VMCA & weight

[TANGO TANGO]

How does weight affect VMCA????

[Mad (Flt) Scientist]

The search function should give many examples of this topic, but the short answer is that VMCA increases at lighter weights. The published VMCA is, however, usually picked as the highest applicable value and if so must be applied to all weights. As a result the light weight takeoff case is usually much more critical with respect to VMCA considerations.

[HazelNuts39]

Tango;

Drawing on my memory, Vmca is affected by weight because 5 degrees of bank is allowed in the demonstration. A Vmca determined at zero bank would not be affected by weight. However, I don't remember the exact mechanism, or in which direction. Perhaps someone else can explain.

As explained by MFS, whether Vmca is scheduled as a function of weight is another matter.

regards,
HN39

[mattpilot]

a google search would of probably given you a much quicker answer - usually the case with questions that can be found in normal textbooks.


Vmca is lower with higher weight because (1) heavier aircraft are more stable, higher momentum resisting yaw/rolling, and (2) heavier aircraft produce more lift on the wings, which means when you bank into the 'good' engine you have a greater component of lift acting against the yawing/rolling force, which reduces the requirement of having to use rudder.

[Mad (Flt) Scientist]

Vmca is lower with higher weight because (1) heavier aircraft are more stable, higher momentum resisting yaw/rolling,

Not relevant. VMC is a static manoeuvre, mainly, and VMCA is almost always determined through a static balance of forces and moments. Inertial effects are therefore usually irrelevant.

and (2) heavier aircraft produce more lift on the wings, which means when you bank into the 'good' engine you have a greater component of lift acting against the yawing/rolling force, which reduces the requirement of having to use rudder.
This is closer to the truth, combined with HN39s point.

To summarize in one place:

VMCA must be demonstrated/determined within a constraint of not using more than 5 degrees of bank. (It invariable turns out that banking into the live engine is the direction which is most favourable).

When banked, there is a sideways component of the aircraft weight, when resolved into the (banked) aircraft axes. In order to balance this weight component, an aerodynamic sideforce is required. This means that there must be some sideslip on the aircraft. The heavier the aircraft, the more the component of weight in absolute terms, the larger the sideforce and, all other things being equal, the larger the sideslip.

Sideslip generates, in addition to sideforce, both rolling and yawing moments. By banking into the live engine, the yawing moment generated is invariably helpful, in that it assists the rudder in generating the yawing moment required to counter the engine-out moment.

So at a given speed, a heavier aircraft gets more rudder "help" by banking, and so needs less rudder to trim. Alternatively, the heavier aircraft can go to a slower speed before running out of rudder.

This means that a heavier aircraft can maintain a heading within the constraints of the VMC requirements to a lower speed. hence heavier aircraft "have a lower VMCA".

[john_tullamarine]

.. some extra comments to hammer a few points home ..

(a) Vmca/Vmc in the AFM is the certification figure - the "real world" figure on the day for a given configuration should be a little lower

(b) Inertial effects are therefore usually irrelevant.

Vmca generally is worked up progressively to determine the static value (a bit like the typical pilot Vmca demonstration thing). However, once the TP folk have defined what they reckon is the static value, the dynamic value is determined with an engine failure. This may or may not cause the first value to be revised to end up with the book figure

(c) VMCA must be demonstrated/determined within a constraint of not using more than 5 degrees of bank.

Unless the book says something different, presume that the value is with 5° bank into the live engine(s). More importantly the variation of Vmca with variation of bank is SIGNIFICANT - if you are near to the book figure and playing engine failures, make sure that you crank in the bank .. lest you find the Vmca for your actual bank to be something faster than your actual speed. The 5° limit is to prevent folk cheating and ending up with an unrealistically low Vmca by using excessive bank angles.

As I recall, for a well-known many-engined bomber, the Vmca goes up something like 40-45 kt if the aircraft is banked the "wrong" way ... (or was it 30-35 kt ? ... either way, a very significant increase),

Also, the 5° figure is linked with Vmca handling, not OEI climb performance. Generally the optimum bank for OEI climb is around 2-3° and the performance wings level is much the same as that for 5° into the live engine(s). As a consequence, unless you really need that last bit of performance, the usual OEI climb technique is wings level.

The oft cited "requirement" for 5° bank during OEI climb (which one sees in every second GA operations manual) is tied up with the Industry's wealth of OWT rather than any objective engineering validity.

[Escape Path]

So let's see if I got this one:

My understanding of what MFS said is that since a heavier aircraft has to produce more lift for obvious reasons, once you are OEI by banking into the live engine the horizontal component of lift (being greater than the one on a lighter aircraft) actually helps counteracting the yaw caused by the loss of thrust in one engine thus resulting in less rudder input required and causing the Vmca to decrease. Is that basically right?

Thanks in advance :8

[john_tullamarine]

Let's go back a step or two and think about what is going on ...

(a) forget about engine failures for the moment.

Imagine that you are flying along on a nice summer's afternoon minding your own business and enjoying the view.

What happens if you set up a roll angle by using some aileron input ? You start slipping into the low wing and set up a yawing moment in that direction. If you don't do anything to stop it happening .. you will start turning as well.

Tuck that away in the mind for later on ...

(b) now, let's think about an engine failure.

