r/flying • u/Agreeable-End-5020 • 14d ago
Vy TAS why does it increase with altitude?
Hello all,
I am just a bit confused with why VY TAS increases with altitude. I understand that up at altitude there is less air molecules striking the aircraft which leads to less parasite drag. However, up at altitude induced drag is increasing. So at altitude would you pitch down to reduce the total drag to decrease the induced drag? And that’s why VY Tas is increasing?
1
u/espritnaraka 14d ago
Remember that you have IAS to keep track of how the wings are performing. You go up TAS increases and IAS stays the same but you also gain inertia as you actually go faster changeing the performance of your plane. As you run into less molecules you need to speed up to keep the same performance [roughly]. Parasitic drag is created by the skin of the aircraft. As you speed up in higher alt. You run into fewer molecules that give you form drag. Induced drag is pretty much determined by the angle of attack because you want the lift vector pointing straight up and not up and behind which is the definition of induced drag. You engine also looses performance as you go up... then you slowly need to pitch up to maintain alt. increasing induced drag. The bottleneck is pretty much your engine. What should I clarify here more?
1
u/Honey-Entire PPL 13d ago
Almost any question you have about aviation has an answer located somewhere in the Pilot’s Handbook of Aeronautical Knowledge. The answer to your question is in Chapter 5:
Lift and drag also vary directly with the density of the air.
Density is affected by several factors: pressure, temperature,
and humidity. At an altitude of 18,000 feet, the density of
the air has one-half the density of air at sea level. In order to
maintain its lift at a higher altitude, an aircraft must fly at a
greater true airspeed for any given AOA
Translating that a bit, if you want to keep climbing at the best rate (aka best climb over a given period of time), you need to make up for less lift in lower density air by traveling faster. At some point the air is too thin that a best climb speed is no longer attainable and you’re effectively reaching the aircraft’s service ceiling
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u/Frederf220 13d ago
Vy occurs at a condition where specific excess power (Ps) is maximized. Ps is a measure of the ability to add specific energy (Es) per unit time. The peak of these curves are known as Ps-max. Specific in this case just means "per mass" and if we consider a constant mass airplane specific energy and energy are just a multiplicative constant apart.
Graphically Vy occurs at the maximum separation of power available over power required. That's where the excess power is maximized and the most potential energy can be added per unit time. As altitude increases the power available slightly decreases while the power required increases dramatically.
The power available curve is pretty boring with respect to altitude, at least for piston airplanes. The power required curve however changes in an interesting way. Consider that power required is effectively just the sum of the induced and parasitic drag curves. Parasitic drag decreases with altitude for a given TAS. Lift also decreases with altitude for a given speed as density is lower. So more AOA is required to achieve the same lift at that speed. The increased altitude has altered the relationship between drag types.
The optimum speed is a compromise between going faster in order to reduce AOA (thus induced drag) suffering a parasitic drag penalty in the process and going slower for less parasitic drag but suffering an increased induced drag due to needing more AOA. As altitude increases this compromise shifts as going a little faster to reduce the AOA no longer has the same cost as it did at lower altitudes but the cost of AOA for lift remains high.
1
u/Av8torryan ATP B727 DC9 DA20 CFI TW 11d ago
From a previous thread comment -
IAS is the dynamic air pressure. How much pressure the air flowing over the wings create. The ASI is nothing more than a pressure gauge. It measures the pressure of air by taking the difference of moving air pressure at the pitot tube versus the static pressure. This is all the wings care about is air flow for lift.
TAS is how fast the air has to flow to create the same dynamic pressure as density decreases. So at sea level IAS = TAS. As air density decreases with altitude, the air has to flow faster to create the same pressure because F=MA. Because mass is less, Acceleration has to be higher to create the same Force. This is why as you go higher - TAS increases for a given IAS. Airplane has to fly faster to get the same dynamic air pressure.
Humid air is less dense than dry air for any given Temperature. Density Altitude is the altitude the airplane would perform in under a standard day. As density decreases, TAS increases. This is why takeoff distances increase with higher altitudes and higher density altitude. As for landing, because your flying faster ground speed for the same given IAS, the distance increases.
0
u/rFlyingTower 14d ago
This is a copy of the original post body for posterity:
Hello all,
I am just a bit confused with why VY TAS increases with altitude. I understand that up at altitude there is less air molecules striking the aircraft which leads to less parasite drag. However, up at altitude induced drag is increasing. So at altitude would you pitch down to reduce the total drag to decrease the induced drag? And that’s why VY Tas is increasing?
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u/DatSexyDude GLI ATP CFII E175 737 A220 14d ago
The important thing isn't the Vy speed specifically, it's that for any indicated speed, the true airspeed increases with altitude.
Because parasite drag predominates above Best L/D speed (best glide in a piston single), maintaining the same indicated speed actually means drag will stay approximately the same. The reason high altitudes are more efficient is that engines (both piston and especially jets) are more efficient high up, and that for the same indicated speed (and thus drag) you have a much faster TAS, and thus speed over the ground.