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JT-8, JT-8T and JT-8S
(T for Twin, Tandem and Trainer, S for electricity)
Page updated 2012
A page for the motorglider + homebuilder enthusiast!
For the past 27 years I have flown a lot of hours with 2-cycle power, first a Solo Hirth in an ASK-14 and then a Rotax in my JT-6. While the 2-cycle powerplants are simple I would hardly say they are reliable. I have never dared to fly long distances with 2-cycle power, the maximum being about 100 km in situations like retrieval flights in deteriorating soaring weather. There are other reasons, too, like high fuel consumption and noisy cockpit.
But I enjoy dreaming. For years I have been longing for reliable 4-cycle power in a new simple motorglider concept with no retractable engine or propeller belt drive but with good enough soaring performance to fly a 500 km triangle. Tinkering led me to a 3D modeling project, Virtual Motorglider JT-8 and JT-8T. It is a pod and boom concept with a pusher engine installation with an automatically folding 2- or 3-blade propeller. I modeled and rendered the virtual JT-8 using formZ RenderZone software.
Advantages of the JT-8 concept:
JT-8 is not the first pod and boom motorglider concept with a pusher propeller but I feel there has not been one with a modern laminar airfoil and low drag fuselage with high refinement in engine installation details. L/D of 35 should be possible with this concept using a 15 m wing and from 38 to 40 using an 18 m wing. You may feel this a bit low compared to retractable engine motorgliders, but the JT-8 concept offers a number of other advantages:
1) Safety!! The airplane total drag with the JT-8 concept would be equally low in all the three important flight regimes:
- Powered flight
- Engine restart phase!
The last one of these three is the most important. I know of several accidents where a retractable engine glider was destroyed by failing to restart when too low. The drag increase with the engine extended is very high (L/D drops from 40 to 14). With the JT-8 the pilot could continue engine restart attempts down to the last moments of final approach since there is no drag penalty and high sink rate when restarting the engine.
2) Economy by power! Long distance flights could be made with really low fuel consumption.
3) Simplicity! No time wasted to extend or retract the engine. No complicated mechanism required to do that. Just push the button to restart the engine, the propeller unfolds automatically!
4) Reliable 4-cycle engine! The engine of choice for the JT-8 would be a Japanese HKS 700. With dual carbs it has a 3 min take off power of 60 hp at 6200 rpm and a continuous power of 56 hp at 5800rpm. Since 40 to 50 hp would be enough for a single seater I would use a single carb for simplicity. The HKS 700 has a dry sump and requires a separate oil tank. Since the HKS cylinder heads are oil-cooled a separate oil cooler is necessary, too. Both are supplied with the engine. The engine package weighs 53 kg (116 lbs) without the prop. A separate air intake for the oil cooler may be necessary. A low drag NACA type, flush with the cowlings , would be the best choice.
HKS is a geared engine with gear ratios of 2.5 or 3. This results in a propeller rpm of 2400 which is low and requires a 3 bladed (may be 4-bladed) folding propeller to absorb the power with a diameter of 1.2 m but offers the extra benefit of very low propeller noise. The propeller tip speed would be only about Mach 0.4.
A more suitable gear ratio for JT-8 would be about 2 resulting in a max prop rpm of 3100 and max tip speed of Mach 0.57. A two bladed folding prop could be used. If suitable gears are not available from HKS they could be specially made. Gear machining shops can be found in all industrialised countries.
An alternative engine is available, the Hexadyne Aviation P60.
Click the pictures to view the bigger size!
5) Easy maintenance! With hinged engine cowlings the engine would be easily accessible, see picture below. In some of the newest retractable engine motorgliders the engine is not so easily accessible.
6) From the soaring drag point of view a pusher engine is a better arrangement than the one in RF-4, Motorfalke, Grob 109 and Carat. The engine and prop (even feathered) on the nose destroy the laminar air flow around the fuselage right from the start.
Turbulence and drag caused by engine cooling requirements could be further reduced by designing sliding doors for the trailing edge of the engine compartment (see picture below). To operate the doors by a mechanical hand lever takes a mere second. Microswitches to prevent inadvertent engine starting when the doors are in out position should be installed in the lever mechanism
7) Good visibility! In the JT-8 the pilot is sitting forward of the wing. This is not so with gliders having the engine on the nose. Compared even to the normal gliders, the visibility would be better, too, because the seating is more upright in JT-8 due to the bigger vertical dimensions necessitated by the engine and propeller.
