V1 vs vmcg

[baobab72]

Hello
I am having a brain farth, trying to figure out why vmcg increases as weight increases, while v1 decreases until they meet.

Anyone can shed some light on the topic?

[5LY]

Only because you're likely using more thrust. Otherwise it doesn't.

[bubbers44]

V1 decreases with increased weight? It never has in my plane.

[Gremlin1991]

At first, remember that v1 the speed, where take-off has to be resumed/rejected in case of an engine-failure, therefore "V1" and coming after it "Continue".
So, the heavier you are,, the more the brakes have to take care of, therefore: more weight, Less V1, because more mass needs more breaking distance.
Hope it's clear? Correct me, I'm a Student and absolutely not free of mistakes ;-)

[Easy Street]

As aircraft mass increases, the stopping distance from 'x' knots increases (because the force provided by the brakes is fixed, regardless of mass, and therefore the decelerating effect is less on a heavy aircraft). Therefore V1 reduces as aircraft mass increases - because you have to abort earlier in the takeoff run in order to stop.

Acceleration = force / mass. Ignoring the effects of derated takeoffs, a heavier aircraft will accelerate more slowly, so will eat up more runway in reaching 'x' knots - again this reduces the stopping distance available, so V1 reduces. These 2 effects are compounding.

Vmcg is a function of rudder and nosewheel steering authority against asymmetric thrust moment. Mass doesn't come into it.

[baobab72]

Hello
First thanks for your replies.
To set things straight, as far as v1 is concerned as weight increases the aircraft carries a greater inertia and in a balanced field condition results in a greater stopping distance thus v1 must be reduced to bring the airplane to a complete stop. On the other hand, the heavier the airplane the greater is the thrust required to accelerate, the stronger is the yaw resulting from an engine failure and thus the greater is the rudder authority needed to counteract the yawing moment.
In the challenger 605 these two speeds meet at 111 knots, representing the minimum v1 speed.

[galaxy flyer]

V1min must be 105% of Vmcg, so V1 will never equal Vmcg. Next, Vmcg is calculated using rated TO thrust, so thrust does change not the Vmcg. CL-605 Vmcg is 105 KIAS, V1min is 111. Vmca is 114-119 KIAS, the lower number is good up to 34,500 pounds. The BFL calculation, by definition, balances the stop AND go cases, heavier gross weights must have a higher V1 to make the go case be "balanced".

If V1 were reduced to account for the stopping distance, the "go" distance, under OEI acceleration, would get very much longer. The stop calculation does limit how much we can increase V1, but we must increase it with ncreasing gross weight.

[Mir]

V1min must be 105% of Vmcg, so V1 will never equal Vmcg.

Aren't you mixing things here? Isn't it Vr which has to be 105% of Vmca? I think that V1 can actually be equal to Vmcg.

[Teddy Robinson]

Vmcg is fixed.

[bubbers44]

We need to know that V1 is V1 and that is all we have to know. Don't make it complicated. Engine failure before V1 abort, after V1 continue as it has been for decades.

[galaxy flyer]

V1min is defined as

(2) V1 , in terms of calibrated airspeed, is selected by the applicant; however, V1 may not be less than VEF plus the speed gained with critical engine inoperative during the time interval between the instant at which the critical engine is failed, and the instant at which the pilot recognizes and reacts to the engine failure, as indicated by the pilot's initiation of the first act

So, I stand corrected, but V1min will not be equal to Vmcg, as of the current FAR 25 amendments.

[172_driver]

V1 is a decision (action) speed - it only has to do with accelerate-stop. V1 can affect your takeoff climb but only when it affects your required VR speed, as VR cannot be less than V1.

That's not right.

Possible V1 speeds is usually a spectra between V1-Go & V1-Stop.
V1-Go being the minimum speed where you can continue the take-off following E.F. considering you now only have 1 eng to get you airborne.
V1-Stop being the maximum speed where you can still abort and stop on the runway.

