Turn Performance, Cornering Velocity and Energy Management: The Voices of Experience

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Aerodynamics, as it relates to high performance maneuvering, is especially important. There is no question about it. In the warbirds, you are flying high performance military airplanes that are not certified. They are not designed for the average Cessna pilot. They are designed as military war fighting machines.

So, aerodynamics and turn performance have even more significance than in flying a Decathlon, for example, or a Pitts. You are working in the same basic altitude spectrum: 250 feet is 250 feet. And yet, 250 feet in a jet going 500 knots is different from 250 feet in a Decathlon or an Extra. Things just happen faster.

Strong, basic aerodynamics aren’t really addressed in the normal certification process. So, when you take your commercial flight test, cornering velocity is not an issue. You have to go in [on your own] and sort of get that and learn it and understand how it applies to your airplane.

We all talk about the reverse Cuban 8 in a high performance airplane. It’s a good reversal or turnaround tool. Once the airplane is in the vertical or near vertical, you are totally committed. So, how does somebody learn to do that? You don’t just go up and say, “Well, here are my charts and here are my graphs. Now, at 1800 feet above the ground, I am going to roll and I am going to pull and it is going to work out fine.”

You’ve got to start up at 5,000 feet and say, “Okay, it is 5,000 feet DA. What does it take? How can I do that?” And, in my world, I’ve got my numbers, not just because I calculated my numbers, but because I went up and proved my numbers, in my style of flying and in my airplane. Then I pad those numbers, and from that pad, I put an additional pad.

I never pull to the buffet. But, if I have to pull to the tickle of the buffet for max coefficient of lift, I now have flaps available, which will make a substantial difference in the rate and radius of the airplane. So I’ve got two safety margins built in. But that was all determined by going up and doing it and practicing it under all conditions. So, you cannot do this by getting into a graph in the P-51, and then bring it down and expect it to work well. The calculations are a guideline. They are not the bible.

When the ground is approaching, are you looking at your altimeter or looking at the ground? You are looking at the ground. How do you learn to do that? It comes with years of experience and seeing it in less critical areas. That is, starting someone at 800 feet, then 500, 250, whatever. I just think that, in a lot of senses, the incoming air show guys are too impatient to get where they are trying to go.

Another question is, “What is the stall speed of your plane?” 70 knots? Great, but at what G? Because you can stall a plane at any G. If someone comes back to me and says, “It is the square root of the load times the one G stall speed,” they are already out on the far end of the knowledge base.

This stuff is essential for warbird training because these planes have no stall warnings, bells, buzzers, beepers, whistles, shakers, pushers, pullers, all those things that are on normal airplanes.

I went up and looped a Mustang with a guy. We went up and set a bench line altitude of 10,000 feet and we entered the loop at 260, then we entered the loop at 250, then 240 and kept taking down a point over and over again. Even though I could calculate that if I sat down with a super computer, it is incredible what I learned just by going up and doing 40 loops in an hour and a half, all being documented. Then we came down and put the data on the board.

“Okay, what are we looking at here? Wow, there is a learning curve.” And it pretty much supported the formulas and things. But it was a wakeup call even for me. This stuff is pretty interesting. But does the average person go up there and look at that? 

Dale Snodgrass

Cornering speed is an important number for fighter pilots. Basically, it is an indicated airspeed in which you have knots sustained turn rate. At a given airspeed and a maximum thrust, you can sustain your best G loads. It is not necessarily your best turn radius, but it is your best sustained turn radius. It is all about sustained energy, not instantaneous energy.

Instantaneous turn rate is based on just putting the stick in your lap, essentially, and generating as much nose movement as possible. But when you do that, you are obviously going to lose energy. Your airspeed is going to diminish. So cornering speed is an airspeed in which you can sustain your turn rate, which means degrees per second.

In high thrust to weight airplanes, fighters and stuff, we use cornering speed to compare airplanes. We use that as a tool to fight other airplanes, because — given a wing, and thrust to weight ratio — the cornering velocity of airplanes may vary significantly. Like for instance, in a MiG 21, the best cornering velocity is like 450 knots. In an F-14, it is 325 knots. In an F-18, it’s about 385 knots. In an F-16, it’s 425. So, that is where a combination of thrust and wing loading meet for a best sustained turn rate.

But, in the air show business, it is a number which is lost on a lot of people, because — in the majority of planes we have — the cornering velocity is semi-immaterial. The airplanes don’t have the thrust to weight to sustain the turns. Or it is so low that it doesn’t really matter. In other words, if you are in a Decathlon, your best sustained turn rate is probably at 110 knots. But your G loading at that speed would probably be less than 2 Gs. So everything is sort of instantaneous Gs. It is kind of weird, because, down low, airplanes like a Pitts — or even an Extra — turn so well at slow speeds that it is not as critical a term as it is for a guy flying a more high performance fighter-type airplane.

