vmca vmcg

[Torstennnn]

preparing the jar fcl exam i came across several answers that confused me!
can you explain how vmcg and vmca behave regarding changes in temperature an pressure-altitude?

[FROMAIBTOBOEING]

As a rule of thumb, you can say that Vmca and Vmcg are more restrictive with a higher DA (density altitude). So the higher the altitude, or the higher the temperature, the higher the minimum control speeds will be.

In plain language, you could say that with a higher elevation or temperature, air is thiner, so a smaller mass of air is going by the rudder, and this makes the rudder less effective. To have the same mass of air passing by the rudder, you need to increase speed.

Hope I was helpful and good luck with your exams.

[Torstennnn]

makes sense,
thanxs

[EnzoC]

Sorry FROMAIBTOBOEING, I have to disagree ;-)

VMCG and VMCA are the minimum speeds on ground/in the air, where the airplane can be controlled with the most critical (in terms of yaw-momentum) engine inoperative by aerodynamic means (i.e. rudder) only.
Thus, VMCG and VMCA behave similar to the installed takeoff thrust of the engine:
with increasing OAT, VMCG/VMCA get lower (thrust gets lower)
with increasing PA, VMCG/VMCA get lower (thrust gets lower)
However, this is not linear. Typcial jet engines are flat rated up to a certain "break point" temperature (e.g. ISA+15-17) and above that they are full rated, with the thrust rapidly decreasing over temperature. With increasing PA this curve shape is moved towards less thrust.
Typical example would be:
oft, VMCG 126kts up to 30degC, then falling to 112kt at 54degC
10000ft, VMCG 111kt up to 10degC, then falling to 100kt at 35degC

There is an additional restriction for VMCA concerning VStall (VMCA <= 1.2xVStall) but I'm not sure about the exact number.

On some airplanes VMCG might also change with bleed settings (A/I and A/C) and T/O weight.

[Wizofoz]

I have to agree with EnzoC on this one.

FROMAIB, VMCA and VMCG are both Indicated Airspeeds(IAS). When you talk about air being thinner, you are describing an increase in Density Altitude, which occurs when either temperature is increased or ambient air pressure is reduced (or both!!). This will indeed mean less control effectiveness at a particular True Airspeed.

IAS, however, is a measure of Dynamic Pressure, and naturally corrects for changes in DA. Therefore at a given IAS, the same amount of rudder authority is available.

If the asymmetric thrust is constant (say, a flat rated turbo-prop or turbo-normalized piston) VMCA and G should remain the same (The only variable here might be nose-wheel steering for VMCG, and it is a certification consideration as to whether it is or isn't counted).

If the thrust from the critical engine reduces, so does the necessary rudder authority, and thus VMCs will be less. This would be the case for most Jets and normally aspirated pistons at higher DAs.

CG also plays a part, as a rearward CG reduces rudder authority.

[FROMAIBTOBOEING]

I have to rewrite my answer. It's true that thrust is a basic part of the equation that I have left out.

As we know, thrust decreases as Temp and/or Altitude increase. The lower the thrust, the lower is the momentum created by the sudden engine failure. So we need less rudder efficacy to control the aircraft. But is also true that the rudder will need more air speed to control the aircraft's momentum.

So what we need to know know is which of the 2 (thrust or rudder) will have a bigger influence, as DA (Density Altitude) increases.

In Jet Turbine aircraft minimum control speed DECREASE with altitude, this means that in jets, with increasing altitude, we lose more thrust capability than rudder aerodynamical capability. I remeber that in Airbus FBW aircraft (320/330 and 340) you could say that Vmca decreased 1 Kts per each 1000ft increase in Airfield's elevation. The variation in Vmcg in Airbus FBW planes is very similar, but at higher altitudes it decreases almost 2 Kts per 1000ft.

I'm trying to find out how does this thing work with Temp.

Hope this clears my previous mistake, think I was talking more about V1 (that is what I have in mind when I talk about Vmcg) rather than Vmc.

Thanks for watching over my shoulder EnzoC!!!

