View Full Version : Vmcg/vmca


Duffer2007
28th Jun 2010, 19:49
Can VMCG and VMCA vary with aircraft weight? I read somewhere that the jaa prefer to say it doesn't but the faa say otherwise. Hence I'm a little confused.



Dave Clarke Fife
28th Jun 2010, 21:59
Weight classically makes Vmca worse as it reduces, due to the 5 degree bank limit. The lighter the aircraft, the less sideslip is a result of a 5 deg bank and hence the less restoring yawing moment can be generated to help the rudder.

Weight is nominally neutral for Vmcg.

See also here for further discussion...............


http://www.pprune.org/forums/showthread.php?t=234326&highlight=weight+affect+vmcg

Photon85
28th Jun 2010, 22:17
Hm. Oxford has it the other way around.

As weight increases, a higher AoA is required to keep the airplane in SaL flight, making the p-factor higher and the yawing moment higher for a given speed

Photon85
28th Jun 2010, 22:28
you sure?

Reading out of a JAA ATPL book (principles of flight)

quote: "any weight increase will require a higher angle of attack at a given speed"
paraphrase: higher p-factor, higher yawing moment from live engine = higher Vmca.

then from JAR 25.149

Vmca may not increase 1.13 Vsr with:
- maximum sea level take-off weight

Dave Clarke Fife
28th Jun 2010, 22:37
Vmca is most critical; at MINIMUM weight. This is because the amount of bank into the live engine is limited - by regulation - at 5 degrees.

For a given weight, the sideslip to balance 5 deg bank can be determined; the heavier the aircraft, the more sideslip you need to balance 5 deg bank. Since that sideslip HELPS control the asymetric engine, it follows that the critical case is where the sideslip is minimized, i.e. the lowest weight. Therefore Vmca is highest at the minimum weight.

Photon85
29th Jun 2010, 00:20
Well since the airplane is heavier can you say that the increased sideslip has increased in a higher proportion than the aircraft weight?

Obviously oxford aviation training disagrees with you, and JAR regulations disagrees with you.

To be honest, I'm not sure. In the US my instructor thought me the principle of a higher horizontal component of lift because of a higher weight, but you need more force to turn a heavier aircraft anyway, so can you say that it increases in a larger proportion than the weight itself?

The JAA books method of explaining it, saying a higher AoA is needed for higher weight giving a more pronounced p-factor, and by that a longer arm to the live engine to yaw the airplane adversely, is also a good argument, is it not?

To be honest you just reiterated your previous post without taking any of mine into consideration...

To put it like this:

An airplane with mass A and an airplane with mass B both fly at the same airspeed with the same angle of bank, airplane A is heavier.
Airplane A will obviously have a higher force turning the airplane, meaning that since it is heavier, a higher force is generated as horizontal component of lift when turning it.
However. Since rate of turn and radius of turn is only dependent upon TAS and bank angle, they both turn at the same rate, and they both turn in the same radius...

So, can you say that an airplane that is heavier has a helping hand in regards to the 5 degree bank when in proportion they are the same?

I'm leaning towards the JAA theory here

JAR 25.149 Minimum control speed

(See ACJ 25.149)

(a) In establishing the minimum control speeds required by this paragraph, the method used to simulate critical engine failure must represent the most critical mode of powerplant failure with respect to controllability expected in service.

(b) VMC is the calibrated airspeed, at which, when the critical engine is suddenly made inoperative, it is [ possible to maintain control of the aeroplane with that engine still inoperative, and maintain straight flight with an angle of bank of not more than 5. ]

(c) VMC may not exceed 12 VS with -
(4) The maximum sea-level take-off weight

selfin
29th Jun 2010, 01:21
Photon85, the post from Dave Clarke Fife was written by Mad Flt Scientist on the abovementioned thread (http://www.pprune.org/forums/showthread.php?t=234326&highlight=weight+affect+vmcg).

JAR-25 has been replaced by CS-25. See CS 23.149 and CS 25.149 respectively on minimum control speeds. The reference to MTOM is for the purpose of Vs. The regulations make reference to Vmc being "determined with the most unfavourable weight and centre of gravity position" with almost identical language used in the counterpart FARs.

Dave Clarke Fife
29th Jun 2010, 09:06
Amazing what you can find with the search function isn't it?? ;)

Photon85
29th Jun 2010, 09:41
cs 25.149 says excactly the same thing? MTOW?
it lists a myriad of things in reference to 1.2 Vs, which I deduce to mean the most critical power-plant failure possible...

in any case, other than referring to the regulations, if they don't specify weight according to the Vmc speed, I still have a book based on the JAA learning objectives that specify that more weight is unfavorable in regards to directional control...

Dave Clarke Fife
29th Jun 2010, 14:25
cs 25.149 says excactly the same thing? MTOW?
it lists a myriad of things in reference to 1.2 Vs

Vmc must not exceed 1.2 Vs" is for the tail designer only, not for pilots; the
tail should not be made too small.

Photon85
29th Jun 2010, 14:29
ok, but if we take the regulation out of the picture then, what about the explanation the JAA ATPL books give you?

Dick Whittingham
29th Jun 2010, 17:16
Point 1. If Mad(flt)Scientist says it you'd better believe it. He's a legend

Point 2. Vmca is a single figure, determined under standard conditions and published for certification. Minimum control speed is what you get on the day. With luck it will be lower than Vmca, but you intend to stay above Vmca anyway, don't you?

Point 3. Looking at a well known set of ATPL notes I see "Factors affecting minimum control speed: A lighter TOM reduces the element of aircraft weight balancing the lateral rudder forces.."

Dick

Photon85
29th Jun 2010, 18:22
tbh I don't know what to think.

After googling it and looking at other topics on other forums you are left with two opinions.

1. A lighter weight gives a smaller contribution to horizontal lift hence increasing Vmc

2. A larger weight gives the airplane a larger AoA at the same airspeeds as an airplane with a smaller weight, and therefore increases P-Factor, giving a larger moment arm to the live engine, increasing the yaw, and also blankets the tail in more disturbed air from the wings given the higher AoA, lessening the effect of the rudder/fin

These two statements about AoA are taken directly from the Oxford ATPL Principles of Flight book

I'm not a engineer, nor a professor in Aerodynamics, so I only know what people tell me. However I have this from an ATPL course, and to me, both seem plausible tbh

Dave Clarke Fife
29th Jun 2010, 18:41
Hi Photon,

Forget the mathematical equations (unless you like this sort of thing) and have a peruse through this courtesy of Harry Horlings.


http://www.avioconsult.com/downloads/Effect%20of%20Bank%20Angle%20and%20Weight%20on%20Vmca.pdf

Photon85
29th Jun 2010, 18:54
that's a bit too much on the mathematical formulas for my part (I have enough with the ones in the exams :p), but I'll take it as during testing he figured out less weight was bad, and someone writing this ATPL book did a poo-poo