NASA Headquarters NACA Oral History Project
Edited Oral History Transcript

Christopher C. Kraft
Interviewed by Rebecca Wright
Houston, Texas – 15 August 2014

Wright: Today is August 5, 2014. This oral history session is being conducted with Chris Kraft in Houston, Texas, as part of the NACA [National Advisory Committee for Aeronautics] Oral History Project, sponsored by NASA Headquarters History Office. Interviewer is Rebecca Wright, assisted by Sandra Johnson, and we thank you for agreeing to come and talk to us this afternoon.

Kraft: My pleasure.

Wright: We’d like to talk to you about your time at NACA, and actually, even before that, when you were still at VPI [Virginia Polytechnic Institute, (Virginia Tech), Blacksburg, Virginia]. I remember in your book that you talked about using and learning from those NACA reports during that time. Can you talk to us about that experience of what you learned from those reports, and actually, what you learned about NACA and why you thought that might be a place to work?

Kraft: What I said there was an overstatement, so I have to back up a little bit. When you study aeronautical engineering in 1944, when I graduated, what we were studying was basic aerodynamics, basic physics, basic things that you would eventually use in aeronautical engineering. Our capabilities, our teachers, were not very versed in the problems of aeronautics of the time. There was a lot of reasons for that. A lot of the research was confidential, so they couldn’t know it. NACA reports were confidential, so we couldn’t know it. As an example, it was obvious that the use of wing sweep was going to be a major part of the future airplanes of that age, and all of that was confidential. The Germans were doing it. You could almost get as much data out of the German reports as you could NACA reports because if you had their reports, it was okay, but to have the NACA reports, you had to put them under lock and key.

Things like turbulence, the first NACA report was about turbulence, flying through turbulence, believe it or not. The early reports were basic. A lot of things on structures, a lot of things on wing shapes, on air flow shapes. There were a number of books that were published by NACA that were just hundreds and hundreds of cord shapes, of wing shapes—cusp and flares and all that kind of fancy stuff that you could do to change the basic stability of the wing as opposed to the lift of the wing. They were published, and those were almost catalogues, and they were fundamental to the designer of the times—the designers, remember now, being late 30s, early 40s. That’s what I was referring to, really.

As I was trying to design a small, light airplane, which was one of my classes, I wanted to get what the wing shapes were, the effects of aspect ratio, things like that, which I knew little about. The types of airfoils that were the most efficient and give you the least drag, and would couple generally with a light airplane. That’s what I was referring to. On the other hand, structures, as an example, was something that was coming on at that time, and we had a heck of a good airplane structures prof [professor], and I learned a lot from him, and so did NACA when I got there, because I had had that background.

The books were pamphlets, big, thick pamphlets put out by a couple of the aviation companies. They were very, very informative. That’s where I got my early knowledge. I really have to say, I was not very smart. I didn’t know how NACA went about doing their work. I didn’t know how they went about publishing their information. I didn’t know that their product was reports, basic writing. I’m pretty glad I didn’t, as a matter of fact, because NACA reports were very dull. One of the first reports I wrote at NACA was on the [Republic] P-47 [Thunderbolt] airplane. It was a lousy airplane from a flying qualities point of view and, really, a control point of view, and I said that. In the beginning of my report, I wrote that down. The editorial office says, “You can’t say that.”

I said, “It’s a classical example of what you should not do to design an airplane.”

They said, “You can’t say that. You can say what it is, but you can’t say what you said.” They were kind of dull, but fundamental.

All the fundamentals were catalogued by NACA. As you know, the cowling, as an example, was an invention of NACA, and it changed the look of modern airplanes in those days. It changed the drag, eventually, and increased the efficiency of the engine, as a matter of fact. NACA was very prominent in anything that aviation did. Later, after I got there, and airplanes started to go high subsonic, supersonic, those airplanes were actually designed in the NACA tunnels. The [Vought] F8U was a classic example. That airplane, which is the first Navy supersonic airplane, which I was in charge of for NACA in flight test, not as it came through the tunnels, but every nuance on that airplane was an NACA nuance. That was quite impressive. Go someplace else—talk about the [Bell] X-1, as an example.

Wright: NACA wasn't your first choice to work.

