Vmc: Flap and Gear Positions, 5 degrees of bank and Max Gross

[RiddlePilot]

To go back to the keel effect, it's my understanding that with the gear down, the CG moves down and forward. Moving the CG in those directions will always increase stability. Think about when you play darts. The weight is always at the tip. If the weight was near the back, you'd get no accuracy...i.e. less stability. When the CG moves down with gear extension, that's akin to flying an airplane with the engines mounted above the CG...which is always stable, compared to an airplane with engines mounted below the CG, which is less stable.

That's the explanation that was drilled into my head while getting my multi.

[sixpack]

C650CPT:
[ QUOTE ]
we explore static Vmc not Dynamic Vmc

[/ QUOTE ]
Would you agree then that the Aft-CG effect applies only to the dynamic case, since the plane is not rotating about the vertical axis until it has already reached it's VMC. The rotation (yaw) of the airplane occurs only when the sum of all forces about ANY vertical axis become unbalanced. Shifting weight in the airplane does not change these forces.

On a lighter note, here's my cut on flaps.
Drag behind the airplane will prevent yawing. Drag in front of the airplane will enhance yawing. Since the flaps are well behind the point of thurst (on a typical twin), they tend to pull the back of the plane toward the back, keeping the plane pointed forward.
In an extreme example, attach a parachute to the rear tie-down clip of the plane, and you'll see that it will help stabilize yawing. Conversely, if you were to attach that same parachute to the nose of the airplane, look out.
Gear also causes drag behind the thrust. The keel effect is often associated with the gear doors, which are fairly large plates aligned with the relative wind.

[EDUC8-or]

Up to 5 degrees of bank is a very good thing. You are using your weight to create a zero sideslip condition. This decreases the drag on the fuselage and creates a greater angle of attack on the rudder, making it more effective.

[ananoman]

The problem with 5 degrees of bank is that it will decrease Vmc compared with a zero bank condition, but in most airplanes it actually causes a sideslip into the good engine. This raises drag and in a light twin with 200 fpm climb or less at sea level, this can really hurt. Best performance is usually found at 2-3 degrees of bank. The only way you can really tell if you are doing it right (without a yaw string) is to look at the VSI. If you keep the ball centered and the wings level, your climb performance will be much less than predicted, at sea level in a heavy plane you can go from 150 fpm to zero climb by flying in this way. If you fly with 2-3 degrees of bank in a zero sideslip condition, the VSI will show the increase in climb, and when you add more bank than necessary you will sideslip the other direction and loose climb performance again.

The certification requirements for Vmc have nothing to do with performance and people often think that 5 degrees of bank is best, because this is what is in the regulations. However, this much bank actually hurts performance.

[sixpack]

When demonstrating the VMC maneuver, it would make sense to use as little bank as allowed in the PTS. This raises the VMC speed and puts some distance between stall speed and vmc speed.

This is much like limiting the rudder. It causes you to VMC at a safer speed. (wish the PTS would go back to the days of limited rudder).

[say_speed]

[ QUOTE ]
The problem with 5 degrees of bank is that it will decrease Vmc compared with a zero bank condition, but in most airplanes it actually causes a sideslip into the good engine. This raises drag and in a light twin with 200 fpm climb or less at sea level, this can really hurt. Best performance is usually found at 2-3 degrees of bank. The only way you can really tell if you are doing it right (without a yaw string) is to look at the VSI. If you keep the ball centered and the wings level, your climb performance will be much less than predicted, at sea level in a heavy plane you can go from 150 fpm to zero climb by flying in this way. If you fly with 2-3 degrees of bank in a zero sideslip condition, the VSI will show the increase in climb, and when you add more bank than necessary you will sideslip the other direction and loose climb performance again.

The certification requirements for Vmc have nothing to do with performance and people often think that 5 degrees of bank is best, because this is what is in the regulations. However, this much bank actually hurts performance.

[/ QUOTE ]

I have nerver heard anywhere that 5 degres of bank hurt the performance, but 2 to 3 is good. The 5 degres of bank story has been demonstrated over the years by test pilots, wind tunnel tests and so on... Where did you find or read that 2 to 3 degres of bank was better than 5? And you have apparently verified it in flight; well I have to admit, I can't even see the difference on my AI between 3 and 5 degres, nor can I maintain that bank with enough precision to get to that conclusion.

