V2

[172driver]

Can someone help me to understand exactly what V2 is and how it is calculated?

I know it is takeoff safety speed but what exactly is safe? Where must V2 be reached in order to consider it a safe takeoff? Is V2 maintained after takeoff or just a speed you pass through on the way to Vyse?

Another question: If you lower V1, does that mean more overall runway will be necessary to reach V2? Example...V1=120 kts and balanced field is 6000 ft. V1=110 kts and balanced field is 7000 ft? This sound right or not? Why or why not?

 

[Fr8Dog]

Ok I will do my best with both questions.

On the first question about V2. I don't know exactly how it is calculated, but as far as I know it is the best single engine climb in a specific configuration. As an example with flap 15 for takeoff V1 may be 109, Vr 112, and V2 115. We would normally climb out at V2 until 400agl or obsticale clearance, then excelerate to bsec, which is best single engine climb with flap 0, put flaps to 0 and continue climbing out. So I guess you could say V2 is calculated at what ever speed gives you the best rate of climb with a specific flap configuration.

As for V1 here is my best shot. V1 is calculated at whatever speed will give the best balanced field legenth and shortest distance to clear an obsticale and give you the required climb performance to do so if an engine fails at V1. Here is the example our ground instructor gave us in class.

Say you are instructed that you have to go out and calculate V1 for a new aircraft. So at first you decide to try 80kts. So you accelerate down the runway, fail the engine at your 80kts and come to a complete stop and you figure your total distance to be 3000ft. You then go accelerate to 80kts fail the engine and try and climb out, but you use 6000ft to clear your obsticle. So you decide to make V1 120kts. You then go fail the engine at 120kts and it takes you 5500 feet to stop. The next run you fail the engine at 120kts and climb out and you clear your obsticle at 4000ft. When you figure a V1 speed that would let you accelerate to V1 loose an engine and stop, and get to V1 loose an engine takeoff and clear an obsticle in the same distance that would then be your V1 speed for a specific weight. So that is how V1 is calculated.

Now I could be completely wrong on both of these so if I am, someone please speek up and tell me.

 

[typhoonpilot]

I'll give your questions a go.

V2, the Takeoff Safety Speed, is the lowest speed at which the aircraft complies with the handling criteria associated with climb after take-off following the failure of an engine.

V2 is also defined as the target speed to be attained at the screen height with one engine inoperative and is maintained to the point where acceleration to Flap Retraction Speed is initiated.

Most piston engined aircraft have a V2 which was determined by a margin of 15% above the power off stall speed but with jet aircraft the benefit of propeller wash over the wing is lost. To make up for this the margin was increased to 20%. More recently it is calculated as 1.13 X Vs @ 1G.

V2 is not a performance related speed per say, but one which is determined by the handling characteristics of the aircraft.

For question two the answer is yes. If you lower V1 and have an engine failure at that lowered V1 it will take longer to accelerate thus it will take longer to reach V2.

Hope that helps.

Typhoonpilot