Engineering Problems of a Small Pusher Design

INTRODUCTION

THE ADVANTAGES OF A PUSHER -- automobile easy entrance and egress, excellent vision, quietness, etc. -- have been recognized for a long time, yet few successful aircraft of this type have been developed. Many designers have purposely avoided the pusher because of the many unknown problems that might be encountered with it, just as the now conventional low-wing arrangement was avoided before sufficient research data were available.

The pusher does have problems of its own, but all are capable of solution. A few of the more important ones are: (l) Wing-body interference; (2) balance; (3) cooling; (4) soundproofing; (5) unconventional structure.

Although the time required for the development of a pusher is undoubtedly greater than for a more conventional design, the inherent advantages of such an airplane should contribute much to the enjoyment of personal flying.

GENERAL DESCRIPTION

The Anderson, Greenwood 14 is a two-place, all-metal pusher. It is powered with a Continental C-90, 90-hp. air-cooled engine. The fixed tricycle landing gear has a long (12-in.) stroke and the nose gear is steerable through the wheel-type aileron control. The tail is carried on twin booms that attach to the wing center section.

Passengers are seated forward of the wing leading edge in a 44-in. wide cabin. Baggage and fuel are located near the center of gravity of the airplane. Baggage is carried in a compartment aft of the seat and below the wing. Fuel is housed in a 23-gal. compartment integral with the nose of the wing center section.

The power plant is located just aft of the center section monospar and is connected through a short spacer to the pusher propeller. Cooling air is ducted from just below the wing leading edge to the engine and exhausts from the engine compartment directly into the propeller.

DESIGN CONSIDERATIONS

In present-day airplanes vision over the front is limited by the engine, and because of this the instrument panel is brought almost up to this line of sight. Some pushers have been built with a normal instrument panel height and vision was little better than that from the better tractor airplanes.

If really good vision is to be had, the instrument panel must be lowered, and unless roominess is sacrificed, it must be reduced in depth. In larger airplanes with more complicated instrumentation, this reduction may present a problem, but, if careful layout is used, a satisfactory solution may be obtained for small personal aircraft.

Lowering the instrument panel does increase the area of the windshield and tends to increase the drag slightly, but the larger windshield serves a useful purpose, and the slight increase in drag can be offset by attention to some of the other aerodynamic details.

Vision to the rear is also desirable and the shoulder-high position of the wing was chosen for this. In the 14 only a small area is blocked by the wing, and this can be cleared by shifting the head position.

The wing of Model 14 is somewhat unorthodox. The aspect ratio, 9.6, is probably the highest that has yet been used on a small personal airplane. Reasons for this wing design might prove interesting.

The first requirement was for a high span to increase ceiling and rate of climb and decrease speed for maximum rate of climb. The small area was chosen because gust loads are inversely proportional to wing loading and comfort at cruising speed could be improved. The smaller wing area decreases drag and improves cruising performance. In addition, it results in a lower wing weight. Wing flaps are employed so that the smaller area does not result in decreased low-speed performance. A relatively thick airfoil was selected because of structural weight considerations and because of improved stalling characteristics.

The original flaps designed for the 14 were double-opening split flaps (Fig. 1). Their purpose was to minimize the change in trim due to raising and lowering flaps. Later, single-opening plain flaps were tried, and practically no difference could be detected as against the double flaps. No change of trim was noticeable with either set of flaps.

PROBLEMS CONNECTED WITH PUSHER DESIGN

Wing-Body Interference

Unless a high-wing configuration is used, wing-body interference becomes a major consideration in a pusher design. The stumbling block in many pusher developments is in attaining a reasonably high power-off maximum lift coefficient. A low-wing arrangement often proves disastrous in this respect, particularly if the converging body is terminated in the vicinity of the wing trailing edge; the combined adverse pressure gradients near the trailing edges prove too much of a barrier, and early separation is almost a certainty at higher angles of attack.

This situation was present to a lesser degree in early tractor low-wing arrangements, and many failures resulted before the expanding fillet was developed to remedy the situation. The expanding fillet has been tried on several low-wing pushers, but, because of the adverse conditions, it is doubtful if much improvement was noted. If a long extension shaft is used so that the body has less convergence and terminates somewhat aft of the wing, the use of an expanding fillet could conceivably cure completely the body-wing interference troubles.

