Weight and Vmc?

[FSIGRAD]

I working on my MEI and I'm putting together a brief on the "principles of engine inoperative flight" and I'm getting kind of confused about how weight effects Vmc. This is how I understand it and correct me if I'm wrong (or please offer a simple way to explain it to a student)
The heavier an Aircraft is the lower Vmc will be for several reasons:

More yaw force will be required to lose directional control, AC with greater weight will resist yaw do to the increased energy(inertia) required to swing the nose over and therefore lower Vmc.

For any given angle of bank a heavier AC will have a greater component of weight acting along the wing to counter yawing tendencies. (I'm a little confused here) In the FAA's 5 degrees of bank max. for certification a heavier AC will in effect lower Vmc due to this component of weight in a sideslip.

I know that Vmc deals ONLY with control and is independent of performance but am I missing anything as far as weight and Vmc is concerned?
A million thanks for any help!

 

[~~~^~~~]

Your explanation sounds correct to me and is the way it has always been explained. A heavy aircraft (loaded within limitations) is more stable - more resistant to a change in direction.

I hope this does not confuse the issue further, but think of it this way. If an airplane subject to VMC was heavy enough, it is likely it would stall before encountering VMC. The heavier the airplane the more the vertical component of lift is emphasized over the yawing motion.

 

[fulcrum]

#1) see when the airplane is in st and level flight weight does not affect Vmc



#2 )but banking into the operating engine creates a horizontal component of lift this component pulls the airplane in the direction of the operating engine ,
counteracting adverse yaw ,therefore requiring less rudder pressure , the heavier the airplane the stronger the horizontal component of lift and the lower Vmc becomes
( taken from faa-h-8083-3 )

fulcrum

 

[bobbysamd]

Vmc

I haven't thought about Vmc in more than eight years, but it sounds to me that you're on the right track. Don't forget - you need Lift to overcome Weight.

Review your factors of Vmc. Isn't one of the factors of Vmc max takeoff weight? Know Vmc and your Seminole systems, and you'll be about set for your oral.

Good luck with your MEI.

 

[SentryIP]

Vmc

Minimum control speed is a direct function of thrust and not of weight. Although what you said is correct about inertia (a heavy airplane is more stable and requires more force to make it yaw), that's only true for a static situation. Once the yaw becomes dynamic, it'll take a lot of force to stop the yaw and return it to the static condition. Weight affects Vmc only in the fact that it takes more thrust to maintain airspeed. Vmc is the calibrated airspeed required to maintain heading, with full rudder and five degrees of bank toward the operaive engines at a certain thrust setting. Remember these facts about Vmc:

Vmc is increased by - increasing thrust on asymmetric engine(s), banking toward inoperative engine(s), decreasing airspeed, using less than full rudder and aft centers of gravity(UNDESIREABLE ACTIONS);

Vmc is decreased by - decreasing thrust on asmmetric engine(s), banking toward operative engine(s), increasing airspeed, using full rudder and forward centers of gravity (DESIREABLE ACTIONS); realize that you may have to sacrifice altitude to decrease thrust, use more bank and increase airspeed.

Hope this helps. Take care and fly safe.

 

[DoinTime]

<<Weight affects Vmc only in the fact that it takes more thrust to maintain airspeed.>>

Thrust/power in this situation is fixed at max t/o. Pitch is used to control airspeed. The ONLY reason increased weight decreases Vmc is because of the horizontal component of lift created by the 5 degree bank into the operative engine. The increased inertia only slows the onset of the yaw, and it dosen't slow it down much.

 

[Simon Says]

With regards to inertia and Vmc weight does decrease Vmc, but once directional control is lost a heavy airplane requires more airspeed to regain control due to the increased inertia.

 

[Hugh Jorgan]

I've always thought the only entering arguements into a Vmc air chart were density altitude, bank angle and thrust. Weight only determines how much thrust is required for level flight. What am I missing?

 

[Hugh Jorgan]

After re-reading Sentry IPs post, I think that's exactly how I would have explained it.....

 

[SkyWestDude]

......

The reason the airplane is banked into the good engine is to counter the sideforce from the rudder. Look at the airplane from the top and there is a large sideforce from the vertical tail that gets countered by the horizontal component of lift from the bank angle.

When you do training tape a piece of yarn just in front of the windshield like they do in gliders. (Don't leave the tape on for more than a day or two or it can mar the paint.)

Before you start instructing in twins I highly, highly recommend you spend a couple HOURS reading the accident reports for the duchess, seminole, baron, etc. You'll notice a couple of things.

1- Don't do single engine go-arounds.
2- End all single engine approaches with a landing, not a missed.
3- Don't shut down an engine unless your easily within a single engine drift down to a suitable airport.
4 - Don't let anyone touch anything (like the flaps) until your off the runway and stopped. (especially in a Baron.)

I appoligize if I'm preaching, but the same accidents happen over and over again.

