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Marske Aircraft Pioneer I

marskpione


Based on experiments with the XM-1 glider, Jim Marske, aided by Bill Daniels, flew the Pioneer I in 1968. The Pioneer is the second Marske tailless sailplane to fly. This glider was distinguished from the earlier one by a higher aspect-ratio wing of trapezoidal planform, with a slight forward sweep. The Pioneer I used a NACA 23112 modified by raising the trailing edge. Roll control was by spoilers, but the machine used a more conventional fuselage-mounted rudder for yaw control. This was later modified after the first fight tests by adding a fixed portion in order to improve its effectiveness. The span of the glider was also increased, justifiying a change of designation to Pioneer I-A.

Both the XM-l and the Pioneer I were low wing loading gliders, designed as proof-of-concept designs incorporating the maximum safety of the test pilot through very low take-off and landing speeds.

The controls were elevators hinged to the inboard trailing edge of the wing, conventional rudder/fin mounted on the centerline (originally, an all-moving rudder without fin), and spoilers for roll control.

The initial flight tests were conucted at the El Mirage dry lake to make long auto tows and long straight glides without worrying about obstacles. In March of 1968, in eight flights in a day it progressed from low, straight glides to 1000-foot altitudes and shallow S-turns without encountering anything but minor problems. One problem was a slower roll rate than hoped for, and the other was that the all moving rudder would oscillate at about 1Hz when free. The rudder could, however, be controlled easily so long as the pilot kept his feet on the pedals. Pitch control proved to be docile. The most apparent thing was the lack of inertia, about the pitch axis, allowing the pitch changes to be made almost instantly without any tendency to overshoot the new attitude.

The next day provided the first opportunity to try the Pioneer at Soaring. An auto tow to 1500 feet it into a small thermal. The climb took it to 11,500 feet ASL, (9500 above terrain).

Trying a gentle stall straight ahead, the stick reached the rear stop before a break occurred. With the stick held back, the ship flew straight ahead with no tendency to drop a wing. A pronounced low frequency buffet came from somewhere to the rear. Trying again with the nose well above the horizon resulted in a gentle break with surprisingly little loss of altitude, again no wing drop. Stalls from turns were no less docile. The ship maintained its bank angle and recovered flying sped immediately. It was becoming clear that while the machine could be momentarily stalled, it could not be held in a stalled condition.

This is apparently due to the fact that the elevator is part of the wing and is affected by the flow separation that starts at the trailing edge as the glider approaches a stall. As this flow becomes turbulent and separates, the up elevator authority is sharply limited. It is therefore not possible to increase the angle of attack to the stall. The effect is to limit the minimum airspeed to a safe value.

An objection is sometimes raised to this saying that if the up elevator authority is limited to prevent a stall, the landing speed will be too high. However, with the flying wing, during the flare before touchdown, the airflow under the wing produces a low-pressure area under the elevator. This is due to the venturi-like shape of the area confined by the under surface of the wing with a raised elevator and the ground. This low-pressure area occurs only when flying in ground effect and greatly assists the flare so the flying wing can be landed a very slow airspeed.

It was stable about all three axes, that it responded logically to all control inputs, that it would not spin however provoked, and that it retained its pitch stability on speed runs to100 mph. If the stick were released in straight and level flight at the trimmed airspeed, the ship would continue straight ahead for up to a minute, then gently enter a spiral. If the controls were released at a speed other than the trimmed speed, the ship would gently oscillate in pitch about the trimmed speed, with a 17-second period. This would damp out in three or four oscillations in smooth air. The oscillation was never noticed unless the stick was free. If the ship were placed in approximately a 30-degree yaw angle relative to the direction of flight and the controls released, the nose would swing back into line within three seconds, overshooting slightly.

