(Written in answer to Pete Bower's write-up in [Western] Sport Flyer, March 1985; Dave Sinclair, Publisher, P.O. Box 98786, Tacoma WA 98498-0786 - VWP)
In 1928 I was working for an aircraft school in Chicago, nowadays called a Ground School. I was in charge of the woodworking and repair shop of this school teaching repair of aircraft in the shop, pIus 2 hours classroom on aircraft general design, and basic aerodynamics.
To help the students pay their tuition, we helped them get paying jobs, and held school at night. Some of the lectures put them to sleep.
I thought of a way to get their interest and helped them to stay awake. I owned several copies of "National Advisory Committee for Aeronautics", published in 1928 and before: such as Report 244, 1927. On page 197 - ref. 511 we found the M-6 airfoil. It had a high L/D Ratio = 23 and a remarkably stable Center of Pressure Travel. We were very attracted to this airfoil because of these characteristics. Many designers also were interested, one built a small racer. One designer talked Waco into building the Waco Taperwing -- later he joined Great Lakes and designed their biplane.
I used many of the NACA reports that dealt with wings and shapes, also wingtips, and taper of wings, etc., to interest the students in how an airplane should look and be efficient.
We showed all the many types of airplanes: mono, biplane, triplane, also tandem wings with explanation of the advantages or disadvantages of each, often referring to the NACA reports to prove it. We all wondered why the then-thought-of "modern designers" did not use the suggestions of NACA?
The wing tips for instance were square. NACA showed: square, elliptical, negative rake, positive rake, and combinations, such as the Knight Twister uses the elliptical and negative rake, for best efficiency, for 1928. In the airplanes of that day (commercial) we saw no evidence that those manufacturers had ever referred to the NACA reports, they did study their competitors.
Some designers studied NACA airfoils, getting away from USA-27, RAF 15, and tried the Clark Y, as was used in Lindberghs "Spirit of St. Louis".
Cub used the USA 35B for many years.
We chose the M-6 with taper and the combination elliptical and negative rake for our wing design, also made it a biplane of unequal spans. The next study was what type of stagger. The English had a negative stagger biplane used in World War I, it got fair top speed but also had fast landing. The positive stagger biplanes were slower landing and better for small fields. The positive stagger seemed to act like a wide blanket with corresponding drag when near the ground, this was their explanation of slower landing. We chose the positive stagger for this reason, also for its look of speed which we admired.
Naturally we went for full cantilever tapered wings but chose to add an "I" strut, leaning forward to help support the illusion of fast flight. In later years the Department of Commerce, Aviation Section, suggested to Arrow Sport to add struts or an "I" strut on their biplane to prevent losing a wing due to wing vibration. The "I" strut helps because the longer top wing has a different vibration period than the shorter bottom wing.
When the Parson's Knight Twister won the first two biplane races at Reno, they started to work out some new rules, so more sport biplanes would feel they had a better chance in the races. I made a few suggestions, which ruled out the Parson's Knight Twister. First, that the wing loading must not be heavier than 12-lbs./sq. ft. of wing area and there must be a stabilizing strut, such as an "I" strut. The wing area need not be more than 75 sq. ft.
We redesigned the wings of the Knight Twister, top wing 17.5' span and bottom span of 15.5'. This gave us approximately 76 sq. ft. We still did very well in the races. We are the only sport biplane -- not a strictly racing plane -- in the Reno Races. Some biplanes are highly modified sport planes, ours is not, and still is in the winners circle.
One other thing we started out with in the original Knight Twister biplane was a large gap-to-chord ratio. This was denied by Pete Bowers. The small Knight Twister had an average chord of 25.9". In 1929 we had a gap of 34 inches, 34/25.9 gives a gap/chord ration of 1.33. This is 33% more than what was called the least efficient ratio, which is 1 to 1. We always had a good efficient, or better and more efficient ratio, than any biplane of its time. In 1946 some pilots asked to raise the top wing to allow the tall pilot to see under that wing more comfortably. We compromised and raised it 2" or a total of 36". 36/25.9 = ratio of 1:39. This was not done because we thought the original was inefficient, as Pete implies.
