Knight Twister – Hale Wallace’s Baby Biplane Bullet

(From Sport Aviation, 10/1999, Page 44)

By Budd Davisson
Photography by Mark Schaible and Hale Wallace

Before there was an EAA. Before Pearl Harbor. Before monoplane fighters. Before there was a Hale Wallace there was Vernon Payne and his little airplane. The year was 1928, the airplane was the Knight Twister and, to a world accustomed to hulking, blunt nosed, wire-braced, bi-winged polywogs, the Knight Twister must have looked as if Buck Rogers had come to town.

Vernon Payne was an interesting individual and someday his story needs to be told. However, an indication of his devotion and ingenuity can be seen in the Knight Twister. Often maligned, more often admired, the design incorporated some inspired engineering and some absolutely edge of the envelope thinking. Even today, for instance, you wouldn’t find many engineers willing to design an airplane for the masses with 15 foot wings incorporating only 55 square feet of wing.

The airplane hasn’t been without its detractors, most of whom have never flown the airplane. However, a casual look at the accident statistics seems to bear out the nay-sayers. The airplane has had its share of accidents and even Payne’s son perished in one. The most common accident was the classic stall-spin on approach.

Was the airplane really that bad? Not if you talk to any who have flown the airplane for any length of time. Listen to them for a while and a somewhat different picture emerges. The late Tony Sablar, whose KT-85 epitomized the classic 1950’s homebuilt, put over 500 hours on his and liked to point out that his flying background prior to the KT was nothing but Cubs and C-140’s. Bill Nagle, who is still flying the Twister he bought in 1951, swears by the airplane and he has flown it from the original 85 hp, to 125 hp and now 160 hp with a constant speed!

In today’s kit oriented market, the Knight Twister is not only practically unknown, but is still shrouded in mystery. What is the truth to how it flies and its construction?

Enter Hale Wallace of Steen Aero Lab and winner of this year’s Reserve Grand Champion — Plans Built award with his Knight Twister. Wallace is probably the world’s leading biplanaholic because, besides being the Skybolt plans and parts supplier, he also owns the rights and sells components for the Pitts S-1C/D/SS line. A few years ago he was offered the rights to the Knight Twister, which included boxes and boxes of engineering drawings and plans. It had two wings so naturally he couldn’t pass it up and added it to his bi-winged stable. Incidentally, Hale says, “There is mention in this material of Payne offering a stress analysis of the Twister for sale. I’d love to find a copy of that.”

It’s interesting to note that Steen Aero Lab is also the exclusive purveyor of Brunton flying wires in the country, but the Knight Twister, as designed, uses no flying wires.

Wallace openly admires Payne’s work and what he accomplished, as he was continually designing and redesigning the airplane right up until his death in the late 1980s. He even designed a yet-to-be-built two place.

Wallace says, “I’ve identified at least thirteen different variations of the airplane, including a straight wing version, a bunch of different tails, several landing gear variations and a hopeless number of fuselage lengths.”

But, what about the airplane’s reputation? Was it really that bad? There was only one way to find out. “We decided to build the baddest version of the airplane, the short wing, short fuselage model and find out how the airplane actually flies.”

There are actually three different wing lengths for the single place airplane, for which Hale offers plans; 15.5 feet (Akro) which Hale built, the 17.5 foot (Imperial) which has the 75 square feet of area required for biplane racing and the 19.5 foot sport wing. For reference, the short wings are three feet shorter than those on a single-place Pitts!

The construction of the airplane emphasizes light weight and utilizes balsa wood as streamlining material. As designed, the “I” struts, for instance, are a single piece of 1/4″ plywood faired to shape with balsa. The tail surfaces use 1/16″ ribs with 1/16″ ply cut in the outline of the trailing edge shape, laid flat and inserted, model airplane style, into slots in the rear end of the rib. This is then faired with balsa to give a wedge shape. Many parts of the airplane bear an eerie resemblance to a model airplane. Considering that at 15.5 foot span, many model airplanes are larger, it would appear that the gap between models and real aircraft has closed.

The wings are actually designed to fly without either wires or the “I” strut, but Payne put the “I” strut in for peace of mind and additional rigidity. Also, most Twisters built have at least one flying wire per side. Again, un-needed. Hale built his as designed, without flying wires just to show it could be done the way ol’ Vernon said it could.

Both wings are single piece units with the space between the two spars sheeted with 1/16″ plywood top and bottom making the unit into a rigid torque box. The leading edge is designed to be sheeted in balsa or aluminum and Hale says, if he does another one he’s going to continue the torque box wood right to the leading edge. The area aft of the rear spar is fabric covered.

Although an estimated 75 Knight Twisters have been built over the years, Hale says building the wings perfectly straight is critical and he thinks some of the airplane’s reputation may have come from lack of attention when building the wings. Part of the wing difficulty comes from the fact that the spar (seven or eight 1/8″ laminates) is stepped and tapered in three dimensions with the top surface, which is straight span wise, curved chordwise to follow the M-6 airfoil. Hale bought a 20 foot long, aluminum “I” beam to use as a datum to work from and to use as a router guide when working different parts of the spar. Also, every single rib is not only a different size but is made in three pieces and has to be jigged square to the spar. And then, just to make things more fun, the top wing has two degrees of washout built in.

