Vmc calculation and weight

[bobbysamd]

Vmc

The above two posts bring home the point that you must be able to explain factors of Vmc to the same level of detail for your practicals.

As an aside, I do not remember having to explain Vmc to such depth when I took my multi and MEI practicals fifteen years ago. At that time practical test standards were just being introduced. But my examiner was a former manager of the OKC FSDO and had a reputation for being tough. I went to him three times because I got along well with him and he was extremely fair. Then, I remember Riddle multi stage checks to be extremely brutal regarding Vmc, much more so than the FAA. Just be prepared.

 

[100LL... Again!]

Bobbysamd-

I think it it is slightly helpful to know the nine factors and also know how changing them affects Vmc, i.e. gear down lowers vmc, cg fwd lowers cg, etc.

However, to some degree I think that it is a lo like the old days of airline orals where you had to memorize tons of irrelevant info - such as turboprop gear reduction ratios, etc.

The big question is then why we do not go into all the factors behind other aircraft limitations, such as Vne, Vno, Vy, Vx, etc.
We know the number and when it is used, but no concern is given to how it is arrived at (thank goodness).

I still hope someone proves me wrong and comes up with a scenario where knowing one of the factors changes how you fly the airplane, or creates a no-go decision.

The only no-go I can come up with is too hot or too heavy to climb on one engine, or too little runway for accel-stop and -go.
BUT these are not Vmc issues. Hah!

Looking forward to more discussion.

 

[Flymach2]

Hi...

Here are a few thoughts regarding the different opinions on the subject of weight vs. Vmc.

It should be obvious that within this discussion that there are more than one point of view.

As mentioned, FAR § 23.149 indicates the most unfavorable weight in the range of takeoff weights in addition to the other factors.

When an airplane departs at maximum gross weight as opposed to a lower weight, the angle of attack required for flight will be at its highest in comparison to any other weight. Subsequently, asymmetrical thrust or P-factor will also be at its highest in comparison. The increased P-factor will have the effect of increasing Vmc.

Another point of view is this.

Since the airplane is at its heaviest, (max. gross weight), the total lift generated by the wings is also greatest. A 3-5 degree bank into the operative engine will result in the horizontal component of that lift being greater, subsequently having a greater impact in the reduction of Vmc.

Food for thought.

Regards

 

[172driver]

The big question is then why we do not go into all the factors behind other aircraft limitations, such as Vne, Vno, Vy, Vx, etc.

The reason we don't go into those factors is because those numbers are relatively constant. Sure, Vx and Vy change by a few knots with altitude but that's it. Vmc can change from the published value by 30+ knots. That's why we need to know about it.

Here's your scenario...

Take a high performance twin up, preferably turbocharged. Load it all the way aft and away from the operative engine, take minimum fuel and pax, windmill the prop, flaps and gear up, bank ten degrees into the inop engine, apply full power to the operative engine. Think you'll be safe if you're flying above the red line? At the blue line? Let us know.

 

[bobbysamd]

Immaterial head nonsense for practicals

Bobbysamd-

I think it it is slightly helpful to know the nine factors and also know how changing them affects Vmc, i.e. gear down lowers vmc, cg fwd lowers cg, etc.

Agreed. Absolutely. I wasn't discounting your points. It's basic aeronautical knowlege that every multi pilot should know, however . . . .

However, to some degree I think that it is a lo like the old days of airline orals where you had to memorize tons of irrelevant info - such as turboprop gear reduction ratios, etc.

Which is the point I'm trying to make.

All your points are well taken.

Shall we turn this into an anti-Riddle (or FSI - which wasn't quite as bad - or anti-Comair or anti-wherever) stage check discussion?

 

[172driver]

Shall we turn this into an anti-Riddle (or FSI - which wasn't quite as bad - or anti-Comair or anti-wherever) stage check discussion?

We can if you like but we're talking about the importance or nonimportance of being able to explain the factors of Vmc. The FAA has deemed it to be important so, we as pilots, stage check pilots, CFI's, must ensure that Vmc is being taught and learned in detail. This obviously includes not only the factors but the principles behind them.

The reason we should know how the airplane is configured when certified is that we've got to know how they determined where to put that red line. We need to know that Vmc can occur at speeds much higher than the red line. In my opinion, it's as important as knowing that we can stall an airplane at speeds much higher than the bottom of the green arc.

PTS:

b) Effects of density altitude on Vmc
c) Effects of airplane weight and CG on control
d) Reasons for variations in Vmc
e) Relationship of Vmc to stall speed

 

[100LL... Again!]

172-

You've posted a scenario, but I don't see how you have made your point that knowledge of Vmc factors changes the way the pilot approaches the flight. Also, how do you get such an aft CG with such a low weight? Could happen, but hard to do in most aircraft.

Also, Unless you are carrying bowling balls inthe nacelle locker, I don't think you would likely be able to shift the CG enough laterally to make an appreciable difference.

I think the point you are trying to make is that rudder effectiveness would be reduced, but power would remain the same, plus the laterally diplaced CG, etc could cause Vmc to rise above redline.

Even if the aircraft can produce the same power at altitude (or more, in the case of a Seneca II), the prop efficiency is still reduced. Rate of climb diminishes with altitude even if manifold pressure and rpm remain constant. Your scenario is remotely plausible, but highly unlikely.

Last point: What in the world would anybody be doing flying at redline? (Other than training).

If a pilot does not possess the skill or sense to maintain blueline or better, all bets are off.

 

[Deftone45075]

However, to some degree I think that it is a lo like the old days of airline orals where you had to memorize tons of irrelevant info - such as turboprop gear reduction ratios, etc.

26:1 on the Garret TPE-331's.

 

[mrflyguy]

So if the manufacturer must use minimum weight, what would they use for minimum weight? It seems you could always take a little more fuel out or find a little lighter test pilot.