LWF Flight Control Laws

LWF Flight Control Laws                                      19 November 2012

 

The initial LWF flight control law concept was: For up and away flight, forward and aft stick force commands normal load factor and side stick force commands roll rate. For takeoff and landing fore and aft stick force commanded angle of attack rate.

Hands-off the control stick resulted in 1-G flight and zero roll rate. Back force on the control stick commanded more Gs, releasing the stick allowed the G level to return to 1-G flight, regardless of pitch attitude.

Hands-off the control stick resulted in zero-roll rate. Right force on the control stick produced right roll rate and releasing the stick allowed the roll rate to return to zero regardless of roll attitude

For takeoff and landing, (when the landing gear was deployed), the hands-off stick held the angle of attack constant and back force on the control stick produced an increase in angle of attack, and releasing the stick force would hold the new angle of attack. Forward or aft stick force produced an alpha rate command.

 

Later, the takeoff and landing pitch force commanded pitch rate and releasing the force allowed the resulting pitch attitude to be held.

 

The stick forces in pitch and roll were rather light and you did not need to  hold the stick while not maneuvering. Pitch and roll trim were provided on the control stick.

 

In addition of company pilots, we used several military pilots in flight simulators and in actual flight to assist in determining the flight control concepts. Some of these pilots had high hours in F-4 fighters.

There was a considerable amount of refinement of the flight control laws during the YF-16 program as well as the F-16 program as you might expect.

 

Our definition of Fly-by-Wire Flight Control Systems is:

Fly-by-Wire Flight Control Systems rely entirely on electrical control of the aircraft without any mechanical backup.

 

Others at the time of the LWF concept designs provided what we called Command Control Systems which augmented the mechanical cable control of the Pitch/Roll Control of the Horizontal Tails, with electrical wing roll control (which could be called partial fly-by-wire). That’s what we did on the F-111.

We used redundant Command Control for the pitch/roll horizontal tails with pure electrical control of the wing-mounted spoilers. The spoilers were only used at low-wing-sweeps.

 

Later in the program(s), we man have signed up for many specifications, but in the very beginning, we were allowed to work with our Best Practices as specifications. These Best Practices were based on many years of working with very stringent specifications and regulations. We developed applicable specifications for the relaxed static margin, considering upsetting gust strikes, horizontal tail control power with angle of attack and sideslip. The LWF flew just like a regular airplane – the pilot never could see any affect of the use of relaxed static margin except in the resulting maneuvering performance. For example, at supersonic speeds, the longitudinal static margin was about 15 percent instead of 30 percent MAC, which resulted in the ability to sustain a much higher turning load factor. This was one of Mr. Widmer’s objectives.

The LWF flight control system was designed for feet-on-the-floor maneuvering. The rudder pedals were for taxiing and cross wind landings. However, many of our high-experienced pilots would make a great deal of rudder-pedal inputs as they rolled which gave us some unexpected results. They soon learned that rudder-pedal inputs were not needed. It took a while for these aggressive fighter pilots to learn to fly the YF-16, which turned out to be a great deal easier. The started out flying the airplane by “stirring the stick” and jamming the rudder pedals, but soon learned to relax and let the airplane fly them.

 

Redundant electrical control for nose wheel steering was provided.

 

Several engineers of my Flight Dynamics Group went on to work the YF-16 detailed design, build, and test phases. Tom Paniszczyn* was the principal aerodynamic stability and control lead engineer and Richard Roberts was a key engineer in the detail design and build of the quadruple-redundant flight control computer(s).

Design Considerations for the Light Weight Fighter

12 November 2012

Design Considerations for the Light Weight Fighter

 

1.     What were the requirements for the LWF aircraft given by the Air Force?

a.     What were the design goals that were derived from these requirements?

2. What compromises were made to reach the design goals?

 

There were no “hard-and-fast” design requirements relative to the Stability and Flight Control characteristics other than our “best design practices” that I can recall. Deputy Secretary of Defense David Packard was promoting high performance and was willing to accept technical risk. We included maneuvering performance as one of his objectives. Mil-F-8785, Mil-F-9490D, etc, specifications were not required, as such.

