Diego
Topic Author
Posts: 135
Joined: Mon Apr 23, 2001 7:33 pm

V1

Sun Sep 17, 2006 9:30 pm

hello everybody
a silly question that just popped out in my mind:

how does weight affect V1, well i know that an increase in weight results into an increased V1 but why is that? is it because of the Vmcg or because of the Vs? and how is V1 is affected by the runway condition or by the pressure altitude?
Another question: why do you need a balanced field condition? ASDA = TODA?

thanks
 
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Starlionblue
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RE: V1

Sun Sep 17, 2006 9:50 pm

Does increased weight lead to increased V1 speed? I would think it is the other way around. Since increased weight leads to increased inertia both accelerating and braking, you reach the point of no return along the runway slower, and thus at a lower speed.

Is my thinking off? Still not quite awake.  Wink
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
BoeingOnFinal
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RE: V1

Mon Sep 18, 2006 12:07 am

It affects Vr, because the wings need more lift to make up for the weight. To get an acceleration vertical: Lift > Weight.

But I would assume as Starlionblue mensions here, that V1 would be lower because you need more time to hit a certain speed and to loose it again because the increased weight.
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jush
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RE: V1

Mon Sep 18, 2006 12:25 am

Quoting Starlionblue (Reply 1):
Since increased weight leads to increased inertia both accelerating and braking, you reach the point of no return along the runway slower, and thus at a lower speed.



Quoting BoeingOnFinal (Reply 2):
But I would assume as Starlionblue mensions here, that V1 would be lower because you need more time to hit a certain speed and to loose it again because the increased weight.

Sounds logical to me. You eat up more runway with more weight so you do at least don't have more runway/time to stop the masses. So V1 should be reach ealier than at lower weights.

Regds
jush
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SlamClick
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RE: V1

Mon Sep 18, 2006 12:51 am

V1 being limited by accelerate-stop distance ONLY explains those rare (one in a thousand or so) takeoffs where your allowable takeoff gross weight actually is limited by accelerate-stop distance. Most of the time it is not. What about those hundreds of twelve thousand foot runways out there? What about all those low-gross weight takeoffs? On a typical takeoff you might have a V1 of 120 knots but realistically you could accelerate to tire speed limit (about 195 knots) and still stop in the remaining runway.

I don't have an up-to-date copy of FAR Part 1 handy at the moment but V1 is established for weight AND configuration by the manufacturer with reference to VEF which is the speed at which an engine is assumed to have failed. VEF itself is established with respect to V2 which, in turn, refers to VS and VMC which are both directly affected by gross weight and configuration.

The published V1 is pretty much a function of those other speeds - BUT it still must permit accelerate-stop on the available runway.

A fairly recent revision also states that V1 is the point when the pilot must have begun the process of rejecting the takeoff. After that point the takeoff must be continued even if the engine is 4th-of-July on fire! This does not necessarily mean that there is not enough runway left to stop the airplane! It just means for sure that it is safer to continue, get the plane ready and make a more normal landing and deceleration.
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113312
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RE: V1

Mon Sep 18, 2006 1:17 am

SlamClick is correct. V1 is called Takeoff Decision Speed but it really is Go-Or-Stop DECIDED speed.

V1 can never be more than V2. V1 must allow the aircraft to clear a 35 foot barrier and achieve V2.

Under normal circumstances, V1 usually relates mostly to getting airborne since the brakes on most modern aircraft are very effective in stopping. However, a short runway or one contaminated with ice, snow, or standing water might be difficult to stop which will drive V1 to a lower value.

In many circumstances, you could assume that there are two numbers. One is the highest speed that would allow for a safe rejected takeoff. That is, to allow for stopping on the pavement. The other is the lowest speed, after which a successful takeoff could still be achieved with the failure of an engine.

Since VR and V2 are directly related to weight, for an average runway, V1 will also be higher as weight increases since acceleration and deceleration are both a function of weight. However, at some point, increased weight may cause V1 to decrease as the stop factor becomes limiting.

It is also fair to point out that this is all based upon Indicated Airspeed. Should a tailwind gust be experienced, while the aircraft is rolling down the runway, the airspeed indicator will stagnate. This situation would define a windshear on the runway and would destroy the concept of using indicated airspeed as a marker for the takeoff or reject decision. But that is a whole other subject.
 
cptspeaking
Posts: 567
Joined: Mon Apr 24, 2006 2:49 pm

RE: V1

Mon Sep 18, 2006 4:03 am

Quoting 113312 (Reply 5):
SlamClick is correct. V1 is called Takeoff Decision Speed but it really is Go-Or-Stop DECIDED speed.

A great example of V1 happens hundreds of times each week as you drive. Approaching EVERY trafic light you go through, your mind calculates a V1 "speed" (or position based on speed) at which if the light turns yellow it would be better to go through or stop. If you're in a little Honda, your "V1" would be closer to the light than if you were in an 18-wheeler just because of weight and momentum.

I realize there isn't any confusion to that here, just a neat little teaching tool for those not so familiar I thought I'd share!  Smile

Your CptSpeaking
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Jetlagged
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RE: V1

Mon Sep 18, 2006 6:06 am

If V1 reduced with weight then you would eventually get to a point where it was exceeded when taxiing  Smile
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411A
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RE: V1

Mon Sep 18, 2006 12:08 pm

It also might be interesting to a few to realise that sometimes, where an obstacle is present in the takeoff flight path, and excess runway is available for the aircraft weight concerned, V speeds are increased slightly (4 knots, give or take) to allow a greater climb profile (rate), to enable the obstacle to be cleared.
In these cases, takeoff weight can be increased over what normally would be the case, to enable a larger payload.
Zurich, runway 16, is a classic case with some heavy 3 or 4 engine aircraft.

These takeoffs are called 'overspeed' or 'improved climb', depending on the wording used by the aircraft manufacturer or performance data provider.
 
flymatt2bermud
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RE: V1

Wed Sep 20, 2006 1:17 am

Point of reference: V1 is reduced (typically by 2 to 6 knots) on wet or contaminated runways to reduce the likelihood of your aircraft running off the end of the runway in the event an abort-able item appears during the takeoff roll at high speed. Therefore, you will get to V1 (all other factors the same) earlier on a wet runway, just have to respond accordingly.
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