Our first Knight Twister with 45 horsepower was efficient enough to get off, even up hill slightly, in a small sod field, and landed safely. It did things well, even with less than 50 horsepower and we still claim 45 mph landing. The total weight of the first Knight Twister was 550 lbs. with a wing loading of 9.17 lbs. and a power loading of 12.2 lbs./hp.
Remember all the efficient ideas we included in the design, also the fact that positive stagger gave slower landings. Then add the effect of some head wind plus ground effect, and once we landed up hill. We did land at about 45 mph.
Now if you use just the coefficient of lift from Report No. 628, page 39, NACA 1938, the airfoil can be landed at 20 degrees angle of attack and get a coefficient of lift of 1.4 at Reynolds No. 3,030,000, but we discounted this to Cl = 1.25 as the Knight Twister was not flying at the higher Reynolds Number. So with 60 sq. ft. of wing area and 54 mph we get 559.8 lbs of lift. The first Knight Twister weighed 550 lbs. Without a wind and without an uphill landing in a sod field. It could be done and we were highly elated, probably forgetting the uphill help. We did not think someone about 53 or 54 years later would nit-pick on this subject, when at that time, all manufactured airplanes had really bragged about higher figures. We took it as it came.
The present day Knight Twister flown by Don Fairbanks claims a stall speed in the air of 50 mph. This is what we call the Imperial Knight model. It is heavier than other Knight Twisters, but only 12 lbs. per sq. ft. of wing area in the races. In the Reno Races 1984 this plane came in 2nd place at 185.349 mph. The 1st place winner came in at 189.972 mph, in a highly modified Mong Sport. Both the Knight Twister and Mong had 150 horsepower.
Everything changes with time. In 1928 we had only 45 hp and 60 sq. ft. of wing area. In 1984 we have 150 hp and 76 sq. ft. of wing area. Also, we only had 550 lbs. of weight then, now we have 900 lbs. in the Reno Air Races.
When a good looking airplane gets attention all over the world, it means that something about it has been admired by millions and Pete Bowers helped by selling pictures of it, the one he claims was revised in 1956. The owner and builder of this long-nose Knight Twister is Tony Sablar and it was built in 1946-1947. He just visited me 3/8/85. He purchased some new drawings of the 2-place Knight Twister. His original Knight Twister with 85 hp is 38 years old. He is just now reworking and recovering it.
Many times someone out of these thousands of people who write to me have asked if I copied or scaled down some production model airplane such as the Curtiss Hawk, or the Boeing job that Hughes used in his movie called "Wings", my answer is definitely "No!" The Knight Twister is an original. I do know the man who designed the Boeing biplane, Robert Menshall. I met Bob in 1921 when I was in the service, stationed at University of Washington, Seattle as an instructor of ROTC. Bob was finishing up some engineering studies to get his degree, and was in my radio class. We were very close to the same age and both just married. When I left the service he tried to get me to come to Boeing, but I went back to Chicago.
I later heard that Bob got the job as chief engineer and after a few more years Bob was president of Boeing.
As to Pete's statement, "The Knight Twister is a classic case of scaling down big- airplane construction to small size." He uses this statement as it was speaking of some that showed lack of knowledge of what crime they are committing. I have been a design engineer for over 60 years. His imaginations should not be put out as facts, but statements of his opinions. I thank him for condescending to give us a few nice compliments if he did not proceed to tear it all down afterwards.
The power was not scaled down, we used what we had to play with and could afford. We had hoped for 50 hp, but found a second hand Salmson 45 hp at about $350.00 and used it.
The first landing gear we used was what we called a knee action shock system, as automobiles at that time were using this term. The gear was not too satisfactory. It was a beautiful soft landing when going straight down the runway but if the pilot turned before coming to a slow roll, the gear would lean the wrong way and the plane would bank the wrong way and scrape the wing tip. We immediately corrected this.
Pete is not original in stating, "The Knight Twister was a handful to fly." The problem was we, as well as other designers, who over controlled the M-6 airfoil. It is an inherently stable airfoil, so we did not need so large a control surface to balance and fly properly. We needed less control and we finally came up with controls that allowed more movement of the control stick and less movement of the control surfaces.
We are still somewhat more sensitive than the average training airplane. The Knight Twister does not change ends when taxiing, if the pilot does not fidget and keep working his feet. Better to just line up on a straight line down the runway and do not play with the rudder pedals - let her go straight and gradually put on power, she wants to fly.
