VMC-Max WT or Light Weight?

[PHX767]

A practical look at Vmc

Perhaps the best way to look at the factors affecting Vmc is to look at the certification requirements individually.

I.e; In discussion with a student, I would begin with: Aircraft gross weight - Certified at max TOW. Since you cannot be heavier than that, what happens to Vmc at lighter weights? Goes up or down?

Address each certification item in that manner and that should eliminate any ambiguity.

And then... How about the factors that make the critical engine critical?

 

[ROB-x38]

G'day jetalc

The variation of the minimum control speed (Vmc) with weight is closely related to the 5 degree AoB limit set down in the rules when determining Vmc.

As I'm sure you're aware when you bank "five to the live" or "raise the dead" there is a small horizontal component of lift (a force) which is added to the force produced by the rudder. The horizontal component of lift supplements the force produced by the rudder and aids in maintaining directional control to counteract the assymetric thrust/drag. So for a given assymetric thrust condition, when a bank is introduced the force produced by the rudder can be less which translates to a lower Vmc.

There are a couple of reasons the regulations impose the 5 degree AoB limit for determining Vmc:

Firstly banking 5 degrees has a negligible impact on the vertical lift generated to oppose weight (the exact amount can be worked out with trigonometry) which means elevator forces are minimal.

Secondly if the 5 degree rule didn't exist it would be possible for manufacturers to publish a Vmc figure which was much less. Obviously, ignoring stall speeds, if you were able to bank the aircraft to 30 degrees, that horizontal component of lift which is supplementing our rudder is now much greater, so the rudder force required is even less which, again, translates to a lower Vmc. The problem with this is the non-practical or unrealistic handling methods which would needed to be used in the event of engine failure (ie: significant angles of bank, large elevator forces etc...)

Hopefully some of the above makes sense, and if it does it should be clear that for a heavier weight that horizontal component which is helping our rudder is going to be greater, which means less force needed from the rudder which means less airflow over the rudder which means lower Vmc.

 

[jetalc]

Rob -


Thanks - it's been a while since I've looked at this stuff, and I appreciate the answer. Yes, it makes sense, as do the other factors - I just hadn't seen the FAA admit that there was a direct correlation between those factors and the actual Vmc speed. It's not that I don't understand Vmc, or that I don't have my own theories for how each of those factors affects Vmc...it's just that I hadn't seen where the FAA had put its neck on the line and actually come out and said it.

Thanks for the explanation - it's obvious you put a lot of effort into your response, and I appreciate it.