Vmc & 23.149

[BlackHawk11]

Hey everyone,

I know that there is a reason for this, and maybe I'm just having some brain spasms trying to figure out the reason, but why can Vmc not exceed 1.2 times Vs1? Obviously, you do not want to stall first before reaching Vmc, as you can enter a spin. What effect does Vmc have if it is faster? (provided that Vxse and Vyse are still faster)

 

[flyboy6585]

Stalling prior to Vmc is not an issue. Your average 172 stalls prior to Vmc...that is why you can control it. If you have 1 dead engine and 1 engine at idle your Vmc is going to be way lower than Vs1 and if you stalled the aircraft you would have sufficient rudder authority to maintain right side up (assuming stalling your specific aircraft doesn't lead to some other issue like some jets).

A fighter jet doesn't have a published Vmc even though the asymmetrical thrust is noticeable and requires some rudder/aileron input to neutralize you have such a big and powerful rudder that you stall before the asymmetrical thrust causes a departure from controlled flight.

That is why the biggest factor in a Vmc recovery is to reduce the operating engine...you have eliminated most of the asymmetrical thrust and therefore have reduced Vmc to some number below your current speed.

So, to sum up, a stall speed greater than Vmc is not a bad thing! (EDIT to clarify: unless your stall speed and Vmc speed are the same or nearly so, then you could get a Vmc roll as you are stalling which would lead to a potential spin)

 

[ahw01]

Stall before Vmc is fine, we are all taught how to recover from a stall.

Vmc is all about directional control (not performance) and the conditions part 23 sets mean the manufacturers do everything they can to lower Vmc under that 1.2 Vs1 threshold. If they cant they need a bigger rudder (or smaller engines, or a change in CG envelope, which are less likely).

Vmc is often 15 knots higher than placarded, as every degree of bank into the good engine reduces Vmc 3 knots (as less rudder is required with the aileron helping) - how often are you 5 degrees banked in when that engine fails? (the test pilots are).

Just some food for thought, as you may find in real life, you could go full power gear up and flaps up and CAUSE Vmc, as you have given the aircraft more power than it can maintain directional control with - several accidents appear to have occurred with a loss of engine on final, surely we should just maintain the glide with partial power on the good engine as needed? (always leave yourself an out)

http://www.cfidarren.com/p8740-25.pdf

 

[TwoTwoLeft]

You lose directional control at a higher airspeed that's all.

The Vmc roll is often confused with the "snap" that occurs due to a stall/spin condition caused by the pilot stalling the wing with the yaw/roll coupling forces present created by the operating engine.

A stall before reaching Vmc is really not a good thing. It's actually the LAST thing you want to have happen.

A Vmc roll is simply a loss of directional control due a lack of rudder authority to counteract the yaw and rolling moments from the running engine. The amount of rudder authority is directly proportional to your airspeed and CG location. But obviously most direct fix is going to be with airspeed.

With a Vmc roll you have a yaw/roll coupling but the wings are still flying. Simply pitching to nose down while simolutaiosly reducing power on the good engine will start the recovery. Hopefully you have enough altitude below you.

From a energy management standpoint, the most efficient way to recover from a Vmc roll is to leave the power on and just pitch the nose down. Directional control will be sacrificed, but by pitching the nose down you will reduce P-Factor and airspeed will be gained faster with less altitude loss due to the thrust of the good engine.

If you hit Vmc with the wings stalled you're stalling the airplane with a Yaw/Roll coupling and setting everything up for a spin with decaying rudder authority. Not a good place to be.

A simple Vmc recovery uses a hell of a lot less altitude than a stall or spin recovery in the same engine out condition.