July 2008 Cover Story:
The Indus Thorpedo:
A Modern Take on a Classic Design

Story by Jay Wischkaemper

To say that the market for light sport aircraft is crowded would be an understatement. Close to 50 different companies manufacture aircraft that qualify under light sport rules. Most of them are excellent designs that naturally push the limits of performance and weight allowed under the regulations. Some are adaptations of existing designs, mainly from Eastern Europe. Some are clean sheet designs. One is a rebirth of a classic design, born over 60 years ago by famed aircraft designer John Thorp: The Indus Aviation Thorpedo.

John Thorp designed the T11 SkySkooter in 1944. The design was certificated, but was unable to get a foothold in a market dominated by Cessna and Piper at the time. The war came to an end and so did military production. The plane was later resurrected by the homebuilt market, and several were constructed. More recently, rights to the design were obtained by Indus Aviation, and with some minor tweaking, the aircraft was certified under the light sport rules. In fact, it was the first American-manufactured aircraft to obtain that distinction.

But why resurrect an old design? The answer is simple. It’s a good design. John Thorp went on to work for Piper, and was the main design force for the Cherokee line. In fact, some of his innovations found in the Thorp design, primarily the all-moving tail (later called the stabilator), became part of the Cherokee design. Sitting a Thorp beside a Cherokee, it’s not hard to notice the family resemblance.

Simplicity was a design trait of Thorp, and simple usually translates to dependable. As a result, the Thorp has not had an AD note in over 60 years. If you are looking for a good aircraft design, one with that kind of record isn’t a bad place to start.

If you are going to enter the light sport market, especially today with all the choices available, how do you set yourself apart? Indus is trying to do it in several ways. First is the history of the design. While there aren’t Thorps on every tarmac, the design has stood the test of time, and a number of them have been flying for years. It may not be the sexiest airplane around, but it flies, and flies well.

The second thing that sets Indus apart is customer service. When I visited the factory, there was a plane sitting on the factory floor that I assumed was a new aircraft about to be delivered. I found out that it was in fact a customer’s aircraft that had been brought back to the factory to be updated with several design changes that had been incorporated in newer aircraft to improve performance, and the bulk of the expense of the update was being borne by the factory. They didn’t have to do that, but they did because it was the right thing to do. Service like that is a rare commodity it today’s world.

Third is innovation. Indus is the first light sport company to offer a diesel engine in their design. By the time you read this, they hope to have certificated the British built WAM 100 three-cylinder, two-cycle, turbocharged, intercooled diesel engine. Even though the airframe design may be old, Indus is a forward thinking company.

Last but not least is by getting people to fly the airplane, because comparing the flying characteristics of the Thorp with other light sport aircraft makes the design very attractive, especially for the training market.

I had an opportunity to spend a day at the factory to fly their diesel prototype as well as the Jabiru powered models. While there, I was also given an opportunity to take the original prototype Thorp that John Thorp himself built in 1946 around the area. This aircraft has been obtained by Indus, and is flown regularly. Interestingly, other than the old fashioned T handle pull starter and some very old gyro instruments, the airplane is much like the new models coming off the assembly line, and except for a few modifications, it is the same airframe.

The main thing Indus had to do to certify the Thorp design under LSA rules was to bring the stall speed down. This was done by adding a six-inch wingtip extension on the end of each wing, adding enough wing area to lower the stall. The engine was also changed. Here is one place Indus runs counter to the trend in the LSA marketplace, since most other light sport aircraft are powered by Rotax. Indus chose the Australian built Jabiru engine. Two versions of the Jabiru are used, the 85 hp J2200 four-cylinder engine, and the 120 hp six-cylinder J3200. The airframe is the same for either engine. Why not be like everyone else and use the Rotax? It goes back to the philosophy of simplicity. The Rotax is an excellent engine, but it’s not a simple engine. There’s the reduction gear box. There is water cooling. There are two carburetors. The Rotax TBO is 1200 hours. The Jabiru, on the other hand, is a direct drive, air-cooled engine with one carburetor. It was developed by Jabiru, another light sport manufacturer, because they couldn’t find an acceptable engine for their own light sport design. It is a smooth running, dependable engine with a 2,000 hour TBO.

