PPRuNe Forums Measuring Vmcg?

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 24th Mar 2010, 12:59 #1 (permalink) Join Date: Feb 2010 Location: Heart of Darkness Posts: 985 Measuring Vmcg? How do you measure Vmcg? Since most aircraft have both nosewheel and rudder linked to pedals, how do you get around the requirement that the Vmcg is determined with "nosewheel steering inoperative"? Presumably also you have to determine Vmcg in linear deceleration, as it would not be possible to accelerate to Vmcg if the nosewheel steering is inoperative? Or do you just place strain-guages on the steering tie rods to determin the speed where nosewheel steering input is required to maintain the aircraft under control?
 24th Mar 2010, 14:05 #2 (permalink) Join Date: Mar 2010 Location: Henley on Thames Age: 65 Posts: 10 Trim Stab Hi ! I saw your other posting,about the Canberra ; I shall be visiting a pub in Bucks tonight,and will enquire of "mine host", Mike, a former Canberra helmsman. He is A) not in the first flush of youth and B) from somewhere in the Far North of England,and may well not know what you are talking about,but it's worth a try !
25th Mar 2010, 19:57   #3 (permalink)

Join Date: Jul 2007
Location: Germany
Posts: 1,211
My type has Vmcg speeds for consideration in perf planning

Quote:
 Ground minimum control speed is the minimum speed during the takeoff run at which the aircraft’s engine most critical to the directional control can fail with the remaining engines operating at selected thrust and still maintain directional control. Additionally, a deviation of no more than xx feet from the ground path originally intended can result, using the elevator, aileron, spoiler, rudder controls, nose wheel steering on a dry hard surface runway, with a 1 second pilot reaction time. The diminished advantage of nose wheel steering will be considered on wet, icy, or austere runways. Ground minimum control speed during rotation is calculated assuming no credit for nose wheelsteering.

 4th Apr 2010, 06:43 #4 (permalink) Join Date: Oct 2004 Location: australia Posts: 20 Nose wheel steering is de-activated for the tests. I can remember doing a few test in B737-200/300. I don't remember full detail, but if the undercarriage lever was selected to "neutral" the NWS was inoperative to the satisfaction of the regulatory authority. I think this may have been done with the undercarriage pins in place. Don't recall how we handled it with other manufacturers.
 5th Apr 2010, 19:48 #5 (permalink) Join Date: Dec 1998 Location: UK Posts: 50 Could someone also confirm that all trials are conducted with zero crosswind. How does this then equate to the real world. Are these then demonstrated aircraft design limits for certification only. When I am approaching VMCG on a take-off roll with a limiting crosswind and then lose an engine , what can I expect?
 5th Apr 2010, 22:32 #6 (permalink) Join Date: Oct 2004 Location: australia Posts: 20 Firstly at Vmcg ie 150lbs rudder force the maximum centre line deviation permitted is 30 feet. It MAY be, in a particular type there is still some rudder available if forces above 150 lbs are used. I have been involved in flight testing of a number of transport aircraft, both jet and turboprop, for approval of operations on 30 metre wide runways. Included in this testing were Vef failures, of the critical engine, during maximum crosswind takeoff. Unlike Vmcg testing the use of nose wheel steering was permitted. Most of the aircraft tested could be confined to the 30 metre runway. Those that didn't were given the option of overspeeding Vef, using a higher, less penalizing, Vmcg (with the take off distance penalty) or to operate on narrow runways at a reduced maximum crosswind. My experience would indicate that, on normal width runways, all aircraft that we tested would remain on the runway with failure of the critical engine at Vef in maximum crosswind. However, it may be a fairly exciting few seconds waiting for Vr to come up. As I recall, the increases in Vef required in those cases where the aircraft could not be contained to the runway were comparatively minor ie around 5kts or less. Aircraft tested during these exercises included B737 200/300, Fokker 50, DC9, G11, Cessna Citations, Jetstream 31, Challenger and others I don't immediately recall.
 6th Apr 2010, 01:03 #7 (permalink) Moderator   Join Date: Apr 2001 Location: various places ..... Posts: 4,957 I'm presuming that I know zzuf from some of those earlier runway tests ? My worry would be the wet runway case with an aft-ish CG at Vmcg .. the OEM data we obtained for the Diesel, in particular, showed a quite rapid divergence in the last few knots and the runway head observations suggested that staying on the runway under critical conditions might be more than a problematic issue with an adverse crosswind for any of the aircraft for which Vmcg was critical.
26th Apr 2010, 08:36   #8 (permalink)

Join Date: Feb 2005
Posts: 496
Quote:
 Originally Posted by Trim Stab How do you measure Vmcg?
My employer just finished doing this for a 19 seat turboprop. The process was fairly straightforward - accelerate down the runway, cut an engine, don't use nosewheel steering or aileron, and see if the aircraft can be kept within the allowed 30 foot deviation.

