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Factors affecting Vmc

Last post 04-02-2009, 8:00 PM by oldengineer. 8 replies.
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  •  10-29-2003, 6:50 PM 2380

    Factors affecting Vmc

    Hi, I'm a new multi-engine student trying to gather more info on factors affecting Vmc.
    From what I've learned so far, it seems as though everyone has their own theories on what causes Vmc to increase and/or decrease. I've been taught by some that increasing weight will increase Vmc for the following reason:
    To maintain lift, the aircraft must fly at a higher angle of attack when weight is increased(true), and therefore left turning tendancies will be stronger at the higher angle of attack, hence increased weight will cause Vmc to go up. Is this really true??? Does anyone really even know? I've been taught by others that Vmc will decrease with weight for other less obvious reasons. Which is right and which is wrong? I can't find any detailed information on any of this stuff. Sounds to me like a bunch of people just sit around and try to think of stuff without any actual testing being done to determine their theories. Can anyone help me out with this? And do you know of any good references available?
  •  10-30-2003, 9:24 AM 2382 in reply to 2380

    Re: Factors affecting Vmc

    Hi ndg

    Yes it is true and also tested.

    Vmc is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with that engine still inoperative and then maintain straight flight at the same speed with an angle of bank of not more than 5 degrees.

    When weight goes up the aircraft will have to fly at a higher Angle Of Attack (AOA) to produce the extra lift needed. Increase in AOA also means an increase in P-factor which is the reason Vmc is increased.

    Propellers have what's called "P-factor" when the airflow into them is not parallel to the propshaft, such as when the airplane is at a high angle of attack during climb. The skewed airflow relative to the prop disk causes the blades on one side of the disk (the downward-moving blades in the case of a nose-up climb) to see a higher angle of attack and airspeed than the blades on the other side. This causes them to make more thrust than the blades on the other half of the disk. The unequal thrust tries to yaw the plane towards the side of the disk that has less thrust, typically to the left for one of the right-handed props we typically use in the USA.
  •  10-31-2003, 5:46 PM 2387 in reply to 2382

    Re: Factors affecting Vmc

    Thanks for the reply!

    Here's the other part of what some have said about Vmc:

    I have read in a book (I won't say which one) that increasing weight will decrease Vmc, basically because of intertia. A heavier airplane will require a greater force to cause it to be moved. In other words...An object in motion will tend to stay in motion until acted upon by another force, hence, a heavier airplane will not want to roll over at the "certificated" Vmc. Vmc would actually decrease because the higher weight will make the airplane 'more stable'. There was something else about that argument that I was going to mention, but now I've forgotten it. Frankly, I'm not really buying into that statement. It may be valid, but wouldn't the left turning tendencies of the higher AOA more than outweigh this argument?

    Any comments about this one?

  •  11-01-2003, 12:12 AM 2388 in reply to 2387

    Re: Factors affecting Vmc

    First of all - Why don't you want to tell the name of the book?

    I can follow your theory, but am pretty sure that the P-factor far outweighs this. Especially in the size plane you are going to fly. Maybe on a big multiengine airplane the factors are more equal.

    You said you are a multi engine student. How far are you in the progress? The school/instructors you are using should give you a very detailed education on the particular factors affecting the airplane that you are flying and their effect on Vmc. If that is not the case you need to pick another school.
    There are a lot of bad flying schools out there that doesn't teach their students crap - Make sure that isn't the case here.
  •  02-10-2004, 8:30 PM 2439 in reply to 2380

    Re: Factors affecting Vmc

    Dear friend,
    flying multi-engine is fun and rewarding.
    to start, controllability and performance are two different and opposite things. what is good for one is bad for the other one.
    Part 23 requires manufactureres to test Vmc under certain requirements.
    Vmc promises only that if your speed is equal or above it, your airplane will fly straight. there is no guarantee of perfomance what so ever. i.e. Piper Seneca PA34 has a Vmc of 80mph. this promises that if you have an engine failure, while at max wt, good engine full power, bad critical engine windmilling, bank 5 toward good engine......with all this Vmc promises that you will fly straight. you might not be even able to maintain altitude, but you will crash going straight. no loss of directional control. that is the logic behind Vmc.

    Now when wt increases Vmc decreases. In the Seneca Vmc is 80mph. this assumes banking not more than 5 toward good engine. when you bank toward the good engine you are using less rudder (rudder is the first line of defense to prevent yawing toward bad engine) by utilizing the horizontal component of lift. when acting at an angle, total lift is divided into components vertical and horizontal. the vertical one has to equal wt to maintain altitude. the heavier you are the more vertical lift you need, however, if your bank is still 5, then this will lead to an increase in horizontal lift as well, same bank angle but because more total lift is produced, then more vertical and horizontal lift you will have. this will lead to less rudder needed, which means you will have a lot of reserve rudder. now revisiting the simplified definition of Vmc. it is simply when the rudder stalls. when the airplane becomes slow enough that full rudder cannot produce enough lateral lift to prevent loss of directional control so airplane continous yawing. if you have full wt then horizontal lift is big enough for you to use partial rudder versus full. this means you need to slow down even more before you have to use full rudder and then slow down on notch more to loose directional control. so more weight means lower Vmc.
    Wt is always a penality on performance because the heavier you are the less thrust available you have and climb and performance charactiristics will degrade. so you will fly straight if you hold Vmc or above. if you are unable to maintain altitude with Vyse you will crash but while having directional control. email me with any questions. mfalhout@hotmail.com
  •  02-15-2004, 10:59 AM 2443 in reply to 2380

