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Hangar 9’s new F-22 Raptor basic trainer is the newest aircraft in their Progressive Trainer System (PTS) program. The first PTS airplane was the now famous P-51 Mustang PTS trainer (photo 1). The Mustang PTS has proven to be an excellent basic trainer and one heck of a great sport-scale airplane (photo 2).

Photo 1             Photo 2

The PTS concept is simple in its purpose and complex in its execution. Maybe that is why there are so many “doubters” out there in the RC community. The PTS idea is to provide the beginning RC pilot with a gentle, forgiving basic trainer that resembles a popular “scale” airplane instead of the traditional boxy basic trainer airframe. Then, as the student progresses to solo and then advanced flight, the PTS trainer can be converted, without any further purchases, to an exciting sport-scale airplane with real aerobatic performance.

A great idea and one that required some very advanced design work. The Mustang PTS used both large flaps and speed brakes to slow the airplane while adding some lift. It also used clear, drooped leading edges that converted the outside portions of the wing from a fully symmetrical airfoil to a flat bottom section. This greatly increased lift and added some drag. The drooped leading edges also increased the outboard wing area which helped some to reduce the Mustang’s usually fast roll rate and the airplane’s tendency to snap out of a tight turn.

The result was a very good basic trainer that looked just like the popular WW II fighter. But converting a propeller-driven WW II fighter into a basic RC trainer, while technically challenging, is really just converting one similar airplane into a gentler version.

A Mustang has a high(er) aspect ratio wing as does a basic trainer. The Mustang’s airframe is designed to use a front-mounted propeller for power. A Mustang has large wing flaps to slow it for landing. The airplane uses only the lift produced by its wings (remember, the stabilizer is also a wing) to fly as does a trainer. The P-51 has the same general airframe layout; single vertical fin and horizontal stabilizer, standard wing and front-mounted engine as do most basic RC trainers. Finally, the Mustang uses the same basic flight controls to maneuver.

But the F-22 is not even close to a basic trainer layout. The full-size Raptor has a short, trapezoidal, low aspect ratio wing: twin, angled vertical fins: tiny wing flaps and is powered (overpowered?) by twin pusher jet engines. The Raptor uses vectored thrust (the engine thrust is directed to increase pitch control) for maneuvering along with its control surfaces. Unlike a trainer, the fuselage is extremely wide (it provides lift for the full-size aircraft) and is flanked by giant air intakes. The ailerons are tiny and the airplane uses twin stabilators (the entire stabilizer moves as there is no elevator) for pitch control.

So how can anyone transform a high-speed, stealth jet with almost no wing area into an RC basic trainer? Simple. The Hangar 9 design team played a lot of tricks on the airflow by adding some clever devices. My guess is that if Kelly Johnson and the Lockheed “Skunk Works” design team played as many tricks with the F-104 Starfighter’s airframe, they might have been able to turn that demanding aircraft into aT-37-like, basic trainer.

Photo 3               Photo 4

Photo 3 shows that both of the full-size Raptor stabilators are located almost completely behind the fuselage. Hangar 9 moved them forward and replaced the stabilators with the traditional fixed stabilizer equipped with huge elevators (photo 4). They also stretched the fuselage to increase the control surface moment arms. The full-size Raptor just doesn’t have a lot of wing area but at the supercruise speeds the F-22 is designed to best utilize, small wings provide enough lift and are assisted by the fuselage.

As photos 3 and 4 show, Hangar 9 increased the wing area significantly. They also made the ailerons and flaps much larger. A part of the increased wing area is formed by the removable leading edge/wing tip droops. Unlike the Mustang PTS, the PTS Raptor’s drooped edges add about 2 1/8 inches of wingspan per side. The actual wing tip is cuffed at the ends to help stabilize the airplane by providing an inward directed force that seems to act something akin to minor dihedral in a standard trainer.

Photo 5

But the most effective trick that the Hangar 9 design team played was the altered airfoil shape that those removable leading edge “cuffs” created. The Mustang PTS has droop cuffs that turn the outer wing section into a flat-bottomed airfoil similar to that of a standard basic trainer. The Raptor PTS has leading edge cuffs that go one step further. The Raptor’s cuffs create an under-cambered airfoil similar to some gliders and old, WW I aircraft (photo 5).

An under-cambered airfoil provides more lift per wing area than does even a flat-bottomed trainer wing. However, nothing is free. The price for all this extra lift is a huge increase in airframe drag. This extra drag will have an effect on the airplane’s performance and will be discussed later on.

Photo 6              Photo 7

As photo 6 illustrates, the full-size F-22 has huge fuselage air intake ducts to feed its massive Pratt and Whitney F-119-PW-100 35,000+ lb. thrust engines. Even though its version is propeller driven, Hangar 9 retained the large fuselage ducts for both appearance sake and to add some more drag (photo 7). Without all its drag devices, the Raptor PTS would be too fast for a good basic trainer.

Building the F-22 Raptor PTS

So that’s how Hangar 9 did it. By using larger control surfaces, flap deployment, devices to increase lift and drag plus some careful engineering, they worked hard to transform a supercruise jet into an RC basic trainer. But to find out how successful the transformation was, we first have to build the thing.

Photo 8                 Photo 9

Everything needed to get the Raptor built and fully equipped for flight comes in the box shown in photo 8. The equipment supplied is a step higher than usually found in most Ready-To-Fly (RTF) aircraft. The transmitter is the popular JR Sport SX600 6-channel computer system. The SX600 features digital trim with built-in memory, throttle auto cut, servo reversing, 10-model memory, Flaperon mixing, dual rates and exponential, a trainer system for dual controls, sub trim and most of the usual computer transmitter features.

There are no programmable mixes in the SX600. That makes it ideal for the first-time pilot or first-time computer transmitter user. The most important features are here but none of the confusing stuff. The SX600 is set up to control the flaps should the pilot desire to install the retractable version.

Photo 10

The RS600 6-channel receiver is an FM receiver with enough capability to direct both twin aileron and twin flap servos. The servos are the JR ST47 standard sport servos featuring 47 oz. /in. of torque with a 0.24 second transit time. The Raptor has all five ST47 servos factory installed and connected (photo 10).

Photo 11

The engine is the Evolution Power Treainer System "A" (actually the Evolution .46 version with mixture limiters) 2-cycle powerhouse. This engine has equipped the Mustang PTS, the Alpha 40 trainer and the Arrow advanced trainer RTF airplanes. It is well proven, immensely powerful (13,000 rpm turning an APC 11 x 5 in. propeller) and very tractable with a positive idle under 2,300 rpm. Mixture limiters prevent the new pilot from operating the engine either too lean (not enough fuel for lubrication) or too rich (too much fuel for a proper idle and engine speed changes).