In our case, we began the testing at a speed well above what we expected Vmcg to be, and then kept reducing the speed. It turned out that the demarcation between "easily keeping the aircraft on the centerline" and "oh-oh, here come the weeds" was pretty sharp - a three knot speed difference was all there was between the two extremes.

The Vmcg for our aircraft turned out to be very low. The only reason we needed to establish this speed (which was never established during initial certification) was to enable development of performance charts for contaminated runways that use a reduced V1.

It's kind of funny to note that although the test aircraft had a huge flight data acquisition suite in the back, multiple GPS receivers, etc., the very best idea for measuring deviation from the runway centerline came from one of the AMEs - he suggested that we just connect a large water bottle to a hose leading out to behind the nose wheel, and open the tap at the start of the takeoff roll. We could then observe the path left on the runway by the stream of water, and measure centerline deviation to the inch... no need for all that fancy GPS stuff.

 26th Apr 2010, 10:45 #9 (permalink) Join Date: Oct 2004 Location: australia Posts: 20 I would far prefer to measure the deviation from the projected ground track at the instant of failure, could be a very different result from just measuring centre line deviation. Another point is that not using the nose wheel steering may not be the same as having it disabled to the extent that the nose wheel is free to castor, which is really what the standard is aiming at. Cheers Last edited by zzuf; 26th Apr 2010 at 12:08.
 26th Apr 2010, 12:21 #10 (permalink) Moderator   Join Date: Apr 2001 Location: various places ..... Posts: 4,957 connect a large water bottle to a hose leading out to behind the nose wheel Years ago for some runway width testing, this sort of approach was tried with not terribly useful results. Boeing et al used the high tech approach. We just used a runway head video with a long lens (1000-2000mm focal length). It was child's play to scale off lateral deviations along the runway to an accuracy of around 100 mm or so. I believe that Cessna ended up using the Citation work from that period for the AFM generally.
 26th Apr 2010, 14:32 #11 (permalink) Join Date: May 2002 Location: Cheshire, UK Age: 50 Posts: 174 On a four-engined regional airliner I was involved with, we measured VMCG like this: Usually light weight and aft CG. Shut down (cut to fuel off) at progressively lower speeds until the deviation just exceeded 30ft, sometimes a cliff edge would be reached and the deviation would suddenly be very large. The aircraft had a VMCG that went up as the weight came down, due to tyre footprint effects we believed. We did not disengage the nosewheel steering in case it was needed in a hurry. The rudder was not connected to the nosewheel. We allowed “instinctive” use of aileron. We always briefed that in the case of a large deviation we would throttle back the opposite outboard (the benefit of a four-engined aeroplane) We had the luxury of lines painted the runway 30ft either side of the centreline and would offset the aircraft one side or the other, depending on the crosswind as we knew which engine was going to be cut. The camber wasn’t significant. The deviation was determined by use of the painted lines and the fact that on this particular type the distance from the centre to the outboard edge of the flaps was 28ft. Latterly differential GPS was used. Hope this helps.
26th Apr 2010, 20:57   #12 (permalink)

Join Date: Feb 2010
Location: Heart of Darkness
Posts: 985
Quote:
 Usually light weight and aft CG
Presumably you need to measure it throughout the range of permissible CG?

Vmca will be highest at the most aft CG, but does it necessarily follow that Vmcg will also be highest at the most aft CG? If CG is very aft, the assistance given by the nosewheel (even unsteered) to directional stability will be highest. I am therefore wondering whether an aircraft with a very long wheelbase might actually record highest Vmcg with the CG not at its most rearwards?

 27th Apr 2010, 20:57 #13 (permalink) Join Date: Sep 2002 Location: La Belle Province Posts: 1,859 I vaguely recall seeing a position paper by Airbus (perhaps as part of a sresponse to a special condition?) which endeavoured to show that neither weight nor cg were critical for VMCG. It was public domain - I think it got presented at a conference - and may have been referenced on a previous VMCG thread...
8th May 2010, 08:17   #14 (permalink)

Join Date: Sep 2007
Location: Was NW England now Quebec
Posts: 28
Quote:
 Vmca will be highest at the most aft CG, but does it necessarily follow that Vmcg will also be highest at the most aft CG? If CG is very aft, the assistance given by the nosewheel (even unsteered) to directional stability will be highest. I am therefore wondering whether an aircraft with a very long wheelbase might actually record highest Vmcg with the CG not at its most rearwards?
This is true but the moment arm of the rudder (which provides the path recovery force) is at it's smallest at aft cg.

 8th May 2010, 12:48 #15 (permalink) Join Date: May 2002 Location: Cheshire, UK Age: 50 Posts: 174 I would have thought that on the ground the aircraft tends to rotate around the gear rather than the CG. Not much difference I know. One of the issues at aft CG is that the nosewheel has less reaction - more of the aircraft's weight is on the mains. This may be why CG is important rather than rudder arm.

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