What decreases VMC

[C-5 MEM]

Here is a question I could not find. What would decrease VMC speed?

 

[deceptibum]

higher altitude........less air pressure up there means less horsepower output of the working engine means less yaw tendency.

 

[C-5 MEM]

Ok Thanks for the reply. It has been a very long time since I had to get into the books for an interview.

 

[bobbysamd]

Decreasing Vmc

Couple of things. Using zero sideslip lowers Vmc; in other words, raising the wing on the dead engine. Think "raise the dead." Slipping raises Vmc.

Gear extended. Sort of like the keel on a boat. Increases stability.

Feathered prop on the "bad" (inop) engine. Windmilling prop increases drag and Vmc.

Go back to your factors of Vmc and they should suggest the answer to your question.

Hope that helped a little. Just like the above, I haven't taught multi in nine years. Miss it, too.

 

[bigD]

A hotter day, higher weight, and more forward CG will do it too.

 

[172driver]

1. Gear extended
2. Flaps down
3. Higher density altitude
4. More bank into operative engine
5. Lower power on operative engine
6. Being in ground effect
7. Higher weight
8. Forward CG
9. Lateral CG toward operative engine
10. Inop prop feathered

Remember to never associate Vmc factors with performance. Vmc is only about rudder effectiveness and thus heading control. As you can see...if the airplane is in the above configuration, you will be able to maintain heading but will be falling to the ground like a rock! When learning the factors you will want to know how the airplane is configured for certification, rather than what makes Vmc the lowest.

Oh...just read your profile...you probably already know the basics. Hope the list helps.

 

[gnx99]

I've been looking for a good take on what structural ICING does to Vmc. Anyone know?

 

[172driver]

Depends what it does to the CG, I would guess. If you have ice on the vertical/horizontal stabilizer...baaad. Nose...goood. Wings=more rearward CG...baaad. Although if you've lost an engine in icing you better keep your speed up anyway.

 

[flyboy]

If you take out bank, VMC actually increases. Try doing a VMC demo without putting your 5 degrees of bank and it will prove this true. You'll VMC 15 knots faster. Also, remember that the 5 degrees of bank is just part of the FAA regulation, you can put more than that and lower VMC even more.

 

[Hovernut]

Regs

Remember, you'll find what the certificated configuration is in 23.149

 

[Nocturnal1]

Vmc

I used to ask this to new PIC's I was instructing:
Today's not your day and you're going to lose an engine on takeoff. Which situation would you choose and why? Lightweight airplane and cold day or heavyweight airplane an hot day?

Believe it or not most "newbies" chose lightweight/cold because that would give them more thrusties for the engine-out climb. Not bad rationale, however, they didn't take into account Vmc. More importantly what more thrusties does to Vmc.

This was in a Part 25 airplane.

 

[172driver]

If it's after liftoff, at climb speed, without usable rwy, I would definitely choose light and cool. On the runway or below Vmc would be another story.

 

[Checks]

Maybe I am an idiot but I would also pick cold and light. You shouldnt be rotating before VMC anyways.

I use this acronym to remember the factors involved in determing VMC-SWAT-MUM

S-Standard day at Sea Level
W-Windmilling Prop
A-Aft CG
T-Takeoff configuration
M-Max Pwr on the operative engine
U-Up to 5 degrees bank into the operative engine
M-Max gross weight

Not included is OGE and cowl flaps in the takeoff position(usually open)

 

[bigD]

Hmmm....I'm surprised Avbug hasn't poked his head in here yet. Usually anytime anyone asks a Vmc question - his response is something like, "Vmc doesn't change! It's a number set by the manufacturer of the aircraft. Now, you may lose rudder authority at airspeeds higher or lower than Vmc, but the actual value of Vmc doesn't change at all."

Of course, don't get him started about what TBO really means!

Juuuuust kidding Avbug!

 

[Timebuilder]

Ah, you see? You have benefitted from Avbug's wisdom, already.

I vote for cool and light, too. It gives me better options, including whatever single engine climb I may need for obstacle avoidance.

Since published Vmc is something of a "worst case scenario", the actual speed where control is lost will likely be lower. I would only experiment with the region below redline with a brick s***house directly in my path, and immediately pitch back to blueline, if possible. Just as in the Vmc demo, at the moment you sense a loss of directional control, you MUST pull the power to avoid the rollover. You may survive if you land cockpit-up. You won't if you land cockpit-down.

Summary: Land if possible, continue takeoff if not possible, and know what performance may be conservatively expected before you taxi onto the runway. You might have to wait until later in the day to safely depart in a light twin.

 

[cvsfly]

When you you have fininished ME training and ME instructing you probably will never do VMC demos again. (Of course we are always training, right?) Hopefully you will never be at a speed even close to VMC if you ever loose an engine unless you are already on the ground.

 

[Hovernut]

Weight

Checks,

The weight isn't necissarilly max gross. 23.149 says "weight most unfavorable."

Typically this will be lightest! Here's why. If you remember your H.S. physics, you'll be familiar with the equation F=M*A. Force equals mass times acceleration. Rearranging, F/M=A, so for the same force (takeoff thrust), a reduction in mass yields greater acceleration....VMC yaw/roll tendancies! You can displace a light a/c easier and faster than a heavy a/c. Hold a brick in one hand and a tissue in the other, blow on both and see which one displaces easier!

Regards

 

[172driver]

Aircraft certified before 8/8/93 are allowed to use max gross weight for certification.

The new reg begs the question...what is the most unfavorable weight? Enough gas to get around the pattern and a 90 pounder flying? Doesn't seem very standardized.

 

[Timebuilder]

Acording to what I have seen, the interpretation is max takeoff weight, "or less if required for the demonstration", before 8/8/93.

 

[172driver]

That is the old reg. The newer one definitely specifies 'most unfavorable weight.'