Kraft: No. I was going to work for Chance Vought [Corporation], and I don’t know, can’t tell you why. I was a young student out of college; I didn’t know where I was going. I said, “NACA is right next door to me, I’ve been living here all my life, why don’t you go someplace different?” I couldn’t get hired. You read that in my book. They wouldn’t hire me. I had two job offers. One was with NACA, one was with Chance Vought. I could have probably had more, if I wanted them, but I just took Chance Vought, and so I didn’t even mess around with it.

I did take the one from NACA as a backup, and fortunately, that was a good thing to do. When I got to [NACA] Langley [Aeronautical Laboratory (now Langley Research Center), Hampton, Virginia], most of the people knew who I was, as a kid. Where I grew up, everybody knew, it was a small town business, and everybody knew everybody from everybody else. They knew my name, and so they were very kind to me. I remember when I went to see the personnel guy, he was very nice to me, and he said, “Well, I think you’d fit best in flight research.” He said, “What are you interested in?”

I said, “Well, I’m not quite sure. I’m not sure I know what I want to do. I’m interested in stability and control and things like that.”

He said, “Well, I think you’d be best fit in airplane flight test.” I didn’t know what that meant. He said, “We’ll put you over in the Flight Research Division. They got a bunch of smart guys over there that’ll teach you how to be an aeronautical engineer, and I think you’ll make out well over there.” He was absolutely right. That was a wise thing for me to go there to work because they were working on all the modern airplanes. They could work you on the P-47, the [North American] P-51 [Mustang], and the [Bell] P-39 [Airacobra]. They had those airplanes flying. It was in the old hangar—they built the new hangar while I was there for the Flight Research Division. It had a Japanese [Mitsubishi A6M] Zero. It was a great place to learn, for me.

Wright: I’m sure it’s a great place for those pilots that got to fly all those different airplanes as well. You worked close with the pilots.

Kraft: Yes. Back in those days, at NACA, test pilots were not very prevalent. NACA taught several of their aeronautical engineers to be test pilots. Jack [John P.] Reeder, who was one of the best test pilots NACA ever had, was just an aeronautical engineer in the full-scale tunnel, and they made him into a test pilot. Mel [Melvin N.] Gough was an aeronautical engineer. He had graduated from Johns Hopkins [University, Baltimore, Maryland], had a job at NACA and saw what was going on, resigned from NACA, went across the river, across Hampton Roads, to join the Navy to learn how to fly Navy airplanes, and came back to NACA as a test pilot, and eventually became their chief test pilot.

Those guys were not too unlike me. I was a young, wet-behind-the-ears aeronautical engineer and didn’t know tiddlywinks about airplanes. I was a baseball player. I wasn’t an engineer, and airplanes didn’t interest me. I’d been there all my life, watched them fly over me and go to the air shows and so forth, but I never was interested in airplanes. That professor at Virginia Tech got me interested in airplanes.

It was a great place to learn. NACA was a really great place to learn, and I’d learned about flying qualities that [Robert R.] Gilruth had written [Requirements for Satisfactory Flying Qualities of Airplanes, NACA Technical Report 755]. I went to class at night, at Hampton High School, had classes at night in various elements of aeronautical engineering. [William] Hewitt Phillips, who was my boss, taught flying qualities. I would go there two or three nights a week and learn what flying qualities was. I realized how great a man Gilruth was. He and Mel Gough had developed flying qualities, and that was the Bible for how to build an airplane from a standpoint of flying it, and it was the Bible of how to test an airplane.

The Army [Air Corps] and the Navy adopted that document as the basis upon which they were going to buy airplanes. That put me in the know on what all airplanes were. The significant fact that was a part of your teaching, and was knowledge, in NACA in those days, was everything was static. When you talked about the forces that operated on an airplane, you were talking about the static conditions. What were the balancing forces? You measured those forces in as static a condition as you possibly could. This was before anybody even thought about doing a dynamic analysis. You’d write the equations of motion for an airplane, it was just the static forces you were dealing with.

By a year or so after I got there, the biggest thing about the dynamics of an airplane was what is the time to damp to half-amplitude? You do a step input with the stick or with the rudder or with the ailerons and see what happened. You would see how it damped. In other words, if you pull back on the stick, if it damped in 1.5 cycles, that was the test. That was the only dynamic test.