[FROMAIBTOBOEING]

Wizofoz, I think that JAR state that in the determination of Vmca and Vmcg you have to consider that this are Calibrated Airspeeds, uisng only primary aerodynamic controls (no NWS for Vmcg) and always with the most unfavourable CG.

[Torstennnn]

so let me see if I understood correctly:
vmca and vmcg are unaffected by environment when seen as IAS
but in TAS vmca and vmcg are highest @ cold and low take offs
?!?!?

[chornedsnorkack]

Let´s consider it this way:

So long as the engine thrust is flat rated at a fixed force

the rudder moment is independent of density altitude at any fixed IAS

thus the control speeds in terms of IAS are independent of density altitude, while control speeds in terms of TAS increase with density altitude.

When the engine thrust is not flat rated and the force derates with density altitude

the control speeds in terms of IAS decrease with increase of density altitude
while the behaviour of control speeds in terms of TAS depends on how fast thrust derates.

Does anyone know how the TAS control speeds depend on density altitude for non-flat-rated engines?

[EnzoC]

Torstennnn,

get a sheet of paper and paint my example as a graph.
Put OAT on x-axis and speed (IAS) on y-axis.
Then draw in my example above (one curve for 0ft and one for 10000ft).
The speeds in my example were in kt IAS.
Each curve consists of a flat part at lower temperatures and then a falling part at higher temperatures.
The edge denotes break-point temperature.
This gives you a rough example of VMCG on a jet powered airplane.
VMCA looks similar to VMCG, with the speeds a bit higher.
I hope that helps a bit

[Wizofoz]

Wizofoz, I think that JAR state that in the determination of Vmca and Vmcg you have to consider that this are Calibrated Airspeeds

Yes, Calibrated airspeed being Indicated airspeed corrected for Instrument error, NOT changes in Density altitude. You are, therefore, still incorrect in stating:-

But is also true that the rudder will need more air speed to control the aircraft's momentum.

It will need more TRUE airspeed, but the same Calibrated (and, therefore, for all practical purposes, Indicated Airspeed).

You don't seem to grasp the difference between TAS and CAS/IAS, the last being what the pilot needs to know in order to fly the aircraft.

Momentum is also an incorrect term here. The rudder is needed to counteract Thrust, which is a force, by producing an equal force in the opposite direction.

[chornedsnorkack]

Momentum is also an incorrect term here. The rudder is needed to counteract Thrust, which is a force, by producing an equal force in the opposite direction.

Force is certainly wrong.

What would you call the product of a force and its applicable lever arm?

Thrust has its lever arm from the relevant engine to airplane centreline. But what is the proper lever arm for the rudder aerodynamic force? Where is the pivot point - at main landing gear (in contact with runway and independent of CoG location) or the CoG location of the airplane (so that lever arm is shorter with rear CoG)?

[Wizofoz]

Chorn,

A force applied through an arm is Torque. In flight the rudder would work about the Centre of Gravity. On the ground it would be a little more complicated to measure, as the anti-rotational force of the gear and the C of G would all effect the resultent required force.

Thrust IS a force, and the Rudder needs to produce a force in order to counteract it, but you are right in as far as, seeing as they would work through different lenght arms, they could be different values and produce the same equal and opposite Torque.

[bArt2]

CG also plays a part, as a rearward CG reduces rudder authority.

Correct, but as far as I know, VMCA, VMCG is determined at the most unfavourable CG location (Aft limit)

I think that JAR state that in the determination of Vmca and Vmcg you have to consider that this are Calibrated Airspeeds

CAS is IAS corrected for instrument error (internal inaccuracy) and position error (measurement of static pressure) so as said before density plays no part here.

[EnzoC]

wizofoz,

of course it's a "torque" I was talking about. "momentum" is wrong - my mistake, sorry for confusion.
In german we use "Moment" as a short form of "Drehmoment" (=Torque).
So I was trapped in the idea the word might be the same in english.

Same thing with Germans using the word "football" when they mean "soccer" instead ;-)

[chornedsnorkack]

as the anti-rotational force of the gear and the C of G would all effect the resultent required force.

