Flex Thrust - Increase in V-speeds (CRJ 900)

[dcoded]

Dear all,

I am flying on the CRJ and there is one question which I can't find an answer to.
We are using the Standard speedcards provided by bombardier, and runway analysis tables (provided as PDFs)

Now to my question.
When making a flex TO, with an assumed temp of, lets say 48 degrees.
According to the booklet I need to Inscrease V1 with around 4-5kt (depending on weight)
The Vr with around 4-5kts, and the V2, Decrease with -1kt or sometimes leave it unchanged (also depending on weight)

WHY do we need to increase the speeds UPWARDS to a higher value? :confused:

Can some-one shed some light on my confusion?

Thanks!

[FlightDetent]

With less thrust you need higher V1 to begin from, in order to satisfy GO performance after an engine failure. That's my guess, not CRJ rated.

[Amadis of Gaul]

It's been four years now since I've flown "The Glorious Nine" as we called it, and we used aerodata numbers via ACARS, but I don't seem to remember flex temps quite that high. I seem to recall the highest we would see was fortyish.

[Mad (Flt) Scientist]

The Vr-V2 relationship is pretty much a direct function of the available thrust (or the T/W). If one or the other is constrained by other criteria (whether it be the V2min of 1.13Vsr or some other value) then that will stay fixed, and the other of the pair will have to adjust accordingly.

So if at a given weight your V2 is essentially already at V2min, then it can't go down, only up. But with less available thrust your acceleration from Vr to V2 will be less, so to ensure you achieve the minimum V2 at the required height, you need to increase Vr a bit so that the lower acceleration is compensated by starting the rotation a bit higher in speed.

That then opens up the possibility (but not a necessity) to increase V1, since Vr is always an upper bound on V1. As FD says, increasing v1 improves GO performance (at the expense of worsening the STOP case) so with less (assumed) thrust the GO case is made worse while the STOP case is hardly affected. therefore you can "rebalance" the GO and STOP cases by shifting V1 up a bit to try to get a better overall distance.

[dcoded]

The Vr-V2 relationship is pretty much a direct function of the available thrust (or the T/W). If one or the other is constrained by other criteria (whether it be the V2min of 1.13Vsr or some other value) then that will stay fixed, and the other of the pair will have to adjust accordingly.

So if at a given weight your V2 is essentially already at V2min, then it can't go down, only up. But with less available thrust your acceleration from Vr to V2 will be less, so to ensure you achieve the minimum V2 at the required height, you need to increase Vr a bit so that the lower acceleration is compensated by starting the rotation a bit higher in speed.

That then opens up the possibility (but not a necessity) to increase V1, since Vr is always an upper bound on V1. As FD says, increasing v1 improves GO performance (at the expense of worsening the STOP case) so with less (assumed) thrust the GO case is made worse while the STOP case is hardly affected. therefore you can "rebalance" the GO and STOP cases by shifting V1 up a bit to try to get a better overall distance.
Mad (Flt) Scientist:

Thanks for a very informative answer!!
Made my day!

I have another question regarding performance aswell, but I will open a new thread to keep things separate!
Maybe you will be so kind to have a look at that aswell? :)

Thanks!

[dcoded]

Dear all,

I have a question regarding the altitude capability of the CRJ900 vs the climb speed.
There are Tables in the QRH regarding this.

My question is:
Why can you climb HIGHER for the same given weight with a climb speed of .77 than .74?

I would like to think that it would be possible to get higher with a slower climb-speed?

If some one could give me an informative answer, in regards to aerodynamics and performance-wise how it can be like this?
1
Thanks

[Goldenrivett]

Hi dcoded,
Why can you climb HIGHER for the same given weight with a climb speed of .77 than .74?
As you climb higher with the same Mach No, your IAS reduces.
MAX FL at fixed Mach No considers the buffet margin over stall warning.
The higher Mach No (& higher IAS) enables a higher cruise FL.

[dcoded]

Goldenrivett

Thanks alot, it makes sense now!