Do not horse around with the stick, just easy movements and she answers very well. You will soon relax and enjoy it.
Speaking of safety records, why not think of the many unqualified pilots that got the wrong advice, also, many who were afraid to admit they were afraid of what they heard, from unqualified advisers, but dared themselves to show his friends he could fly and got into the Knight Twister and when it acted in answer to his over control he became excited and did the wrong thing. I know of one owner of a Knight Twister who later learned how and loved his Knight Twister, and bragged about it later.
He sold it a few years after, and that owner wrote to me stating, "It was the most perfectly balanced airplane he ever came across." He was born too late.
Pete stated that the wings were built with built-up truss ribs. Not so, they were too small for that type of construction. They were built of 1/16" plywood, with spruce reinforcement at the rib caps, so that the plywood skin could be glued to the ribs. Also, to provide 1/4" width so we could nail the skin temporarily until the glue dried, then remove the nails afterward, which was no big deal to accomplish.
The Knight Twister wings were very strong, well over 9 G capability. In fact, one forced landing due to someone draining the gas tank the night before. The pilot was over a graveyard and had to go through the tops of trees to get to golf grounds to land. The wing tip, on landing glide, struck a newly planted small tree and it swung the plane so that the forward side touchdown folded the landing gear under and the L.H. wingtip dug in 16 to 18 inches (soft turf) up-ending the plane, which did a complete cartwheel -- L.H. wing to nose to R.H. wing -- and came to rest on the tail wheel and belly. Pilot walked away. We lifted it, placed it on a small two-wheel cart, and rolled it to the hangar. The L.H. wingtip rib and skin was wiped out with two spar tips sticking out like spears. Later we found the front spar had a longitudinal crack from tip to bolt hole where the "I" strut is attached, which is 24 inches long. We rebuilt it and flew it in a week.
We made changes, many times just to please the prospective owner. For example -- at first nobody wanted to listen when I stated that the wings were fully cantilever and did not need wires. The general public preferred biplanes with wires. So I put on one lift wire and one anti-lift wire on each wing -- just for show -- up until the beginning of the Second World War. After World War II we took the wires off. Before this was done, some Dept. of Commerce aircraft inspector saw a Knight Twister in St. Louis and asked me if I didn't know that there should be two wires not just one? My reply was, "Suppose I take them off." His surprised question -- "Are they cantilever?" My answer was "Yes." He had no more questions.
In 1956 I had a phone call from a Knight Twister owner near Seattle. He stated that while he was flying he had a flying (lift) wire break. My answer was "I doubt it, I believe it was the wing fitting", and he agreed. I asked him "What did you do then?" Why, he just flew back to the airport. "Now what does that prove?" I said... "You do not need wires, so just take them off." He did.
We did not lengthen the fuselage for more directional stability. We needed to move the pilot back 9" to allow him to see over the wing tip on racing around a Pylon.
We were not designing a new fuselage to (as Pete says) "Get away from problems from Curtiss Hawk shrink" - we wanted more speed and comfort for the pilot. We designed the cockpit with more depth to come up around his shoulders, and the top view of the fuselage outline is a laminar airfoil shape, with a wide portion at the pilots cockpit. Airfoil is 64-021. It seems that Pete Bowers and many others think they can guess or make believe they know what the right answers are, even assume they know more about our original design and later changes, than we do. Pete is even wrong about the sport biplane races at Reno in 1984. The Knight Twister came in second at 185.349 mph.
The straight wing Knight Twister built by Walt Redfern was not my idea, but I did a little consultation with Walt. Walt had a small Knight Twister and asked for more wing area on the rebuild. Walt is a very good friend.
I was never a very good salesman, and never had the money needed to pay help to assist in experiments, etc. So I did not take care of using my advantage at the time to create a big desire to own Knight Twister airplanes.
The first Knight Twister with a small radial was minus a NACA cowl because we did not have $10.00 for a second-hand one. Being a designer, I was satisfied I built something that got worldwide acclaim and I pointed the way for others.
Thank you Pete Bowers for the few nice things you said, but for the other things, this statement of facts about the Knight Twister is my answer.
Vernon W. Payne