question on Vx and Vy

[newcadet23]

If we are flying at high altitude say 6000ft and want to climb to 7000ft why do we need to maintain Vy and not Vx?!

thanks for the help!

[FlyingStone]

Comparing Vx and Vy, Vx is the speed which gives you the best climb gradient (or flight path angle, whichever you prefer) for specific configuration (flaps) and Vy is the speed, which gives you the best rate of climb, but the climb gradient is somehow decreased compared to Vx.

Usually, the only time we use Vx is when we want to clear the obstacles, and climb gradient achieved by flying at Vy isn't sufficient. This usually happens at takeoff, initial climb, or if we have to meet published altitude constraints of a SID in IFR flight (for example, when reaching 5 DME from XYZ VOR, be at 5500ft or above). The use of Vx has its downsides, reduced forward visibility (since nose of the aircraft is pointed high up in the sky) and engine cooling. Most aircraft piston engines used in GA aircraft are air-cooled and depend on the airflow to cool the engine (mainly oil and cylinders). If we reduce the speed from Vy to Vx, the airflow reduces and it may not be sufficient to provide adequate engine cooling. Therefore, we usually climb at Vy or even higher (depends on the aircraft type and the cylinder head temperatures) so that engine temperatures remain within operational limits.

In your case, you are cruising at 6000ft and you would like to (whatever the reason), climb to 7000ft. If you have near obstacles and require steep climb gradient, you obviously fly at Vx, so you don't collide with terra firma, but usually during cross country flight, you want to climb and get somewhere at the same time, so Vy is a good compromise between ground speed, climb gradient and engine cooling. Also, usually ATC requires you to be at let's say FL100 in 5 minutes, very rarely in next 5 miles, so you use Vy to get to FL100 in the least amount of time.

[Intercepted]

Everything as explained by FlyingStone, but I would like to add a third way to climb "cruise climb". This is cruise speed or somewhere between Vy or cruise speed. This gives you a comfortable climb rate without loosing to much of your cruise speed and it will keep your pitch at a level where a good lookout can be maintained. A 6000' - 7000' climb is as typical candidate for this type of climb.

[DB6]

If there is no need to make the altitude change quickly I would fly at neither Vx or Vy, but use a cruise climb i.e. power up a bit/full power, speed more or less cruise speed, nose up a bit, retrim, and let it trickle up there. This has the added benefit, if you are navigating, of not changing the leg timing too much.
If you are talking theoretical it is because, as mentioned, Vy gives you better horizontal speed.

[IO540]

The reality is that engine management takes over and unless one is heading towards a mountain one is not going to change from enroute flight to Vx or Vy just to climb by 1000ft.

Also most planes will overheat the engine if climbing at Vx or Vy for any length of time, and one needs to trim for a higher airspeed to get the engine cooling.

In "traditional teaching" one climbs at Vx until clear of obstacles and then climbs at Vy until top of climb. In practice one climbs at Vx until clear of obstacles (which in most parts of Europe is less than a minute or so) and then one trims ASAP to a speed well above Vy to get the engine cooling.

[dublinpilot]

If we are flying at high altitude say 6000ft and want to climb to 7000ft why do we need to maintain Vy and not Vx?!



As others have said, you wouldn't normally use one or the other for such a climb.

However let's imagine that your only two choices are Vx and Vy.

Vy will result in you having a faster forward speed than Vx(so get to your destination faster than Vx), and have a faster climb rate than Vx (so got to the desired altitude quicker than Vx).

So to turn your question back at you, why would you ever choose to use Vx unless you needed to clear an object in a given distance (not time) which wouldn't be cleared by using Vy?

I hope that helps to answer your question.

[SNS3Guppy]

6,000' is "high altitude?"

[Genghis the Engineer]

6,000' is "high altitude?"

It is to most English based light or microlight aeroplane pilots, we get a nosebleed (or worse, end up in controlled airspace) above 3,500ft.

