Another Vmc question

[JustinS]

"the common folk are going to conclude that weight affects Vmc because a heavy, sluggish airplane will react more slowly to aerodynamic forces, because that's an easier concept to grab onto, which is even more wrong."

To which I completely agree.

To which I would agree as well, because it's not an aerodynamic force... That's raw engine force acting on an arm between the engine and the C.G... And it's not that it will react "more slowly" It will react LESS, period, because it's the force acting against a mass that has more inertia resisting a change in direction...

 

[shdw]

an aerodynamic force...is...force acting on an arm between the xyz and the C.G

I did a little reconstruction of your words there, underlining my two changes. What do you think the rudder, elevator, and ailerons do? They supply a force on an arm between their surfaces and the CG. Perhaps it would help to define aerodynamic force? An aerodynamic force is a force acting on a body submersed in a gaseous fluid.

 

[Houston]

I'm not sure why published numbers are not as meaningful as published words,

I'm not sure what you're trying to say there or at any point in this thread. Even after reading what you wrote a dozen times, it still looks like you have taken positions on both sides of the issue in the same thread. You have ducked or danced around answering questions while writing in a manner that seems more intended confuse than clarify. Is it possible for you to state your position in clear concise words?

 

[JustinS]

This thread just gives me a headache...

Increased weight causes Vmc to be lower due to HCL, and inertia... That's all I got out of this thread...

 

[Houston]

This thread just gives me a headache...

Increased weight Vmc to be lower due to HCL, and inertia... That's all I got out of this thread...

Which, if you note from the votes, puts you ahead of 11.1% of the people who said that an increase in weight will cause an increase in Vmc. The rest is just why does it do that.

 

[shdw]

Increased weight causes Vmc to be lower due to HCL, and inertia... That's all I got out of this thread...

Without bank HCL wouldn't exist, would it? Transitional* motion wouldn't exist either either, so inertia also wouldn't exist. Summation, weight wouldn't play a role in a wings level scenario. You should also take that away from this thread.

*See note on next reply.

 

[shdw]

I'm not sure what you're trying to say there or at any point in this thread.

1) I take the standing that both inertia and HCL are valid explanations to weight's role on Vmc in a banked condition.

2) I favor using HCL because inertia carries baggage and can easily be misinterpreted to mean more than it does.

3) Weight does not play a role in a wings level Vmc condition. Weight plays a role because HCL, and inertia due to transitional motion*, exists when you introduce a bank. The greater the bank, the more substantial the role.

*Note: Transitional motion means movement along, not around, an axis.

 

[Houston]

1) I take the standing that both inertia and HCL are valid explanations to weight's role on Vmc in a banked condition.

2) I favor using HCL because inertia carries baggage and can easily be misinterpreted to mean more than it does.

3) Weight does not play a role in a wings level Vmc condition. Weight plays a role because HCL, and inertia due to transitional motion*, exists when you introduce a bank. The greater the bank, the more substantial the role.

*Note: Transitional motion means movement along, not around, an axis.

Good, clear, concise, and understandable. Thanks.

 

[Houston]

A google search has yielded people saying both things,

When you came across the various items in your Google search, were they just the opinions expressed by individuals or were they references to published works?

 

[durind]

When you came across the various items in your Google search, were they just the opinions expressed by individuals or were they references to published works?

Well there is this site: http://www.avweb.com/news/airman/184438-1.html?redirected=1 which basically talks about both sides of weight, but doesn't take a side.
This is just a thread on a forum but seem to have people on both sides: http://www.avitop.com/cs/forums/thread/2380.aspx

But yeah, I would say overwhelmingly, just like the poll in this thread most sources support that an increase in weight disregarding all other factors will decrease Vmc.

 

[Houston]

I tried to go beyond substituting opinions for facts. The reason was best stated by Clint Eastwood in the 1988 movie The Dead Pool when he defined opinions. Looking just at published authors, there were are a great many who explain that the change in Vmc with weight is due to the increased horizontal componenet of lift and never mention inertia as even a contributing factor.

ASA produced the standard oral examination guide for multi-engine practical tests. This is what they write on the subject in Chapter 2:

Vmc is unaffected by weight in straight and level flight. Vmc will be affected by the aircraft's weight in turning (banked) flight. When an aircraft is banked, a component of the aircraft weight acts along with the horizontal component of lift to create a more effective sideslip toward the operative engine. For a given bank angle, the greater the aircraft's weight, the lower the aircraft's Vmc.

Others such as Piper Aircraft Corporation:

http://ehfc.net/SenecaIIManual.pdf

Embry-Riddle Aeronautical University

https://eta.erau.edu/fif/DB/pa44_meg.pdf

CAOP as quoted in AVweb:

http://www.avweb.com/news/airman/184438-1.html

Even the world's most poorly prepared flight instructor teaches it that way on youtube:

However, as noted earlier, there is one published author who states that the reasons rests with inertia and does not mention the horizontal component of lift. That is Paul Craig. I wrote to him at Middle Tennessee State University asking him how he formulated the view that he wrote in his book. After two weeks, I have not received a reply.

Each of us can formulate our own opinion, but until demonstrated otherwise, I will stick with the preponderance of published literature.