8) Good taxiability! My present JT-6 motorglider is fully taxiable and I am happy I designed it like that. However, there are two circumstances where JT-6 is not so good for taxiing, namely on soft ground and on taxiways with tall lights.
The JT-8 has a low propeller thrust line which should prevent tipping over on soft ground when the throttle is pushed forward.
There are three possible landing gear configurations:
a) The JT-6 style wing tip wheels and tail wheel would be quite acceptable for taxiing on normal glider ports. The drawbacks of this configuration are that you cannot use taxiways with tall lights and the main landing gear tyre wears rapidly on one side (see a photo of JT-6 tyre).
b) For operations on bigger and busier general aviation airports an electrically operated retractable main landing gear could be installed. The type shown below was used by Molt Taylor in his "Mini Imp". Press the picture to see details. The drawback of this configuration is the added complexity and weight.
c) Retractable outrigger wheels under the wings are another possibility. They could be operated together with Shemp Hirth air brakes. This type of system I had in my JT-6 for many years. It's drawback is the drag increase although very small. See the three landing gear types pictured below.
The JT-8T (Twin, Tandem, Trainer)
The author preparing the JT-8T for a flight with a virtual passenger on board.
As a two place tandem version the JT-8 concept offers even more advantages than the single seater. The propeller could be made bigger (ca 1.5 m dia) with better efficiency. The rear seat for the instructor could have a higher position as in tandem fighters offering good forward visibility. The main landing gear could be fixed with only the outrigger wheels being retractable with the air brakes.
Assuming a wing span of 16 m and a grossweight of 650 kg the rate of climb with a 60 hp engine would be better than 2.5 mps. With the 90 hp Rotax 912 the climb rate would be around 3.5 mps.
Comparing the soaring performance with some existing trainers the JT-8T would not be as good as Grob 103 Twin Astir (L/D 37) but much better than Grob G 109 (L/D 25) offering a true tandem trainer feel and performance (L/D better than 30).
The JT-8S (S= sähkö, electricity in Finnish)
Electric propulsion has emerged to the sport aircraft field during the past 10 years. The motorglider is a very favourable aircraft type for electric propulsion since the engine power is needed for a fairly short time only, to climb to the soaring altitude. Some electric motorgliders already exist, the Lange Flugzeugbau Antares, the Alisport Silent and the Yuneec E430.
With suitable electric motors and battery packs available today, I also wanted to take a look at what a JT-8 might look like with Yuneec Power Systems electric motor and battery pack installed. I therefore did some remodeling of the JT-8 engine compartment and this is how it looks like with a Power Drive 20 motor, a Power Block PB 20 and a 4,6 kWh Battery Pack installed. The total weight of this system should not exceed 50 kg with all the cables and necessary instruments included. A retractable air scoop is necessary for engine cooling. I also increased the propeller diameter for better efficiency.
With this systen the JT-8S should be able to reach about 2000 m altitude gain, about the same as Alisport Silent but less than the Antares (3000 m).
How could it be made?
The JT-8 concept could probably be worth small scale production but it would be a time-consuming and risky project. A faster way to realize a proof of the concept aircraft would be to modify an existing glider. But new gliders are expensive and too old ones are not up to date in aerodynamics. So how could one be made? One method is as follows:
1) Buy a "kit." Every now and then insurance companies get damaged gliders which are offered for sale to anyone willing to start a repair project. A glider with a badly damaged fuselage but with wings and tail section intact (or having only minor damage) would be an "economical kit" to start with. You have to build a new fuselage anyway. The best choice would be a 15 meter glider with 17 or 18 meter wing extensions. You would not use the extensions but they guarantee that the wing spar is strong enough to carry the extra load of the HKS engine and fuel. Extensions indicate that wing roots have been designed for higher bending loads.
2) Strip all undamaged metal parts, fittings, landing gear, control system and instruments from the damaged fuselage. Remove even damaged parts to use as templates, you may have to reproduce a copy of them. Repair parts as necessary.
3) Redesign the control system to fit the low wing position by using as much of the old parts and fittings as possible.