G

[SNS3Guppy]

I see. Where I learned to fly, the field elevation was five thousand, and the density altitude in the summer was closer to 9,000' to 10,000', for the takeoff. Local terrain was above 12,000'. I think it was ten years before I ever flew close to sea level.

Vx and Vy are functions of power; in the light normally-aspirated airplane, power decreases with altitude, and the values of Vx and Vy change, as well.

I suspect that the original poster may be thinking of published sea level values of Vx and Vy, and asking why these aren't used at altitude. By the time one reaches 7,000', the values for Vx and Vy are coming closer together.

Cooling is one reason, as IO540 noted, though with cooler temperatures, cooling isn't as critical at 7,000 as it may be at sea level on a warm day. Cooling isn't as effective, however, because of reduced flow and a higher angle of attack with reduced power, lower air density, etc. For that matter, it's often advantageous to climb most light airplanes at speeds well above Vy; during a hot day in Las Vegas (USA), for example, 100 is the minimum speed one should climb, for engine cooling. The original poster is from UAE, apparently, and will see similiar temperatures in UAE during the hot season.

If climbing to altitude quickly is the mission of the flight, then Vy as far as one can take it is the answer; getting the maximum altitude available in the minimum amount of time. If, however, getting from A to B is the primary mission, then the best forward speed one can get is the order of the day. One doesn't necessarily need to climb as fast as one can, because climbing isn't the primary goal; getting there as quickly as possible is usually the goal. Otherwise, you might walk.

If you're in cruise at 6,000 and wish to climb to 7,000, you're be much better off with a cruise climb. You want to keep as much speed as you can, and climb at a minimum value to get to the next altitude. Generally five hundred to a thousand feet per minute is the desired rate. You're not going to get that in a light airplane like the 172, so get a climb going, whatever you can manage, and accept a slight speed loss in the process.

If you're cruising at a reduced power setting (not always reasonable in light airplanes), then you can keep your cruise speed and simply increase power, because the technical climb function is excess power beyond that necessary for level flight at a given angle of attack (airspeed, in this case).

If you're using full power in cruise, which is often the case in light normally-aspirated airplanes, then you're probably going to need to find a compromise between best forward speed, and a reasonable rate of climb, taking care not to slow down too much.

[FlyingForFun]

Some excellent answers already, not much to add... except that one or two people are making the (all to common) mistake of thinking that what works in one aircraft/scenario will work in all aircraft/scenarios.

In many types, it is true that you would normally use a cruise climb, with a speed well above Vy, when climbing en route. But there are also plenty of types with low-powered engines, where engine cooling is not a problem but climb performance may be, where making every climb Vy (unless Vx is called for) might be appropriate.

I can also think of examples where Vx would be appropriate for an en-route climb. Two that immediately spring to mind would be climbing through a gap in clouds, where Vx might be required to maintain VMC through the climb; and a climb to comply with an ATC requirement such as "cleared to enter controlled airspace level at 7000 feet", where you are perhaps too close to the boundary of the airspace to be able to be at 7000 feet in time unless you climb at Vx.

One size does not fit all!

FFF
------------

[BackPacker]

because the aircraft will only climb at one speed.

Well, actually, at absolute maximum altitude, the aircraft will not climb anymore. But in that situation you're flying at Vx which now equals Vy, with full power, and that's the only speed that will allow you to sustain that altitude. Any slower or any faster and you descend. For that reason the absolute maximum altitude is virtually impossible to reach and is of theoretical interest only.

The service ceiling of an aircraft is defined as the altitude where the aircraft is no longer able to sustain a certain minimum rate of climb. (Wikipedia suggests 100 ft/sec). At this service ceiling there's still some excess thrust so there's still a slight gap between Vx and Vy.

But you are right: The higher you go, the closer Vx and Vy will become. It's just that for various practical reasons you will never be able to actually fly at the point where Vx and Vy intersect. (Unless you have infinite fuel and infinite patience I guess.)

Ceiling (aeronautics) - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Ceiling_%28aeronautics%29)

[IO540]

As noted by Silvaire1, to reach the operating ceiling one will finally be climbing at something like Vy, which is itself close to Vbg (best glide speed).