[say_speed]

[ QUOTE ]
C650CPT:
[ QUOTE ]

On a lighter note, here's my cut on flaps.
Drag behind the airplane will prevent yawing. Drag in front of the airplane will enhance yawing. Since the flaps are well behind the point of thurst (on a typical twin), they tend to pull the back of the plane toward the back, keeping the plane pointed forward.
In an extreme example, attach a parachute to the rear tie-down clip of the plane, and you'll see that it will help stabilize yawing. Conversely, if you were to attach that same parachute to the nose of the airplane, look out.


[/ QUOTE ]

You should know by now that flaps have nothing to do with parachute; I understand that you are trying to assimilate the drag they create to the effect of a chute... Remember though, 90% of additional lift is created with the flaps extended up to 10 degres, and then drag slowly takes over at about 20 or so degres. To study the effects of the flaps on VMC speed, you have to consider the different flap setting of your airplane, and you will see that a low flap setting has a different impact on VMC than a larger one.
There are certain configurations (dictated by the FAA to demonstrate VMC) for which you simply cannot say that they increase or decrease VMC. They are the flaps, cowl flaps, ground effect... You have to demonstrate VMC in the take-off configuration (the most likely moment to face VMC). If your airplane is approved for normal t/o flaps retracted, then that is the way you have to do it; wether it decreases or increases VMC, unless you take it up and try it, you won't know it. No DE has never had any problems with me teaching that to my students, and my students argumenting that during a ride. It all depends on the particular a/c you are flying.

[sixpack]

[ QUOTE ]
You should know by now that flaps have nothing to do with parachute; I understand that you are trying to assimilate the drag they create to the effect of a chute... Remember though, 90% of additional lift is created with the flaps extended up to 10 degres, and then drag slowly takes over at about 20 or so degres

[/ QUOTE ]
I never said ANYTHING about lift. I was referring to the drag created by the flaps.
You are wrong to say that flaps have nothing to do with parachutes (aka drag chutes). Flaps cause drag and so to parachutes! (yeah, flaps have other effects like lift, wake production, stall speed changes, but I wasn't talking about these effects because they don't really apply to my point).

Flaps create drag towards the rear of the plane, and tend to stabilize yaw.

[ananoman]

You can look right in the Seminole POH. In the performance chart for single engine climb it list 2-3 degrees of bank in the associated conditions. It is not uncommon to find other aircraft with this in their performance charts.

Since most aircraft don't have a yaw string and the attitude indicators are rather small, the best you can do is hold a very slight bank into the good engine and keep the ball 1/2 out. Once things are under control it is possible to experiment a little and see what works best on the VSI. Very small changes in bank and rudder can equal 50-75 fpm in climb.

[say_speed]

[ QUOTE ]
I never said ANYTHING about lift. I was referring to the drag created by the flaps.

Flaps create drag towards the rear of the plane, and tend to stabilize yaw.

[/ QUOTE ]

My point exactly... You need a flap setting well above the one required to demonstrate VMC to talk about drag; The flap setting you have to use for VMC demo create 90% more lift than drag. So the real question is: does lift help decrease VMC?
As for the drag, like I said, I understand your point of stabilizing the yawing moment... I was trying to say to say that unlike chutes, flaps also create lift, that's all.

[say_speed]

[ QUOTE ]
You can look right in the Seminole POH. In the performance chart for single engine climb it list 2-3 degrees of bank in the associated conditions. It is not uncommon to find other aircraft with this in their performance charts.


[/ QUOTE ]


Could you tell me what page you werelooking at? I went over the entire POH, and did not read anything about 2 to 3 degrees, all I have seen is max 5 degrees bank. I am sure I am missing something... Ok, I am looking at POH for PA44-180, Date of Approval 03/23/1978.

[ananoman]

It is in the new Seminole POH, which is currently on revision 17. This is truly one of the worst POH ever. A retarded monkey could have made done a better job on the emergency checklist.

Notwithstanding the quality of the rest of the POH, this is not the first place that I have seen this information. There have been several articles written over the years, and some large turboprops also specify this.

[say_speed]

Ok, I haven't seen that new revision yet...
Like you said, I also have heard talks about the 2 to 3 degres, but I haven't found any article backed by some flight test; do you have any web links or else? I would appreciate it, that got my curiosity teased...
Thanks

[Soonermurph]

Also the FAA standard set forth for manufacturer's to determine Vmc state 5 degrees into the operating engine.

