Which V speeds to use?

[framer]

Here's the scenario.
B737-400
Take-off weight 55T
The max we can lift out with bleeds on is 54.8T
Decision made to go bleeds off.
On this chart we can get 57T at full power and the associated speeds are
144 149 and 154 (just guessing those).
There is also a slight reduction available at 55.2 T with speeds 142 147 152.
Question; if the take-off is made at full thrust, at the take-off weight stated, bleeds off, which speeds do you use?
I had this scenario in Ausi just a few days ago and I wanted to use the speeds 142 147 152 because they were closest to our weight, my side kick wanted to use the higher speeds because that was the thrust setting we were using. Have I gone mad or do you use the speeds closest to but not lower than your weight?
The way I see it the aircraft weighs x amount and there will only be one set of correct speeds for that weight regardless of what thrust setting you use to get there....(acually V1 will change but not the other two). If you take it to the extreme and go full power in an empty aircraft your V speeds are def not going to be high like the full thrust limiting weight speeds......I'm not crazy am I?

[mutt]

Use either set of speeds, they are both right !

Mutt

[411A]

If you take it to the extreme and go full power in an empty aircraft your V speeds are def not going to be high like the full thrust limiting weight speeds......I'm not crazy am I?

No, not crazy, in fact, quite correct.
I would suggest that the Vspeeds used should be for the weight at the time of takeoff, subject however to increased speeds in the case of an overspeed/improved climb situation.

[framer]

.....................cheers

[Mad (Flt) Scientist]

....acually V1 will change but not the other two....

Just to be pedantic, all the speeds are subject to change with a change to the thrust being used. Depending on whether the data is Vr-limited or V2-limited, the other will have to adjust to allow for the different acceleration capability between Vr and V2. (i.e. if you are using minimum V2 speeds, then in order to assure you achieve V2 at 35ft you need to start at a faster Vr when you reduce the thrust, and so on)

Oh, and I agree, use the speeds for the nearest weight (interpolating or rounding up per preference or table instructions).

I disagree with mutt on this; use of speeds from a weight too far from your actual weight may screw up trim settings and rotation behaviour. (I this case a couple of knots here or there wouldn't likely be critical though)

[framer]

Thanks Mad S,
The V speed performance is more complicated than a lot of people think. I am aware that it is something I need to give some attention to in my study time. There are about 100 subjects like that that I plan to square away over the next ten years or so . I try to do the most critical ones first. I'm not saying I have no idea what the speeds mean but I don't have an in depth understanding of how they are calculated and how they effect each other like in the example you gave above. You explained it well though and it makes total sense to me.
For now I am comfortable that my basic understanding is sound .
Thanks again.:ok:

[photons]

Normally, once the performance page has been satisfied the FMC calculates the v speeds for you.
And if not, a V speed chart which is temperature, pressure and weight sensitive with slope corrections.
A temp chart column reference allows you to select the column, and this can be further refined by selecting your wieght in Ib's.
And then another chart allows you to select the flap settings for the takeoff, a higher flap setting would require a lower speed.

As for your TO thrust, this is calculated when selecting your OAT with PMC's active, this too is temperature sensitive and when manipulated can produce a derated thrust.

Without your OAT or pressure altitude you cannot determine your V speeds correctly.

And please note, increasing your TO thrust won't alter your v-speeds but will alter your to ground run and climb rate should any obstacle clearances or minimum slope gradient be considered in the climb.

[BOAC]

And please note, increasing your TO thrust won't alter your v-speeds- back to school, I think:). try post #5?

[framer]

Cheers Photons. I'm pretty happy with how to use the charts. I just came across a situation the other day where we used full thrust but at our weight a reduced thrust was possible. With that in mind I used the V speeds just above our actual weight on the chart but used full thrust. ie the V speeds used were closer to the top of the page than the thrust developed at the actual ambient temp....I hope that hasn't confused things even more .....lets not talk about increasing our Vr towards V2 when windshear has been reported ha ha......and yes....I know there are limits to how much you can increase it!:ok:

[mutt]

MFS, you dont agree with me......:{:{:{

Well lets see what Mr Airbus has to say....

