Riley, a CFI candidate’s non-pilot flying companion from Florida, asks: “So what’s rotating when you say ‘rotate’ during takeoff? It seems to me to be more of a pull-back than a spinning motion.”
I have no idea whatsoever. Next question?
What? What’s that? Oh, my editor is telling me that I need to do better than that.
Ohhhhkaaay….I guess we need to start with something called “vee-speeds,” which are performance and maneuvering numbers all pilots need to learn for any aircraft that they fly.
V-speeds are highly variable, airplane specific, and sometimes change with the weight or configuration of the airplane. Some V-speeds are coded with numbers — such as V1, which is the speed at which you can no longer safely abort a takeoff — while others are coded with letters.
Some of the letters make sense, such as VFE for the max speed for extending flaps, but others not so much. VB deals with wind gusts. I don’t know about you, but “B” doesn’t exactly speak gusts to me. Maybe whoever came up with it was a fan of Winnie the Pooh and the Blustery Day.
Anyway, VR is one of those VSPEEDS that does make sense. “R” is for “rotation.”
But that’s where the “sense” breaks down. Because while rotation is one of the most common terms in aviation, and anyone with a pilot certificate (including student pilots) knows what it is, no one — with any degree of authority — knows what it means.
And by that, I mean that all of us know that rotation is the act of making whatever inputs are required in a given airplane to transform it from a fast rolling ground-based vehicle to slow-moving airborne one. And you are correct that it’s a pull-back kinda thing, generally entailing some degree of “back pressure” on the yoke/stick/sidestick, which changes the pitch of the airplane, which in turn allows aerodynamic magic to occur with the wings, which allows the airplane to generate lift (according to whatever theory of lift you personally ascribe to).
But, to your question, what are we actually rotating when we, you know, rotate?
That, no one seems to agree on.
Yeah. This is what I love about aviation.
But here are the three main “camps:”
- The aircraft is rotating around its center of gravity.
- The aircraft is rotating around its lateral axis, alternately stated as moving on its pitch axis.
- The aircraft is rotating on its main landing gear.
Let’s quickly look at the strengths and weaknesses of each theory, starting with the last one first, which is actually a bit complicated and requires some discussion.
The landing gear camp says to fly, one must lift the nose, and to do that, the plane needs to rotate on its mains. This is technically true for tricycle-gear aircraft, but maybe not so much so for taildraggers.
And supporting the notion that rotation is largely a trike kinda thing is the fact that the word “rotation” is not found in older flight texts from the tailwheel-dominated days, at least in the context of a takeoff. The older books largely talk about “liftoff,” while rotation is the standard term in newer books, and modern flight training is dominated by tricycle gear aircraft.
When did that change? I’m not sure, but by the early 1980s, looking at the FAA’s Flight Training Handbook, “liftoff” is still standard, but we also find the statement, “When all the flight controls become effective during the takeoff roll in a nose wheel-type airplane, back elevator pressure should be gradually applied to raise the nose wheel slightly off the runway, thus establishing the takeoff or liftoff attitude. This is often referred to as ‘rotating.’”
It might be possible this same language is used in the 1965 version of the same book, which would be interesting to know, but I don’t have a copy of that. Readers?
Of course, one of the problems with this whole thesis is that the tailwheel crowd also talks about rotating. Still, that could simply be from exposure to the modern term, which has largely replaced “liftoff.”
What about rotation on an axis? To fly, the aircraft, again focusing on the trikes, needs to change the angle of its longitudinal axis, raising its angle on the pitch axis, which is movement on the lateral axis.
Good Lord, no wonder none of us ever sort this out.
Still, in aerodynamic terms, any motion on any axis is defined as rotation, so it makes sense that the displacement of the longitudinal axis during liftoff could properly be defined as aerodynamic rotation.
Lastly, what about movement around the center of gravity (CG), which is technically different from movement around an axis or around the landing gear wheels?
One of the problems here is that it’s arguable whether or not CG comes into play on the ground. CG is generally defined as the point at which an aircraft in flight would “balance.” But of course, at rotation, you are no longer ground-bound…but you’re not really flying either. You are in the nether regions between taxi and flight and the rules and forces at play are…well…murky.