[Soonermurph]

Sixpack is correct on this issue. Flaps down do increase Vmc. A clean AC will yaw easier than a dirty AC.

[Alchemy]

If the airplane will yaw more easily clean, wouldn't that make the adverse yaw from the critical engine more pronounced?

Perhaps the logic in having the flaps up and gear up is to most accurately simulate a "climbout" configuration, the point where a VMC-induced loss of control is most likely to occur.

[sixpack]

[ QUOTE ]
If the airplane will yaw more easily clean, wouldn't that make the adverse yaw from the critical engine more pronounced?

[/ QUOTE ] Yes. Clean means more yaw, which means higher VMC speed (need more airspeed on rudder to counter-act).

[ QUOTE ]
Perhaps the logic in having the flaps up and gear up is to most accurately simulate a "climbout" configuration, the point where a VMC-induced loss of control is most likely to occur.

[/ QUOTE ] Yes.

[say_speed]

[ QUOTE ]
Sixpack is correct on this issue. Flaps down do increase Vmc. A clean AC will yaw easier than a dirty AC.

[/ QUOTE ]

I think you meant flpas down do decrease vmc, since a clean a/c will yaw easier than a dirty a/c.
But I still do not agree on that. Flaps set for take-off create more lift than drag. Therefore there is NO keel effect or chute effect or however you want to call it. Drag helps in stabilizing the a/c around the vertical axis, we all do agree on that. But the drag is minimal compared to addition of lift when the flaps are selected to a take-off position (required to demonstrate vmc by the faa). The effect of flaps on vmc vary with the flap setting, and vary with the airplane type as well. It is like adding flaps on a single engine or multi engine, some have a pitch up moment, some have a pitch down moment; there is no generic answer to the question of what is the effect of adding flaps on vmc.

[Ralgha]

Doesn't matter that takeoff flaps add more lift than drag, they still add drag, and that adds keel effect.

[Soonermurph]

Vmc is not a stall. It's simply the loss of directional control with the critical engine inoperative. To put it simply, there is not enough air flowing over the rudder to overcome the yawing and rolling tendancies encountered during an engine out at low speeds. When the rudder runs out the a/c is unable to fly in a straight line any longer. The result; turning and rolling into the dead engine. Drag is relevant to Vmc only to the extent it creates more yawing effects, i.e. windmilling props. etc.

[say_speed]

[ QUOTE ]
Doesn't matter that takeoff flaps add more lift than drag, they still add drag, and that adds keel effect.

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Yes it does matter. If you want to talk about the effect of flaps (take off position) on vmc, talk about lift, not drag. What is the result of the addition of lift on vmc speed?? If you can answer that question, you have solved the problem of this thread.

[say_speed]

[ QUOTE ]
Vmc is not a stall. It's simply the loss of directional control with the critical engine inoperative. To put it simply, there is not enough air flowing over the rudder to overcome the yawing and rolling tendancies encountered during an engine out at low speeds. When the rudder runs out the a/c is unable to fly in a straight line any longer. The result; turning and rolling into the dead engine. Drag is relevant to Vmc only to the extent it creates more yawing effects, i.e. windmilling props. etc.

[/ QUOTE ]

No one said that vmc was a stall.(I hope I didn't...). But this is a great definition of vmc.

[rausda27]

Remember that when you have flaps extended you are only concerned primarily with the effect of the accelerated slipstream from the operating engine creating airflow over the flaps increasing lift and thus increasing drag. This drag on the operative side conteracts the assymmetrical thrust of the operative engine, reducing the yaw and thus decreasing Vmc...this is at least true in the seminole I believe..

[say_speed]

[ QUOTE ]
Remember that when you have flaps extended you are only concerned primarily with the effect of the accelerated slipstream from the operating engine creating airflow over the flaps increasing lift and thus increasing drag. This drag on the operative side conteracts the assymmetrical thrust of the operative engine, reducing the yaw and thus decreasing Vmc...this is at least true in the seminole I believe..

[/ QUOTE ]

Ok... I don't know why people keep arguing about the drag created. The lift represents 90% of the addition, drag is only 10%. Which has the greatest effect? When you are cooking, if you put 90% more pepper than salt, do you think the disgusting taste will result from the salt?
Hey, I love cooking...

[davidhigbie]

where are you getting those figures? I was under the impression that with full flaps the lift to drag ratio resulted in less lift and more drag...

By the way..this should be rausda27..higbie is letting me use his PC