the rudder. We'll define the lever arm between the C.G. location and the rudder as "rudder arm". Recall that an airplane rotates about its C.G. along all three axis (in all three planes). The shorter the rudder arm, the more rudder that is required to counteract yaw, so rudder effectiveness is at a minimum, which necessitates higher airspeeds in order to increase the airflow over the rudder to maintain control and therefore the higher Vmca.

The flaps in the takeoff position and the gear up are stipulated because they are indicative to the takeoff scenario. The gear and the gear doors extended tend to act like rudders and act to decrease Vmca. Vmca will increase as we raise the gear. The Flaps add drag and help resist the yawing moments set up by the operating engine. Incidentally, statistics show that most engine failures occur during takeoff. Obviously, due to higher air densities and closer proximity to the ground, having an engine failure on takeoff would be a worse case scenario. Also, if the gear were down, not only would it have a stabilizing effect on the aircraft, but the pilot would most likely still be in a position to land straight ahead, which after pulling both throttles would eliminate the Vmca problem altogether.

Exploring the takeoff scenario further, if the pilot is late or forgets to raise the landing gear on takeoff and to make matters worse, the pilot is climbing out at a slower airspeed trying to figure out why the aircraft isn't performing as well, and then still worse the engine fails. The aircraft loses control when the gear comes up.

Let's look at the compounding events that caused the grim results. It was the combined effort of three things that caused the aircraft to lose control. They are slow airspeed to start with, application of full power, and the gear coming up. First off the aircraft was climbing out at a slow airspeed with the gear down. Advancing the throttles to full position is good only if the airspeed is above Vmca. Advancing the throttles when the engine failed is good because the airspeed was slow but never apply power below Vmca.

Full power, when added, will increase the climb performance, and being so close to the ground you would want to climb to a safer altitude. However, recall that excess thrust over drag is what's needed to climb. Vectorally, thrust equals drag, the aircraft is in equilibrium. When the gear came up, that equilibrium was altered. This helped increase performance. However, the pilot did not consider Vmca. As the gear came up, we lost the stabilizing effect and therefore control was lost.

The pilot should not have raised the gear until sufficient airspeed was attained. The additional airspeed over the rudder would have controlled the yaw.

FAR Part 23 requires that Vmca be determined with the critical engine failed. The critical engine is defined as simply the engine that if failed would most adversely affect the handling and performance of the aircraft. The latter definition rules out the common misconception that the critical engine is the engine with the only alternator, hydraulic pump or some other accessory. It may well be that the critical engine in some aircraft has less accessories than the non-critical engine but the accessories, or lack of accessories, is not what made the engine critical.

On most American made multi-engine the clockwise rotating propellers (as viewed from the cockpit) is the determining factor. The left engine is the critical engine because the left engine would adversely effect the aerodynamic control of the aircraft more when failed than the right engine would. In the Dutch made Fokker Friendship F-27, the right engine is the critical one because of the counterclockwise rotating propellers, as viewed from the cockpit.

Four Factors That Make The Left Engine Critical (Conventional Twin)

P-FACTOR: Both props turn clockwise when viewed from the cockpit. The decending blade produces more thrust than the ascending blade. Even though both props produce the same thrust, the descending blade on the right engine has a longer moment arm, or greater leverage than the descending blade on the left engine. The yaw produced by the loss of the left engine will be greater than the yaw produced by the loss of the right engine, making the left engine the critical engine. See Fig below:




SPIRALING SLIPSTREAM: A spiraling slipstream from the left engine hits the vertical stabilizer from the left, counteracting the yaw caused by the loss of the right engine. However, in the case of a left engine failure, the slipstream from the right engine does not counteract the yaw towards the dead engine. See Fig below:




ACCELERATED SLIPSTREAM: Because P-Factor results in a longer moment arm to the centerline of thrust of the right engine than the left, the centerline of lift is also farther out on the wing, resulting in a greater rolling tendency with the loss of the left engine. Also, the rudder is more effective with the left engine operating. See Fig below:




TORQUE: For every action there is an equal and opposite reaction. Since the props rotate clockwise, the aircraft wishes to roll counterclockwise. If we lose the right engine, the aircraft will yaw right but torque wants to roll it to the left, thereby trying to cancel each other. However, if we lose the left engine, the aircraft will yaw left and roll left, into the dead engine furthering the need for more control deflections. See Fig below:




To Summarize: When losing the critical engine, both directional control and performance will suffer. In discussing directional control problems, the focus is on the concept of Vmca. In discussing erformance, the focus is on the concept of Vyse. It is important to keep the two ideas separate to fully understand them.


Last edited by K.Haroon on Mon Dec 11, 2006 9:27 am; edited 4 times in total
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