Adam the point of the Vmc demo is to demonstrate that with wings level, climbing, as you approach Vsse you will reach full rudder deflection. By applying around 3 degrees angle of bank towards the live engine, directional control can be maintained with less than full rudder deflection, at Vsse.
The 2-3 degrees AoB, or around 1/3rd ball side slip, uses the horizontal component of lift on the around 170 square feet of wing to reduce substantially the side slip beta on the tailplane and rudder drag (around 30 square feet), fo a very small reduction in lift and a slight increase in aileron drag. The aircraft is now flying straight with minimal fuselage drag. As lift=weight in steady un accelerated flight, the closer to MAUM, the stronger is the horizontal component of lift, hence being closer to MAUM will result in a lower Vmc which is cpunter intuitive.
It is a principles of flight demonstration and not lets show the student a departure from controlled flight at Vmc.
The classic mis-management scenario of the demo is in a summer environment, high density altitude, the instructor ignoring Vsse, going to an incipient departure of controlled flight, except that Vmc is below Vso (more pronounced due to less power on the live engine because of density altitude, and therefore Vmc well below Vs) and the aircraft departs into an unrecoverable flat spin.
The PA-30 AFM is quite explicit.
Cttime
OK, I had it wrong. But I do recall that Vmc decreases with gear down.
RobertL18C wrote:
Vmc is certainly not taught as safe
Agreed, but why are we demonstrating Vmc at all in training then? All it does now is to give false sense of security to pilots that a plane is controllable as long as you stay above it. It’s not true.
AdamFrisch wrote:
Vmc speed for certification purposes (and the one published in the POH), is based on the 3-5 degree bank angle into good engine, and if I’m not mistaken, also with gear out and flaps out
It’s the opposite. Gear retracted and flaps in the takeoff position according to FAR 23.
https://www.law.cornell.edu/cfr/text/14/23.149
Ted wrote:
I used S&L to mean straight flight (the FAR definition), when as you point out there is no requirement to maintain altitude. I have edited my post for clarity.
Actually in certification under Pt23, there is no requirement to be able to climb out feathered or unfeathered single engine if below 6000lbs.
RobertL18C wrote:
No requirement for a multi engine with an unfeathered propeller on the inoperative engine to maintain altitude, so Vmc only require ability to maintain control, not altitude.
I used S&L to mean straight flight (the FAR definition), when as you point out there is no requirement to maintain altitude. I have edited my post for clarity.
No requirement for a multi engine with an unfeathered propeller on the inoperative engine to maintain altitude, so Vmc only require ability to maintain control, not altitude.
AdamFrisch wrote:
is based on the 3-5 degree bank angle into good engine
It is 5 degrees and if the manufacture could get away with a greater bank angle they probably would… However Vmc must include an allowance for the engine to fail and the pilot to take corrective action (using normal responsive skills) so that there is no dangerous attitude and there must not be more than 20 degrees of heading change during the process. You are not permitted to change power, or feather during this demonstration.
For example if the engine failure occurs at VMC (in the respective configuration. i.e. take off) the pilot should have sufficient aileron and rudder authority to re-establish Straight flight while not deviating by more than 20 degrees. To hold the aircraft on heading at VMC he may need up to 5 degrees of bank, and probably will need that much if at the same limiting configuration that the test was conducted. I don’t think there is any bank limit set out in re-establishing Straight flight, other than you cannot assume a dangerous attitude to get back to Straight flight.
This will not of course result in maximum climb performance, which is a different topic.
This is true for FAR 23, it gets more complex for bigger types.
In summary the certification standard has both a static (can you maintain Straight flight) and dynamic test (can you regain control)
Vmc is certainly not taught as safe – after hundreds of training fatalities during Vmc demonstrations the FAA introduced the concept of Vsse (single engine safety speed) as the minimum to use during the Vmc demo, giving a safety margin. Instructors who ignore Vsse still suffer fatalities in recent times.
Vmc is certified gear up, flaps in take off configuration, OEI propeller windmilling, max power live engine, up to 5 degrees bank towards live engine. Density altitude, gross weight (Vmc goes up the lighternyou are!), bank, use of controls, all come into play.
The German youtube the real value of Vmc remains one of the best discussions of Vmc.
It is worth remembering, and keeping in back of your mind, that the Vmc speed for certification purposes (and the one published in the POH), is based on the 3-5 degree bank angle into good engine, and if I’m not mistaken, also with gear out and flaps out (which increases stability). Anytime that’s not the case, and it can very well be such a scenario in real life, Vmc occurs much earlier. So you can easily be on the wrong side of this number that has been taught of as “safe”. This is why they push blue line so hard, even when you don’t need to climb.
But there are scenarios where blue line is not possible – during takeoff for example (you don’t want to hug the rwy until that speed as that would cause unnecessary wear and tear) in high performance aircraft (Aerostar was a good example of a high blue line aircraft), nor is your Vmc number going to correspond to the certified one as you’re not 5 degrees into the engine and full flap during takeoff or early climb. So somewhere in-between these numbers is your real takeoff speed – the more, the better, but without wearing on the undercarriage too much. It’s a balance.
Been following Martin for some time. I like his style and he’s very friendly when messaging him!
The MEP videos are nicely done, thanks for linking them here.
That „drill“ sure has a lot of components. Would it be possible to simplify it?
„Boom“
Blueline
Gear Up
Flaps Up
Feather
and then do the rest using a checklist?
