VMC vs. Weight

[pilot.90]

hey guys ...

here is the question ...

How is the weight dose effect the VMC speed ..? and why ?

please guys i need a good explaination for my Multi Flight test.!

[Strega]

Think about it,,, really think... you'll figure it out...

[Bede]

Think about it,,, really think... you'll figure it out...

That's quite some insight. I'm pretty sure he really thought about it, and so he came on here.

Here's the answer, about 3/4 pagde down:
http://www.avweb.com/news/airman/184438-1.html

[Hedley]

I wrote that 14 years ago I didn't know it was still around!

[FlaplessDork]

Most unfavorable weight and center-of-gravity
position. VMC increases as the center of gravity
is moved aft. The moment arm of the rudder is
reduced, and therefore its effectivity is reduced,
as the center of gravity is moved aft. At the same
time, the moment arm of the propeller blade is
increased, aggravating asymmetrical thrust.
Invariably, the aft-most CG limit is the most
unfavorable CG position. Currently, 14 CFR
part 23 calls for VMC to be determined at the
most unfavorable weight. For twins certificated
under CAR 3 or early 14 CFR part 23,
the weight at which VMC was determined was
not specified. VMC increases as weight is
reduced.

Basically its harder to move (Yaw) a heavier object.

[Bede]

I wrote that 14 years ago I didn't know it was still around!

Seriously, I have seen that document on more than one occasion. Good on you.

[floats4fun]

I believe it has to do with the heavier the aircraft, the more lift it has to generate to keep flying. The added lift (compared to a lighter a/c) is what (is said...Im not a engineer) to reduce VMC. I may be wrong..

[iflyforpie]

I wrote that 14 years ago I didn't know it was still around!

Yeah you did. You quoted it not too long ago.

Excellent article!

[Buzz Lightyear]

And 14 years ago they already called you a veteran....ouch!

[YOWza]

When you're in the zero sideslip configuration, the higher weight promotes higher horizontal component of lift , giving you more authority and decreasing Vmc.

[x15]

Basically its about forces.

In the "test configuration" the aircraft is banked into the good engine 5 degrees or so. This generates a horizontal lift component. Since lift is greater with an increase in weight. The heavier the airplane the greater the horizontal force generated when you bank the airplane.

Since the horizontal force is going to pull the airplane into the good engine the demands on the rudder to maintain straight flight are decreased. So you can manage a lower speed and still keep the shiny side up. However! an increase in weight will lower VMC from a controlability point of view only. In light twins as weight increases so does the power loading. So you may be able to control the airplane but you may not have any performance to climbout.

[Dagwood]

From the link above:

So, we can see that the angle of bank into the good engine in addition to affecting performance, most definitely affects Vmc. What else affects Vmc?

Well, the published Vmc figure is determined at maximum gross weight.

Sometimes you will see the VMC test is done at Minimum Practical Weight or Minimum Test Weight.

Those refer to the airplane loaded as lightly as you can (worse for VMC), yet with a pile of weight in the rear for the most aft C-of-G (again, worse for VMC). Hence, the airplane is not at BEW, but some higher value that gives the worst case scenario for both light weight, and most aft C-of-G.

[TC Aviator]

Dagwood is correct on this one.

Please read the bottom of Page 63 of AC 23-8B, Section 23-149 at the following link:

http://www.airweb.faa.gov/Regulatory_an ... ghlight=ac 23-8b