Power tends to decrease the unfavorable pressure gradient of the wing-body intersection and to prevent separation, so that this is not a problem at the lower cruising lift coefficients. The favorable effect of power is absent, however, in landing when the largest possible lift coefficient is needed.

The wing-body arrangement in the Anderson, Greenwood 14 avoids the difficulties of a converging body in the vicinity of the upper wing surface. The only part of the body projecting above the wing is the part that houses the flat engine, and convergence is kept to a minimum. Below the wing, the body is highIy tapered and separation might again become a problem on the lower surface of the wing at low lift coefficients (corresponding to cruise and high speed flight) were it not for the favorable effect of power.

Actually, at 100 m.p.h. with power off, there is a small region of separation at the body-wing juncture near the trailing edge. Tuft observations of this were made with a mirror attached to the lower surface of the wing. The application of power cleans up the flow completely. At the higher lift coefficients the flow is smooth with or without power. The cooling air intake ducts help fillet the wing-body intersection beneath the wing.

In addition to the precautions mentioned above, an airfoil was selected (NACA 4418) which is least susceptible to wing-body interference troubles.

A high-wing configuration could have been used for the 14, but vision to the rear would not have been nearly so satisfactory as with the semihigh-wing configuration used.

In spite of the precautions taken on the 14, tbe maximum lift coefficient is dependent upon the smoothness of the wing-body intersection and upon the smoothness of the body-wing intersection. As originally flown, the stalling speed of the 14 was approximately 7 m.p.h. faster than the improved design.

Stalling speed and stall progression are now considered satisfactory. The stall begins at the body-wing intersection and spreads outward to the wing booms. The outer portion of the wings stall only at extremely high angles of attack.

Balance

The problem of balance is an important one in the case of the pusher. The center of gravity still has the same relation to the wing position as in the tractor, but that is about as far as the similarity goes. The location of the major weight items -- structure, useful load, and power plant -- are completely new.

Let us first consider the useful load items. Their location is now forward of the airplane center of gravity. So that the center of gravity does not vary too much, these items were placed as near to the weight empty c.g. as was possible. The exception to this is the weight of the pilot, which, for flight purposes, was treated the same as a weight empty item; he will be in the airplane whenever it is in flight. Location of the useful load items near the weight empty c.g. is good insofar as the center of gravity shift is concerned, but, in order to obtain the correct balance with minimum useful load, it is necessary for the weight empty c.g. to be as far forward as possible.

The major item in the weight empty which is subject to design movement is the power plant, and it was located as far forward as possible, immediately aft of the main wing spar. A further movement forward would have involved moving the wing, which would defeat the purpose of providing a forward position of the center of gravity with relation to the wing. The short chord of the wing allowed a propeller to be attached directly to the engine through a short spacer with the engine in this forward location. The spacer used on the 14 is approximately 3 in. Iong and is a glorified aluminum washer. This installation has proved completely satisfactory in over 200 hours of flight testing.

The position of the center of gravity with the useful load out is also of importance, in that, since the useful load is all forward of the center of gravity, its removal shifts the center of gravity decidedly aft. Unless careful consideration is given to this problem, it might result in a shift aft of the main wheels and cause the airplane to sit on its tail when empty.

The balance as worked out for the 14 is from 16 per cent to 26 percent of the mean aerodynamic chord in flight, and a weight empty position is such that there is a load of 40 Ibs. on the nose gear. The load on the nose gear increases to 200 lbs. at full gross load. The center of gravity shift in the flight conditions is a little misleading as given above because of the short mean aerodynamic chord. Actually, the shift is approximately 4.5 in.

Cooling

Another problem critical to the pusher is power-plant cooling. This is by no means a simple one, and, be.cause of its complexity, it will be discussed only briefly in this article. A cooling fan could have been used, but the increased expense and complication ruled it out.

The problem of ground cooling with the propeller is the reverse of that in a tractor. In a tractor, the propeller directly forces the air through the system, while in the 14 the low-pressure area ahead of the propeller is used to induce the cooling airflow. In order to accomplish this the cooling exit area was located in the immediate vicinity of the propeller. To date no cooling troubles have been encountered during normal ground operations.

Cooling air intake ducts are located below the leading edge of the wing. This location has given excellent results under all flight conditions. Cooling air flows from the ducts into the lower engine and accessory compartment, upward through the cylinder baffles, and then to the exit areas in front of the propeller.