Wallace, who is still a very serious model builder and was on the 1970 U.S. World Scale Model team, utilized a lot of his model building techniques in building the wings. For instance, rather than leaving anything to chance, he built cradles for each rib location to sit in, both top and bottom, so when he laid the unskinned wing down, on either side, it remained straight. Then, when he skinned the wings he was virtually guaranteed it would be straight. This is critical because, unlike most biplanes, any twist cannot be rigged out by twisting the wings with washers at the “I” struts. He says the surfboard sized wings are stiff as concrete. He also says he can’t imagine how they used to get the wings true before the days of smart levels. He credits them for a tremendous increase in accuracy.

The relatively tiny fuselage is quite wide for its length and utilizes side opening clam-shell doors to allow larger than normal pilots to board. The doors close up around the pilot’s neck, easily clearing wide shoulders.

When it came to the tail, Hale found some confusion because of the different horizontal tail spans shown on different plans. He opted for the shorter six foot tail and reduced both the chord and the travel of the elevators. Now that he’s flown the airplane quite a bit, he says he’s glad he made those changes as it has more elevator authority at all speeds than it can possibly use.

One of Payne’s better pieces of engineering is the way he mounted the spring landing gear on his later models (earlier ones used a bungee gear). A common problem with spring gear is cracks developing in the longerons next to the leg. This is because of the bending that occurs fore and aft because of spin-up loads on the wheels at the end of the long moment arm that is the gear leg. Most designers simply keep adding steel in that area of the fuselage until it stops cracking. Payne solved the problem by bolting a long, welded box-like bar which parallels the lower longeron to the bottom of the gear leg. The bar goes back one bay to the next cluster and is bolted to a bushing there. That way the twisting moments are transferred through that arm to the cluster taking them entirely out of the gear mount area. Inspired engineering!

The original control system called for 1920’s style bellcranks and cables, but Hale couldn’t see doing it that way. His Twister’s control elevator system is 100% Pitts Special using a torque tube coupled to push rods. According to Hale it’s much more rigid and reliable. Aileron controls are ball bearing push rods per plan.

Hale also praised Payne for his ability to draft pieces that fit. He said all the full sized patterns were absolutely dead on.

Knight Twisters have been built with everything from 40 to 180 horsepower, but Hale settled on the O-235 Lycoming and had one crafted specifically for the airplane by Monte Barrett that pumps out 132 horses on a dyno and is equipped with all B & C lightweight electrics. This particular O-235 is reputed to be the actual engine which was mounted in the original PA-12 factory demonstrator.

The detail that’s on the outside of the airplane follows right into the engine compartment as everything is not only exquisitely finished but each line going through the firewall has its own identifying engraved placard.

Originally Hale was going to cowl the engine completely in aluminum but delays at his tin-beater forced him into hand forming a fiberglass cowling only a few weeks before Oshkosh. Most of the fairings, however, are aluminum.

He wanted a particularly slick cover job so he double covered it without tapes the way Jim Younkin and a few others do. Initially he laid down a coat of 1.7 ounce Dacron glider cloth. Then he did all rib stitching using flat stitching cord and (get this!) cut little pieces of tape to fit between the stitches so everything would level out. He hit this with three coats of nitrate, then laid on another layer of Sig Coverall, which is a 1.2 ounce Dacron model cloth and brushed a 50/50 mix of cement and nitrate through it. He says, “To get the cloth and finish down into all the edges so it doesn’t bridge anything and create voids means you can’t be afraid to get your fingers wet. That’s the only way to get the cloth pushed into all the nooks and crannies.”

Then it was four or five coats of silver with 98% sanded off. Then a bunch of sealer and primer, sanding lightly between every other coat.

Considering the quality of the finish, you’re going to find this next statement a little hard to believe; there is only one (count’em, ONE) cross coat of color on the airplane. He used Durathane II. Pretty amazing!

The airplane came out weighing 720 pounds against the original design weight of 600 pounds when using a C-85 and no electrics.

When Hale talks about the first flight, he laughs a lot.

“For one thing, we have too much prop on the airplane. It only turns 2,100 rpm static and that drops to 2,000 when it starts rolling. It took forever to get off, but the second it lifted off it accelerated to 110 mph like a shot. Then I just held that speed and it was going up at better than 1,000 feet per minute. I took it up high and stalled it a few times and really had to watch the skid ball because the rudder is so light and effective. It mushed forward at about 70 mph with the stick all the way back and was completely normal. But then I had to land it.” This is where he starts laughing.

“I thought I’d fly a circling Pitts type approach and didn’t even come close to the runway. I couldn’t get it to slow down! I made eight approaches before I got it down to 100 mph low enough to the runway. Finally, I just killed the mags and it still floated. This thing is clean!”