 

In addition to excellent flying qualities and handling qualities, our objectives were to operate to high angles of attack without fear of departure in yaw or pitch. Positive directional stability to high angles of attack was sought after. Aircraft nose shape as well as vertical tails were studied. The location of the engine-air flow inlet was pushed back as far as possible because of its affect on directional stability. We studied and tested twin and single vertical tails. We liked the twin vertical tails for several reasons. However, we finally selected the single vertical tail because it was clear that the height of the vertical tail(s) was very important at the high angles of attack. This was primarily because the fore-body strakes that were used by most of our case studies. The tail had to extend up, out of the composite strake-wing high-angle-of-attack wake. We had to make sure that the rudder had enough control power to provide for cross-wind landings.

 

Fore-body strakes were used to extend and linearize the CL/Alpha curve to achieve higher lift. The classical CL/Alpha curve breaks and limits the useable lift prematurely. The fore-body strakes also affect the CM/Alpha (pitching moment) curve. We used the most effective fore-body strakes that did not adversely affect the pitching moment curve. The fore-body strakes resulted in an unstable tendency at high angles of attack in the pitching moment curve. This is an aerodynamic art.

Mr. Jack Buckner*, supervisor of the Aerodynamic Lift and Drag Group was pushing for larger and more effective fore-body strakes, but we had to stop this tendency when it began to adversely shape the CM/Alpha curve. Our Aerodynamic Stability and Control lead man was Tom Paniszyzyn*.

 

We participated in the wing design because Ailerons are not effective with stalled wings and we didn’t want to use spoilers for roll control. We had performed two roll-control studies for the Flight Dynamics Laboratory that proved helpful. Dr. Jack McAllister provided this expertise. We didn’t use the desired wing tip design because AIM-9 missiles were required and the wing tip is a very good place to attach missiles. Forty-degree wing leading edge sweep was used (instead of 31-35 degrees) in order to allow a more rounded wing leading edge to delay separation of airflow as we go to higher angles of attack. Also, programmable leading edge flaps were used. High-angle-of-attack design was a high priority.

 

We had studied Fly-by-Wire flight control when we prepared for the AMSA (B-1) proposal effort and saw its many advantages. David Packard was quick to smile on our leaning to adopt this concept. When we decided to adopt the CCV Relaxed Static Stability concept, there were no more objections to using fly-by-wire flight control.  We designed and built the quadruple redundant flight control computer in-house for the YF-16

 

The greatest risk factor involved the decision to use the CCV Relaxed Static Stability concept by moving the 280 square-foot wing 14+ inches forward. This amounted to a 15-percent change in the longitudinal natural static margin. We went from a 5-percent positive static margin to a 10-percent negative static margin at Mach 0.8 @ Sea Level.  Classically, the aerodynamic center will shift aft about 25-percent when supersonic speed is achieved. The aircraft is then stable, but not as much. This means that there is not as much down-load on the horizontal tail while maneuvering and therefore not as much drag, etc. There was great risk involved in the decision to use this concept. We experienced opposition and concern from all levels of management. Before the proposal was submitted, we traveled to the AF Flight Dynamic Laboratory to speak with Mr. Bob Johannes, who was in charge of the Control-Configured-Vehicle (CCV) Advanced Development Program. We needed to “feel him out” about his potential response to questions relative to risk and his opinion as to our capability to actually accomplish this feat safely. We also spoke with Mr. Vernon Schmitt and others of the Flight Dynamics Laboratory, relative to his/their opinion of us proposing to use the fly-by-wire concept for the LWF. Either one of these gentlemen could have caused us to loose the contract by giving an opinion that the risk was too great or that General Dynamics engineering was not capable of doing the job.

 

With the use of fly by wire, it made sense to consider a side-stick controller. We selected a quadruple force transducer with limited motion for pitch and roll control.

Many very excellent fighter pilots had a hard time adjusting to these new and better ways to fly.

 

1.a. What were the design goals?