Soundproofing

The problem of noise in a pusher is one that is generally misunderstood. The common misconception is that with a pusher engine the noise is "left behind." This is undoubtedly true at speeds above the velocity of sound, but at the current speeds of small personal airplanes this is a minor influence.

For example, assume a cruising speed of 100 m.p.h. The ear of the occupant is traveling away from the noise source at this same speed. At sea level, the noise is traveling toward the ear at 760 m.p.h. This means that the intensity of the noise heard is the same as would be heard in the airplane were it motionless and the occupant was one-seventh farther away from tbe noise source. Passengers in a pusher are usually located nearer to the noise source than in a tractor, so little reduction in noise should be expected at speeds of 100 to 200 m.p.h.

The big advantage of the pusher is that adequate soundproofing can be installed between the noise source and the occupants. This is not possible in the tractor arrangement because of the necessity of windows between the noise source and occupants.

Much of the soundproofing in the pusher is obtained without additional cost, since upholstered seats are extremely effective in absorbing sound and additional soundproofing need be added only in areas not protected by the seats. For the same weight of sound proofing material the pusher normally should be substantially quieter.

Unconventional Structure

The large field of vision, forward and down, possible in the pusher design is not compromised in the 14 by an overly conventional shaping of the forward fuselage or by excessive width in the window corner posts. It is possible to have convenient entrance and egress with the passenger cab close to the ground, forward of the wing, and unendangered by the propeller. High structural body sides are not allowed to detract from this convenience.

These demands on the structural design, that vision and entrance be excellent, are met in Model 14 by employing a keel beam as the load-carrying member forward of the seat (Fig. 2).

Nose-gear loads are the major loads in the forward keel beam, since there are no large masses forward of the passengers. The keel beam is reacted horizontally through the baggage floor, and vertically between an open bulkhead in the plane of the seatback. The shear connections between forward and rear bulkheads are the body side skins in this region, the baggage floor, and the wing leading-edge bottom surface.

The wheels are well back under the engine section. Part of this distance back from the nearest convenient structural pickup has been covered by "sweeping" the landing-gear legs back from the trunnions. In order to avoid the weight penalty of cantilever trunnions, a rear support point has been incorporated into a welded tube structure, which also comprises the engine mount.

Similar rear trunnion point supports are also incorporated in this tube structure for the oil-spring shock absorbers. The forward trunnion support points for landing-gear legs and oil-spring units are on the diaphragm bulkhead. Since this same bulkhead incorporates the engine mount pickups and the main spar wing pickup points, it can be seen that the 14 is structurally rather short-coupled. Engine mass loads are transferred through the engine mount to the landing gear during landing and to the wing spar in flight. In flight, engine inertia couple loads go through the bulkheads, through the body skin between the bulkheads, and into the wing.

The engine location, chosen to balance the airplane and to give the best chance of avoiding separation difficulties, places the engine in space normally reserved for wing structure. The structural problem of the wing in this region is to carry through the shear, bending, and torsion loads in the relatively small wing cross-section forward of the quarter chord. It has proved possible to carry the wing trailing edge through the engine section as a tension member, which helps greatly in carrying the forward wing bending from high angle of attack flight conditions.

The wing center section of Model 14 (Fig. 3) has a monospar at approximately 27 per cent chord and a light auxiliary rear spar. The spar in the center section portion of the wing is a 24ST extrusion with caps and web extruded as one piece. This has the advantages of simplicity and low cost. Simplicity and elimination of rivets are particularly important in this case, since this spar web also forms the back side of the structurally integral fuel tank.

The torsion and tank section, in the center wing region, is the "D" formed by the spar and the leading-edge skin. This "D" section has been kept as free of internal projections as possible in case a fuel cell is ever used. Fore-and-aft corrugations in an internal doubler sheet, which have been used to bring the shear strength of the skin up to the required value, start just inboard of the tank end ribs and run to the body torsion panels. The corrugations are not required inboard of these body panels, since the torsional shear is taken in the panels which, in effect, are external wing ribs.

The leading-edge torsion load is taken out of the leading edge by the end ribs, which are just forward of the wing boom connections at the outer end of the center section. These ribs have shear web, as well as top and bottom cap continuity with the boom attachment boxes that are at either end of the center section behind the main spar. These boom attachment boxes also serve as outer ribs of the center section. The boxes are made up of two parallel vertical planes and two parallel horizontal planes, which are extensions of the planes of the booms. Each boom box provides four forged fittings at the trailing edge to which the booms attach. A bulkhead in the boom box is attached to the center section rear spar and to the rear spar of the outer wing. At the wing main spar the boom boxes are secured top and bottom by the bolts that hold the wing joint terminal fittings onto the main spar extrusion.