Obviously, there are air show airplanes out there with high thrust to weight values, but — at the same time — their turn radiuses can be very low. It’s not a big factor unless you are flying a P-51 or a war bird or something like that. Then it is a little more applicable. But, even in those airplanes, the thrust to weight is so low that the G loading at sustained turn rate is at whatever speed the plane turns the best.

The best would be, if you are at full power, what is your best airspeed to complete a split S? That is probably a more applicable number.

If I was setting it up in a low energy airplane, which is pretty much everything on the air show circuit, what is my [turn radius] minimum altitude to execute a Split S?

I’m transitioning from this to something that is more applicable for us. Then that number is driven by very critical things. It’s driven by your entry airspeed. So let’s assume that, if you entered a split S at Vne, your turn radius is going to be a lot more than if you entered at 110 knots and got up to 130 or 140, where the plane does its best turn rate. Cornering velocity is the airspeed at which a plane turns the best, and you are using gravity to help you do that. In other words, if you are doing a nose-low turn or even a split S, then it would be nice to know what that number is. And is that a full power event or a partial power or an idle event?

The problem is that, in a lot of these airplanes, the manufacturer doesn’t generate those kinds of numbers, like they do in a fighter, obviously. You don’t get a tactical manual with your Marchetti or your Extra 300, where you have all your massive amounts of energy management graphs. In a fighter, you have massive documents a couple inches thick with all that information that you pore over and absorb and apply when you operate the airplane tactically. They tell you exactly how your aircraft functions with airspeed and G in various configurations at various altitudes and thrust levels, whether you have drop tanks on, how many bombs and so on. But, in our case, in these airplanes, there is zero point zero for that.

The T-6 didn’t have that and the L-39 doesn’t really have it, and the Sabre, and the older jets didn’t have it either. It sort of came on later. So, I basically went up to altitude; I knew I could get to 10,000 feet in an F-86 and 5,000 feet in a T-6. Then, at a couple different airspeeds, I would do full power and idle-power split Ss and said, “Okay, I know exactly what my mins are now.”

If I’m on top in an F-86 and I’m at 130 knots, I can get through this. I can pull through, if I have to. When you get into your performance and are practicing down low, then you cross check your numbers both on the entry of a maneuver and at the top. So you know you have the correct energy package to execute the maneuver, then you cross check your altitude and airspeed at the top, so you always know you have an out.

So, for instance, if I’m in my F-86 and I’m coming over the top, if for any reason maybe I am at 2400 feet, because I’m trying to push it because of clouds, or something. I’ve got an undercast or an overcast I’m trying to squeeze below and I end up at 2,400 feet. I know that I’m not going to make it. It is too close. Then I execute a half Cuban kind of thing or I come out nose low, oblique away from the crowd or something to make sure I have the turning room to miss the ground.

And that is something in cornering velocity that goes back to another fighter term that is called turning room. So, what does that mean? I have the turning room in the vertical to make sure I can get through a vertical maneuver and recover above the ground.

There are a couple classic cases of not having enough turning room. These were jet guys. One was a guy in an F-86. He decides to do a reverse Cuban. He enters real fast. He is going like 500 plus knots. He pulls up. He goes to like 3,000 feet. But he is still going 350 knots and he commits nose low to do the reverse Cuban, which — once he rolls inverted and pulls — now it is a split S, by definition. And his energy state is so high that his turn radius/turning room is so big that it doesn’t meet his altitude requirement. Once you get the nose committed 90 down, you’re in a total commitment event and there is no option. So all you can do is pull, and he is going so fast that his turning radius is so big that that he didn’t have the turning room to recover, so he kills himself.

Same thing with both a Thunderbird and another F-16 demo guy doing an exact similar thing, a reverse Cuban and didn’t have enough altitude to pull through. They don’t have enough turning room and they hit the ground. So, turning room for all of us is very critical. We all instinctively know about it, particularly those of us who have been around. But I’m not sure how many people actually figure out what it takes to do a Split S, and how airspeed and power impact that Split S.

If I’m full power in an AT-6 and I’m at 180 knots and I do a split S, what is my turn radius, versus if I’m at 110 knots and at idle power as I come down? That magic airspeed, that original corner number, corner velocity, varies. It is hard to determine with little airplanes, it’s sort of a wider band in most cases. But, if I’m in an F-14 or I’m in my MiG 17, if I’m at the top pulling through and I wind up at 350 knots, for whatever reason, I’d be at idle and boards on that pull. If I were at 150 knots, I’m better off being in full power, because I’m adding energy to the airplane and keeping the airplane closer to where it turns the best.

So, it is just a function of trying to figure out those numbers that work for your airplane, then you cushion in that with everything. So that in my flying, I actually fly very low, but if I’m coming out of a loop or a reverse Cuban or a split S or something, the last 6, 7, 800 feet I would try and feather it. It’s not a maximum turn radius event. You are already through. You could be through the maneuver at, at least, 1000 feet. In a jet, it might be 1000 feet above the ground. The last 800 feet or so, you are just driving it toward the ground. You are feathering it down there. In other words, you are driving it to a point. You’re not having to pull hard just to finish the loop.