From the outset, the objective of the LCF/ADF/LWF aircraft project was to provide a very high performance air-to-air day fighter with superior flying and handling qualities at the lowest possible price. In other words, a “real Dog Fighter”. This was not a high visibility program – The Air Force had already selected the McDonnell Douglas FX proposal as the F-15. But there were some in the Air Force (Civilian and Military Pilots) who believed that the F-15, while an excellent Air Defense Fighter, was too expensive to acquire a large number of fighters. The LWF Project (not yet a  program) was seen as a threat by the F-15 zealots; therefore, there was not a lot of money to promote the effort. The Air Force was counting on companies like General Dynamics, who urgently needed the business, to agressively pursue the idea. The LWF was to have guns and Aim-9 missiles and be the best “Dog Fighter” in the world. Our approach was to use a superior wing design enhanced by leading edge flaps and fore-body strakes. The directional stability had to be  excellent to very high angles of attack. The aircraft was to be able to sustain high normal load factor turns even at supersonic speeds. We considered horizontal canards but gave that up when we decided to move the wing forward and embrace the CCV Relaxed Static Margin (High Risk) concept. We had already decided to use Fly-by-Wire Fight Controls because it offered significant weight reduction and freed-up volume wthin the fuselage. This would allow the aircraft to be smaller and lighter or to allow for more fuel and/or equipment. The concept of modular hardware construction was embraced which allows potential fuselage streatch for future requirements. Extra wing attach fittings were incorporated on the fuselage and the wings were simply bolted on. For example, the F-16XL program resulted in two aircraft with radically different wing planforms. The LWF was designed with a 9.0 g limit load factor intead of the usual 7.3 g. The two YF-16s were designed to pull 9.0 gs during the initial flight test evaluation. The Fly-by-Wire flight control system was designed for very high reliability and was “two-fail-operate”

 

1.b. What compromises (decisions) were made?

Best practice design procedures were used instead of Mil Specifications; however, safety, performance issues were not allowed to be compromised.

We  improvised the electro/hydraulic control surface actuators because of the very short time we had to build the YF-16s. F-111 servos were modified for use on these two aircraft. New, specially designed and built electro/hydraulic control surface acturators were used for the F-16A/Bs.

The two YF-16 aircraft were designed and built in a special dedicated area of the General Dynamics Fort Worth facility. Also, because of the short time to deliver aircraft, we designed and built the quadruple redundant analog flight control computers in our AeroSystems Laboratory. Richard Roberts was the principal design engineer. For the F-16 program, the flight control computer hardware was subcontracted out.

I don’t figure that we made any significant compromises but we did make some quick and very bold decisions that proved to be good decisions.

We studied twin and single engines configurations and selected the single engine design. This offered lower weight and lower drag for the thrust required. Twin-engine “boat tail” drag is significantly more that fore a single engine design. Also, we picked that same engine as the F-15 uses, which was considered a logistically important decision. The LWFs could operate along side the F-15s in a complementary fashion, using the same engine shop. Turbofan engines were getting very reliable even though it feels good to have two engines. Neil Anderson flew the No. One YF-16 across the North Atlantic Ocean in May of 1975 for a sales tour at the Paris Air Show. That shows what we thought of the reliability of the P&W F100 TurboFan engine.

 

2.     What led to the decision to implement relaxed static stability?

a.      What other options were considered to improve the aircraft’s handling characteristics?

b.     How did the available control options factor into the choice to implement RSS?

c.     Did the use of strakes affect the longitudinal stability?

 

There was no serious management consideration of the use of CCV (Relaxed Static Stability) on the LWF until one Saturday morning in the early 1970s in the pre-design room at General Dynamics, Mr. Bob Widmer* was reviewing wind tunnel test data. He was disappointed at test results concerning aerodynamic data that would affect turn rate at Mach 1.2.  He reacted very negatively when it was suggested that we move the wing forward as a way of achieving the desired Mach 1.2 turn rate. This suggestion was coming from the supervisor of the Flight Dynamics (Aerodynamic Stability and Control and Flight Control System Design) Group. He exclaimed that we of all people should know that “we can’t do that, the pilot would not be able to control the airplane”.  We reminded him that we had been studying Relaxed Static Margin for a couple of years. After some wiping of his brow, he stated… do you really think that we could that?  I told him that if we don’t do it now, then we will probably never do it. He quickly said, how much instability do you recommend? I told him –15% MAC. He said, let’s make it 10%.

