V1 Vr corrections

[alatriste]

I have registered in the forum recently and would be happy if someone out there could help me regarding few topics. Here goes the first one:

Wherever we land we use same landing and aproaching IAS speed regardless OAT or PA. TAS are going to be higher if DA increases. Therefore, Why do we have to increase V1/VR when taking off on high DA conditions? Why are we supposed to increase speeds when using TO flex ratings?

[747FOCAL]

No offence bud, but are you a pilot like your post implys? If you are you can't me serious man. :}

[alatriste]

Dear 747 focal-brahmi:

your answer was very nice and clarifying, anyhow I would like to know about your great knowledge.
Lets figure out for the same aircraft type:

AIRPORT X OTOW 152.000 lbs (V1/VR/V2) OAT 15ºC Pa 1.000
TORA 7.000 feet

AIRPORT Y OTOW 152.000 lbs (same speeds) OAT 30ºC Pa SL
TORA 13.000 feet.

In both cases operating on Balanced criteria. In the second scenario we have to increase V speed (IAS) few Knots (1/2), even though it requires much higher TAS to reach same IAS.
If ASDR as well as TODR are going to be higher in the second example, make sense to increase speeds?.

I do appreciate your answer I am eager to learn, it seems you are eager to teach.

[BOAC]

Alatriste - not sure what 'DA' is, but VR increases as you 'flex' because the lift component of thrust at rotate is less so you need more knots.

[Mad (Flt) Scientist]

From the context of the first post, I'm surmising PA - "Pressure Altitude" and DA - "Density Altitude"

At higher DAs or flexed, in boith cases you'll get a lower thrust/weight - either because there isn't any more to give (DA) or you choose not to use it all (flex).

Assume now that your aircraft is V2 limited (stall speed or Vmca, it doesn't matter for this). Therefore the V2 will be the same whatever conditions prevail.

Vr must be chosen such that the aircraft can achieved V2 at 35ft. This is dependent upon the pitch attitude you need to get to and the acceleration of the aircraft, because if you're accelerating at 3kt/sec and take 4 secs to rotate and get to 35ft, that's a 12kt margin right away between Vr and V2. This margin - the 'speed spread' - is obviously quite dependent on thrust/weight ratio.

Considering the flex and non flex cases, if 'un-flexed' I am accelerating at 3kt/sec I need my 12kt Vr-V2 margin. If by flexing I drop down to 2.5kt/sec accel then I only need 4*2.5=10kt margin. So now Vr is set 10kts below V2, rather than 12kts. Hence Vr is higher when flex is used.

edit: getting it all straight :)

[quid]

For the V1 case, it's also due to the lower (sometimes signifcantly) thrust output of the remaining engine.

You say you are using balanced field numbers. (As most carriers do.) Then, unless you are at Max T/O weight, you are not runway limited. You will (in most cases) see a slight increase in indicated V1 due to the unequal effect of DA on thrust vs. brakes.

For a very simple example, let's say the increase in DA (at the same weight) will cause an increase in takeoff distance of 200 feet due to the above mentioned lower trhust. But...the increase in stopping distance (due to the higher TAS) is only 100 feet. The V1 speed will be increased by a knot or two to once again balance the takeoff.

In a nutshell, at a higher density altitude, the degraded thrust on the remaining engine is more of a factor than braking from the higher TAS. :ok:

[SuperRanger]

alatriste,

i can't really figure out your examples but regarding your Q of the need to increase V speeds when using flex, this is how i understand it.

the reason you can use flex TO is because you have excess runway for that particular TOW at full TO thrust (that's why you use less thrust to save eng life).

also with excess runway, you can use 'increased V speeds procedure' to increase your TOW (you can do this if you are 2nd segment climb limited as opposed to field length limited. this is very often on a 2-eng airplane)

when using flex TO, it's like putting on a smaller engine, as a result your climb performance is compromised. in order, to get your original climb limit weight (at full thrust), you will need to increase your V speeds very similar to 'increased V speeds procedure'.

i hope my explanation helps more than it hurts :)

SR

[alatriste]

Thank you very much for your time and explanatios.@ SR, QUID, BOAC and MAD SCIENTIST@. A couple of ideas were really interesting. These are the type of fellows I was looking for.

Take your time 747 FOCAL but I am becoming anxious looking forward your wise reply.