McKinnon’s Most-Modified Goose – Part 2

modifiedN150M - 1960 McKinnon model G-21D serial no. 1251 at McKinnon Airpark near Sandy. OR in 1965 (courtesy of Gary L. Killion)

N150M – 1960 McKinnon model G-21D serial no. 1251 at McKinnon Airpark near Sandy. OR in 1965 (courtesy of Gary L. Killion)

McKinnon’s Most-Modified Goose – Part 2

Written by Dave Marion – The first major change that McKinnon made to N150M after its initial re-certification as a new model G-21C, serial no. 1201 was relatively simple in one sense but in another was still quite significant from the FAA’s point of view. During the first half of 1960, McKinnon decided to try to expand the seating capacity of the new, four-engine model G-21C into which he otherwise had built such a large useful load. His first step was to put it back into his shop and cut off its entire nose section forward of the windshield. Next he spliced into it a new, 36-inch longer section in the middle of what had been the original forward baggage and anchor compartment, which on his more powerful version had a load capacity of more than 600 pounds. Into that new space, he mounted provisions for four extra passenger seats, installed four new windows (two on each side) and framed the left rear new window as a hinged loading hatch and emergency exit.

To compensate for the additional weight built into the nose section of the aircraft, McKinnon modified the tail to increase the span of the horizontal stabilizers and elevators by an extra 12 inches. With just those changes to its nose and tail, N150M was re-certified as another completely new model, the 14-seat G-21D, on June 29, 1960 and once again it was assigned another new and correspondingly unique serial number: 1251. At that point too, McKinnon model G-21C serial no. 1201 officially ceased to exist as such.

Incongruously, about a year later, Popular Mechanics magazine published a story in its June 1961 issue called “Backyard Eagles” about the “latest designs from the wacky world of experimental aircraft” in which it christened McKinnon’s conversion of N150M the “Golden Goose.” The photo included in the article showed it configured as a four-engine but still short-nose model G-21C. Note, that magazine article is the only instance that I have ever found in which the first McKinnon G-21C conversion was referred to as a “Golden Goose.”

McKinnon profile drawing showing changes made to convert N150M from a 1958 model G-21C (s/n 1201) to a new model G-21D (s/n 1251) in June 1960.

Even after the second major conversion and re-certification of N150M, McKinnon was apparently still not yet satisfied with all of the details of his now two new models G-21C and G-21D. The obvious target of his continued dissatisfaction was the engines. The weight and complexity of the four Lycoming supercharged piston engines was less than ideal and they were not proving to be a particularly effective selling point either. Historically too, that series of engines earned a reputation for being tricky to operate, difficult to maintain economically, and with any kind of improper operation, also somewhat less than reliable in their own right too.

The answer to McKinnon’s powerplant dilemma came in the form of the new, powerful yet lightweight gas turbine engines that were being developed in the early 1960’s. In 1962, United Aircraft of Canada Ltd. (later becoming Pratt & Whitney Canada Ltd.) came out with its new PT6 series of gas-turbine engines, the first of which, the 500 shp model PT6A-6, was certified by the FAA on December 31, 1963. The PT6 engines were unusual in that they had reverse-flow – meaning that the air intake was on the aft end and the exhaust was toward the front of the engine. They were also “free” or uncoupled engines meaning that there was no direct mechanical connection between the power section and the propeller drive gearbox. Instead of a direct connection, the hot exhaust gases from the power section flowed over and turned a turbine wheel connected to the input drive shaft of the propeller section gear box.

The pair of 450 hp P&W R-985-AN-14B series radial engines ultimately used by Grumman on the last military models JRF-5 each weighed 676 lbs. (dry) for a combined total of 1,352 lbs. The 340 hp Lycoming GSO-480-B2D6 engines that McKinnon used initially on his new models G-21C and G-21D each weighed 513 lbs. but there were four of them for a total weight of 2,052 lbs. By comparison, the new 550 shp United Aircraft (PWC) PT6A-20 turbine engines weighed only 286 lbs. each for a combined total weight of only 572 lbs. When taking into account propellers, accessories, engine mounts, fluids, and other actual installation factors, the weight differences were even more significant – and as history has shown, the reliability factor of the new turbine engines has been unmatched ever since their introduction.

Fabrication and assembly of the new, 36-inch longer nose section used to create the unique model G-21D in McKinnon’s shop. (from McKinnon archives via Antilles Seaplanes)

After watching a couple of other projects get underway to re-engine different aircraft with the new PT6A turbines, McKinnon rolled N150M back into his hangar in the summer of 1965 and began once again to make some major changes to it. In addition to the powerful new turbine engines, McKinnon had read about a new aerodynamic theory being explored by a Peruvian-born Stanford University engineering graduate named Alberto Alvarez-Calderon. After collaborating for a while, they decided that the Goose would be a good test bed on which they could test the new theory.

Alvarez-Calderon believed that any or even all twin-engine aircraft in general would benefit to a significant degree from a whole new approach to mounting the engines on the wings. Instead of the traditional engine nacelles mounted on inboard stub wings approximately the radius of the prop arc away from the sides of the fuselage, he believed that a large amount of undesirable turbulence and drag that was generated by the restrictive channel formed by the nacelle, stub wing, and side of the fuselage.  He further believed that turbulence and drag could be eliminated by also essentially eliminating the stub portions of the wings and mounting the engine nacelles right up against the sides of the fuselage, but canted as necessary for propeller clearance from the fuselage.

