Vmc...

M

meritflyer

Well-Known Member
Is one of the most babbled about, most misunderstood concepts in aviation.

Cowl flaps raises Vmc. Gear lowers Vmc. Flaps raise and/or lower Vmc. Aft trim lowers Vmc.

Way too much misunderstood water cooler talk around this subject.

Thanks for listening.
 
Landing gear would really be aircraft specific. Your statement assumes the landing gear has a stabilizing effect and also does not consider the landing gear design and its relation to the aerodynamic center of lift.
Landing gear is assumed to be up for Vmc certification testing purposes ONLY. The reason behind this is that as soon as the airplane is airborne it is assumed that the gear will be retracted to reduce drag. If an engine fails at this point a pilot is not going to lower the gear to reduce Vmc, even if its design was stabilizing. You can not jump from certification purposes to "the landing gear lowers Vmc", though many have.
 
I'd like to hear a bit about the Vmc characteristics of a Piaggio.

I've milled it over a bit, but all things involved seem convoluted.
piaggio_p180_avanti_ii_a.jpg
 
Codyjp said:
I'd like to hear a bit about the Vmc characteristics of a Piaggio.

I've milled it over a bit, but all things involved seem convoluted.
piaggio_p180_avanti_ii_a.jpg
Click to expand...

Pot Stirrer. We are talking about real, american airplanes here. :D

I think most folks get bent around the axle about Vmc because the basis of certification is based on lowest airspeed that allows directional contollability, not necessarily best performance and safest operation of the aircraft.
 
And to add to the pot stirration:

Discuss the merit's of two critical engines on aircraft such as:
P-38-1.jpg

P38.jpg
 
jml2992 said:
Pot Stirrer. We are talking about real, american airplanes here. :D

I think most folks get bent around the axle about Vmc because the basis of certification is based on lowest airspeed that allows directional contollability, not necessarily best performance and safest operation of the aircraft.
Click to expand...

Just to clarify, Part 23.149 in the FARs generally achieves a "high" airspeed, not a low speed. A low Vmc would be better for controllability, not worse. The certification process basically isolates as many variables as possible that reduce controllability and therefore, raise Vmc. There are a few exceptions regarding the variables (i.e., out of ground effect) but the vast majority of them produce a higher minimum controllable airspeed.
 
How about:
The nitpicky debate about the effects of flaps, gear, and cowl flaps is irrelevant because the flaps and gear need to be up to provide best performance, and their effect is probably so small that it is practically irrelevant anyway. Cowl flaps need to be open on the good engine to prevent cylinder head meltage (on most piston twins) and closed on the dead engine (if you remember it in the heat of the moment) so really the .025 knots that they might help Vmc is also irrelevant.

Weight and CG are only academically relevant, because the pilot has no control of those once airborne.
 
AviatorGator said:
Just to clarify, Part 23.149 in the FARs generally achieves a "high" airspeed, not a low speed. A low Vmc would be better for controllability, not worse. The certification process basically isolates as many variables as possible that reduce controllability and therefore, raise Vmc. There are a few exceptions regarding the variables (i.e., out of ground effect) but the vast majority of them produce a higher minimum controllable airspeed.
Click to expand...

exactly! all the factors that must be met to during certification to determine Vmc for an aircraft are actually set up to be the least favorable conditions (to create the highest Vmc speed). Except for the maximum 5 degree bank, all of the other conditions would be unlikely to happen all at once. So, the published Vmc speed would more than likely NOT result in loss of directional control in real life (a good thing!)
 
Codyjp said:
I'd like to hear a bit about the Vmc characteristics of a Piaggio.

I've milled it over a bit, but all things involved seem convoluted.
piaggio_p180_avanti_ii_a.jpg
Click to expand...

I wonder if during certification, they had to meet the same criteria for determining Vmc that conventional "puller" twins had to meet?
 
SIUav8er said:
exactly! all the factors that must be met to during certification to determine Vmc for an aircraft are actually set up to be the least favorable conditions (to create the highest Vmc speed). Except for the maximum 5 degree bank, all of the other conditions would be unlikely to happen all at once. So, the published Vmc speed would more than likely NOT result in loss of directional control in real life (a good thing!)
Click to expand...

What is your basis for the first part of your statement? I do not see anything in part 23 that states this, but I could be missing something. Here is the link. See if you can find a reference. Or maybe the FAA has written something elsewhere supporting this statement about certification.
http://ecfr.gpoaccess.gov/cgi/t/tex...&view=text&node=14:1.0.1.3.10.2.62.28&idno=14
 
Blackhawk said:
What is your basis for the first part of your statement? I do not see anything in part 23 that states this, but I could be missing something. Here is the link. See if you can find a reference. Or maybe the FAA has written something elsewhere supporting this statement about certification.
http://ecfr.gpoaccess.gov/cgi/t/tex...&view=text&node=14:1.0.1.3.10.2.62.28&idno=14
Click to expand...

here:

23.149 Minimum control speed.

(a) VMCis the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.
Click to expand...
 
There has to be at least one Piaggio pilot on this board... Lets hear it!

On paper, that plane "just aint right!"

Technically though, all things would be based on the CG location, which should be a bit forward of the engines, which would give the rudder adequate leverage in a SEO scenario.
 
Blackhawk said:
Read what you quoted. Something about critical engine, but nothing about landing gear or other configuration being critical with respect to controllability. Engine only.
Click to expand...

it continues to say:

VMCmust be determined with the most unfavorable weight and center of gravity position and with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with—

(1) Maximum available takeoff power initially on each engine;

(2) The airplane trimmed for takeoff;

(3) Flaps in the takeoff position(s);

(4) Landing gear retracted; and

(5) All propeller controls in the recommended takeoff position throughout.
Click to expand...

While it doesnt state that the flaps, gear, trim, and power have a negative affect, it is true that these conditions which must be met are unfavorable. Vmc would be lower if the aircraft is not configured this way
 
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