How does the Thorpedo fly? The short answer is, “Not like any other LSA I’ve flown.” While my experience in the category isn’t extensive, the models I have flown have been very similar. They get off the ground quickly. They handle crisply. They are a pleasure to fly, but they feel very light, as perhaps the moniker “light sport” would indicate they should. That’s not the case with the Thorpedo. From the moment you seat yourself in the cockpit, you have the feeling that you’re in a “real” airplane. Panel space is abundant. The cockpit is roomy if not spacious. Nosewheel steering is through a direct linkage. There are no toe brakes. Brakes are controlled by a handle coming out of the center console, somewhat like the brake lever on a Cherokee. Control sticks are substantial. The airplane simply feels much bigger and heavier than it is.

Both the Jabiru and the diesel engines fire up immediately. The diesel is a little different in that you have a glow plug button that has to be held down for a few seconds before the starter is engaged. The Jabiru is a very smooth engine, being a six-cylinder design. The diesel on the other hand, can run like a diesel, especially at low RPM settings, which is to say a little shaky. However when power is increased to 1800 rpm or so it smoothes out, although there is never any doubt about which mill is under the cowling. You would never guess there are only three cylinders at work however, and at full power, vibration is not noticeable.

The Jabiru has no mixture control. The carburetor is altitude compensating, so the only thing to check during the runup other than everything being in the green and controls being free are mags and carb heat. Of course, on the diesel, you don’t even have that, since there are no mags or carburetor. On takeoff, the airplane doesn’t jump in the air like other light sport models I’ve flown. Book figures notwithstanding, in the real world, it’s going to take you 700 feet or so to get airborne, but once you do you experience a solid, heavy aircraft feel. Climb is decent, at about 1,000 fpm with the Jabiru. With a useful load of 615 pounds, you can fill the tank and the seats. There is one 22-gallon tank, located behind the pilot, and filled from the top of the fuselage. Control harmony is good, not squirrely. Rudder is required if the ball is to be centered. Controls are not as crisp as on some other models, but you also don’t feel like you have control of a kite. Flight characteristics are more in line with a Cessna 152 or a Cherokee. This airplane is not a glider, and in descent mode, it is a lot like a Cherokee, with a significant power-off sink rate. In some ways, it even reminded me of the Bellanca I normally fly, which glides like a brick. In fact, in its current configuration, the diesel model requires power all the way to the flare. Stalls are not only a non-event, they are practically non-existent. Power off, the airplane simply mushes down, never dropping the nose or a wing. Power on, it hangs on the prop, refusing to break. The airplane has flaps, but they aren’t used unless you need to get down fast. It takes off and lands fine without them. The only slightly bad trait it has is the high sink rate if airspeed is allowed to get too low on final. Of course, that can be said of any airplane, and the solution is to not let airspeed get low on final. Even if a student were to drop it in from a few feet however, minor drops are easily absorbed by the oleo struts that are designed to take a little pounding. Airspeed is easy to control. It is a very easy to fly, forgiving airplane.

Is that good or bad? Well, it depends on what you’re wanting. If you’re looking for sex appeal, short field performance, blinding climb rates, and aerobatic type controls, this may not be the airplane for you. On the other hand, if you’re looking for a good flying, solid, fun, dependable airplane, it just might be. If you’re looking for an excellent, economical trainer, you’ve found it. While I have said that other light sport models might not be good primary trainers, the Thorpedo changed my attitude. A person who received their private ticket in a Thorpedo would have no more difficulty transitioning to something more complex than they would had they trained in a 152. Don’t let the light sport certificate fool you. This airplane is light only in name, not in handling. It would be a fun airplane for a student to learn in, and an economical airplane for a flight school to acquire and operate.