Within a year, I was writing equations of motion in the airplane. It was all dynamics because it was obvious to NACA, particularly, that as you approached higher speeds, the dynamic aspects of the airplane were equally important as the static aspects of the airplane. All the wind tunnels, all the flight tests began to look at the flight dynamics, as opposed to balancing forces. Of course, that changed all the mathematic analysis, also.

When I was in college, I’d never heard of a Laplace transform. I took operational calculus, which is all I needed when I got to NACA because that’s how you wrote the equations of motions of the airplane, but wasn’t too long after that, that the dynamic parameters which you left out of the equations of motion had to be considered. They became as predominant a force on the airplane, naturally, as just doing it as a balance stick. The wind tunnels, everything started becoming interested in dynamics instead of balancing.

The other thing, however, was that, as I alluded to, it was obvious that in order to get to the problems that you were having at high subsonic speeds—as the Mach number began to increase, most airplanes at that time period, the high-speed airplane was about 550 miles an hour and Mach 7/10. If you increased the speed, increased the power, and the jet engine was not going to allow you to do that, get a lot more power and efficiency out of a jet engine than you could with props, and propellers were limited in their aerodynamic design as well, it was obvious that you were going to be flying at higher Mach numbers. What is the airplane going to have to do to deal with higher Mach numbers?

They began to look what the Germans were doing. The Germans were sweeping their wings. Immediately, NACA started sweeping wings and seeing what that did to the wind tunnel and how that changed the dynamics of the machine, how that changed the drag, how that allowed them to prolong, put off a swept wing. All you’re doing is you take the angle of the sweep, and multiply that times the speed. You find that, well, the force is coming along the cord, and so that allowed you to design the airplane to fly at Mach 9/10 while the wing was going to 0.75. Just that simple aerodynamic feature. The Germans figured that out pretty quickly, I guess by rote, by trial and error, because I don’t think they had the wind tunnel capacity NACA did.

As soon as we started mucking around with sweep, we found out that was a very prominent thing that was going to be in the new airplane. The other thing was that [Richard T.] Whitcomb—Whitcomb was an aerodynamicist of my age, maybe a little older, not much, he just died recently. I think it was the [McDonnell] F-101 [Voodoo] and the [Convair F-]102 [Delta Dagger], one was a delta wing and one was a straight wing, and they found that the doggone drag was a lot higher than they thought. They had drag interference between your fuselage and the wing, interference drag, they call it.

Whitcomb figured out that if he slimmed the fuselage down where the wing came in, that the drag went down. They didn’t understand that, but it worked, and then he figured the math that went with it. That’s how he became famous. At that time, those two airplanes would not go high subsonic, into the transonic range, and they put the Coke bottle shape in there, and lo and behold, they reduced the heck out of the drag and they were able to get a lot, almost 1/10 of a Mach number higher, speeds out of it. That’s the kind of gimmicks we were getting to, to try to get close to the transonic range. We all knew that the one block that we were facing was, what did we call it? When you get to Mach 1?

Wright: The sound barrier?

Kraft: Sound barrier, right. Everybody said, “Well, we got the sound barrier right there,” and all the math said that drag became infinite at that speed, and of course, it didn’t. We knew that bullets would go through the speed of sound, but we didn’t understand the math. We didn’t understand the design. The wind tunnels that we had would not measure the forces in the throat of the tunnel because they got these shockwaves coming off of the wing, and that screwed up the flow. The flow became turbulent. If you know anything about aerodynamics, at any speed you’re flying at, the air is not only worried about what’s there, but it’s sending out signals in front of it. It literally sends out a signal that I’m coming, and tells the molecules of air what to do. The closer you got to transonic and high supersonic speeds, the signal wasn’t there anymore. It was just a flat shockwave coming off the flat plane of the wing.

That fact, you put a configuration in the wind tunnel and the shockwave would come off and the flow would be destroyed. All the measurements you would make on the wing or the tail, on the whole airplane, were in error because the flow was wrong. It wasn’t what you were going to see when you actually got flying in free air. That’s where John Stack became very famous at NACA. John Stack invented the throated tunnels. What he did was put a force outside the tunnel and suck that air off, suck the turbulence off, so that you could measure things at transonic, supersonic speeds.