So what does the FAA say? Now, in my defense, I’ve not read everything the FAA has ever written, so I might have missed it, but it would appear they don’t know either or have chosen to duck the subject. When discussing VR in the Airplane Flying Handbook, our Friendly Federal Authorities simply say that rotation speed is the “speed at which back pressure is applied to rotate the airplane to a takeoff attitude.”
They aren’t sayin’ what we’re rotating either.
So there you have it, something is rotating at VR, but your guess is as good as the next pilot’s as to what — specifically — you rotate to take to the sky. I guess it falls into the category of “we don’t need to know everything there is to know to fly the plane.”
“I guess it falls into the category of ‘we don’t need to know everything there is to know to fly the plane.'”
Oh, but speaking of what we know and don’t know, most pilots incorrectly believe that the “V” in V-speed stands for velocity. It doesn’t — at least, not technically.
It stands for vitesse, another one of those French words that snuck into aviation (itself a French word). So no velocity speeds, folks. They’re vitesse speeds.
Of course, in fairness, vitesse is one of a number of French words for speed, although it might more closely be translated as “rate,” while the engineers among us will point out that “velocity” is properly defined in kinematics as having a particular direction, and therefore inappropriate for use in a discussion of V-speeds.
If the French had wanted us to have V-speeds as most English-speaking pilots conceptualize them, we’d have R-speeds instead, as “velocity” in French is rapidité, at least according to Google.
Ironically, we’d then have RR — but we still wouldn’t know what we’re actually rotating.
Simply a misuse of vernacular.
Rotate implies within a certain confine and/or about a given point, horizontal or vertical plane. A gear rotates on it’s own axis. Tires are rotated horizontally within the confines of the vehicles, the tires themselves rotate vertically and individually on their own axis. As used it would be to rotate the nose in relation to the tail, limited to exceeding AOA.
Elevate for liftoff would be a more appropriate term.
There is a rotation during takeoff in either type aircraft, tricycle or taildragger. Anytime motion occurs around an axis contained within the object, or its periphery, that is the very definition of a rotation. In flight, forces that do do pass directly through the CG cause rotations about the three-axes passing through the CG. But on the ground, where aircraft are constrained by contacts with that surface, things like the tires cause additional twisting moments.
In addition to having to consider the net result of all those moments on the airframe and its motion, the ground contact temporarily constrains the liftoff rotation, to an axis that passes through something other than the CG itself. Those constraints are removed upon liftoff, when the ground loses its ability to exert forces. I don’t understand the controversy. This subject is clearly understood by the designers of airplanes. The proof is seen in every successful takeoff.
What makes more sense to me is how subtle is the takeoff in a taildragger, due to less obvious rotations. Were it not for the proper angle of incidence, and some ability of the tailwheel spring to compress, conventional gear aircraft would either remain rolling to the airport boundary, or the first substantial obstacle. Where, in a moment of sudden awkward realization, the pilots would rightly be upset that some engineer had not done his figures correctly.
1965 AFH only uses ‘ takeoff ‘——- no ‘ rotate ‘ anywhere in that section.
If “what’s rotating” gives you pause, i can’t imagine how you’d explain the motion in the Artificial Horizon (AH) insturment.
I suspect once in the air, the elevator causes rotation around the center of lift axis,
The answer to this question is really quite simple, and it has nothing to do with the airplane itself. At takeoff you are rotating the Earth out from under the airplane. This answer applies equally well to both tricycle and taildragger aircraft. You can easily confirm this by looking out the side window as you “rotate”. 😉
Good one. 57 years of flying and now I know.😂
CG definitely comes into play on the ground. Ask anyone who’s ever groundlooped a taildragger.
Now let’s move on to why something would be called a Compass Locator. It’s right there on top of the instrument panel!!
Dad gum, I was staring at it all along.
The word “rotate” in this context, I believe derives from The FAR Part 25 (14 CFR 25) certification requirements for large aircraft (12,500 pounds and up) and turbojet aircraft. V1 (takeoff decision speed – stop before V1, continue at V1 or above)), Vr (rotation speed), and V2 (takeoff safety speed with one engine inoperative after V1) are all defined with specific performance requirements. V2 provides the best performance after the landing gear is retracted when operating with an engine failure beyond V1 speed.
