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If my speed for best angle of climb is 60 and rate is 79, if I kept a 500 fpm climb to my cruise altitude of let’s say 4500, would those speed change?

The flights are being done on separate occasions.

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  • $\begingroup$ Related: aviation.stackexchange.com/q/73997/34686 , aviation.stackexchange.com/q/53081/34686 $\endgroup$ Aug 14, 2023 at 12:53
  • $\begingroup$ If you are targeting a specific vertical speed and adjusting power to maintain 60 and 79 knots respectively, just know that you are not climbing at Vx and Vy. That should go without saying, but… $\endgroup$ Aug 14, 2023 at 19:43
  • $\begingroup$ I'm sorry, I didn't see the part of the question that reads "if I kept a 500 fpm climb to my cruise altitude". I thought you were just asking whether Vx and Vy changed with altitude, and I answered accordingly. Question appears to be a mishmash of something about how Vx and Vy change with altitude, and something else about what speed you have to fly at to get a 500 fpm climb rate and how that changes with altitude. Needs more clarity. $\endgroup$ Aug 14, 2023 at 19:53
  • $\begingroup$ Do you want to know the speed that gives the best climb angle given that you are maintaining a 500 fpm climb rate? Obviously you will fly at max power, and obviously you get the best climb angle at the slowest speed that will achieve your target climb rate. (So if you can get the target climb rate on the "back side" of the power curve rather than the "front side", you'll fly on the "back side".) Are you trying to craft a question about that particular situation, and how the speed you end up flying changes with altitude? But if so, why introduce Vy into the picture at all? Needs clarity. $\endgroup$ Aug 14, 2023 at 20:01
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    $\begingroup$ It's very confusing to ask something about Vy (speed for best climb rate) in the context of a scenario where the climb rate is constrained to a given value (e.g. 500 fpm). $\endgroup$ Aug 14, 2023 at 20:09

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Yes the speeds would change-- because they must converge at the aircraft's absolute ceiling, where no more climbing is possible. Vy (best climb rate speed) decreases more than Vx (best climb angle speed) increases, but they both change.

If thrust were absolutely constant with altitude, then Vx would remain constant.

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I assume you ask for a piston-powered airplane with constant speed propeller. Next, I assume you want those speeds as IAS, so you need not correct for density effects. Then the grah below shows the general behavior of the optimum climb speeds for different power loadings (ratio of thrust over weight).

Climb polars at different power loadings

Climb speed polars (green) over indicated airspeed for different power loadings. The two black lines show the variation of best climb speed (v$_\text{y}$ - highest point of each curve) and best angle / steepest climb speed (v$_\text{x}$ - best ratio of climb speed over flight speed) with changing thrust. Note that the lowest line is for a glide. Since the green lines are limited by the stall speed, v$_\text{x}$ becomes constant over thrust at high enough thrust levels. The diagram has this oblong aspect ratio in order to make the graphic solution for the best angle speed possible.

If you interpret each power loading with the appropriate altitude, the lines also show the behavior of both speeds over altitude. In all cases the engine is run at maximum continuous power.

Now you want to climb at a constant climb rate. This means you will need to throttle back in order to limit climb speed and stay at the optimum speed at lower altitudes. This means that you will fly at the same power loading and need to adjust power with altitude to keep climb speed constant. This also means that you will stay at the same power loading and both optimum speeds should not change over altitude.

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Both Vx and Vy vary somewhat by density altitude. Check your POH. It should have a table.

It doesn't vary much, though. Somewhere in the neighborhood of 7 kts per 10,000 feet.

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If one wishes to climb at (a rather modest) 500 feet per minute than one may not be constrained to fly an airspeed between Vx and Vy at all.

Why not try a cruise climb.

Rate of ascent will be dependent on throttle setting and pitch to the horizon. A combination of these two will yield 500 fpm rate.

Rather than wallowing along at Vx at full throttle it may be better to fly at Vy, using the throttle to determine ascent rate. Here's where it gets interesting, as lower engine RPM will be more efficient at lower airspeeds (with fixed pitch, to maintain optimal prop AoA to the relative wind).

But if you're only interested in 500 fpm up to 4500 feet, you wish to try an airspeed faster than Vy. On this part of the drag curve, one can slow down to Vy as engine power decreases with altitude to maintain 500 fpm climb.

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