The ribs at the body side are fire resistant, and, because of the engine well discontinuity, they have been tied into the body torsion panel to allow the transfer of load from the body panel to the rear torsion panel of the wing.

The stiffening in the intraspar portion of the center section is in the form of a spanwise intercostal spar, used to avoid the contoured parts that members in the chordwise direction would have required. For aerodynamic reasons, the trailing edge of the center section is internally stiffened rather than corrugated. The large well-rounded trailing edge has been found to stand up well under the vibration stresses imposed by the nearby propeller.

The attachment of the wing to the body consists of the body torsion panel and a bolt on each side of the airplane running in a for-and-aft direction through an angle extension of the diaphragm bulkhead and the web of the main spar. The spar is built up by a pad to increase the bearing loads where the bolt passes through. The tank is sealed around the bolt by a beveled washer trapping an "O" ring against the bolt. The body torsion panel acts as a wing rib putting vertical load in the wing leading edge and maintaining the wing cross section against "parallelogramming."

Vertical loads from wing to body go through spar bolts to the bulkhead and through rivets between the vertical part of the leading edge and the body torsion panel. Horizontal shear loads are carried through the body torsion panel, and these loads require considerable shear strength between the upper and lower edges of the torsion panel. This has necessitated fairly thick shear doublers in the small neck of the torsion panel opposite the wing leading edge.

The vertical sides and flush-to-wing bottom surfaces of the tail booms (Fig. 4) were dictated by the aerodynamics of the wing boom intersection which called for an extremely clean surface detail. At the forward end of the boom two bulkheads and several shear doublers put the loads into four forged fittings that mate with similar fittings in the boom attachment boxes. The front and rear spars of the vertical tail form the bulkheads in the rear of the boom that load the boom with the horizontal and vertical tail loads. A fairing cover to the top pan provides aerodynamic cleanness and accessibility to the tail cable controls and allows a beam design with good accessibility for fabrication.

The horizontal stabilizer is composed of two spars, two end ribs, one rib on the airplane centerline, and beaded skin. The beads run in a for-and-aft direction and increase the ability of the 0.016 skin to carry the local air loads. Three ribs has proved a sufficient number to make the spars work together.

All the movable surfaces -- ailerons, elevators, flaps, and rudder -- are similar to each other in construction, and all are attached to their respective fixed surfaces by short lengths of piano hinge. The typical construction is a spar of channel or "Z" form, with corrugated skins (4-in. spaced, 1/4-in. deep, "tent" corrugation), an inserted 0.040 trailing edge strip, and end ribs with operating horns attached. It has proved desirable to reduce the hinge deflections by a clip that triangulates from the outer edge of the flange under the hinge to the flange web intersection across the spar. This stiffening reduces control system deflections and is a safeguard against fatigue failure in this area.

Like the center section, the outer wing is monospar with an auxiliary spar (Fig. 5). A torsion cell is formed by the two spars and top and bottom skins. A second "D" torsion cell is formed forward of the main spar by the leading-edge skln and is stiffened against buckling by leading-edge ribs and spanwise intercostal spars.

At the joint between the center section and the outer wing the torsion load from the two wing cells is collected in a rib with a reinforced cap strip. The torsion toad is then reacted against the tail load through the main spar terminal and the rear spar terminal, both of which tie into the center section boom attachment box.

The main and auxiliary spars are connected by four main ribs in the outer panel, though each interrib skin area is broken up into three smaller panels by ribwise channel stiffeners. These stiffeners, as well as the ribs, are gusseted to the front and rear spars to take the corner loads from the panels when the light gage skin buckles under load.

The 0.016 gage skin from the main to the auxiliary spar is one continuous sheet from the inboard to the outboard of the panel. The leading-edge skin is also continuous. The outer wing has no trailing edge, since all of the area aft of the auxiliary spar is taken up by movable surfaces.