Then he said, when can you tell me how far to move the wing forward. I told him, Monday morning. The number was 14+ inches.  We elected to calculate the forward wing movement using the same horizontal tail size and geometry. That amounted to a 15% shift in the stability margin ie from 5% stable to 10% unstable. Mr. Widmer*, VP, Engineering made this very risky decision one Saturday morning without advise from anyone but me. This was a very risky discussion for me, as well. This resulted in superior turn rate at supersonic and subsonic speeds as well as reduced drag while cruising. No changes were necessary in the overall control system implementation except the control laws and the provision of the high reliability. We had already decided to go fly-by-wire. The use of a side-stick-controller was obvious selection at this point.

In conjunction with L/D Aerodynamics Group, we studied (too many to count)  nose and fore-body strakes for improved lift characteristics. The longitudinal stability, i.e., the slope of the CM/CL curve was essentially unchanged at low angles of attack.  However, the higher influence of the strakes tended to produce a negative break in the CM/CL curve at the higher angles of attack. The general characteristic of the curve did not change much with the center of gravity location. The curve would just rotate counter-clockwise with moving the wing forward, or reducing the longitudinal stability.

 

3.     How did the design decision lead to what became the YF-16?

a.      Single engine vs Twin engine

b.     Single tail vs Twin tail

c.     Wing strakes

d.     Canard vs conventional empennage

 

We selected the single engine design because it gave us the most performance per pound and dollar. We accepted the risk of just one power plant. Since the fly-by-wire concept is electrical, we had to provide an emergency power supply for the electrical and hydraulic operation. The emergency electrical and hydraulic system was powered by a hydrazine driven turbine, which was to provide about 30 minutes of operation without the turbofan engine running.

 

We preferred twin verticals, but the single (very tall) vertical tail provided superior directional stability in the presence of the fore-body strakes at high angles of attack

Twin (fixed) ventral surfaces were added to enhance the directional stability at low speeds and supersonic speeds.

 

After extensive testing and tweeking, we selected fore-body strakes that extended the CL/Alpha curve, providing greater (vortex) lift with lower drag at high angles of attack. The pitching-moment curve (CM/CL) was also enhanced by the use of fore-body stakes, by delaying the stable break in the curve at the higher angles of attack. The use of strakes did not noticeably affect these relationships at low angles of attack.

 

We like canards, and studied them for this application. However: the use of Relaxed-Static-Margin with aft mounted horizontal control surfaces proved to be much more suitable for this design concept because of the “up-load” of the aft mounted horizontal control surfaces at subsonic speeds.  It proved to produce lower drag with lower weight for this design.

 

4.     How did the requirements for future upgrades affect the design?

 

Initially, there were no requirements for future upgrades. We did design the fuselage bulkheads 16 inches apart and provide additional (more than actually required) wing attach brackets which came in handy when we later designed and built two F-16XLs. (These aircraft are presently at NASA, FRC, Edwards AFB, CA. The wing area was increased (from 280 square feet) to 300 for the F-16A/B aircraft. The area of the horizontal tails were increased slightly after the F-16 were tested in the early test phase.

We used the GD Low Speed Wind Tunnel and CALSPAN Wind Tunnel for the  design work for the LWF. After we got the contract to design and built the YF-16, the AEDC wind tunnel was used which yielded somewhat different results.

Also, the fuselage was stretched a little over a foot and the nose was enlarged slightly to install a search radar, with little affect on the aerodynamic characteristics.

Our design was such as to allow only a small variation in the center of gravity with fuel usage and external store attachments.

The two YF-16s were structurally designed to allow early flight testing to the full 9-g load factor.

The F-16 was later required to provide the multi-role capability of air-to-ground in addition to air-to-air capability. The initial armament was the M-61 20mm cannon and AIM-9 Sidewinder missiles. It now can carry and deliver Sparrow missiles, AMRAAMs,  and a vast assortment of dumb and smart bombs.

 

 

Andrew, this has been a bunch of rambling comments and I hope that you can make some sense of it. I will try to look it over and straighten it out some. Keep in mind that all these comments are from my personal observations and with my own personal bias. Should have any questions about my comments or about other things, pleas let me know.

Best regards, John

So, Who am I?

