Wet Vmcg

[galaxy flyer]

Yes, I'm a sorry type that cannot get off the topic, but

Going thru the various threads recently, there have been many on performance, crosswinds, wet runway compromises, BUT no discussion on what effect wet surfaces have on Vmcg. And the effect is not insignificant; all the forces opposing the failed engine having to be applied thru the tires until rotation occurs.

I was doing a very wet, crosswind take-off from a hilltop airfield quite near London recently. Wet data V1 was near the certification Vmcg, add in a 22 knot crosswind component and an engine failure near the posted V1 was going to be interesting.

Any estimates on the effect on Vmcg a wet runway has.

GF

[AerocatS2A]

Unless you are using brakes, what forces are being applied through the tyres?

[Maurice Chavez]

I think he means the directional control forces placed on the tires.....As for the original question, the airplane manufacturer I fly has that covered in its In Flight Performance. Do a google search for Slippery Runway by Boeing, nice power point presentation with the info you're looking for....It seems to be a topic that most pilots are struggling with, I'm NOT saying you are.

[galaxy flyer]

The aerodynamic forces that are opposing the failed engine. I have a flight manual that does show about a 18 knot increase on a wet runway, what else can cause the increase on a wet runway?

GF

[Maurice Chavez]

I've heard or certain aircraft increasing V1 on a wet runway, I have never seen it on the types I fly, we reduce V1 and weight obviously.

[aerobat77]

vmcg is related to the pure aerodynamical effect of the rudder, no diff braking, no noseweheel steering and so vmcg is the same for a dry or wet runway.

[Mad (Flt) Scientist]

That's the first order idea, sure.

But if all Vmcg involved was a yawing moment due to an engine asymmetry and a countering yawing moment from the rudder, the manoeuvre would track straight down centreline, with the nose first yawing one way then the other, but no deviation. This, of course, is not the case.

What makes the aircraft deviate - primarily, it's the sideforce generated by the yawed tyres. Vmcg on an infinitely slippery runway would have a much smaller deviation, but it'd also be much harder to stop the deviation once it was happening. Traditionally it's assumed that the reduced sideforce capability of the gear cancels out as the runway becomes more slippery.

To be honest, while there may be an effect, it would be swamped by any kind of crosswind effect, which is a definite real world factor and also ignored.

[john_tullamarine]

Worth having a look at some bits in AC 25-7A. The ACs give background to one acceptable means of doing certification work (in this case FAR 25 flight test work).

Talking about

(a) some aspects of landing tests -


The nose wheel should be free to caster, as in VMCG tests, to simulate wet runway surface conditions.


(b) thrust levels presumed -



Airplane Flight Manual values of VMCA, VMCG, and VMCL (and VMCL-2, as applicable) should be based on the maximum net thrust reasonably expected for a production engine. These speeds should not be based




on specification thrust, since this value represents the minimum thrust guaranteed by the engine manufacturer, and the resulting minimum control speeds will not be representative of what could be achieved in operation. The maximum thrust used for scheduled AFM minimum control speeds should represent the high side of the tolerance band, but may be determined by analysis instead of tests.


(c) what the basic exercise is setting out to achieve -


It must be demonstrated that, when the critical engine is suddenly made inoperative at V


MCG during the takeoff ground roll, the airplane is safely controllable if the takeoff is continued. During the demonstration, the airplane must not deviate more than 30 ft. (25 ft. prior to Amendment 25-42) from the pre-engine-cut projected ground track.

(d) crosswind -


At the applicant's option, to account for crosswind test conditions, the runs may be made on reciprocal headings, or an analytical correction may be applied to determine the zero crosswind value of VMCG.





(e) controls -



Control of the airplane should be accomplished by use of the rudder only. All other controls, such as ailerons and spoilers, should only be used to correct any alterations in the airplane attitude and to maintain a wings level condition. Use of those controls to supplement the rudder effectiveness should not be allowed. Care should also be taken not to inadvertently apply brake pressure during large rudder deflections, as this will invalidate the test data.



(f) weight -



VMCG testing should be conducted at the heaviest weight where VMCG may impact the AFM V1 schedule.





(g) CG and NWS -



VMCG testing should be conducted at aft c.g. and with the nose wheel free to caster, to minimize the stabilizing effect of the nose gear. If the nose wheel does not caster freely, the test may be conducted with enough nose up elevator applied to lift the nose wheel off the runway.





(h) pre A/L42 certifications considering wet runways -



For airplanes with certification bases prior to Amendment 25-42, VMCG values may be demonstrated with nose wheel rudder pedal steering operative for dispatch on wet runways. The test should be conducted on an actual wet runway. The test(s) should include engine failure at or near a minimum VEF associated with minimum VR to demonstrate adequate controllability during rotation, liftoff, and the initial climbout. The VMCG values obtained by this method are applicable for wet or dry runways only, not for icy runways.





(i) takeoff on unpaved runways -



Rudder pedal nose wheel steering may be used, provided the runway surface for the test represents the "worst case" anticipated for operation. The aerodynamic moment applied to the airplane by the rudder, combined with the use of rudder pedal nose wheel steering, may result in the nose wheel “plowing” the unpaved runway surface. This can result in the runway surface elements impacting critical airframe and powerplant surfaces. The test should be closely monitored to ensure this damage source does not exist.



If the test is conducted with rudder pedal nose wheel steering:



(A) Credit may be taken for any performance benefit provided; and



(B) Dispatch without it is prohibited, regardless of whether credit for any performance benefit is taken.



(j) lateral balance -



When determining VMCA, VMCL, and VMCL,-2, consideration should be given to the adverse effect of maximum approved lateral fuel imbalance on lateral control availability. This is especially of concern if tests or analysis show that the lateral control available is the determining factor of a particular VMC.






I'm a sorry type that cannot get off the topic



.. several of us with that disease, mate.



no discussion on what effect wet surfaces have on Vmcg



.. it depends



all the forces opposing the failed engine having to be applied thru the tires until rotation occurs



the certification does its best to give some attention to a reasonable worst case (although we have some reservation regarding crosswinds) - at the end of the day, the Vmc figures are just lines in the sand marking bottom boundaries for the envelope. Generally, we can largely ignore the effect of NWS in a critical case but the mains on dry and wet may well have some interesting differences.



Wet data V1 was near the certification Vmcg, add in a 22 knot crosswind component and an engine failure near the posted V1 was going to be interesting.



I reckon. One DC9 model has a crosswind variation in Vmcg of around 0.5kt/kt. Given your contacts, I would check with the relevant folk to see if the certification program looked much at this metric for your Type. A twin, 20kt and min speed would see me looking to delay the departure - wet or dry. Then again, one would be looking at the CG and whether a flex thrust takeoff might be available to load the dice towards the pilot's interests.



Unless you are using brakes, what forces are being applied through the tyres?



Assuming that the NWS steering is useless (which may not be the case dry with a forward CG), the mains can produce a significant lateral slipping force. I am of the view that modelling is a bit average unless you have pretty accurate test data to validate models.



so vmcg is the same for a dry or wet runway



maybe not .. unless the OEM has the flight test validation to support such a contention.



MFS comes from the OEM side of things so one should take his comments with some consideration.

(Having some difficulty forcing font changes for some reason - will revisit later and try to tidy things up a bit)