    Re: Factors affecting Vmc

    Captain Mo is correct, it is an increase in weight that will decrease Vmc, not the opposite. You must seperate performance and control. Vmc is about directional control, nothing else. When an engine is brought to idle with the other running at take off power, you will see a very strong yawing tendancey, yes with roll as well. The natural instinct is to apply opposite rudder. However, when you are slow, you must apply more rudder to get the desired effect. When you run out of rudder travel and lose directional control, that's Vmc. The red line is Vmc under specific conditions. It is meant to be used as a reference.

    Anyway, back to your question about weight. When you bank the aircraft, you create a horizontal component of lift. The reason we lift the dead engine is to allow this horizontal component of lift to assist us in counteracting the yaw which was created by the good engine. When we increase weight, we increase total weight, which creates more horizontal lift. The more horizontal lift, the more lift counteracts yaw. Will you maintain altitude, it doesnt matter. We are talking about Vmc, otherwise you could crank it over to 45 degrees and you'll get Vmc very low. This isnt very safe now is it. But the increased weight at 45 degrees, you may even rudder in the same direction as the dead engine. But this is all about Vmc, not performance; or safety in this case.

    Back to weight, this is also not to say that you should fly around at max weight. It will dampen your performance. How often will you be flying at Vmc, hopefully when you are on the runway and its a split second as the needle passes over it. But if you want to minimize Vmc, then by all means, load that bad boy up.

    Another intresting topic, did you know that having the gear down will decrease Vmc as well? Now thats counter-intutive.
  •  03-15-2004, 8:18 PM 2458 in reply to 2380

    Re: Factors affecting Vmc

    I am in late so I hope some are still watching. As a 24000 hr instructor pilot, I am amazed at the misinformation given out. Vmc is not affected by weight as much as one might think.. As weight increases, the CofG moves aft. Vmc is a function of the amount of lift that the fin and rudder can produce to offset the assymetric thrust of the engines. When the CofG moves aft, the moment arm is less, therefore the rudder has a shorter moment arm to work with, erga, a higher Vmc. In a Lear, as weight is increased, the CofG moves forward because the weight is added forward of the wing, and the Vmc decreases. Drag from the windmilling prop will increase, erga, a higher Vmc. Beech B200 has a Vmc of 87Kts but with a 4 blade prop, if the prop does not feather, Vmc can be as high as 108 kts. Landing gear will create some drag and have either little or no no effect. The amount of power the operating engine is producing will have a profound effect. Get below Vmc and cannot maintain directional control, reduce power on the operating engine, lower the nose to gain airspeed and increase power as required. The Navajo with BLC vortex generators has a lower Vmc because the generators increase the effectivness of the fin and rudder, therefore a lower Vmc. Counter rotating props reduce the P factor on the right engine to match the left so there is no critical engine. All "traditional" American engines rotate clockwise when viewed from behind. "traditional' British engines rotate the opposite. All American airplanes, the left engine is critical, British the right is critical. Not so much for Vmc as performance. Vmc is always the worst case. Do anything to improve and Vmc will be less. All airplanes with a max take-off weight less than 6000 lbs and a stall speed in landing configuration, when measured at 5000ft ISA(+5C) configured for single engine cruise do not have to demonstrate the ability to climb or maintain altitude on one engine, so be careful. Do not attempt to do something the airplane is not capable of doing.
  •  03-16-2004, 9:22 AM 2459 in reply to 2458

    Re: Factors affecting Vmc

    I sure am happy that I am flying a single engine fighter :o)
  •  04-02-2009, 8:00 PM 7390 in reply to 2459

    Re: Factors affecting Vmc

    I have worked in flight test engineering in the past and am a certified multi instructor.   I like Capt Mos explanation of the weight effect.  There are actually seven factors that we teach new multi drivers to reduce VMC. 1) Higher AC weight 2) Lower density altitude 3) Landing gear down 4) Flaps down  5) Lower power setting 6) CG located forward in the envelope and 7) Prop feathered on the sick engine.

    1) Mo explained well.  2) Lower DenAlt gives better control at lower speeds due to higher dynamic loads on the control surfaces 3) & 4) Give additional lateral stability 5) Is the obvious P factor and torque issue 6) With the CG forward the rudder has a longer lever arm through which to create anti-adverse-yaw moment.  and 7) With the prop feathered, there is less pro-adverse yaw moment created in the direction of the sick engine. 

    Of course some of these factors which reduce VMC might be inadvisable, given the aircraft situation at the time; but, if you are in a corner and need additional control of the aircraft, these will get it for you.  Remember, it is better to maintain control of the airplane even in a challenging situation.

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