A few points were not to liking. For one thing, while the roll spoilers' alone produced coordinated turns at 60 kt, they tended to produce skidding turns above that speed. The effect was gentle, but noticeable, and required rudder input to keep the yaw string centered. It is odd to need opposite rudder to make a coordinated roll into a turn at high speed. In slow turns it showed the normal glider tendency to overbank and opposite spoiler was necessary to stop it. This was disturbing because it resulted in loss of lift when it was needed most. In spite of circling in thermals with one spoiler open, the ship seemed to climb extremely well.

On the plus side it was found that the ship was taking less of a pounding during high speed runs than might be expected for a light wing loading machine flying in strong desert turbulence. The panel-mounted accelerometer showed a maximum of +2g, -1/2g for the 100mph runs, although pitch attitude changes were noted in turbulence. It seemed that the nose went up in down gusts, and up in down gusts, reducing the wing's angle of attack relative to the local flow quickly enough to prevent the machine from absorbing the load. This is no doubt due to the low moment of inertia about the pitch axes. On several occasions, while flying in formation with other sailplanes, they broke off saying it was too turbulent to be going so fast.

On the slow end of the airspeed envelope, the Pioneer proved to be a surprising performer. The Pioneer 1 would fly with the airspeed needle rested on the peg and refusing to indicateas the speed was below the instrument's threshold. This slow flight ability proved a tremendous asset in small, weak thermals, where it allowed extremely small diameter turns to be made.

From May to August of 1968, the Pioneer was modified to correct some of problems found in the original configuration. The opportunity was taken to lengthen the wings from 40 to 46 feet. This brought the wing area to 192-sq. ft. Considerable effort was made to reduce the moment of inertia about all axes. This resulted in an overall weight reduction of 10 pounds to 430 empty even with the longer wings. A new airspeed was installed that would indicate down to 10 knots. As a result of these modifications, the ship is now called the Pioneer IA.

First flight tests of the modified version began in August 1968. The handling qualities were generally the same as the 40-foot span Pioneer, with the exception that the rudder was stable and surprisingly effective considering the short moment arm. However, the roll rate was still disappointingly slow even though the spoilers had been moved forward and outboard.

It was decided that the roll rate might be improved if air were allowed to escape (in a "puff") from within the wing as the spoiler was opened. It was thought this would "trip" the flow quicker. They used gaskets to seal the spoiler box completely when it was closed. This small modification almost doubled the roll rate, cutting the time required to perform a coordinated roll from a 45-degree turn in one direction to a coordinated 45-degree turn the other way from 9 seconds to just under 5 seconds, measured at an airspeed of 50 knots.

The performance gain with the longer wings was quite noticeable. Flight comparisons seemed to suggest that it achieved just under 30:1 L/D at 50 Kts combined with very good climb performance. With the new ASI, the minimum airspeed was found to be an indicated 28 knots. At this speed the sink rate was not significantly more than at the minimum sink speed of 40 knots. Soaring flight has been achieved on all except one attempt, with an average time per flight of just less than two hours. Most of these flights were made from auto tows.

The over banking tendency noted in the early tests seemed to be less noticeable in the Pioneer IA. Glide path control on the Pioneer 1A was provided by dive brake/flaps hinged at the mid chord point on the underside of the wing. These acted to increase drag and to increase the coefficient of lift. At the speeds used for approach and landing, trim changes are negligible. At higher speeds there was a noticeable nose-down pitching moment.

The problem with the P1A is that the rudder and roll spoiler response axes didn't meet at anything close to 90 degrees. In fact, they were almost parallel. In effect, the P1A had two rudder systems and no ailerons.

The Pioneer 1A flew on for several more years. After moving to Colorado in 1969, the P1A flew on several long cross country flights. The ability to use very weak lift and, if that failed, land almost anywhere helped. On one flight, it reached 32,000' in the Pikes Peak wave. In this time almost twenty pilots to fly the P1A.

Pioneer I
Wing span: 40 ft

Pioneer IA
Wing span: 46 ft
Wing area: 192 sq. ft
Empty weight: 430 lb
Aspect ratio: 10.8
Fuselage length: 12.5 ft
Wing loading: 3.5 lb/sq.ft

 

 


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