Vx vs. Vy

[superrav]

had a question for you experienced CFI's...

We all know the difference between Vx and Vy...my question is...Teaching a student to do Vx climbs and gaining enough altitude and being closer to the field or a field that is close to the airport incase of an engine failure at the point that you are over your emergency landing area (if you have one picked out) is wrong? and that climb out at Vy and getting to the safe altitude faster but being beyond your emergency landing area is the correct way to do it?

Here's an example you takeoff to do pattern work, and about .25miles off the end of the runway there is a safe place to put down if you have a engine failure. Your making left traffic and you climb at Vx to gain some altitude so if you do lose the engine at the point you can make a nice gentle left turn and set the plane down...correct?

I'd appreciate all the input i can get ..thanks guys

 

[vja217]

I'm no experienced CFI, but I was taught by one, who taught acceleration to Vy asap after you rotated on a normal departure (assuming not short/soft or with obstical). The rationale for this was better engine cooling as well as better visibility while in a busy VFR pattern. The difference between 80 and 65 kts in terms of pitch attitude is significant in itself. Also, since most engine failures on departure occur at the first power reduction, we would continue to climb at takeoff power in a non-complex aircraft, or wait until 1000-1200' agl in a complex before any power adjustments were made. This way, you ensure safe gliding back to the runway regardless of whether your climb was Vx or Vy, at least in the planes we flew. And also, if you're climbing at Vy, you can trade that extra airspeed for altitude if you identify the engine failure quickly.

They say as a pilot, you are a carbon copy of your flight instructor. That's what I was taught for those reasons, and for now I'm stickin' to it.

Max

 

[tarp]

Super:

Was hard to follow your question, but let's go right to the answer of why Vx?

We teach Vx out of an airport for the mythical FAA short field with a 50ft obstacle at the end of the runway. This is the only time that Vx makes sense. I regularly operate my airplane out of a 1600 strip with trees at each end. There is no "time" to mess with Vy (I'm playing with words here). I need the airplane to gain the greatest possible altitude in the shortest possible distance. This is Vx. I know I'm not climbing as fast as I should be (and I would use Vy in all other circumstances) but my first job is to clear those trees.

In the old days when Mountain Empire down at Wytheville, VA had a much shorter runway, taking off uphill to the southwest used to be the adventure of a lifetime trying to hold Vx and climb over that 300ft mountain at the end. Tough to make an old 150hp C-172 do that.

By the way, once I clear an obstacle, I really have no further use for Vx and the airplane can be lowered to Vy for the afore mentioned increase in altitude per unit of time and to help with cooling on the engine and allowing one a chance to look over the cowling.

PS, the textbook short field takeoff has the pilot rotate smartly to Vx, climb to 50ft and then lower the nose to Vy. This is that FAA 50ft tree that all us old guys used to laugh about.

 

[flywithruss]

Vy is the way to go ...

As a flight instructor, I do feel the need to weigh in on this one. The FAA's logic is (believe it or not!) correct here ... the appropriate use of Vx climbs after takeoff is limited to short-field and/or obstacle clearance scenarios. Otherwise, a Vy climb should be used, in accordance with FARs, POH recommendations, and common sense.

The reasons have essentially been covered ... unless there is an obstacle at a fixed point (like the FAA tree, or a mountain, a ship mast [a la BOS, PHL, etc.]), the increase in altitude over TIME is the prime consideration, regardless of distance covered. In the "Engine Failure After Liftoff" scenario, landing straight ahead is essentially your ONLY option in a single-engine airplane below 500 feet ... whether you are over the airport or the grocery store across the street. In less altitude than that, one can not make the 180 turn for a downwind landing before reaching terra firma.

To illustrate, let's say the engine fails 20 seconds after liftoff. In a Vy climb scenario, I may be at 400 feet just beyond the airport boundary, while a Vx climb could have me at 300 feet in that time, but still over the airport property. So, we're a glider now ... our Vx glider may be over the airport, but it does us no good ... we don't have the altitude to turn around, and there's not going to be enough airport ahead of us to be useful, so we land off airport, just like the Vy glider. Now, the Vy glider has an extra 100 feet (translated to a number of extra seconds based on descent rate, as well as a number of horizontal feet based on glide ratio) to find a place to land. Now, it may not make that big a difference, but it could make all the difference in the world, and if I'm in the hot seat of that glider, I want the extra time, height, and distance ... every extra second is a little more time to pull the proverbial rabbit from the hat.

This is why the Vy climb makes sense from a safety standpoint, to say nothing of the improved visibility and engine cooling arguments, which are good reasons to use Vy by themselves.

When all else fails, fall back on the simple maxim which states that our two best friends in an airplane are airspeed and altitude. The Vy climb gives us MORE of BOTH in less time than we'd get at Vx.

Fly safe, y'all ...

R

 

[avbug]

Why worry about it? Be prepared to execute a forced landing, any time, any place. This question is similiar in thinking to the rationale that one should always be able to land on the runway when flying near the airport. Poppycock.

