A simple matter? Building a reduced-scale replica presents its own challenges

One of the popular areas of the homebuilt aircraft movement is the designing and building of reduced-scale replicas.

For the most part, these are reproductions of well-known single-seat fighters which, when built, are right in the size-weight-power range of the typical single-seat homebuilt. However, this is not just a simple matter of scaling the original down in exact proportions as is done with flying or exhibition scale models.

There are three major problem areas involved here, so let’s take a look at each one separately:

The first is a matter of pilot fit. While the airplane is scaled down, generally to three-quarters to one-half size, the occupant stays full size. This means that the proportions of the scaled-down replica often have to be distorted so that the pilot can get into it. Getting him in is one thing, giving him enough room to operate the controls is something else. For example, take a famous single-seat fighter of the between-World Wars era, the Curtiss “Hawk” that was built for the U.S. Army as the P-1 and P-6 series from 1925 through 1932, with a few experimental variants under different P-numbers.

This had a fuselage only 24 inches wide at the cockpit, and even then was a snug fit for a pilot in ordinary military flying gear. In those days, the pilot’s weight was considered to be 180 pounds. The pilot of a three-quarters-scale replica would probably be that, or even more, and there is no way that he is going to squeeze into a cockpit that is only 18 inches wide. So, the fuselage is going to have to be fattened out of scale proportion in that area. This problem will be even more acute on some World War I designs.

On some later designs, notably World War II, when the single-seaters were a lot bigger, the scale proportions can be pretty well retained, as on the Hawker Hurricane, but the proportions of the canopy will have to be enlarged. This isn’t a problem for open cockpit types: the pilot’s head and shoulders just stick up farther above the top of the fuselage than on the original.

The second problem is that the powerplant to be used in the replica is not usually a proportional reduction of the original, and is usually of an entirely different form. The first generation of reduced-scale replicas mostly used modern flat-four engines in the 65-125 hp range. In some cases these were wider than the original V-types or in-lines, but could usually fit under a scale-size round cowling when the replica was a design that used a radial or a rotary engine.

When substituting a flat four for a radial or a rotary, the distance from the firewall to the back of the propeller is usually greater than scale. An oddity of the rotary was that its induction system was all behind the firewall, so the cowlings used were extremely shallow.

Since the original V or in-line engines were under tight cowlings, it is natural to want to have the flat four of the replica hidden under a closed cowling. In some cases, as with the Hurricane, this ends up looking pretty much OK, but for designs with narrow fuselages like the Hawk, a wide closed “Cheek”-type cowling grossly distorts the nose. It would be better in some cases to leave the cylinders out in the open with Cub-type airscoops for cooling. Some current replicas are going to converted V-type automobile engines, but these have not become a significant part of the movement yet.

The final problem is the weight distribution on the replica. It’s not much of a problem when the pilot of the original sat right of the center of gravity (CG). The weight reductions of the engine and rear fuselage structure are generally in proportion, so the final balance is pretty much OK without resorting to ballast. Also, with the pilot right on the CG, individual variations in pilot weight will not upset the balance.

The major problem here is that the weight of the smaller engine is not in proportion to the reduction in the scale of the airplane. The 435-hp Curtiss D-12 engine in the P-1 Hawk, a 12-cylinder water-cooled V-12, weighed 693 pounds dry. A 65-hp Continental weight 170 pounds, only 24% of the D-12, not 75%. A Continental 0-200 weighs only 188 pounds and a Lycoming 0-290 weighs 263 pounds. The overall weights are way out of scale – The P-1B Hawk grossed 2,932 pounds; a three-quarters-scale replica with a 0-200 engine should not be expected to exceed 1,000 pounds. With the lightest engines, it may be necessary to lengthen the nose a bit for balance, with consequent spoiling of the looks.

On planes like the Hawk, with the pilot seated quite a way behind the CG, the problems of a scaled replica can become almost impossible to overcome if reasonable appearance is to be retained. With a 180-pound pilot aft of the wings in a three-quarters-scale replica, there is no way that a 170-pound or even a 263-pound engine in scale position is going to keep such a design balanced properly. Also, with the pilot displaced from the CG a significant distance, variations in pilot weight will have a serious effect on the balance. In the 2,932-pound Hawk, a 180-pound pilot made up only 6% of the total weight; in a 1,000-pound replica, a 180-pound pilot makes up 18% of the total.

PHOTOS
EASILY ADAPTED: A British Hawker Hurricane, a famous World War II fighter, is easily adapted to a reduced-scale replica. Note the tightly cowled V-12 Rolls-Royce Merlin engine and that the pilot is seated right on the center of gravity under a small canopy.

A STELLAR JOB: Fred Sindlinger of Seattle did a magnificent job with his 5/8-scale replica Hurricane. Note the necessary enlargement of the canopy and the use of bulged cowing cheeks to hid the 150-hp Lycoming 0-320 engine.

A POPULAR REPLICA: A Fokker Triplane of World War I, powered with a 110-hp Oberursel rotary engine. Note the shortness of the nose and the location of the cockpit slightly behind the center of gravity. Fokker tripes are among the most popular replicas, but most of them have been full-size, not scaled down.

THE ORIGINATOR: Hobie Sorrell of Rochester, Wash., started the reduced-scale movement in the late 1950s with his 3?4-size Fokker. Note the lengthened nose for the 65-hp Continental, which hides successfully under a round cowling. He kept the rudder full size for better control.

THE FORTUNATE FEW: Few replica builders are fortunate enough to have small radial engines for their small planes. Hobie Sorrell had a rare antique French Salmson radial of 40 hp and designed this 3?4 size- Nieuport 17 to use it and the “right size” wheels that he had.

OVERCOMING OBSTACLES: The Curtiss P-1B Hawk of 1926, seemingly a logical candidate for a scaled-down replica, has some serious design obstacles: the narrow 24-inch fuselage, the far-aft location of the pilot, and the great weight discrepancy between the big Curtiss D-12 engine and the small flat fours that would be used in the replica.

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