He also believed that in such a configuration, even in an emergency situation such as the failure of one engine, the prop wash and airflow from the remaining good engine would still be directed over the vertical stabilizer and rudder, enhancing their effectiveness and directional control of the aircraft during what otherwise would be a problematic mode of operation. The high-wing G-21 series Goose airframe, with its main wings mounted closely behind the windshield, proved to be an ideal test bed for the canted engine nacelles of Alvarez-Calderon’s “clean wing” theory, which was so named because it kept a larger portion of the wing’s airfoil uninterrupted and aerodynamically “clean.”

N150M – McKinnon G-21D serial no. 1251 over Mt. Hood in 1960

Because of weight and balance issues, the much lighter PT6A series turbine engines that were replacing the heavier, old piston engines had to be extended forward on longer nacelles in order to achieve the same “moment” which is defined as an installed component’s weight multiplied by its arm or distance from the aircraft’s center of gravity. It’s a matter of leverage; in terms of aircraft balance, a small weight on a long arm has the same “moment” or overall effect on the balance of the airplane as does a large weight on a short arm. In final practice, even with the engines canted both upward and slightly outward, the props mounted on the new turbine engines reached in front of the windshield over the top of the nose of the aircraft and came within a few inches of each other on the subsequent McKinnon “Turbo” Goose installations.

Technically speaking, by converting N150M back from four piston engines to two turbine engines, McKinnon risked having the FAA invoke the regulatory requirement to have the “new design” re-certified once again under a whole new type certificate in accordance with 14 CFR §21.19. Instead McKinnon petitioned the FAA for and was granted an exemption to that regulation that allowed the airplane to remain certified as a model G-21D under TC 4A24 Section II and the engine swap was approved instead by means of a new supplemental type certificate (STC) no. SA1320WE.

The application for the new STC was submitted by McKinnon to the FAA on July 28, 1965 and final approval for it was eventually granted on February 16, 1967. In addition to the turbine engine swap that was eligible to be made on any McKinnon model G-21C or G-21D aircraft already converted and re-certified under TC 4A24 Sections I or II respectively, STC no. SA1320WE also permitted the installation of a new type of wing flaps on the McKinnon model G-21D – of which of course N150M was the only example ever built.

Excerpt from a 1960 McKinnon Enterprises Inc. sales brochure for the G-21D showing the interior and performance specifications

The new wing flap system, like the “clean wing” engine nacelles, was also designed by Peruvian aerodynamicist Alberto Alvarez-Calderon. His idea for the new wing flap system was to achieve the performance benefits of Fowler flaps without the complexity and weight of the extended flap tracks along which Fowler flaps have to deploy. Traditional “simple” flaps have a single axis hinge arrangement and “split” flaps are those in which only the bottom portion of the inboard wing is hinged, as with which all Grumman Gooses were originally equipped. However, such traditional style flaps are usually the least effective means of adding either lift or drag to enhance the performance of aircraft at lower airspeeds.

Fowler flaps are beneficial in comparison to other types because in addition to increasing camber (effective curvature) of the wing, adding lift at lower airspeeds, and adding drag to help the aircraft slow down for landing, by running both back and down on their tracks as they are extended, Fowler flaps also increase the effective chord (fore and aft measurement) of the wing which also adds to wing area and increases lift without adding much parasite drag – which is also useful during take-offs especially when runway distance is limited.

In order to achieve the benefits of Fowler flaps and be able to increase both the camber and chord of a wing without the complexity of the roller tracks used for Fowler flaps and also keep an essentially more simple, single-axis hinge arrangement like traditional simple or split flaps, Alvarez-Calderon went way “outside the box” of conventional thinking and decided to move the hinge line of the flaps to the trailing edge of the wing.

In that configuration, the wings flaps stowed up underneath the wing just as they had previously on other Gooses, but when deployed, they folded both down and back through an arc of more than 90 degrees so that both the effective chord and camber of the wing were greatly increased. The thing is, they must have generated an unbelievable amount of turbulence during their initial extension (or retraction) through the first 90 degrees or more of travel and during which they must have made for some incredible and none-too-subtle air brakes.

N150M – 1960 McKinnon G-21D serial no. 1251 after conversion to two 550 shp PT6A-20 turbine engines per STC SA1320WE circa April 1966 (courtesy of Gary L. Killion)

After completion of an experimental flight test program that was conducted between July and December of 1966, the latest changes to N150M were proven and validated to the satisfaction of the FAA. With the final approval of STC no. SA1320WE in February 1967, the airplane itself was finally able to be re-certified with a new Standard certificate of airworthiness as well.

The only significant change to its operating limitations that was necessary because of the installation of the new turbine engines was due to the fact that the two PT6A-20 turbine engines totaled only 1,100 shp combined compared to the 1,360 bhp of the four Lycoming piston engines that had been installed previously. As a result, in its new model G-21D “Hybrid configuration as McKinnon described it once it was modified by STC SA1320WE, the maximum allowable gross take-off weight of N150M was reduced just a little bit from 12,499 lbs. to 12,200 lbs.

Finally after all of that work, in the fourth major configuration of its lifetime, this Goose was finally ready to get back out of its nest and start flying again as it should….

N150M on April 20, 1966 after completion of initial conversion to two 550 shp PT6A-20 turbine engines per STC SA1320WE (courtesy of Gary L. Killion)

Dave Marion is the Technical Content Editor at As A&P and IA with 29 years of experience in aircraft maintenance, he is also a Commercial Pilot with Airplane, Single & Multi-Engine, and Instrument ratings. He has a BA from Colgate University in 1984 and also graduated cum laude from Embry-Riddle Aeronautical University (DAB) with a BS in Aviation Technology in 1990. He can be reached along with all of the editors via E-Mail: [email protected]