We people in flight tests were trying to come up with, well, what can we do—Gilruth was thinking about those kinds of things—to measure what’s happening to the airplane at transonic speeds? He came up with these two techniques that we were using, which is what I started working on, almost from the time I got to flight research. That is, we would take a high fineness ratio body and put a wing on it, and sweep the wing to various angles, put the wing at various locations before and after the max [maximum] diameter of the fuselage, take it up to 35,000 feet, drop it from a [Boeing] B-17 [Flying Fortress], initially, and we had a balance inside the fuselage to find the high fineness ratio body, and we put telemetry in there.

Nobody had ever used telemetry before. We tracked it with radar as we’d drop it, as it fell. That way, you got into free air to measure the aerodynamics, and with the radar, we could measure the velocity very accurately. We could do all kinds of configuration testing. We could express our measurements on the body, on the high fineness ratio bodies, and you could locate the wing at different positions back and forth. Lo and behold, there was the Coke bottle.

We found, hey, we put the wing behind the maximum diameter of the body, the drag decreased because it reduced the interference drag between the fuselage and the wing. That was a big invention of the time. Sweep, high fineness ratio bodies, and location of the wing relative to the fuselage. A lot of airplanes then started using that to get through transonic speeds. The theory said that what happens as you approach those Mach 1 is that you get separation of flow. If the air separates from the wing or the tail, either, it loses its effectiveness, and you don’t know what that is. You can’t measure it in the tunnel, and we can’t predict it. We can’t write the equations which predict what’s going to happen. That promoted the thickness of the wings and the tail.

Up till that time, their initial thought was, “Well, you got to have thick wings because it’s got to be really strong to withstand these forces that you’re going to get;” high Q forces that you were going to get at high speeds, but that was wrong. What we needed was thin wings, so that makes sense, doesn’t it? Thin wings are going to have less drag, and that’s what happened.

We didn’t know what kind of controllability we would have, or whether we could predict [the controllability] in order to design the stability and control of the airplanes. We didn’t know what that was going to be. We’re all searching for how to measure the forces that acted on the configuration you had at transonic speeds. Gilruth came up with body over a P-51 and the drop technique, and then he took the rocket, put the bottle on the end of the rocket, and then you could get to Mach 3 easily, by firing it on the end of a rocket.

Everybody was looking at those kinds of techniques. Then, that further exacerbated the stability and control problems, the dynamics problems, and that’s when everybody started working on automatic stability and control. How can we provide stability and control when we know the configuration, but even if we don’t know the configuration, we could measure what’s happening and we could use that information to provide forces to correct the forces which are about to send you off somewhere.

Now, about 1951, ’52, ’55, that’s where, in that period, automatic stability and control became prominent. You mix wing sweep, new ideas about drag, and artificial stability and control together, that’s the modern airplane of the time. That developed at the NACA from ’46 to spaceflight. Everything that we were doing at NACA was going in that direction. It was slow.

The other thing I’ve forgotten, did not talk about much, is the jet engine. With the jet engine, you’d get reduced drag, and so, therefore, how can we get high, efficient jet engines? That’s the other thing that NACA began to get into, was how to reduce the fuel consumption of a jet engine. The first jet engine airplane was a Bell [XP-59A Airacomet], Bell Aircraft built one, a low-wing monoplane. I think it could only fly about 30 minutes, was highly inefficient. Everybody jumped on that, to try to improve the jet engine.

We had all these bi-flows and air flow changes and prop design inside the jet engine, the vanes inside a jet engine. That became also a requirement of the NACA, to improve the efficiency of the jet engine. All those things were coming, being driven by the jet engine; you could get to the speeds you wanted to get to, but you couldn’t fly there. The F8U airplane would fly at supersonic speed for 6 minutes and run out of fuel, so you needed to improve the efficiency, needed to improve the drag, you needed to improve the stability and control, and you needed to invent automatic stability and control. Those were the design requirements that developed in the time period when I grew up in that age.

Wright: Can you explain to us how all of you that were working on these projects exchanged information?

Kraft: We had this exchange of information between the divisions, we had