1: For a given weight it is possible to select a temperature lower than the maximum determined one and keep the speeds defined at the maximum temperature.........

or

2: Takeoff must be performed with TOGA (in this example MTOW = 259000 kgs 137/157/166) takeoff speeds for the actual TOW 252000 kgs are increased to MTOW speeds.....

or

3: As the performance limited MTOW of 279000 is greater than the structural MTOW of 275000. 275000 has to be retained as the MTOW. Enter the chart with the MTOW to find the v-speeds.

So as I said, both methods are correct! :)

Mr Photons, what aircraft type are you talking about? It sounds incredibly strange....



Mutt

[framer]

Is there any such guidence from Mr Boeing?
It seems to me that Mr Airbus is saying that it is permissable to do this.
What would an aerodynamics purist do?....or more importantly, what will the aircraft do?
If there are no obstacle or climb limitations, just a flat desert, which speeds will put you highest 20nm from the field?....

[Mad (Flt) Scientist]

MFS, you dont agree with me......:{:{:{

Well lets see what Mr Airbus has to say....

1: For a given weight it is possible to select a temperature lower than the maximum determined one and keep the speeds defined at the maximum temperature.........

So, by extension, you could use the fully flexed speeds and do the takeoff with full power (as the extreme case of selecting a lower temperature? That would seem to imply that there should be an effect on the speed spread, but you don't adjust Vr/V2 accordingly. That would seem only to be acceptable if you aren't scheduling for minimum V2. Maybe this aircraft is always min Vr?

2: Takeoff must be performed with TOGA (in this example MTOW = 259000 kgs 137/157/166) takeoff speeds for the actual TOW 252000 kgs are increased to MTOW speeds.....

Yet this example shows the speeds being driven by the thrust considerations, which would seem to imply that speed spread IS a concern. Though even here it won't be quite correct as you've got the right thrust but now wrong weight, and speed spread is really a T/W thing at heart.

3: As the performance limited MTOW of 279000 is greater than the structural MTOW of 275000. 275000 has to be retained as the MTOW. Enter the chart with the MTOW to find the v-speeds.

No arguments here - numbers above the structural MTOW can't be used (and shouldn't even exist!)

Not arguing that those are what AB tell you to do, but I think there's something else going on to make those numbers work. :confused:

In general I'd also be concerned over trim/rotation characteristics if a speed were used which was appropriate to a (very) different weight - at some point you do invalidate the cert takeoff technique demonstrations.

[mutt]

Dear Mod, can you bounce this thread into the Tech Forum so we can get the input of OS and JT.

Thanks

Mutt

[photons]

http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/smile.gif[/IMG]. try post #5?]

The only speed effected by this is VR, but you still use the same V speeds, is a slow rotation allowed prior to reaching V2?

Corrections that must be made include tail winds, a + or - slope, those kind of things, V2 will of course follow a lower v/s.

To determine whether to use a bleeds on or off takeoff, you MUST include 4,000 Ib's to you actual gross weight. A bleeds on takeoff with the assumed method must also include this correction.

To determine whether to use the assumed temp method, refer to your flex temp gw column of the runway in use.

Many more corrections/factors need to be made before you can dtermine any further and make the decision.

I don't have any data regarding the 400, so I don't want to comment further. Besides I don't want the criticism.

James

[framer]

The only speed effected by this is VR, but you still use the same V speeds, is a slow rotation allowed prior to reaching V2?

Photons, I don't think you are correct . It may be permissable to use the same speeds but in reality the V1 will be different, you've already mentioned that Vr will be different, and by my way of thinking V2 may change based on the different Vr and the increased rate of accelleration.
I would really like to see someone like Mad Scientist do a comparison of two thrust settings to see what the differences are. Might be a bit tricky to get across on here though.
To determine whether to use a bleeds on or off takeoff, you MUST include 4,000 Ib's to you actual gross weight. A bleeds on takeoff with the assumed method must also include this correction.