He says the current prop has a 75″ pitch and he’s going to bring that back in an effort to increase takeoff performance and increase drag on landing.

Hale had a problem when it came to getting the airplane to Oshkosh because he wanted to bring his big engine Skybolt too. Looking around, he found Bob O’Haver, an FAA pilot, who was both willing and qualified to bring the airplane. Between the two of them, they now have nearly 40 hours on the airplane and feel they are ready to comment on how it flies. [Webmaster note: See the article entitled ‘Old-Timey Flying Helps FAAer Fulfill Promise to a Buddy’ for more about Bob O’Haver and the Knight Twister.]

Both agree that anyone who can fly an S-1C Pitts or anything similar will have no problem flying the airplane. Bob says the ailerons have just enough breakout force to let you know where center is and the roll rate is about that of a two aileron Pitts. The rudder, however, has no breakout force, is extremely powerful and has practically no pressure. Bob says he slows the airplane down for approach by using at least one high G turn going into the pattern and getting it down to 95 mph at which speed it is quite controllable. He says it is much quicker on pavement than grass, as would be expected, but that the Haigh locking tailwheel makes the roll-out quite controllable. In all situations, however, he feels its runway manners are much better than most people would expect.

SOME KNIGHT TWISTER HISTORY The Knight Twister went through an interesting series of iterations in the early days. Remember that in 1928 there was no such thing as a flat, four-cylinder Continental or Lycoming. The original airplane used a tiny 50 hp radial engine. Then in the 1930’s it could be equipped with a variety of converted Ford 4-cylinder water cooled engines that on a good day might give 70 hp, but weighed a ton. During WW-II Vernon Payne wrote, “We had planned to sell the completed airplane of 50 hp at $1,450 and… 95 hp at $2,150.” Kit parts had listed prices such as welded fuselages at $145 and complete wings covered and doped at $161.50.Payne also mentioned such interesting asides as… “Drawings were sold all over the world. I was visited by several Germans, one a captain in Hitler’s Air Service and… one Japanese was trying to sell me the idea of putting up a factory on some Japanese owned island. The foreigners were interested in my idea of folding the wings and storing it on a submarine and using a single pontoon. We were going to use a gear type engine, horizontally opposed cylinders, with a .50 caliber machine gun firing through the hub of the propeller.” Vernon was nothing if not optimistic and always thinking of something new to do with his little airplane.

Both also agree that they really like flying the airplane but that the current windshield is too small as the turbulence beats them up too much. To cure that at the last minute, Hale made up a small spoiler he clips to the edge of the windshield which diverts the wind out away from the pilot.

On the trip to Oshkosh they found the airplane would hit 175 mph flat out and cruised effortlessly at 160-165 mph at 2,300 rpm. The airplane has 22-23 gallons in the main tank with a three gallon wing tank which feeds into a fuel selector through cabane struts which double as fuel lines. Pretty slick! The main tank is actually two tanks welded together with a flop tube in the bottom one. How much range is that? Bob says he doesn’t know for sure, but it is more than he has. Something over 3-1/2 hours is possible.

He said it was fun when they landed to get gas on the way to and from Oshkosh because he could predict what the gathering crowd at the gas pump would say. They’d start out with, “What is it?”

He’d answer, “A Knight Twister.”

They’d say, “What’s that?”

We’d like to make an editorial comment here concerning the Knight Twister and its reputation. During most of the airplane’s history, there really wasn’t a comparable airplane in which a pilot could get experience. Pilots would be stepping out of Cubs and such thinking the Twister would be just another airplane and it wasn’t. Today, we have so many other high performance airplanes in our inventory that a modern pilot has no excuse for not being properly prepared. A few hours in a two-place Pitts would probably make most tailwheel pilots Knight Twister capable.

Hale says there may be another factor that contribued to earlier accidents and that is that the CG envelope is really narrow because of the narrow chord wings. If the CG is allowed to creep back just a little, the pitch sensitivity goes way up and an inexperienced pilot could get in trouble.

Is Steen Aero Lab going to be making kits available? Hale usually twists up his face when asked and answers slowly, “Yeah, we’ll probably make components, if asked, but I’d think long and hard before I’d build a wing for someone. We could get the spars cut for them, which is the hard part, then they’d be on their own.” Then he thinks a little and says, “Yeah, we might build a wing too.”

Considering the small amount of material in the airplane, it could be one of the less expensive, high performance airplanes available in the homebuilt catalog, especially considering any engine in the 85 to 150 hp range could be utilized.

This makes Hale’s second Reserve Grand Champion — Plans Built award. Is he disappointed? “Yeah, a little, but then I went and looked at the airplane that beat me and I don’t feel so bad anymore.”

Looking at his airplane, we’re just glad we weren’t the judges who had to make the final decision.

NOTE: You can view more articles by Budd Davisson on his website: Airbum.com