Probably the most interesting thing I learned by reading a lot of twin accident reports is the lack of accidents caused by the loss of directional control on one engine (Vmc). It essentially never happens outside of the training eviornment.

Scott

 

[A Squared]

Sky west dude wrote:

>>>>Probably the most interesting thing I learned by reading a lot of twin accident reports is the lack of accidents caused by the loss of directional control on one engine (Vmc). It essentially never happens outside of the training eviornment.


Huh????? Why would you make a statement like that? Go to the NTSB website and do a search for fatal accidents for any common light twin. (PA 31, PA-23, C-310) I have, and I assure you that you will find a significant number of VMC accidents when not training. Granted, they are not as plentiful as CFIT fatalities, but they are there in numbers that far exceed "essentially never"

Regards

 

[cheater1239]

the explanation i use is in a Kershner book. my copy is out on loan so i can't provide the correct title or page info. it is either an "advanced pilot " manual or a CFI manual. 23.149 is the applicable FAR

 

[FlyinBrian]

I thought I'd make a comment about bank angle and Vmc. The previous poster was correct when he said bank is used to counteract the rudder IF DONE CORRECTLY. the greater the bank, the lower the Vmc because the horizontal lift component counteracts yaw caused by the operating engine. So your Vmc is lower at 30 degrees bank than at 5 degrees bank. However, this totally dessimates your climb performance. This is why the FAA does not allow more than 5 degrees bank when publishing Vmc. It prevents manufacturers from publishing a very low Vmc at the expense of single engine performance. Ideally, you only want enough bank to get to 0 sideslip, which minimizes drag, and makes for the best climb performance. Incidentally, the ideal bank angle is typically about 2-3 degrees in a light twin. Additionally, an ERAU study showed that in most light twins, every degree of bank beyond that decreases climb performance by as much as 30 fpm. This means that at 5 degrees of bank, you could unneccesarily be sacrificing 90 fpm. Here in AZ on a hot day, that could be the difference between life and death. Many MEI's teach their students to always go to 5 degrees bank and a half ball deflection. That 5 degrees comes from conditions under which Vmc is tested, and has nothing to do with good technique. Just some things to think about...

(Many Thanks to Mr. G Henrie for making me a better CFII and MEI.)

 

[Timebuilder]

Great post, Brian. I have found some chief flight instructors and DE's (the same person in one instance..) who adhere to the idea of 5 degrees of bank and a FULL ball width, rather than teaching the student that these ideas are transitory, and should be used as appropriate to avoid compromising single engine performance. MEI candidates should also realize that when banking into the good engine, the student will have to reduce the anti-yaw rudder they had first applied in order to hold their heading while banked.

 

[sstearns2]

Vmc accidents

The previous post was...

*******
Huh????? Why would you make a statement like that? Go to the NTSB website and do a search for fatal accidents for any common light twin. (PA 31, PA-23, C-310) I have, and I assure you that you will find a significant number of VMC accidents when not training. Granted, they are not as plentiful as CFIT fatalities, but they are there in numbers that far exceed "essentially never"
******

I guess it depends on one's definition of 'essentially never', but I just searched thru all 372 C-310 accidents and found 6 that were due to loss of directional control on one engine. There were about the same number of accidents caused by pilots turning the fuel pumps to high, flooding the engines and loosing all power. My point is that multi engine training, while providing significant single engine work, should reflect the fact that the general complexity of the airplane is much more likely to kill you than a mechanical failure of one of the engines.

Scott

ps- C-310s seem to have an excessive number of landing gear collapses.

 

[A Squared]

Scott,

Yes, there are other types of accidents which kill more people. Characterizing it as "essentially never" might give the impression that it isn't something that really needs to be taken seriously though. VMC rollovers can and do kill people and you need to train for them, although dying while training for engine failure is counterproductive. I looked through the PA-31 accidents and I didn't count, but it seemed like that there were comparatively more VMC rollover accidents listed for the Navajo than the C-310. In fact, a good friend of mine died in precisely that sort of accident in a Navajo. He was a non-pilot passenger. (LAX97FA217)
You make a good point that the complexity of the aircraft should be taken as seriously as engine failure. You will probably see more fuel exhastion accidents for the C-310/320 too. It has a fairly complex fuel system and if you're not paying attention you can dump a bunch of your fuel overboard. The landing gear is a problem with the 310/320. The gear legs are relatively tall and weak (especially the earlier 320s), and it's a fairly complex electromechanical system. It needs plenty of maintenence to keep it rigged correctly. I've had my share of gear problems in that airplane, none of them ending in bent metal, thankfully.

 

[sstearns2]

.......

I can't belive cessna let the fuel system for the 300 series out the door. It's an accident waiting to happen.

Your post got me to thinking that maybe we shouldn't be thinking about the number of Vmc rollover accidents there are compared to all accidents, but on how many engine failures end in a Vmc rollover situation. I'd estimate 10-20%.

Very sorry to hear about your friend.

Scott