Wing Vortex Question

Q: Could you explain to me why the vortex does not appear to be coming from the tip of the wing, but rather several feet closer to the fuselage on this Boeing 777?

Boeing 777 vortex – Unknown photographer

A: Good question. The answer is that the vortex is visible where the change in lift distribution -and thus, shed vorticity- is highest. The flaps are extended, which creates a sharp discontinuity in the wing geometry and lift distribution.

Here’s the physics:

  1. The extended flaps increase both the wing area and the effective angle of attack for the inboard wing. (see: Thin airfoil theory)
  2. The increased area and angle of attack increase the lift being generated on the inboard panel.
  3. Shed vorticity is proportional to the spanwise derivative of the lift distribution.
  4. The vortex rotation decreases the local air pressure and temperature below the dew point. Water vapor condenses into a fine mist. We see this fine mist.
  5. The vorticity is transported downstream (i.e. Helmholtz rule #3)

More information:

  • Notice the spanwise lift coefficient is visually displayed with a vapor cloud above the upper wing. This cloud confirms that the spanwise lift coefficient has the largest decrease at the flap tips.
  • You should remember that the entire wing is shedding vorticity. We see the vortex at the flap tip. If the humidity were higher, we might see additional vorticies.
  • Ground effect is responsible for the slight outboard track of the visible vortex. As the aircraft descends further, the shed vortex will likely be pushed further outboard; induced drag (for a given CL) will decrease.
  • It is not true that a vortex is only generated at wingtips or flap tips. Physics demands a smooth lift distribution (regardless of what we see).

A Summer in Greenland (Part 1: Getting there)

In 2019, I spent the summer in Greenland at EastGRIP on the permanent ice sheet. This is a overview of the deployment. The Remote Sensing Center where I worked received funding from the University of Copenhagen’s Niels Bohr Institute (NBI) and NSF to develop ice and snow radars. Our objective was to perform fine resolution ice layer measurements with radar systems mounted on a surface vehicle. At the end of the summer, our project deliverables were: 4 systems built and operated including the first known ice-layer survey in the L-band (1-2 GHz). This was a unique and enjoyable opportunity.

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The Mystery Proceedings

At the end of 8th grade, I anonymously received the 1989 Proceedings of the National Space Society’s Eighth Annual International Space Development Conference.

To this day, I have no idea who sent it.

Someone knew of my interest in aerospace. I have no idea why they picked these particular proceedings, as it was well past 1989. I still have the book. Thanks to anonymous! Sometimes the mysteries of life are never known.

Aircraft Dutch Roll with No Dihedral?

Months ago, an excellent question appeared on the Modeling Sciences sub-forum of rcgroups.com concerning an flight-dynamics aerospace engineering topic: Is dutch roll possible in an aircraft with zero effective dihedral?

The answer is yes.

Simplified Yaw-only Analysis

The dutch roll flight mode shows up in a yaw only behavior driven in frequency by the yaw stiffness Nβ and in damping by yaw damping Nr. A pilot would identify the behavior as a snake dominated dutch roll behavior. With zero effective dihedral, we could also reasonably expect only little to modest yaw-roll coupling through the rate terms, which would be primarily driven by the vertical offsets of surfaces. For the engineers, this simplified 2DOF model of dutch roll has a frequency and damping term approximated as: (derivation)

Interestingly enough, the dutch roll behavior seems to appear even if the aircraft has zero effective dihedral AND zero effective yaw stiffness, provided the product of yaw damping and sideforce derivatives are positive. Both Nr and Yβ are almost always expected to be negative.

Coupled Roll-Yaw Analysis

The dutch roll flight modes show up in higher fidelity dynamics models. The lateral 4DOF model below contains the spiral, roll, and dutch roll modes with sideslip, roll rate, yaw rate, and roll angle perturbation states:

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[Book Review] Failure Is Not An Option by Gene Kranz

Failure Is Not An Option describes the Mission Control career of Gene Kranz, the archetypal spacecraft flight director. The book covers the period from Mercury to Apollo with additional chapters of Gene Kranz’s early life and USAF pilot experience.

This book is a gem in that Kranz tells the story of the people, the machines, and the infrastructure. He clears shows the challenges, the solutions, the trials, the magnificent success of the Apollo program, and how NASA moved on.

I particularly enjoyed the discussions of infrastructure development (e.g. flight computers, tracking networks). Several of his insights were introduced into my own R&D group, as we see similar challenges. This book would be useful for startup managers and those people developing the infrastructure needed for a complex engineering or science program.

Strongly recommended for aerospace engineers and program managers.

“The Kranz Dictum” after Apollo 1

Spaceflight will never tolerate carelessness, incapacity, and neglect. Somewhere, somehow, we screwed up. It could have been in design, build, or test. Whatever it was, we should have caught it. We were too gung ho about the schedule and we locked out all of the problems we saw each day in our work. Every element of the program was in trouble and so were we. The simulators were not working, Mission Control was behind in virtually every area, and the flight and test procedures changed daily. Nothing we did had any shelf life. Not one of us stood up and said, “Dammit, stop!” I don’t know what Thompson’s committee will find as the cause, but I know what I find. We are the cause! We were not ready! We did not do our job. We were rolling the dice, hoping that things would come together by launch day, when in our hearts we knew it would take a miracle. We were pushing the schedule and betting that the Cape would slip before we did.

From this day forward, Flight Control will be known by two words: “Tough” and “Competent”. Tough means we are forever accountable for what we do or what we fail to do. We will never again compromise our responsibilities. Every time we walk into Mission Control we will know what we stand for. Competent means we will never take anything for granted. We will never be found short in our knowledge and in our skills. Mission Control will be perfect. When you leave this meeting today you will go to your office and the first thing you will do there is to write “Tough and Competent” on your blackboards. It will never be erased. Each day when you enter the room these words will remind you of the price paid by Grissom, White, and Chaffee. These words are the price of admission to the ranks of Mission Control.

USS Alabama Battleship Memorial Park

The USS Alabama Battleship Memorial Park in Mobile, AL displays an interesting collection of WW2 ships and aircraft not normally seen. The collection includes several rare museum items. The park is well worth the visit. https://www.ussalabama.com/

The Battleship Memorial Park visit is contained in 3 parts:

USS Alabama (BB-60)

My visit operated under a slightly modified charter, as it was a sponsored program by the BSA and included an overnight “opportunity”. The ship is designed around three 16 inch gun turrets.

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[Book Review] Roskam’s Airplane War Stories

Dr. Jan Roskam’s Airplane War Stories is a seriously good book. The lessons come in bite sized tales (“War Story #”) with the wonderful treat of providing the moral (“Lesson”) in his own words.

This book won’t teach you how to analyze aircraft -Dr. Roskam’s other books take that charge- but once you do know the mathematics, the concept, and the application, this book is a lot of fun to read.

The first part of the book is a bit better than the later part. It is slightly disappointing that the book does not cover as many aircraft design topics with specific discussions of the aircraft features.

Thank you Dr. Roskam for this and other books. The book is available through several sources:

Recommended.