You are flying along still minding your own business and enjoying the view and one engine goes quiet on you.

First off you note the yawing moment's causing a turn (important to keep in mind that the aeroplane wants to turn toward the failed engine) .. so you apply some rudder input toward the operating engine(s) to re-acquire the original heading and increase the throttle setting somewhat to minimise the IAS loss. With any sort of good fortune and a bit of juggling of controls and throttles, you hope to settle down to a reduced IAS, constant heading flightpath.

For the exercise, let's consider that you are maintaining wings level.

Now, if you are at a higher IAS, you only need comparatively little rudder deflection.

However, at a lower IAS you needed comparatively more rudder deflection. Indeed, if you are in a situation where the IAS is reducing due to the thrust capability's being a little on the low side, then, progressively, you will be increasing the rudder input to maintain heading.

[We are playing with something approximating the static Vmca exercise here - a bit like the usual endorsement demonstration].

If you end up with full thrust on the operating engine(s) but the IAS continues to reduce, eventually you will get to the stage where you no longer can maintain the heading.

Now, what can you do ?

(i) descend for IAS control - fine. (This is much the same as reducing thrust .. but that means you are going to descend as well so the situation is much the same)

(ii) continue slowing until you lose control - not a good idea so let's not go there

(iii) re-read (a) and play with bank angle.

- if we bank into the dead engine we make things worse as the bank causes a yawing moment which wants to see the aircraft turning into the dead engine (which was the original problem just made worse) - not a good idea so let's cancel that idea

- if we bank into the operating engine(s) the the yawing moment associated with the bank wants to see the aircraft turn toward the operating engine(s) which means that we can reduce the rudder input a little or tolerate a further slight reduction in IAS. Is this not a useful thing to do ?

The inclined lift vector is important in setting up this yawing moment via lateral acceleration and sideslip.

When we come to looking at gross weight (in level flight) the heavier aircraft (for the same bank angle) will generate a slightly higher lateral accelerating force and a bit more sideslip (for the same IAS) which normally should see a slight increase in the yawing moment which, in turn, should see a benefit for the rudder requirement.

Now it becomes pretty obvious (?) that more bank angle results in more lateral acceleration and sideslip and there exists the potential to reduce the Vmc figure even more if we use a larger bank angle. However, too much sideslip is a good way to visit spins and such so we need to keep a happy balance - hence a limit on how much bank may be used for certification.

Which, I guess, is a long-winded way of saying yes ...

[Escape Path]

Which, I guess, is a long-winded way of saying yes ...

But you have massively helped to the fact that I'm going to bed tonight a bit less ignorant about the subject thanks to that rightly worded and straight forward explanation.

So for that, I thank you JT :D

[FerrypilotDK]

In The Real world, climbing out of a balked landing and with a dead engine, you are not near the Vmc, but the best rate or angle of climb. Why then do people teach the 5 degree bank angle under these circumstances, as the rate of climb is less? ...and at this point, the climb is the point, I would think?

After all, the certification limitation of 5 degrees bank, is also predicated on the rear-most C of G as well, which is not usually the real-life situation either. Seems this limitation in certification has somehow "bled over" into pilot technique and training, but is in fact inappropriate aerodynamically.

...or am I missing something?

[mattpilot]

When i taught multi-eng students i always told them 5* is the max allowable for certification standards (as explained above), but i always added that the best bank angle that gives optimum performance varies with each aircraft (if i recall correctly, the seminole is 2.8* or something close - been a while, its in the AFM). In practice, it matters little if you fly wings level or try to maintain that ~3* that seems best for most twins, but if you fly 5* or the 0* on climbout, you'll be suffering the same performance penalty as the optimal * is in the middle.

Another argument for not neglecting that bank angle in favor of an 'easier wings-level climbout' is to maintain positive control of the aircraft. I had one particular student who constantly would relieve control pressure when he looked down to check the checklist or do something. Aircraft would begin to roll on him everytime.

[john_tullamarine]

Another argument for not neglecting that bank angle

Perhaps your student might try the usual technique of trimming out the loads rather than trying to fly an untrimmed machine ?

[mattpilot]

Perhaps he should... or perhaps he did? Or.. perhaps you don't know what he did? Perhaps that wasn't the point?

People have different reactions to stressful situations - especially when not anticipated. My point was that i'd rather have the student fly the plane first before he thinks about to much else. In response to "Why then do people teach the 5 degree bank angle under these circumstances, as the rate of climb is less? ...and at this point, the climb is the point, I would think?", i'm merely saying that i hammer that 'positive bank angle' towards the good engine into my students so they don't forget to fly the plane. Saying 'don't worry about it, wings level is just as fine' might lead to some issues. That said, i only had one student that had these issues.

[john_tullamarine]

I think we are in agreement.

There is no general problem with a one-size-fits-all initial action response with a followup refinement once the initial problems are brought under control. Depending on the particular Type's margin between Vmca and the target speed, it may be wise to be conservative and crank in some bank in case one inadvertantly finds oneself back towards Vmca.

Likewise, many folk don't trim OEI initially (me included - other than for a generic rudder trim input) with the intention of doing so once things have been sorted out. The penalty is that the stick and rudder workload increases if you want to do other things at the same time.