Venr, Vyse

[JetDriver2727]

A better analogy for V2 is best angle of climb speed with gear up, flaps takeoff, and takeoff thrust. It's all about the climb gradient. This is the 2nd segment of the climb, the 1st having ended with gear retraction/35 feet above runway, at V2. Indeed, the required minimum climb gradients for these segments is highest during the "V2 segment", i.e., during the second segment.

Venr is the target speed for the next segment, which involves flap retraction, reduction to max continuous thrust, and acceleration to Venr. This final segment generally begins at 400 agl.

I have not seen Vyse in the manuals for the Citation. While the rate of climb could be (is, I believe ) higher at Venr than at V2, the gradient is shallower because of the higher forward speed. I just looked at an FSI Citation handout, and Venr is defined therein as "Minimum Single Engine Enroute Climb Speed".
These are certification speeds in the event of an engine out scenario.
Vyse might be an okay analogy to Venr, although it would be just an analogy, as there is likely no published Vyse for the airplane, anyway.

You're right, I hastily left out the part about takeoff thrust, and even more carelessly I said V2 is best rate as opposed to best angle. Oops! even I make mistakes at 2:30 in the morning when I posted that.....But, I think everyine got the idea. You are correct that Venr provides for a higher climb rate, but shallower gradient then V2. I don't recall any Vyse speed being publised for any turbine powered part 25 airplane that I have flown so I agree with you there too. I have always been one to build analogies to keep things strait in my head, and I always remembered V2 as Vxse and Venr as Vyse. You are correct that it is only a loose analogy, but it helps visualize what you are trying to obtain with the aircraft.

 

[MauleSkinner]

As JetDriver2727 said, for the most part, jet performance numbers are not based on aerodynamic figures like Vx, Vy, etc.

Let's look at some specifics...

For the Hawker 800, at sea level, 22000 lbs, and 25 degrees C (numbers chosen out of pure laziness), V2 with flaps retracted is 133kias, and the chart shows a 6.2% net climb gradient. 6.2% of 135kTas, we end up with about 825 feet per minute.

Then, we look at Venr under these same conditions. Configuration is the same...the only difference is that we don't have APR activated, so thrust may be up to 2% less (I didn't think to check those numbers at the time). Venr at this weight is 154kias, and the chart yields a 6.4% net climb gradient. Boys and girls, that's a STEEPER climb, with LESS thrust. Working this further, we get a rate of about 985 feet per minute, again higher than the figure for V2. Obviously, V2 is NOT best angle for the configuration.

The reason for this is simple...it WILL climb at V2, and while it may not be at the best angle, accelerating to Vxse would require several miles of runway/clearway, which is generally not available. It's simply the best compromise for the situation.

Taking this a little farther, at this same weight, the Hawker Crew Manual gives a single-engine long range cruise speed of 220kias. There isn't a comparable chart for this operation, but theoretically, this SHOULD be roughly best L/D for the same configuration. Since the amount of lift produced doesn't change appreciably in an unaccelerated flight condition, and "thrust" being the counter to "drag", this is the "minimum thrust" speed...or the speed at which maximum EXCESS THRUST would be available. It's been a long time, but I think that's the definition of Vx...maximum excess thrust available. If my thought process is correct here, this would make Vxse more than 80 knots faster than V2. Imagine how much runway/clearway it would take to accelerate to 220kias in ground effect in order to make the best angle of climb...kinda kills any takeoff I've made

Note that, so far, the airplane has been in an identical configuration for each case...0 flaps, gear up, engine out...the only difference is possibly some extra thrust at V2.

I haven't looked at the case for jets, but in piston airplanes, Vy is always faster than Vx...I'm assuming the same is true for jets. This, to me, indicates that there is no way that Venr can be Vyse, because it would need to be faster than 220 knots.

I haven't got any reference for single-engine ops, but the climb profile for the 800 is 250kias until intercepting a mach number in the high 20's. Once we're above 10,000 we generally let the airplane accelerate (those airliners HATE being slowed to 250 behind us) to 270 or 280, and once stabilized, we actually get a higher climb rate than we do at 250. Again, the problem is that we climb so slowly while accelerating that we're altitude ahead to maintain 250.

Bottom line, Vx and Vy figures really aren't used in jets, or at least the ones I've flown...performance numbers are a simply a compromise built on the composite angle from "full power prior to brake release" to whatever altitude you're computing.

Fly safe!

David