Historical notes on the Jim Van Loo - SIG Chipmunk C/L Stunter
Written by Mike Gretz
Based on discussions with
Chipmunk designer Jim Van Loo, and
SIG Designer and Nats winning Chipmunk flier Mike Stott.

In the 1960s, Jim Van Loo of Sioux City, Iowa was one of the best C/L stunt fliers on the Midwest circuit.  One of Jim’s first designs to gain national recognition was the Mystere, which appeared in a construction article in the January 1967 Model Airplane News.

According to Jim, he first conceived the idea of a semi-scale Chipmunk stunter after seeing an article in the August 1966 issue of FLYING magazine about aerobatic pilot Harold Krier and the modified DeHavilland Chipmunk that he’d used to win the national aerobatic crown.  Starting with a stock Canadian-built DH Chipmunk trainer, Krier had clipped the wings and installed a 175-hp Ranger inline engine with an Aeromatic constant speed prop.  With a beautiful red, white and black checkerboard color scheme, the Krier Chipmunk was the hit of the air show circuit in ’66.

Inspired by the FLYING magazine article, Jim immediately drew up his first semi-scale Chipmunk stunter.  He built two prototypes during the fall and winter of 1966-67, both with a 57-inch span built-up balsa wing.  Jim flew one of these prototype Chipmunks at the 1967 Nats in California.  A picture of Jim's Chipmunk at the Nats appeared in the November 1967 Model Airplane News.

Jim had a young protégé on the Midwest stunt circuit named Mike Stott from Mankato, Minnesota.  Jim and Mike had known each other for several years, ever since Mike had first appeared on the contest circuit as a Junior age flier in the early 1960s.  Mike flew in his first Nats in 1964 and was soon designing his own airplanes.  His futuristic Fantom Foam Stunter was published in the July/August issue of SIG AIR-MODELER magazine.  One of the highlights of Mike's early Nats flying was when George Aldrich gave the Fantom 9 out of 10 points for "originality" during appearance judging at the 1966 Nats.  Mike, along with his father Arnold, owned and operated a part-time hobby shop and foam wing cutting business called Foam-Flite.  Foam-Flite was the first business of its kind, producing completely finished foam core wings for C/L and R/C customers nationwide.  Foam-Flite wings were used by some of the biggest names in stunt from the mid-60s to mid-70s (Bob Lampione, Bill Simons, Gene Schaffer, Jack Sheeks, Jim Silhavy, Dawn Cosmillio, Windy Urtnowski, etc.).

After seeing Jim Van Loo's Chipmunk during the summer of 1967, Mike Stott decided that he had to build one for himself.  During the winter of '67-'68, Jim and Mike got together to build two new identical Chipmunks, using Foam-Flite foam wings with a 55-1/2 inch total wingspan.  Working as a team, Jim cut out the fuselage parts and tail surfaces while Mike made the foam wings for both models.  Both guys assembled, sanded, covered and painted their own airplanes.  Mike painted his Chipmunk in the original Hal Krier air show paint scheme, like Jim had used on his first two Chipmunks.  Keeping up with Hal Krier’s latest change, Jim added checkerboard to the top of the wings and stab/elevator of his new Chipmunk.  They powered their models with Super Tigre .40 G-21 series engines.  Jim and Mike competed with their new Chipmunks during the 1968 contest season, including both of them flying their Chipmunks at the ‘68 Nats in Olathe, Kansas.

Also during the winter of '67-'68, Glen Sig, President of SIG Manufacturing Co., worked out a deal with Jim to kit a slightly smaller .35-size version of the Chipmunk, to compete with the Top Flite Nobler.  SIG announced their new CL-3 Chipmunk kit with an ad in the March 1968 issue of Model Airplane News.  Jim had also written a construction article on his Chipmunk stunter, and for maximum exposure and marketing impact, it was carried in the same March 1968 issue of MAN.

