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Gyro Variations
JT-11, JT-12, JT-13 and JT-14
Jukka Tervamäki 2009

This page is a modified and expanded version of the original JT-11 page (2004). The reasons for the additions were that I downloaded a new modeling software "Bonzai" from Autodessys for beta testing and at about the same time I received two emails suggesting me to design a twin engine push-pull autogyro and a very light and simple autogyro for beginners (See comments 13 an 14). So, I decided to fiddle with these ideas using the Bonzai beta. The JT-11 part of the page is from 2004.

Click the picture to see JT-11 QuickTime movie on the page bottom

JT-11, a 2-seat tractor gyro with a pusher engine!

The JT-11 autogyro concept is a 3D modelling project with the purpose of evaluaiting a side by side tractor gyro design with no compromises in pilot visibility, propeller efficiency, gyro loading or the CG position.

Why a tractor? Pusher gyros have two main benefits, good helicopter like visibility from the cockpit and a fairly simple load carrying structure, with all main loads (engine, pilot, fuel tank and main landing gear) attached to the rotor mast. The biggest drawback of pusher gyros is the limited space (between keel tube and rotor) for an efficient propeller. In a tractor gyro one can use an airplane-like big diameter propeller offering a much improved efficiency.

Side-by-side seating is a much preferred seating arrangement by many pilots. However, in gyros, pushers or tractors, this arrangement creates a problem with the CG position when the gyro is flown with one very light pilot or with two heavy ones. In tandem gyros this problem is avoided. The pilot balances the engine weight and the passenger is seated right in the CG-position. The gyro can be flown with or without a passenger with minimal CG-shift.

With the JT-11 concept I wanted to achieve the following goals:

1) A tractor design with a big, efficient propeller
2) Side-by-side seating
3) Good forward visibility for both pilots. Rotor mast, prerotator and control rods should not be in the pilots view
4) A short prerotator shaft to the rotor
5) No (or minimal) longitudinal CG shift with one or two pilots on board
6) Fuel tanks should be right in the CG position, so that there is no CG shift when the fuel is consumed.

I achieved these goals by placing a part of the power plant (the engine) in the back and another part (propeller, reduction unit, cooling radiators and the battery) in the front of the aircraft. This arrangement balances the empty aircraft so that the longitudinal CG position is right where the pilots are sitting and by placing fuel tanks under the seats all the changing usefull load is in the CG.

I am not the first one to figure out this arrangement. Although no such gyros exist today, Harold Pitcairn was the designer of the thirties who extensively used this arrangement in his last gyro designs. The Pitcairn PA-36 or AC-35 (?) is shown below. One can easily recognize the cooling air intake behind the pilot seats and the very small cowling on the nose which only offers enough space for a reduction unit. The propeller shaft also is higher than the engine revealing the power train principle. I have not seen the Pa-36 in the museum nor any drawings of its power train.

Pitcairn Pa36

The JT-11 power train.

The JT-11 utilises a 1.5 m long drive shaft from the engine to the reduction unit. It is a well known fact that long drive shafts create problems. Torsional vibrations are the most serious of them and can ruin the whole power train unless dampers are installed. A V-belt or a cogged belt drive reduces vibrations to some extent but often additional means are necessary.

Molt Taylor used Flexidyne couplings in his Aerocar, Imp and Mini Imp designs. The German Stemme S-10 motorglider has a power train which includes the following components:

1) V-belt drive
2) Flexible coupling
3) Carbon-Fibre drive shaft contained in a Kevlar tunnel
4) Splined sliding joint
5) Torsional Vibration Absorber
6) Centrifugal Clutch to avoid stop/start shock-loads

The JT-11 power train should be something similar. Of course, this adds weight and complexity. But this is the trade-off one has to accept to achieve the above goals.

3-view drawing in pdf-format is available for downloading. The drawing size is ISO/Din A0 for printing with the biggest plotters.

Click the pictures to view the bigger size!  Pictures 7, 8 and 9 below show a revised control system giving more space for pilot shoulders.

JT-12, a twin engine push-pull gyro.

This design offers similar CG advantages as the JT-11 concept with the addition of twin engine safety. Two HKS 700 engines could be installed and the gyro should be able to fly level with one engine only. Drawbacks: Complexity and cost.

JT-13B, a side by side design powered by a Rotax 914 engine.

