Vmca-Vmcg

[alexban]

HY
Is it possible to have Vmcg bigger than Vmca? Why?
Any particular plane?
Thks Alex

[747FOCAL]

I think so, but there are others here that can explain that better than me. You also might search around here for it. I know it has been discussed before.:)

[catchup]

There is a difference between VmcG (ground) and VmcA (air) - merely because, on the runway (ground), airplane cannot be banked to assist in directional control... Therefore, VmcG is ALWAYS higher KIAS speed than VmcA is... - VmcG is always the LOWEST possible V1 speed.

[Mad (Flt) Scientist]

It IS possible to have Vmcg either higher or lower than Vmca.

Despite the close similarity in the names they are DIFFERENT requirements, undertaken in very different "flight" regimes (one being on the ground!)

I have worked on aircraft where it was both ways around, so I'm pretty confident it can be either way.

[Mad (Flt) Scientist]

To add a few more details and also address catchup's post....

... Therefore, VmcG is ALWAYS higher KIAS speed than VmcA is... - VmcG is always the LOWEST possible V1 speed.

Sorry, I know of aircraft where Vmcg is higher than Vmca, for quite legitimate reasons, which I'll address below. And V1 may be being determined by facors other than Vmcg, even at the lightest weights, so it's a little unsafe to say that Vmcg is the lowest possible V1; you can't necessarily derive Vmcg from a knowledge of V1.

The reason why Vmcg may be higher or lower than Vmca arises from a number of considerations, both theoretical and practical. To address the practical:

Both Vmcg and Vmca are artifical, declared certification values for an aircraft. There is no requirement for the manufacturer to determine absolute truth; rather, it must be demonstrated (usually by test, plus supporting analysis) that a given speed meets the requirements of the regs. But it need not be the lowest speed which would meet the requirements. If I go out and demonstrate that I can meet the centreline deviation requirement for Vmcg at 100kts, and I am confident that I'll never need to use any lower value to achieve the performance I need for an economic product, I may never go trying to squeeze out the last two or three knots of Vmcg. Especially since it's an inherently risky test.
Similarly, Vmca is also a risky test and achieving the performance-driven target may well be enough. The practical upshot of that is that the declared Vmca and Vmcg may not represent the theoretical values.

As to the theory, because the two speeds are defined with different manoeuvres, it is possible that different factors may prove to be limiting the capability of the aircraft. This may be seen more easily by considering a more unusual form of configuration than is usually the case for a FAR25 transport.

If one were to have an aircraft with significant "powered lift" - either by a jet exhausting across the wing (as the Antonov short field twin jet did, I think) or, more usually, a turboprop type - then when an engine is failed in addition to the "normal" directional control problem there is a significant lateral control requirement to counteract the large asymmetric lift now present. If the roll controls on the aircraft are relatively weak, while the directional control (rudder) is powerful, one may find that maintaining adequate bank angle control actually limits the in-air minimum control speeds. However, the landing gear will usually absorb any rolling tendency on the ground and so the directional control is the dominant requirement for Vmcg.

In that case one would expect the Vmca speeds to be higher than the Vmcg speeds.

Essentially, Vmcg is pretty much a single axis control balance between rudder and engine-induced yaw. Vmca is a more complex problem involving the roll axis. Therefore scope exists, by changing the relative power of roll and yaw controls, to vary the relationship between Vmca and Vmcg.

To identify the specific aircraft:

Bombardier's Global Express has a slightly higher Vmca than Vmcg. The CRJ-700 has a lower Vmca than Vmcg. (In both cases, the differences in the speeds is small).

The difference may be partially attributed to inherent variation in the practical test results, and partly to differences in relative control powers. The geometry of the two planes is not that different, so it can be seen how sensitive this Vmca/Vmcg relation can be.

[Alex Whittingham]

Here's another. The L1011-1 has a lower Vmca than Vmcg, usually by 3 or 4 kt.

[mutt]

Mad (Flt) Scientist,

Would you care to give us your inputs regarding the validity of flight testing VMCG with zero cross wind. Do you think that this is practical?

From the CRJ700 AFM, the VMCG and VMCA for Flaps 08 are 119 KIAS. For Flaps 20 the VMCG is 119 KIAS while the VMCA is 111 KIAS. I'm surprised that these are fixed values rather than values based on temp vs alt!


Mutt.

[Mad (Flt) Scientist]

Well, of course certain people in the UK aren't too happy with the harmonised rule removing the old adverse Xwind approach. :)

Personally, I have no great dispute with it; I'm not aware that non-CAA aircraft suffer significantly more runway excursions, so regardless of the theoretical merits of the CAA method, the practical effect doesn't seem to have been to increase safety.

Now one might argue that using the historical data may be less valid now, when the test techniques are more sophisicated, and we're probably shaving more margin off in other places as we "learn more". There's a world of difference between DGPS and a guy dropping a bag of chalk to measure deviation, after all. :) But it's all we have to go on.