For instance, I can loop the T-33 in 2,600 feet, if I do everything right. But when I normally do the loop, I want to see like 3,500 feet or even 4,000 feet at the top. Then I can float it over the back side and pull the power. I can manage my airspeed properly. Then at the end, I am feathering it down, reducing the pull so I am finishing at show center at 30 feet.

I’m just reducing the G and bringing it in so the bottom of the loop is very, very controlled and very benign. It’s not a five G, six G pull that is terminating at 30 feet. I’m coming from three to four to five Gs to three Gs to two Gs to a one G pull out.

I’m not a big aerodynamicist or a test-piloty kind of guy, but I think the biggest thing that people need to do is they have to know what their minimum turning room is. And turning room is the minimum altitude you need to execute the maneuver, so that you recover without hitting the ground. You need to know what the turning room of your airplane is. So, as opposed to doing lots of reverse Cubans and stuff, I teach war bird guys that — when they finish a maneuver — to then turn 30 to 45 degrees off the show line, away from the crowd. Then go vertical again and turn back in.

Ninety percent of them do that. What that does is, as your energy depletes, it allows you to still have the turning room to come back, because now you are not using your turning room. You need extra turning room to come back. But now you are using the oblique, as opposed to the pure vertical, which gives you more time. You can extend your radius and gain energy. So it increases your turning room and you can come back in the oblique. Say you go to the left corner. You see in all the fighters the guys all go out to the corners, then they turn back in to get on the show line. Well, that is to make sure they have the correct amount of turning room, which allows them to regain their energy for the next maneuver.

I always ask guys, “What is your airspeed at the vertical?” Maneuvering in the kind of airplanes I fly, the critical event is your entry airspeed. What is your energy package when you pull and then how do you pull? If you fly the profile with the right energy at the bottom, I guarantee you that, at the top, you’re going to have the right energy and more than enough room to come back down hill. So, the energy package is what is critical.

Now, if you are in crazy airplanes that are super light and have super thrust to weight ratios, like Sean Tucker’s airplane or some of those airplanes, they sort of defy everything we are talking about here. There are only a few of those guys and, most of the time, they practice a lot.

You learn by practice, by what pulling too hard does to your airspeed. You know, coming down the back side, feathering also keeps your speed and keeps your energy, as opposed to pulling things too tight and so on.

Like, for instance, if you are doing a loop, or something, kind of pointing the nose down and get through 5/8ths of it, or 3/5ths of it, you’re already through the vertical. You come through and now your nose is somewhere around 45 degrees nose low, so you’ve gone through most of that turn. You’ve gotten through 150 degrees or 120 degrees of that turn in the first one third of your altitude loss, or half of your altitude loss. Then the last 45 degrees to get back to wings level, you are using at least half of your altitude or no less than a third of your remaining altitude. So you’ve gotten your turn. Your turn is executed past vertical, but you still have anywhere from a half to a third of your altitude left.

Ideally, I like to have about a third of my altitude left, so at that point I can come off of the G and now I am just driving to a point down at show center. In my mind, I put a little tiny square in front of me, a little window that I am driving the airplane to.  And I want to go through that window wings level, so I am all set up. I am driving through, like a little bridge down there and I want to go under the bridge. But, I don’t want to go under the bridge while I’m pulling through. I want to be wings level going down to the water.

Wing loading, how many square feet your wing area is, basically tells you how tight your airplane will turn. It basically gives you how much lift the wing will provide. The ultimate high wing loading airplane on the air show circuit is the F-104, because it simply has no wing. So what does that mean?

Even at high thrust, its turn radius and its turn circles are very large. That is why the Star Fighters never do any overheads, because the wing loading is so high. They just go out and go away for three minutes and then come back. The reason is that the wing loading is so high that it takes a long time to turn the airplane around. In the two-seater version of that airplane, you go to half flaps at 500 knots. And, if you are at 450 knots and you have the flaps up, you pull and you get to two Gs and the airplane is already in buffet and stalling. So that is a classic where you have a very high wing loading. The ultimate light wing loading on the air show circuit is the gliders. They have very low wing loading. That doesn’t necessarily mean they can turn tight, because to turn tight you have to have the correct amount of elevator to make that move.  So airplanes vary in combinations.

Wing loading is the square footage of your wing versus how much your airplane weighs. So a normal jet fighter like an F-14, F-15 or F-16 has a wing loading of about 80 to 90 pounds per square foot.

Thrust will compensate for high wing loading to a certain extent. The more thrust you have, then the more you can compensate for high wing loading. A Decathlon is very low thrust to weight, but it also has a very low wing loading.

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Deb Gary
Deb Gary is a former air show performer, member of the ICAS Foundation Air Show Hall of Fame and freelance writer whose work has been published in Air Shows Magazine and Smithsonian Air & Space Magazine.