Flight characteristics of the diesel are a little different. The engine creates a lot of torque, and absent rudder trim, a strong right leg is required. You don’t notice it so much on the takeoff run, but when you get in the air, it wants to not only turn left, but to a certain extent roll left. This was especially noticeable on my first takeoff when we didn’t realize the airspeed indicator on the prototype was reading high, and pulled it off a little before I should have. At the slower airspeed, it really wanted to roll. Waiting until it was ready to fly helped, but there was still a slight roll tendency, but nothing a little right rudder couldn’t handle. The diesel currently swings a three-blade prop which adds to the torque as well. The prop choice isn’t final, and could change, but most likely p factor will be significant whatever propeller is installed. The diesel also has a higher sink rate, but when I flew the plane, the oil cooler and radiator provided a rather large frontal area that will soon not be there, and that might cut down on some of the drag. It will be interesting to see what differences there are in the two when the design of the diesel configuration is final.

Performance figures are probably going to be similar between the two engines. Currently, the initial rate of climb on the diesel appears to be a little less, possibly because of weight, drag, and the prop. The diesel is at least 70 pounds heavier than the Jabiru. However, the diesel is turbocharged, so as it climbs higher, it should climb better. Airspeed is going to be limited by the light sport rules, and since the Jabiru-powered airplane is near the maximum speed anyway, the diesel can’t be much faster. The main advantage of the diesel is going to be fuel burn, TBO, and fuel availability, especially in other countries. Fuel burn is about 3.5 - 4 gph, as opposed to 5 in the 120 hp Jabiru. Anticipated TBO is a minimum of 2,000 and likely as much as 3,000 hours. At this point, the diesel has to be sent back to the factory in England for overhaul. However, as more of them go into service, it is anticipated that field overhaul will be available.

The Thorpedo can be flown with the canopy open. It has to be closed for takeoff and landing, but at all other times, your hair can blow in the breeze. I tried it, and while it’s a little noisier, it’s not bad, and the wind inside the cockpit isn’t bad at all. If I owned one, I’d probably have the canopy open a lot, especially on summer days. It would be a great mount to cruise at 500 feet on a cool summer evening, experiencing the sights and smells of the earth below.

Indus Aviation has a global marketing perspective.They currently have multiple aircraft deals working in several countries around the world, and with the need to train pilots in third world countries, that will likely be a good market, especially for their diesel. There is a manufacturing plant in Bangalore, India that produces some of their parts, mainly skins and other stamped metal parts for the U.S. operation, and produces full aircraft for overseas demand. Many of the machined parts are being produced locally in Dallas. Indus maintains a full inventory of parts for the plane.

One unique aspect of the Thorpedo design is the ribbed wing skin. At first glance, you wonder if John Thorp tore down somebody’s barn and used the tin for his skins. There is, however, method behind his madness. The purpose of the ribbed skins is strength and simplicity. The Thorpedo wing only has four ribs, the rest of the wing stiffening coming from the unique skin. This cuts down on weight and complexity, while not compromising strength. According to Indus, while it doesn’t impact flight characteristics it doesn’t make for a particularly attractive design, and Scott Severen, sales manager, admits they may have lost some sales because of it. Will they ever change the wing? Scott was noncommittal, but didn’t rule it out. Making the change would probably add weight without adding anything other than eye appeal.

One thing is certain about the light sport market. There are too many designs for the market to support. I believe the Thorpedo will be one of the survivors. Indus is targeting flight schools, and I think you’re going to start seeing more of them on ramps, not just for light sport training, but for private and even instrument training. Indus is selling airplanes, with a several month backlog in orders. Indus has a number of advantages, including the near-term availability of diesel power, especially overseas where avgas is difficult to find.
I’ll admit to a bit of skepticism when I initially looked at this design, but having looked close and flown these airplanes my skepticism has been replaced with respect, not only for the design, but for the people behind the design.

If you’re in the market for an LSA, you might want to check this one out. I think you’ll be impressed as well.