There's other compromises in the Vmcg tests, too, other than Xwind. Wet runway would be another issue; nosewheel steering off removes the steering, but the mainwheels are still working on a dry runway.

I think the way we do it now is probably as defensible as any other; you could argue, for instance, that in a Xwind landing the pilot may be more active/aggressive in heading control, and perhaps faster to react to the initial heading swing; that makes a huge difference, of course, to the deviation.

In fact, tests in a (mild) Xwind might even be artificially improved; if it's a bit gusty too, there's a risk that the pilot is applying a input in the "right" direction when the engine is failed, and your deviation might be optimistically low. At least low wind conditions are going to be consistent (as much as Vmcg ever can be!)

-----

Regarding Vmca, there are issues with those tests too; it's common to test at lightest practical weight, as that is the classical limiting case. But I believe I saw a paper from Airbus some years ago that suggested that alpha effects on lat-dir aerodynamics could be important (among other concerns), which might make a heavier weight more critical for the same CAS.

-----

And in the case of both tests, the way the industry approaches the thrust to use for test is a bit odd too; the guidance material isn't exactly logical when compared to the regs. It makes ASSUMPTIONS about the design when stating the test method, which can lead to contradictions between the intent of the regs and the method in the guidance. Much debates about whether it's valid to follow the letter of the guidance, even if it doesn't (strictly) meet the reg, since the guidance is stated to be a means to comply with the reg......

Regarding the CRJ VMC's. I don't think any Canadair/Bombardier products use alt/temp scheduling; it's only an option, after all. Why don't we? Well, on the older aircraft Vmca is declared as not limiting, so there's no real point in doing the extra extrapolation and/or testing. I think the merits for doing so were debated on the 700, but it must have been of limited perf value, because as you say it's a declared single value.

[catchup]

Yes, you are right. I have to correct myself. It's possible that VMCA >VMCG.

Concerning V1, V1 must be at least equal to or higher of VMCG or VMCA, whichever is greater.

regards

[john_tullamarine]

Some observations ..

(a) (catchup) - you might like to revisit your contention that V1 ties in with Vmca - refer FAR 25.107 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=74594d24e0a3033398c1038f7a2a06ee&rgn=div8&view=text&node=14:1.0.1.3.10.2.155.11&idno=14) for details

(b) the sensible way to test Vmcg is dead calm for repeatability. The Brit strategy to reduce Vmcg to 7kt crosswind conditions is laudable, but increases the certification workload, cost and difficulty for little, or no, realworld advantage ?

(c) as is regularly discussed in this forum, however, there is a quite reasonable need for crew to have an idea of the crosswind effect on Vmcg so that sensible risk minimisation strategies can be put into place for very low weight takeoffs in aircraft which are Vmcg-limited.

[Milt]

john_tullamarine

In my day as a military/some civil TP the conditions to be observed in determining both Vmcs included a delay of 2 seconds before taking corrective action following an engine failure. This often was enough to result in an exciting ride as things could be quite pear shaped after 2 seconds. The time delay was supposed to cater for the pilot in the system having the slowest reaction time.

Is the 2 seconds still part of the requirement in determining Vmca/g and if not, then why not?

[Alex Whittingham]

Something I always wondered, is rudder fine nosewheel steering disconnected when testing for Vmcg?

[Mad (Flt) Scientist]

re 2 seconds delay:

25.149 (at the amendment to hand) makes no reference to time delays in discussing the VMC determination. AC25-7A also appears not to require any pilot delays for this. As far as I'm aware, our testing doesn't include a deliberate pilot delay time. A 2 second delay on Vmc would be ... interesting. Is that an old BCAR requirement, too?

re nosewheel steering. AC25-7A requires it to be OFF unless a pre amdt 25-42 aircraft, on a wet runway. We turn ours off, consequently. This provides a degree of wet runway simulation, although it's not perfect of course.

[i]edit: and another example of Vmca/Vmcg variation. With modern type rudder systems, incorporating travel limiters or FBW type controls, it would be easy to conceive of an aircraft with very different rudder authority limits in-air and on-ground. As a result one might have very different Vmca and Vmcg values, which would allow them to switch magnitudes.

[john_tullamarine]

Milt,

Probably better to leave that question to one of the site's TP fraternity who is in the thick of such things.

Such exposure as I have had to testing has seen instinctive rudder reaction permitted although thrust changes post 42 have included the delay.

Thinking back to some specific runway deviation testing in years past ... a delay in rudder application would have been considerably more than just interesting ... I still have some videos taken (with BIG lenses) from the far end of the runway ... quite sobering to see airliners dancing left and right like a dainty ballerina ... even have a picture of the illustrious Red ambling across the runway a mile or two from the camera.

By the way, I am in your town periodically .. if you see it as appropriate, do let me know a contact and we can catch up for a well-deserved quiet one or three sometime ...