Past, Present and Future

Past,

Born at home, the fourth of five children, in a small town in Northeast Louisiana.
I remember a few things from my early childhood. I blew up a balloon, tied a knot in it to hold the air in. To make sure the knot didn't come out I placed my ring over the knot and then I threw the balloon into the air. When I picked up the expended balloon, the knot was untied and the ring was gone, never to be found.
I dreamed that I was riding my bicycle in the Delhi train depot parking lot. There were not many or any cars in the parking lot so it was natural that I would ride my bicycle as fast as I could which was pretty fast. I happened to see a penny on the cinder turf and I tried to stop as fast as I could, but when I did stop and go back to get the penny, I couldn't find it. This was a recurring dream, I can't tell you how many times I had this dream, over and over again, sometimes more than once in a single night. Psychoanalyze that will you? I remember my mother driving me to school in our 1929 Chevrolet sedan. I remember one day at school recess, I was playing with some other kids and my mother came up behind and startled me and as I turned around I accidentally hit her in the face somehow. I doubt if she felt anything. She was checking up on her little boy.

I frequently visited my Grandpa Hudgins' farm and rode his ole gray mule with no saddle. I remember one time the mule decided to get a drink of water out of the slew and as he lowered his head to drink, I slid right down his neck into the slew, head first. How embarrassing! Otherwise, no damage done. Grandpa had a 40-acre farm about 4 miles south of Delhi and his main crop was cotton along with a small amount of garden vegetables. He was the only grandparent that I ever knew. He was a Christian preacher and a farmer.

When I was about 6 or 7 years old, we were having a birthday party at our house, I think that it was mine. One of the little girls who attended my party wanted to go to the bath room. My Aunt Patsy led her to the outhouse which was about 30 yards from the house. My dad built the house that I grew up in in circa 1930. It was well- built but didn't have a bathroom or running water. The water pump was beside the house about 10 yards away. My Dad continually made improvements to the house, like enclosing corner porches, adding a bathroom and a kitchen sink. He asked me since I was getting a little older which facet was for the hot water and which was for the cold. I told him that the left was for the cold water and the right knob was for the hot water and so he hooked it up that way. Of course, that was not right but the water flowed, you just had to remember which was which.
Another milestone in my life was when my little brother, George, was born at home when I was 11 years old. Talk about a bubble bursting. All of a sudden I was not the baby of the family anymore and I was just another middle child with all the benefits.

About the time I was a senior in high school, we got a telephone, the number was 345W. Later on, it became 318-878-3459. When I was about 12 years old, I did two things that had a significant affect on my life. I joined the Boy Scouts of America and I made a public profession of my faith in Jesus Christ as my Lord and Savior. Mrs. Ola McKinnis was my Sunday School teacher and led me to believe in Christ.

Dad bought a broken down Service Cycle motor bike and we installed a washing machine gas engine in it and I drove around town and North on highway 16 (they changed the number to highway 17 in about 1955). I had several older friends who had motor cycles like the Harley-Davidson 45,61,80, etc. I remember riding the Harley 45 over an elevated railroad crossing at a fairly high rate of speed. That was quite a thrill, going airborne for a few seconds.

