perfo question: increased V2!!!

[inner]

Hello ,

i've read that an aircraft can increase its weight by increasing its V2 because this would give a better climb gradient.

I know that the formula of a climb gradient is: (thrust minus drag) divided by weight.

Now what i don't understand is: does the increased V2 increases our thrust or decreases or drag??

is there someone who can explain this a bit.

thx

[BOAC]

http://www.pprune.org/forums/showthread.php?t=239526&highlight=improved+climb

[Old Smokey]

Great link BOAC, sorry if I can't resist adding a little more......

V2min is arbitrarily pegged at 1.2 X Vs for older aircraft, and 1.13 X Vs for some more modern aircraft. This provides for a good compromise between satisfactory Field Performance and Climb Gradient Performance. This speed inevitably falls below the best Gradient speed, which is very close to Vmd (for jet aircraft), and thus V2min provides for less than optimum climb gradient performance. (Vmd is usually in the range 1.35 to 1.40 X Vs).

Following this, IF there is sufficient Runway available to achieve a V2 between V2min and up to Vmd, improved Climb Gradient Performance results. Of course, the aircraft must have been certified for this second performance schedule.

It is also because of this, that AFMs advise that if engine failure occurs above V2 (typically V2+10 for 1.2 Vs aircraft, and V2+15 for 1.13 Vs aircraft), then maintain that increased speed, or reduce to it if above. It is note-worthy that the same AFMs don't caution against using the 'additive' speed if already using the Increased speeds V2 schedule - THEY SHOULD!

It is important to emphasise that if engine failure occurs at V2min or at less than V2 plus the approved additive, that existing speed should be maintained, and no attempt made to increase to the +10 or +15 speed. To do so is a potential CFIT in the making, to achieve the acceleration to the higher speed, it would be necessary to 'duck under' the required flight path, which may be obstacle limited.

inner, you asked "does the increased V2 increases our thrust or decreases or drag??". As described above, drag decreases. Thrust will actually be less, due to the effects of the thrust equation, and V2 being well below that when Ram recovery begins to take place (usually around about M0.5), and this is why the best gradient speed is SLIGHTLY below VMD (but not so much that you'd notice).

Regards,

Old Smokey

[BOAC]

OS -our very best wishes to you for 2007.

NEVER hesitate (from my POV) to expand on our posts, please!:ok:

[Old Smokey]

BOAC et al @ PPRuNe,

My very best to you, and all Prooners for the coming year. May PPRuNe continue to prosper in 2007, not just as a mere pilot/industry 'chat line', but as a VERY valuable contribution towards safety and standards in our industry, always in good humour of course! A good interchange of ideas and opinions inevitabely leads to improved understanding and competence, and PPRuNE is an invaluable interchange of ideas and opinions.

I'm a 'Technocrat', not so well versed in other not-so-technical areas of aviation, so even after 43 years in the industry, continue to learn more and more on a daily basis from these forums.

You never fail to boggle my imagination with your ability to, at a moment's notice, ressurect an associated thread for a new post.:ok:

Fly smart, fly safe everybody, in 2007 and beyond!:D

Regards and a Happy New Year,

Old Smokey

[MrBernoulli]

Thanks to both BOAC and Old Smokey for their enlightening views on the 'darker' corners of aviation. Do you guys have degrees in this stuff? Or are you just terribly, terribly professional:ok:

[BOAC]

their enlightening views on the 'darker' corners of aviation- don't want to go there, thanks! Misplaced praise for me there, Mr B! I think that should be directed at others. Merely the 'librarian'.