The Model Airplane News article included a plan drawn by magazine draftsman Al Novotnik showing a downsized Chipmunk with a built-up balsa wing spanning 54 inches.  That was 3" smaller than Jim's first two Chipmunks, and 1-1/2” smaller than the ‘67-’68 foam wing Chipmunks.  I’ve had the opportunity to take extensive measurements of Mike Stott’s original ‘67-'68 Chipmunk (which is the sole remaining example of a pre-MAN, pre-SIG Chipmunk), and I've found that there are a lot of significant dimensional differences between that airplane and the MAN plan.  While most of the differences can be attributed to the downsizing, there are a few items on the MAN plan that really stand out as different from the ‘67-'68 scratch-built Chipmunks.

1) The MAN Chipmunk plan shows a bizarre “lifting” stab.
2) The MAN Chipmunk plan shows an extra large bubble canopy.
3) The MAN Chipmunk plan shows a rib mounted main landing gear.
4) The MAN Chipmunk plans calls for the elevator counter balance to be glued to the stabilizer. 

1) The “lifting” stab on the MAN plan defies logic, and even Jim couldn't tell me how it got on there.

2) The large bubble canopy shown on the MAN plan never existed.  According to Jim, during one of his many visits to the SIG factory he had his friend Glen Sig mold the early Chipmunk canopies for him off a wood mold that Jim had carved.  This was over a year before the SIG kit even existed.  When the SIG Chipmunk kit went into production, Glen had several duplicate production canopy molds made off Jim’s original wood mold.  So all of the SIG Chipmunk kits had canopies derived from Jim’s original mold, which is the same mold that had made the canopies for Jim’s scratch built Chipmunks.  In other words, there never was an extra big bubble canopy as shown on the MAN plan.

3) Jim's first two balsa-winged Chipmunks had a rib mounted main landing gear as shown on the Model Airplane News plan.  On the other hand, the two '67-'68 foam wing Chipmunks had a "torsion-bar" style mounting, as did the SIG kit.  Notice on page 2 of the MAN construction article that the plan shows a rib-mounted main gear, while the picture on the same page, right below the plan, shows the torsion bar mounting.  Close study of the pictures in the MAN article reveal two different airplanes being shown in the article.  Notice the difference in pin striping on top of the wings of the airplanes on page 1.  I believe that the airplane in the lower left corner of page 1 is one of Jim's first balsa-winged prototypes, while the airplane in the upper right corner is his new foam-wing Chipmunk.  Likewise, judging by the difference in pin striping, I also believe the picture of the torsion-bar landing gear mounting on page 2 is the foam-wing airplane.

4) I think this is simply a case of the draftsman drawing the arrow to the wrong point.  Jim Van Loo never built a Chipmunk with the elevator counter-balance glued to the stabilizer.  It's always part of the elevator, same as the full-size Chipmunks.

Whether these differences between the ‘67-'68 scratch-built Chipmunks and the MAN plan were deliberate, accidental, or simply bad drafting is hard to tell.  I believe some of these changes may have been made just so that the MAN plan would be different from the SIG plan for copyright reasons (I remember when I wrote a construction article for MAN a few years later, editor Walt Schroeder insisted that I draw a completely new plan, different than the SIG kit plan, so that there wouldn’t be any copyright overlap).  To my knowledge, neither Jim Van Loo or Mike Stott ever built a Chipmunk exactly like the Model Airplane News version.

There are also significant differences between the MAN plan Chipmunk and the SIG CL-3 kit version.  The SIG kit Chipmunk had a 54-3/8” wingspan and called for a Fox .35 Stunt engine, ala the Nobler.  In addition to the wingspan there were many other changes in wood sizes, undoubtedly made to make the kit more economical to produce.   The size of the flaps and tail surfaces, and the fuselage moments are all different.  The cowling was changed from an all-balsa “Nobler” style unit that included an under-the-nose portion, to an ABS plastic molded cowl for the engine compartment only.  The SIG kit wheel pants are also molded out of ABS plastic, whereas the scratch-built early Chipmunks and the MAN plan wheel pants are balsa.

After the Chipmunk project Jim Van Loo quit flying C/L Stunt to concentrate on radio control models.

Mike Stott continued to campaign his original 55-1/2 inch span foam-wing Chipmunk in 1969.  Mike took first place in Senior Stunt at the '69 Nats with this model.  He narrowly missed taking the Walker Cup from ‘69 Open Stunt winner Bob Lampione, in spite of a sudden lean engine run caused by a speck of dirt in his needle valve.  Mike also flew his Chipmunk at the '69 FAI Team Trials, as seen in a photo in the December 1969 issue of Model Airplane News.