The outer wing vertical loads come through fittings from the main spar of the outer wing to the main spar of the center section. These fittings are heat-treated steel slabs of simple design. One fitting design serves 16 times per airplane, two upper and two lower on outer wing and center section, left and right wings. At the spar end, in addition to the steel fittings just mentioned, there is an aluminum column bar from the upper main bolt hole to the lower and tied into the shear web of the outer wing spar. The function of this member is to take bending out of the terminals and to pick up the outer spar shear load. The bolts connecting the wings are in a fore-and-aft direction in holes reamed on assembly through the wing fittings. The outer wing and center section rear spars are bolted together through a fitting to carry the tension and torsion loads.

An interesting design detail is the outer wing spar. This member is made up of extruded caps, tapered by machining and riveted to a corrugated web. The corrugations, spaced at 4 in., serve as spar web stiffeners. While low cost by elimination of parts and rivets is possibly the chief advantage of this web, it has also turned out to have a low weight. A series of tests were necessary to accumulate enough information to design this shear web.

The wing tip is plastic-impregnated fiberglas and is attached to the flange of the tip rib in the outer panel by screws into Tinnerman nuts.

The landing gear of Model 14 is designed to land the airplane safely from a rate of descent of 15 ft. per sec. (900 ft. per min.), which is considerably in excess of C.A.A. requirements. It was originally conceived as an added factor of safety but, in addition, has worked out to give a pleasant landing.

With the extreme vision over the nose, a normal landing gives the visual sensation of rapid descent, and with flaps down the approach is rather steep. The pleasant, gentle nature of ground contact comes as a surprise. The passenger has the feeling that the airplane has merely been turned into a more horizontal path.

The control system is a conventional cable type with dual control wheels. Rudder pedals have been provided on the left-hand side for the pilot who wants to achieve perfect turns, but there are none on the right because the airplane can be very nicely handled without any and because their absence gives the passenger more leg room. The elevator tab is a hand-operated crank. The brakes are hydraulic and are foot-operated from a pedal on the floor, although some thought has been given to a combination brake-throttle, which may be included before certification is complete.

The engine section is of interest as a distinctly pusher problem, though it has much in common with a submerged installation. Underwing ducts of triangular cross section pick up air at the wing-body intersection near the wing leading edge. The inducted air is carried along to a point aft of the main bulkhead, flows through a pressure-tight compartment under the engine, up through the engine baffles, and out on top of the engine. The air exits through passages just forward of the propeller. This relation to the propeller is counted on to draw air through the system for ground cooling, eliminating the need for a blower.

Accessibility to the top of the engine is through an upper cowl built all in one piece and hinged alligator style off the cabin roof near the main bulkhead. Lower engine compartment cowls are individually removable with Dzus fasteners.

The exhaust system is muffled for carburetor and cabin heat. The exhaust gasses pass through the airplane bottom and are directed downward to prevent them from impinging on the propeller and causing noise and vibration. The carburetor inlet has what at first appears to be a rather odd location since it is on the airplane centerline facing directly aft. Pressure checks reassure that the location is favorable, and this location makes for the simplest possible connection to carburetor.

Captions:

TITLE ILLUS.

FIG. 1. Double-opening split flaps were originally designed for the Anderson, Greenwood 14 to minimize change in trim due to flap operation. Landing gears are long-stroke, oil-spring type; the main legs are X4130 steel welded into a box form and heat-treated. The nose cylinder is attached to nose-wheel fork (a T220 aluminum sand casting) and passes through two oilite bearings held in nose gear pickup casting. Cylinder also rotates in these bearings as gear is steered.

FlG. 2. A keel box beam is employed as the load-carrying member in the Model 14 body as shown in this struetural diagram. Completely below the floor level, it permits a full-sized low level door in body side and large front and side windows with narrow nonstructural corner posts, allowing convenient entrance and egress, as well as a large field of vision.

FIG. 3. Wing center section is composed of a monospar and auxiliary rear spar. The spar in the center portion of the wing is a 24ST extrusion with caps and web extruded as one piece. This has the advantages of simplicity and low cost. Simplicity and elimination of rivets are important in this case, since this spar web also forms the aft side of the structurally integral fuel tank.

FIG. 4. Boom structure consists of a top pan, sides and bottom made from one bent-up sheet, "W" sections added at bottem corners to increase compression strength of boom, and an intercostal midway between top pan and bottom to increase buckling strength of sides.

FIG. 5. Structural detail of outer wing showing monospar and auxiliary spar. In the outer wing, auxiliary spar picks up movable surfaces and also carries local air loads to four main ribs, which in turn carry loads to main spar.

FIG. 6. Model 14 plan view.