[chornedsnorkack]

Great link BOAC, sorry if I can't resist adding a little more......
V2min is arbitrarily pegged at 1.2 X Vs for older aircraft, and 1.13 X Vs for some more modern aircraft. This provides for a good compromise between satisfactory Field Performance and Climb Gradient Performance. This speed inevitably falls below the best Gradient speed, which is very close to Vmd (for jet aircraft), and thus V2min provides for less than optimum climb gradient performance. (Vmd is usually in the range 1.35 to 1.40 X Vs).
Following this, IF there is sufficient Runway available to achieve a V2 between V2min and up to Vmd, improved Climb Gradient Performance results. Of course, the aircraft must have been certified for this second performance schedule.
It is also because of this, that AFMs advise that if engine failure occurs above V2 (typically V2+10 for 1.2 Vs aircraft, and V2+15 for 1.13 Vs aircraft), then maintain that increased speed, or reduce to it if above. It is note-worthy that the same AFMs don't caution against using the 'additive' speed if already using the Increased speeds V2 schedule - THEY SHOULD!
It is important to emphasise that if engine failure occurs at V2min or at less than V2 plus the approved additive, that existing speed should be maintained, and no attempt made to increase to the +10 or +15 speed. To do so is a potential CFIT in the making, to achieve the acceleration to the higher speed, it would be necessary to 'duck under' the required flight path, which may be obstacle limited.
May be, but it is not always. The airplanes seem to be restricted to WAT limit weights even if there are no obstacles ahead...

inner, you asked "does the increased V2 increases our thrust or decreases or drag??". As described above, drag decreases. Thrust will actually be less, due to the effects of the thrust equation, and V2 being well below that when Ram recovery begins to take place (usually around about M0.5), and this is why the best gradient speed is SLIGHTLY below VMD (but not so much that you'd notice).
Regards,
Old Smokey

And what is the best V2? With zero operating engines, obviously the best gradient would be at Vmd... best gradient speed with all engines operative is at somewhat below Vmd (because thrust derates), and thus, the best gradient with one engine inoperative would be at a speed slightly faster than the best gradient with all engines, but slower than Vmd...

[Old Smokey]

chornedsnorkack,

I wish that I could fly in your operation, the vast majority of mine are runway or obstacle limited, in fact, I can't remember the last time that the full WAT limit was available to me. Even some apparently 'obstacle-free' departures are not so, an example, you have a 3000M runway Up-Sloping at 1.0%, you only need 2000M, and there's no trees, nothing beyond the runway. Airborne after 2000M with an engine failure at the WAT limit, can you out-climb the Up-Sloping runway during the 1st Segment?

On your second point, I'm not being defensive, but I did quote in the last line - ".....the best gradient speed is SLIGHTLY below VMD (but not so much that you'd notice).". Because of thrust degredation with speed, the actual Best Gradient speed may be 10 knots or so below VMD, but as the Drag curve is 'bottoming out' at it's minima in this region, +/- 10 knots or so speed variation will have minimal effect upon achieved Climb Gradient - hence "not so much that you'd notice".

For the jet aircraft, the Net Thrust line hardly plummets like a Disneyland ride, that's more in line with what is expected of a propeller aircraft. The gradient of the Net Thrust line is actually quite shallow, and for general discussion purposes is usually taken as horizontal.

It's pretty hard to make a comparison in generic terms between Vmd AEO and Vmd OEI, as they present two entirely different Drag Curves, you cannot use the one curve for both instances. In the OEI situation, windmilling engine and offset rudder drag markedly modify the profile drag, yielding a 'new' drag curve with Vmd at a lower speed than for the AEO case. BGC speed may well be lower than for the AEO case, but this will depend on the type of aircraft.

A good discussion.

Regards,

Old Smokey

[john_tullamarine]

The airplanes seem to be restricted to WAT limit weights even if there are no obstacles ahead...

WAT - maximum Weight for Altitude and Temperature - limit is the Design Standard "bottom line" MTOW for the day and usually cuts you back from the certification MTOW at higher temperatures and altitudes.

Reason for WAT limits is to ensure that you have a modest OEI climb capability regardless of other considerations and checks that the minimum certification OEI climb gradients are available in each segment.

As OS suggests, for most operations you will be limited further by the other limits.