If you're departing and a good field is off the end of the runway to land in if an engine failure occurs, what to do you do if you experience a failure before or after the point where you would make the field? Easy. Land some place else; deal with it as it occurs.

Should you climb at Vx or Vy? It all depends what you're trying to accomplish. If it's hot out and you're heavy, try climbing at a higher speed than either one; keep that engine cool. The practical use of Vx for an initial climb is limited except for brief periods during training for short field work; climb at Vy or higher most of the time out of a need for safety, and respect for the engine.

Don't let your flying revolve around one single little landing spot...a failure could occur anywhere; the world is your landing spot. Use it.

 

[hyper]

I agree with Avbug's comments. Why worry about it? Be ready anytime. Using the rationale for Vx in relation to being closer to the airport or a field close to the airport isn't really justified. You could have just as good a field farther from the airport whilst climbing at Vy without the temptation of trying to 180 back to the field as well. 2 notes about climbing at Vx when unnecessary; 1) with the growing debate over noise abatement, the prop and engine make much more noise and you stay closer to the ground over a given period of time with Vx. 2) Reaction time and a stall speed buffer in the event of an engine failure.

Factor in an "oh $hit" time of a few seconds where your mind has to tell your body that you just lost an engine on climbout and close to the ground. Most unsuspecting pilots (and some expecting) will not react for the first few seconds because it takes that long for you to believe it's actually happening to you before you react. That airspeed disappears real quick with a high pitch, Vx attitude doesn't it? Add that to the fact that an inexperienced pilot's first initial reaction is to pull up and the rest can get messy. Sure, the same thing happens at Vy, but you've got a few knots to spare while you react appropriately before stalling.

 

[Austpilot]

Sometimes climbing at Vx can be good insofar as getting through an initial cloud layer, low lying band of weather (beware icing)etc after takeoff, at least this is what I was taught once upon a time.

As for Vy, well, this is an important speed for a multi - and time and time again you will hear people saying accelerate and climb at Vy(se), however, think of this, if a donk does fail, you will lose airspeed, by the time you have realised this, the speed will be way below blue line, requiring you to lower the nose to accelerate to blue line with a resultant loss in altitude - doesn't work for me at low altitude.

In a modern piston twin, climbing at 10kts or so above blue line will result in a negligable loss in ROC (at normal weights), however will provide you with a good buffer if one does fail, or at least mean you won't sacrafice as much altitude.

Common sense is really what you need to have in regards to your choice of speed for climb, and as mentioned, be it hot weather, obstacles in the takeoff/go around splay, freezing level,turbulence etc, use your judgement and think of the pro's and cons of the choice of speeds for your operations.

Know your speed schedule and escape route, emergency plan etc inside out before you attempt the takeoff.

 

[172driver]

Slightly off topic but fits well here. Does anyone have an easy explanation of why indicated Vx and Vy converge with altitude? I seem have lost it after my initial CFI and can't find it in the books.

 

[Eric]

I have the same question

I am working on my CFI now and am looking for someone to shed some light on this one. I asked another CFI, and he knew that they converged but couldn't give a good reason why.

For pisotn a/c I know that as altitude increases, the thrust available curve and the thrust required curves shift so the point of 'most excess thrust' (Vx) now lies at a higher airspeed.

Vice versa for excess power (Vy). Those curves shift so 'the most excess power' lies at a slower airspeed. The two converge at the a/c's absolute ceiling.

As for a simple explanation why, I'm at a loss. What causes those curves to shift? Aero for Naval Aviators touches on it and says that these shifting curves due to gross weight, config., and alt. are key to exploring range, endurance, and climb performance, but I need a simple way to explain it to a pre-private (as well as myself ).

Enduring the Florida monsoon.....Eric

 

[TIMP]

172 Drv and Eric,

If you want a simple and practicle explanation without the confusing aerodynamic buzz words like "thrust avail," etc., then here it is:

For a given power setting (lets say full power), as you climb have you noticed what happens to your "angle of attack" and "airspeed?" The air at altitude becomes less dense, which means you need to increase your angle of attack to maintain the climb rate (and power if you have any left, but we are assuming that we are already at full power). So as you keep climbing toward your "absolute ceiling," and continue to increase pitch, you'll notice that your starting to approach the "critical angel" and your "airspeed" is decreasing. As you reach the "CA" you can not increase pitch any farther without stalling. Now, if you started the climb out at Vy you'll notice that there is only one direction to go, as far as airspeed is concened, to maintain the climb.........and that is SLOWER, which means Vy is naturally converging on Vx. Vx is already backed up against the wall and can't really get much slower on the power curve, other wise the aircraft will decend. Now you can see that Vy is decreasing because of aircraft limited performance.

To sum up: Vy can ONLY get slower as you climb because of both engine limited performance and angle of attack limitations. So as Vy gets slower it will eventually meet Vx, who is already at the end of the line. At the absolute ceiling, in order to maintain the altitude you will be flying at the "Critical Angle of Attack" at max forward throttle. Only one airspeed will be available to you for sustaining altitude. If you pitch the nose down to increase AS you will start to decend, if you pitch the nose higher to decrease AS you will stall.

Hope I didn't confuse you.

Timp