This variation was inspired by the availability the 115 hp Turbo Rotax engine.  This engine is installed in many 2-seat gyros (like Magni M24, Xenon, etc) but unlike in the other gyros I wanted to try the possibility to install the cooling radiator on the nose of the aircraft.  The cooling hoses are inside the aerodynamic casing under the belly. The air flow to the tail planes would be smooth with minimum turbulence and a simple valve in the wall between radiator space and cockpit would provide cabin heating which is a necessity up here in the north.
I also wanted to see if the readily available aluminum rotor blades could be installed in the JT-type rotor head. The answer appears to be yes.


JT-14, a minimum gyro with 200 kg gross weight.

This is my idea of a minimum gyro. Yes, Bensen made it already 60 years ago (B-7m) powered by a 40 hp Nelson engine. Today an even lighter engine is available, the Aixro rotary engine. It already has flown over the Mount Everest in a powered paraglider. So the Aixro engine should  be a good choice for a very light autogyro.  More info on the Woelfle web site.
In my design concept the canopy tilts forward for easy pilot access to the seat.

If you feel these designs are interesting, please send me your comments


1) JT-11 looks very interesting! And well conceived. You will not find a PA-36 museum reference as the only flying model was cut up for scrap in 1942-3. Harold Pitcairn had the cutting up filmed. It marked the end of his Autogiro development/involvement. There was a second aluminum body constructed by Luscombe, but it too was cut up and the aluminum donated to the war effort.
Bruce Charnov, Hofstra University, New York

2) I love the JT-11 concept. It sure would be great if someone would put this into production. It is a beautiful design. The configuration is very compact and should have a very good power/weight ratio. This also allows complete aerodynamic enclosure of the engine - good performance?! Would the weight of the drive system offset the configuration weight savings?
Greg Gremminger, USA.

3) I saw the design concept for your JT-11 Autogyro on your website. It is brilliant!
Clifford Rock, USA

4) Gyrokonseptisi näyttää näin keskinkertaiselle heko-experimental miehelle hienolta (tuttuun ammatimaiseen tyyliisi). Olisi kyllä hieno saada joku rakentamaan sellainen. Onko kukaan ollut niin pitkälle kiinnostunut (vielä)?

5) Kylläpä olisi hieno projekti!!!! Sellainen pitäs ehdottomasti saada tänne Suomeen. Meitä ainakin himottaisi tollainen kone.

6) Nice design.. As soon as I finish my ultra light tractor gyro, I plan on attacking just this sort of machine. The drive shaft issue has been proven to work if done properly. Look forward to seeing this design taken to the next level !!
Timothy Blackwell, Mentone, IN.

7) A beautiful concept! Tom Hughes Olympia, Wa.

8) I couldn't believe my eyes! I have sketched out on paper something to build just like this! Only main difference... I had the re-drive in the back by the engine... I think I like you idea better (Lower drive shaft! Also, it could spin slower...). Maybe we'll see this thing come to life...

9) Well done once again, this is exactly the configuration the world needs. I hope you will be able to provide at least "advisory" drawings
so others may fill in the "missing bits", share information and get the JT - 11 airborne.
Yours in great esteem Jim Sherlock

10) The concept seems excellent. I think this is an autogyro that would be much desired by the rotor community, as a plans built, or as a kit. I hope you continue to develop the design, and look forward to seeing plans offered.
Lawrence A. Sciortino, USA

11) A brilliant design, sometimes a little blast-from-the-past is just the key. I too wonder if the re-drive (all or part) should not be in back by the engine. Thank you, I will be following this design development!
Douglas White

12)  Let's say...your brain is a volcano! Thanks for brainstorming for us all... My question is: how may I get plans to build the JT9 and/or the JT11 rotorcraft?  As I'm a quite well proven aircraft home-builder, it could be sufficient also a not well finished set of drawings.  May I get plans of such beautiful birds?
Best regards   Pierre Marcucci

13) I was playing with the my idea of a push-pull gyro, when I remembered your Virtual Gyro exercise: so I started messing with Paint to adopt your JT-9, marvel to my imagination...
What an incredible cruise machine would it be ! I'd love to build one.  Again, congratulations, Vincenzo Praturlon

14)  Have you ever tought about an ultralightversion of your JT-9 concept? A taildragger tractor built like the gyro bee or the affordaplane, based on a single square tube?! And engines like the Citroen Visa or Honda, Briggs and Stratton engines could be used, they do not cost the world, and the airframe could be built for some €uros, without ruining a houshold and scaring away the wife ! ;-)  Would you have an idea for such a JT-...?
My very best rgds from France.  Erkki Müller

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