the SOGGI Senator ... a 30" rubber freeflight

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Bob H
Posts: 183
Joined: Tue May 12, 2009 10:46 pm

the SOGGI Senator ... a 30" rubber freeflight

Post by Bob H » Fri Sep 02, 2011 2:43 pm

KeilKraft’s venerable Senator is a classic rubber freeflight, designed and drawn by Albert E. Hatfull around the year 1950. His design produces an airplane entirely without vice, and I tip my hat to him for a job well done. His artistic talent is also apparent from his drawing. A surviving copy of his KeilKraft drawing is provided here for your reference by clicking on
Keilkraft Senator.pdf
You will need to have Adobe Reader installed on your computer before you can open the drawing file. The no-cost Adobe Reader can be obtained from: .
SOGGI Senator1.JPG
We have several local examples of fine-flying Senators built to the original KeilKraft/Hatfull design. Typical for kits of that era, the KielKraft Senator drawing omits details of certain useful features on the assumption that any builder would know how to create them according to his/her own preferences. Around here, experienced freeflighters are in short supply and the necessary refinements are in danger of becoming a lost art. To help preserve the tradition, I've made a new drawing named the "SOGGI Senator", that provides some additional details of my own model in its' “as-built” configuration; the sole exception to "as-built" being my use of a Montreal Stop and 12.2D x 15.6P folding propeller, rather than the 12 3/4 D x 15 5/8 P freewheeling propeller that is illustrated on the new drawing. Local Senator flyers have been using freewheeling propellers similar to the one illustrated on the new drawing, with good success.

"SOGGI" refers to the Southern Ontario Glider Group Inc (SOGGI), an R/C Soaring club that also includes a small rubber freeflight contingent.

The "SOGGI Senator" drawing is offered free of charge to fellow scratch-builders and I'd be pleased to exchange e-mails with anyone who takes up the challenge. My e-mail address is given at the end of this article.

To view or to download the SOGGI Senator, click on
SOGGI Senator.pdf
If you want a full size paper copy of the drawing, download a copy of the .pdf file and provide that copy to your local blueprint shop.
Surviving examples of KielKraft's drawing seem to be graphically distorted, as evidenced by the drawn size of objects compared to their written dimensions. This distortion has probably arisen because copies were made from copies, rather than from the original master. The SOGGI Senator drawing attempts to correct the graphical distortion. New details contained on the SOGGI Senator drawing reflect my own preferences, but other intermediate sport flyers may find them of interest:
  • Patterns for all of the required parts are nested in 3" wide sheets
    A dethermalizer … you'll need one of those. Otherwise, Senators can easily depart to distant places.
    A carving blank and a freewheeling mechanism for a propeller have been created using methods set out in Don Ross' book Rubber Powered Model Airplanes.
    Jacking-bolts are shown for fast, precise and repeatable trimming of pitch, yaw and thrustline
    The locking mechanism for the rubber-peg reduces the chance of a destructive accidental release of a wound rubber motor
    A "Reverse-S" propeller-shaft-hook prevents the rubber motor from climbing up the shaft, thereby preventing an imbalance and vibration
    A spar web has been added at the wing centre-section to distribute contact loads from the fuselage
The Senator’s stabilizer is located on the bottom of the fuselage. I have hinged the SOGGI Senator's entire rear fuselage with attached tailgroup, in order to achieve the necessary dethermalizer-motion of the stabilizer. For improved reliability and streamlining, the SOGGI Senator's dethermalizer motion is driven by a tiny coil spring, rather than by the more typical rubber band. Instructions for making the spring are given.

Southern Ontario summer days can be oppressively hot and humid. On the field, anything that makes it faster and easier to set up the model is a big plus. A byproduct of hinging the fuselage is that both the front and back ends of the rubber motor are easily accessible. Having access via both ends of the fuselage makes changing out motors on the field pretty painless. The drawing shows the necessary mechanical details for winding the motor from the rear access, rather than from the more usual nose-access. A spike driven into the ground serves as a winding stooge. To restrain the model while winding the motor, the propeller shaft retainer is hooked onto the spike. I have used variations on this system successfully on several models and now prefer it.
Winding from the back.JPG
My model including all modifications and mechanisms, weighs 110 grams. That is 5 grams (5%) more than Mr Hatfull’s original prototype. Contained within that 5% overall weight increase, is a folding propeller mechanism plus a 13% increase in the weight of rubber (compared to his original). Presumably this extra rubber delivers close to 13% more stored energy, and the folding propeller reduces drag during the glide. I consider the weight increase to be a good trade off.
Bobs Senator1.jpg
My model first flew in early June of 2011. By the end of August it had accumulated about 70 flights. It is a capable thermaller and has a beautiful glide for its size and type. The D/T has already prevented several fly-aways. The model is simple to rig at the field, is rugged, easy to trim, and has very repeatable behaviour; prized characteristics in any freeflight model. Currently I'm testing different configurations of rubber motors. The Spike-stooge fits into a small flightbox. In two hands and in one easy trip, this compact airplane and all of its necessary field support gear can be carried to the upwind end of the field for launching. It's a big field.

Thank you to Dick C. for the second photo, Ann T. for the last photo, and to my wife for photos one and three appearing here.

Bob Hammett ... Southern Ontario Glider Group Inc. (SOGGI) ... September 2, 2011
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