Does the gear exert anti-rotational force?

On a tricycle, the main gear is behind the CoG (or else the plane would fall on tail) and therefore the rudder is closer to main gear than CoG. This means that the main gear ought to exert a torque tending to rotate the plane. In other words, for exact same rudder sideforce, when the main gear is off the ground the plane would tend to slip sidewards and resist yawing, while a contact between the main gear and ground would resist sidewards slipping and cause the plane to yaw instead of slipping.

[Mad (Flt) Scientist]

"Moment" is a perfectly acceptable English term for the product of a force and a distance; the three non-force components used in aircraft six degree of freedom modelling are normally called "pitching moment", "rolling moment" and "yawing moment" - and it's the balance of the yawing moment due to the thrust asymmetry with the aerodynamic yawing moment arising mainly - but NOT solely - from the rudder that is important in most VMC considerations.

Incidentally, the attempts to compare VMCA and VMCG which I've seen earlier in this thread are somewhat misplaced. There is NO specific relationship between the two, as the conditions for the determination of the two speeds are very different aerodynamically, and either VMCA or VMCG may be higher or lower depending on the specifics of the aircraft - and even of the configuration.

[Daniel_11000]

1) Dynamic Vmc : the a/c is with flaps TO and gear UP, dynamic engine cut, not more than 15° heading change without exceeding 180 lbs rudder pedal force. This test is more critical at low weights (less inertia, so more swerve). and AFT CG (less vertical stabilizer restoring moment).Pilot should not apply corrective action for 1 second. In this condition, also the 180 lbs limit is normally not a factor.
2) Static Vmc : same aircraft contitions as 1) but , with one engine cut, the a/c is slowed with max 5° bank toward the good engine, until rudder pedal force reach 180 lbs(or lat. control force reaches its limit, which is seldom the case).Normally in this situation the most critical CG position is FWD.

All those tests must be verified for all the approved take-off flap configurations.
The published Vmc is the higher off all the above speeds.

For jet a/c, the Vmc starts to decrease at the OAT/Alt combination where rated EPR cannot be maintained, but this is not true for turboprops, also if flat rated to whatever altitude, because if it is true that the rated power can be maintained up to, say, 20.000 ft, propeller thrust always decreases with increasing TAS. This means that, for the same CAS , TAS decreases with altitude, so does the thrust. For turboprops the Vmc is so tested at different altitudes and then linearly extrapolated at sea leve/ISA conditions (in order to avoid extremely risky tests at low altitudes

I lived five of my best professional years doing engineering flight test, and I am pretty sure of the above.

[Mad (Flt) Scientist]

I think you'll find that the critical cg for the in-air demonstrations is aft cg.

And I've never heard of a one second time delay in the dynamic VMCA/L manoeuvres, nor does AC25-7A recommend or require such a thing. It would be inconsistent with the Vmcg demonstration requirements, where that is also a dynamic manoeuvre and a one second delay would be horrendous.

Also be careful about attemting to define specific test conditions or requirement, such as the rudder force value. Those are subject to variation with the specific regs in force at any time ...

[FE Hoppy]

Daniel

667 N of force (150 lbf) in CS25

no mention of a second either:

1 The determination of the minimum control speed, VMC, and the variation of VMC with available
thrust, may be made primarily by means of ‘static’ testing, in which the speed of the aeroplane is
slowly reduced, with the thrust asymmetry already established, until the speed is reached at which
straight flight can no longer be maintained. A small number of ‘dynamic’ tests, in which sudden failure
of the critical engine is simulated, should be made in order to check that the VMCs determined by the
static method are valid.
2 When minimum control speed data are expanded for the determination of minimum control
speeds (including VMC, VMCG and VMCL) for all ambient conditions, these speeds should be based on
the maximum values of thrust which can reasonably be expected from a production engine in service.
The minimum control speeds should not be based on specification thrust, since this thrust represents
the minimum thrust as guaranteed by the manufacturer, and the resulting speeds would be
unconservative for most cases.