Typical scenario in a jet is for the pilot flying (PF) to have his or her hand on the power levers until the V1 call by the pilot not flying (PNF). At V1 both hands are on the yoke, and at Vr the PNF calls “rotate”, at which point the PF “rotates” the nose upward to a specific deck angle. (This differs from a standard takeoff technique in a light aircraft where the pilot will sometimes apply gentle back pressure on the yoke an allow the aircraft to fly off the runway when sufficient lift is developed. Usually takeoff distance is not an issue as there is is often more than enough runway to land back if an engine fails immediately after takeoff. What happens between shortly after liftoff and 500-700 feet is discussion for another day.) With a side-stick control, the PF keeps a hand on the power levers until the V1 call by the PNF. After that the PF is flying “one-handed” until after Vr. Exactly when the PF resumes control over the power levers in both cases depends on a number of things, including climb power requirements, auto-throttle settings, etc.
Some FAR Part 23 certified turboprops, while not required to meet Part 25 takeoff performance requirements, or to have published charts or tables that mimic Part 25 requirements, nonetheless are provided with performance charts that allow for operations similar to a jet, with the appropriate field length data for both accelerate-go and accelerate-stop distances, and one-engine-inoperative speeds at various weights and atmospheric conditions. When I flew a King Air 200 in the early 1980’s, we did have such charts and operated the aircraft with two pilots, even though it was certified under Part 23 (with a max takeoff weight of 12, 499 pounds) and could be flown single-pilot. Cockpit procedures, takeoff speed considerations, and call outs were similar to the jets that we also operated.
I’m getting a headache !
Never did get the point for V stuff, so I never memorized them. But I have a simple Cherokee 140. There are color codes also, red, yellow and green , ” like keep it in the green” . Their are flap speeds also, “.one notch on down wind, two notch Cross wind, three notch on final” or a no flapper all around the amount of wind blowing on the ground level.. I also flew over 3600 hours on a KC-135 as an inflight refueling operator. The pilots use a different lingo in places, like he calls out S1. I’ve had CFI ask questions about v speeds etc and get P-O when I didn’t know them!!! I own this airplane and I’m also a licensed A&P . I know my airplane inside an out.. I’m Old now , I still own the old Cherokee but I don’t fly any more. Probably cain’t pass a physical I/A/W the FAA RULES. I’ve owned the the Cherokee since about 1979, Just keep putting money into something that I love?????
Even in the PA-28 you have Vfe, Va, Vno, Vso, Vs, Vne, Vy and Vx that are important to know.
I suspect “Rotate” came from the early turbine airliner days where you rotated the plane to liftoff attitude, pushed the dingy button, asked the stew for a cup of coffee, checked that there is still enough runway, and waited for positive rate of climb before telling the co-pilot to lift the gear knob.
You said “ They aren’t sayin’ what we’re rotating either.” Yes they did, “rotate the airplane”.
BTW, you can rotate about an axis and the CG at the same time. Or should the axes always be defined through the CG, wherever it is, and not the physical center of the airplane. That might be more accurate.
Maybe Vb also has a French connection – b for bouffée.
The beauty of the English language as spoken in the USA is that words have many meanings. Words that sound the same may be spelled differently.
Just an example— tires are rotated meanings reattached in a different location on the vehicle. This balances the wear on the rubber caused by the road.
But being in another place is where which is answered by here. Did you hear that?
Truth is what PEOPLE think happened. Facts are what happened.
Jack Webb ( Joe Friday ) said ” Just the facts”.
Of course Joe is coffee and Friday is a day of the week. Weak minds.
Four letter words can be good, bad or evil. Words spoken can mean different, often opposite.
Politicians say “friend” which often means enemy. Call friend because if you called them a dirty, rotten sob they wouldn’t negotiate.
Speaking of witches, which witch? The good witch or the evil witch?
Back in the Stone Age when I went through training, rotate was limited to twins and larger aircraft. Singles were simply flown off of the ground.
FAA & college speak. AKA lawyers and insurance policies.
In the older books elevators were flippers
We all know flippers are either pancakes or a dolphin. 😷😛🇺🇸