I've taken the TB20 to FL200 a few times and there is really only one way to get up there: leaning all the way up in a constant-EGT climb and at the very top watching the IAS very carefully, and at the very top of the climb the stall warner is intermittently coming on. ISA +/- a few degrees makes a significant difference too.

Takes a good half an hour...

But I don't think the OP was interested in this :)

[dublinpilot]

The service ceiling of an aircraft is defined as the altitude where the aircraft is no longer able to sustain a certain minimum rate of climb. (Wikipedia suggests 100 ft/sec).

Ha! I think by that defination, most SEP's would have reached their service ceiling at sea level!

I think the more normal defination of service ceiling is 100ft/minute (...though I have 50/min in the back of my mind for some reason) :p

[BackPacker]

Okay, you got me. Good reminder not to post anymore after midnight...

[IO540]

I vaguely recall 100ft/min and 200ft/min by the CAA and FAA respectively, or vice versa, or something like that. Which is why my TB20 will go to 20k with a G on the side but only 18k with an N on the side :)

[SNS3Guppy]

But you are right: The higher you go, the closer Vx and Vy will become. It's just that for various practical reasons you will never be able to actually fly at the point where Vx and Vy intersect. (Unless you have infinite fuel and infinite patience I guess.)

One can easily fly above the published service ceiling, and easily above the absolute ceiling for the airplane. One only needs a source of lift.

I've flown Cessna 150's well above the published altitudes; in fact I've flown them to what used to be the base of the Continental Control Area (14,500' in the USA), and to what's now Class A airspace (18,000 in the USA), or formerly "Positive Controlled Airspace." Convective lifting action and oxygen, and a few hours to kill over Kansas. No problem. It was a part of an experiment, and a successful one.

Bear in mind that while service ceiling is defined by certification standards (max sustainable 100 fpm in single engine airplane, and 50 fpm for multi engine airplanes on one engine for single engine service ceiling), absolute ceiling is the point where the airplane can no longer climb under it's own power. This is an environmental question, given temperature and leaning of the piston engine.

Various other types of ceilings also apply.

[Mark1234]

Quite. On one particularly hot (40deg C at sea level) day in Aus, with a PA28-161 at near MTOW and serious thermal activity I found the only way to climb meaningfully was to dolphin fly - the thermals were clocking around 10kts (1000fpm approx), up and down. Needless to say in the down bits the climb rate was negative!

Push the nose down and fly fast through the sink, haul it up and go as slowly as possible in the lift - maximising time in rising air, minimising time in sink. I did draw the line at standing it on a wingtip and circling in deference to the poor passengers who were already melting :) We eventually got to 9500 where it was still something like +18, but a lot more comfortable - and smoother!

[BackPacker]

I found the only way to climb meaningfully was to dolphin fly

I guess glider pilots never forget their tricks, do they?

I wonder if a MacGrady ring would exist for the PA28...:ok:

[Big Pistons Forever]

For PPL I teach the following rule of thumb. After lift off accelerate to Vy and hold untill 1000 AGl and then transition to a cruise climb which is that speed which will maintain a rate of climb of 500 feet per min for the existing conditions. This speed must be at least 10 kts above Vy for engine cooling and forward visibility. If a speed lower than Vy + 10 is required to maintain the 500 FPM than reduce then reduce the rate of climb to preserve the Vy +10. When the aircraft ROC has reduced to 100 feet min at Vy + 10 (corrected for altitude !) you have reached the practical operational service ceiling for that flight.

For descent I tell my students to leave the throttle at the cruise RPM value and set and trim a descent attitude which will give a 500 FPM rate of descent, monitor the ASI so as not to exceed VNO or Manoevering speed (if applicable). Enrichen the mixture slightly and reduce the throttle as necessary as the aircraft descends in order to maintain the same cruise RPM setting. As the destination is approached reduce the airspeed to an appropriate value to enter the control zone/circuit.