I'm not sure what you're saying here. I think there may be some sort of language barrier. Are you talking about "padding " the figures or are you saying that with bleeds off you can lift 4000lbs more? From the charts I use you can either do it or you can't, the performance engineers have already done the maths on what can be lifted. The only reason we would add any weight to our "actual gross weight" as you say, is to avoid having to do the numbers again if our zero fuel weight changes between our provisional load sheet and our final load sheet.
To determine whether to use the assumed temp method, refer to your flex temp gw column of the runway in use.

I think everyone commenting on this knows how to do the basic calculations on a day to day basis. What I am hoping is to get a bit better understanding of exactly what the relationship is between thrust and weight and the V speeds. I will do some research tonight and try and impress the clever ones on here with my knowledge tomorrow :8

[photons]

The bleeds on takeoff correction just allows you to determine whether you can satsify both the runway and climb limits.

ie Actual weight 110,000 Ibs + the correction 114,000 Ibs, takeoff runway limit 135,000 Ibs and a climb limit of 129,000Ibs.
It is therfore okay to use a bleeds on takeoff and removes the need to reconfigure at 1500feet AGL.

[framer]

Photons the charts that I use account for climb, obstacle, and runway limits all on one page. The pilot doesn't have to add anything. The chart automatically shows the most restrictive limit for that runway.
Can I ask what part of the world you are flying in because a lot of the things you describe sound very different to how things are down at the bottom of the globe.
The only time we will depart with the bleeds off is if the limit weight (be it runway, climb or obstacle) is lower than our take-off weight at either F15 or F5, and full thrust for the ambient temperature. And when we do we rarely reconfigure at 1500ft because we are normally in amongst the hills at that stage and concentrating on other things.
It is very interesting to hear how differently things can be done.
Cheers, Framer

[mutt]

Just to come back to the Boeing way of doing things, these are for the 777, fixed weight, first 2 are balanced V-speeds with Full Thrust and 20% Derate, you can see that there is a slight differnce in V2.

#3 and #4 are OPTIMUM Vspeeds, with Full Thrust and 20% Derate, once again you can see a diffence in V2. So saying that V2 is constant regardless of the Thrust is incorrect.

Speeds are taken from a certified electronic flight manual.

Takeoff Speeds
--------------
V1 = 147.8 KIAS
VR = 153.9 KIAS
V2 = 160.5 KIAS

Takeoff Speeds with 20% Derate
--------------
V1 = 154.4 KIAS
VR = 156.6 KIAS
V2 = 159.4 KIAS

Critical Limit Weight Speeds
----------------------------
Min V1 = 151.6 KIAS
Max V1 = 166.5 KIAS
Bal V1 = 160.2 KIAS
VR = 166.5 KIAS
V2 = 171.3 KIAS

Critical Limit Weight Speeds with 20% Derate
----------------------------
Min V1 = 169.9 KIAS
Max V1 = 169.9 KIAS
Bal V1 = 169.9 KIAS
VR = 174.7 KIAS
V2 = 177.2 KIAS

Photon, once again i have to ask, what aircraft are you talking about? You have a pretty strange way of doing these calculations. Packs on/off to justify if you need to reconfigure at 1500 feet is pure and utter BS!

Mutt

[seejonfly]

Hola!

Maybe I'm still a bit addled from muscle relaxant...but what did you? It sounds to me that you used the numbers for Full Thrust with Bleeds Off even though you were at a lighter weight and used reduced thrust. Is this correct?

NEVER TRY TO USE AB DATA FOR A BOEING AIRCRAFT OR VICE VERSA!!!!!! Why? Several good reasons, first none of the data is correct (lift, drag, etc.) and second both manufacturers use very different techniques and procedures for their calculations.

Best wishes to all,

Jon

[framer]

Seejonfly,
More relaxant for you man.
It's all there at the top of the thread if you want to read it once you've come down.
If anyone was dumb enough to use bus data for a boeing or the other way around then they would not be able to cause much damage at all. The worst thing they could do is go off the end of the runway and then a quick reset would see them back at the start of the runway blaming microsoft for their failure.
Framer

[Jinkster]

I fly a 737-300

Assumed Temperature - goes for bleeds off weight, QNH correction, Anti-Ice etc

V Speeds - Actual weight.

Hope this helps!