4) Build a fuselage plug using a wooden frame with plastic foam covering for proper shape. Use filler for finishing. The tail boom could be made of metal sheet. Add the original vertical tail to the boom. Use part of the fuselage plug to make a canopy male mold
5) An important detail is the wing root fitting. Try to remove one from the old fuselage and use it in the new fuselage or in the fuselage plug. Make weight and balance calculation and carefully check the wing and horizontal tail angle of incidence and position before bonding. Battery placement can be used to fine tune CG position in the finished aircraft.
6) Carefully wax your finished fuselage plug and laminate two mold halves onto the plug. The mold can be quite light weight and disposable, if you only make one fuselage in the mold. For other items like the pilot seat and instrument panel, temporary molds made of plastic foam or metal sheet will do the job.
7) Make a strength analysis and determine the required number of fiberglass fabric layers on various parts of the fuselage. A good set of computer programs would help in this process, but you can learn a lot from the damaged old fuselage by cutting sample pieces on different areas and by measuring, weighing and counting the number of fabric layers.
8) Power plant design. You have to design and make an engine mount and a folding propeller, take a look at point 4) above.
Once you have made the two fuselage mold halves, the work continues like any fiberglass/epoxy aircraft project made in female molds. It involves a lot of know how but the details of that process are quite another story.
And Up We Go!
Story of the JT-8 was published in December 1999 issue of the Finnish aviation magazine ILMAILU.
Download a beautiful Desktop picture(MacOS) or Screen Background (Windows) for 19-21 inch screens, jpg 130 kb
Past and present motorgliders or motorglider projects resembling the JT-8
(If you know other similar projects, please, send me e-mail!
1) SZD-45 OGAR (1, 2, 3)
2) Alex Strojnik S2A
3) Terry Wells Motorglider
4) Exel by Alpaero
5) J5 Marco and J6 Fregata from Poland
6) Piuma Motorgliders
7) Aerostructure Lutin 80
8) Technoflug Piccolo
9) Bagaliente by Valter Bagalini, Italy
10) S 45 Mystere, France
Some of the Comments I have receivedNice job on the 3D rendering and concept design of your JT-8 Motorglider. I started soaring last spring in the state of Minnesota in the US. I have become enamored with the sport of Soaring and Motorgliders in particular. I like the JT-8 design because of its realistic long rage capability and retained L/D especially in the 18M configuration. The safety factor of the quick engine restart is also a great feature. Who knows, maybe someday in the future, when I am independently wealthy, I can be the one to finance the building of the JT-8.
Thanks for sharing your design ideas with the world,
Blake S. Miller
You have an very interesting site and project in the JT-8. I am currently building a Windrose motorglider, that I hope I can finish soon.
Hyvä artikkeli ja sait varmaankin aikaan jonkinlaista kuhinaa Otaniemessä. Ennenkaikkea mallinnus on onnistunut oikein hyvin.
I just came across your web page and I am very impressed with the JT-8 and your graphics. I am building a motorglider that looks almost identical except it is a high wing. Gross weight 700lbs(I hope) 15 meters, 40 hp Rotax.
I would like to pursue the JT-8 concept. I am particularly interested the application of new alloys to the concept. I feel the aircraft could be made with the materials I have in mind. I am particularly sold on your engine choice. Do you have plans for sale? license? or some other arrangement.
I came across you pages while I was doing a search for the Grob 109. Bravo, you have done an outstanding job designing your JT-8 Motorglider. It is a beautiful airplane and I hope you will build it. It reminds me of the SZD-45 Ogar of which I am very fond.
Good luck with your project,
Hello from Australia!!
Your concept for the JT8 is beautiful! I started building a Strojnik S2A about 5 years ago................a big job. I have fuselage tube, bulkheads etc built, main spar etc, so I am a bit far down the track to be changing plans. Unlike Strojnik's fuselage pod design, (timber, foam glass with flat wrap canopy) I designed a plug and hence mold for the fuselage. This fuselage was designed using Peter Garrison's Loftsman software. I have attached a jpg for your interest. It is very similar to your concept JT8. Keep up the good work
Yes, Jukka, it seems that EXEL and JT-8 share many ideas, and especially the "pod and boom" formula, for the same reason (economy and security). That was also Alex Strojnik's choice, before us...
You're also a great modeler with 3-D software. Your work is fantastic. If only we could have such skills in our team !