After winning Senior Stunt at the Nats in ‘69, Mike Stott built a new Chipmunk, stock from the SIG CL-3 kit, and flew it at the 1970 Glenview Nats. It was his first year in the Open class.  Mike's new Chipmunk carried the color scheme of another new Hal Krier inspired Chipmunk that had recently appeared on the full-scale aerobatic scene, belonging to air show pilot Bill Lumley. Unfortunately, Mike Stott's new Chipmunk was to be short lived. After it crashed during the 1971 contest season, Mike went back to flying his ‘69 Nats winning Chipmunk. He quit flying stunt competitively after the 1973 season to get ready for the 1974 FAI C/L Scale World Championships.

The SIG CL-3 Chipmunk remained basically unchanged from its introduction in 1968 until 1973.  By that time, in the full-scale aerobatic world, Hal Krier was dead and Art Scholl was now the king of aerobatic Chipmunk pilots with his “Super Chipmunk”. Art Scholl’s Super Chipmunk had an even larger opposed-cylinder engine, which necessitated a new wide curvaceous cowling.  The Super Chipmunk also had a larger rudder with an airfoil shaped top.  Scholl’s original Super Chipmunk had the standard fixed landing gear, but he eventually built another Super Chipmunk with retracts.  So in order to keep up with what was going on in the full-scale aerobatic world, Mike Stott did a major redesign of the SIG CL-3 Chipmunk kit into the Super Chipmunk in March of 1973.  This was the CL-19 Super Chipmunk that SIG still kits today.  Even though the CL-19 Super Chipmunk was definitely trading on the older Chipmunk kits' history and name, it was basically an all new design.  None of the parts of the CL-3 and CL-19 kits are interchangeable!  The only thing the CL-19 Super Chipmunk has in common with the earlier SIG CL-3 version is the airfoil shape.

There are several different significant versions of the "Jim Van Loo - SIG Chipmunk":
   1) Jim Van Loo's two original prototype Chipmunks (57" wingspan) built in the winter of 1966-67, with built-up balsa wings.
   2) The Van Loo/Stott twin foam wing Chipmunks (55-1/2" wingspan) built in the winter of 1967-68.  Mike Stott's Nats winning model still exists.  In Mike’s opinion, these were the best flying of all the Chipmunks.  Mike Stott was a fierce competitor!  I will never forget seeing him wind fly his Nats winning Chipmunk through the last 3 maneuvers of the pattern at a very windy contest in Des Moines, Iowa after his engine quit prematurely.  Granted the maneuvers didn’t look like much, but the fact that he saved his pattern points was a fantastic thing to watch (including two laps between each maneuver!).
   3) The Model Airplane News Chipmunk (54" wingspan) had a built-up wing and several other differences with the foam wing Chipmunks and with the SIG kit.  As far as I know, no one has ever built a Chipmunk exactly per the MAN plans!
   4) The 1968-1973 SIG CL-3 Chipmunk (54-3/8" wingspan) had a built-up wing and showed a Fox .35 Stunt engine.  This airplane has many differences in structure and wood sizes to the earlier Chipmunks for kit production reasons.
   5) The 1973 SIG CL-19 Super Chipmunk (53 1/2" wingspan) was a new design by Mike Stott.

FOOTNOTE:  The purpose of this document was primarily to record the sequence of Chipmunk variations up to the Super Chipmunk’s arrival in 1973.  However, I would like to make note of the splendid record that Super Chipmunk pilots have added to the contest legacy of the SIG Chipmunk line since 1973.

1st place Senior Stunt 1969 Nats by Mike Stott

1st place Junior Stunt 1976 Nats by Dave Fitzgerald
1st place Senior Stunt 1977 Nats by Dave Fitzgerald
1st place Senior Stunt 1978 Nats by Dave Fitzgerald
1st place Senior Stunt 1979 Nats by Dave Fitzgerald
1st place Senior Stunt 1980 Nats by Dave Fitzgerald
1st place Senior Stunt 1983 Nats by Jim McClellan
1st place Senior Stunt 1984 Nats by Jim McClellan
1st place Senior Stunt 1985 Nats by Jim McClellan