Why Vx and Vy Change With Altitude

Recently, someone asked about why Vx and Vy change with altitude. This isn't necessarily an easy thing to understand since it involves several variables. So here's a modified "quick" version of the explanation on this topic that is covered in great detail in my “How to Fly an Airplane Handbook.”

Figure 43 shows the power curve vs. rate of climb for three different altitudes. The top of the power curve represents Vy and the tangent’s contact with the power curve represents Vx. The speeds at the bottom of this curve are true airspeeds which makes it easier to represent performance at all altitudes.

Notice that Vy and Vx both increase with an increase in altitude as true airspeeds. Now just ask yourself what indicated airspeeds are necessary to generate the true airspeeds shown for Vx and Vy in Figure 43 (use a 2% increase in TAS for every thousand feet of altitude gain). I’ve done this for you in Figure 49. It’s clear than Vx and Vy converge with altitude when measured as indicated airspeeds.

Clearly Vy increases as a TAS with altitude (Figure 43) but it does so relatively slowly compared to Vx as a TAS. Therefore, Vy as an indicated airspeed can decrease by 1% per thousand feet (Figure 49) while still resulting in the slight increase in Vy as a true airspeed as shown in Figure 43. That’s why the Vy (IAS) line tilts to the left slightly in Figure 49.

To learn more about this and other topics and have fun in the process, check out my "Rod Machado's How to Fly an Airplane Handbook." It covers both basic and advanced flying skills, stick and rudder skills as well as lost flying techniques that are not taught in flight schools today. Learn how to master your airplane by reading this book.

By Rod Machado | | CFI Resource Center, Learning to Fly | 0 comments
next post → ← previous post

Comments

Leave a comment

Stay in Touch

Physical Product Ordering Only (800) 437-7080

If you'd like to order a PHYSICAL product by phone, please call the number above. Digital (downloadable) products can't be ordered by phone.

Latest Posts

  • Rod's Letter to the FAA and ACS Committee Members

    Yes, really! The Private Pilot ACS adds 363 Risk Management items to the PPL practical exam testing requirements. Students are responsible for knowing the answers to each and every one of these items, all of which require highly subjective answers (according... read more

  • Minimum Cost Private Pilot Certificate

    Greetings Folks:Below is a recent letter from a young man named Joel Thomas. Joel earned his pilot certificate at a very low cost using many of the recommendations I've made over the years. Yes, it's entirely possible to earn a... read more

  • Why Vx and Vy Change With Altitude

    Recently, someone asked about why Vx and Vy change with altitude. This isn't necessarily an easy thing to understand since it involves several variables. So here's a modified "quick" version of the explanation on this topic that is covered in... read more

  • How to Flare Any Airplane Any Time and Anywhere

    Princess Buttercup and I were walking on the Redondo Beach pier last month and unknowingly stumbled onto the “live” movie set of Big Momma 2. As I passed one of the props, an ice cream kiosk, I stopped to buy... read more

  • The Airman Certification Standards

    As most readers of my blog know, I'm not a fan of the new Airman Certification Standards (ACS) for many reasons, not the least of which I originally posted with the FAA in 2013 (click here to read that response). Apparently, the... read more

  • Risk Management? Really?

    Is the Emperor Naked? It Sure Looks That Way The illusion here is based on the FAA’s belief that student pilots can be taught risk management skills. Learning to manage risk, however, requires prerequisite knowledge that student pilots typically do... read more

  • Hot Props

    Crocodiles and Propellers - Not Much Difference Sometimes we need to be reminded just how dangerous a propeller can be.  The first video below shows a fellow pulling a propeller through to check for nicks when the engine started and... read more

  • Striving for Too Much, Too Soon - Fantasy Flight Training

    Early in the previous decade, the FAA began heavily promoting a flight training concept known as scenario-based training (SBT). SBT was billed as a highly structured script of real-world experiences to address aviation training objectives in an operational environment. Hailed... read more

  • Rod Machado's "FREE" Private Pilot Flight Training Syllabus

    I'd like to offer you two different syllabi (FREE). The first is my Private Pilot Flight Training Syllabus. This is intended for use by the flight instructor as well as the student. Both should have a copy for their own... read more

  • Flight Instructor Training Resources

    I'm often asked about resources for CFI applicants. Here are a few resources that you might find useful. When I discover more, I'll be happy to list them here.  Rod Machado’s Private Pilot Handbook - A  must have book to help... read more

  • In Defense of Stick and Rudder Training

    It happened in the early 1990s. That was the time we saw the diminishing influence of WWII flight instructors (and their instructional progeny). Our pilots didn’t fly jets during that war. Instead, they flew airplanes that demanded exceptional stick and... read more

  • Bad Instructors: How They Behave and Why You Should Know

    I'm curious about your experience with bad flight instructors and hope you'll add to this blog with  your comments. Of course, most flight instructors do a fantastic job. I just can't say that LOUD enough: Most flight instructors do a fantastic... read more