The chosen V1 can be anywhere within this spectra depending if you are Go-minded or Stop-minded. E.g. if you've got a cliff or water at the end of the runway you should perhaps be more Go-minded choosing a lower V1. If you've got a big mountain in front of you it could be wise to be more Stop-minded (choosing a higher V1), as this will increase your margin in case your flying skills are less than perfect.

In practice, your company should provide you with the necessary tools. In my case we have paper RTOW charts that gives us the limiting weights for a given temperature & speeds (V1, Vr, V2). As a pilot, that is all you need to know.

For the original question, I think weight will very mildly affect Vmcg. To my understanding, the main contributors to change of Vmcg are:
- Thrust
- Pressure altitude
- Temperature

[italia458]

172 driver,

I agree that there is a range of V1 speeds that are appropriate. I guess I could have been clearer.

[bubbers44]

Balanced field length of course is what V1 is predicated on. You can stop on a runway or be at 35 ft at the end. Most of the time you have additional runway so balanced field length isn't important. Taking off from MIA with 13,000 ft of runway we still called V1 as a decision speed but knew we had tons of runway left if we chose after V1 to stop. The FAA wouldn't like it but it would work just fine.

[Lord Spandex Masher]

We need to know that V1 is V1 and that is all we have to know...

Err until your computed V1 is less than VMCG. The only way to know that is to know VMCG as well!

[Fabo.sk]

What is V1? It's the speed at which you either act to abort the takeoff by applying brakes or reducing power, or it's the speed at which you do nothing and continue with the takeoff. If you do nothing at V1, how does it affect the accelerate-go distance? It doesn't. It's the speed at which you rotate that will affect the accelerate-go distance.

It does actually. Lower the decision speed is, the higher the distance to Vr in ef case is, since you are accelerating on one less engine for more.

As such, V1go increases as mass increases, it might or might not increase past Vmcg, and it might or might not influence V1min.

As for the original question, I can only imagine that by putting more pressure on tyres, the lateral resistance/friction increases helping steering effectiveness.

[italia458]

Fabo,

Yea, you're right. I forgot about that!

[italia458]

I deleted my previous post because it wasn't correct in some ways. I'll attempt to be correct here! :}

Here is a graphic representing the V1 speed changing with respect to weight: http://i.imgur.com/i77J28F.png

Essentially, you need to remain in the lower part of the blue 'X', where I've drawn the red and green lines. If you go outside the bottom two lines that form the blue 'X', you'll be at a weight and V1 speed combination that won't be able to either satisfy the accelerate-stop portion or the accelerate-go portion. The line labeled "Continued takeoff" is the accelerate-go limit line and the line labeled "Rejected takeoff" is the accelerate-stop limit line.

This example is specifically for a fixed runway length. If you're at the red weight and would like to increase your takeoff weight to the green weight - the V1 speed can either 1) remain unchanged, 2) increase, or 3) decrease. If you had a V1 speed that was on the dashed "Balanced field Limit V1 speed" line, increasing your weight from the red line to the green line would not require a change in V1 speed. Once you figure out the balanced field limit V1 speed, it doesn't need to change if you decide to takeoff at a weight less than your balanced field limit weight.

If you were using a V1 speed that was very close to the accelerate-go limit at the red weight, increasing to the green weight you would see the V1 speed increase. If you were using a V1 speed that was very close to the accelerate-stop limit at the red weight, increasing to the green weight you would see a V1 speed decrease.

[FE Hoppy]

https://lh3.googleusercontent.com/-itG1mV8eMuI/T1CkVtzcjzI/AAAAAAAAABU/PkT_El3j10Y/s640/V1.001.jpg

[italia458]

I do like the inclusion of VMCG and VMBE! :ok:

[PEI_3721]

See the presentation / notes in V1-Go-No-Go (http://www.scribd.com/doc/128452927/Boeing-Perf-Cse-V1-Go-No-Go)