Another Vmc question

[shdw]

Just checking. Anyone yet been able to come up with anything from a published author

Yes, I linked you a published formula that expresses inertia's presence. You just choose not to accept it. Repetition: the existence of weight in a calculation implies the existence of inertia. It doesn't need to be spelled out.

 

[Houston]

Yes, I linked you a published formula that expresses inertia's presence. You just choose not to accept it. Repetition: the existence of weight in a calculation implies the existence of inertia. It doesn't need to be spelled out.

The presence or lack of presence of Inertia is not the question, or even the subject. The question is has anyone been able to come up with anything from a published author, and aircraft manufacturer, or a noted school that says the reason Vmca changes with weight is because of anything other than the change in the horizontal component of lift?

 

[shdw]

The presence or lack of presence of Inertia is not the question, or even the subject.

So if inertia is present and plays a role doesn't matter because it hasn't been spelled out for you in a publication?

 

[Autothrust Blue]

-blinks-

Perhaps this would be an inappropriate time to say something simple like "don't go slow on one engine."


SSSSSSEENT FFFFROMM FMS KEYBOAAAAAAARDDDDD

 

[JustinS]

-blinks-

Perhaps this would be an inappropriate time to say something simple like "don't go slow on one engine."

But first we need to know if you can takeoff from a conveyer belt single engine with a unicorn strapped to the prop.

In all seriousness good advice.

BTW @shdw In free space, the heavier pole would take more force to rotate due to inertia... I haven't been in outerspace, but I'm pretty sure inertia does play a role....

I'll try to find some aviation publication. Right now I'm working crazy hours. I'll try to find some time...

Justin

 

[CharlieRomeo]

Multiengine Flying by Paul Craig talks about inertia only when discussing how gross weight affects Vmc.

 

[CharlieRomeo]

Now, a heavier aircraft will have a greater lift vector than a lighter aircraft. This means that, given the same bank angle, a lighter aircraft will have a smaller HCL than a heavier aircraft. This means that a heavier aircraft will require less rudder pressure to maintain control at a given bank angle than a lighter aircraft (hence the lower Vmc at a higher weight). However, all of this must assume a constant bank angle. If a pilot in a lighter aircraft simply compensated for the shorter lift vector by increasing the bank in "bank and ball", this effect would be negated because the HCL would be increased to match what it would have been in a heavier aircraft. Therefore, if we can assume a perfect technique is used (which may be impossible - I'm not sure), then I believe that the HCL argument is invalid.

Agreed. You set the bank angle to give you the correct amount of HCL to put you in zero sideslip. You don't set the same bank angle and then just end up with extra HCL at a higher weight. That would result in another slip.

 

[shdw]

You don't set the same bank angle and then just end up with extra HCL at a higher weight.

No, you must have a greater total lift to make VCL match the now greater weight. Along with it comes an increase in HCL. If the total vector gets larger, both it's x and y vector components must also increase.

@JustinS In the wings level case we are dealing with rotation, or changes in angular velocity. Not transitional. In other words, we would need to look at moment of inertia. Not inertia. Now moment of inertia exists when the rotation point is other than the CG. I'm not 100 percent certain of it's role when the rotation point is on the CG. However, I've multiple texts that mention wings level conditions are independent of weight. So far I've posted supporting material from AFNA and The Mechanics of Flight.

 

[CharlieRomeo]

No, you must have a greater total lift to make VCL match the now greater weight. Along with it comes an increase in HCL. If the total vector gets larger, both it's x and y vector components must also increase.

So would you say that a heavier airplane has a higher rate of turn at a given bank angle than a lighter airplane, since the HCL would be greater?

 

[shdw]

So would you say that a heavier airplane has a higher rate of turn at a given bank angle than a lighter airplane, since the HCL would be greater?

This is part of your commercial training. I would say your turn rate is a function of your bank angle and velocity. Here:

These relationships define the turn radius, r, and rate of turn, ROT, as functions of the two principal variables: bank angle, theta, and velocity, V (TAS).

Weight, and HCL, play no part in the discussion of rate and radius. If it helps, think about taking a bucket of water and spinning it around so it maintains some degree off the vertical (your body) axis. A heavier bucket would take more strength (total lift = HCL + VCL...more of both) on your part to keep the same rate and radius than that of a lighter bucket.

 

[CharlieRomeo]

I would say your turn rate is a function of your bank angle and velocity. H

Ok, good. So, if weight does not affect rate of turn for a given bank angle, a point on which we both agree, can someone explain to me how the HCL decreases Vmc more at a higher weight? After all, we are cancelling out a given rate of turn using bank, and you just stated correctly that weight does not affect rate of turn for a given bank angle.