I decided that I would take my flying lessons at the local airport when I was 16 years old. My instructor, Bill Clinton had an Aeronca tandem trainer that we used for my first flights. This was right after the war and Bill had raised just enough money to buy a new Piper Cub J3C-65. With a little over 5:25 hours of dual flight instruction, told me to take it around the field. This meant that he wanted me to solo, so I taxied over to the end of the runway and got still for a minute. I twirled my high school graduation ring several times and said a little (but sincere) prayer. And suddenly the throttle was moved full forward and the Cub began to accelerate down the grass runway. I made three takeoffs and landings in 15 minutes and taxied up to the gas pump where Bill was standing with several others interested in flying. I was wearing a khaki shirt. I remember because the guys pulled out my shirt tail and cut it off to hang on the hanger wall. (We didn't have a hanger.)
My flight instructor wrote my name on it, signed it and dated it May 4, 1946. I was 16 years old, a senior in high school and an airplane pilot.
That same year, I was awarded the BSA Eagle Scout rank and graduated from high school. I was the president of my senior class.
So, there I was, I wanted to be a fighter pilot in the U S Army Air Forces, like my brother Jim. But, I was too young to go into the military. I decided to go to Northeast Junior College of LSU in Monroe, Louisiana, just 40 miles away. What academic field should I take? Agriculture, No!, Business, No!, Engineering, what is that?, Aeronautical Engineering, Yes, whatever that is, it must have something to do with airplanes. So, that is what I enrolled in. Since the war was over, there were a lot of veterans attending NJC on the GI Bill, many of them were former U S Army Air Forces pilots, etc. I hung out with them. My brother Jim was still in the service. When I was 18 I joined the USAF Reserves so I could fly with my friends out at Selman Field there in Monroe. Mostly, I flew in the back seat of an AT-6 trainer. One very good friend in particular, fancied himself as an instructor pilot and gave me many hours of instruction and practice in "under the hood" instrument flying. When we completed the course of instruction at NJC, He went on the Louisiana Tech and I went to LSU. I never saw him again, but we kept in touch occasionally. He has passed away but I still keep in touch with his son by email and facebook.
The reserve unit that I joined went on a two-week active duty trek that summer at Carswell AFB. We were part of the 579th Bomber (VH) Squadron that flew B-29s. It was a way of maintaining proficiency. I made some night flights out of Carswell in T-6 trainers. Across the field, was the No. 1 Convair B-36 that I saw on several occasions, but I didn't see it fly. After the two-week active duty assignment, our squadron CO had agreed to fly me to Baton Rouge so that I could enroll at LSU. However; something came up and he was not able to fly me there. So, I had to ride a bus, with my footlocker all the way to Baton Rouge and LSU campus. I got there at the last minute and was able to enroll but I couldn't get into the AFROTC because all the draft deferments had been subscribed. (Actually, I didn't need a draft deferment because I was already in the USAF Reserves.) This delayed my graduation from LSU from the summer of 1950 to January 1951. That was good however, because the Aeronautical Engineering course was part of the Mechanical Engineering department and therefore, I almost got the full coursework in both Mechanical and Aeronautical Engineering. I had 179 semester hours when I graduated. More to follow.....

New Photos

We made some new photos for the church directory.

Really Big Catfish.

Last Friday, nine of us went to Squaw Creek which had recently been reopened after 9-11 security shutdowns around nuclear facilities. We took two boats, five of us on one and four on the other. Each boat had a fishing master that managed the boat and baited the hooks, etc. We did hold the pole and real them in. You know, the fun stuff. We caught hundreds of catfish, mostly about 1 to 2 pounds. In addition, I caught two rather large catfish, maybe as much as about ten pounds each, maybe less. We didn't have any good fish scales. The guys gave me the fillets of the large catfish and I thought that was right. However, when I go home with them, Lee told me that we should have thrown the larger catfish back and kept the smaller ones. They taste better. So anyway, it was fun. One of the guys said that it wasn't fair for me to catch both the big catfish.

History of the Mach 2.1 B-58 - "HUSTLER"

Tuesday, 15 June 2010, 1900 hours

Mr. Jay Miller, International Aviation Historian and Photographer presented previously unpublished information and visuals of the Convair B-58 Supersonic Bomber at 3500 Camp Bowie Blvd, Fort Worth, Texas. There were about 150 people in attendance, mostly grey haired men who used to work for Convair, General Dynamics at the bomber plant. He showed some old film clips of the No. 1 B-58 first flights and the Air Launched Ballistic Missile (ALBM) tests based from Eglin AFB, Florida. The No. 1 B-58 launched the ALBM at Mach 1.46 and 2.0. , four launches total. The program was considered a success even though there were several problems. These test launches were conducted about 1958/59. I was the flight control technical rep operating in the control tower during the first flight that occurred during the afternoon of 11 November 1956. The meeting lasted over two hours, including free discussion, questions, and answers from the audience. We talked about the loss of test pilot Ray Tenhoff in 1960, the loss of B-58 No. 5 over Lawton, Okla, the development and test of the crew module with black bears as occupants during the test. A good time was had by all.

February 2010 in Ridglea

It snowed in February 2010 in Fort Worth, Texas. I measured 12 1/2 inches using a yard stick. It started snowing before daylight and snowed all day. His is a picture of the snow from our front porch looking towards the SouthSouthEast.