New ideas are incorporated in this tiny biplane which should appeal to the lightplane fan, and the author tells you how he arrived at this design.
(From Popular Aviation, 07/1933)
By Vernon W. Payne
A SINGLE-SEATER small biplane, with big performance! Have you ever had a friend tell you that a small plane just landed at the local airport, and then take a run out there to see it?
Well, I have too, and quite often it is not the kind of an airplane that you wanted to see. Many of us would like to see a small biplane very nearly representing a Boeing Pursuit, reduced in size, but most of the small planes today, are monoplanes of a span ranging from 26 feet to 35 feet and with an overall length of from 17 feet to 20 feet.
They are not so small when it comes to paying hangar rent, especially if they charge according to the span of the ship.
Well fellows, I’d like to tell you of a small biplane, that, I hope, fills your needs and embodies all the best design practices of today. The span of the upper wing is 15 feet, the lower is 13 feet. The chord of the root rib is 30 inches, and the tip rib is 18 inches. The overall length of fuselage is 10 feet, 9 inches.
How is that? Is it small enough? The wings are tapered and plywood covered, then cloth is doped on over the plywood. The wings are designed as cantilever wings, like the Lockheed, but to further strengthen them against wing flutter, we put in an “I” strut between the wings.
We also put in one flying wire for additional safety for some fellows are afraid of the looks of a biplane with no wires. Cantilever wings are a little heavier than the regulation biplane wings, but you have a lower resistance because you do away with a lot of brace wires, out in the air. It takes less gas to pull a ship with low resistance, so we will try to make that kind of ship.
Note the clean lines and the neat arrangement of the structural parts, particularly, the landing gear. If desired, the interplane bracing can be omitted for both wings are sufficiently strong to carry the load without this bracing.
NOTE: larger versions of this 3-view are available on the Knight Twister 3-Views page. It is listed as the KT Prototype.
Although I am very much in favor of a plywood fuselage, this ship is built with a fuselage of steel tubing welded. The landing-gear is a mono-strut type, fully streamlined. The engine is a 9-cylinder Salmson, which develops 50 H.P., and which is provided with an N.A.C.A. cowl. The stabilizer is adjustable from the seat while in flight. The span of the stabilizer is 6 feet and is a full cantilever, or in other words it has no brace wires. If you want to make it stronger you can put brace wires on it.
The wheels can be airwheels 12″x5″x3″ or 16″x4″ high pressure tires. The drawings show 16″x4″ tires. The airwheels make it easier on the bumpy landing fields. This ship may land a little faster than some amateurs are accustomed to, for it has a landing speed of 48 to 50 m.p.h. and a top of 150 m.p.h. The wing section used is the N.A.C.A. M-6, which is known as an inherently stable airfoil. The center of pressure is practically always at 25 per cent of the chord from the leading edge of the wing, throughout all ordinary angles of flight.
A stable airfoil is one in which the center of pressure moves to the leading edge when the ship dives, and then moves toward the trailing edge when climbing. When an airfoil has this kind of center of pressure movement, it has a tendency to level itself out of a dive or a stall. The majority of airfoils are not that way. The Clark “Y” has a center of pressure movement towards the trailing edge in a dive condition. This C. P. movement aggravates the dive, making it steeper, but on an airplane, the tail surfaces counteract this unstable tendency. In a climbing angle of attack, the Clark “Y” has a C. P. movement towards the leading edge, this tends to increase the angle of attack or to stall and the tail surfaces of the airplane have to right this condition.
In designing this ship, we wanted a nice appearence and a fairly high top speed with economy. The neat appearence meant a small pursuit type biplane and economical high speed meant a minimum of parasitic resistance. Parasitic resistance is the total resistance of all the parts of the ship that do not give lift. That means we must cut out all wires and braces if possible. We first meant to have no lift wires, but you will now find one wire.
The landing gear is small and incased in sheet aluminum pants. The wheels have “spats” or aluminum wheel fairings. The pilot’s head-rest fairs back to the vertical fin. The wings are tapered and faired into the fuselage. The wing strut is an “I” strut of stream-lined shape and faired at the point of contact with the wings, for anything which is in contact with a wing causes about four times the resistance that part would have by itself when alone in the wind.
For good visibility, we placed the upper wing in line with the pilot’s eyes. If you are 6 feet, 4 or 5 inches tall, the wing will be in line with your “Adam’s apple,” so don’t stop suddenly. The ailerons are on the lower wing only and controlled by tubing-no wires here. The ailerons are welded steel tubing cloth covered, but as I mentioned, the wings are plywood covered with spruce spars.
Notice the size of propeller that this engine swings. It is a six footer, and that means a high setting airplane when on two points. When on 3-points, it looks as if the nose of the ship is sure pointing skywards. The lower wing is reinforced near the fuselage to provide a step to get in and out of the cockpit. The tail surfaces, the elevators and rudder, are controlled by cables. The elevator cables are inclosed in the fuselage and vertical fin. The rudder cables come out of the tail of the ship to the rudder horns. The stabilizer is controlled by tubing.
When deciding on the shapes of the tail surface and wings, and you consult such authorities as Walter S. Diehl, Charles N. Monteith or Alexander Klemin, you will find that the tail surfaces should be elliptical in planform which combines the good features of both the rectangular and triangular shaped tail surfaces. Tail surfaces that give maximum stabilizing and control effects are thin sections. A good aspect ratio for the horizontal tail surfaces is span divided by chord equals three. A large aspect ratio in the tail means that more of the tail surface will be outside of the propeller slip stream and a smaller difference will be noted between power off and power on. Too large a span of tail surface requires heavy struts as braces or several wires.
Flat spins are bad and on investigating data on ships that had tendencies to flat spin, it seems that biplanes should not have wings of equal chord without stagger. A biplane with stagger, tapered wings, and an upper wing of greater span than the lower is a desirable one. Large tail surfaces are good aids in recovering from spins. The weights in the ship should not be too spread out.
The most desirable shape of wing, for efficiency, is one that is tapered and has an elliptically shaped tip — or better still — a combination of elliptical and negative rake tips. This tip will not cause wing flutter as square tips do.
A tapered wing requires a lighter structure compared to a rectangular shaped wing, and is easier to control laterally. The tapered wing has a higher L/D. This is good for climbing. With tapered wings, smaller ailerons can be used than with rectangular shaped wings. Some thin wing sections require the area of the ailerons to be 12 per cent of the wing area.
Some large airplanes with tapering, thick sectioned wings require the aileron area to be only 7 percent of the wing area. The small ailerons have small chord and large span, or 15 percent to 20 percent of the chord of the wing, is the chord of the aileron. Our aileron area is about 10 percent of the total wing area.
The better airplanes have a tail length of three times the average chord of the wing, not less than 2-1/2 times chord, measured from the center of gravity back to the tail post.
We made the horizontal tail surfaces area equal to one-sixth of the area of the wings, or in other words 16-2/3 percent of the wing area. The vertical tail surface area is slightly less than one-half the area of the horizontal tail area. These are what we call fairly large tail surfaces considering that the all length is over 2-1/2 times the chord.
The “Knight Twister” described here incorporates the experience of the writer in building other lightplanes, several of which have been described in past issues of POPULAR AVIATION.
One of these early planes, shown in the June issue of P. A. has a monocoque fuselage.