Aviation Safety Corner
this month's topic, I would like to continue our discussion on, "Initial Climb Strategies." Exploring
the pros and cons of using your airplane's best-rate versus best-angle climb speed after liftoff. This is Part
II of IV.
Now, let's discuss, "Mysterious Relation." So, what is Vx in relation to stall speed? Well, nobody really can tell from their available printed material.
Even your AFM, owner's manual or pilot's information manual is never very specific concerning the relationship of the power-on, best-angle of climb air-speed to either the power-on or power-off stall airspeed. One thing is for sure: Vx is always slower than Vy until you have climbed to the absolute ceiling of your airplane. At that (high) altitude, Vy and Vx are the same IAS (indicated airspeed).
Vy is at its highest IAS value at sea level. The IAS for Vy drops slowly as altitude is increased. Vy is the airspeed at which there is the most excess horsepower available to give the best climb rate. In other words, Vy is at the peak of excess-power curve or approximately at the "bucket" of the power-required curve. If you climb at a faster airspeed than Vy, you are on the "front side of the power curve." You will get more airspeed, but reduced climb rate.
Climbing on the front side of the power-required curve is safe, in that if you hold the pitch attitude a little too nose-low, you'll lose some climb rate, but you'll have a higher IAS and a greater margin above stall. A cruise climb is on the front side of that power-required curve, and the airplane will perform normally should you then pull the nose a little higher. The airspeed will decrease, but the climb rate will increase. Remember that climb rate is dependent on excess horsepower. Not so when you try to climb at Vx. Climb angle is dependent on excess thrust available (not excess horse-power). For a propeller airplane, that point occurs at a lower airspeed than the "bucket" of the drag curve. This occurs because propeller thrust decreases with an increase in airspeed.
So, now, at Vx, you're on the backside of the airplane thrust-required curve and into the region of reversed command. If you pull the nose even a smidgen higher, you'll increase drag; you'll not only lose airspeed, you'll lose both climb rate and climb angle. And you may arrive at power-on stall speed.
Note: The proximity of Vx to stall speed (Vstall). Remember that climb angle is dependent on excess thrust. I'm not going to delve into the math derivations and calculations for this climb rate vs. climb angle determination. If you're interested in the math and have a copy of H.H. Hurt's textbook, Aerodynamics for Naval Aviators (NAVWEPS 00-80T-00), look up the section on climb performances.
Now, let's discuss, "Vanishing Horizon." While you're trying to climb Vx, the airplane's nose will be so high that you can't establish a good visual pitch attitude, because you won't be able to see the horizon ahead. You won't have a good control of airspeed. Should a wind gust cause airspeed to drop just a few mph or knots, you might be right into stall onset. And, of course, should the engine quit or lose partial power with the nose way up there, and with the airplane already close to power-on stall speed, you may find yourself immediately into a power-off stall!
Another concern: At the slow airspeed for Vx, your aileron control will be at its weakest. Rudder and elevator/stabilator control probably will still be good so long as the engine is at full power, but the effect of P-factor will likely mean that you aren't using enough right rudder to hold zero side slip at this high AOA (angle of attack). Any sideslip angle, plus an inadvertent stall, will probably mean the beginning of a spin and complete loss of control. Those "weak" ailerons may not be able to counter any sudden roll, but they'll still be able to provide lots of adverse yaw into an incipient spin. (This might happen when you're so close to the ground that accident investigators will not detect, from the wreckage, the incipient spin condition).
Think about the last time you last "practiced" stalls (hopefully, with a good CFI who could critique your technique and keep you out of trouble). Try to remember that departure stall, the one you did with lots of power from the engine(s) and at an IAS just a little above where you normally lift off. You had to keep pulling the wheel/stick back. The nose kept getting higher. You couldn't really see ahead. The airspeed was slowly dropping. You probably were too busy to watch the altimeter and VSI (vertical speed indicator), which were trying to tell you that you were on the backside of the power and thrust curves, with decaying climb rate.
Now, in conclusion, remember you were getting close to stalling and, maybe, even going down before you heard the stall horn or felt any buffet. That's what you are asking for when you try to fly a Vx climb after takeoff!
Don't miss next month's Aviation Safety Corner, when we'll discuss, "Cutting the Margin" and "Trees in the Window". This will be Part III or IV.
Larry G. Harmon
FAA AVIATION SAFETY COUNSELOR
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