V1, Vr, V2.....again

[some_dude]

One other thing to consider in regard to a discussion of V1 is the effect on takeoff distance. The lower your V1, the shorter the accelerate-stop distance, but the longer the accelerate-go distance. If you increase V1, then obviously your stopping distance is longer and your going distance is shorter.

In most jet aircraft, V1 is set so that the accelerate-go distance is equal to the accelerate-stop distance, thus giving you a "balanced field." This will also equal the shortest absolute runway requirement.

 

[EagleRJ]

Vef - The speed at which the critical engine is said to have failed

Is this a specific number listed somewhere or is it just, you roll down the runway and the critical engine quits and you go, "hey, that makes ___kts Vef!"

Vef is simply an arbitrary speed at which the engine is assumed to fail during certification tests. Vef can be any speed provided it is greater than Vmcg (Vmc on the ground).

Also, if the plane doesn't have a critical engine, do you still have a Vef?

Yes- Vef is established for every aircraft certificated under Part 25. If there is a critical engine, that must be the engine that fails during testing, otherwise, either engine can be used.

Vr - Doesn't every plane have a Vr? A speed which you begin to rotate? Like in a 152 you start to raise the nose at 50kts...when the plane has a sufficient AOA, it lifts off (Vlof). I would think that would make 50kts Vr in a 152 right?

Every plane has a speed that you normally rotate at, but Vr is a regulatory speed determined for Part 25 aircraft. Vr is adjusted so it is neither too slow (stall or inability to lift off) or too fast (unnecessary runway used). Part 25 says Vr can't be less than 110% Vmu (105% Vmu OEI), and it needs to be fast enough to let the aircraft reach V2 by the time it is 35 feet above the runway.

Vlof - Is this also a known speed based on Vr and weight and such or is it another "oh hey we lifted off at ___ knots so Vlof is ___kts!"

Vlof is like Vef. It is simply a reference to the speed at which the aircraft leaves the ground during that particular test. In everyday operations, it varies by a few knots due to rate of rotation, wind gusts, etc.

Vyse - Why is it that we shoot for Vyse if we lose one rather than (if there is one - I'm guessing) Vxse? Wouldn't you want to gain the most altitude in the least space? Avoid obstacles? stuff like that? Or is it too close to Vmc?

You do want to gain as much altitude as possible before a possible engine failure, which is why many pilots fly Vyse as an inital climb speed in light twins. After an engine failure, Vyse will give you the best possible performance after you establish zero-sideslip, clean up and everything. Keep in mind that since small twins aren't required to have positive SE climb performance, all Vyse might do is give you the shallowest possible descent angle. It's important to look at the charts in the POH before you take off to see if you will have SE climb performance or not.
Vxse should only be used if you have an obstacle immediately ahead to avoid. It's usually very close to Vmca, so if you get just a few knots slow, you'll be in trouble.

Vmc - Is this just simply the speed at which you can't put in enough rudder and aileron to stop the plane from rolling or does something worse happen?

Yes, Vmc is the point for both Part 23 and Part 25 aircraft where the differential thrust can overpower the flight controls. Regaining control means reducing power on the operating engine, or lowering the nose to increase airspeed, both of which will reduce climb performance.

Vmu - So this is the speed which the control surfaces become "effective"? So if you pull back at this speed, the nose would come up, but not rotate (eh...the plane wouldn't get off the ground?) ? Is this because the AOA would have to be increased due to the low airspeed and induced drag would be too much? Or is it a different concept altogether?

Vmu is the minimum speed at which the aircraft can physically leave the ground.
If you've ever seen videos of large aircraft like the 747 or 777 doing Vmu testing, it's pretty impressive to watch. At the target speed, the test pilot raises the nose into the air- so high that the tail starts scraping down the runway. The airplane still accellerates and slowly staggers off the runway with a very high deck angle.

Induced "airflow/lift" - Read something somewhere about twins having an "induced airflow/lift" factor regarding the engines. Something about the slipstream from the prop going over the wing and creating more lift??? How does this work? Is it pretty simple or is it more complex than that?

Yes, induced airflow from engines that blow over the wing can delay airflow seperation. In some light twins, you can actually maintain level flight five knots below the published stall speed by using induced lift. Several experimental aircraft have used "blown wings" to enhance STOL performance. Run a Google search on "Coanda effect".

Vmc conditions - Okay, so when you do a Vmc demo, you use all sorts of flap/gear/prop configurations...what would happen if you couldn't feather the prop? I would assume that performance would be severely affected...how bad does it get?

You can experiment with this, doing Vmc demonstration with the prop feathered or zero thrusted vs. windmilling. The difference is dramatic.
Part 25 propeller powered aircraft are certificated with auto-feather systems, and the takeoff performance assumes the auto-feather will function properly. If it doesn't, you probably aren't climbing very fast!