In case it is unclear, I believe that inertia is the primary reason weight affects Vmc. This is best explained in the book Multiengine Flying by Paul Craig.

 

[shdw]

So, if weight does not affect rate of turn

The discussion of weights effects on Vmc deals with opposing forces. Is rate a force? What about HCL? We know the thrust on the good engine is a force trying to rotate and push us toward the dead engine. So what force (other than rudder) counters that force?

 

[Houston]

So if inertia is present and plays a role doesn't matter because it hasn't been spelled out for you in a publication?

Once again, you failed to answer the question.

The question is has anyone been able to come up with anything from a published author, and aircraft manufacturer, or a noted school that says the reason Vmca changes with weight is because of anything other than the change in the horizontal component of lift?

 

[Houston]

Multiengine Flying by Paul Craig talks about inertia only when discussing how gross weight affects Vmc.

Any chance you could copy or otherwise quote that portion of the text?

 

[CharlieRomeo]

Any chance you could copy or otherwise quote that portion of the text?

"A heavy airplane is a stubborn airplane. The more mass the airplane has, the more inertia it will have." Then he gives a bowling ball vs. volleyball illustration.

"If one engine of a light twin fails, the entire airplane will start to yaw. But a heavy airplane will resist any movement. It will take a greater yaw force to move the nose of a heavy airplane than that of a light airplane." Then he gives a cruise ship vs. rowboat illustration.

"Any yaw force caused by a failed engine will have less effect on a heavy airplane. When yaw has less effect, there is less force required to balance forces and therefore less airspeed required."

This is from Multiengine Flying by Paul A. Craig 3rd edition.

 

[Houston]

"A heavy airplane is a stubborn airplane. The more mass the airplane has, the more inertia it will have." Then he gives a bowling ball vs. volleyball illustration.

"If one engine of a light twin fails, the entire airplane will start to yaw. But a heavy airplane will resist any movement. It will take a greater yaw force to move the nose of a heavy airplane than that of a light airplane." Then he gives a cruise ship vs. rowboat illustration.

"Any yaw force caused by a failed engine will have less effect on a heavy airplane. When yaw has less effect, there is less force required to balance forces and therefore less airspeed required."

This is from Multiengine Flying by Paul A. Craig 3rd edition.

Thank you. I appreciate that.

 

[CharlieRomeo]

Thank you. I appreciate that.

Not a problem.

 

[shdw]

a heavy airplane will resist any movement. It will take a greater yaw force to move the nose of a heavy airplane than that of a light airplane.

Agreed except that, for reasons already posted.

@Houston Exactly as the equation I posted depicted. I'm not sure why published numbers are not as meaningful as published words, but that's neither here nor there anymore. I will note that the only thing this author is missing is that weight does not exist in the calculation of wings level Vmc because it's not a factor. It is present in the posted formula only because that formula deals with calculation of Vmc in a banked flight condition.

FWIW I had some words, and disagreement, with tgray on this subject. He stated a few things you might be interested in:

"The reduction in Vmc only happens when the aircraft is banked; since the regs limit the bank to 5 degrees, a heavier airplane has a strong sideslip towards the good engine, maximizing rudder authority.

So weight is only a factor because lift must equal weight."

--

"If you throw around the term "inertia", you're giving other people permission to keep thinking about this issue as one in which the airplane is sluggish to rotate. The term carries a lot of baggage."

--

"So my view is that using the word inertia in this context is wrong.

Even ignoring its wrongness, if you use the term "inertia", 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.

I will say he hit the nail on the head with the thinking it's sluggish to rotate, which is false. That was evident with the contention that inertia plays a role in Va, or in wings level Vmc for that matter. And why I said earlier that I agree in using the HCL definition. I just don't agree with him in saying that this definition is wrong, though it certainly carries some baggage.

 

[JustinS]

I'm not sure why published numbers are not as meaningful as published words, but that's neither here nor there anymore. I will note that the only thing this author is missing is that weight does not exist in the calculation of wings level Vmc because it's not a factor.

Mass has an effect on Vmc during level flight.

 

[shdw]

Mass has an effect on Vmc during level flight.

I've posted multiple recognized texts, both a pilot and engineering geared text. As well as a good friend whose responses I posted verifying the contrary. You'll forgive me if I say you've no merit in your standing. If you wish to believe so, go ahead. It will however remain an unfounded belief as far as I can see.

This is precisely why, while I may argue to your side on inertia being a factor, for this precise condition; remembering the caveat that it is a transitional, not rotational, existence. I'll stand by Houston, and Taylor, in saying that HCL is the most effective way to teach weights effects on Vmc because: (paraphrasing):

"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.