 

[TonyC]

I love the way these threads evolve. You light airplane drivers need to make sure you aren't confusing some things here. Remember, light (under 12,500 lb) aircraft are not certified to the same rules or criteria as the large (12,500 lb +) and turbine-powered aircraft.

Part 23 airplanes (light aircraft) have no guarantees when it comes to takeoff performance. Period.


...


99% of professional pilots, myself included, don't fully understand this topic. We know it well enough to function in day-to-day operations, but misconceptions abound. For example, if I were to ask most pilots to define V1 I would expect the majority to answer that it was the takeoff decision speed. Wrong answer.

Grrrrrr.... I just lost a lengthy response to this post. I guess I'll boil it down to a quick point or two.

Nice job quoting FAR Part 1 - - did you look in Parts 23 or 25?

I ask because you made a big deal about drawing a distinction between Part 23 (you say "light", the FAA says "Normal, Utility, Acrobatic, and Commuter") and Part 25, when the descriptions of V1 in those parts are almost identical to each other. You may be interested, in fact, to see what FAR 23 Sec. 23.51(c)(1)(ii) has to say about V1 for commuter category airplanes:

The takeoff decision speed, V1, is the calibrated airspeed on the ground at which, as a result of engine failure or other reasons, the pilot is assumed to have made a decision to continue or discontinue the takeoff. The takeoff decision speed, V1, must be selected by the applicant but must not be less than VEF plus the speed gained with the 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 application of the first retarding means during the accelerate-stop determination of Sec. 23.55

(emp. added)

I think I'd go a little easy on a guy that called it Takeoff Decision Speed. You can research earlier versions and find the same verbiage with and without the label. The meaning of V1 hasn't changed. They've only removed the label.


Now to demonstrate my ignorance... DO bizjets fall under Part 23 Commuter Category or Part 25 Transport Category?

 

[Donsa320]

Ifgc

4.) Gear down - This is the only thing helping you. The gear acts like a keel on a boat trying to stop the yaw.


I think you misspoke...yes the gear is stabilizing so "Gear Retracted" would be worst case.

And obviously you would not want the act of retracting the gear to cause loss of control, as it might, if "Gear Down" was the demonstrated configuration in determining Vmc

Regards,

D.C.

 

[saltyworks]

one reason that V1 might be much lower thatn VR, in on a wet or snow covered short runway, you would need a low V1 so you can stop, but because of the low V1 there might take a while to get to VR.

 

[IFlyGC]

Donsa,

As wierd as it may sound, the gear is actually left down for Vmc certification. They are trying to simulate a sea level takeoff and not very many people retract the gear so soon after leaving ground effect.

One other thing I forgot last night is the flaps...they are set to the correct take-off position.

 

[Donsa320]

Donsa,

As wierd as it may sound, the gear is actually left down for Vmc certification. They are trying to simulate a sea level takeoff and not very many people retract the gear so soon after leaving ground effect.

One other thing I forgot last night is the flaps...they are set to the correct take-off position.

Hmmm, well, unless you are referring to Vmcg, everything I have found for Vmca has the gear retracted.... to obtain the worst case, highest value Vmca.

I'd like your source for the opposite.

Thanks,

D.C.

 

[Lead Sled]

DO bizjets fall under Part 23 Commuter Category or Part 25 Transport Category?

Tony...
Sorry that I didn't write a doctoral disertation on all of the various nuances of this topic. I tried to keep it simple and paint it with a broad brush so to speak. Sure, the definition of V1 was changed a while back in Part 1, the testing standards were also changed to reflect the change. It used to be the the various tests were run with comparitively new brakes, tires, etc. Now the tests are run with worn (not worn out) brakes, tires, longer recogniton and reaction times, etc - more reflective of the "real world" and the typical pilots that fly these aircraft.

The point that I personally feel important to stress is one of inherant performance capability. "Light" piston twins have precious little to none after the loss of any engine. This is not the case when it comes to transport category jets as long as they're flown "per the book". I've never flown a commuter category airplane so I can't speak to them.

As far as answering your question. I don't know of any bizjets certified under Part 23 Commuter category. Most of them are certified under Part 25 Transport Category, however there are exceptions - back in the 60's, the original Lear 23 was certified to Part 23. There are a few others.

'Sled

 

[IFlyGC]

Donsa,

My initial response was "well, that's the answer I gave on my MEI ride and I passed first time, as did all my multi students" but I did some digging regardless.

23.149 - This requires piston aircraft weighing LESS than 6000lbs to have the gear UP when certifying Vmc and MORE than 6000lbs has the gear DOWN.

23.66 - This is the climb to 5000' requirement for aircraft greater than 6000lbs and also requires the gear to be down (initially at least).

It appears that we're both right, depending on the aircraft in question. Shall we come to a compromise?

 

[Donsa320]

IFYGC

Thanks, yep, we'll compromise. I'da got back to you sooner but I was out flying my Champ...rotated that sucker at 49mph and climbed at 51 'till clear of all obstacles and....and...etc..<big grin>.

D.C.